Top 5 Purging Tips for Injection Molding
Purging an injection molding press, whether it be for color changes or removal of carbon, can be very time consuming and expensive. The lost production time and the scrap resin cuts into the company’s profit margin making it more difficult to compete in this difficult economy. Along with using a commercial purging compound there are a few basic tips a company can follow in order to help reduce the down time and scrap. These five recommendations are easy to do and require no monetary investment:
- When performing a dark to light color change increase the temperature of the nozzle and hot runners (all zones). The amount to increase the temperatures depends on the application; this should be discussed with your purging compound representative. The reason for increasing the temperature is to help melt the dark color that is being removed, thus reducing the viscosity and making it easier to be removed from the press.
- Reduce the shot size when performing a color change on large tonnage presses (greater than 200 ton). Reducing the shot size to 10% of the maximum allowed shot size will work the check ring more allowing it to clean up faster. Also, the short bursts through the nozzle (or hot runners if applicable) will pull out more color.
- Establish a preventative maintenance program in order to combat carbon build up. Black specks can be a nuisance for companies processing thermo-sensitive resins. It is important to remove the carbon before too many layers of degraded material form on the screw. After a certain point of carbon build up the only way to truly clean the screw is to pull it and manually clean it. Using a PM program, whether it is every couple days or every couple weeks, will help prevent carbon build up and removal of screw.
- Shutdown the injection molding press properly to make start ups easier. When shutting down presses remember to first purge the machine. Purging a machine before shut down will prevent material from degrading on the screw, resulting in a much faster start up.
- The easiest and most basic way to reduce scarp and downtime is to have a protocol established to clean the hopper and conveying system of the previous production color. Many times the previous color will hang up in hopper or conveying system and it will slowing be released into the press resulting in contaminated parts. Also, make sure there is no contamination in the resin being used in production.