Authors: Mateusz Imiela & Kamil Maszczyk
The Technology of the Foaming Process
Elastomeric foams, compared to foams made from thermoplastics in general, have unique properties like tensile strength leading to their widespread use, higher flexibility, energy absorption capabilities, resistance to abrasion, and strength-to-weight ratio. These properties mean that these materials have found a wide range of applications, including thermal insulation, energy absorbers, pressure sensors and absorbents.
Three main techniques can be distinguished to produce elastomeric foams: injection moulding, extrusion and batch foaming. The first two methods are commonly used for large-scale production at an industrial scale. The third one is mostly used for small production runs, laboratory investigations, fundamental studies or bulk production.
Foam Injection Moulding
Conventional injection moulding and foam injection moulding are commonly used methods sequentially for non-foamed and foamed products. The process starts with the addition of a rubber compound to the screw, the injection unit melts the compound and maintains pressure while filling the mould. The closing unit then precisely opens or closes the mould at the appropriate clamping force.
Injection moulding differs from extrusion by the possibility of reciprocating movement during injection moulding. During the addition of the rubber compound, the screw rotates and retracts due to the filling at its end. With sufficient filling, pressure and temperature, the screw moves towards the mouthpiece which causes the compound to be injected into the mould.
Injection moulding is the most promising process because it can produce products with variable geometries and sizes, from microchip sockets to car door modules. Furthermore, foamed rubber mouldings are used in many applications due to their low material consumption, high dimensional stability, low back pressure, wide range of mechanical properties and high stiffness-to-weight ratio.
Rubber compounds are fed into the extruder through a hopper and drawn into a rotating screw. The speed of the screw and the temperature profile of the drum control the flow rate and temperature. Crosslinking and foaming reactions are strongly temperature dependent. Therefore, drum temperature and screw speed are two important parameters that also affect the self-heating of the material through viscosity dissipation. On the other hand, surface quality is an important feature of rubber foam profiles. During extrusion, high shear stresses in the extruder die can exceed the tear strength of the rubber compound, causing surface defects (cracks) in the extrudate. In addition, the foaming reaction during vulcanisation can produce small bubbles on the surface layer, causing undesirable roughness.
The temperature profile and speed of the screw control the vulcanisation and extrusion process, especially foaming by extrusion. In most cases, the final part of this process is the calibration and cutting of the extruded profiles. The products of extrusion foaming are insulating foam sheets, pipes and other products that can be made by profile extrusion.
Batch foaming, also known as “solid-state foaming”, as the name suggests, is a batch process in a closed system. Firstly, the pure rubber must be masticated to reduce the molecular weight and viscosity, allowing simple and homogeneous mixing of the additives. Then all the ingredients except the curing agent are added and mixed for a few more minutes. Finally, the curing agent is added and mixed in. The prepared compound is kept overnight at room temperature to release the residual stress of the rubber particles during mixing. The compound is then placed in a mould and hot pressed at a specific pressure, temperature and time depending on the curing and foaming kinetics. The polymer granules are placed in a stainless-steel mould with microdots. The mould is then placed in a high-pressure vessel (autoclave). After saturation (specific conditions), the foamed parts are obtained by rapid expansion. The samples produced are usually in the form of round discs or rectangular/square plates. Sample thicknesses typically range from 5 mm to 50 mm, which is a very important dimensional factor related to gas diffusivity.