Purging: cutting downtime during color and material changes

Every injection machine that switches color or raw material retains some leftover material from the previous run in the screw, barrel and runner channels. Purging is the process of reducing that residue to a level that won't contaminate the new run. Inadequate purging shows up as color streaks, black specks, mechanical weakness from mixed-in foreign material, and scrap.

Methods

• Mechanical (natural) purging: Continuously feeding new material at the same screw speed and temperature to physically push the old material out. Cheap but slow; takes a long time for very different color/material transitions (e.g., black to white).
• Chemical purging compounds: Special pellets with foaming or abrasive additives strip sticky residue off the screw surface faster. Saves time for businesses with frequent material/color changeovers.
• Mechanical cleaning (screw pull): For deep-rooted contamination, burnt material, or buildup of glass-fiber-reinforced compound, the screw is pulled and cleaned by hand; the most thorough but slowest method.

Which method, and when?

• Close colors within the same family (e.g., shades of gray): a short natural purge is usually enough.
• Switching to a lighter color (black to white, to clear): a chemical purging compound saves time.
• Switching after glass-fiber-reinforced or high-temperature engineering plastics: reinforcement fibers left on the screw surface may not fully clear even with a purging compound; check the screw if in doubt.
• If burnt/degraded material is suspected (odor, black specks): going straight to mechanical cleaning is faster than losing time on scrapped parts.

Things to watch

• Set the temperature during purging according to the compound's data sheet; some compounds only work within a specific temperature range.
• The sprue bushing and hot runner channels get contaminated too; think about the whole melt path, not just the screw.
• Quality-check the first few shots after purging; a visually clean appearance doesn't always mean the chemical/mechanical cleaning was complete.

Note: Purge duration and compound choice depend on the color difference, material family change, screw design and machine barrel; what's given here is a general roadmap — the compound manufacturer's data sheet takes priority.

You can use the calculator to see how screw and runner temperature/pressure settings affect cycle time.