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Tooling Corner: Got hoist rings? Changing molds safely

May 3, 2006

5 Min Read
Tooling Corner: Got hoist rings? Changing molds safely

Suppose you and your working partner are mold setup technicians faced with 20 to 30 mold changes per week. This includes startups and shutdowns. Whatever brought this about could be called the Setup Tech?s Full Employment Act.

Take one transition for example. You?ve moved the new mold to the machine, flushed out the water lines of the previous machine, unhooked everything, and brought the crane over and now?.. ?Where the [insert rude language here] are the hoist rings?? you ask.

For the moment let?s ignore the question of whether the mold chains/straps are on the crane hook. Now the fun begins. First you go on the equivalent of an Easter Egg hunt, trying to find enough rings to install on the mold in order to pull it out of the machine it occupies at the moment. Second, you hope you?re not trying to force a NPT standard threaded bolt into a metric tapped hole. Third, you are also praying that the hole is tapped deep enough so that you don?t bottom out the hoist ring before its collar engages the mold. Last, you really hope the ring isn?t bent, has microfractures, or anything else wrong with it so it doesn?t decide to fall off when it?s a few yards over your head. You also know that your steel toed boots will only act like a bagel slicer on your toes.

Safety hoist rings generally come in two flavors: First is the old standby?the cast/forged ring. While inexpensive, if they are not fully bottomed out on the mold they tend to bend when exposed to excessive weight. Only by sheer luck will the eye?s orientation be facing the hook. Thus it will be loosened slightly to line up properly. While some techs improvise by putting washers on the eyebolt to compensate for the length of the bolt and a shallow hole, this logic rarely works. Loose bolts (even slightly) or the use of washers will cause side stresses on the forged ring. This will in turn ultimately fatigue the steel to the point of a fracture. The consequences are unpleasant or fatal.The second type of hoist ring offers a solution to the problem of bending the ring. This is a high tension bolt with a fat donut collar on it along with the ?U? shaped bar that swivels to accept the hook from the crane. The fat collar distributes the tension of the crane picking up the mold. Because the swivel U can turn, it doesn?t cause side stress on the bolt as the mold is lifted.

How do we stop the Easter Egg hunt and rude language? While simple and obvious, most people don?t see it?mold specifications. First, standardize the size and depth of the hoist holes. Intentionally specify the holes deeper than necessary to avoid any problems in bottoming out. Second, specify the purchase and installation of good safety hoist rings on every mold you buy. With a little thought you can standardize on three sizes of hoist rings: ?small?, ?medium?, and ?large?. Spray paint the donuts of each size a different color on the mold, leaving an overspray area on the mold. This way it is easy to match the size of the hoist ring to any particular mold.

Ideally (I?ve done this) you further specify that the lift rings be welded to the mold. This way you never have to hunt for them. However if space doesn?t allow it, you?ll very quickly have a large enough collection so that the Easter Egg hunt won?t take too long.

When specifying your safety hoist rings always err on the side of over-specification. It never hurts to have a hoist ring that is rated well above the weight of the mold, rather than one that is right on the ring?s safe lifting limit.

Overhead cranes are both good and a constant problem. Murphy?s law for setup techs always says the crane isn?t available when you need it. Well, actually it usually is. If you have to, video tape a complete setup from mold shutdown to mold startup. Look at the absolute amount of time you really need the crane. It is a very few minutes. Yes it would be nice once you?ve pulled a mold to bring it down to floor level then use the crane to commute it over to the mold storage area. But that takes up time.

You keep the crane in place while you level the mold. I?ve actually seen this operation take 10-15 minutes because the techs are using spirit (bubble) levels. Newsflash?your machine?s platens probably aren?t square to the ground. Level the mold with a tape measure using the threaded holes on the platen for a point of reference. Once you?ve found it, put clamps on the top and bottom and move off the crane. Other techs level the mold, clamp the stationary side then open the press and install the ejector rods. Why? Install the ejector rods before you clamp up and level. It saves time both in setup and the use of the crane.

I?ve seen the ubiquitous fork lift with pig iron loaded on the back end as a counterweight almost at the tipover point, both hanging and pulling molds. This goes well beyond the guts-to-brains ratio for any tech: The chains are usually too long so the forks have to be fully extended both forward and upward. Now you have a pendulum of several hundred pounds on a high center-of-gravity base. This is unsafe, unnecessary, and an invitation to somebody getting hit by a falling forklift, mold, or both; not to dwell on the damage to the machine, mold, and secondary equipment during the crash and the Workman?s Comp scenario that follows. Frame hoists, homemade or purchased, are sometimes difficult to maneuver but always safer than forklifts. However they can be easily overloaded and fail well before the hoist itself stalls. On small molds, scissor jack tables can be used by lowering the mold under the tie rods and raising the new mold into position.

These tables only have the shortcoming of putting the mold on wooden rails so that the platens don?t close on the mold table.

Bill Tobin is a frequent contributor and consultant who also publishes an independent monthly e-newsletter with tips on injection molding. E-mail [email protected] to request to be put on the circulation list.

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