Product Design
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.
Radii
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.
Ribs and Gussets
When designing ribs and gussets, it is important to follow the proportional thickness guidelines shown in
If the rib or gusset is too thick in relationship to the part wall, sinks, voids, warpage, weld lines (all resulting in high amounts of molded-in stress), longer cycle times can be expected. The location of ribs and gussets also can affect mold design for the part. Keep gate location in mind when designing ribs or gussets. For more information on gate location,
Ribs well-positioned in the line of flow, as well as gussets, can improve part filling by acting as internal runners. Poorly placed or ill-designed ribs and gussets can cause poor filling of the mold and can result in burn marks on the finished part. These problems generally occur in isolated ribs or gussets where entrapment of air becomes a venting problem.
Note: It is further recommended that the rib thickness at the intersection of the nominal
wall not exceed one-half of the nominal wall in HIGHLY COSMETIC areas. the dimension of the rib at
the intersection of the nominal wall should not exceed one-half of the nominal wall. Experience shows that violation of this rule significantly increases the risk of rib read-through (localized gloss gradient difference).
Bosses
Bosses are used in parts that will be assembled with inserts, self-tapping screws, drive pins, expansion inserts, cut threads, and plug or force-fits. Avoid stand-alone bosses whenever possible. Instead, connect
the boss to a wall or rib, with a connecting rib as shown in Figure 31. If the boss is so far away from a wall that a connecting rib is impractical, design the boss with gussets
dimensional proportions for designing bosses at or away from a wall. Note that these bosses
are cored all the way to the bottom of the boss.
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