Why don’t you use air pressure to move the piston ?
It is a way to have high pressure with initial pressure about 8 bars, using surpressor process : with 2 sections you can generate high pressure : P1 = (P2xS2) / S1
P1 : pressure for injection
P2 : initial pressure from compressor ( 8 bars )
S2 : section of first piston (this piston move with de 8 bars from compressor)
S1 : section of second piston (this piston push the plastic for injection)
Erwan from http://www.reve-de-porsche.fr, and very interested to PET recycling.