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Injection Molding: Troubleshooting

Approach to Troubleshooting

Proper troubleshooting should use a systematic approach to resolve problems. There are two kinds of issues: those encountered during startup and those involving "quality control."

Startup problems occur during the launch of a new tool or machine. "Quality control" issues occur when parts have been successfully produced in the past, but are now out of specification. These problems are the result of something in the process changing. To resolve them, one must determine what has changed and restore the process to its proper balance.

Startup Issues

To resolve startup problems, the material's process window must first be determined to ensure that there is a set of conditions that can make good parts.

Start by setting the process conditions to the middle of the material's process range and then adjust the process to fix any observed problems. If successful parts are not possible, determine what combination of variables must be changed to resolve the problems. These changes may include material selection, machine selection, and/or tool redesign.

Quality Control Issues

Most quality control issues are caused by process conditions, the material, or machine and tool maintenance. If the problem occurred after a change in the material lot number, try a different lot of material. It is advisable to store a retain of the previous lot of material.

If the mold setting is from a previously successful part, check the original setup conditions. If the mold is used in a different machine, minor adjustments to account for the new machine may be needed. If the mold was modified during its downtime, process adjustments may be required.

Startup Issues                               

Observation

Possible Causes

Corrective Actions

Poor Knit Lines

Poor venting.

  • Place vents at last place to fill and at converging flow fronts.
  • Increase size of the vents.

Too cool a melt.

  • Increase the melt and/or mold temperature.
  • Increase injection speed.

Part Shorts
and has burn marks

Gas trapping/ poor venting.

  • Improve venting and/or relocate to burned area.

Part Shorts
No burn marks

Not enough material.

  • Increase shot size, if possible. If not, move to a larger machine.

Blockage in flow at the feed-throat.

  • Reduce rpm and back pressure.
  • Decrease the barrel temperature in the rear.

Not enough injection pressure.

  • Increase injection pressure.

Material too high in viscosity.

  • Increase injection rate.
  • Increase process temperature.
  • Use a lower viscosity TPE.
  • Increase gate and runner size.

Part Flashes

Too much injection pressure.

  • Reduce injection pressure and time.

Too much material.

  • Decrease shot size.

Material viscosity too low.

  • Reduce injection speed.
  • Reduce process temperature.

Tool too loose

  • Machine or dress the parting line.

Not enough clamp capacity.

  • Use a higher viscosity TPE.
  • Increase clamp tonnage.
  • Reduce thickness of the vents.
  • Move to a larger machine.

Voids or Sinks
Voids occur inside the part. Sinks pull away from the mold wall.

Material shrinkage & insufficient supply of molten material.

  • Decrease wall thickness.

Not enough pack pressure during material solidification.

  • Increase the pack pressure and time.
  • Increase the gate size.
  • Relocate the gate to the thickest section.
  • Increase the runner size.

Bubbles
Part surface bulges above a bubble.

Gas entrapment.

  • Relocate the gate or modify the flow path.
  • Add a pin in the area to eliminate the gas trap.

Flow Marks
Back fills

Filling from thin to thick sections.

  • Reposition the gate to a thick section.

Flow Marks
Shadowing

Surface irregularity.

  • Radius dimples.

Flow Marks

Folds

Uneven filling of section.

  • Relocate gate to balance the flow or reduce the runner diameter.

Jetting

High viscosity flow.

  • Increase process temperature.
  • Increase injection speed.
  • Decrease gate size.
  • Change type of gate.
  • Relocate gate to impinge.

Warped Parts

Anisotropic shrinkage.

  • Relocate the gate so flow occurs in only one direction.

High molded-in stress.

  • Increase the process temperature.
  • Reduce the pack pressure.
  • Increase the mold temperature.

Surface Defects
Uneven shine
Beach marks

High molecular weight TPE in highly polished mold.

  • Change to a lower molecular weight TPE.
  • Texture mold cavity surface (EDM, sandblast, etc.)

Surface Defects
Silver streaking or splay marks

Contaminated material.

  • Check for moisture (condensation) or dry the material.
  • Check regrind for contaminants or moisture. Dry if required
  • Reduce injection speed.

High shear in the material.

  • Increase the process temperature.
  • Increase the gate size.

Ejector Pin Marks

Parts soft during ejection.

  • Increase mold close time.
  • Reduce mold temperature.
  • Reduce process temperature.
  • Texture mold surface for better release.
  • Increase size of pins.
  • Increase draft on part.
  • Reduce wall section.
  • Use a TPE with mold release.
  • Use a harder TPE.

Burnt Smell
Parts have a yellow cast

Degraded material.

  • Purge machine and observe whether problem reoccurs.
  • Reduce process temperature. (Particularly in rear of machine).
  • Reduce regrind level.
  • Reduce residence time.
  • Purge machine after shutdown.
  • Move to a smaller machine.
  • Reduce hot runner system temperatures.
  • Minimize dead spots in hot runner manifold.

Part Sticks in "A" Half or Stationary Side of the Tool

Insufficient extraction force.

  • Sandblast "A" side of tool.
  • Polish "B" side.
  • Run "A" side cooler.
  • Put keepers in "B" half of tool.
  • Increase draft on part in "A" half of tool.

Part Sticks During Ejection

Insufficient ejection force.

  • Increase mold closed time.
  • Reduce mold temperature.
  • Reduce process temperature.
  • Reduce pack and hold pressure.
  • Sandblast "B" side.
  • Increase size of ejector pins.
  • Increase number of ejector pins.
  • Increase draft on part.
  • Provide air assist.

Non-Uniform Color

Poor dispersion.

  • Increase back pressure and/or screw rpm.
  • Change the color concentrate carrier to a material with a lower melt temperature.

Contamination.

  • Check for clean regrind.

 

Quality Control                               

Observation

Cause

Solution

Shorts

Inadequate shot size.

  • Material blockage at throat - lower feed zone temperature.

Vents blocked.

  • Clean vents.

Flashing

Too much material.

  • Check shot size.

Clamp pressure low.

  • Check for dirt on the mold parting line of low hydraulic pressure.

Injection pressure too high.
Injection speed too fast.

  • Check injection pressure and speed.

Burns

Dieseling in the mold.

  • Clean vents.

Non-Uniform Color

Poor concentrate mixing or dispersion.

  • Check barrel temperatures.
  • Check back pressure and screw rpm, increase if necessary.

Off-Color Part or Odor

Contamination.

  • Check for contamination.

Material degradation.

  • Reduce the level of regrind.
  • Reduce the rear zone temperature.
  • Reduce the residence time.
  • Add delay to screw recovery.

Dimensions Out of Specification

Loss of control of shrinkage.

  • Check hold pressures.
  • Check melt and mold temperatures.
  • Check that cushion was maintained.

Surface Defects (Streaks)

Moisture in part.

  • Dry material.
  • Check for contamination.

Possible mold sweating.

  • Raise mold temperature.

Voids or Sinks

Inadequate amount of material.

  • Make sure there is a cushion maintained.
  • Increase shot size.
  • Increase pack and hold pressures.
  • Increase hold time.

 

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