Design engineers don’t have to be fluid dynamics experts to injection mold plastic parts without a hitch. The following rules will help engineers avoid problems when designing injection-molded plastic parts:
Rule 1: Keep Wall Thickness Consistent
Plastic part walls must be uniform in thickness. This is the most basic design parameter, and strict adherence to it will eliminate many manufacturing problems. Parts with uniform walls will not warp, will fill properly and will fit together because variable shrinkage is minimized. Wall thickness variations should not exceed 10% in high mold shrinkage plastics. In fact, even this slight disparity can introduce processing and quality problems.
Rule 2: Provide for Proper Gate Location
If varying wall thickness cannot be avoided, then designers should provide for proper gate location. If this is not supplied, then attaining uniform pack of the molded part will be nearly impossible. The most effective gate location is when the melt enters at the thickest part of the cavity and then flows to the narrower areas.
Rule 3: Determine Optimal Wall Thickness
Theoretically, there is no maximum wall thickness for injection-molded parts. But designers are more concerned with determining the minimum wall thickness because thinner is almost always less expensive. Two factors contribute to this: first, thinner parts require less raw plastic material, and second, they cool faster. To determine the most suitable wall thickness, engineers should first consider product requirements. Generally, strength dictates the wall thickness. Engineers can also rely on a finite analysis to select the optimal wall thickness.
Rule 4: Radius Corners Generously
During injection molding, the molten plastic has to navigate turns or corners. Rounded corners will ease plastic flow, so engineers should generously radius the corners of all parts. In contrast, sharp inside corners result in molded-in stress—particularly during the cooling process when the top of the part tries to shrink and the material pulls against the corners.
If the inside and outside radii of a part are each equal to half of the nominal wall thickness, a uniform wall around the corner can be achieved. Both sides of the corner will display equal amounts of shrinkage, and sink marks will be avoided entirely. Moreover, the first rule of plastic design—uniform wall thickness—will be obeyed. As the plastic goes around a well-proportioned corner, it will not be subjected to area increases and abrupt changes in direction. Cavity packing pressure stays consistent. This leads to a strong, dimensionally stable corner that will resist post-mold warpage.
Rule 5: Select Suitable Draft Angles
From a cost and manufacturability viewpoint, the ideal draft angle is the largest angle that will not lessen the customer’s satisfaction with the product. The minimum allowable draft angle is harder to quantify. Plastic material suppliers and molders are the authority on what is the lowest acceptable draft. In most instances, 1° per side will be sufficient, but between 2° and 5° per side would be preferable. If the design is not compatible with 1°, then allow for 0.5° on each side. Even a small draft angle, such as 0.25°, is preferable to none at all.