Purging Compounds: Types and How to Choose

The short answer

A purging compound is a material you run through a molding machine or extruder between jobs to push out the old resin, color and carbon faster than the next resin would on its own. Most purging compounds work in one of two ways: scrubbing or chemical action. Mechanical grades scrub with a stiff, often filled, carrier resin. Chemical grades react in the heat to loosen deposits, and some also foam to reach dead spots. The right one depends on your process, the temperature range, whether you run clear parts and whether the tool has a hot runner.

Types of purging compound

Type	How it works	Fits best	Watch for
Mechanical (scrubbing)	A stiff carrier resin, often with a mineral or glass filler, pushes and scrapes old material off the screw and barrel	Fast color and resin changes on the barrel and screw. Works right away, with no soak time	Filled grades may not pass small gates, hot runner tips or screen packs. Check before you run it through the tool
Chemical	Reacts in the heat to break down and loosen residue. Often needs a short soak	Carbon build-up, hot runners and low-shear areas a scrubbing grade cannot reach	Soak time adds minutes. Follow the temperature limits on the sheet
Foaming (expanding)	A foaming agent makes the melt expand so it fills and sweeps dead spots	Check rings, end caps, manifolds, dies and other hang-up points	Gives off gas. Never trap it in a closed barrel or a blocked nozzle, and follow the supplier’s venting instructions
Concentrate	A pellet or liquid you blend with your own resin at a set ratio	Shops that want one product across several resins, and lower freight cost	Mixing has to be accurate and even
Shutdown or sealing grade	Stays in the barrel during a stop and keeps air off the steel	Weekend shutdowns, and heat-sensitive resins that leave carbon	Use only a grade sold for sealing, with the heats off or lowered as its procedure says. Glass-filled and foaming grades are not for sealing

General categories. Many products combine more than one of these. The supplier’s sheet for the exact grade comes first.

How to choose one

Temperature window. Grades are sold by range. A grade made for PE, PP or polystyrene may not be rated for polycarbonate or nylon temperatures, and a high-temperature grade may not melt at polyethylene settings. Check the range on the data sheet. Give the lowest and highest barrel temperatures on both sides of the change.
Process. Injection molding, extrusion, blow molding and blown film each have suitable grades. For extrusion, ask whether the screen pack and breaker plate must come out.
Hot runners. Ask for a grade rated to pass through the manifold and gate size you have. Many shops purge the barrel first with the injection unit pulled back, then the hot runner with a suitable grade. Keep the gates open, never soak a gassing grade in a closed manifold, and check the hot runner maker’s guidance.
Clear and light parts. Clear polycarbonate, acrylic, clear polystyrene and clarified PP show any residue. Use a grade made for transparent resins.
Food-contact work. Some grades carry food-contact statements for their ingredients. Ask for the letter for the exact grade, and purge the compound itself out fully with the next resin. A statement for the compound does not make parts that contain purge fit to sell.
What is being removed. A dark-to-light color change usually suits a scrubbing grade. Carbon and black specks usually call for a chemical or foaming grade with a soak. A resin change with a large temperature difference needs a grade rated for both ends.
The resins involved. Some purging compounds must not be used with acetal, PVC or flame-retardant grades. Say so if any of these is on either side of the change, and use only a compound rated for them.

Using it well

Empty and clean everything above the screw first: hopper, loader, dryer, hoses and feed throat.
Run the barrel empty, then feed the compound at the temperatures on its sheet.
Vary the screw speed, take short shots and use some back pressure. Pushing more material through at a steady rate cleans less. For chemical grades, allow the stated soak time.
Purge until the melt runs clean. One to two barrel capacities is a common starting amount, meaning the melt the barrel holds and not the shot size. Some suppliers say up to three, and stubborn changes take more.
Push the compound out with the next resin until none remains before making parts.
Write down what worked for each changeover so the next shift repeats it.

The full routine, with the places old material hides, is on purging and color changes. If the problem is specks, see black specks and contamination.

Safety during a purge

Hot purge can spatter. Keep the purge guard closed, wear a face shield, long sleeves and heat-resistant gloves, and stand clear of the nozzle. Never look into the nozzle or the feed throat.
Purge piles stay molten inside long after the skin cools. Leave them until they are cold.
PVC and acetal must never meet in a hot barrel. Purge one out completely with a neutral resin, such as polyethylene or general-purpose polystyrene, before the other goes in, and clean the hopper and loader too.
Check the purging compound against your resin. Some compounds are not rated for acetal, PVC or flame-retardant grades.
Foaming and chemical grades give off gas. Never block the nozzle, and stop the machine if it clogs. Keep the area well ventilated, and do not leave these grades sitting in a hot barrel.
Stay inside the temperature limits on the compound’s data sheet, and read its safety data sheet before the first use.
Do not raise barrel or hot runner temperatures while PVC, acetal or a flame-retardant resin is still in the system.

Working out the cost

Compare it with what a changeover costs now. Add up the resin you scrap, the machine time lost and the rejects that show up hours later when old material lets go. A purging compound costs more per pound than most resins. If it shortens a long change, or stops black specks from coming back, the saved scrap and machine time can outweigh the price. A short trial on your worst changeover gives a real number: weigh the scrap and time the change with and without it.

What to send for a quote

Process and machine size. Injection, extrusion, blow molding or film, and the screw diameter or shot size.
The change. The resin and color before and after, with barrel temperatures for both.
The tool. Hot runner or cold runner, and the smallest gate or screen.
Special needs. Clear parts, food contact, glass-filled resins, carbon and black specks, or PVC, acetal or flame-retardant resin on either side of the change.
How much you use. Changeovers per week, or what you buy now. Purging compounds are usually sold in bags or boxes. Pack sizes and minimums are confirmed with the quote.

Common questions

What is a purging compound?

A material run through a molding machine or extruder between jobs to clear out the old resin, color and carbon deposits faster and with less scrap than flushing with the next resin alone.

What is the difference between mechanical and chemical purging compounds?

Mechanical grades scrub with a stiff, often filled, carrier resin and work right away. Chemical grades react or foam in the heat to loosen deposits and reach low-flow areas, and usually need a short soak. Many products combine both.

Can I use purging compound in a hot runner?

Only a grade rated for it. Ask for one that will pass through your manifold and gate size. Filled scrubbing grades may not be suitable for small gates.

How much purging compound do I need?

One to two barrel capacities is a common starting amount, and some suppliers say up to three. A hard change, such as black to clear or a barrel with carbon build-up, takes more. Follow the supplier’s sheet for the grade.

Can I just purge with regrind or natural resin?

For easy changes, often yes. For dark-to-light changes, clear parts, hot runners and carbon, a purging compound usually clears the machine in less time and with less scrap. Compare the cost of the compound with the scrap and downtime you have now.

Do purging compounds remove black specks?

They can remove the carbon layer that sheds specks, especially chemical and foaming grades used with a soak. On a badly carbonized screw the first purges can shake loose more specks before it cleans up, and heavy build-up may still mean pulling the screw. If specks return later, look for a hang-up point such as a worn check ring or a dead spot in the nozzle or manifold.

Can you quote purging compound?

Yes. We quote purging compounds through a supply partner. Send your process, resins and temperatures. A polymer chemist will recommend a product that fits and quote it, or tell you if nothing in the range fits.

Talk to a polymer chemist

Slow color changes, black specks, or a resin change with a big temperature step? Tell us what you run.

Send your process, the resins and the temperatures.
A polymer chemist recommends a purging compound that fits.
We aim to quote within one to three business days.

Rather talk? Call (678) 310-6899 or text a photo.

Tell us about your changeover

A polymer chemist reviews every request, recommends a purging compound that fits and quotes it.

What to send for an accurate quote

Process and machine size
Resin and color before and after the change
Barrel temperatures for both
Hot runner or cold runner
Clear parts or food contact, if any
Changeovers per week and ship-to city

Purging compounds for injection molding and extrusion