Extrusion guide Semi-con
This guide applies to our semi-conducting products including the following grades:
SX528, SX539
| Equipment | ||
| Extruder | Ideally 20 to 24 L/D ratio. | |
| Extruder head | Ideally with as small a volume as possible. A head assembly with a diverter valve is beneficial. | |
| Screw | Low compression ideally 1.2:1. | |
| Extension piece | If there is an extension piece connecting the end of the extruder and head then this needs to be heated to 160oC by heater bands or similar. | |
| Tooling pressure | Short land length; no longer than 3mm. The flow path should be as gentle as possible with the adequate separation of the core and ring die. | |
| Tubing | Double taper designs are preferred. Drawdown ratios of no greater than 1.5:1 is recommended. | |
| Processing conditions | ||
| Drying | Pre-drying is necessary, preferably using a desiccant air or vacuum dryer, for a minimum of 16 hours at 40 to 50ºC. | |
| Temperature profile | Ranging from 95 to 110oC from the feed to the end of the extruder with 170oC on the head and 170oC on the die. | |
| Extrusion speed | The aim is always to keep the residence time in the extruder to a minimum. On initial start up screw speed should be at a minimum of 5 rpm and during production at a minimum of 20 to 30 rpm. When changing tooling or during other stoppages every attempt should be made to keep material moving through the extruder. |
|
Troubleshooting guide
|
Problem |
Cause |
Solution |
| Poor surface finish | Temperature low | Increase barrel and head temperatures Use higher shear screw |
| Melt fracture |
Increase barrel and head temperatures |
|
| Extrusion O/D too low | Reduce haul off speed | |
| Change to smaller die (run with slight die swell) | ||
|
Material reactive |
Compound wet | Vacuum dry at ambient temperature |
| Replace material | ||
| Melt temperature too high | Check extruder heating and cooling for faults | |
| Reduce temperature profile | ||
| Weld line on one side | Material cold | Use highly polished die/point |
| Material flow path in head too short | Re-design head | |
| Stagnation in head or die | Purge or clean head | |
| Inconsistent flow around circumference | Material flow in head too short | Re-design head |
| Poor concentricity | Adjust dies | |
| High motor amps | Is motor rating sufficient? | Upgrade motor |
| Incorrect screw design | Use lower shear/compression design | |
| Material crosslinking | See reactive material above | |
| Gauze pack too harsh | Remove some/all gauzes | |
| Head design restrictive | Change head design | |
| Material too stiff | Increase temperature profile | |
| Check heater bands/thermocouples | ||
| Use higher MFI material | ||
| Gels in extrudate | Stagnation | Use process aid |
| Use highly polished die and point | ||
| Check screw and head design | ||
| Gels in extrudate | See reactive section above | |
| Bubbles/porosity in extrudate | Moisture in compound | Dry compound |
| Cores sticking during curing | Cure temperature too high | Reduce cure temperature |
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