When designing a plastic injection mold, the mold designer must consider the type of resin and the resin properties that will be used in the molding process. Thermoplastic engineering resins are the most common type of plastic resin used for the injection molding process. An experienced mold design engineer will take into consideration the various properties of the resin such as elasticity, fatigue strength, shrink rate, resistance to corrosives or high temperatures, and finish such as matte or polished.
Elasticity and Fatigue Strength
The resin used to mold a particular product is chosen due to its properties. For a product that needs a rubber like property with some room to bend or give a little in its application, the print may call for a Thermoplastic Elastomer (TPE) with high elasticity properties. This type of resin is often used in over-molding applications where a vertical press will mold a rubber like finish around an insert, using a shuttle mold design.
The injection mold design engineer must be aware of the elasticity of the resin and any critical areas when designing the tool to meet part specifications.
Shrink Rate of Thermoplastic Resins
The shrink rate of the chosen resin is extremely important, as the cavities would have to account for shrinkage after ejection while still meeting part specification. The cooling rate is also an important property for ejection as the part must be completely solidified prior to ejection.