Purging Compound Selection — Mechanical, Chemical and Abrasive Grades

Direct answer

Choose the purge by the contamination you are removing, not by the machine. Mechanical purges rely on high viscosity and shear to push melt out — they are the safest choice for routine colour changes on injection moulding and are the only sensible option when you cannot hold the machine at temperature. Chemical purges foam and expand at temperature, reaching the low-shear dead zones behind flights, in accumulator heads and in hot runners where carbon builds; they need a soak of three to fifteen minutes and a temperature window the supplier specifies, and they are what you use for black specks and degraded polymer. Abrasive-filled purges physically scour hardened deposits from screw flights and should be used sparingly — they will also wear a nitrided barrel if run as a routine daily purge.

When each type earns its cost

A purge compound is bought against downtime, not against price per kilogram. The comparison that matters is kilograms of virgin resin plus minutes of press time consumed by the old method versus the purge cost.

• Light colour change, same resin family. Mechanical purge, no soak, one to two barrel capacities. Typical saving is 40–70 % of the resin normally wasted on a natural-resin flush.
• Dark to light, or pigment carry-over on a white or transparent part. Chemical purge with a soak. Natural resin alone rarely clears a strong red or black pigment from the screw root.
• Black specks, streaking or degraded material after a weekend stop. Chemical purge, then a mechanical chase. Specks are carbonised polymer from dead zones, and only a foaming grade will lift them.
• Resin change with a large temperature gap (for example PP to PC, or PA to PE). Use a purge rated for the higher processing temperature and step the barrel to that temperature before purging, not after.
• Shutdown. Purge out any resin that degrades on hold — PVC, POM, PVDC and most flame-retardant compounds — then leave the barrel filled with a stable purge or PE. Leaving PVC in a hot barrel is the most common cause of a corroded screw and an unplanned rebuild.

Compatibility rules that prevent damage

Never mix a PVC-bearing purge into a barrel that has run POM, and never leave POM residue where PVC will follow: the acid gas from PVC decomposition and the formaldehyde route of POM combine into a violent, exothermic reaction. Confirm the maximum processing temperature of the purge against your highest zone setting — a commodity purge run at 320 °C will itself degrade and add to the problem. Where the part is food contact or medical, the purge must carry the same declaration you require from the resin, because trace purge remains in the first shots after the change.

What to specify on your enquiry

1. Process — injection, extrusion, blow moulding, or hot runner system.
2. Barrel size (shot weight or throughput) and the number of changes per week.
3. Highest barrel temperature and the resins on either side of the change.
4. Problem being solved — colour change, black specks, resin change, shutdown.
5. Screw metallurgy, if abrasive grades are being considered.
6. Regulatory declaration required — food contact, medical, or none.
7. Annual volume in kilograms.

Related knowledge

• Screw and barrel cleaning between production runs.
• Mould care, corrosion and release for the tool side of the same changeover.
• Processing aids and lubricants when purge frequency is driven by melt instability rather than colour.