Extrusion guide SX unfilled
This guide applies to the following grades:-
SX408, SX409, SX490, SX522A, SX505, SX510, SX523, SX554, SX558
| 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 2:1 (up to 3:1 for SX522A) | |
| Extension piece | If there is an extension piece connecting the end of the extruder and head then this needs to be heated to 190oC 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 without losing concentricity. | |
| Tubing | Double taper designs are preferred. Draw down 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 190oC from the feed to the end of the extruder with 190oC on the head and 210oC 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 satsifactory extrusion is achieved. Initially catalyst should be used at 3% and if this runs satisfactorily then then levels can be increased to a maximum of 5%. A rough finish may be exerienced at extruder speeds below 10 rpm but this shoud 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 |
|
| Use lower draw down ratio | ||
| Reduce extrusion rate | ||
| 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 calibaration | |
| Die drool | Low moecular 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 | |
| Fails hot set test | Shear breaks (locked-in stresses too high) | Reduce draw down 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 |
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