Purging and Color or Material Changeovers Without the Waste

The logic sounds airtight on a busy floor: purge compound is expensive, so why buy it? Just run the screw dry, drop in the new material, and mold parts until the old stuff flushes out and the new color comes clean. You’ll make some mixed-material junk along the way, but it has to be cheaper than buying a special compound for every changeover.

It usually isn’t. Here’s a case that makes the point. A shop with dozens of large machines ran exactly this way — run dry, load the new color, mold until it cleared. They had a few things going for them: color match wasn’t critical, they were on pre-colored resin so there was no liquid colorant to chase, and it was a color change rather than a material change. Then they went from a forest green to a fire-engine red on a large flat part. They ran that machine for more than eight hours, and you could still see green stripes ghosting through the red.

Eight hours of a large press making scrap is not a cheap changeover. It’s one of the most expensive things that can happen to a job. This article doesn’t reproduce historical WJT Associates material; it applies the same shop-floor reasoning — count the real cost of the changeover, not just the price of the compound — to color and material changes.

Why “run it dry” hides the cost

Running the screw dry feels free because the cost doesn’t show up as a line item. There’s no purge-compound invoice. But the cost is real and it lands in three places that are easy to ignore:

• Scrap. Every mixed-material, streaked, or contaminated part during the transition is wasted resin and wasted machine time.
• Press time. The press earns nothing while it flushes. On a large machine, hours of flushing is hours of lost production at full burdened rate.
• The long tail. The last 5% of the old material is the hardest to clear, because it’s hiding in the dead spots — behind the non-return valve, in the nozzle, in the corners of the feed throat, packed against the barrel wall. That’s where the green stripes were coming from eight hours in. Mechanical flushing alone may never fully reach those spots.

A purge compound exists precisely to scrub those dead spots that fresh resin pushes past rather than clears. The question is never “compound or free.” It’s “compound, or the hidden cost of flushing — which is usually higher.”

What makes a changeover hard

Not every changeover needs the same effort. Knowing which factors make a transition difficult tells you when to reach for compound and when running through might genuinely be fine.

The dangerous combination is a high-contrast color change and a large machine and a critical color match — which is roughly the case that ran eight hours. When several of these stack, “run it dry” stops being a shortcut and becomes the most expensive option on the table.

When purge compound pays for itself

Purge compound is a tool, not a religion — but it earns its cost in specific, predictable situations:

• High-contrast color changes, especially dark-to-light, where streaks are obvious and tolerance is low.
• Material changes between incompatible resins, where leftover polymer can degrade at the new material’s temperature and contaminate parts with black specks or splay.
• Large machines, where the volume and dead-spot geometry make mechanical flushing slow and incomplete.
• Color-sensitive or cosmetic parts, where even faint streaking is a reject.
• Shutdowns, where you’re sealing the barrel against degradation rather than just changing what’s in it.

The decision rule is simple arithmetic: estimate the scrap and press hours a run-dry changeover will cost on this particular transition, and compare it to the cost of the compound plus a short, controlled purge. On an easy changeover the run-through wins. On a hard one the compound wins, usually by a wide margin.

Material changes need more than purging

A color change is mostly a cleanliness problem. A material change is also a process problem, and treating it like a simple swap is how you make scrap on both ends.

• Temperature. Different polymers run at different melt temperatures. Load a high-temp resin into a barrel set for a low-temp one and it won’t plasticize; load a low-temp resin into a hot barrel and the leftover high-temp material may flow fine while the new resin degrades. Stage the barrel temperatures for the transition, not just the destination.
• Degradation risk. Some resins sitting in a hot barrel during a slow changeover degrade and seed black specks into every part that follows — a contamination that outlasts the changeover by hours.
• Drying. The incoming material has its own drying requirement. A changeover that nails the purge but skips proper drying of the new resin just trades a contamination defect for a moisture defect.

The clean sequence for a material change: stage temperatures, purge with compound matched to the materials, confirm the new resin is dried to spec, then qualify back to the documented process window rather than re-discovering it at the press.

A practical changeover checklist

1. Classify the changeover. Color or material? High or low contrast? Large or small machine? That tells you whether to run through or purge.
2. For hard transitions, use a matched purge compound — and follow its procedure rather than improvising.
3. For material changes, stage barrel temperatures for the transition and confirm the new resin’s drying before you start.
4. Clear the dead spots, not just the barrel — nozzle, non-return valve area, feed throat. That’s where the last of the old material hides.
5. Qualify to the documented window, not by eye, so the new job starts in control.

The goal is to make the changeover a short, planned event instead of an open-ended flush that quietly eats a shift.

FAQs

Is it cheaper to run the screw dry than to use purge compound?

Usually not, on a difficult changeover. Running dry avoids the compound invoice but replaces it with scrap, lost press time, and a long tail of contamination hiding in the barrel’s dead spots — which mechanical flushing may never fully clear. On an easy changeover (a small machine, a low-contrast color change on the same resin) running through can be fine. On a hard one — high contrast, large machine, or a material change — the compound almost always costs less than the hours of flushing it replaces.

Why do I still see streaks of the old color hours into a changeover?

Because the last bit of the previous material isn’t in the main melt stream — it’s lodged in the dead spots: behind the non-return valve, in the nozzle, in the feed-throat corners, packed against the barrel wall. Fresh resin flows past those areas rather than scrubbing them, so they bleed old color into new parts long after the bulk has cleared. Purge compound is formulated to scour exactly those spots, which is why mechanical flushing alone can take hours and still leave faint streaks.

What makes a color change harder than another?

Contrast and direction. Dark-to-light and high-contrast changes (like a deep green to a bright red) show every trace of leftover color, so the tolerance for residual contamination is very low. Light-to-dark or similar-shade changes hide minor carryover. Liquid colorants are harder to clear than pre-colored pellets because they film the screw and barrel, and large machines are harder than small ones simply because there’s more volume and more dead-spot geometry to flush.

Is changing materials different from changing colors?

Yes. A color change is mainly a cleanliness problem, but a material change is also a process problem. Different polymers run at different melt temperatures, so you have to stage the barrel temperatures for the transition, not just set them for the new material. Some resins degrade if they sit in a hot barrel during a slow changeover and contaminate later parts with black specks. And the incoming material has its own drying requirement. A material change needs temperature staging, a matched purge, proper drying, and re-qualification — not just a swap.