Welcome to our comprehensive guide on troubleshooting common issues in injection blow molding. This resource helps manufacturers identify and resolve defects such as short shots, sink marks, striations, and more. Injection blow molding is a process used in plastics manufacturing to create hollow objects like bottles. Problems can arise from machine settings, material issues, or mold conditions. Use the interactive tool below for quick diagnostics, or scroll to the static FAQ for a full overview.
Key terms: Parison refers to the initial tube-shaped plastic; core rod is part of the mold; venting ensures air escape. Always prioritize safety and consult material specs. This guide is based on industry best practices for efficient problem-solving in blow molding defects.
Interactive Troubleshooting Tool
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Full List of Injection Blow Molding Problems and Solutions
For search engines and non-JavaScript users, here’s a static version of all troubleshooting information. Expand each section for details.
Short Shots
Machine Side/Processing
- Cause: Hopper or barrel out of material
Action: There is typically a sensor to warn you but do a visual inspection to be certain - Cause: Material is bridging in hopper or blocking throat
Action: Inspect the hopper for pellets that have fused together and created a blockage that is impeding the flow of pellets. This can happen when the hopper begins to reach internal temps high enough to soften the material. - Cause: Material in screw is too cold
Action: With a hand probe verify that barrel temps. This can happen if a thermal probe or heater fails. Also check the manifold and nozzle tip temps. - Cause: Programed shot size too small
Action: Ensure that you are not loosing injection pressure or that it is set too low. Increase settings as needed. Verify that there is no loss of hold pressure or reduction of screw RPM. - Cause: Too much screw suck back
Action: No corrective action provided. - Cause: Failure of one or more temperature control units
Action: With a hand probe check the temperature leaving and returning from the control unit. Check for flow.
Mold Side
- Cause: Nozzle tip has frozen off
Action: Use hand torch add heat to nozzle and restart process. - Cause: Nozzle tip has an obstruction
Action: Swap nozzle for a new one. Ensure that the new nozzle is drilled out to match the one removed. - Cause: Nozzle tip drilled too small
Action: Move to next size larger drill diameter - Cause: Venting is dirty or inadequate
Action: Wipe part lines on parison cavity. Ensure that the neck ring is slightly higher than cavity parting line.
Sink on Cast Parison
Machine Side/Processing
- Cause: Melt is not homogeneous
Action: Increase the melt and mold temperatures, raise the screw speed and/or back pressure to enhance mixing and shear. Optimize the injection speed and velocity profile. Ensure the material is thoroughly dry and free of contaminants. - Cause: Pack time and/or pressure need to be increased
Action: Increase timer slightly or injection pressure one at a time. Waite several cycles between adjustments.
Mold Side
- Cause: Venting is dirty or inadequate
Action: Wipe part lines on parison cavity. Ensure that the neck ring is slightly higher than cavity parting line.
Striations or Streaking
Machine Side/Processing
- Cause: Wrong grade of material or regrind
Action: It is critical that only an Injection Blow Mold grade be used. Verify with the material manufacturer how much, if any, regrind may be used. - Cause: Melt is not homogeneous
Action: Increase the melt and mold temperatures, raise the screw speed and/or back pressure to enhance mixing and shear. Optimize the injection speed and velocity profile. Ensure the material is thoroughly dry and free of contaminants. - Cause: Injection pressure too high/fast
Action: Reduce slightly and wait several cycles before next adjustment. - Cause: Melt temperature too high or low
Action: No corrective action provided.
Mold Side
- Cause: Buildup on molding area (dirty)
Action: Wipe down molding area with a mold polish compound - Cause: Cavity damage
Action: Block off cavity if possible. Send mold for repair as soon as possible. - Cause: Inadequate polish on cavity molding area
Action: No corrective action provided.
Gate Stringing or Drooling
Machine Side/Processing
- Cause: Melt temperature too high in manifold or nozzles
Action: No corrective action provided.
Mold Side
- Cause: Nozzle tip drilled too large
Action: Swap in a new nozzle and step down one nominal size.
Parison Sticking to Core Rod
Machine Side/Processing
- Cause: Melt temperature too high
Action: Reduce slightly and wait several cycles before next adjustment. - Cause: Parison mold cooling lines set to high of temperature
Action: Reduce slightly and wait several cycles before next adjustment. - Cause: Core rod external cooling not adequate
Action: Turn on internal and/or external core rod cooling. Increase cooling time. - Cause: Resin has insufficient lubricant
Action: Check with manufacturer for proper ratio natural to additives such as zinc stearate. - Cause: Pack and hold pressures too high or low
Action: Adjust slightly and wait several cycles before next adjustment.
Mold Side
- Cause: Mold cooling inadequate
Action: Ensure proper and adequate flow though water lines. - Cause: Chrome plating worn off
Action: Send mold to tool shop for stripping and replating.
Parison Tip Pinned to Bottom Plug
Machine Side/Processing
- No causes available.
Mold Side
- Cause: Core rod opening too great
Action: Decrease valve opening by .010″ at a time until it is fixed. - Cause: Nozzle land (a.k.a. Slug) length too long
Action: Move manifold closer to cavities - Cause: Blow mold has inadequate blow distance
Action: Consult with mold maker for possible error in design.
Blowouts, Bulges, Thin Center of Push Up or Warped on Bottle Bottom
Machine Side/Processing
- Cause: Nozzle gate temperature is too high
Action: Reduce nozzle zone temp slightly and wait several cycles before next adjustment. Ensure proper flow in the line. - Cause: Parison body temperature is too high
Action: Reduce zone temp slightly and wait several cycles before next adjustment. - Cause: Preform sticking to cores
Action: (See section above – Parison sticking to core rod) - Cause: Injection or pack time too short
Action: Increase slightly and wait several cycles before next adjustment. - Cause: Injection or Back pressure is too high
Action: Reduce slightly and wait several cycles before next adjustment. - Cause: Too much screw suck back
Action: Reduce slightly and wait several cycles before next adjustment. - Cause: Exhaust time too short
Action: Increase slightly and wait several cycles before next adjustment. Check machine for obstructions at exhaust port. - Cause: Poor cooling
Action: Ensure water units are actually outputting cooling at the set temp. Check for flow on each unit.
Mold Side
- Cause: Insufficient venting
Action: Test by adding shim on part line. Recut tooling as needed based on test.
Thick Center of Push Up
Machine Side/Processing
- Cause: Nozzle gate temperature is too low or Core rod tip too cold
Action: Increase nozzle zone temp slightly and wait several cycles before next adjustment. - Cause: Parison body temperature is too low
Action: Increase slightly and wait several cycles before next adjustment. Check water units for flow and actual temp of lines.
Mold Side
- No causes available.
Inconsistent Push Up
Machine Side/Processing
- Cause: Blow hold time too short
Action: Increase slightly and wait several cycles before next adjustment. - Cause: Blow pressure too low
Action: Ensure pin 125 PSI. Step up 10 PSI at a time. - Cause: Insufficient cooling
Action: Ensure flow through bottom plugs. Use chilled water - Cause: Parison nozzle gate temp too high
Action: Reduce slightly and wait several cycles before next adjustment. - Cause: Core rod cooling not being used
Action: Enable core cooling at pick off station. - Cause: Overall cycle time is too short
Action: Increase timers to allow more blow hold time.
Mold Side
- Cause: Vents are dirty or too shallow
Action: Clean vent around bottom plug. Test by adding shim on part line. Recut tooling as needed based on test.
Rocker Bottoms
Machine Side/Processing
- Cause: Parison mold is flashing
Action: Ensure proper clamp pressure on parison station. - Cause: Manifold too hot
Action: Check for uniformity of heat at each nozzle. - Cause: Not enough mold cooling
Action: Increase blow hold time. Check for flow. Use chilled water. - Cause: Exhaust time too short
Action: Increase timer slightly
Mold Side
- Cause: Blow exhaust path restriction
Action: Increase exhaust path depth and/or width - Cause: Core rod not opening enough
Action: Increase valve opening 0.005″ to 0.010″ at a time until it improves - Cause: Vents are dirty or too shallow
Action: Increase vent depth and clean as needed - Cause: Mold halves not aligned
Action: Check for mold mismatch. Use interlocks in cavities.
Cold Folds
Machine Side/Processing
- Cause: Blow pressure too low
Action: Ensure pin 125 PSI. Step up 10 PSI at a time. - Cause: Injection pressure too high
Action: Decrease injection pressure slightly one at a time. Waite several cycles between adjustments. - Cause: Temp too low in parison
Action: Increase body or should temperatures. Waite several cycles between adjustments.
Mold Side
- Cause: Core rod not opening enough
Action: Increase valve opening 0.005″ to 0.010″ at a time until it improves
For more resources, visit our services page or contact us for custom mold solutions. Images of common defects coming soon.
