Part Design Guidelines for Thermoplastics
Even though part design for thermoplastics might be laborious, adopting a variety of primary guidelines can help you eliminate troubles throughout molding as well as in functional effectiveness. The guidelines specified here are generally standard. Depending upon the specific demands of the particular part, it may not always be possible to follow these tips. However, these rules can help you successfully resolve most of the more commonplace pattern issues.
Nominal Wall Size
For items produced from nearly all thermoplastics, nominal wall thickness should not exceed 4.0 millimeters. Walls thicker than 4.0 millimeters may cause longer cycle times due to the extended cooling period, increase the possibility of voids, and significantly reduce the specific qualities of the component.
When a design requires wall thicknesses higher than the recommended limit of 4.0 millimeters, structural foam resins should be evaluated, although additional processing technologies 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 incorporating transition zones.
Transition areas will eliminate stress concentrations, which can significantly minimize the impact durability of the component. Additionally, transition areas reduce the likelihood of sinks, voids, and bending in molded items. A wall thickness variation of ±25% is tolerable in a product made with polycarbonate having a shrinkage rate of less than 0.02mm/mm. If the shrinkage rate exceeds 0.01mm/mm, then a thickness variation of 15% is permissible.
Radii
It is best to avoid designing parts with sharp corners. Sharp corners act as notches, which emphasize strain and reduce the part’s impact durability. A corner radius will improve the durability of the corner and enhance mold filling. The radius should be in the range of 25% to 75% of the wall thickness; 50% is recommended as a function of the ratio of part radius to wall thickness (R/T).
Draft Angle
To ensure components can be easily ejected from the mold, walls must be designed with a slight draft angle. A draft angle of one degree for each side is the absolute minimum for acceptable results. A two-degree draft for each side is considered standard practice. Smaller draft angles can lead to difficulties in removing finished items from the mold. However, any draft is better than no draft at all.
Items with a molded-in deep texture, such as leather-graining, will require more draft. Typically, an additional draft should be provided for every 0.025 millimeter depth of texture.
Ribs and Gussets
When designing ribs and gussets, it is essential to follow proportional thickness guidelines. If the rib or gusset is too thick relative to the component wall, it may cause surface sinks, warpage, internal voids, and weld lines—all of which produce high levels of molded-in tension, resulting in longer cycle times.
The placement of ribs and gussets can also impact mold structure for the unit. Keep the entrance site in mind when generating ribs or gussets. Well-positioned ribs in the flow line and gussets can enhance item filling by acting as internal runners. Conversely, poorly positioned or ill-designed ribs and gussets can lead to inadequate filling in the mold and may cause burn scars on the finished part.
These problems often occur in isolated ribs or gussets where air entrapment is a ventilation challenge. It is also recommended by experienced plastic injection mold manufacturers that the rib thickness at the intersection of the nominal wall not exceed one-half of the nominal wall in highly cosmetic areas. Experience shows that violating this rule significantly increases the risk of rib read-through (localized shine slope change).
Bosses
Bosses are used in parts that will be assembled with inserts, self-tapping screws, driving 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. When the boss is too distant from a wall for a connecting rib to be practical, design the boss with gussets.
Results from various studies provide recommended sizing ratios for designing bosses at or away from a wall. Remember that these bosses should be cored up to the bottom of the boss.
Threads
Molded-in threads can be integrated into items made from engineering thermoplastic resins. Threads should always have radiused beginnings and should not have feather edges to avoid stress concentrations. We will demonstrate examples of good design for molded-in external and internal threads later.
Threads also create undercuts and should be managed so that the component can be pulled from the mold, possibly using unscrewing means, retractable cores, etc. Every effort should be made to position external threads on the mold’s separating line where costs of injection mold supplies and mold stability are most reasonable for the injection molding companies.
Undercuts
Due to the hardness of most engineering plastic resins, undercuts within a part are generally not recommended. However, if a design requires an undercut, ensure that the undercut can be relieved by a cam, core puller, or other mechanisms when the mold is opened.