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.

About the Author

John Bozzelli
Injection Molding (IM) Solutions
1019 Balfour St.
Midland, MI 48640-3227
Phone: 989-832-2424
Fax 989-832-8743
John Bozzelli is a graduate of Marietta College (BS) and Ohio University (MS). His studies were interrupted for a stint in Vietnam (US Army, Purple Heart; Silver Star). Twenty years in Dow Plastics provided extensive experience in polymer synthesis, development, production, and processing. John has been a seminar leader with RJG Associates, Injection Molding Magazine, University of Wisconsin Milwaukee, General Polymers and John Klees. Competent in resin characterization and analysis, his specialty is practical, hands-on injection molding training with both small and large machines. National recognition has come through ten patents, over 60 papers covering plastics, processing, machine specifications, and over 12 years on the national seminar circuit. Feature articles such as the “Productivity”; by Plastics World and ”Scientific Molding” by Injection Molding Magazine October, November and December 1997, have highlighted a couple of exemplar case histories. Check out the August 2001 issue for applications of The Universal Set Up Sheet.John is the initiator of Scientific Injection Molding and teaches the plastic’s point of view for design and processing with a passion you will remember. Take some of your valuable time to learn practical molding techniques that improve your profits tomorrow while eliminating the state of ”fire fighting“ currently found in many molding facilities. Let us keep plastic manufacturing strong in North America.

The views, opinions and technical analyses presented here are those of the author, and are not necessarily those of UL, or While the editors of this site make every effort to verify the accuracy of its content, we assume no responsibility for errors made by the author, editorial staff or any other contributor. All content is subject to copyright and may not be reproduced without prior authorization from Prospector.

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