Injection Molding: Safety First—Know the Compatibility of Different Resins

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 accidently 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.

The resulting acrid odor is well known to Crusty Sr., my archetype of the grizzled processor. Find someone who has some experience processing PVC or acetal; ask about his or her experiences. 

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.

No matter how hard you try to purge the previous resin from the system, small amounts will remain on the screw flights.

The question of resin compatibility is not well publicized and rarely comes up out on the shop floor. So, any time you purge to change resins, you need to ask the question: Are these resins compatible or do they react like PVC and acetal?

I’ve developed a list of plastics that I know could possibly react with one another. My bet is there are others that I do not know about. If you know of any others, please let me know.

Here’s my list:

• Acetal with PVC or CPVC.
• Ionomer with acetal, PVC or CPVC.
• TPV with acetal, PVC or CPVC.

If the same machine barrel and screw must process both these resins, disassemble and clean them properly before resuming processing. Even small residues can be hazardous.

Of course, safety in injection molding extends beyond material compatibility. High temperatures and pressures of 20,000 to 45,000 psi are needed to melt and inject plastic into complex shapes. While these machines are built robustly to handle these temperatures and pressures, it’s best take a serious look at the equipment on a regular basis and not assume all is OK. Do you review the machine/mold upon startup or during production to make sure these pressures and temperatures do not reach out and touch you or a coworker? For example, does the nozzle tip mate properly with the sprue bushing? Is a temperature zone overriding?

Are you sure all is OK before you look down into the hopper?

Who is responsible for your safety? Look in the mirror.

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ABOUT THE AUTHOR: John Bozzelli is the founder of Injection Molding Solutions (Scientific Molding) in Midland, Mich., a provider of training and consulting services to injection molders, including LIMS, and other specialties. Contact john@scientificmolding.com; scientificmolding.com.