Last updated: 2026-09-04 · 5 min read
How Do You Change Color Without Contamination? — Injection Molding Purging Procedure
A color change on an injection press is a contamination race against what the old resin left behind — not just the bulk in the barrel, but films on screw flights, degraded pockets in the nozzle tip and non-return valve, and old pigment smeared through hot-runner channels. Purging is the engineered sequence for displacing all of it, done in the right order it costs ten minutes.
Key Facts
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Parting line — The line where the two mold halves (A-side and B-side) meet. Parting line location affects: gate placement, ejector pin marks, witness lines, and cosmetic appearance. A well-designed parting line minimizes visible witness marks. The related guide covers adjacent topics.
Where Does Old Material Hide Between Runs?
Five hiding places, each requiring its own eviction method:
| Hiding Place | Why It Survives | How to Evict |
|---|---|---|
| Nozzle tip and bore | Dead flow zone at exit | Disassemble and scrape |
| Screw flights and root | Thin smeared film | Staged purging shots |
| Non-return valve | Clearances trap degraded polymer | Recovery strokes; pull if needed |
| Hot-runner channels | Long narrow paths, low shear | Sequential drop-by-drop shots |
| Previous run’s degraded residue | Already dark; sheds as specks | Lower temps first, then purge |
The nozzle tip is the worst offender: its internal step creates a dead zone where polymer sits at processing temperature for hours, slowly darkening, and no amount of bulk flushing moves it until the tip is pulled and cleaned. Every molder who has chased mystery black specks that come and go has eventually traced them to this pocket.
The screw film and valve clearances are subtler. A thin layer of old resin rides on the metal, overheats faster than bulk, then breaks into gels and specks a few shots into the new run — after the purge looked clean. Hot runners add a geometric problem: manifold and drop channels are long, narrow and unpolished, so old color leaves slowly — which is why a color change on a hot-runner mold is planned in stages rather than one flush.
What Is the Correct Purge Sequence?
Four steps, in order:
- Set temperatures for the old resin first. Bring barrel temperatures down to the low end of the old resin’s range, so what you push out stays intact instead of degrading further. Empty the machine with the old resin — it is the best solvent for itself. Then choose the transition route: where the next resin is compatible, purge directly with it; where viscosities or chemistry differ, step through a purging compound or inexpensive transitional carrier that strips films without fighting the new process.
- Purge in staged, low-volume shots. Bulk does not clean; motion does. Run short recovery strokes and small shots repeatedly rather than one long stroke, because repeated acceleration and reversal across the screw and valve does the scrubbing. Vary back pressure slightly to change the shear pattern on flight surfaces. With a purging compound, follow its own procedure — soak-and-shot and mechanical grades work differently. Watch the purgings exit: you are looking for the moment the old color and dark degraded streaks disappear.
- Displace the hot runner channel by channel. If the mold has a hot runner, the color change is not finished when the barrel is. Bring the manifold and drops to temperature, then run sequential shots so fresh resin flows through each channel long enough to displace the old rather than mixing with it. For natural-to-black or black-to-natural changes, plan the largest purge allowance and schedule them for planned tool changes rather than mid-shift improvisation.
- Verify clean, then restore process and document. Run several short shots and inspect purgings and parts against an agreed standard for specks, streaks and color ghosting under good light. Only when consecutive purgings come out clean do you set the machine to the new resin’s full process sheet. Log the change: resins in and out, purge method and compound used, shots consumed, verification result. That log turns purging from crew folklore into a tunable procedure.
Cycle time — The total time for one injection molding cycle: injection + packing + cooling + mold open/close + ejection. Typical range: 15–60 seconds. Cycle time determines production rate and cost per part — reducing cycle by 5 seconds saves 10–15% annual production cost. The related guide covers adjacent topics.
A purge changes what is in the barrel; it cannot clean a scar it never touches. The durable fix for recurring specks is mechanical — pull and clean the nozzle and valve on a schedule, and treat any persistent source machine by machine.
Frequently Asked Questions
Do purging compounds pay for themselves?
For frequent color or material changes on shared machines, usually yes. The economics are clear once you count all inputs a purge consumes: machine time at full rate, resin shot through as waste, and labor standing by during the transition. A purging compound cuts transition shots substantially on difficult color gaps and on sticky or glass-filled resins where plain resin barely moves screw films.
The honest way to decide is to log current changeovers for a month — shots to clean, minutes consumed, scrap created — then compare with compound-assisted purges including the compound cost. Where changes are rare and easy, natural-resin purging remains valid; where they are frequent and dirty, the compound earns its place.
Why do black specks appear days after a color change?
Because the source was not bulk resin — it was a pocket of polymer that had already degraded, hidden where flow never reaches: the nozzle tip dead zone, valve clearances, a scratched spot in the hot runner, or a damaged flight. Those pockets shed dark flakes intermittently, which is exactly why the specks come and go and why a purge that looked clean did not end them.
The durable fix is mechanical: pull and clean the nozzle and valve on a set schedule, inspect those surfaces for damage when specks recur on the same machine, and treat any persistent source individually. A purge changes what is in the barrel; it cannot clean a scar it never touches.
How do you sequence colors to minimize purge waste?
Plan the week so every transition moves toward darker or more saturated colors, and save the hardest reversal — dark to light or natural — for a planned window with time for a thorough purge or nozzle strip-down. A sensible color ladder runs natural, yellows and reds, blues, greens, then blacks, and re-schedules rather than backtracking mid-shift.
Combine color sequencing with material sequencing where possible, keeping compatible resins adjacent so the machine never has to jump chemistry and temperature at once. The savings from good sequencing are real: fewer transition shots, less compound, fewer borderline first-off parts, and a schedule that stops punishing whoever draws the dark-to-light change.
Takeaways
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Color changes done right are a ten-minute procedure; done wrong they consume a shift of scrapped parts.
The difference is not equipment or compound — it is following the sequence and verifying before restoring process.
Sources
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Sources: Plastics Technology · Manufacturing.net · Society of Plastics Engineers (SPE) · ThomasNet