Lightweight polymers produced in injection molding

Polymer foams are a class of cellular materials composed of a solid polymer phase and a dispersed gas phase. They offer advantages over solid materials due to their light weight, excellent energy absorption capacity, and good insulating properties. Today, we encounter polymeric foams in everyday life, which necessitates the optimization of their manufacturing processes and the investigation of structure-property relationships.

The key parameter for defining this type of material is its relative density (the ratio of the density of the expanded polymer to that of the solid polymer). Foams are classified as low-density if the relative density is less than 0.40, or high-density if the relative density is between 0.40 and 0.70. High-density foams are also referred to as “lightweight polymers.”

Lightweight polymers have become important products, particularly in the automotive industry. Injection-molded parts combine weight reduction with satisfactory mechanical properties such as stiffness and impact resistance.

For this reason, production processes for lightweight polymers in injection molding have evolved significantly in recent years. They can be produced chemically, using blowing agents, or physically, by directly injecting gas into the screw/cylinder system. In general, the production of lightweight polymer injection-molded parts involves the following steps:

• the selection of polymers and/or polymer blends;

• the choice of foaming process (chemical or physical);

• the definition of injection parameters (hydraulic pressure, screw temperature profile, injection speed, etc.);

• the use of innovative techniques to control expansion (mold opening, gas backpressure inside the mold, etc.);

• determining the properties of the resulting materials and optimizing the process.

This article focuses on the production of lightweight polymers via injection molding, primarily polypropylene. The article begins with an introduction to the most important characteristics of polymer foams and a description of chemical and physical foam injection molding processes. Foaming control processes such as partial mold opening, known as “core-back,” and cavity pressurization prior to injection are examined. An analysis of gas-polymer interactions (diffusion, nucleation, plasticization, growth, expansion, etc.) helps to understand the physics of the process, supported by a simple bubble growth model. Finally, several mechanical properties are analyzed, and several avenues for further research are proposed to advance our understanding of this innovative material....