Plastic Product Design Chapter I
Objectives •Gain an insight into the Design Fundamentals of Injection Molded Plastic Parts •What to do •What not to do
Objectives •Gain an insight into the Design Fundamentals of Injection Molded Plastic Parts •What to do •What not to do
Injection Molding Machine
• Mold halves closed • Clamping force increased • Reciprocating Screw advanced
Injection Molding Cycle I Phase
• Clamp remains closed during solidification • New charge brought forward • Clamp opened, part released from mold
Injection Molding Cycle II Phase
THE PROCESS: •The plastic material is received by the molder in granular form •It is placed in the hopper of an injection-molding machine, from which it is fed to a heated cylinder. • Mold defines final shape as material cools and solidifies
THE PROCESS: •As the granules heat in the cylinder, they melt, or plasticize(Tg) •A typical melting temperature is about 180°C (350°F), although this varies with different materials and molding conditions •The mold, usually of steel, is clamped in the machine and is water-cooled
THE PROCESS: •The mold is opened, and the molded part with its attached runners is removed •The process, with the occasional exception of part removal, is automatic •It requires about 45 s / cycle, more or less, with most of that time being devoted to cooling of the material in the mold •Very high pressures, on the order of 70,000 kPa(?) (10,000 psi) or more, are required during injection
Can this assembly be replaced by a single piece molded design?
Draw the part as molded
TYPICAL CHARACTERISTICS OF INJECTION- MOLDED PARTS: •Intricate parts in large quantities(?) •One molded part can replace what would otherwise be an assembly of components (Moped Engine Shield)
Characteristics of injection- molded parts: •Color and surface finish often can be molded directly onto the part, so that secondary finishing operations are not necessary(Painting???) •Generally thin-walled. •Heavy sections and variable wall thicknesses are possible, though they are normally not recommended.(Molded Hand?)
Characteristics of injection- molded parts: •Generally less strong than metals •More apt to be found in less highly stressed applications •Housings and covers are common uses rather than, for example, frames and connecting rods.
Characteristics of injection- molded parts: •Gradually being developed with better and better strength characteristics and are increasingly finding themselves used for moving parts and in more structural applications (Engine Parts? Boeing Parts?)
Characteristics of injection- molded parts: •Some “Engineering plastics” •Nylon •Polycarbonate •Acetal •Phenylene oxide •Polysulfone •Thermoplastic polyesters & others, particularly when reinforced with glass or other fibers, are functionally competitive with zinc, aluminum, and even steel.
Effects of Shrinkage •All thermoplastics exhibit shrinkage on cooling and solidification (Slide on shrinkage; Mat.Change?) •Causes various irregularities and warpage in the molded part •Most common such defect is the sink mark, or surface depression, opposite heavy sections.
Please write down what you consider is the Range of % Shrinkage of different Polymers Only Maximum & Minimum
What are the three Effects of Shrinkage?
One Effect of Shrinkage
Second Effect of Shrinkage
Third effect of shrinkage
Please write down the “Economic Production Quantity with Injection Molding Process”
ECONOMIC PRODUCTION QUANTITIES: • >10,000 (Production must be large enough so that the mold cost can be amortized over the quantity manufactured) •A unique mold for each part
Relationship between Mech. Properties of Plastics & Moldability Mechanical Properties Low (PE, PP, PS) High (PC, Alloyed Grades..)
Moldability High Low
Suitable Materials: •PVC, though low in cost and having very good physical properties, is more difficult to injectionmold than many other materials •PVC’s prime drawback is a narrow temperature range between its melting and degradation points
Please write down the specific gravities of •LDPE •MDPE •HDPE?
Properties,Cost & applications of Thermoplastics
Properties,Cost & applications of Thermoplastics
DESIGN RECOMMENDATIONS: Gate and Ejector-Pin Locations •The designer(?) should consider the location of these elements •Ejector pins usually can be located on the underside of a part if it has an outside and an underside
Various Gating systems
Various Gating systems
Various Gating systems
Difference?
Draw a cylindrical bush & indicate the gate location which will result in no weldline
Various Gating systems Thin part
No weldline for circular part!
Various Gating systems
Gate and Ejector-Pin Locations
•Desirable gate locations for trouble-free trouble-free mold filling •Center gating of round and cylindrical parts •Near-center gating of other large-area parts
Suggested Wall Thickness •Generally •Generally,, thinner thinner walls walls are more more feasible feasible with small parts rather than with with large large ones •The limiti limiting ng factor factor in wall wall thinness thinness is is the tendency for the plastic material in thin walls to cool and solidify before the mold is filled •The shorter the material flow, the thinner the wall can be
Maximum Length of Part?
Suggested Wall Thickness •Walls also should be as uniform in thickness as possible to avoid warpage from uneven shrinkage. •When changes in wall thickness are unavoidable, the transition should be gradual, not abrupt.
Redesign
Comment on this design
Design Recommendations: Holes • Holes are feasible in injection-molded parts but are a complicating factor in mold construction and part quality • “Knit” or “weld” lines adjacent to the hole often develop, and flashing also may occur at the edge of the hole
Holes •Minimum spacing between two holes or between a hole and side-wall should be one diameter. Comment on this design?
Holes •Minimum spacing between two holes or between a hole and side-wall should be one diameter.
•Holes should be located three diameters or more from the edge of the part to avoid excessive stresses.
Comment?
•Holes should be located three diameters or more from the edge of the part to avoid excessive stresses.
Design Guidelines: Holes •A through hole is preferred to a blind hole •Why?
Design Guidelines: Holes •A through hole is preferred to a blind hole •Because the core pin that produces the hole can then be supported at both ends •Resulting in better dimensional location of the hole and avoiding a bent or broken pin.
Design Guidelines: Holes • Holes in the bottom of the part are preferable to those in the side because the latter require retractable core pins
Design Guidelines: Holes •Blind holes should not be more than two diameters deep. If the diameter is 1.5 mm or less, one diameter is the maximum practical depth.
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Design Guidelines: Holes •Blind holes should not be more than two diameters deep. If the diameter is 1.5 mm or less, one diameter is the maximum practical depth.
•To increase the depth of a deep blind hole, use steps. This enables a stronger core pin to be employed.
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•To increase the depth of a deep blind hole, use steps. This enables a stronger core pin to be employed.
•Similarly, for through holes, cutout sections in the part can shorten the length of a small-diameter pin. (See Fig. 6.2.13.)
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•Similarly, for through holes, cutout sections in the part can shorten the length of a small-diameter pin.
Comment on this design
•Use overlapping and offset mold-cavity projections instead of core pins to produce holes parallel to the mold-parting line (perpendicular to the mold-movement direction).
Comment on this design
What should be (Rib Thickness / Nominal Wall Thickness)%? Please write down your answer
Design Guidelines: Ribs •Reinforcing ribs should be thinner than the wall they are reinforcing •To prevent sink marks in the opposite side of the wall. •Recommended rib thickness : Between 40% and 60% of the wall thickness.
Design Guidelines:
? •Ribs should not be higher than 2.5 to 3.0 times the wall thickness.
Design Guidelines: •Ribs should not be higher than 2.5 to 3.0 times the wall thickness.
Design Guidelines: Ribs •Two ribs may be used, if necessary, to provide the extra reinforcement that would otherwise be provided by a high rib. •The ribs should be two or more wall thicknesses apart (??)
Design Guidelines: Ribs •Two ribs may be used, if necessary, to provide the extra reinforcement that would otherwise be provided by a high rib. •The ribs should be two or more wall thicknesses apart
Design Guidelines: Ribs •Ribs should be perpendicular to the parting line to permit removal of the part from the mold.
How to camouflage sink marks?
• Sink marks caused by ribs can be disguised or hidden by grooves or surface texture opposite the rib.
Write down the values of the draft angle for ribs & the root radius
Design Guidelines: •Ribs should have a generous draft of 0.5deg to 1.5 deg per side •There should be a radius at the base of 25% to 40% of ‘t’
