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Cycle time increases with thickness

Posted by admin on July 29th, 2012
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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.

FIGURE 1. A thick part (left) can lead to excessive shrinkage and warpage (center). A recommended alternative design with uniform thickness (right).

Thin, uniform parts improve surface quality

A 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 (see Weld lines and meld lines) that produce defects on part surfaces. In addition, Sink marks and voids will also arise in thick sections without sufficient packing.

Reducing part thickness

To shorten the cycle time, improve dimensional stability, and eliminate surface defects, a good rule of thumb for part thickness design is to keep part thickness as thin and uniform as possible. The use of ribs is an effective way of achieving rigidity and strength while avoiding heavy cross-sectional thickness.

Part dimensions should take into account the material properties of the plastics used in relation to the type of loading and operating conditions the part will be subjected to; the assembly requirements should also be considered.The diagrams shown below provides several examples of designs we recommend vs. designs you should avoid.

FIGURE 2. Designs with thick sections (left) and alternative designs that use ribs to achieve uniform thickness (right).

 

Author: ZHILIAN MOULD
Page Link: http://www.zhilianmould.com/blog/cycle-time-increases-with-thickness/
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