Designing plastic injection molded parts is a complicated process with a wide array of considerations that must be taken into account. In addition to designing your parts to fulfill their intended purpose, you must also be aware of the unique problems and challenges associated with the injection molding process itself. Failure to design injection molded parts with these constraints in mind can result in casts that are damaged as they are removed from the mold or that cannot be cleanly removed at all.
Two of the most important features to design into any injection molded part are draft angles and rounded edges. Both of these features are required to ensure that the part can be removed from the mold cleanly and that the plastic is able to flow properly through the mold.
Always Use Draft Angles
A major potential issue with plastic injection molded parts is friction during the release process. Completely vertical walls along the edge of a part will potentially rub against the edges of the mold, creating friction that can damage the part or prevent it from being removed from the mold at all. In addition to the damage that may be caused to the part, the mold itself will be worn down by this rough ejection process. This can lead to a reduction in mold life and ultimately greater production expenses as molds must be replaced more often.
The solution to this problem lies in draft angles. Draft angles are angles that are added to any exterior vertical wall of a part. The angle is designed to taper down from the top half of the mold, with the narrower portion of the part lying deeper inside the mold. This guarantees that the part will separate entirely from the walls of the mold once it has been released, with only a small amount of friction at the initial point of release.
Since draft angles are key to all injection molded design, it is important that all members of a design team be aware of this constraint early in the design process.
Round All Interior and Exterior Edges
When designing any injection molded part, it is important to remember that material will be injected into a mold and must be able to flow cleanly through it. Straight corners can potentially create issues with material flow through the mold, resulting in portions of the part that are weaker and prone to damage from stress.
The combat this problem, all interior and exterior edges of a part should be rounded. The size of the radius should be chosen so that the curved edge of the wall has the same thickness as the straight portions. Ensuring that all edges are designed in this fashion will result in stronger parts that are less likely to suffer fractures during the molding process.