TPE Two Component Processing
Thermoplastic elastomers are bonded to engineering plastics by melt adhesion to the most widely used structural plastics. 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 if the two parts were assembled together after each had been produced separately. There are two main processing procedures used to achieve bonding, overmoulding and co-extrusion.
When overmoulding with soft components, the surface of the hard component softens and a diffusion of the molecules in the outer layer takes place. A prerequisite for this is that the two materials, i.e. the TPE Compound and the engineering plastic, must be compatible with one another, meaning that they must not reject one another’s molecules. As the molecule mobility increases, the molecules of the two materials will diffuse and this gives rise to so called melt adhesion. The molecules of the two materials form a network at the surface layer and the network forms a cohesive bond between the materials.
Engineering plastics, to which Dryflex, Mediprene and Lifoflex TPE Compounds have documented good adhesion, include PP, PE, PA6, PA 66, PC, ABS, PC/ABS, ASA and SAN. Polyamides and ABS may be either reinforced or non-reinforced. The adhesion between the TPE Compounds and the engineering plastic may often differ, depending on the manufacturer that supplied the engineering plastic.
The adhesion of PA varies particularly widely. So it is important that adhesion tests are done on the relevant materials. The following information provide recommendations how to improve the bonding in the overmoulding as well as in the co-extrusion process.
The best processing method, to ensure good adhesion in the overmoulding process is the use of an overmoulding machine in which the whole process takes place automatically, i.e. the engineering plastic is first injected, the mould is then turned and the TPE Compounds are finally injected into the mould. It is also possible to inject TPE Compounds in the first step and in the second step the engineering plastics. Another option is to first produce the engineering plastic part and then manually move the part over to a different mould, into which TPE Compounds are injected. The latter method is much more work-intensive and also demands external contact with the engineering plastic part when it is moved between the moulds.
Overmoulding Setting Parameters
The following recommendations are intended to improve the adhesion between Dryflex, Mediprene and Lifoflex TPE Compounds and the engineering plastic. The parameters are applicable when the TPE Compounds are moulded onto the engineering plastic, but they may also be applicable if the procedure is reversed.
- High mix temperature.
- High injection rate.
- The mould temperature of the TPE part should be in the range of 20 – 60°C.
- Avoid excessively high holding pressures.
- Avoid shrink marks.
- Highest possible injection pressure.
- Pre-drying of hygroscopic (moisture absorbing) materials.
- Homogeneous preheating of the engineering plastic (approximately 100°C is ideal).
- Dry, clean and smooth surface of the engineering plastic. The surface should not be structured.
- Sufficient thickness (at least 1.5 mm) of the TPE part.
- Venting of the mould.
- Location and design of the gate.
The setting parameters in 1 – 6 are easiest to implement, since they involve only the machine settings. The parameters in 7 – 9 are pretreatment stages, which are also simple to perform. These stages can be simplified if production of the engineering plastic part and the application of TPE Compounds take place in a continuous process in an overmoulding machine. The parameters in 10 – 12 concern the mould and should thus be considered at the mould design stage.
Unlike the conditions in overmoulding 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 it is in overmoulding in which the molten material must melt the solid material in order to achieve adhesion. The molecules of the two materials diffuse into one another’s surface layers when the materials are in the molten phase. The molecules thus lock the materials to one another when solidification takes place.
Co-extrusion Setting Parameters
In co-extrusion there are also certain parameters that can improve the adhesion. However, these are not as many as in overmoulding. It is still the high temperature of the melt which is the most important factor. The materials are the same as in overmoulding, i.e. an engineering plastic and Dryflex, Mediprene or Lifoflex TPE Compound.
- High temperature of the mix.
- The highest possible pressure at the nozzle.
- Pre-drying of hygroscopic (moisture-absorbing) materials.
Further details on overmoulding and co-extrusion are available to download in the TPE Processing Guide for Adhesion