This review-paper outlined the work done on bead foams both from an industrial and scientific viewpoint and showed the trends and perspectives of state-of the-art materials and machinery, as wells as new material developments.
Currently, EPS is the most widely used bead foam. It is used for commodity applications requiring cost effective part production in huge quantities as packaging and insulation. EPS has a high specific modulus and strength at the drawback of low elasticity. In contrast to EPS, the other two commonly used bead foams EPE and EPP are more expensive, but have a much higher elasticity making them suitable for the packaging of more sensitive goods. Especially EPP is gaining more and more attention as new fields for its application besides the traditional ones as structural material are found. Nowadays, EPP is used for furniture or multi-material combinations. An advancement of the recent past is ETPU, since thermoplastic polyurethane is one of the most elastic thermoplastically processable materials. Bead foams made of this material shine out with extraordinary elasticity making parts from ETPU an ideal candidate for damping high impact forces as found in shoe soles. ETPU bead foams will open new areas as the foaming reduces TPU hardness without the use of plasticizers. However, all state-of-the-art bead foams have a low usage temperature in common. Therefore, current developments aim to tackle this issue by making technical thermoplastics like PBT ready for bead foaming. Bead foams made of bio-based polymers like PLA are also a key-focus of current research.
Beside research on new materials or their optimisation, the other focus of investigation lies on the processing of bead foams. The trends in machinery show that steam-chest moulding machines are made ready for new types of bead foams requiring high steam temperature, which is critical since mould, steam chamber and injectors have to withstand much higher steam pressure. Optimisation towards lower energy consumption to facilitate an efficient production of huge quantities of bead foam parts is also a challenge to ensure efficient production. More efficient steaming and cooling cycles are key features for new steam-chest moulding machines.
Although bead foams are quite mature in terms of time on the market, there are still many open scientific questions regarding the effect of their unique morphology on properties, physical phenomena during steam chest moulding and how to achieve suitable welding properties.