Purging compounds have many benefits for your facility, including faster changeovers, reduced machine downtime, effective removal of contamination and the reduced need for screw pulls.
However, a time likely will come when your purging process doesn’t go as planned. Maybe your purging compound begins working less effectively, or you notice contamination during production runs.
When you notice an issue with your purging process, it’s important to address the problem immediately. The longer you wait, the more detrimental the effects will be on your production runs and bottom line.
Here are four common problems plastics processing companies experience and purging tips to help you troubleshoot each one:
1. Purging Compound Doesn’t Clean The Way It Should
There are several possible reasons your purging compound isn’t cleaning properly. If you’re following the proper purging procedures as outlined by your vendor, you can solve this issue in one of two ways.
If you’re using a mechanical purging compound, use maximum safe screw speed while purging and keep the screw completely forward. If you’re using a chemical purging compound, use the recommended soak time, as provided by your supplier.
2. Extended Machine Downtime And High Scrap Rates
If you’re experiencing extended downtime and high scrap rates due to contamination, seal the machine using a heat-stable purging compound during temporary or extended machine shutdowns.
During shutdowns, the presence of oxygen in the barrel, combined with high heat, causes degradation and eventual carbonization of residual plastic. Sealing your machine with a heat-stable purging compound will create an “airtight” environment in the barrel, preventing oxygen-related degradation from occurring.
3. Issues When Cleaning Hot Runners
If you’re experiencing issues when cleaning your hot runners, you may need to re-evaluate the type of purging compound you’re using.
Not all commercial purging compounds are suitable for purging hot runner systems. Before attempting to purge a hot runner system, consult with your supplier to ensure the grade you’re using is compatible.
Most purging compound suppliers offer product grades suitable for purging hot runner systems. In most cases, the moldability of a purging compound is what you should look for because the closed-mold purging method often provides a more thorough cleaning of the hot runner system, due to higher system pressures.
In general, it’s recommended that hot runner temperatures are raised to help loosen deposits when purging. Short, high-velocity shots are also helpful when purging using open-mold procedures.
4. Purge Comes Out Clean, But You Still Get Contamination
If you’re still getting color or carbon contamination when you go back into production, nozzle or check ring hang-up of color or carbon may be the issue.
If you’re using a mechanical purging compound, raise the temperature of the nozzle to 20-30 degrees Celsius (35-55 degrees Fahrenheit). If you’re purging a heat-sensitive resin, do not exceed the safe processing temperature of the material.
Next, feed a small amount of purging compound – usually less than half a barrel – into the machine, and use short, high-velocity injection shots until the purge emerges from the nozzle clean.
It’s important to remember that several different types and grades of purging compounds are available. Always consult your supplier if you have any questions about how to troubleshoot an issue with your machines.
Learn how the right purging compounds and procedures impact your process efficiency.