Troubleshooting guide for Blow Moulding Applications
Blow Moulding Troubleshooting
Problem
Probable Cause
Suggested Remedy
Temperature Setting
Temperature setting are crucial parameter for proper plastic processing ,hence one should check temperature controller, thermocouple and solid state relay(SSR)
- when you cold start the machine ,wait for 2hours.
Parison stretch is more
1. Melt temperature high
2. Slow extrusion rate
3. Low melt index material
1. Reduce material temperature
2. Increase extrusion rate
3. Use right melt index materi
Rough surface, melt fracture or chatter marks in parison
1. Low melt temperature,Melt fracture of material, die causing fracture of material
2. Extrusion rate too high
3. Melt fracture or instability
1.Increase melt temperature, Raise parison bushing temperature, Reduce extrusion rate, Streamline die approach
4. Decrease the extrusion rate
3. Use higher flow resin if possible
Uneven wall thickness and thinning of sidewall near the base of bottle
Pin not centered in the die bushing, die design wrong, uneven melt, parison necking down, low melt strength of polymer, too high a melt temperature, bottle design requiring too great a length for the thickness of parison employed
Centre 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 non-uniform
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
Contamination
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 below bushing, 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 streamline the flow of material, check for hot spots
Poor surface finish
Mould finish rough, blow pressure low, proper 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 incorrect
Clean the orifice, reduce extrusion rate, decrease shaping Geo decrease shaping land, increase shaping width, reduce re 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 condensacion 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
Warpage
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 toc 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-off angle
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 Distortion warping
- Blunt knife, knife far off from parison, knife speed not enough
Shrinkage difference in axially asymmetrical parts, shrinkage difference due to variation in wall thickness, in oval parts, warping
- Sharpen knife, locate knife near parison, increase knife speed, reduce parison temperature
- 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
