DPI takes EVA foam moulding into new areas

Injection moulder DPI, based in Geldrop, the Netherlands, has started moulding EVA foam, the very light material used in Crocs shoes.

DPI claims to be the first company in western Europe to process EVA material, aside from contract moulders producing shoes in Italy, Romania, Bosnia and Hercegovina for Crocs.

In September, DPI exhibited at the Kunststoffen 2011 fair in Veldhoven, and it held a technical event at its Geldrop facility in June to demonstrate the process to customers.

At these events, DPI showed the children's EVA cycle seats it is moulding for Dutch company GMG. The Yepp seats are available in a variety of bright colours.

EVA is suitable for these seats because it is waterproof, shock-absorbing, has a high insulating capacity and is easy to clean. EVA is about 70-80% lighter than traditional plastic products, but is also tough, UV resistant, flame retardant and easy to colour.

DPI has invested in KS series machinery from KingSteel Machinery in Taiwan to produce the seats for GMG.

It says EVA can be moulded for many consumer products, such as footwear, toys, bicycle saddles, bags and belts. The company can also mould products in smaller runs of about 500 to 1,000 items.

The temperature of the EVA compound in the injection unit is only 90C, while the mould temperature is 170C. This is necessary because the EVA compound contains two additives - a foaming agent and a crosslinking agent - which only react if the temperature is sufficiently high. So once it enters the mould, the higher temperature causes foaming and crosslinking of the EVA material. When the mould opens after injection and cooling, the EVA bubbles grow and the product expands by 170%.

The cycle time is longer than when injection moulding other plastics, and to address this DPI uses a mould with eight cavities located side by side. Two independent barrels are attached to the injection unit and cavities are filled in a sequence, increasing productivity.