Design in thermoplastics

Although component design in thermoplastics is complex, following a few fundamental principles will help you minimize problems during molding and in part performance. Of course, the guidelines given here are general.

Depending on the particular requirements of the part, it may not always be possible to follow all of our suggestions. But these guidelines, in furthering your understanding of the behavior of thermoplastics, can help you effectively resolve some of the more common design problems.

Nominal Wall Thickness

For parts made from most thermoplastics, nominal wall thickness should not exceed 4.0 mm. Walls thicker than 4.0 mm will result in increased cycle times (due to the longer time required for cooling), will increase the likelihood of voids and significantly decrease the physical properties of  the part. If a design requires wall thicknesses greater than the suggested limit of 4.0 mm, structural foam resins should be considered, even though additional processing technology would be required. 

In general, a uniform wall thickness should be maintained throughout the part. If variations are necessary, avoid abrupt changes in thickness by the use of transition zones, as shown in Figure 25. Transition zones will eliminate stress concentrations that can significantly reduce the impact strength of the part. Also, transition zones reduce the occurrence of sinks, voids, and warping in the molded parts.

A wall thickness variation of ± 25% is acceptable in a part made with a thermoplastic having a shrinkage rate of less than 0.01 mm/mm. If the shrinkage rate exceeds 0.01 mm/mm, then a thickness variation of ± 15% is permissible.

Radius

It is best not to design parts with sharp corners. Sharp corners act as notches, which concentrate stress and reduce the part’s impact strength. A corner radius, as shown in Figure 26, will increase the strength of the corner and improve mold filling. The radius should be in the range of 25% to 75% of wall thickness; 50% is

suggested. Figure 27 shows stress concentration as a function of the ratio of corner radius to wall thickness, R/T.

Draft Angle

So that parts can be easily ejected from the mold, walls should be designed with a slight draft angle, as shown in Figure 28. A draft angle of 1 ⁄ 2° draft per side is the extreme minimum to provide satisfactory results.

1° draft per side is considered standard practice. The smaller draft angles cause problems in removing completed parts from the mold. However, any draft is better than no draft at all.

Parts with a molded-in deep texture, such as leather-graining, as part of their design require additional draft. Generally, an additional 1° of draft should be provided for every 0.025 mm depth of texture.

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