Page 33 - Plastics News July 2018

FeAtures

Injection Molding: Safety First—Know the Compatibility of Different

Resins

Certain materials don’t play well together when mixed. So it’s best not to. Here are some guidelines.
JOHN BOZZELLI

ecause of the growth of the plastics industry, there has walls, etc. The gases produced upon resin decomposition
Bbeen an uptick in number of new hires for processing generate extreme pressures, which can and do blow
plastics over the last few years. With new employees injection molding machine barrels into shrapnel. The
comes the challenge of training—not only on company accompanying photo shows the front zone of a machine
policies and processing duties, but on safety. With that in barrel that blew into pieces. Imagine the damage if it had
mind, this column will focus on a topic I feel does not get hit someone! This may not happen often, but it does occur,
sufficient attention. In fact, it is a topic on which most of and you need to be aware its possibility. The question
us do not have all the details needed. In my experience, of resin compatibility is not well publicized and rarely
searches on the internet certainly proved inadequate. comes up out on the shop floor. So, any time you purge
That topic is the mixing of different resins in the barrel, to change resins, you need to ask the question: Are these
hot runners, and hot tips. resins compatible or do they react like PVC and acetal?
This is an issue whenever you change jobs to a new mold I’ve developed a list of plastics that I know could possibly
with a different resin. Are the two resins compatible or react with one another. My bet is there are others that I
do they react with one another? No matter how hard do not know about. If you know of any others, please let
you try to purge the previous resin from the system, 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
small amounts will remain on the screw flights. Purging material compatibility. High temperatures and pressures
compound may help, but even a small residual amount of 20,000 to 45,000 psi are needed to melt and inject
of the previous resin can provide a catalytic effect. For plastic into complex shapes. While these machines are
example, if you accidently mix acetal (POM) with PVC, you built robustly to handle these temperatures and pressures,
and the rest of the people on the shop floor will have an it’s best take a serious look at the equipment on a regular
extremely unpleasant emotional and physical experience. basis and not assume all is OK. Do you review the machine/
Small amounts of either of these two resins catalyzes the mold upon startup or during production to make sure these
decomposition of the other polymer to gases. pressures and temperatures do not reach out and touch

The resulting acrid odor is well known to Crusty Sr., my you or a coworker? For example, does the nozzle tip mate
archetype of the grizzled processor. Find someone who has properly with the sprue bushing? Is a temperature zone
some experience processing PVC or acetal; ask about his overriding? Are you sure all is OK before you look down
or her experiences. You will hear true stories of hoppers into the hopper?
being blown to and through roofs, endcaps blown through Who is responsible for your safety? Look in the mirror.

33 July 2018 Plastics News