South Asian Petrochem Limited
Customer Interaction Program
@ ASPET Edited by: Y.Somasundaram, B.E, PGD-PE, PGD-EE, PGD-TQM, PGD-MM
First Edition: 12/03/2005 Second Edition: 20/10/2005 Third Edition: 20/03/2006 Fourth Edition: 20/12/2006
South Asian Petrochem Limited Preface The aim of this technical guide is to provide our customers with thorough expertise in PET processing to Preforms & Bottles. The experience accumulated by various Resin Manufacturers, Preform Makers, and Bottle Blowers, has been collected and assimilated into this technical guide. PET resin to package encounters three unique factors in the polymer industry, the Intrinsic Viscosity (IV) drop, Residual Acetaldehyde (AA) during Preform making and the Strain hardening during blowing of preform, these needs to be effectively controlled during the process to get a perfect package, the guide will throw some light in these topics, to help to process PET better into a ideal package. The guide starts with the basics of Polyesters, Zimmer’s manufacturing technology. The main focus of this technical guide is to provide the Preform Makers and the Bottle Blowers adequate technical background for various processes during conversion of PET resin to a bottle. To enable them take decision, when they encounter processing problems, and solve it with easy. The guide also provides tips in design of preforms and bottles in the development of new preform / bottle designs for new packaging applications. Hope this will be an informative resource for engineers in the PET industry. In this fourth edition, we have added ―Single stage blow molding‖, based on the interest from our single stage customers. We thank all our customers, who have provided their valuable comments for improvement. We also thank Dr. Sushil K.Verma, Director General, CIPET for his comments for improvement, accordingly, we are also added a topic on ―Product testing‖. Please let me know your suggestion for improvement for this guide in terms of new topics to be added, which will help us in bring out our fifth edition more informative.
Fourth Edition Date: 20/12/2006
Y.Somasundaram Customer Technical Service
[email protected]
South Asian Petrochem Limited Content 1
Polyesters
4
2
Zimmer Technology
5
3
ASPET resin grades
6
4
ASPET 21CF: Energy saving & Easy blow
7
5
ASPET 19C: Low AA applications
10
6
Quality Parameters
12
7
PET Drying
14
8
PET Injection Molding
22
9
Stretch Blow Molding of PET
30
10
Single stage – Injection Stretch Blow Molding
37
11
Product testing – Preform / Bottle
41
Annexure 1: PET Injection Stretch Blow Molding – Thermal history
49
Annexure 2: IV of Preform / Bottle
51
Annexure 3: Definition of Natural Stretch Ratio
53
Annexure 4: PET Injection Molding Screws
54
Annexure 5: PET Preform Design
57
Annexure 6: PET Bottle Design
61
Annexure 7: ASPET 22CJ
62
Annexure 8: Coloring of PET
63
Annexure 9: Trouble shooting in Injection Molding
67
Annexure 10: Trouble shooting in Stretch Blow Molding
71
Annexure 11: Trouble shooting in ISBM
77
South Asian Petrochem Limited
Polyesters have hydrocarbon backbones with ester linkages, hence called polyesters.
The structure above is called poly(ethylene terephthalate), or PET for short, as it contains ethylene groups and terephthalate groups.
The ester groups in chain are polar, with the carbonyl oxygen atom having a negative charge and the carbonyl carbon atom having a positive charge. The positive and negative charges of different ester groups are attracted to each other, allowing nearby chains to align up in crystal form, so they can form strong fibers.
South Asian Petrochem Limited 2. Zimmer technology Pure Terephthalic acid (PTA) is reacted with ethylene glycol (EG) in the presence of catalyst, to form ethylene terephthalate, which is polymerized in melt and further in SSP (Solid State Process) to form Polyethylene terephthalate in form of pellets suitable for bottle grade application. Paste preparation: PTA is mixed with Ethylene Glycol and catalyst into a paste Esterification: The paste is fed to esterification reaction system consisting of two-esterification reactor. Esterification is carried out at atmospheric pressure. Water is the byproduct of the reaction. The byproduct is rectified and expelled. Pre-poly-condensation: Esterified product from esterification stage is fed to Pre Poly-condensation where poly-condensation reaction takes place under vacuum. The pre-poly-condensation takes place in two reactors. Final Poly-condensation: Polymer from pre-poly-condensation reactor is fed to the final polycondensation reactor where desired viscosity is achieved by temperature and vacuum. Final polymer is filtered though polymer filters and granulated by cutters. Solid State Poly-condensation (SSP): The chips produced from melt phase poly-condensation are amorphous in nature with lower IV. The low IV amorphous chips are converted to bottle grade chips in solid-state poly-condensation reactor. In solid-state poly-condensation process, amorphous PET chips are crystallized in Crystallizer 1 and Crystallizer 2 and then fed to SSP reactor. Desired molecular weight is achieved by temperature and residence time in a Nitrogen atmosphere. The reaction products and volatile impurities such as Acetaldehyde are removed by diffusion to gas stream. Chips from reactor out let are cooled and de dusted and conveyed to the storage silo for bagging.
2.1: Key Features of ASPET resin:
One largest Poly condensation melt line producing 500 MT/day.
Two SSP reactors working in tandem with Poly condensation line, ensures two product IV‟s at same time.
Zimmer‟s low temperature profile and high residence time ensures low residual AA in resin.
Zimmer‟s technology ensures better “L” color.
Low oligomer & Vinyl ester content ensures, lower AA generation in down stream injection molding process.
South Asian Petrochem Limited 3. ASPET Resin Grades Parameter
Units
Intrinsic Viscosity Carboxyl end group Acetaldehyde Crystallinity Dust content Moisture content
.dl/g .meq/Kg PPM % PPM
3.1: General grades: ASPET 19C ASPET 20C
ASPET 21C
0.76 +/- 0.02 30 max
0.80 +/- 0.02 30 max
0.84 +/- 0.02 30 max
1 50 100 max 2500 max
1 50 100 max 2500 max
1 50 100 max 2500 max
1.7 248 +/- 2 90 -0.5 to 1.2 Low AA Package
1.7 248 +/ 90 -0.5 to 1.2 General purpose
1.7 2 90 -0.5 to 1.2 High strength / pressure applications Carbonated soft drinks
PPM Weight / 100 chips Melting point L Color .b color
Gms o C CIE CIE
Applications Flat / Mineral water
Parameter
Units
Intrinsic Viscosity Carboxyl end group Acetaldehyde Crystallinity Dust content Moisture content
.dl/g
Weight / 100 chips Melting point L Color .b color
Gms
.meq/Kg PPM % PPM
Pharmaceutical container Liquor bottles Wide mouth containers APET sheet Strapping
3.2: Special grades: ASPET 20 HF ASPET 21CF
Sparkling water Beer Packaging
ASPET 22CJ
0.78 + / - 0.02
0.84 +/- 0.02
0.90 +/- 0.02
30 max
30 max
30 max
1 50 100 max 2500 max
1 50 100 max 2500 max
1 50 100 max 2500 max
1.55 ~ 1.7
1.7
1.7
PPM
o
C CIE CIE
Applications
Performance Highlights
253 ~ 255 90 -0.5 to 1.2 Hot fill bottles
248 +/- 2 82 -1.5 to –0.5 CSD + Power Saving Carbonated soft drinks Beer packaging 25% power saving in blow molder Low blowing temperature, o less by 8 C
244 +/- 2 90 -0.5 to 1.2 Thick wall containers 20 ltr water bottles. Thick wall cosmetic bottles. Clear preforms for wall thickness up to 10 mm Low processing temperature. High co-polymer content, 5.5%
South Asian Petrochem Limited 4. ASPET 21CF Power Saving & Easy Blow ASPET 21CF has inbuilt additive, to enhance absorption of Infra red radiation from IR lamps, during the Reheat Blow Molding Process. Enhance IR absorption of preform during re-heat provides following advantages;
Lower preform temperature during blowing. Lower oven temperature & less oven ventilation. Heating energy saving of 18 ~ 24 % in CSD preforms (High wall thickness). Heating energy saving of 14 ~ 18 % in water preforms (Low wall thickness). Increase in machine speed possible to a tune of 3 ~ 5 %. Better stretching of material around gate, due to better heat absorption, enhancing the stress cracking resistance. Wider process window due to uniform heat distribution across the wall thickness of the preform. Better wall thickness distribution and lower temperature processing, improves the overall strain hardening of the bottle wall and hence better gas barrier properties.
ASPET 21 CF Performance in Sidel SBO 20 Trial conducted at Partex Beverages, Dhaka Preform 29.3 gms 500 ml Contoured Rc Cola Bottles Parameters for No. comparison Unit ASPET 21C 1 Oven temperatures % Kw Zone 7 % 7.40 3.70 Zone 6 % 28.40 14.20 Zone 5 % 13.20 6.60 Zone 4 % 20.50 10.25 Zone 3 % 51.20 25.60 Zone 2 % 54.00 27.00 Zone 1 % 32.30 19.38 2 3 4 5 6 7
Oven Temperature Preform Temperature Delta (Preform Vs Oven) Power meter (Total) Blowing Rate Blower Throughput
o
C C o C o
Amp % BPH
106.73 254 94.3 159.7 311 100 30682
Date
17/06/2005
ASPET 21 CF % Kw 6.70 3.35 18.60 9.30 13.10 6.55 17.70 8.85 36.70 18.35 40.70 20.35 29.50 17.70
Advantage
84.45
20.88 38 10.3 27.7 21.22 0 0
216 84 132 245 100 30682
9.46 34.51 0.76 13.66 28.32 24.63 8.67
Table 3: Power saving achieved in ASPET 21CF resin in SIDEL SBO20, with 29.3gms preform.
South Asian Petrochem Limited 4.1: ASPET21CF - Injection Molding Preform processing conditions are very much similar to processing of ASPET 21C. The additive does not play any role in the injection molding process. The preforms made with ASPET 21CF are dark in color due to the presence of IR heat absorbing additive. Normally it is not distinctly visible, when blown into a bottle.
Recipe file Product Preform Box. No.
Zone 9 8 7 6 5 4 3 2 1
1
Date Place Size Date
500 ml contour - Rc Cola Partex - ASPET 21C
2
3
1 1
Penetration Oven 4 5 6 7
1 1
1 1 1
1 1 1
Oven settings % 10 11 12
8
9
1 1
1 1
1
1
1 1 1
1 1 1
1 1 1 1
1 1 1 1
Preform temperature Oven Temperature Room Temperature Relative Humidity
94 254 32 65
80 60 35 75 80 75 50
1 1 1 1 1 1 1
o
C C C %
o o
Process Time, in Seconds Preblow Exhaust Blowing Compensation Process
Stretching 14.00 Diameter 8.00 Shoulder diameter 120.00 Shoulder height Gap between rod and mold base 2.50 1.06 Stretching Speed Preform ASPET 21C Material Clear Resin color 28PCO Type of neck 29.30 gms Weight 100.00 mm Total length 21.00 mm Preform Neck height 23.00 mm Exterior diameter Neck diameter 25.00 mm
0.150 0.400 0.960 1.560 1.630
1 1 1 1 1 1 1
17/06/2005 Partex 29.3 gms 15/06/2005
Distribution Oven 14 15 16 17
13
1 1 1 1 1 1
1 1 1 1 1 1
1
1
1 1 1
1 1 1
18
1
19
20
1
Production start up Ventilation Correction frequency Correction coefficient
% overall Heating 0.00 0.00 7.40 28.40 13.20 20.50 51.20 54.00 32.30
80 % 80 % 50 0.3
Angular position, in degrees Point 0 55.9 Point 10 67.95 Start of preblow 55.31 -0.5/10 Nozzle down 26 Nozzle up 293.8 Stretching up 180 Stretching Speed 1.059 m/s Pressures
mm mm mm mm m/s
Preblow Preblow rate Blow pressure Blowing SV control pressure Capacity Type Shape VXBO On mold - Neck & Bases On mold - Bodies Blowing rate
13 3 39 Min
Max
5.7
6.7
bar rev bar bar
500 ml Standard Contour - Rc Cola Partex 7126 12
degC degC
30682
bph
Table 1: SIDEL SBO20 blow molder setting for ASPET 21C, with 29.3gms preforms for 500ml Carbonated Beverages.
South Asian Petrochem Limited 4.2: ASPET 21CF - Blow Molding The blow parameters need to be retuned to reduce heat input by 14 ~ 24 % depending on the wall thickness of the preforms. High wall thickness preforms (4 mm), will require switching off of heaters in the first column of Penetration Zone & last column of Distribution Zone of the oven in Sidel machines, in machines like SIG all ovens are integrated together, hence switching of heater may be tried in the first column. Subsequently the heater percentages may be tuned to match the blowing condition of preforms. Water preforms and other preforms with ~ 3mm wall thickness; the preform blowing conditions may be achieved by reducing the heater percentages. Recipe file Product Preform Box. No.
Date Place Size Date
500 ml contour Rc Cola Partex - ASPET 21CF
17/10/2005 Partex 29.3gms 15/06/2005
Oven settings Zone 9 8 7 6 5 4 3 2 1
1
2
3
Penetration Oven 4 5 6 7
1
1
8
1
1
1 1 1
1 1 1
9
10
1
1
1 1 1 1
1 1 1 1
12
13
1 1 1 1 1 1 1
1 1 1 1 1 1 1
1 1
1 1
1 1 1 1
1 1 1 1
216 oC 84 oC
Oven Temperature Preform temperature Process Time, in Seconds Preblow Exhaust Blowing Compensation Process
Stretching Diameter Shoulder diameter Shoulder height Gap - rod and mold base Stretching Speed
Distribution Oven 14 15 16 17
11
0.150 0.400 0.960 1.560 1.630
1 1
18
19
20
% Heating
6.70 18.60 13.10 17.70 36.70 40.70 29.50
1 1
95 70
Production start up
Ventilation
% %
Angular position, in degrees Point 0 55.9 Point 10 67.95 Start of preblow 55.31 -0.2/10 Nozzle down 26 Nozzle up 293.8 Stretching up 180 Stretching Speed 1.059 m/s Pressures
14.00 8.00 120.00 2.50 1.06
Preform ASPET 21CF Material Grey Resin color 28PCO Type of neck 29.30 gms Weight 100.00 mm Total length 21.00 mm Preform Neck height 23.00 mm Exterior diameter Neck diameter 25.00 mm 48.00 hrs Preform age
mm mm mm mm m/s
Preblow Preblow rate Blow pressure Blowing SV control pressure Capacity Type Shape VXBO On mold - Neck & Bases On mold - Bodies Blowing rate
13 3 39 Min
Max
5.7
6.7
bar rev bar bar
500 ml Standard Contour - Rc Cola Partex 7126 12
degC degC
30682
bph
Table 2 SIDEL SBO20 blow molder setting for ASPET 21CF with 29.3gms preforms for 500ml Carbonated Beverages.
South Asian Petrochem Limited 5. ASPET 19C Low AA in Preform / Bottle Acetaldehyde commonly known as AA, is present in all citric juices, and is not harmful to humans for consumption. AA normally gives off taste and flavor to flat water. AA (Acetaldehyde) is generated during the melt polymerization and most AA generated during the melt phase is driven out during the SSP (Solid State Polymerization) process and the net resultant AA in PET chips of water grade resin is less than 0.60 PPM. Major portion of AA in preform / bottle is generated during the melt processing of SSP chips in extruder to make preforms. Since, the functional end groups in PET are highly sensitive to temperature. The high temperature and shear in screw / barrel, breaks Hydroxyl groups to vinyl ester groups. The reaction is as below.
R-CO-O-CH2-CH2-OH R-CO-O-CH=CH2 + H2O The new vinyl ester groups, further degrades to produce AA (Acetaldehyde) in the preforms.
H2O+ R0-CO-O-CH=CH2
AA + R0-COOH
The AA thus generated remains in the preform / bottle wall, after the processing is completed. This residual AA in the container, migrate slowly into the content of the container, when the temperature is above the boiling point. The AA in the bottle is reduced in long-term storage, by migration to air. But, there will hardly any change in AA during the Stretch Blow Molding of Preform to Bottle. o
The boiling point of AA at normal pressure is 21 C.
5.1: ASPET 19C – High Lights
Manufactured in latest Zimmer Technology with low temperature profile ensures low vinyl ester & oligomer content. Low residual AA in pellets around 0.6PPM. Low temperature processing due to higher copolymer content, so low AA generation, easy blow-ability. Low AA generation in molding due to low vinyl ester content. Low fines content. Low catalyst Low Global Migration.
South Asian Petrochem Limited 5.2: Tips for low AA in Preform / Bottle:
Reduce the barrel set temperature. Optimum temperature of processing of PET changes from machine to machine. Suggestion will be to reduce the temperature, without the haziness or un-melt. While lowering temperature, if you encounter actual temperature overshooting, more than set temperature, generation of higher shear heat is indicated, which can induce higher AA generation. Hence lowering can be discontinued.
o
Try inverse barrel temperature profile. .i.e., Feed zone temperature 5 ~ 10 C, higher than metering zone temperature. Reduce melt pressure during plasticization (backpressure). The lowering of backpressure will reduce the shear, but caution should be exercised that bubbles are not formed in preform. The optimum melt pressure will be 50 ~ 150 bar for low AA processing, though it may vary from machine to machine.
Reduce the screw speed. Lower screw speed will reduce the shear and thus reduce the AA generation. Avoid too low speed leading to increase in cycle time.
Screw idle time should be less than 2 sec, if higher, reduce screw speed to match. Inject the melt at slower rate. This will help to reduce the shear heating. Ideal injection rate for water preforms are < 10 g / sec. Minimum cushion for extruder and shooting pot is 5 mm. Reduce the manifold and NT temperatures of hot runner to minimum.
The above are some of the tips for reducing the AA, a combination of the same will help to have the AA under stipulated norms of 4 PPM for water preforms / bottles. Drying has considerable impact on AA generation. Higher moisture content will lead to hydrolytic degradation. Hence moisture content to be maintained less than 40 PPM, for lower AA preforms.
South Asian Petrochem Limited 6. Quality Parameters The vital parameters deciding the quality of PET resin are Intrinsic Viscosity, Brightness (L) and Yellowness (b) and Residual Acetaldehyde content. 6.1: Intrinsic Viscosity (IV): Intrinsic Viscosity (IV) is an indirect measure of polymer molecular weight. This characteristic helps customer to choose the right resin for right application. Resin with higher I.V crystallizes slowly, and has higher melt viscosity. The preforms from high I.V resin are stiffer and require higher force to stretch blow. The containers from higher IV resins have better mechanical strength and barrier properties. The properties are reverse with low IV resin. IV of PET is determined by solution viscosity techniques, where, a very dilute solution of polymer is passed through glass capillary viscometer, and time required for specific volume of solution and solvent is measured. Intrinsic viscosity of resin is computed using formula. Following are the computation procedure for Intrinsic Viscosity. Relative viscosity (RV) = Flow Time of Solution / Flow Time of Solvent Specific Viscosity ( Reduced Viscosity (
Sp)
RV – 1
=
Red
Intrinsic Viscosity (IV) =
)= Red
Sp /
c
at Zero Concentration
The intrinsic viscosity value is very specific to a solvent and set of condition, because, interaction of PET with different solvent is different; there are various solvents and conditions used in the PET industry. ASPET method Equipment: Ubbelohde Viscometer Solvent: Phenol + 1-2 Dichlorobenzene (60 : 40 w/w) Temperature: 25°C 6.2: Color (L, a, b): L, a, b are the three-dimensional characteristic for appearance of an object, light source. ―L ―defines the lightness, ―a‖ & ―b‖ defines the chromaticity. “L” denotes - Black to White: 0, for Black, and 100 denotes white. “a” denotes - Red to Green: +ve value indicates Red & -ve value indicates Green. „b‟ denotes - Yellow to Blue: +ve value indicates Yellow & -ve value indicates Blue.
The color values are measured using colorimetric spectrophotometer, which measures the wavelength distribution of reflected or transmitted light by the sample. Overall appearance of Resin, Preforms & Bottles is based on the various combinations of the above said color components. These values will vary from technology to technology and recipe to recipe.
South Asian Petrochem Limited ASPET measures sample in powder form. Measuring color values of resin in ground form helps to get the true picture of color values for the core polymer / Pellet. 6.3: Acetaldehyde (AA) Content: Acetaldehyde (CH3CHO) is a colorless liquid below room temperature with a boiling Point of 21°C. In melt phase poly-condensation, AA is produced as by-product and gets entrapped. The concentration is reduced drastically by diffusion at elevated temperature in SSP. ASPET method Gas Chromotograph: Cryogenically ground resin. Capillary: Carbowax – 30 M Incubation Temperature & Time: 150 oC & 30 minutes ASPET resin has residual Acetaldehyde around 0.60 PPM, ensured by Zimmer’s process. 6.4: Typical Value for PET Bulk Density(kg/m3)
817(poured)
Pellet Shape / Size o Melt Density at 285 C(g/cm3)
881(Vibrated) Cubical(2.8mmX1.8mmX2.7 1.2 g/cc
Density (Amorphous) Density (SSP chips) Heat of Fusion Thermal conductivity – Melt Thermal conductivity – SSP chips
1.33 g/cc 1.38 g/cc 58 J/g 0.213 W/m.K 0.242 W/m K
mm)
Specific heat, J/g.K
Specific Heat of PET 2.20 2.00 1.80 1.60 1.40 1.20 1.00 J/g.K
0
50
100
150
200
250
280
1.13
1.26
1.51
1.70
1.88
1.99
2.05
Temperature, C
South Asian Petrochem Limited 7. Drying of PET PET is very hygroscopic. PET granules can absorb as much as 3000ppm of moisture, under humid conditions. Moisture adversely affects the I.V, during plasticization of polymer, due to hydrolytic degradation of the molecular chain, with lower resultant IV of end product. Typically, the preform should have an I.V. of approximately 0.69 dl/g or greater to prevent problems such as haze, thinning bottle sidewalls, or brittleness. It is always better to have lowest possible IV drop, to have the full benefit of the resin paid for. The critical factors, which play a vital role in drying of PET, are
Dew Point of Drying Air Pellet Dwell Time Airflow Rate Inlet Air Temperature.
7.1: Dew point of hot air Most advance de-humidifying dryers have the dew point meter, to measure dew point of hot air at hopper inlet. A hand held dew point meter can be also used for dew point measurement. Engineers who are conversant with the age-old practice of measurement of dew point with dry ice can do the same with the help o of thermometer and dry ice in a glass tube & stirrer. The recommended dew point for hot air is – 40 C, and in o no condition the dew point should drop below – 20 C, for effective drying and good product quality. Following experimental graph gives the IV of the preform produced with various dew point values, from a base resin IV of 0.74 dl/g, with a moisture content of 1500ppm.
These curves indicate that having a low dew point is important, but it is not as critical as it was once thought. If the air temperature with flow rate, and pellet dwell time are optimum, a reasonably low I.V drop can be obtained. Even so, the dew point should always be kept as low as possible, preferably –40ºC or lower.
South Asian Petrochem Limited 7.2: Pellet residence time in hopper The pellet dwell time in the hopper can be calculated with the hopper full capacity, and resin consumption of the injection molder. 3
3
Hopper full capacity (Kg) = Hopper full volume (M ) X 850 (Kg / M ) 3,
Where, 850 Kg / M is the bulk density of the PET chips. Hopper full capacity (Kg) -------------------------------------------Consumption of Molder (Kg/hr)
i.e. Pellet dwell time, hrs =
This give the actual pellet dwell time in the hopper, but the effective dwell time for the calculation of drying should be arrived at by reducing the time required for increasing the pellet temperature to the drying temperature. This for practical purpose is approximated to ½ hr. Thus the effective pellet residence time in the hopper
7.3: Airflow rate The amount of air required to dry pellets to less than 40 ppm of moisture content by weight, depends of following factors,
o
Dew Point of hot air. C Pellet Dwell Time, hrs. o Inlet Air Temperature, C Moisture content of pellets. % wt.
South Asian Petrochem Limited Devices such as the Pitot tube, anemometer can be used to estimate air velocity and thus airflow rate. But, since the dryer hose has very short straight portions and bends, these tools may not be much suitable, due to turbulence in the airflow. The standard recommended air flow rate for drying is 0.062 cMm /Kg / hr (.ie. 0.062 Cubic meter of air per minute for every Kg / hr of PET dryer output) The other method for the estimation of the airflow rate in dryer, conventionally used by the processor is estimation of the return air temperature of hopper. This has been effectively proved working. The following graph provides effective tool for the estimation of airflow rate in a dryer.
South Asian Petrochem Limited
South Asian Petrochem Limited
7.3.1: Procedure of airflow rate calculation: Note: The dryer should have processed at least its full capacity for recording parameters for this calculation. (.i.e. approx. 8 ~ 10 hrs from the start of the dryer, depends on the temperature used) Steps for the calculation 1. Calculate the pellet residence time in the dryer. 2. Measure the actual air inlet temperature of the dryer. 3. Measure the air exit temperature at the exit hose of the dryer hopper. When fresh pellets are dumped into the dryer, the return air temperature will typically drop 15º–25ºC. The temperature will start to recover and peak will be reached at the just before the next dumping of pellets. The peak temperature must be used in estimating airflow rate. 7.3.2: Deducing the airflow rate from the experimental curves Once the pellet dwell time and the return and inlet air temperatures have been determined, the appropriate graph for a given pellet dwell time is selected. The point at which the inlet and return air temperatures intersect will indicate the approximate airflow rate. For example, assume the following values have been determined:
Pellet dwell time = 4h Dryer inlet air temperature = 155ºC Dryer return air temperature = 95ºC
By referring to Figure A-1, it is seen that the point the temperature values intersect will indicate an airflow rate of approximately 0.037cmm/kg/h. In such a case, an investigation should be conducted to determine why the airflow rate is so low. Potential causes include:
Air filters needs cleaning. Perforated screen in the bottom of the dryer needs cleaning. Dryer blower is undersized for the PET throughput rate.
South Asian Petrochem Limited 7.4: Effect of Moisture in Pellet
The air flow rate change with the dwell time because the drier has constant flow rate of air and with increase in resin throughput the specific air volume for Kg of resin processed / hour decreases and vice versa.
The second experimental graph has the entry moisture level of pellets increased from 0.15% wt to 0.35 % wt. Higher moisture of chips has resulted in the narrowing of optimum processing region, and the dryer process window gets reduced, and it requires close control for effective drying of pellets and thus the final preform IV.
South Asian Petrochem Limited 7.5: Effect of increase air flow rate in dryer process window From the experimental graphs, we could infer following facts for effective drying. Dew point of less than -20 C (-40 C recommended) Optimum drying temperature is range bound for a particular drying time. Higher temperature always does not ensure lower IV drop, though it may ensure better drying (.i.e. lower moisture content). In a similar way residence time of pellets in the hopper at a particular temperature is range bound beyond which the product IV will once again drop due to thermal degradation of pellets. In case of increased moisture in the pellets, the dryer process window gets reduced. A considerable improvement in the dryer process window can be effected by increasing specific airflow rate to the dryer hopper. The following experimental graph provides the significance of increased airflow rate.
Considering all conditions, let us assume a process engineer encounters a wet bag, where the moisture content may be as high as 1.0 % due to external moisture on the pellets, then how does he effectively process the material. The best possible procedure for the lowest possible IV drop in product may be as follows.
Opt for a mold with minimum throughput, so that the residence time can be kept more for the dryer. It also does another added benefit, which is very crucial, the specific airflow for dryer throughput, increases, as the blower capacity is constant. In case the dryer residence time shots beyond 11 hours reduce the hopper fill height to control the residence time to 10 ~ 11 hrs. Select a dryer inlet air temperature of 130 C, and maintain the pellets hopper inlet temperature of not less than 125 C. Under these conditions, always maintain the lowest possible screw speed, as the resin has to pick up additional 30 C from the extruder feed zone, as compared to normal optimum temperature of 150 ~ 165 C at the feed throat of extruder. A lower speed will help to reduce resultant shear with lower temperature of resin at hopper throat. Under these conditions, ensure that the specific air flow rate in the hopper is about twice that of normal (0.10cmm/kg/hr. min)
South Asian Petrochem Limited Thus, a product with minimum IV drop could be achieved (0.03 ~ 0.04 dl/g). Which otherwise could have been in the order of 0.10 dl/g, due to the combined effect of hydrolytic degradation at the hopper at high temperature, residence time & moisture. Also, degradation during melting due to very high end groups, in the dried resin initiating further chain breaking at extruder. 7.6: Dryer selection & design Following are the key aspects to be considered during selection of a dryer.
The dryer must be a regenerative desiccant type capable of maintaining a dew point of –40ºC. The hopper height to diameter (h/d) ratio is of utmost importance. As the h/d ratio increases, drying becomes more uniform because the pellets start to approach "plug flow," meaning all pellets experience about the same dwell time. The opposite is true of short hoppers with large diameters where "channeling" will occur and proper drying is practically impossible. This is because pellets near the hopper wall will have a very short dwell time. The h/d ratio should be at least 2:1, and preferably 3:1. The optimum dwell time for drying PET pellets is approximately 8.5 hours. With this dwell time, relatively low process air temperatures should ensure proper drying. The lower temperature translates into a very large savings in electrical energy. Therefore, in addition to providing more effective drying, a dryer capable of an 8.5 hour dwell time should reduce the drying cost. A good hopper dryer should have facility to control the level of the pellets in hopper, this will help us in reducing the hopper capacity, when the throughput is very low. This can help to avoid over drying and subsequent degradation in the dryer. In cases, where a hopper of 8.5 hour dwell time is impractical because of space limitations, it is strongly suggested that the dryer provide at least 6 hours dwell time. If this is also not accommodated in the shop floor two small hoppers connected in series should be ideal choice, one on the floor and one on the molding machine. It is also important that the hopper be well insulated and the dryer blower large enough to provide at least 0.062 cmm of air per kg for maximum intended processing capacity of the injection molder (pellets processed per hour). Having a built-in dew point alarm is also a desirable feature.
7.7: Dryer Maintenance After selecting a good dryer, it is very important to maintain well, so as to achieve best performance. Following checklist is suggested to observe optimum performance; 1) Air filters - Check daily. Fines or other contaminants will clog the filters and thus reduce the airflow. A flow rate of at least 0.062 cmm of air per kg of pellets being processed per hour is essential. 2) Dew point - Check daily. Air having a low dew point (–40ºC) is needed, so that the air can absorb the moisture from the pellets. A high dew point is usually caused by a) Air leak, in the dryer circuit. b) Poor regeneration of the desiccant, or a bad desiccant. If the desiccant is believed to be bad, the dryer manufacturer can provide assistance in testing it. Most manufacturers suggest changing the desiccant every year or two. 3) Heaters - Check weekly. This includes process air heaters and desiccant regeneration heaters. Consult your dryer manual for the proper regeneration temperature. Normally, it should be around 220ºC. 4) Hoses and connections - Check weekly. Air leaks can increase dew point and reduce airflow through the dryer.
South Asian Petrochem Limited 8. PET Injection Molding Injection molding is a process, where the polymer is melted under heat and shear to make a homogeneous viscous fluid, which is injected under pressure into the mold, with the shape of the product and where it is cooled to the final product shape and dimension. A basic injection-molding machine has following parts, Hopper: Where the plastic pellets are held ready for processing. In PET a de-humidified dryer hopper replaces it, where the material is dried to moisture level of less than 40 PPM, to make it suitable for injection molding.
Barrel: The barrel is a hollow tube, which houses the screw. Screw does the function of pumping and plasticification of pellets into a homogeneous melt. The barrel is surrounded by heaters, which supply around 33 % of the heat required for plasticization of the polymer. The barrel heater does the critical function of softening the pellets during start up, to enable the screw to start rotation at a lesser torque, without any damage. The heater also maintains the surface melt temperature and prevents from sticking to barrel. Nozzle: The nozzle forms the front end of injection barrel, it injects hot melt into the cavity, either directly through a sprue, or a Hot runner manifold as in case of a multi cavity mold. Nozzle normally houses the Shut-Off valve, which closes the extruder during the plasticizing process and avoids drooling of melt through the nozzle. Shut-off valve is present in all PET injection molders, as PET melt has low viscosity, which causes it to drool during refilling.
South Asian Petrochem Limited 8.1: Injection Molding Preforms The quality of the preform determines the quality of the final product the bottle. Thus the quality of the preform is very significant stage in the making of the final bottle. Following are the critical factors, which have effect on Final product (Preform): 1. Injection molding machine and equipments. 2. Resin characteristics, IV 3. Critical processing parameters; a. Drying temperature & Time b. Use of regrind c. Process temperatures d. Process Pressures e. Process durations & Cycle time. Injection molding equipment can be categorized as follows, based on the function. 1. 2. 3. 4. 5. 6. 7.
Dryer Plasticization & Injection system Mold Clamping unit Preform Handling De-humidifier Chiller
8.1.1: Dryer: The function of the dryer is to dry the material to less than 40 PPM of moisture, so that the material does not undergo hydrolytic degradation / deterioration in properties. (Drying is discussed in ―Drying of PET‖) 8.1.2: Plasticization & Injection system: The plasticization & Injection system may be same in case of normal machine, where extruder also functions as injector for the melt into the mold cavity. There are machines having independent shooting pot to inject the material into the mod cavity, where the extruder function is to only plasticize and supply the melt into the shooting pot. Some systems have packing ram in tandem with extruder, which takes over during hold phase, so extruder can start plasticizing melt during hold phase and thus saves cycle time. Extruder should have variable backpressure for optimum melt plasticization, depending on the IV and product requirement. Backpressure should not be excess to cause shear degradation of melt or too low to allow passage of air / bubbles into plasticized melt. 8.1.3: Hot runner: The hot runner forms most vital part of the mold. Hot runner conveys the plasticized melt from the barrel to cavities; this should be done with utmost care to avoid any melt separation, which can result in poor clarity, degradation & separation of colors. To ensure the same, following aspects needs to be considered during design and manufacturing; 1. Melt should not encounter sudden sharp corners. 2. Melt velocity should be constant from the moment it enters the sprue to the time it enters the cavity. o 3. Melt temperature should not drop or increase more than 5 C, of melt temperature at metering Zone. Any major change will lead to separation of melt layers, color, black specks and splay defects.
South Asian Petrochem Limited 8.1.4: Mold: The essential feature of good mold is the polish and the air venting. Air venting in particular is very critical, as melt enters the cavity at 10 ~ 12 g / sec, and the entrapped air has to be exhausted at the same speed, or else, the leading edge of the polymer melt can burn due to high temperature & pressure and cause burn marks. Typically a white mark found in the thread area of preform. Inadequate, air venting can also considerably increase the injection pressure, and may also result in flash at the neck ring opening due to use of high pressure used for packing the cavity. 8.1.5: Preform Handling: Advance systems have robots for collecting the preforms from the core pins and dumping on the conveyor. Most of these have cooling provision for preforms in the robot arms, so that the cooling time can be reduced in the mold, this saving the cycle time. When such cooling robots are available the preforms are ejected at surface temperature just below the Tg of o o PET, i.e. 78 C (the inner core may be of much higher temperature). Further cooling to < 45 C for packing is done in the Robot cooling. 8.1.6: De-humidifier: o
To maintain air around the mold at less than 8 C dew point. Key requirements / Advantages: 1. Avoids dew formation & moisture ring defect at low chilled water temperature. o 2. Lower cycle time possible due to cooling of mold with low temperature water 8 C. 3. Lower crystallinity of preforms & better blowing. o
o
The de-humidifier supplies air with dew point from – 2 C to 2 C. This air forms an envelope around the mold and prevents formation of dews on the cold mold surface. 8.1.7: Chiller: To supply cold water for the mold and robot handling system for quenching the melt to preforms. o
Operating temperature range 6 ~ 10 C. 8.2: Resin Characteristics The key parameter, which affects the process and the product, is the IV (Intrinsic Viscosity) of the polymer. Generally all bottle grade resins are made with IV ranging from 0.72 dl/g to 0.84 dl/g. An increasing IV has following effects on the process ability of the PET. 1. Higher processing temperature. 2. Higher back pressure required to break the crystals. 3. Higher injection pressure to inject the melt into the mold.
South Asian Petrochem Limited As IV shifts lower following are the effects on the bottle. 1. Lower IV resin or higher IV drop in processing will yield hazy preform. While increasing temperature will usually reduce or eliminate the problem, it is not a suggested as this will further reduce the IV of the preform. Lower preform IV will result in narrow blow molding process window. Such preforms eventually yield defective bottles. a. Uneven wall thickness distribution. b. Thick shoulder and base, with thin sidewall thickness. c. Brittleness in shoulder and gate area. d. Bottles failing in drop test. 2. Lower burst strength of the bottle. 3. Inconsistent blowing, need to frequently change the blowing parameters. Preform IV less than 0.68 dl/g is difficult to blow consistently. Resin IV less than 0.70 dl/g, is difficult to process, in normal injection molding. 8.3: Use of Regrind: Though use of hot runner molds in PET has greatly reduced the generation of regrind material, from runners and sprue. There is need to consume small quantity of regrind generated from start up and testing. Critical aspects of using regrind. 1. The maximum allowed regrind is 10 %. Any higher dosage can reduce the mean IV of the mix due to lower IV of the regrind. The mean IV should always be maintained above 0.76 dl/g. 2. Essential to maintain constant mixing ratio for uniform blow molding performance, as variation in IV due to variation in level of regrind will affects the consistency of preform blowing. 3. Proper feeding screw should be used in the hopper for uniform feeding, incase of using bottle flakes as these flakes tends to separate. Any separation of flakes in dryer or feed hopper will result in inconsistent preforms. 4. The effect of IV drop in the molding system (Dryer + Molder) also plays a critical role in the processing of regrind. If the system has very low IV drop, then higher dosage of regrind can be safely done. In no case the IV of the preforms should be allowed to drop less than 0.68 dl /g. 5. Some studies suggest that the use of 2 ~ 3 % regrind can reduce the AA level in preforms. But higher regrind dosage has always resulted in higher AA in preforms due to the higher AA content in the regrind and also the higher end groups. 6. As regrind do not have smooth corner, they tend to generate more fines while transportation to hopper and in the hopper. Thus process air filters need to be cleaned frequently. May be even once a shift. 7. Precautions to be taken, while using regrind in following aspects. a. Cleanliness of regrind. b. Moisture content of the regrind. Flakes can have higher moisture content and thus a cause for hydrolytic degradation during processing. Monsoon and high humid environments needs special care to avoid haziness and low IV preforms. c. The bottle and preform regrinds needs to be thoroughly sieved before loading into hopper dryer, as higher fines content can i. Choke the drier filters. ii. Dust marks in the preforms, as fines are difficult to melt under normal processing conditions. Thus, use of regrind even though financially rewarding, can open up a big box of rejection, if not handled with care.
South Asian Petrochem Limited 8.4: Injection Molding machine – Cycle time Break up Mold open
Ejection
Mold Closing Clamping Side
Cooling
Injection side
Screw Idle time Plasticization Injection Hold
Cycle time Break up for Direct Screw Injection Machine 8.4.1: Term Definitions: Dry Cycle time: The dry cycle time basically comprises of mold closing, clamping, unclamping, opening of mold and ejection stroke. Normally they are from fraction of second in advanced machines to 2 seconds in slow systems. Mold closing: The moving platen moves forward and closes with the fixed platen; on completion of closing the clamping tonnage is applied. Mold Opening: The clamping tonnage on the mold is released and the moving platen moves backward separating the two halves of the mold so that the product can be ejected. Ejection: During the process the components formed during the molding process is ejected from the core side. Basically the components stick to the core side as the product shrinks on cooling. The product may be ejected down or can be collected by a robot. Plasticization: Plasticization is the process of melting the polymer and mixing the same into a homogeneous melt. Injection Phase: During this phase the hot plasticized melt is injected into the mold, at high pressure, for PET the injection pressure range is 600 ~ 1400bars. Hold Phase: This phase starts after the entire cavity is filled with the melt. During the hold phase a reduced melt pressure is applied, about 30 % of the injection pressure. The additional material required for compensate the product shrinkage is filled in hold. Cooling: This time is given to cool the preforms, so that it could be handled without deformation in the handling system. This may differ from machine to machine. In case of machine with ROBOT handling o system, the preform may be ejected at around 80 C, whereas in case of direct ejection system, the o preforms may be cooled to as low as 45 C.
South Asian Petrochem Limited Ejection: The ejection of preforms to the conveyor or to the ROBOT handling system.
Mold Closing
Mold open
Ejection
Mold Clamped
Clamping Side
Cooling Injection Shooting Pot Hold
Plasticization
Extruder Side Plasticization Transfer
Screw Idle time
Cycle time Break Up for Shooting pot Injection Machines Screw idle time: This is the time when the screw is idling without plasticizing or injecting / transfer. This should as low as possible, may be 1 ~ 2 seconds. Transfer: This phase is only in machine having a separate shooting pot for injecting the melt into the cavity. During this phase the plasticized melt in the extruder is transferred to the shooting pot through the distributor. Packing: This phase is also applicable for the shooting pot type machine. During this phase, the shooting pot is packed with polymer under pressure. This follows the transfer phase, to adequately pack the polymer into the Pot. 8.5: Preform Molding – Process Setting Parameter Significance Typical Values Process Temperature 1 Hopper Throat First zone in the feed section of the The temperature set should not induce the screw. The function of temperature in softening of pellets, leading to this zone is to increase the heat agglomeration, hence the temperature, o content of the material, aiding the should be lowest, about 15 C less than melting in the transition zone plasticizing temperature, 270 ~ 275 deg C. 2 Barrel The barrel heaters provide around ASPET 19C: 275 +/- 5 deg C temperature 33% of heat required for ASPET 20 C: 280 +/- 5 deg C plasticization. Balance is provided by ASPET 21 C/CF: 285 +/- 5 deg C screw shear. ASPET22 CJ: 270 +/- 10 degC 3 Distributor The melt flow through distributor 2 ~ 5 deg C more than the Last zone in temperature without any change. Hence, maintain the extruder. temperature equal to melt temperature, to avoid melt separation 4 Shooting Pot The melt is stored here for next shot. 2 ~ 5 deg C more than the extruder temperature Temperature set should maintain the temperature. melt without separation, as crosssectional diameter is high.
South Asian Petrochem Limited 5
Injection unit Nozzle
6
8
Sprue Temperature Manifold temperature Nozzle NT
9
Screw Speed
10
Back pressure
11
Screw Idle time
12
Extruder Cushion
13
Transfer pressure
13
Packing pressure Packing time.
7
14
The narrowest cross-section melt Normally 5 deg C more than the Extruder channel in injection unit. Any ambient temperature. temperature variation will have remarkable effect on the melt temperature. Mold temperature Sprue transfers the melt from Injection 5 deg C more than the extruder Nozzle to Hot Runners. temperature. Runner Manifold transfers melt to 5 deg C more than the Extruder. individual cavities. Nozzle NT, is normally a Time Depends on mold / Preform weight / Proportional Heater. Keeps the melt Cycle time / incoming melt temperature. from crystallizing & settling. Setting (%) depends on volume of melt flow, temperature and condition of incoming melt. Plasticizing Parameters The screw rotation feeds, melts, and For 100mm screw speed is typically in plasticize the pellets. Screw speed the range of 50 RPM. depends on diameter of screw, it should be maintained minimum, for target cycle time. The backpressure aids in uniform Low IV & AA grades: 20~150 bar. plasticizing of melt, and uniform mixing High IV: 50 ~ 300 bar. of colorants. Backpressure may be higher by 50 bar, when colorants are used, for uniform dispersion of color. The waiting time of the screw before The screw Idle time should be as start of Injection / Transfer. Low idle minimum as possible, in the order of 1 ~ time, reduces separation of colorant 2 seconds. and melt. Ensure homogeneous melt. Extruder cushion is the length of melt in 10 ~ 20 mm front of the screw at end of Injection / Transfer. Extruder cushion avoids of variation in shot weight due difference in melt conditions & feed. Protects screw tip from damage. Transfer Parameters Pressure applied to transfer melts Typically 800 Bar or less than or equal to from extruder to shooting pot. maximum injection pressure. Recommended to maintain the pressure equal to Injection pressure or lower. The pressure used for packing the Normally, 65 ~ 85 % of the transfer melt into shooting pot. pressure. The duration of packing pressure. 0.5 sec
South Asian Petrochem Limited 15 16
Injection Pressure Injection velocity Profile
17
Injection time
18
Hold pressure
19
Hold Time
20
Cooling time
21
Transition position
Injection Parameters The maximum pressure for injecting the melt into cavity, in a laminar flow. The injection velocity profile is set to give a constant flow in the cavity, so that the molecules are arranged in a linear fashion.
The time required for filling the cavity with melt. Pressure required for packing the melt into the cavity, to compensate shrinkage during solidification.
The time required for packing the melt, to compensate shrinkage during solidification. Time required to cool preforms to packing temperature or handling temperature in case of robot m/c. The position of the Screw / Ram at which cavity is filled. Any additional melt injected further goes for compensating the shrinkage of Preform on solidification. Melt density is 1.2gms/cc, and Preform density is 1.33gms/cc.
22
Shot Size
The shot size is equal to weight of the product produced in each shot + Cushion
23
Cushion
The material in the front of the screw / Injection Ram
600 ~ 1400 bar, depending on the mold. Preferred: Three stage. Stage 1: Size / Volume equal to the end cap Preform. Speed 1: 30 ~ 35 m/s Stage 2: Size / Volume equal to body of the preform. Speed 2: 50 ~ 70 m/s (To match injection time) Stage 3: Size / Volume equal to Neck Finish of Preform. Speed 3: 30 ~ 35 m/s The rate of Injection for PET is 10 g/s ~ 12 g/s 30 ~ 40 % of Injection Pressure. Recommended: Three stages. Stage 1: 40% Injection Pressure. Stage 2: 35% Injection Pressure. Stage 3: 30% Injection Pressure. 1.5 times the wall thickness of preform. Divided into three equal portions, for each hold pressure. 2 For 3 Stage Robot machines: t /4. 2 For machines without Robot: t t = Wall thickness of preform. Typically: (Cushion + 10% of shot weight) 80 ~ 100% of the value represents the transition position. Low wall thickness preform ~80% High wall thickness preform ~100% Shot Weight: Preform weight X no. of cavities. Shot Volume: Shot weight / 1.2 Shot size: (Shot volume / Projected Area of screw or Injection Ram) + Cushion. 5 ~ 10 mm.
Process parameter setting for injection molding is critical, as the machines gets faster and faster and their throughput ever increasing any time consumed in setting a process right will increase the waste generated. To start the machine and run it with least start up waste, it is very important to have the basic right. Following are the basic parameters in injection molding, for which the values can be even set before starting the machine, with the experience the PET industry accumulated.
South Asian Petrochem Limited 9. Stretch Blow Molding of PET Stretch Blow Molding process involves the production of hollow objects, such as bottles, having biaxial molecular orientation. Biaxial orientation enhanced physical properties, clarity, and gas barrier properties. There are two distinct stretch blow-molding techniques.
Single stage. Two stage.
In the one-stage process, preforms are injection molded, conditioned to the proper temperature, and blown into containers—all in one continuous process. This technique is most effective in specialty applications, such as wide mouthed jars, cosmetics, where production rates are low. In the two-stage process, preforms are injection molded, stored and blown into containers using a reheatblow (RHB) machine. This technique is best suited for producing high volume items. 9.1: Benefits of Stretching PET There are two major reasons for stretching PET. The first is purely economic. Stretching allows thinner, more uniform sidewalls and thus less expensive containers. Second, with the stretching orients the polymer matrix, and hence dramatically improving the physical and barrier properties. Orientation is a physical alignment of the polymer chains in a regular configuration. Figure 1: Polymer Matrix
9.1.1: Typical Properties of Oriented & Un-Oriented PET ASTM Method
Unoriented
Oriented
—
0.25 (10)
0.36 (14)
F372
6 (0.4)
2.3 (0.15)
Oxygen Permeability, cm3·mm/m2·24h·atm (cm3·mil/100 in.2·24h·atm)
D3985
5.1 (13)
2.2 (5.5)
Carbon Dioxide Permeability, cm3·mm/m2·24h·atm (cm3·mil/100 in.2·24h·atm)
D1434
28 (70)
14 (35)
Tensile Modulus of Elasticity, MPa (psi)
D882
2,200 (320,000)
4,960 (720,000)
Tensile Stress @ Yield, MPa (psi)
D882
57 (8,300)
172 (25,000)
Property Thickness, mm (mil) Water Vapor Transmission Rate, g/m2·24h (g/100 in.2·24h)
South Asian Petrochem Limited 9.2: General Stretching Behavior of Polyesters Most materials fall into three basic categories of stretching behavior: elastic stretching, viscous stretching, and plastic stretching. Figure 2: (A) Stress vs. Strain
Elastic materials, like rubbers have memory effect, they return to the original shape after the force is relieved. In the contrary the ―Viscous materials‖, like the tar, does not have any memory it retains the newly formed shape, readily. PET falls into ―Plastics‖. A typical stretching sequence for PET is shown in Figure 3. As force is applied to the material, there is a region where very little stretching occurs (A). Here PET behaves somewhat like an elastic material: if it is stretched only a small amount and released, it will shrink back to its original size. If stretching continues past the yield point (B), however, the material will start to stretch and become thinner, causing permanent deformation. In this region the stretching continues at almost constant force. Once the material has been stretched past its natural stretch ratio (NSR) (D), a dramatic increase in force is required for additional stretching to occur. This is known as the strain-hardening region (E). It is here that the highly desired increases in physical properties are maximized. Therefore, it is critical that the natural stretch ratio be surpassed slightly during the stretching process. Figure 3: Typical Plastic Behavior
9.3: Factors affecting the Stretching Behavior of PET Preforms A number of factors can affect the stress/strain curve. Four major factors are
Intrinsic viscosity (I.V.) Copolymer level. Temperature of blowing Moisture level of PET preforms.
South Asian Petrochem Limited Figure 4: Effect of Intrinsic Viscosity (I.V.)
Figure 5: Effect of Copolymer Level (CL)
Figure 6: Effect of Temperature (T)
Figure 7: Effect of Moisture Level (ML)
Since, all four factors have effect on the ―Stress – Strain Curve‖, a fundamental of stretch blow molding process, a thorough understanding and control of these parameters is necessary to have a good bottle.
South Asian Petrochem Limited Intrinsic Viscosity: Slight variations in I.V. can be compensated, by adjusting the preform temperature. As I.V. goes down, the reheat temperature should be reduced. If the preform I.V. is too low, however, it will become impossible to blow high-quality bottles. Copolymer Level: the resin manufacturer maintains Co-Polymer levels constant and they do not change during the processing, hence the customer need not worry on the effect of the same, if he is not changing to a different resin. Temperature: By adjusting the lamp settings, preform temperature can be optimized. Typically, temperature changes should be made in small increments; allow at least 5 minutes for the machine to equilibrate before assessing the effect of the change. The temperature of the preform should be maintained, not more than 2 degC above the temperature at which the preform yields the pearlescent bottles (this is the lower processing temperature of the preform, can be called as (P+2)). Ideally (P+2) temperature will result in better strain hardening for the particular preform / bottle combination, and will be different for different Preform / Bottle combination. It is also important that the temperature of the preforms being fed into the reheat blow machine is constant. Preforms that are colder than set; will tend to yield bottles that are pearlescent. Preforms that is hotter than set will tend to yield bottles that are hazy. To eliminate the need to adjust the temperature of the machine to compensate for varying preform temperatures, maintain a constant preform storage temperature. In cases where preforms have been stored in unusually cold or hot conditions, allow the temperature of the preforms to reach the normal preform temperature before feeding them into the machine. This process is known as conditioning of the preforms. Normally the preforms are conditioned for 6 hrs in the blow molding room, for a tropical environment, were the temperature changes are not more than 5 deg C. Moisture Level: PET preforms start to absorb moisture almost immediately after they are molded. Within a few days, the amount of moisture absorbed can start to have a rather dramatic effect on the natural stretch ratio (NSR) of PET preforms. Reducing the reheat-blow temperature will help compensate for higher NSR due to moisture. However, if the preforms get too "wet" because of long exposures to high humidity, it may become impossible to make high-quality bottles from them. Therefore, the best results can be obtained when storage time and conditions are consistent after molding— preferably no more than 2 days at 24°C with low humidity. This condition does not exist in Indian environment, were the preforms are blown at the filler location, depending on the production need. To have a uniform blown properties of the bottle, preforms of not more than 2 days of production interval should be mixed during the continuous loading in the hopper. If different age of preform is to be blown, the earlier preforms should be emptied from hopper before next batch is loaded. The process parameter shall be retuned to suit the preform with different age (moisture level). 9.4: Re-Heat Blow Molding The critical stage of the Re-heat blow molding process is the design of the preform to match the bottle design. For all the discussion under this topic, we assume that the preform is suitably designed for the blow shell. In the stretch blow molding process, the preform is placed inside the blow mold and preblow (low pressure) air is introduced. As pressure starts to build within the preform, there is very little initial expansion. The stretch rod pulls the preform down till the base. An aneurysm (bulb) starts to form in the middle of the preform. At this point, high-pressure blow air is used to finish blowing the prebottle out against the inside wall of the mold.
South Asian Petrochem Limited When blown correctly, the bottle should have a wall thickness that is relatively uniform starting at about the middle of the shoulder and going down the entire sidewall to a point about midway around the base. If the prebottle is not formed, the center section of the sidewall will be much thinner than areas toward the base and shoulder region of the bottle.
Number of variables can prevent the formation of the proper prebottle. These include:
Preform I.V. too low due to improper drying. Preform I.V. too low due to the use of too much regrind. Preform temperature too high. Preform moisture level too high. Preform designed for a different NSR resin.
In SBM, prebottle condition defines the NSR position of the preform. The pre blow pressure and the stretching of the rod will be able to take the preform to the pre bottle condition (NSR), beyond which additional force required would be delivered by the high blow pressure. The surface area of the mold should be more the Pre bottle (NSR, at the particular blown condition for the preform) by 5 ~ 10 %, to get a final bottle with best performance, in tensile & barrier properties. If prebottle is larger than the mold cavity, when preform is blown, the surface of the mold stops expansion of the preform before the NSR. This results in bottles having thin sidewalls and thick shoulders / bases.
South Asian Petrochem Limited 9.5: Re-Heat Blow Molding – Start Up Anomaly & its Significance Pearlescence and haze are both aesthetic defects that reduce the clarity of a bottle. While both have the same general hazy appearance, the cause for each is entirely different. When a clear preform becomes hazy in the RHB process, it is because the preform was too hot and started to crystallize. In the vast majority of cases, crystalline haze is due to low I.V. This is because the crystallization rate of PET increases as the I.V. decreases. Since a preform is heated from the outside, haze occurs on the outside surface.
Pearlescence, on the other hand, occurs on the inside surface. It is due to two factors. First, the inside surface of the preform must stretch approximately 35% more than the outside surface for a 2-liter bottle. Second, the inside surface is the coolest part of the preform and therefore does not stretch as easily as the warmer areas toward the outside. The combination of both factors causes the PET to stretch much too far into the strain-hardening region, producing thousands of very tiny cracks. These micro fractures affect the passage of light and cause a somewhat hazy appearance. Pearlescence can be distinguished from crystalline haze by its luminous sheen that is similar to an oyster pearl. When the preform I.V. is correct, it is relatively easy to find an RHB temperature setting that will heat the inside surface enough to avoid pearlescence but not so hot that the outside becomes hazy. If the preform I.V. gets too low, however, it may become impossible to make clear bottles. In these cases, increasing the temperature to eliminate pearlescence will make the haze worse, whereas decreasing it to eliminate haze will make the pearlescence worse
9.6: Establishing Optimum Reheat-Blow Conditions To produce a uniform bottle sidewall thickness, RHB conditions must be properly adjusted. 1. The stretch rod should be used only to center the gate of the preform in the bottom of the bottle; do not use it to obtain orientation. If the speed of the rod can be adjusted, reduce it to the point where the gate moves off center. Then increase the speed in very small increments until the gate is again centered. Hold that speed setting. 2. Increase the line speed until a slight pearlescence appears. Then adjust individual heater temperatures to make the pearlescence uniform. Clear areas indicate the preform is hotter in that location; temperature in these areas should be reduced. Once the pearlescence is uniform, decrease the line speed to its normal setting. 3. If the machine is normally run at its maximum speed, a different approach will be required. Reduce the temperatures of the heaters until a slight pearlescence appears; then adjust the individual
South Asian Petrochem Limited heaters to make the pearlescence uniform. Once it is uniform, record the settings. Then increase the temperature of each by the same relative amount. For instance, if one heater is set at 80 and another at 40, and an increase of 5% is desired, increase them to 84 and 42 respectively. By doing this in small increments, the pearlescence should disappear in a uniform manner. 4. If the bottle sidewall thickness is not uniform, increase the preblow pressure and check the uniformity again. If this makes the sidewall less uniform, decrease the preblow pressure. 5. Once optimum preblow pressure is established, further improvements can sometimes be made by adjusting the timing of the preblow relative to the timing of the stretch rod. Therefore, change the preblow timing so that it starts sooner, as well as later, in the sequence and assess the results to determine which timing is best. NOTE: The preblow pressure and the time can have a great effect on the thickness of the shoulder and base of the bottle. An increase in both will usually increase base thickness, while a decrease in both will typically increase shoulder thickness. 6. If there is no noticeable change in wall thickness, by changing the preblow pressure and its timing, increase the high-pressure blow delay time. As high pressure may be coming on too soon, thus not giving preblow chance to do its job. 7. If changes in the high-pressure delay do not give the desired thickness uniformity, try increasing the stretch rod speed in small increments. 8. As a last resort, change the heater profile. Normal practice of tuning the heater profile, practiced by many engineers as quick measure, reduces the performance of the bottles, even though it may not be visually found. A drop in properties, like GV, & top load is very much imminent. When adjusting the machine, never change more than one variable at a time.
9.7: Bottle Shrinkage and Creep After a bottle is blown, it will shrink as the internal stresses relax. While most of the shrinkage occurs immediately after blowing, the bottle will continue to get smaller for several days. The reverse of this occurs when the bottle is filled with carbonated soft drink; the high pressure [typically 0.41 MPa] will cause the bottle to "creep" or expand. It is important that the degree of shrinkage and creep be controlled since most bottlers have strict specifications on the maximum limits. Reheat-blow conditions have a great effect on shrinkage and creep. Bottles blown from preforms that were heated to a temperature just slightly above the pearlescent point [approximately 95°C] will shrink the most but will have the best resistance to creep. The opposite is true of bottles made from preforms that were heated to a higher temperature, just slightly below the haze point [approximately 105°C]; these bottles will shrink less but creep more. While preform temperature generally has the greatest effect on the level of bottle shrinkage, it is important to note that the temperature of the blow mold will also have an effect. Warmer mold
Higher post mold shrinkage.
Cold mold
Lower post mold shrinkage.
This is because a warm mold tends to promote relaxation of internal stress. For the same reason, bottles stored in a warehouse will shrink more during the hotter months.
South Asian Petrochem Limited 10. Injection Stretch Blow Molding Single stage machine process resin and provide bottle as output. Single stage machines are basically of two types four station versions and three station versions. These machines mold the preforms in first station and blow them to bottles in their 2 / 3 stations. The main advantage of these machines is that they do not require additional energy to heat the cold preforms for blowing, as in two stage machines. As here the preforms are ejected from the mold at temperature close to blowing temperature and conditioned at the next station, so that the temperature is more or less uniform across the cross section of the preform, as required for blowing. In these machines the preforms are held in position, without any rotary motion imparted to them. Hence it is possible to heat the particular sector of the preform and control the blow-ability of the preform sector wise, which is extremely helpful in blowing, unsymmetrical cross section containers, such as Kidney shape used in liquor, oblong shapes used in cosmetic industry, with uniform wall thickness along circumference. In single stage machines the preforms are held by means of neck rings as it is molded, and hence there is no tilting or variation in positions as they are moved between stations. The accurate position of preforms help them blow uniformly without any gate offset and variation in wall thickness for bottles with volumes less than 60 ml, which becomes extremely difficult in case of two stage machines due to small size of preforms and inability to hold tight at exact position. The containers made out of single stage machines always have less scratches and blemishes, since the preforms do not come in contact with each other. In two stage process preforms are packed together in cartons, rubbing of surfaces of preforms and hard gates of preforms, causes, considerable scratches. Which when blown spoils the surface appearance of the bottle. Functional units of Single stage machine: Extruder: Plasticize the pellets into uniform melt. The L/d ratio of the screw used in these extruders are 21:1 to 25:1, to aid uniform gentle plasticization.
Conditioning Station Condition the preform for blowing
Injection Station make preform Extruder: Plasticize the pellets
Blow Station Blow into bottle
Ejection Station Eject the bottle 4 Station Stretch Blow Molding Machine
South Asian Petrochem Limited Injection Station: The injection station consists of injection mold mounted over a hot runner, which distributes the melt from the extruder to individual cavities, at uniform rate & pressure. The injection mold consists of three functional parts the Cavity, which is mounted over the hot runner; Core, which is fixed to the top moving platen; and the Lip / Neck rings, which forms the threaded portion of the preform, mounted to the indexing table. The number of sets of lip plates is equal to number of stations in the machine. The neck rings in the lip plate carries the preforms, to other stations, till ejection. Conditioning station: The objective of the conditioning station is to increase the temperature of the preform skin, and to maintain uniform along the cross section of the preform. The conditioning station is present in four station machines. In case of three station machines the conditioning process is accomplished by preform design and cooling channel balancing in the injection mold.
Blow Stage: Blow into bottle
Injection Stage: make preform Extruder: Plasticize the pellets
Ejection Stage: Eject the bottle 3 Station Stretch Blow Molding Machine The conditioning station consists of two features; the provision for heating the outer wall of the preform is called a ―POT‖, this normally provides additional heat to the external surface, which is cooled faster in the injection mold, due to higher cross sectional area of cooling water channels. The feature to heat or cool the inner surface of the preform is called the ―CORE‖. Cooling cores are normally used in unsymmetrical cross section bottles. They basically cool the preform interior for the particular sector, for which the thickness needs to be increased, or which forms the farthest surface of the bottle from the center. The conditioning core temperature is maintained by circulating water / oil from thermo-regulator. The schematic diagram shows the operation of a conditioning core, with elliptical cross section bottle. Here, you can see the major diameter of the ellipse is more than 2.5times the minor diameter. Hence the sector, which forms the minor surface needs to be stretched faster, before it comes in contact with the blow shell wall, vice versa, the sector which form the major surface of the ellipse, to be stretched slower, so that it does not thins, thus achieving uniform wall thickness along the entire circumference. To achieve lower temperature at sector which needs to be stretched slower, the conditioning core is brought in contact, to cool the sector. The sector which needs to be kept warm is relieved so as to maintain higher temperatures.
South Asian Petrochem Limited Preform Bottle
Conditioning core: Cool the sector, and increase wall thickness. Conditioning core operation – Cross sectional view
Heating cores are normally deployed in conical preforms, which have thick shoulder area, which needs to be heated more to distribute the wall thickness evenly. They are heated by electrical systems.
Schematic view: Heating core in operation
Heating Core UP: Not in use
Heating core Down: In operation
Conical preform
Heating pot consists of heating elements, heated electrically; there may be 2 to 5 horizontal zones depending on the length of the preform.
South Asian Petrochem Limited Heating Pot Zone 1, 2, 3
Bottle Cap Preform
Bottle
Schematic diagram: Heating pot operation
Blow Station: Blow station, blows the conditioned preforms into blown final bottle. The blowing is accomplished by means of primary blow, which is a low pressure blowing operation, in the order of 4 ~ 10 bars, which helps in forming the pre-bottle, during the stretching operation. The secondary blow or the high pressure blow is in the order of 25 bars to 35 bars depending the bottle stretch ratio and intricacies. Once the bottle is blown it is cooled by the blow mold surfaces to solidify the polymer matrix. Normally the blow mold is maintained at temperature of 20 ~ 25 degC.
Ejection Station: At ejection station the bottles are ejected.
South Asian Petrochem Limited 11. Product testing – Preform / Bottle Bottle forms the final product and preforms is the intermediate product, we need to check the quality at these two stages to ensure proper performance of the product at the final destination. Preform testing is more critical as the PET resin undergone transformation from a highly crystalline stage to an amorphous stage at higher temperatures and pressures, and hence the chances of occurring defects are more at this stage and identifying a defect at this stage will avoid subsequent wastage of material and energy at the next stage (Blowing). Preform quality check need to be conducted at various levels to ascertain the quality of the product; Level 1: Visual inspection (with normal light) Level 2: Visual inspection (With polarized light) Level 3: Dimensional inspection Level 4: Chemical inspection Level1: Visual inspection (Normal Light) Level 1 inspection is carried out with the naked eye with or without the aid of following tools; 1. White light table, with magnification glass, 5 X 2. Preform cutter. Most of the start up defects come under this category, during a regular run, few of the these defects may reappear due to faulty process conditions, which needs periodical check, as per the process machinery consistency, hence frequency of such check to be decided by internal Quality control. Level 2: Visual inspection (Polarized light) Level 2, inspection is carried out with aid of polarized light inspection table, and inference charts. The polarized light inspection indicates the nature of filling of the preforms and packing, by stress pattern made visible by the polarized light. Another critical defect identified by this method is the surface moisture, called the moisture ring due to the unique appearance. Level 3: Dimensional check Dimensional checks are one to confirm whether the preforms meet the nominal sizes and tolerances laid down during design. Dimensional checks are done by measuring devices during initial run of the mold / product qualification and during production normally a Go -No Go gauge is used to speed up the process.
The inspection aids for Level 3 process: 1. Vernier caliper 2. Screw gauge 3. Profile projector 4. Preform cutter 5. Go – No Go gauge
South Asian Petrochem Limited Level 4: Chemical test Chemical test are done to ascertain transition undergone by the polymer during drying, plasticization, and molding has not had any detrimental effect on the polymer, to affect the performance of the final product. The two properties which are monitored at this stage are; 1. Intrinsic Viscosity (IV) 2. Acetaldehyde (AA) The equipments used for this purpose and test methods are similar to the procedure done for PET resin, for details refer ―Quality Parameter – section 6‖. Defects could be further classified as 1. Indicative defects 2. Major defects 3. Critical defects Indicative defects: These defects are indicative of variation in quality of preforms due to variation processing conditions and raw material. These defects may not create problems in final product quality, but if they are not attended on time, may lead to more serious defects. Major defects: These defects can create problems in blowing and have appearance problem in final product (Bottle). These defects need immediate correction. Critical defects: These defects will create problem in blowing as well as functional problems in the final bottle quality. These defects need to be attended by stopping the machine in most cases as any delay in correction will lead to wastage of material. Classification of preform defects & effects No. 1
Defect Bubbles
Status Major
Test Level Level 1
2
Un-melt
Major
Level 1
3
Long gate
Major
Level 1
4
Stringing
Indicative
Level 1
5
Moisture marks
Critical
Level 1 / 2
6
Gate crystallinity
Critical
Level 1
7
Hazy preform
Major
Level 1
8
Knit line / crack in neck finish Black specks
Critical
Level 1
Critical
Level 1
Flash at neck finish
Critical
Level 1
9 10
Effect of defect Bursting. Double layer in bottle wall. Poor bottle appearance. Bursting. Poor bottle appearance Probability to hit upper row of IR lamps. Bottle gate deformation. Sticking to hot preform wall. Surface lines on the bottle wall. Bursting during blowing. White patch or pearlescence like bottle wall appearance. Bottle wall weakness. Stress cracking. Bursting during blowing. Weak gate / failure in drop test. Dirty bottle appearance. Unstable blowing operation. Bursting of neck finish during blowing. Poor seal integrity. Bursting during blowing. Poor appearance. Higher capping force. Improper capping / seal integrity.
South Asian Petrochem Limited 11
Flash below NSR
Major
Level 1
12 13 14
Indicative Indicative Major
Level 1 Level 1 Level 1
15
Gate dimpling Gate peel off Internal gate deformation Burn marks
Critical
Level 1
16
Heat splay
Major
Level 1
17
Preforms buckling Gate pin hole Color variation Spider web Void in gate Wall thickness variation, > 0.2 mm IV drop > 0.03 dl/g
Critical
Level 1
Critical Major Indicative Critical Critical
Level 1 Level 1 Level 1 Level 1 Level 3
Critical, CSD
Level 4
Higher AA, > 4 ppm Neck finish – Go – No Go failure Neck finish ID – Go – No Go failure
Critical, Water Critical
Level 4
Bursting during blowing. Appearance problem. Poor appearance around gate. Bursting during blowing Can cause dancing of preform in oven. Gate offset in bottle. Uneven bottle wall thickness and weakness. Weak bottle. Higher stress cracking probability. Higher gas loss in CSD bottles. Flavor change in water packed.
Level 3
Capping problem may arise
Critical
Level 3
Preform may not be held properly in blowing mandrel. Capping may be a problem in case of inner sleeve caps.
18 19 20 21 22
23
24 25 26
Poor parting line appearance in bottle. Preform holding and transfer problem in high speed blow molders. Poor gate appearance. Poor gate appearance Weak gate. Drop test failure. Bursting during blowing. Poor appearance Higher AA Lower IV / weak bottle Weak bottle. Poor surface marks. Preform cannot be blown
All the defects can be present in various degrees of intensity, which may call for difference in opinion than the ones discussed above, which require expert opinion or the final end user comments.
Shelf life: Normally the shelf life of the preform can be safely taken as 6 months, beyond which it may require further testing to ascertain the suitability for the particular application. The storage ambient conditions of temperature and humidity play a vital role in the shelf life of the preform. High moisture absorption of preforms can lead to increase in ―Natural stretch ratio‖ of the preform, which may lead to not optimally strain hardened bottles, with lower strength, barrier properties etc,.
South Asian Petrochem Limited Preform inspection sheet Date Operator Time Box no.
Machine Mold Cavities Weight Sample No. 1 2 3 4 5 6 7 8 9
Parameter
AQL
Damaged neck finish Gate problems, Pin hole, void Full crystallinity, > 10mm diameter Spider web Contamination, dirt, oil Drag marks, sink marks Wall thickness variation, > 0.2mm Weight variation, > 0.5gms Neck finish, Go No Go failure Sample size, Pcs. 32 50 500
Inspected by
0.65%
Defective 1.5%
4.0%
0 1 1 2 7 8
1 2 2 3 14 15
3 4 5 6 21 22
0.65% 0.65% 0.65% 4.0% 1.5% 4.0% 0.65% 0.65% 0.65% Total defects Status Acceptance Rejection Acceptance Rejection Acceptance Rejection Approved by
Typical preform inspection sheet
Bottle testing; Bottle forms the final product of the PET chain, and it is used in direct contact with the food material, which is stored, transported and protected by the inherent strength of the bottle. Hence the bottle testing should consider the product packed and the conditions it is subjected during the shelf life for design of a specific test method. The bottle test method and specifications will vary from product to product. All PET bottle testing methods are in-process in nature, to help aid the blow molding engineer to tune the machine to produce bottle to meet specifications. 1. 2. 3. 4. 5. 6. 7. 8. 9.
Bottle sectional weight. Bottle wall thickness. Bottle volume. Bottle top load Bottle drop test Bottle burst test. Bottle stress crack test. Bottle head space AA Bottle shelf life.
South Asian Petrochem Limited Bottle sectional weight: Bottle sectional weight measurement is an easy method to control the distribution of wall thickness in the bottle, as it can be done much quicker compared to actually measuring the wall thickness at all locations. Here the bottle is cut into three pieces horizontally; Section 1: Base section: Along the base mold parting line. Section 2: Panel section: Along the top limit of the panel. Section 3: Shoulder section: The remaining top portion. The sectioning is done by hot wire cutter, with wires positioned along the exact position. The weights of each section is calculated during the bottle design stage and adequate tolerance imparted based on the process, so as to enable the blowing engineer to take a quick decision on the process changes.
Base Section
Label Section
Shoulder Section
Cutting Location Typical CSD bottle with sectioning lines
Bottle wall thickness: Bottle wall thickness is measured to ensure that there is no variation in the wall thickness along the circumference of the bottle in a particular plane / section, due to gate offset or any other blowing variations. It is also helpful to ensure proper wall thickness at intricate sections, which are critical for the strength of the bottle like the petaloid legs. Bottle wall thickness is measured using a magnetic wall thickness tester, or by means of CCD scanning. As the scanners are quite expensive in normal circumstances a magnetic probe is used.
South Asian Petrochem Limited Bottle volume: Bottle volume test is done in order to ensure that the bottle is blow properly and also to control the volume of the content, as filling machines fill quantities by means of fill height and not by measurement, hence and variation in bottle volume will affect the product content. Normally bottle volume is measured at two conditions; filled to brim, which is called the brimful volume, and filled up the fill height, called the fill volume. Brimful Volume: In brimful volume the weight of the empty bottle is measured, and followed by weight of bottle with water filled to the brim level. The difference in weight give the weight of water, this value divided by the density at the measuring temperature, gives the volume of the bottle. Brimful volume is measured at time zero (immediately after blowing), and after 24 hrs, allowing for the bottle to shrink and set, the volume at ―T + 24 hrs‖ is always lower than the volume measured at ―T‖. Volume at ―T+24‖ is considered as the final volume. Fill Volume: Fill volume is the volume of the bottle, when filled up to the fill height. The fill level is measured from the lip surface / top of the bottle, and specified in mm. The fill height is as low as 25 mm in case of water bottle, as there is no need for a head space, to as high as 50 mm in case of carbonated soft drink bottle, to allow for adequate head space for carbon dioxide. The procedure for measurement of fill volume is also same as that of brimful volume, except here the water is filled up to the fill point. Bottle top load: Top load test is done to understand the stack ability of PET bottle crate, over another crate during storage. Top load strength or the nominal top load depend on the storage requirements, and differ for different bottle capacities and products. Top load test is conducted on a top load tester, with capacity up to 100Kgs.
No. 1 2 3 4
Typical top load values: Top load (Kgs) Bottle volume Water CSD 500 ml 7.5 35 1000ml 10 35 1500ml 12 35 2000ml 16 35
Bottle drop test: Bottle drop test is done to understand failure of bottle in drop fall during handling of bottles. Drop test are conducted under three conditions; 1. Vertical drop 2. Horizontal drop o 3. Oblique drop (45 ) All the drop tests are conducted for 1.5 meter fall of filled bottle up to the fill point. Vertical drop test: The bottle is filled up to the fill point and capped. The bottle is held by the neck and allowed to fall from 1.5 M on a flat concrete surface. The drop is done for 3 times for the same bottle continuously and the failure if present is recorded. Horizontal drop test: The bottle is filled up to the fill point and capped. Then it is held by the body horizontally and allowed to fall horizontally on the concrete surface for three times continuously and the failure if any is recorded. Oblique drop test: The bottle is filled up to the fill point and capped. Then it is held vertically above a o concrete slab inclined at 45 to the horizontal plane at distance of 1.5 M, the drop is repeated for 3 times and failure if any is recorded.
South Asian Petrochem Limited Bottle stress crack test: Stress cracking is the crazing or cracking that can occur when plastic is under tensile stress. PET material is strongest in a highly oriented state, such as in the sidewall of the container. It is most susceptible to stress cracking when it is in an amorphous state, such as in the area surrounding the center of the base (gate area), and under tensile stress. The following are the factors which can accelerate the stress cracking of bottle and eventually lead to bursting of bottle abruptly, with high force, which is dangerous. Hence it is important to understand the stress cracking strength of the bottle, through accelerated method. High alkalinity of environment Poor material distribution Excessive IV degradation Over carbonation Contact with incompatible chemicals High temperature exposure Objective: To determine the level of resistance to sodium hydroxide induced stress cracking, on a carbonated soft drink bottle. Principle: One known mode of stress crack attack on PET bottles is by hydroxide ion. A bottle that has more resistance to Sodium Hydroxide attack should be more resistant to stress crack initiators that a bottle may be exposed to during its lifetime. Apparatus: 1. 2. 3. 4. 5. 6.
Beaker / containers. stop watch or timer Compressed air regulated to 5.31 bars distilled water bottle closures 0.2% NaOH solution prepared with distilled water and solid NaOH.
Sample quantity: 2 set for each cavity of blow mold. Procedure: 1. Bottles should be less than 2 weeks old, then aged at 50C +/ -1 C and 50% RH for 24 hours. After aging, the bottles should be stored at 22C +/ -1C for a minimum of 16 hours. Label the bottles. 2. Prepare the solution of 0.2% NaOH solution. (Alkalinity 2.4 -2.6 g/l CaCO3). 3. Fill each bottle with the target net contents of water. (2L bottle would contain 2000ml of water) The water should be equilibrated to 22C +/- 1 C. 4. Pressurize each bottle with compressed air to internal pressure of 5.31 bars. 5. 5 minutes after pressurizing the bottles mark the liquid level on each bottle and then gently place each bottle into beaker of 0.2% NaOH solution at 22C +/-1C. The solution must cover the base. Start the timer and check at following frequency. Time Frequency of check 0 ~ 30 Continual check 30 ~ 60 Every 2 minutes 60 ~ 90 Every 5 minutes 6. Record the time to failure in minutes for each bottle. Failure is defined as a burst or a slow leak. A slow leak is evidenced by a visual fill point drop.
South Asian Petrochem Limited Report: 1. Complete fill out report form including Alkalinity of the NaOH solution, Room Temperature, Bottle Temperature, and Caustic Temperature. 2. Preform numbers and blow cavity numbers. 3. Time to failure in minutes 4. Location of failure, choosing one of five categories: a. Gate through or tangent to it. b. Amorphous region (around gate and stretch rod area) c. Oriented region (base of foot) d. Strap area e. Stretch rod impression 5. Type of failure, i.e. catastrophic (burst) or slow leak. 6. Manufacturing defects, if present. Bottle head space AA Bottle head space AA method check the amount of AA that has migrated into the empty space of the bottle over a period of 24hrs or higher depending on the requirement of the customer, under standard temperature (25 degC). This value is used to extrapolate the amount of AA that will migrate to the content after packing in storage. This helps the designers and quality controllers to design and control the impact of AA on the packed contents. Equipments used: Gas chromatograph with capillary column to sense AA in 1 ~ 10 PPB, head space sampler. Nitrogen gas for purging Temperature cabinet to maintain 25 deg C. Septum for sealing nitrogen purged bottles. Procedure: Freshly blown bottles collected from the machine are purged with nitrogen, so that the atmospheric air does not interfere with the analysis. The bottles are closed with septum and kept in controlled environment for 24 hrs. After 24hrs the bottle internal space air is collected by means of a head space sampler with a needle, and injected into the gas chromatograph, which gives the results in PPM (parts per billion). Shelf life: Bottle shelf life: Blown bottles do not normally have a fixed shelf life, and can be used after any duration, excluding the case of ―Hear set bottles‖. But the users may be aware of the fact that bottles undergo continuous secondary shrinkage after the primary post mold shrinkage. Hence brimful volume of the bottles could have considerable variation depending on the period of storage and the ambient conditions. Product shelf life: Product shelf life with the bottle is a complex analysis which depends on the product packed and the requirements of the product manufacturer, which may differ from case to case. In basic term, a shelf life study for product and package combination is to study the period till which the product can be safely stored in the package in a shelf without any degradation in its preset qualities for consumption. In case of a carbonated drink, a loss 15% of gas volume of carbon dioxide is considered as the end of the shelf life. i.e. directly proportional to the outward migration of carbon dioxide from the product. In case of beer packaging, apart from carbon dioxide outward migration, the inwards migration of oxygen is also critical, which causes stalling and settling of beer. Whereas in case of a crisp product the migration of moisture into the package which cause the loss of crispness of the product is considered for estimating the shelf life. Hence the equipment and procedure used for estimating and analyzing the shelf life will also differ from product to product.
