Because of the growth of the plastics industry, there has been an uptick in number of new hires for processing plastics over the last few years. With new employees comes the challenge of training—not only on company policies and processing duties, but on safety. With that in mind, this column will focus on a topic I feel does not get sufficient attention. In fact, it is a topic on which most of us do not have all the details needed. In my experience, searches on the internet certainly proved inadequate. That topic is the mixing of different resins in the barrel, hot runners, and hot tips.
This is an issue whenever you change jobs to a new mold with a different resin. Are the two resins compatible or do they react with one another? No matter how hard you try to purge the previous resin from the system, small amounts will remain on the screw flights. Purging compound may help, but even a small residual amount of the previous resin can provide a catalytic effect. For example, if you accidentally mix acetal (POM) with PVC, you and the rest of the people on the shop floor will have an extremely unpleasant emotional and physical experience. Small amounts of either of these two resins catalyzes the decomposition of the other polymer to gases.
You will hear true stories of hoppers being blown to and through roofs, endcaps blown through walls, etc. The gases produced upon resin decomposition generate extreme pressures, which can and do blow injection molding machine barrels into shrapnel. The accompanying photo shows the front zone of a machine barrel that blew into pieces. Imagine the damage if it had hit someone! This may not happen often, but it does occur, and you need to be aware its possibility.