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Why this part won't run in that machine

Editor's note: Bill Tobin is a molding consultant and a frequent contributor to IMM. In this article, he diagnoses the cause of a common shop floor problem.




I was in a client's shop for a mold tryout. During one of the times we were just standing around, the production scheduler came up to the mold technician with the schedule for mold changes. The technician looked at it and frowned. "This mold won't run in machine number six," he said. "Every time we put that mold in that machine, all we get is scrap!"

"A plastic molecule is really small," I joked. "It's dark inside the barrel, it's very hot, and the plastic ends up in the mold. It is too dumb to know where it is. It only sees heat, pressure, time, and speed. Why should the machine make the difference?"

I asked the technician to tell me the experience behind his remarks. He told me it was a nylon part. Once it had run very well in the machine. However, for the last few runs he could not get the gate to freeze off. I asked him what machine he'd like to run it in. He immediately pointed out another machine (we'll call it #5) that was happily running Delrin with no problems.

Determining Why

I asked the most obvious question: "Why does the part run in that machine and not in the other one? They are both about the same age. They are the same brand and have the same size clamp, injection units, and screws." He gave me a look like a deer caught in headlights. His answer was something along the lines of "it's always been that way." This was the tip-off that he really had no idea.

When we finished the tryout, I asked him to look with me and try to find the differences between the two machines, #5 and #6, because something had to be different. The easiest thing was to look at the sprue. Sure enough, the two sprues were different. Machine #5, where the Delrin job was running, had a shutoff nozzle. Machine #6 had a standard nozzle for running ABS. This was the answer. The nylon job needed a shutoff nozzle. Running it in a machine without one would only lead to an unreasonable amount of scrap.

"But," he objected, "identifying a nozzle when it's new is easy. I can read the stamped numbers. But after a few jobs, it's burned and covered with crud, and I can't read the I.D. anymore. Besides, how am I supposed to know what nozzle goes to what mold?" This sounded like the negative generic excuse, "It's not convenient," (also known as whining).

Solving the Problem

The answer to these questions and complaints is really simple. All you have to do is separate your mold documentation or guide into two sections. The first section should be a setup guide. This has all the instructions for waterlines, clamps, nozzles, and all the stuff you need to set up the job, including the occasional sonic welder, inspection gauge, packaging, and so forth. The second part of your mold guide should be a process guide. This holds all the molding conditions, inspection points, and other information that pertains to running the job.

Get all your nozzles together and engrave them with large letters or numbers. These should be at least 1/16 inch deep and 1/2 inch high or larger on both the bolt portion of the nozzle where you use your wrench and the shaft portion. Then, you simply have to note "Nozzle 13" on the setup guide for mold #243 that goes into machine #6.

This also eliminates the problem of different-size ball radiuses (often conveniently fixed by jamming cardboard between the sprue bushing and the nozzle), as well as using a nozzle not suited to the sprue orifice size or the material requirements.

Now, there might be those who complain that they might need the same type nozzle for running two jobs at once and only have one nozzle of that type. Buy another nozzle for goodness sake! We are here to make a profit. This means having the proper tools for the job. The cost of another nozzle is paid for in less than half a shift by eliminating the scrap generated.

The first time you pull the nozzle that has been in there for a few years out of a machine, you might require a 6-ft extension pipe on your wrench, but it will come off easily the next time, so long as you put the new one on with a generous amount of high-temperature pipe fitter's compound.

You might do the math on what you would save your company balanced against what it would cost you. If you make the right decision, you'll look like a hero.


Contact information
WJT Assoc.
Boulder, CO
Bill Tobin
Phone: (303) 499-3350
Fax: (303) 499-4116

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