Injection Molded Part Defects and Remedies

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There are several variables in the injection molding process and mold design. All of which have implications on part quality. It’s important to know the potential defects, how to prevent them, or process them out. Here you will find a list of common molding challenges and their remedies

Assumptions

When assessing issues with injection molded parts, we want to make sure some core aspects of molding are checked. Below is a list of items that should be checked both during normal molding operations and if defects are detected. Material contamination, for example, can cause several molding issues. Therefore, it’s important to check these things first before digging deeper into the potential issues or changing machine settings.

  • Material is free from contamination
  • Material has been properly dried
  • Material handling equipment is functioning correctly and has been properly cleaned
  • Processing parameters including drying time, drying temperature, and regrind limits are within specification for the selected material
  • The proper material has been selected for the application
  • The molding machine is functioning properly and has been properly maintained.
  • Auxiliary equipment such as temperature controllers and dryers are functioning correctly
  • Mold surfaces are cleaned
  • Mold water channels are clean and flowing properly
  • Previous processing parameters have been recorded and checked

Brown / Black Marks

Description

Brown or black marks or streaks are present on the surface of the molded part.

Cause – Remedies

  • Material from previous runs are stuck in the barrel/screw assembly, nozzle, or hot runner system. Degrading material can also be caused be high resonance time in the barrel.
    • Purge the barrel and hot runner system
    • If possible increase cycle time to reduce material resonance time. Running the mold in molding machine with less barrel capacity is also possible
    • Remove and inspect the check ring
    • Remove the barrel assembly and thoroughly check for damage or ware
  • Melt temperature of the plastic is too high
    • Lower the rear zone temperatures
  • Material is being injected too fast causing excessive shear heating
  • Screw RPM is too high
  • Gates are too small
  • Check for uniform wall thickness to avoid high shear areas
    • Make varying wall thicknesses uniform

Burning

Description

Yellow / black marks at the end of fill on a molded part. Burning is most likely cause by air traps in the mold. As plastic fills the mold, the air needs to escape. If the air gets trapped, it will compress and overheat the plastic.

Causes – Remedies

  • Injection Speed too High
  • Material is too hot
    • Reduce melt temperature
  • Excessive clamping force
    • Reduce clamping force if possible
  • Not enough venting in the mold
    • Check for restricted venting and air traps
  • Dirty Vents
    • Clean the mold surfaces, parting lines, and vent pins

Cracking

Description

The components crack on ejection or during handling.

Causes – Remedies

  • Injection speed is too slow
  • Degraded material
    • Adjust cylinder temperatures
    • Reduce back pressure
  • Melt temperature is too low
    • Increase rear zone temperature
    • Increase nozzle temperature
  • Mold surface is too cold
    • Increase the mold temperature
  • Gate size or runners are too small
  • Excessive regrind present

Flashing

Description

Extra plastic around parting lines and other shut off areas.

Causes – Remedies

  • Injection pressure too high
    • Slow down the injection speed
    • Raise the volume-pressure switch over point
    • reduce shot size
  • Material is too hot
    • Reduce the melt temperature
  • Low or uneven clamp force
    • Make sure sufficient clamp force is available
    • Make sure mold is clamping squarely and fully
    • Make sure tooling assemblies/actions have proper support to resist material pressure
  • Clogged or small venting
    • Check that venting is sufficient

Flow Lines

Description

A rippled surface finish that is usually around a gate but can also be at the end of fill on the parts

Causes – Remedies

  • Slow injection speed
    • Increase the injection speed
  • Low melt temperature
    • Increase the temperature in the front two zones of the barrel
  • Runner and gates are too small
    • Increase the size of the runners and gates
  • Surface temperature of the mold is too low
    • Increase the mold temperature

Jetting

Description

A swirled line coming from the gate of the part

Causes – Remedies

  • Mold surface temperature too low
    • Increase the mold temperature
  • Gate size too small
    • Increase gate size

Short Shot

Description

The molded part is not completely filled out

Causes- Remedies

  • Injection speed to slow
    • Increase injection speed
  • Shot size too small
    • Increase shot size if screw position continuously goes to 0
  • Non return valve/check ring damaged or worn
    • Check for wear or damage if consistent cushion cannot be accomplished
  • Inadequate mold venting
    • Air traps causing material flow to stop. Check for proper venting
  • Long flow length / thin wall sections
    • Check for consistent wall thickness and make sure gate location is optimized for material flow.

Sink Marks

Description

Sinks or voids in the plastic part. Usually located around thick wall sections.

Causes – Remedies

  • Inadequate shot volume
    • Make sure cushion is consistent
  • Inadequate injection pressure
    • Increase injection pressure to flow more material into the mold
  • Inadequate hold time
    • Add more hold time to allow part to fill out
  • High melt temperature
    • Lower the melt temperature
  • Too small of a gate
    • Gate size should allow for proper filling of the part without freezing off
  • Gate position not optimized
    • Gate should feed into larger areas of the part to ensure they fill

Voids

Description

Bubbles trapped inside a part. May not be seen on colored parts.

Causes – Remedies

  • Air traps in the melt / barrel
    • Increase back pressure
    • Reduce screw RPM
    • Reduce decompression
  • Inadequate packing pressure
    • Increase injection pressure
  • High fill speed
    • Slow down the injection speed

Warping

Description

Part shape is warped after it’s ejected from the mold

Causes – Remedies

  • Cycle time too short
    • Increase the cooling time to allow the part to be cooled more thoroughly by the mold
  • Insufficient shot size
    • Increase the shot size
  • Melt temperature too low
    • Increase the melt temperature
  • Insufficient mold cooling
    • Check for proper cooling flow through the mold
    • Increase the amount of cooling in the mold
  • Feed systems to small or misplaced
    • Gates should feed into thicker wall sections of the part and allow for enough material to flow and cool
    • Runner system should be able to feed enough material into the cavities without excessive pressure or freezing

Weld / Knit Lines

Description

Locations where material is “reconnected” during the injection molding process. This is inevitable with holes in plastic parts and multiple gate situations. These can be failure points on the molded parts if not remedied properly

Causes – Remedies

  • Low melt temperature
  • Flow speed too fast or too slow
  • Mold surface is too cold
    • Increase the mold temperature
  • Mold is not vented properly causing slow flow at knit lines
    • Add or increase vents as needed

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