Foam sheets & Coventional Foam Sheet Processing
Plastic foam sheets have many applications. They can be used for thermal insulation or packaging purposes, or can be thermoforming into meat trays, eggs cartons, fast-food plates, and containers.
Generally the production of polymer foams by extrusion consists of the following steps. First, a polymer resin is melted in the extruder barrel by the shear heat generated from the screw motion. A blowing agent, which is a substance that can provide the gas (es) needed to foam the polymer, is premixed with the polymer resin or injected into the polymer when the polymer is melted. There are two types of blowing agents, namely chemical blowing agents and physical blowing agents. Chemical blowing agents are compounds or mistures that release a gas or gases as a result of chemical reactions. Possible reactions include the thermodecomposition, chemical reactions of the blowling agents, or interactions of the blowing agents with other components in the formulation. Physical blowing agents are substances that release a gas or gases as a result of physical processes, such as evaporation, or desorption at elevated temperature or reduced pressure. No matter what kind of blowing agent is used, the generated gas is dissolved in the polymer melt to foam a polymer/gas mixture. The mixture is extruded through the die where foaming and shaping take place. The pressure drop at the die exit causes the solubility of the gas in the polymer melt to drop. Consequently, the dissolved gas is separated from the polymer in the foam of bubbles; this process takes place as long as gas is available in the polymer melt and little resistance is encountered.
Two different foaming processes are used in industry, namely free-rise foaming extrusion and controlled foaming extrusion. In the free-rise foaming process, the extrudate is allowed to expand freely without any restrictions after it leaves the die. The die is designed to deliver the extrudate with a shape and size that approximates the desired profile. This pricess is good for production of a sheet or product with a simple cross section and a wall thickness in the range of 2mm ~ 6mm.
In the controlled foaming process, the extrudate is mechanically restricted by a cooled shaping section located right after the die. Because of the cooling, a solid skin layer of polymer is formed on the extrudated, which means foaming takes place only in the core of the extrudate. This process can be used to produce a sheet, tubing or any desired cross sectional shape with a wall thickness of over 6mm. As for solid plastic parts, most of them made by the process of injection molding.