Co-Extrusion

Thermoplastic elastomers are bonded together to engineering plastics by melt adhesion. Since the materials are bonded together at the production stage, no separate adhesive is needed, which makes the process faster and more cost-effective than a two part assembly.

Unlike the conditions in over moulding, both materials in co-extrusion are in the molten phase when the surfaces come into contact with one another. As a result, the adhesion will be much better in co-extrusion than in overmoulding, in which the molten material must melt the solid in order to achieve adhesion.

The molecules diffuse into one another's surface layers when the materials are in the molten phase and thus lock the materials to one another when solidification takes place.

Setting Parameters

In co-extrusion there are certain parameters that can improve adhesion, it is the high temperature of the melt which is the most important factor.

1. High temperature of the mix. 
The mix temperature is the temperature of the melt when it leaves the gate or the thermoforming channel. If this temperature is high, adhesion will be improved.

2. The highest possible pressure at the nozzle.
A high pressure at the nozzle increases the mix temperature, which leads to better adhesion.

3. Pre-drying of hygroscopic (moisture-absorbing) materials.
Moisture impairs adhesion, so all hygoscopic materials, such as polyamide, should be pre-dried. An extruder with venting is capable of drying plastics with slightly hygroscopic properties.

Processing Guide for Adhesion