Since the invention of polystyrene (PS) foam in the 1930s, foaming technologies progressed and matured. Nowadays, it is possible to produce foams of nearly every polymer resin through one or more of the following processes:

Injection Molding Process

Polymer matrix and blowing agents (BA) are processed conventionally in the screw injection molding equipment equipped with gas leakage preventive measures such as nozzle shutoff valve or additional external control gate. Gases released from BA through thermal decomposition are dissolved in the polymer melt and injected directly into an enclosed mold cavity to produce foam products as the pressurized gases expand when subjected to atmospheric pressure. BA decomposition should occur during plasticizing process and remain dissolved in the polymer melt until the gas-melt mixture is injected to the enclosed mild.
This process often produces foams with a sandwich structure consisting of a continuous foam skin and cellular core and is commonly used to produce thermoplastic polymer foams such as high-density polystyrene (PS) foams, both rigid and flexible, poly(vinyl chloride) (PVC) foams, acrylonitrile butadiene styrene (ABS) structural foams, and thermoset foams such as polyurethane foams.

Reaction Injection Molding (RIM)

Reaction injection molding systems combine two or more liquid components that chemically react in a closed mold to form polymer foam, taking the intricate shape of the mold. Figure 2 shows a simple schematic illustration of the RIM process. The components are precisely controlled at stoichiometric ratio before combined at the mixing head by high velocity impingement mixing. The process is usually carried out at low temperature involving comportments of low viscosity which greatly reduce the pressure required to drive the components into the mold. RIM is widely used in industries for the production of large, complex parts such as bumpers for vehicles, panels for electrical equipment, enclosures for medical devices, and housings for computer and telecommunications equipment. Recently applications are extended to microcellular foam fabrications.

Expandable Polymer Pellets

The polymer pellets are first impregnated with BA during the polymerization process. These pellets are then subject to heat for free expansion or confined expansion. For example, freely expanded PS pellets are commonly used in packaging and as cushion fillings. For confided expansion, the expandable pellets filled up the entire predesigned mold and fused together forming foam products with well-controlled density, shape, and sizes. Disposable foam cups are an excellent example of polymer foams produced by confined expansion from expandable PS pellets.

Extrusion Foaming Process

STYROFOAM™ is perhaps the most popular type of polymer foams known to everyone. However, it should not be mistaken as the white disposable foam cups made from expansion process explained in the previous paragraph. STYROFOAM ™is a registered trademark for a line of extruded polystyrene foam products made exclusively by The Dow Chemical Company for thermal insulation and craft applications (Company DC. It is not a cup).
The typical extruders as shown in Fig. 3 used for bulk polymer products are commonly used with necessary modifications which mix BA into the polymer melt homogeneously with applied pressure. The sudden drop in pressure led to rapid cell growth as soon as the melt is being extruded out from the die’s orifice.
Continuous extrusion is preferred from the economical point of view due to the higher throughput and versatility in the properties and shapes of the products obtained. Besides the traditionally used polymer resins for foam fabrication, trends are moving toward the use of nonconventional foam materials such as biodegradable polyhydroxyalkanoates (Liao et al. 2012).

In general, an ideal foaming system consists of a polymer with a gelation time that coincides with the time required for rapid gas liberation; hence the solidifying polymer will be able to “trap” the gas bubbles inside the matrix. Prior to foaming, it is necessary to characterize the BA and to obtain the viscosity profile in order to establish a balanced system for proper foam fabrication. An example is given in Fig. 4 which illustrates that 220o C is the “ideal” foaming condition in which gas liberation and polymer cross-linking occurred within the same time frame. In contrast, when the polymerization process took too long a time, the rising foam may collapse before the polymer matrix failed to gain enough gel strength to form a stable foam system.
Similarly, if polymerization occurred before gas liberation, the resulting foam may have a density much higher than expected. Cell growth was inhibited when the gas generated was unable to overcome the rapidly increasing gel strength.