How to Select an Extrusion Blow Molding Machine
Work from the finished part to the machine, mold and production line. This checklist helps turn a product idea into a clear equipment brief.
ExploreTechnical guide / Troubleshooting
Extrusion Blow Molding Defects: Causes and Fixes
Find the defect you see, check its likely causes in order, and adjust one variable at a time. Download the table as a PDF for the machine side.
What causes extrusion blow molding defects?
Most defects come from five areas: melt temperature, parison programming, the condition of the die, mandrel and neck tooling, mold cooling, and mold alignment and venting. Identify the defect, check the likely causes in order, and change one variable at a time so you can see what fixed it.
How to use this table
Start with the defect you can see on the part or the parison. Work down the causes from the quickest check to the most involved one, and record each change and its result. Problems that persist after process adjustments usually point to tooling wear, mold design or the resin grade.
Parison problems
| Defect | Common causes | What to check or adjust |
|---|---|---|
| Parison hooks or curls instead of hanging straight | Uneven wall around the parison; dirty or warped die and mandrel; uneven head temperature; drafts or static on the parison | Re-center the die; clean the flow channel; check head heaters and shield the parison from moving air |
| Parison sags or stretches | Melt too hot; resin melt strength too low; parison hangs too long; heavy tail; slow mold closing | Lower melt temperature; shorten drop time or raise extrusion rate; reduce tail weight; close the mold faster; review resin melt index |
| Parison folds or webs (curtaining) | Melt too hot; wall too thin at the top; resin swell does not suit the tooling | Correct melt temperature; reprogram the top of the parison thicker; adjust pre-blow; match resin swell to die and mandrel size |
Forming problems
| Defect | Common causes | What to check or adjust |
|---|---|---|
| Handle missing or webbed | Molds misaligned; parison hooking; parison too narrow for the handle | Realign molds; correct parison drop; lower melt temperature or use larger tooling so the parison reaches the handle |
| Weak or cut pinch-off and weld | Melt or mold temperature too high or too low; mold closes too fast; too little material in the pinch; pinch-off lands too wide or flash pockets too deep | Adjust melt temperature in small steps; slow mold closing; program more material into the pinch; review pinch-off land and flash pocket design with the mold maker |
| Hole, slit or thin wall at the parting line | Mold not fully closed; plugged vents; blow pressure too high for the clamp | Clean mold faces; check mold lock-up and clamp pressure; clean vents; review blow pressure |
| Part does not blow fully | Parison too short; blow-up ratio too high; air leak or restriction; moisture or trapped air in the melt; clamp pressure too low | Increase shot size; review tooling diameter; check blow and exhaust air lines; check material handling for moisture |
| Excessive flash | Parison diameter too large; flash pockets too shallow; mold not closing correctly | Reduce pre-blow or extrusion rate; review resin swell and tooling size; check mold closure and flash pockets |
| Neck flash does not separate | Damaged shear ring, blow pin or cutting ring; not enough shear force at the neck | Replace or sharpen the shear parts; adjust blow-pin position or force |
Appearance problems
| Defect | Common causes | What to check or adjust |
|---|---|---|
| Die lines or streaks | Contamination between die and mandrel; degraded material or carbon in the head; failed heater band; damaged tooling | Briefly open the die gap to purge; clean die and mandrel with a brass or copper tool; check heater bands and raw material cleanliness |
| Rough surface, pits or orange peel | Dirty tooling; melt fracture; poor mold surface or venting; low blow pressure; condensation in the mold | Clean tooling; adjust die gap, extrusion rate or melt temperature; check vents, blow pressure and mold temperature against humidity |
| Bubbles or unmelted particles | Too little back pressure; worn screw or barrel; contamination or fines; melt too cold | Raise back pressure or use a finer screen pack; check screw and barrel wear; check regrind and fines; raise melt temperature |
Dimensions and strength
| Defect | Common causes | What to check or adjust |
|---|---|---|
| Uneven wall around the part | Die not centered; uneven head temperature | Center the die; check heater bands and shield the head from drafts |
| Wall too thin at the top of the part | Parison draws down from slow extrusion or high melt temperature; mold closes too late | Raise extrusion rate; reprogram the top thicker; lower melt temperature; close the mold sooner |
| Warpage, rocker bottom or oval neck | Not enough mold cooling; blocked cooling channels; cycle too short; mold opens before exhaust is complete; thick and thin sections in the design | Increase water flow or lower mold temperature; clean channels; lengthen cooling and exhaust time; review wall distribution |
| Low top-load or impact strength | Uneven wall distribution; part too light; thin parting line or weak pinch; sharp corners in the design; too much regrind | Reprogram the parison; increase part weight; fix the pinch and parting line; add radii; reduce regrind |
| Volume too high or too low | Part weight off target; part ejected too cold or too hot; wrong mold volume; blow pressure; storage temperature | Correct part weight; adjust mold temperature and cycle; check blow pressure; resize the mold or inserts |
| Cap or closure leaks | Worn blow pin or shear steel; blow pin or mold misaligned; wrong neck insert or cap dimensions; pleating inside the neck | Replace worn neck tooling; align blow pin and mold; check neck insert and cap against the drawing; adjust pre-blow |
When the machine is the cause
Some defects point to equipment rather than settings: a worn screw or barrel, a failed heater band, a damaged die or blow pin, or uneven clamping. Keep spare heater bands and neck tooling, and check screw wear periodically. If you are choosing a new machine, the machine selection guide lists what to specify.
Troubleshooting FAQ
What are the most common extrusion blow molding defects?
Parison sag and hooking, weak pinch-offs, thin parting lines, excessive flash, die lines, rough surfaces, warpage (rocker bottoms and oval necks) and closure leaks. Most trace back to melt temperature, parison programming, tooling condition, mold cooling or mold alignment.
What causes a rocker bottom on a blow-molded bottle?
Usually not enough cooling: low water flow, blocked channels, a cycle that is too short, or a mold that opens before the blow air has exhausted. Uneven wall distribution in the design makes it worse.
Why is the wall too thin at the top of the part?
The parison draws down under its own weight, from a slow extrusion rate, a high melt temperature or a mold that closes too late. Increase the extrusion rate, program the top of the parison thicker, or close the mold sooner.
How do you fix a weak pinch-off?
Adjust the melt temperature in small steps, slow the mold closing, program more material into the pinch area, and review the pinch-off land width and flash pocket depth with the mold maker.
What causes die lines in blow-molded parts?
Contamination or degraded material caught between the die and mandrel, a failed heater band, or a nick on the die lips. Purge by briefly opening the die gap, clean the tooling with a brass or copper tool, and check raw material cleanliness.
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