In today’s issue of Cliff’s Chronicles:
The thrill, the exhilaration, and the anticipation of sampling a new material, all were shattered when the OEM dropped one of the 1st
parts on the cement floor of the mold shop and it broke into 3 pieces. Heads swivel all looking at me, with glaring disapproval. The questions start to fly…
The part was a structural frame for a new metal detecting device for the US Army, nearly 1.5lb shot. The material was pre-colored green 43% glass fiber reinforced, PA6/12. As we were developing the material, we quickly ascertained that pigment selection was critical. Early internal test lots were very low in strength and elongation, and we traced the root cause to inorganic green pigments with very high hardness. Just like TiO2, these green pigments severely damaged the glass fiber, causing low properties.
So, as the part lay in pieces on the floor, I knew the brittleness was not due to the composition, because we had thoroughly tested the material both dry and moisture-conditioned. So why was the part so weak? Then it occurred to me, being an “old glass fiber guy from PPG” that maybe the glass fiber was somehow still being degraded.
I ignore the glares and the questions and turn my attention to the press. With the press operator, I am watching the machine outputs and notice that the screw recovery is varying by more than 5 seconds and seems to be increasing. Then, I move to the back of the press and watch the screw rotate. Boom, there it is. The screw starts to rotate, and the screw position isn’t moving, meaning that material isn’t being melted and conveyed, it’s just rotating. Only after “about 10 seconds” does the screw move back, as the shot accumulates. While the screw is not moving back, the contents of the barrel, in its various states of plastication, are literally just spinning. This means the polymer melt with 43% glass fiber is just mashing itself to pieces, making smaller and smaller fibers, as it grinds itself to bits. Now I know why the parts are brittle, we’ve broken up the glass fiber, but what’s going on??
So, what do we do, to address the inconsistent screw recovery? The 1st adjustment we tried was increasing the screw speed, to increase the sheer rate applied to the melt and raise the melt temperature. Recovery time got worse; parts were even more brittle. We went back to the initial screw rpm and raised the barrel temperatures, thinking that the issue was the melt temperature was too low and there was a flow restriction. The same thing, recovery time still increased some, and parts were still weak. The press operator confirmed that the check ring was a high-flow, fluted type, designed specifically for glass fiber compounds. As I’m brainstorming with the press operator, I ask innocently, when was the last time y’all pulled the screw on this machine? “I’ve been here for 7 years, and I’ve never seen this machine a part, let alone not running”! We’ve been running 70G33 on this machine non-stop, he says.
Light bulb! I quietly ask the operations manager for the mold shop, out of earshot of his customer (the metal detector OEM), “When was the last time you scoped the barrel and inspected the screw on this press?” Why he asks? I tell him that the machine is behaving as if the screw and barrel are worn or that the screw has a broken flight. He smiles a wry smile and then announces magnanimously to his customer, I think we have a machine problem. Let’s stop for the day and I’ll move the tool to a brand-new machine.
We start up the next day, “new” press. Parts look better, appearance-wise and Mr. Big Shot from the OEM cannot make even one part break.
The operations manager called me a few days later and said that they pulled the screw from the first machine and the feed section of the screw was worn by 0.25 inches into the root diameter. The barrel OD and screw OD were also heavily worn too.
The moral of the story is: it’s not always a material problem and you should include an annual screw pull and barrel inspection in your preventive maintenance plan, to avoid weeks of unplanned downtime.
Contact The Fixers at PolySource today to learn how we can help you with our superior technical support: https://polysource.net/contact-us/