Product testing – PET Preform / Bottle Y.Somasundaram 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 pr oduct at the final destination. Preform testing is more critical as the PET resi n undergone transformation from a highly crystalline stage to an amorphous stage at higher temperatures and pressures, and hence the chances of occurring defect s are more at this stage and identifying a defect at this stage will avoid subse quent wastage of material and energy at the next stage (Blowing). Preform qualit y check need to be conducted at various levels to ascertain the quality of the p roduct; Level 1: Visual inspection (with normal light) Level 2: Visual inspectio n (With polarized light) Level 3: Dimensional inspection Level 4: Chemical inspe ction 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 t able, with magnification glass, 5 X 2. Preform cutter. Most of the start up defe cts 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 i nference charts. The polarized light inspection indicates the nature of filling of the preforms and packing, by stress pattern made visible by the polarized lig ht. Another critical defect identified by this method is the surface moisture, c alled the moisture ring due to the unique appearance. Level 3: Dimensional check Dimensional checks are one to confirm whether the preforms meet the nominal siz es and tolerances laid down during design. Dimensional checks are done by measur ing devices during initial run of the mold / product qualification and during pr oduction 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. Pr ofile projector 4. Preform cutter 5. Go – No Go gauge Level 4: Chemical test Chemi cal 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 mo nitored at this stage are; 1. Intrinsic Viscosity (IV) 2. Acetaldehyde (AA)
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 def ects can create problems in blowing and have appearance problem in final product (Bottle). These defects need immediate correction. Critical defects: These defe cts will create problem in blowing as well as functional problems in the final b ottle 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. Classificati on of preform defects & effects No. 1 Defect Bubbles Status Major Test Level Lev el 1 Effect of defect Bursting. Double layer in bottle wall. Poor bottle appeara nce. Bursting. Poor bottle appearance Probability to hit upper row of IR lamps. Bottle gate deformation. Sticking to hot preform wall. Surface lines on the bott le wall. Bursting during blowing. White patch or pearlescence like bottle wall a ppearance. 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 b lowing. Poor appearance. Higher capping force. Improper capping / seal integrity . Poor parting line appearance in bottle. Preform holding and transfer problem i n high speed blow molders. Poor gate appearance. Poor gate appearance Weak gate. Drop test failure. Bursting during blowing. Poor appearance Higher AA 2 3 4 5 Un-melt Long gate Stringing Moisture marks Major Major Indicative Critical Level 1 Level 1 Level 1 Level 1 / 2 6 Gate crystallinity Critical Level 1 7 8 9 10 11 Hazy preform Knit line / crack in neck finish Black specks Flash at neck finish Flash below NSR Major Critical Critical Critical Major Level 1 Level 1 Level 1 Level 1 Level 1 12 13 14 15 Gate dimpling Gate peel off Internal gate deformation Burn marks Indicative Indicative Major Critical Level 1 Level 1 Level 1 Level 1
16 17 18 19 20 21 22 Heat splay Preforms buckling Gate pin hole Color variation Spider web Void in ga te Wall thickness variation, > 0.2 mm IV drop > 0.03 dl/g Higher AA, > 4 ppm Nec k finish – Go – No Go failure Neck finish ID – Go – No Go failure Major Critical Critical Major Indicative Critical Critical Level 1 Level 1 Level 1 Level 1 Level 1 Level 1 Level 3 Lower IV / weak bottle Weak bottle. Poor surface marks. Preform cannot be blown Bursting during blowing. Appearance problem. Poor appearance around gate. Bursti ng during blowing Can cause dancing of preform in oven. Gate offset in bottle. U neven bottle wall thickness and weakness. Weak bottle. Higher stress cracking pr obability. Higher gas loss in CSD bottles. Flavor change in water packed. Cappin g problem may arise Preform may not be held properly in blowing mandrel. Capping may be a problem in case of inner sleeve caps. 23 Critical, CSD Critical, Water Critical Critical Level 4 24 25 26 Level 4 Level 3 Level 3 All the defects can be present in various degrees of intensity, which may call f or difference in opinion than the ones discussed above, which require expert opi nion or the final end user comments. Preform inspection sheet Date Operator Time Box no. Parameter AQL 0.65% 1 2 3 4 5 6 7 8 9 Damaged neck finish Gate problems , Pin hole, void Full crystallinity, > 10mm diameter Spider web Contamination, d irt, oil Drag marks, sink marks Wall thickness variation, > 0.2mm Weight variati on, > 0.5gms Neck finish, Go No Go failure Sample size, Pcs. 32 50 500 Inspected by Total defects Status Acceptance Rejection Acceptance Rejection Acceptance Re jection Approved by 0.65% 0.65% 0.65% 4.0% 1.5% 4.0% 0.65% 0.65% 0.65% Defective 1.5% Machine Mold Cavities Weight Sample No. 4.0% 0 1 1 2 7 8 1 2 2 3 14 15 3 4 5 6 21 22
Typical preform inspection sheet Shelf life: Normally the shelf life of the pref orm can be safely taken as 6 months, beyond which it may require further testing to ascertain the suitability for the particular application. The storage ambien t 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 bo ttles, with lower strength, barrier properties etc,. 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 prote cted by the inherent strength of the bottle. Hence the bottle testing should con sider the product packed and the conditions it is subjected during the shelf lif e for design of a specific test method. The bottle test method and specification s will vary from product to product. All PET bottle testing methods are in-proce ss in nature, to help aid the blow molding engineer to tune the machine to produ ce bottle to meet specifications. 1. 2. 3. 4. 5. 6. 7. 8. 9. Bottle sectional we ight. Bottle wall thickness. Bottle volume. Bottle top load Bottle drop test Bot tle burst test. Bottle stress crack test. Bottle head space AA Bottle shelf life . Bottle sectional weight: Bottle sectional weight measurement is an easy method t o control the distribution of wall thickness in the bottle, as it can be done mu ch quicker compared to actually measuring the wall thickness at all locations. H ere the bottle is cut into three pieces horizontally; Section 1: Base section: A long the base mold parting line. Section 2: Panel section: Along the top limit o f the panel. Section 3: Shoulder section: The remaining top portion. The section ing 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 ade quate tolerance imparted based on the process, so as to enable the blowing engin eer to take a quick decision on the process changes. Bottle wall thickness: Bott le wall thickness is measured to ensure that there is no variation in the wall t hickness 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 strengt h 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 a re quite expensive in normal circumstances a magnetic probe is used.
Base Section Label Section Shoulder Section Cutting Location Typical CSD bottle with sectioning lines 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 varia tion 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 brimf ul 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 we ight of water, this value divided by the density at the measuring temperature, g ives the volume of the bottle. Brimful volume is measured at time zero (immediat ely 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 t he bottle, when filled up to the fill height. The fill level is measured from th e lip surface / top of the bottle, and specified in mm. The fill height is as lo w 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 hea d 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 n ominal top load depend on the storage requirements, and differ for different bot tle capacities and products. Top load test is conducted on a top load tester, wi th 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 10 00ml 10 35 1500ml 12 35 2000ml 16 35 Bottle drop test: Bottle drop test is done to understand failure of bottle in dr op fall during handling of bottles. Drop test are conducted under three conditio ns; 1. Vertical drop 2. Horizontal drop o 3. Oblique drop (45 ) All the drop tes ts are conducted for 1.5 meter fall of filled bottle up to the fill point. Verti cal 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. T he 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 p oint and capped. Then it is held by the body horizontally and allowed to fall ho rizontally 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 t o the horizontal plane at distance of 1.5 M, the drop is repeated for 3 times an d failure if any is recorded. Bottle stress crack test: Stress cracking is the c razing or cracking that can occur when plastic is under tensile stress. PET mate rial is strongest in a highly oriented state, such as in the sidewall of the con tainer. It is most susceptible to stress cracking when it is in an amorphous sta te, such as in the area surrounding the center of the base (gate area), and unde r 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 crack ing strength of the bottle, through accelerated method. High alkalinity of envir onment Poor material distribution Excessive IV degradation Over carbonation Cont act with incompatible chemicals High temperature exposure Objective: To determin e the level of resistance to sodium hydroxide induced stress cracking, on a carb onated soft drink bottle. Principle: One known mode of stress crack attack on PE T bottles is by hydroxide ion. A bottle that has more resistance to Sodium Hydro xide attack should be more resistant to stress crack initiators that a bottle ma y 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 p repared with distilled water and solid NaOH. Sample quantity: 2 set for each cavity of blow mold. Procedure: 1. Bottles shoul d be less than 2 weeks old, then aged at 50C +/ -1 C and 50% RH for 24 hours. Af ter 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 fol lowing 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 ea ch bottle. Failure is defined as a burst or a slow leak. A slow leak is evidence d by a visual fill point drop. Report: 1. Complete fill out report form includin g Alkalinity of the NaOH solution, Room Temperature, Bottle Temperature, and Cau stic 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 thr ough 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 A A 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 c ontrollers to design and control the impact of AA on the packed contents. Equipm ents used: Gas chromatograph with capillary column to sense AA in 1 ~ 10 PPB, he ad space sampler. Nitrogen gas for purging Temperature cabinet to maintain 25 de g C. Septum for sealing nitrogen purged bottles.
Procedure: Freshly blown bottles collected from the machine are purged with nitr ogen, so that the atmospheric air does not interfere with the analysis. The bott les 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 sample r with a needle, and injected into the gas chromatograph, which gives the result s in PPM (parts per billion). Shelf life: Bottle shelf life: Blown bottles do no t normally have a fixed shelf life, and can be used after any duration, excludin g the case of “Hear set bottles”. But the users may be aware of the fact that bottle s 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 whi ch depends on the product packed and the requirements of the product manufacture r, which may differ from case to case. In basic term, a shelf life study for pro duct and package combination is to study the period till which the product can b e 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 volu me of carbon dioxide is considered as the end of the shelf life. i.e. directly p roportional to the outward migration of carbon dioxide from the product. In case of beer packaging, apart from carbon dioxide outward migration, the inwards mig ration of oxygen is also critical, which causes stalling and settling of beer. W hereas in case of a crisp product the migration of moisture into the package whi ch cause the loss of crispness of the product is considered for estimating the s helf life. Hence the equipment and procedure used for estimating and analyzing t he shelf life will also differ from product to product.
