Table 5.

Comparison between the main rubber foaming methods: batch, extrusion and injection molding.

Technique	Formation Method	Structure	Properties
Batch foaming	After masticating the neat rubber, all the ingredients, except for the curing agent, are added and mixing is performed for several minutes. Finally, the curing agent is added and mixed. The prepared compound is placed in a stainless-steel mold. Thereafter, the mold is put inside a high-pressure vessel (autoclave) at a defined pressure, temperature and time depending on the curing and foaming kinetics. After being saturated, the foamed parts are obtained by a rapid depressurization.	-Uniform cell size distribution can be achieved.	-Batch-wise process (time consuming). -Easy control for producing specific products. -Relatively low cost process compared to the other two techniques (no complex machine and equipment). -No shear: appropriate for producing sensitive materials. -High expansion ratio can be achieved.
Foam extrusion	The prepared rubber compounds in strip or pellet forms are fed into the extruder through the feed hopper and drawn into the rotating screw. The screw rotation continuously pushes the melt forward and out of the extruder through the die. The final part of this process is calibration and cutting of the extruded profiles.	-Achieving uniform cell size distribution is often difficult (faster cooling of the surface compared to the the middle of the sample).	-Continuous process (more difficult to control, but large scale production). -Thicker skinned foams. -The surface of the foams are often rough or/and have defects.
Foam injection molding	After adding the prepared rubber compounds into the screw in the shape of pellets through the hopper, the injection unit melts the rubber compounds and maintains the injection pressure during mold filling. Then, the gas-filled melt is injected through the nozzle into the mold at high temperature and pressure. Finally, the mold is opened, and a rapid pressure drop occurs leading to foaming.	-Achieving a foam with uniform cell sizes is difficult (the filling and maintaining time are coupled leading to difficult cell nucleation over the sample).	-Semi-continuous process (medium to large scale foam production). -Produce samples with a wide range of geometry and size. -Low material use. -High dimensional stability of the parts. -Low back pressure required. -Wide range of mechanical properties with very high stiffness-to-weight ratio.