Why purge a plastics processing machine?
During thermoplastic processing, color, additives and polymer can remain in low-flow regions, dead spots, nozzles, dies, screw surfaces and barrels. With excessive heat, shear or residence time, retained material may degrade and carbonize. Fragments can later break loose and create black specks, streaking or contamination in the next production run.
Flushing with virgin resin may displace easy-flowing material but may not remove residues in stagnation areas or degraded layers. Commercial purging compounds are engineered to reach and displace these contaminants more effectively.
Three common problems purging should address
Black specks & carbon
Often associated with degraded polymer, pigment or additives retained in dead spots, hot spots or machine surfaces.
Color streaking
Old color can remain trapped in the nozzle, die, barrel or feed zone and continue to contaminate subsequent material.
Resin-to-resin contamination
Differences in viscosity, temperature and thermal stability can increase transition time and leave old resin behind.
Main types of commercial purging compounds
| Type | Main mechanism | Key consideration |
|---|---|---|
| Chemical | Heat-activated chemistry acts on retained polymer/residue. | May require let-down, temperature adjustment or soak time. |
| Mechanical / abrasive | Hard fillers or particles create aggressive mechanical action. | Machine clearances and potential wear must be considered. |
| Mechanical / non-abrasive thermoplastic | Engineered thermoplastic material softens, scrubs and carries contamination out. | Select the correct processing temperature and clearance for the grade. |
How Dyna-Purge works
Dyna-Purge is positioned as an engineered thermoplastic, mechanical/non-abrasive purging compound. Current grades are designed to remove resin, color, carbon, additives and contaminants from process boundary layers and stagnation areas. Depending on grade, the material can be used at the resident resin processing temperature without mixing or soaking.
When should you purge?
- Color changeovers, especially dark-to-light or opaque-to-clear.
- Resin changes involving different viscosity, temperature or thermal stability.
- Black specks, streaking or degraded-resin contamination.
- Preventive maintenance before deposits become a quality problem.
- Before manual cleaning to reduce retained material.
- Shutdown / start-up when the selected grade is approved for this procedure.
What determines purge time?
Material & color
Some pigments, masterbatches and polymers are more difficult to displace; the viscosity relationship between outgoing and incoming resin also matters.
Machine & procedure
Machine size, dead spots, hot runners, die design, maintenance condition and operator procedure can all change transition time.
Evaluate total purge cost
Purging compounds should not be compared on price per kilogram alone. A meaningful cost-per-purge includes compound consumption, post-purge resin, downtime, transition scrap and the time required to return to saleable product.
Current Dyna-Purge grade selection
GKTECH currently focuses on the six grades highlighted in Dyna-Purge's current product selection guide. Final selection should be based on resin, processing temperature, machine configuration and clearance.
| Grade | Temperature | Typical positioning |
|---|---|---|
| L | 143–329°C | Low viscosity; high-cavitation, stack molds and continuous automation. |
| D2 | 160–329°C | Broad-range, versatile grade for color/resin changes and preventive cleaning. |
| E4 | 302–379°C | High-temperature engineering resins. |
| A | 160–260°C | Polypropylene-specific formula. |
| K | 143–287°C | Low-temperature, low-durometer materials. |
| C | 193–310°C | Clear/transparent resins, low-residue positioning. |
Frequently asked questions
Can purging compounds be used through hot runners?
Yes, when the selected grade is designed for hot-runner use and the system meets its clearance requirements. Not every purging compound is suitable for every hot-runner system.
Is Dyna-Purge “FDA approved”?
No. Purging compounds are maintenance/intermediary products and are not FDA-approved finished plastic products. Dyna-Purge states that its formulation uses FDA-compliant ingredients.
Can purge be left in the barrel during shutdown?
Some Dyna-Purge grades are recommended for shutdown/start-up because of thermal stability. Follow the grade-specific procedure rather than applying this rule to all purging compounds.
How should high-to-low temperature transitions be handled?
Select a grade with a suitable transition window and reduce temperature in a controlled sequence after the previous resin and purge have been displaced appropriately.
What if the next resin is too low-viscosity to push the purge out?
The incoming resin may flow through rather than fully displace the purge. A bridging transition or a grade with a more suitable carrier may be required.
What about heat-sensitive resins such as PVC?
Avoid starving the heated barrel and keep the purge within the polymer's safe processing window. Use a grade intended for the relevant low-temperature range.
Need help selecting a purge grade?
Send the resin, color, processing temperature, machine type and current problem. GKTECH can recommend a grade and trial procedure.