Injection molding is a prevalent manufacturing process utilized across a variety of applications, from full-scale productions of consumer products to smaller volume production of large components like car body panels.
The process involves a tool or mold, typically constructed from hardened steel or aluminum. The mold is precision machined to form the features of the desired part, and thermoplastic material is fed into a heated barrel, mixed and forced into the metal mold cavity where it cools and hardens.
Whatever the project, industrial designers, engineers and product designers may face some challenges when designing for injection molding. The following details three mistakes designers should avoid for successful injection molded parts.
On average, the minimum wall thickness of an injection molded part ranges from 2mm to 4mm (.080 inch to .160 inch). Parts with uniform walls thickness allow the mold cavity to fill more precisely since the molten plastic does not have to be forced through varying restrictions as it fills.
If the walls are not uniform, the thinner sections cool first. As the thicker sections cool and shrink, stresses occur between the boundaries of the thin and thick walls. The thin section doesn’t yield to the stress because the thin section has already hardened. As the thick sections yields, warping and twisting of the part occurs, which can cause cracks.
If design limitations make it impossible to have uniform wall thicknesses, the change in thickness should be as gradual as possible. Coring is a helpful method where plastic is removed from the thick area, which helps to keep wall sections uniform. Gussets support structures can also be designed into the part to reduce the possibility of warping.
Not Utilizing Draft
Mold drafts facilitate part removal from the mold. The draft must be in an offset angle that is parallel to the mold opening and closing. The ideal draft angle for a given part depends on the depth of the part in the mold and its required end-use function.
Allowing for as much draft as possible will permit parts to release from the mold easily. Typically, one to two degrees of drafts with an additional 1.5 degrees per 0.25mm depth of texture is sufficient. The mold part line will need to be located in a way that splits the draft in order to minimize it.
Sharp corners greatly increase stress concentration, which, when high enough, can lead to part failure. Sharp corners often come about in non-obvious places, such as a boss attached to a surface, or a strengthening rib.
The radii of sharp corners needs to be watched closely because stress concentration varies with radius for a given thickness. The stress concentration factor is high for R/T values, less than 0.5, but for R/T values over 0.5 the concentration lowers. It is recommended that an inside radius be a minimum of 1 times the thickness.
In addition to reducing stresses, the fillet radius provides a streamlined flow path for the molten plastic, resulting in an easier fill of the mold. At corners, the suggested inside radius is 0.5 times the material thickness and the outside radius is 1.5 times the material thickness. A bigger radius should be used if part design allows.