Part Details Review
One of the first and perhaps the most important decision a designer makes, before looking at any other design feature, is deciding on an appropriate nominal wall thickness for an injection molded part. Depending on the end use, the part thickness must be adequate to have the required functionality in terms of mechanical, optical, electrical, thermal, and flame resistance properties.
As a designer, even though part performance and functionality are of prime importance to you, you can not afford to ignore moldability issues. Flow behavior of the melt has a close relationship to the wall thickness. How well the part is packed is also influenced by the wall thickness. Abrupt changes in wall sections could increase the amount of molded-in stress in the part. Ultimately, the moldability problems could affect part performance and its functionality.
Thicker walls, however, cause part weight and cycle times to increase, which translate into higher part cost. Generally, increasing wall thickness reduces deflection during impact and increases the energy required to produce failure. For some materials, increasing the thickness can increase the part stiffening to the point that the part can not flex enough, resulting in reduced impact strength. The thickness, at which notched impact strength drops drastically, is known as critical thickness for the material. Critical thickness for medium viscosity polycarbonate at room temperature is approximately 3/16 inch as illustrated in the graph.