For trouble-free parts, follow these important rules when molding polyurethane.
?This month I received a small runner with four gates and a part—a plug—attached to each gate. The material was natural polyurethane (TPU) and the parts were as round as could be when using a single small edge gate on each part.
As often happens, there was no note in the box, but I’m getting used to these mysteries. Turns out it was from someone I go way back with and before long he called and filled me in on the problem as it appeared from his perspective.
?This article continues our series of troubleshooting reports from one of the leading on-the-spot problem solvers in the molding industry. Consultant Bob Hatch of Bob Hatch & Assoc. has more than 45 years of experience finding solutions to processing challenges. You can reach him at firstname.lastname@example.org.
?The depth reduction built into the main runner caused defects in these polyurethane plugs, such as voids and sink.
Before the call, it had been a slow week so I decided to take a look at the parts. The natural TPU appeared to be well dried and was being molded in a four-cavity, two-plate mold fed by a heated sprue bushing. Alternately, this runner could have been one of many cold runners being fed by several hot drops in a hot runner mold.
Before we go any further, I refer all of you to one of my many points pertaining to molding. Rule #92 says that we mold big parts in big machines and small parts in small machines, but we do not mold many small parts in a big machine.
If these parts had been from anybody I had known for any length of time, then they were not running these small parts in a large mold; and this single important point almost guaranteed we were not dealing with a big hot runner mold trying to run 48 cavities or so of natural TPU. Don’t get me wrong—I have helped design and set up a 128-cavity, even a 256-cavity hot runner mold before, but it is hard to keep something like that running for very long. Parts stick in the mold, a slide quits moving, or any number of problems with the mold can shut us down in a hurry.
Instead of one hot runner mold with 128 cavities running in a 200- or 300-ton molding machine, I prefer to set up several 30- to 50-ton machines, each running a four- to eight-cavity cold runner, two-plate mold or even a small-cavitation hot runner.
Why? Mold maintenance. If I have a bad cavity in one of my four- or eight-cavity cold runner molds, when I shut it down for maintenance, I am only shutting down 10% or less of my productive output. When I have to shut down a big hot runner mold, my production is down 100%. When the big molds get shut down on the night shifts and don’t start up until the next day, production numbers suffer tremendously. I learned this lesson