Molding Defects and Countermeasures

The main molding defects that may occur, their probable causes, and corresponding countermeasures are listed in Table. 6.1.

Table. 6.1　Molding defects and countermeasures

Defect Cause Countermeasure
Insufficient filling
(short shot)
  1. 1) Insufficient resin amount
  2. 2) Insufficient injection pressure
  3. 3) Low filling speed
  4. 4) Low resin temperature
  5. 5) Insufficient release of air from the mold
  6. 6) Too small runner or gate diameter
  7. 7) Long runner
  8. 8) Thin wall thickness of the molded product
  9. 9) Worn backflow preventing ring.
  1. 1) Increase the resin amount.
  2. 2) Increase the injection pressure.
  3. 3) Increase the injection speed.
  4. 4) Increase the cylinder temperature.
  5. 5) Provide a vent for the unfilled portion.
    (About 5/1000 mm)
  6. 6) Increase the runner or gate size.
  7. 7) Review the runner placement.
  8. 8) Increase the wall thickness of the molded product.
  9. 9) Replace the backflow preventing ring.
Insufficient strength of welds
  1. 1) Gate position
  2. 2) When the flows meet, the viscosity at that portion is too high.
  3. 3) When the flows meet, the pressure applied to that portion is low.
  4. 4) Air is trapped.
  1. 1) Change the gate position so that welds are generated at locations carrying a low load.
  2. 2) Increase the injection speed. Increase both the cylinder and mold temperatures.
  3. 3) Increase the injection pressure. Increase the resin amount.
  4. 4) Provide a vent. Decrease the injection speed.
Sink marks, voids
  1. 1) Insufficient resin density
  2. 2) Short gate seal time (vacuum voids)
  3. 3) Excessive wall thickness (vacuum voids)
  4. 4) Large thermal shrinkage (sink marks)
  5. 5) Large wall thickness difference (convoluted voids)
  1. 1) Increase the back pressure. Increase the injection pressure.
  2. 2) Enlarge the gate, and increase both the cylinder and mold temperatures.
  3. 3) Decrease both the cylinder and mold temperatures.
  4. 4) Decrease both the cylinder and mold temperatures.
  5. 5) Decrease the injection speed at the portion where the wall thickness differs.
Flash
  1. 1) Resin flows into gaps such as air vents.
  2. 2) In the flow process, the flow resistance increases, and an excessive opening force is applied (the mold opens).
  1. 1) Decrease the injection pressure or the injection speed. Decrease both the cylinder and mold temperatures.
  2. 2) Increase the cylinder temperature, mold temperature, and injection speed, and also increase the fluidity.
Surface gloss defect
(mold transfer defect)
  1. 1) Insufficient resin density
  2. 2) The filling speed is low relative to the solidification rate.
  3. 3) Low mold temperature
  4. 4) Poor air exhaust from the cavity
  5. 5) Deposit adhesion (tarnish)
  1. 1) Increase the injection pressure.
  2. 2) Increase the injection speed and the cylinder temperature.
  3. 3) Increase the mold temperature.
  4. 4) Provide a vent.
  5. 5) Decrease the cylinder temperature and the injection speed, and clean the mold.
Internal cracks
  1. 1) Too quick cooling
  2. 2) Occurrence of residual stress
  3. 3) Excessive wall thickness.
  1. 1) Increase the mold temperature, extract the molded product, and then slowly cool the product.
  2. 2) Decrease the injection pressure, and increase the mold temperature.
  3. 3) Decrease the wall thickness, and ensure that it is uniform.
Warpage
  1. 1) Uneven cooling.
  2. 2) Quick ejection
  1. 1) Increase the cooling time, ensure that the mold temperature is balance, and make the product wall thickness even.
  2. 2) Decrease the ejection rate.
Burns
  1. 1) Insufficient gas venting
  2. 2) Large amount of exhaust gas
  1. 1) Provide a vent.
  2. 2) Decrease the cylinder temperature.
Ejection defect
  1. 1) Insufficient draft taper angle
  2. 2) Inappropriate ejector pin
  3. 3) Insufficient cooling of the molded product
  4. 4) Overfilling
  1. 1) Increase the draft taper, and use a mold release agent.
  2. 2) Adjust the ejector pin position and balance, and increase the ejector pin diameter.
  3. 3) Increase the cooling time, and decrease the mold temperature.
  4. 4) Decrease the injection pressure.
Drooling
(resin leakage from the nozzle)
  1. 1) Large amount of moisture absorbed in resin
  2. 2) High nozzle temperature
  3. 3) Small suck back
  1. 1) Perform sufficient predrying.
  2. 2) Decrease the nozzle and cylinder temperatures, and use a shut-off valve.
  3. 3) Increase the suck back.
Silver streaks
  1. 1) Large amount of moisture absorbed in resin
  2. 2) Large amounts of exhaust gas
  1. 1) Perform sufficient predrying.
  2. 2) Decrease the nozzle and cylinder temperature, and decrease the injection speed.
Jetting
  1. 1) Gate position
  2. 2) Low mold temperature
  1. 1) Increase the size of the slug well, and provide a gate at a position where the flow direction changes immediately after the gate.
  2. 2) Increase the mold temperature.
Cold slug
  1. 1) Low nozzle temperature
  2. 2) Small slug well
  1. 1) Increase the nozzle temperature, and shorten the nozzle touch time.
  2. 2) Increase the size of the slug well.
Contamination with foreign matter
  1. 1) Insufficient purge
  2. 2) Long residence time
  3. 3) Use of recycled material
  1. 1) Increase the number of purges.
  2. 2) Shorten the molding cycle, and perform a purge.
  3. 3) Change the material of the blade of the crusher, piping, and so on.