Probable Cause

Suggested Remedy

Parison stretch is more
  • Melt temperature high, slow extrusion rate, low melt index material.
  • Reduce material temperature, increase extrusion rate, use right melt index material
Rough surface, melt fracture or chatter marks in parison
  • Low melt temperature, melt fracture of material, die causing fracture of material.


  • Extrusion rate too high
  • Melt fracture or instability
  • Increase melt temperature,

raise parison bushing temp
erature, reduce extrusion rate, streamline die approach.

  • Decrease the extrusion rate.
  • Use higher flow resin if Possible.
Uneven wall thickness and thinning of sidewall near the base of product.
  • Pin not centered in the die bushing, die design wrong, uneven melt, parison necking away from the die due to low melt strength of the polymer, too high a melt temperature, the bottle design requiring too great a length for the thickness of parison employed.
  • Center the die pin, check die design, melt condition to be made uniform, use material of better melt strength (usually of lower melt flow index), reduce the melt temperature, employ mechanical means to maintain uniformity of parison wall.
Curling of parison, lengthwise thickness nonuniform
  • Non uniform walls, Parison necking down.
  • Align die to give uniform parison thickness
Bubbles in parison.
  • Moisture in material


  • Air entrapped in the melt.
  • Use high melt index resins, reduce melt temperature, increase extrusion rate.
  • Pre dry material before use , use higher screen mesh to increase back pressure
  • Degraded material being used , hold-up spot in the machine , surroundings not clean
  • Clean machine thoroughly, use good quality material, streamline flow of material by removing hold -up spot, maintain clean environment.
Dough nut formation
  • Material temperature low, mandrel not extending below die, mould touching die head, die face not clean,
  • Bring mandrel to system temperature, allow mandrel to reach equilibrium with rest of the system, lower mandrel tip slightly below bushing, clean die.
Streaks on parison
  • Contamination due to hold- up spots in the machine, hot spots in the machine.
  • Clean machine and stream line the flow of material, check for the hot spots.
Poor surface finish
  • Mould finish rough , blow pressure low, vents not provided, die temperature low, mould temperature low
  • Polish the mould, increase blow pressure, provide vents, increase melt temperature, increase die temperature, increase mould temperature.
Rippling in part of parison
  • Contamination in die orifice, extrusion rate high, die shaping in correct.
  • Clean the orifice, reduce extrusion rate, decrease shaping depth, decrease shaping land, increase shaping width, reduce pressure or accumulation.
Product surface shows pits, fish scales, rough grain surface, etc.
  • Trapped air in the mould , water on mould surface(condensation)
  • Thermal degradation, mould temperature very high, inadequate blowing pressure or slow rate of blow.
  • Provide vents, raise mould temperature to reduce condensation, check for air leak, have mould cavities with roughened surface (sand blasting), particularly for polyethylenes,
  • Reduce the melt temperature, reduce the mould temperature, correct blowing pressure and rate of blow.
Indented parting line.
  • Air entrapped , low blowing pressure, low mould temperature
  • Provide vents, increase blowing pressure, increase mould temperature
  • Insufficient cooling, non uniform cooling
  • Decrease mould temperature, increase cooling time, provide uniform cooling, provide better water circulation , use internal cooling
Breakage of seams
  • inadequate weld, melt temperature low
  • Increase melt temperature
Black specks
  • Degraded material, hot spots, hold up areas in head
  • Purge thoroughly and clean, streamline flow of material, check and remove hot spots, Thoroughly purge barred and accumulator, avoid long shutdowns with material at high temperature
Part sticking to mould
  • Mould temperature high, cycle time too short
  • Reduce mould temperature, increase cycle time
Part blow out
  • Too large blow ratio,

Parison blow out,

Mould separation,

Part too hot,

Pinch off very sharp,

Pinch off too hot,

  • Use larger parison die, higher melt strength material to be used, use higher clamping force, longer cooling time to be given, provide wider land in pinch off, cool mould pinch off, use larger die
Parison tail sticking to part
  • Parison very long, mould base pinch off design wrong
  • Shorten the parison,

Design pinch off properly

Excess shrinkage
  • Improper cooling, blow pressure not enough, melt temperature too high, wall thickness too high, high shrinkage material
  • Increase blow pressure for material to have close contact with the mould, reduce melt temperature, reduce wall thickness, decrease material density
Thin wall at pinch off
  • Pinch off too sharp
  • Increase pinch off land width and relief area
Thick wall at pinch off
  • Pinch off clearance too high, pinch off angle too small,
  • Reduce pinch off clearance, open pinch angle 30°
Part removal difficult at under cuts
  • Part over cooled,

Severe undercuts

  • Reduce cycle time, reduce undercuts or use movable inserts,
Die lines
  • Blend of material (being used), scratches in die, restricted flow in die.
  • Use similar melt index material, polish the die surface and mandrel, streamline flow in the die
Insufficient blowing
  • Air supply restricted, pinch off too hot, pinch off too sharp
  • Provide enough air, cool mould at pinch off area, provide enough land in pinch off area
Cutting knife sticks to parison
  • Blunt knife, knife far off from parison, knife speed not enough
  • Sharpen knife, locate knife near parison, increase knife speed, reduce parison temperature


Distortion warping
  • Shrinkage difference in axially asymmetrical parts, shrinkage difference due to variation in wall thickness, in oval parts , warping
  • Design mould properly, compensate a mould shrinkage allowance that increases with wall thickness, give the article a spherical design
Moulding sticking to blow pin
  • Blow pin too hot
  • Reduce the temperature around blow pin.