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Part thickness

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Molded plastics do not lend themselves to solid forms. There are two principal reasons for this. First, plastics are processed with heat but are poor conductors of heat. This means that thick sections take very long time to cool and so are costly to make. The problems posed by shrinkage are equally severe. During cooling, plastics undergo a volume reduction. In thick sections, this either causes the surface of the part to cave in to form an unsightly sink mark, or produces an internal void. Furthermore, plastics materials are expensive; it is only high-speed production methods and net-shape forming that makes moldings viable. Thick sections waste material and are simply un-economic.

 It is not easy to generalize what the wall thickness should be. The wall plays a part both in design concept and embodiment. The wall must be thick enough to do its job; it must be strong enough or stiff enough or cheap enough. But it must also be thin enough to cool quickly and thick enough to allow efficient mould filling. If the material is inherently strong or stiff the wall can be thinner. As a general guide, wall thicknesses for reinforced materials should be 0.75 mm to3 mm, and those for unfilled materials should be 0.5 mm to 5 mm.

Ideally, the entire component should be uniform thickness - the nominal wall thickness. In practice that is often not possible; there must be some variation in thickness to accommodate functions or aesthetics. It is very important to keep this variation to a minimum.

The influences of part thickness are on cycle time, shrinkage and warpage, and surface quality.

Cycle time increases with thickness

Injection-molded plastic parts have to be cooled sufficiently before being ejected from the mold to avoid deformation due to ejection. Parts with thick wall sections take longer to cool and require additional packing.

Theoretically, cooling time is proportional to the square of the heaviest part wall thickness or the power of 1.6 for circular features. Therefore, thick sections will prolong the press cycle, reducing the number of parts per unit time and increasing the cost per part.

Thick parts tend to warp

Shrinkage is inherent in the injection molding process. Excessive and non-uniform shrinkage, however, both globally and through the cross section of the part, will cause the part to warp. Warpage is a distortion where the surfaces of the molded part do not follow the intended shape of the design. The diagrams below illustrate how part thickness affects shrinkage and warpage.

Thin, uniform parts improve surface quality

Combination of thin and heavy part cross-sections can easily produce a "race tracking" effect, which occurs because melt preferentially flows faster along thick sections. Race-tracking leads to Air traps and weld lines that produce defects on part surfaces. In addition, Sink marks and voids will also arise in thick sections without sufficient packing.

                                  part_thickness1                    part_thickness2                        part_thickness3

Figure above ,A thick part (left) can lead to excessive shrinkage and warpage (center).

A recommended alternative design with uniform thickness (right).