One of the key steps in the injection molding process is the packing and holding phase, sometimes also referred to as the compensation phase. During the packing phase material is continuously “packed” into a cavity, thereby compensating for the shrinkage that occurs due to material cooling inside the mold. To optimize a packing profile, it is important to understand the material characteristics of the polymer to be molded. The two types of polymers, amorphous and semi-crystalline, have significantly different material characteristics that affect their behavior under heat during the injection molding process.
Amorphous materials have a random microstructure, meaning their molecular chains have no specific orientation or internal order. When the material is heated, the secondary bonds loosen, and the material begins to flow. Amorphous material molecules have a random configuration and will melt under a range of temperatures.
The molecular chains of semi-crystalline materials are more organized and tightly packed together. The densely arranged molecular chains have few or no side branches which allows the chains to embed themselves into the crystalline areas. The areas that have not crystallized have an amorphous structure. Crystallization takes place above the glass transition temperature. When the temperature drops below this point, the molecular chains lose their ability to move, halting the formation of crystalline areas.
Because amorphous materials have a random molecular structure, they have more compressibility than semi-crystalline materials. This needs to be a consideration when the packing pressure profile is set to avoid over- or under-packing.
Understanding material behavior is important for optimizing a packing profile. Amorphous materials have smooth and gradual packing behavior, the packing pressure profile can have a significant impact on overall part quality. Semi-crystalline materials have narrow transitional packing and cooling behavior, so optimizing the pack profile and volumetric shrinkages needs to be achieved in a shorter time frame.
Optimizing pack pressure profiles can minimize volumetric shrinkage differences and reduce molded-in stress, resulting in a high-quality dimensionally stable part.
Having a basic understanding of polymer material behavior and characteristics is important for successful molding, from part design to mold design to processing. A clip from our “Polymer Material Overview” lesson from Kruse Training is available here.