How to Purge an Injection Molding Machine (2026)
- UniTemp
- 7 hours ago
- 4 min read
A Step-by-Step Guide for Plastics Processors (2026)
Purging an injection molding machine is one of the most important maintenance practices in plastic processing. Proper purging removes residual polymers, colorants, and degraded material from screws, barrels, and hot runners, helping prevent contamination and production defects.
Without proper purging procedures, injection molding operations may experience issues such as black specks, color streaking, inconsistent melt flow, and increased scrap rates.
Understanding how and when to purge injection molding machines is essential for maintaining efficient production and consistent part quality.
Why Injection Molding Machines Need to Be Purged
During normal operation, small amounts of plastic remain inside the machine’s screw and barrel assembly. Over time, these residues can degrade due to heat, pressure, and shear forces.
When degraded material accumulates inside the machine, it may eventually break loose and contaminate production runs.
Common problems caused by poor purging include:
black specks in molded parts
color contamination during changeovers
burn marks or streaking
degraded polymer residue in the screw flights
Routine purging removes these residues before they have time to degrade into carbon buildup.
Processors dealing with contamination issues in extrusion equipment can also review this guide:
When Should Injection Molding Machines Be Purged?
Purging should occur during several common production situations.
Material Changes
Switching from one resin to another without purging can leave residual material inside the machine.
This may cause contamination or compatibility problems between polymers.
Color Changes
Color changeovers often require purging to remove pigment residues from the screw and barrel.
Without proper purging, previous colorants may appear as streaks in the next production run.
Shutdown Procedures
Before shutting down an injection molding machine, purging removes residual plastic that might degrade inside the heated barrel.
Preventive Maintenance
Routine purging during scheduled maintenance helps prevent long-term contamination buildup.
Step-by-Step: How to Purge an Injection Molding Machine
Although exact procedures vary depending on machine size and resin type, most purging operations follow a similar sequence.
Step 1 — Empty the Hopper
Remove the current production resin from the hopper.
Allow the machine to run until the majority of the existing material has cleared from the screw.
Step 2 — Adjust Processing Temperatures
Set the barrel temperature to match the purge compound’s recommended operating range.
For PEKUTHERM purging compounds, the working temperature range covers a wide spectrum of plastic processing applications.
More information about purging compound types can be found here:
Step 3 — Introduce the Purging Compound
Feed the purging compound into the hopper.
Allow the purge material to enter the screw and begin processing through the barrel.
Mechanical purging compounds soften during processing and form a thermoelastic mass that scrubs internal metal surfaces.
Step 4 — Run Purge Cycles
Cycle the injection screw several times while allowing purge material to pass through the nozzle.
This step pushes residual material and contamination out of the machine.
Operators will often observe contamination exiting the nozzle during early purge cycles.
Step 5 — Continue Until Material Runs Clean
Continue processing purge compound until the discharged material appears clean and free of contamination.
At this stage the screw and barrel are typically free of residual polymer and degraded material.
Step 6 — Introduce New Production Resin
Once the purge compound has cleared contamination, introduce the next production material.
Because mechanical purge materials do not rely on carrier resins, new production material can typically run immediately after purging.
Types of Purging Compounds Used in Injection Molding
Injection molders generally use two categories of purging compounds.
Mechanical Purging Compounds
Mechanical purge materials clean equipment through physical scrubbing action.
During processing they soften into a thermoelastic mass that adheres to itself and removes contamination from screw and barrel surfaces.
Chemical Purging Compounds
Chemical purge products rely on reactive additives that generate gas expansion inside the barrel.
These reactions help loosen residues but may require multiple purge cycles.
A detailed comparison of purge technologies can be found here:
Advantages of Mechanical Purging Compounds
Mechanical purge compounds offer several advantages for injection molding operations.
physical removal of degraded polymers
effective color removal during changeovers
reduced contamination risk
wide operating temperature range
minimal chemical residue
Because the purge material adheres to itself, contamination is encapsulated and pushed out of the machine during processing.
Best Practices for Injection Molding Purging
To maximize purging effectiveness, processors should follow several best practices.
Maintain Proper Temperatures
Operating within recommended purge temperature ranges ensures the purge compound performs effectively.
Use Adequate Purge Volume
Purging typically requires approximately 10–15 percent of the screw capacity.
Using too little purge material may leave contamination behind.
Purge During Shutdowns
Purging before shutdown prevents residual polymers from degrading inside heated barrels.
Schedule Routine Purging
Regular purging reduces the risk of contamination buildup and helps maintain consistent production conditions.
More purge procedures can be found here:
UniTemp’s Thoughts
Proper purging procedures are essential for maintaining clean injection molding equipment and preventing contamination issues.
By removing residual polymers and degraded material from screws and barrels, processors can reduce scrap rates, improve production efficiency, and maintain consistent part quality.
Understanding the differences between purge technologies and following proper purge procedures helps injection molding operations maintain optimal machine performance.
For a complete overview of purge technologies used across plastic processing, see this guide: