Most of the scrap in a changeover comes from old material that stays behind in a few places: the hopper and loader, the check ring, the nozzle and the hot runner. A planned run order and a set purge routine remove most of it. Two safety points come before anything else. Purge is hot and can spit, so keep the guard closed, wear a face shield and gloves and stand clear of the nozzle. And PVC and acetal must never meet in a hot barrel.
Also described as: color changeover, purge compound, old color bleeding through, cleaning the barrel.
Quick diagnosis
| What you see | Likely cause | Check first |
|---|---|---|
| Streaks of the old color that fade slowly over many shots. | A layer of old material is clinging to the screw and barrel wall, and the new material is flowing through the middle of it. | The purge routine: screw speed, back pressure and how much material went through |
| The old color comes back hours later, or after a stop or a temperature change. | Material is letting go from a hang-up point: the check ring, the nozzle tip or the end cap. | Pull and clean the nozzle tip; inspect the check ring |
| The old color shows in some cavities only, or always at the gate. | The hot runner manifold or one of its drops is holding old material. | Which cavities; a hot runner purge with the mold open |
| Whole flecks or pellets of the old color in the part. | Pellets or dust left in the hopper, loader, dryer, hoses or feed throat. | Empty and clean everything above the screw, including filters and the magnet |
| Black specks or brown streaks after the change. | Old material stayed in the barrel and degraded, or a carbon layer on the steel broke loose when the temperature or the resin changed. | The temperature sequence used; the shutdown routine before the change |
| Unmelted lumps or cloudy streaks after moving from a high-temperature resin to a lower one. | The old resin is still in the barrel and does not melt at the new settings. | Whether the barrel was purged clear at the higher temperature before the heats were dropped |
| A change from a dark color to a light or clear one never quite cleans up. | Dark pigments tint strongly, so traces that would not show in another color show in a light one. | Run order; a purging compound; whether the screw needs to be pulled |
Checks in order, cheapest first
- Plan the run order. Run light colors before dark ones and clear before opaque. Group jobs in the same resin, and where you can, move from lower processing temperatures to higher ones. A good schedule removes changeovers that no purge routine can make cheap.
- Empty and clean everything above the screw. Hopper, loader, hoses, dryer, magnet, filters and feed throat. One stray pellet of black in a white job is a reject, and no amount of purging the barrel will find it.
- Set the temperatures for both materials. The old resin has to be molten to leave the barrel, and neither it nor the purge may be overheated. Moving up in temperature, purge the old resin out at its own settings and then raise the heats. Moving down, purge at the higher settings with a material that can take them, and then lower the heats.
- Purge with movement, not volume alone. Retract the injection unit and purge with the guard closed. Short shots, changes in screw speed and some back pressure scrub the screw better than one long steady extrusion. Follow the instructions that come with a purging compound, because they differ by type.
- Purge the hot runner as its own step. Open the mold and shoot through the manifold until every drop runs clean. Many hot runner makers allow a temporary rise in manifold temperature for this. Check their guidance and the limit of the resin first.
- Use a purging compound where resin alone is slow. Purging compounds are made to scrub or to loosen deposits. They cost more per pound than resin and usually use fewer pounds. The compound has to be flushed out in turn, and not every type suits a hot runner or a small gate.
- Clean the hang-up points. If the old color keeps returning, remove and clean the nozzle tip and end cap and inspect the check ring. A worn or damaged check ring holds material and should be replaced. Pulling the screw is the last step, and sometimes the quickest one.
- Change the resin last. Changeover scrap is rarely a resin problem. The material choices that do help are about how the color gets into the resin, and they are listed below.
The numbers
Typical melt temperature ranges. Where the ranges of two resins overlap, one can usually push the other out directly. Where they do not, the barrel has to be purged clear before the heats move. Polycarbonate, for example, starts at 536°F, and acetal copolymer must never go above 460°F.
| Resin | Melt temperature | Time limit in the barrel |
|---|---|---|
| HDPE | 380–525°F / 193–275°C | Not published |
| Polypropylene homopolymer | 374–525°F / 190–274°C | Purge if idle for more than 30 min |
| General-purpose polystyrene | 356–500°F / 180–260°C | Not published |
| ABS | 425–525°F / 218–274°C. Flame-retardant grades never above 475°F / 246°C. | Not published |
| Polycarbonate | 536–653°F / 280–345°C | 4–8 min |
| Nylon 6 | 482–536°F / 250–280°C | 10 min |
| Nylon 66 | 527–580°F / 275–305°C | 10 min |
| Acetal copolymer | 370–446°F / 188–230°C. Never above 460°F / 238°C. | 15–20 min above 380°F |
| Rigid PVC | 380–410°F / 193–210°C | 15 min absolute |
Typical values from published processing guides. Confirm against the data sheet for the grade you run. The full list is in the melt temp, mold temp and shrinkage chart.
Safety during a purge
- Purge is hot and can spit. Trapped gas, moisture or degraded resin can throw molten plastic from the nozzle. Keep the purge guard closed, wear a face shield, gloves and long sleeves, and never look into the nozzle or the feed throat.
- Never mix PVC and acetal in the barrel. At processing temperature the two react, and producer guidance warns that the reaction can end in a violent release of gas. Purge the barrel completely with a neutral resin, such as polyethylene or general-purpose polystyrene, between the two, and clean the hopper and loader as well.
- Do not overheat PVC or acetal to speed up a change. Overheated PVC gives off hydrogen chloride and overheated acetal gives off formaldehyde. Purge each out at its own temperature, ventilate the area, and do not shut down with either left in a hot barrel.
- Purge piles stay hot inside. A lump of purge can be solid on the surface and molten in the middle for a long time. Keep piles small and let them cool before anyone handles them.
Where the old material hides
- The melt moves slowest at the steel. Plastic melt flows in layers. The layer against the barrel, the screw and the runner walls hardly moves, so new material passes through the middle and wears the old layer away slowly.
- Some places have almost no flow. The back of the check ring, the nozzle tip threads, the end cap and the corners of a hot runner manifold hold material that no ordinary shot will move.
- A stiff material pushes better than a thin one. A purge with a lower melt flow than the material it follows displaces more of it. A thin, easy-flowing resin tends to run past a stiff one and leave it on the wall.
- Dark pigments are strong. Carbon black and some dyes tint at very low levels. That is why a change from dark to light takes far more material than the same change in the other direction.
- What stays behind degrades. Old resin left on hot steel breaks down and chars over the next run. It comes back later as black specks, often on a job that has nothing to do with the original color.
- Dust travels. Pellet dust and colorant cling to hoppers, hoses and filters by static and let go later. Cleaning above the screw is part of the purge.
Material and scheduling changes, and what each one costs you
| Change | Why it helps | What it costs you |
|---|---|---|
| A purging compound in place of production resin | It is made to clean, so a change usually takes fewer pounds and less press time. | The cost of the compound, and it has to be flushed out afterward. Check that the type suits your temperature, your hot runner and any food-contact requirement. |
| A stiffer, lower-flow resin of the same family kept as a purge | It displaces the old material better than the production grade does. | One more material to buy, store and label, and the purge itself is scrap. |
| Natural resin with a color concentrate, in place of precolored resin | The hopper, dryer and loader hold natural resin only, so a color change starts at the feeder. | Color then depends on your dosing and mixing. Your customer may need to approve the change. See color inconsistency. |
| Longer runs and fewer changes | Each change that is avoided saves its own scrap and its own lost press time. | More finished parts in inventory, and less room to react to a schedule change. |
| A machine, or a screw and barrel, kept for black or for clear | The hardest changes never happen on that machine. | Equipment tied up, and less freedom in scheduling. |
Any resin change moves your dimensions. If a changeover plan includes a different base resin for the part, check critical dimensions before you approve it. The tool was cut for the shrinkage of the resin you run now.
Often confused with
- Black specks. Specks that appear with no recent change of color or resin point to degraded material or dirt in the handling system. See black specks and contamination.
- Poor color mixing. Swirls or streaks of the current color come from dosing or mixing of the concentrate. Streaks of the previous color come from the purge. See color inconsistency.
- Splay. Silver streaks that follow the flow and wipe to the same color as the part are moisture or gas, and are not old material. See splay and silver streaks.
- Delamination. Skin that peels in layers after a resin change means the old and new resins do not mix. The barrel was not purged clear. See delamination.
Quick answers
How do I purge an injection molding machine?
Empty and clean everything above the screw, set temperatures that suit both materials, then retract the injection unit and purge with the guard closed. Short shots, changes in screw speed and some back pressure scrub better than one long extrusion. Purge is hot and can spit, so wear a face shield and gloves and stand clear of the nozzle.
How do I change colors with less scrap?
Start with the run order. Run light colors before dark ones and clear before opaque, group jobs in the same resin, and where possible move from lower processing temperatures to higher ones. Then clean the hopper, loader, hoses, dryer and feed throat, and purge the hot runner as its own step with the mold open.
Why is the old color still coming through?
Old material is usually clinging to the screw and barrel wall, or sitting in a hang-up point such as the check ring, nozzle tip, end cap or hot runner manifold. Streaks that fade slowly point to the purge routine. Color that returns hours later points to a hang-up point, so clean the nozzle tip and inspect the check ring.
Is changeover scrap caused by the resin?
Rarely. Most changeover scrap comes from old material left in the hopper and loader, the check ring, the nozzle and the hot runner. A planned run order and a set purge routine remove most of it. The material choices that help are about how the color gets into the resin, such as natural resin with a color concentrate.
Which resins should not be purged together?
PVC and acetal must not meet in a hot barrel. At processing temperature the two react, and producer guidance warns of a violent release of gas. Purge the barrel completely with a neutral resin, such as polyethylene or general-purpose polystyrene, between the two, and clean the hopper and loader as well.
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