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 in terms of light weight, excellent energy absorption and good insulating properties. Today, we are faced with the use of polymeric foams in everyday life, and this calls for the optimization of their elaboration processes and the investigation of structure-properties relationships.

The main parameter defining this type of material is its relative density (the ratio between the density of the expanded polymer and that of the solid polymer). Low-density foams are defined as those with a relative density of less than 0.40, and high-density foams as those with a relative density of between 0.40 and 0.70. High-density foams are also known as "lightweight polymers".

Lightweight polymers have become important products, especially in the automotive sector. Injected parts combine weight reduction with satisfactory mechanical qualities such as rigidity and impact resistance.

For this reason, production processes for lightweight injection polymers have evolved considerably in recent years. They can be chemical, with the use of blowing agents, or physical, with the direct injection of gas into the screw/sleeve system. In general, the production of injection-molded lightweight polymer parts involves the following stages:

• the choice of polymers and/or polymer blends ;

• choice of foaming process (chemical or physical);

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

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

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

This article is devoted to the study of the production of lightweight polymers by injection molding, mainly polypropylene. The article begins with an introduction to the most important characteristics of polymeric foams and a description of the chemical and physical injection foaming processes. Foam control processes such as partial mold opening and cavity pressurization prior to injection are examined. An analysis of gas-polymer interactions (diffusion, nucleation, plasticization, growth, swelling, etc.) provides an understanding of the physics of the process, supported by a simple bubble growth model. Lastly, a number of mechanical properties are analyzed, and several avenues of investigation are proposed for further progress in mastering this type of innovative material....