Extrusion guide Heatshrink
This guide applies to our silane crosslinkable heatshrink grades to include the following grades:-
SX716, SX738, CT07-28
| 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 | The graft should not be dried. The catalyst can be dried in a dehumidifying drier. | |
| Temperature profile | Ranging from 130 to 160oC from the feed to the end of the extruder with 165oC 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. |
|
| Trial procedure | It is suggested that when conducting a first trial, the graft is run first without catalyst and a satisfactory extrusion is achieved. Initially catalyst should be used at 3% and if it runs well then it can be increased to 5%. A rough finish may be experienced at extruder speeds below 10 rpm but this should disappear above screw speeds of 20 rpm. |
|
| Colour masterbatches | Those from PW Hall are recommended. | |
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 on start up |
Compound shelf life expired | Replace material |
| Compound wet | Vacuum dry at ambient temperature | |
| Replace material | ||
| Catalayst masterbatch too strong or overdosed | Vacuum dry at high temperature | |
| Reduce amount used | ||
| Replace with weaker catalyst masterbatch | ||
| Material reactive during production |
Stagnation in extruder or head | Use process aid |
| Check screw and head design | ||
| Melt temperature too high | Check extruder heating and cooling for faults | |
| Reduce temperature profile | ||
| Catalyst overdosed | Check dosing unit calibration | |
| Die drool | Low molecular weight material exuding during extrusion | Use highly polished die/point |
| Add 1-2% process aid | ||
| 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 | |
| Porosity | Catalyst masterbatch overdosing | check dosing unit calibration |
| Compound surface moisture | Vacuum dry and ambient temperature | |
| Wet colour masterbatch | Vacuum dry | |
| Melt temperature too high | Reduce barrel temperatures | |
| Use screw with lower shear | ||
| Reduce extrusion rate | ||
| Check extruder heating and cooling | ||
| Surging extrudate | Die pressure low | Increase die land length |
| Rear barrel temperatures too high | Adjust and check heaters | |
| Screw speed too high | Reduce extrusion rate | |
| Die land too short | Use die with increased land length | |
| Porosity in extrudate | See porosity above | |
| Take-off inconsistent | check take-off operation | |
| Fails hot set test | Shear breaks (locked-in stresses too high) | Reduce drawdown ratio |
| Reduce vacuum | ||
| Adjust die separation | ||
| High elongation (low cure level) | Increase cure time | |
| Check calibration of catalyst dosing unit | ||
| Use stronger or more catalyst | ||
© AEI Compounds Limited, Great Stonar House, Sandwich Industrial Estate, Sandwich, Kent, CT13 9LY
Telephone +44 (0) 1304 616171 Facsimile +44 (0) 1304 616170
Email sales@aeicompounds.co.uk Website www.aeicomppounds.co.uk
Whilst this document has been prepared in good faith, AEI Compounds Limited accepts no liability of any kind to any person in respect of its contents or any use made thereof. Neither must it be assumed that any use will not infringe any patent. This document shall not form part of any contract with a customer.