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December 8, 1998

5 Min Read
How to crunch a mold

Editor's note: Consultant William J. Tobin has ordered construction of hundreds of molds in his career, and he knows just how easy it is to damage one. Here is some of his preventive medicine for this ailment.

It's common knowledge that most mold damage occurs at startup. Molds with mechanical slides or hydraulic cores are particularly prone to damage; it's essential that the pins on the slide or the core are retracted either before the mold opens or before ejection begins.

Murphy's law usually rules so that even if the odds are one in a million, when damage does happen, it will be at 4:00 a.m. at the end of the week when you have just received a rush order for parts.The mold opens, the ejector plate comes forward, and a loud popping, crunching, and screeching sound comes from the mold as it destroys itself because the actions occurred out of sequence. Let's look at the common causes of this problem and a simple solution.


Generally, the core sequences on the molding machine are standard. However, the hoses tend not to be. Try this tip: Choose the male fitting on the hydraulic cylinder to be the one energized when the pin goes forward, and put a female fitting on the end for the pin to retract. This is known as idiot-proofing. You can't do it wrong, and you can't do it backwards. The only thing you can be guilty of is hooking up only one side or not hooking the cylinder up at all. (Don't laugh and think you will never do it--I have.) It is also considered good form, if the mold calls for a C-sequence, that you set the core set switch to a C-sequence. Any other selection will cause the machine to perform perfectly and destroy your mold.

Mechanical Slides

Slides work on two sliding levers. The first is an angled set of pins. It engages a slightly larger angled hole as the mold closes and moves the slide inward. As the mold is just about closed, the slide's back side is engaged by a heel block. It is this block that actually moves the slide to its final position and keeps it forward. When the mold opens up, the pins hit the other side of the holes and force the slide back as the platens move apart.

When you make slides they should have some common components.

  • The pins should be stout enough to withstand the bending forces of moving the slide. Stout means fat with a ball radius on the end and buried deep enough into the mold base not to wobble. Many people use through-hardened ejector pins for this. They don't bend. However, they do shatter.

    The hole the pin goes into should have enough lead in so the shaft of the pin is in the hole before any engagement begins.In the back position, there should be ball detents or Vlier pins. These are used so when the slide is in the home position it is held there and cannot fall out of position and damage the pins when the mold closes.The slides should be spring loaded with the springs pushing the slide back to the ball detent. If you have a slide that is vertical, it won't fall down. However, this is simply good form to have on all slides.The gibs and slide-ways on the slides should have a designated "wearee" and "wearor." Steel on steel will gall unless one is harder than the other. If both are hardened, grease and grease ways must be employed. Here it is also considered good form to put grease ways with channels and Zerk fittings so that you don't have to take a glob of grease and smear it all over the slide. The folks in medical are always concerned about this eventuality, and they tend to run dry on minimally lubricated slides. The only consequence of this is premature wear. You can get around this if you use self lubricating bronze alloys for your slide paths. Good machining technique is a must. The slide and the slide path must be parallel, allowing no twisting when the slide moves forward and back.

Even with all these precautions, people still crash molds, especially new ones. So what's a setup tech to do? Before you get the mold for the first tryout, go to the toolmaker. Have him or her open up the mold and explain how it works to you. Wiggle and twist the slides. If there is any free play either in them or in the pins, you have an accident looking to happen.

Have limit switches put on all the slides and cores. You can use either conventional switches or some of the nifty microswitches that are now available. There is even one available now that is tied to a ball detent. If the detent isn't in the right position, the signal isn't sent. Tie them in series and hook them to the machine. If the cores need to be pulled before the mold opens, you have just added the insurance policy of requiring that not only the hydraulic signal and pressure are sent to the cylinders but that the cylinder actually came back! Another trick is to hydraulically daisy-chain the cylinders in series. When configured in this manner, all cylinders will move together and not independently.

Limit switches for the slides can be easily wired into the machine to stop the ejectors from coming forward until all the slides are fully open. Again, tie these switches in series. Thus, all the slides must be back before the machine will operate.

What does all this cost? At most, it will set you back a few hundred dollars. This is insignificant when compared to the cost of lost business, tool repair, and a late delivery.

What does it take? Both the discipline to do the work up front and the ongoing effort to use it are required. All of these systems will work whether you plug in the electrics or not. However, the electrics are really the safety net that will save you thousands of dollars of damage and ten times that in lost sales. Tooling standards and setup sheets were written to be used and not filed away. Just remember it's only time and money each time you crunch a mold.

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

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