hot runner system trouble shooting

Hot Runner System Nozzle Trouble Shooting

A Hot Runner System is used to maintain a molten flow of plastic from the molding machine nozzle to the gate of a plastic injection mold.

In general, the system is composed of three main parts; the sprue bush, the manifold block, and one or more hot nozzles. Why need hot runner system for plastics industry? Please check advantages of hot runner system.

The following is a list of common problems and answers for hot runner systems.

1. The part is not filling

  • Cause: Melt temperature too low, injection pressure too low, gate too small, nozzle too small, mold too cold, exit from machine nozzle too small, nozzle blocked.

  • Remedy: Raise nozzle and manifold temperature, raise injection pressure, enlarge gate, raise mold temperature, fit larger nozzle, enlarge hole in machine nozzle, clear blockage.

2. Nozzle drooling

  • Cause: Insufficient suck back, Melt temperature too high, gate too big, insufficient gate cooling, incorrect Nozzle type selected.

  • Remedy: Increase suck back, lower nozzle and/or mold temperature, reduce gate diameter, increase gate cooling, contact your Nozzle supplier for correct nozzle selection.

3. Nozzle not working

  • Cause: Heater failure, Thermocouple failure, Nozzle blockage, Incorrect allowance for expansion of nozzle.

  • Remedy: Check/replace heater, check/replace thermocouple, remove clean nozzle, re-machine nozzle cavity.

4. Poor colour change

  • Cause: Incorrect colour change procedure, wrong type of nozzle.

  • Remedy: See guide for correct colour change below, Contact your supplier for correct nozzle selection.

Recommended Procedure for Colour Change

  1. Increase mold temperature by 25 ℃
  2. Increase manifold and nozzle temperature by 30 ℃
  3. Retract molding machine nozzle
  4. Purge the molding machine as per your standard practice using a purging agent
  5. Re-start normal cycle – 6 shots
  6. Lower manifold and nozzle temperature 20 ℃ – 1 shot Minimum
  7. Lower manifold and nozzle temperature 10 ℃ – 1 shot Minimum
  8. Lower mold temperature 25 ℃
  9. Check the next moulded parts for colour consistency & quality, and if required repeat
    Steps 1-9
  10. New colour is now ready

5. Excessive flash on part

  • Cause: Too high an injection pressure, temperature too high, poor shut off face flatness. Insufficient clamp pressure on molding machine, tool plates flexing.

  • Remedy: reduce injection/pack, lower nozzle/manifold/mold temp, increase machine clamp force, change tool.

6. Burn marks/streaks on part or near gate

  • Cause: Not enough venting in tool, injection speed too high, gate profile incorrect, material not dry.

  • Remedy: Add more venting, lower injection speed, increase “J” dimension on gate profile, dry material.

7. Excessive tip wear in nozzles when using plastics with high glass fill content

  • Cause: Tip material too soft for application.

  • Remedy: Change to Carbide tips, such as MASTIP.

8. Gate vestige too large

  • Cause: Gate too large, incorrect nozzle selection, gate profile machined incorrectly.

  • Remedy: Fit bush/sprue nut to reduce gate, Contact hot runner system supplier for correct nozzle selection, check gate
    machining profile.

9. Gate freezing off too soon, or during cycle

  • Cause: Melt too cold, gate too small for material being used, excessive cooling around gate, too much contact between nozzle and mold, gate profile incorrect or incorrect type.

  • Remedy: Raise nozzle temperature, raise mold temperature around gate, check machining of nozzle cavity and make sure contact is at a minimum, check machining of gate profile and change if needed.

10. Flow lines on large flat part

  • Cause: Incorrect nozzle type

  • Remedy: Change nozzle type

11. Bloom on part opposite gate

  • Cause: Mold too cold, melt too cold, cold slug in part.

  • Remedy: Raise mold temperature, raise melt temperature, use MOT nozzle.

12. Cold slug in part

  • Cause: Wrong nozzle selection, head of the nozzle too cold.

  • Remedy: Contact your hot runner suppier or nozzle supplier for correct nozzle selection, machine cold slug trap opposite gate, ensure contact area on nozzle head is minimum.

13. Intermittent blockage caused by cold slug, tip fails by trying to extrude through nut

  • Cause: Too much head loss through nozzle head.

  • Remedy: Reduce head contact to a minimum, Sit head in thermally insulated material.

14. Plastic sticking to front of bush nut or sprue nut

  • Cause: Not enough contact between nut and mold to dissipate heat.

  • Remedy: Change nut type with increased contact area to dissipate heat from nut.

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