Backpressure Setting on an Injection Molding Machine

By John Bozzelli

I do recommend you use 1,000-psi PLASTIC pressure for backpressure unless there is a reason not to. So on electrics it should be 1,000-psi set point. On hydraulic machines divide the intensification ratio into 1000 to find the correct hydraulic pressure setting.

The reasons to use 1,000-psi plastic pressure for backpressure are:

1. Better melt temperature control
2. Better melt uniformity
3. If blending in color at the press this should give you good color mixing if you are running a correctly designed screw
4. Better shot size position control
5. Better decompression repeatability control, it acts as a brake to stop momentum
6. Better filling of the flights to prevent black specks and degradation behind screw flights, minimize hot spots
7. Prevent screw augering due to pellets wedged between flight land and feed throat

Reasons to use less back pressure:

1. Vented barrel screws
2. Fiber breakage in fiber filled resins
3. Resins sensitive to degradation such as PVC or Acetal
4. Long recovery times are making your cycle too long. This will have to balance with production of acceptable parts. Short cycles are important but worthless if you can not sell the part.