Canadian Plastics

Quick Changes

It goes without saying that the injection molding community in Canada is going through difficult times. Competitive pressures are terrific, and as more shops chase less business, it's also likely that...

January 1, 2009   By Jim Anderton, Technical Editor



It goes without saying that the injection molding community in Canada is going through difficult times. Competitive pressures are terrific, and as more shops chase less business, it’s also likely that shorter production runs will become more common.

While everyone wants that 10-million part business, it is possible to optimize your press changeover procedures, even in a small shop, to win back productivity and make some of the smaller jobs profitable. How?

There are two steps: Change the clamping technology, and optimize that technology for best speed. If the budget can stand it, magnetic clamping is a very fast, clean way to mount molds. Clamping pressures can easily exceed 100 pounds per square inch, which, multiplied by the surface area of the mold’s contact face, can produce truly large clamp forces. If the mold plan form is 18 by 24 inches, for example, the clamping force would be:

18 inches x 24 inches x 100 pounds/square inch = 43,200 pounds

Chances are, 21 tons of clamping force will hold your mold securely. Remember, this is clamping the mold to the press, not the mold closing force, which is controlled by the machine.

Another benefit of magnetic clamping is that by nature, the forces are spread evenly over the tool’s surface area, minimizing distortion and eliminating the need for very high localized clamp forces with conventional edge clamping techniques. Need a three-minute tool change? Magnetics are for you, but bring your banker, because they’re expensive.

The second technique is hydraulic clamping, which is really a semi-automated conventional wedge clamping technology. Hydraulics are much faster than bolt-up, slower than magnetics, but are substantially cheaper. Besides the speed benefit of hydraulics, clamp forces are uniform, helping alignment and reducing the possibility of distortion. Hydraulic clamping also eliminates the phenomenon of under clamping, where a hurried set-up tech deletes a few “extra” clamps or forgets to fully tighten one or more bolts. Don’t ask me how I know this!

Can’t afford either magnetic or hydraulic clamping? There is still much you can do to cut precious minutes from your changeover time. Start with the floor configuration around the press. Are molds racked near the press? Are you using a pneumatic or hydraulic lift table? Most of my career, tables were hand-cranked, and by 3 PM on a busy day, I guarantee that a 40-something tech will be slowing down. A system I worked on 20 years ago halved changeover time at minimal cost with conventional manual clamping. See if it applies for your operation:

1. Use two lift tables.

Two tables allow flow-through changeovers. Bring the new mold to the press first, then push the old tool onto the second table, then pull it aside. Do not move the old tool right away, but concentrate on getting the press up and running. Saving $1,200 on a second lift means removing the old tool, moving it into storage or maintenance, and then fetching the new mold. The press can sit empty for twenty minutes waiting.

2. Use pneumatics tactically.

Impact guns are great for clamp removal. Air ratchets are similarly excellent for spinning up nuts and bolts prior to final torquing. Use both in that sequence, and you can safely use unskilled/marginal labour to remove and spin up clamps while the experienced tech follows, torquing to final spec. This makes it possible to pull non-maintenance personnel off another task to bolster your changeout team temporarily.

3. Don’t re-use clamps, bolts, or nuts.

I don’t mean throw them away, I mean bring a racked set of clamping accessories to the press with the new tool, and then zip off the old fasteners into a bucket or box. Re-organize the removed bits later when dealing with the old tool. I’ve seen junior techs spend 15 minutes on their hands and knees looking for a lost nut. Forget it for now and move on with fresh parts.

4. Think about cooling.

Is the chiller in the way? Can longer lines be used to move it back? When the mold is in repair, is it possible to change the connection ports to move them out of the way? Your set-up tech won’t tell you, so you need to either watch the changeout process to see for yourself, or better yet, get a cheap video camera and tape the process for later study. Either way, be aware that your presence will affect the team’s performance. Review the results with the team and assure them that you’re looking to make their jobs more convenient, not pressuring them for a speed-up.

There are more ideas, but I’m out of space, so more later.

CPL


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