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Blow moulding of thermoplastics
 
 
 Fault Probable Cause Suggested Remedy
1. Rough, grain, surface
1.1 Local air pockets. 1.1 Provide adequate venting.
1.1.1 Have mould cavities with roughened surface (sand blasting), particularly for polyethylenes.
1.2 Thermal degradation. 1.2 Reduce the melt temp.
1.3 Mould Temp. very high. 1.3 Reduce the mould temp.
1.4 Inadequate blowing pressure or slow rate of blow. 1.4 Correct.
  Fault Probable Cause Suggested Remedy
2. Melt fracture or chatter marks in parison.
2.1 Material temp. too low. 2.1 Increase the melt temp.
2.2 Extrusion rate too high. 2.2 Decrease the extrusion rate.
  Fault Probable Cause Suggested Remedy
3. Distortion warping
3.1 Shrinkage differences in axially symmertrical parts, shrinkage differences due to wall thickness. 3.1 Compensate by a mould shrinkage allowance that increase with thickness.
3.2 Shrinkage differences in oval parts, warping. 3.2 Give the article a spherical design.
  Fault Probable Cause Suggested Remedy
4. Formation of an edge along the parting line.
4. Non-alignment of mould halves. 4. Adjust the mould halves carefully.
  Fault Probable Cause Suggested Remedy
5. Non-uniform wall-thickness, particularly thinning of side wall near the base.
5.1 Parison necking away from the die due to low hot strength of the polymer. 5.1 Use material of better hot strength (usually of lower melt-flow index).
5.2 Too high a melt temp. 5.2 Reduce the melt temp.
5.3 The bottle design requiring too great a length for the wall-thickness employed. 5.3 Employ mechanical means to maintain uniformity of parison wall.
  Fault Probable Cause Suggested Remedy
6. Poor weld and excessive thinning at the weld. <
6. Badly designed pinch-off. 6. Redesign the pinch-off to prevent material being robbed from the weld and to ensure a good weld yet at the same time provide for easy removal of flash.
  Fault Probable Cause Suggested Remedy
7. Parison welding or parison sticking to the knife which cuts the parison.
7.1 Blunt knife. 7.1 Sharpen the knife-edge.
7.2 Knife situated too far from the die. 7.2 Bring the knife closer to the die.
7.3 Knife travels too slow. 7.31 Increase the travel speed of knife.
7.32 Reduce the temp.
  Fault Probable Cause Suggested Remedy
8. Unpleasant odour.
8.1 Thermal degradation of the polymer. 8.1 Check the temperature programmes.
8.2 Oil contamination in air used for blowing. 8.2 Use filtered air for blowing.
8.3 Contamination from the extrusion barrel. 8.31 Clean the extrusion equipment periodically.
8.32 Avoid use of reprocessed material.
  Fault Probable Cause Suggested Remedy
9. Rough parison.
9.1 Melt temperature too low. 9.1 Increase heat progressively.
9.2 Melt fracture or instability. 9.2 Use higher flow resin if possible.
  Fault Probable Cause Suggested Remedy
10. Poor glass.
10. Low flow resin. 10. Use higher melt index resin.
  Fault Probable Cause Suggested Remedy
11. Rippling in sector or part of parison.
11. Die shaping incorrect. 11.1.1 Decrease shaping depth.
11.2 Parison extrusion rate too high. 11.1.2 Decrease shaping land.
11.1.3 Increase shaping width.
11.2 Reduce pressure or accumulation.
  Fault Probable Cause Suggested Remedy
12. Warpage.
12.1 Insufficient cooling. 12.1.1 Reduce mould temp.
12.2 Non-uniform cooling. 12.1.2 Provide better water circulation.
12.1.3 Increase cooling time.
12.1.4 Use internal cooling.
12.2 Provide more cooling.
  Fault Probable Cause Suggested Remedy
13. Thinning at parting line.
13.1 Low blowing pressure. 13.1.1 Increase blow pressure.
13.1.2 Eliminate restriction in air line.
13.2 Pinch-off too sharp. 13.2 Widen pinch land
13.3 Air-entrapment. 13.3 Improve mould venting.
  Fault Probable Cause Suggested Remedy
14. Part blow out.
14.1 Too large a blow-up ratio. 14.1 Use large die.
14.2 Mould separation. 14.2 Increase clamp pressure or decrease blowing pressure.
14.3 Pinch-off too hot, no weld. 14.3 Cool mould pinch-off.
14.4 Pinch-off too sharp. 14.4 Provide wider land in pinch-off.
14.5 Part blows too fast. 14.5 Use low-pressure blow followed by high pressure blow.
  Fault Probable Cause Suggested Remedy
15. Parison 'doughnut' formation.
15.1 Mandrel not upto temp. 15.1 Allow mandrel to reach equilibrium with rest of system.
15.2 Mandrel too high. 15.2 Lower mandrel tip slightly below bushing.
15.3 Die face dirty. 15.3 Clean die.
  Fault Probable Cause Suggested Remedy
16. Black specks in parts.
16.1 Degraded material in system. 16.1.1 Throughly purge barrel and accumulator.
16.1.2 Avoid long shutdowns with material at high temperature.
16.2 Hold up areas in head. 16.2 Improve streamline on head.
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