Some aspects of injection moulding of TPE & TPV

The field of thermoplastic elastomers (TPEs) has been growing rapidly during the last two decades. A number of classes of TPEs have been commercialized and widely adopted in many applications across most industries. The thermoplastic vulcanizates (TPVs) are a class of TPEs with a cross-linked rubber phase, which has the performance of thermoset rubbers. The TPVs are growing rapidly and attention to their processing is important to understand TPEs. The other TPEs generally can be described as block copolymer or a polymer blend.  The most commonly used thermoplastic elastomers are:

Styrene block copolymer (SBC) Copolyester TPE (COPE) Thermoplastic urethane (TPU) Copolyamide TPE (COPA)

The injection molding process used for thermoplastics is easily adopted for TPE/TPV. Among all TPEs, TPU was by far difficult to injection mould because of high viscosity. However the grades are commercially available for injection moulding, lately.  The key TPE properties include:

Melt temperature Melt viscosity Shrinkage

The processing melt temperature directly correlates with the melting temperature of the TPE polymer. The melt viscosity at the process shear rates used directly correlates with the injection pressure and packing pressure required. The various TPE classes have different viscosity ranges and temperature sensitivity, and these vary from grade to grade. Therefore, the process and mould must be tailored to the specific class of TPE and the grade selected.  A comparison of the relative moulding parameters for the various TPEs is shown in the table below.

TPE Class: Typical Melt Temperature (°C) Typical Injection Pressure (MPa) Typical Shrinkage Rate (%) SBS block copolymer 190 – 200 1.0 - 5.5 0.3 - 0.5 TPO blend 175 – 205 1.0 - 10.0 0.8 - 1.8 EPDM/ PP TPV 190 – 230 1.0 - 10.0 0.5 - 4.7 NBR/PP TPV 190 – 230 1.0 - 10.0 1.0 - 2.2         TPU block copolymer 175 – 205 1.0 - 7.0 0.5 - 2.5 COPE block copolymer 180 – 260 2.0 - 10.0 0.5 - 1.6 Typical TPE Injection Moulding Characteristics

TPE SBS TPO EPDM / PP TPV TPU COPE Zone: Temperature (°C) Temperature (°C) Temperature (°C) Temperature (°C) Temperature (°C) Feed / Rear Barrel 120-150 185-200 170-190 160-182 200-215 Center Barrel 170-182 190 – 210 170-190 177-199 205-235 Front Barrel 185-195 200-220 170-190 188-210 210-235 Nozzle 190-200 200-220 170-210 190-210 215-240 Melt 190-200 207-216 185-205 185-210 220-245 Mould 22-32 10-50 10-80 38-60 20-50 Typical TPE Injection Moulding Characteristics

The TPVs represent a special case, since the structure does not consist of a single polymer type. TPVs have a cross-linked rubber phase dispersed in a thermoplastic phase. This morphological structure makes them very shear dependent such that the viscosity is quite low at the high shear rates used during injection molding (1,000 to 100,000 sec-1). Then at very low shear (and when static) the viscosity is quite high giving the TPV melt a relatively high stiffness. The grades of TPVs commercially available also vary widely in their injection mouldability. Some are highly tailored to injection moulding and work extremely well. These molding grade TPVs work better in thin and difficult to mould parts. The general purpose grades generally mould well and can be used many TPV applications without resorting to the more expensive moulding grades. But where they are needed, the higher cost is more than offset by the elimination of moulding difficulties.