Thermoplastic processing basic troubleshooting

Define the Problem:

  • Focus effort on defining the exact problem, not the result of the problem.
  • Example: Although short shots present a problem, they are really the result of not enough material in the mold.
  • Example: Although flash presents a problem, it is really the result of poor tooling, weak clamp, improper transfer from fill to pack or a combination.

Quantify with Numbers:

  • The more precisely you can quantify the numbers, the better off you will be.
  • Hot? How hot — 100, 200, 450? F? or C? scale?
  • Melt temperature? — the actual temperature of the plastic.
  • Mold temperature? — the actual temperature of the surface of the mold cavity.
  • Fast? Slow? Injection time — the time from the start of injection until transfer to pack. • Shot to machine capacity? — the weight of parts and runner vs. the maximum machine shot.
  • Cycle time? • Part weight achieved during fill time?
  • Final part weights after pack and (or) holding pressures are applied?

Process Parameters:

  • Understand the process. – What is supposed to happen?
  • Process from the plastics point of view
  • Avoid temperature adjustments because they take too long to reach equilibrium
  • Make machine adjustments only to repeat proven plastic conditions. – Once a set of plastic conditions produce the desired parts, they will continue to do so.
  • Understand when melt and or mold temperature adjustments need to be made. – Rarely and only to repeat proven plastic conditions .
  • Many machines will not develop a consistent pressure or flow until the hydraulic oil reaches an ideal operating temperature range. Action Plan: • Develop a clear written plan.
  • Change only one condition at a time.
  • Allow sufficient stabilization time after each change. Record:
  • Know where you started.
  • Keep an accurate record of each change.
  • Note the results for cause-and-effect analysis.

Plastic Troubleshooting