Here's a case study type question with a straight answer!
The diecutter asked - We do an awful lot of diecutting, especially in plastics. Most of what we cut comes to us in roll form, and most of that material is either PVC or HDPE in thicknesses ranging from .010" to .040" (.254mm to 1.02mm). Our problem is that we currently lose anywhere from ten to thirty feet of material on the end of every roll due to it being too curled up to use. We can get it through the machines, most of the time, but the final product has too much of a curve to be considered good quality. Any suggestions?
This is a very common problem in many materials where the material is wrapped around a core to be delivered to the final converting machinery. Most metals immediately come to mind when thinking about “coil set” problems but yes indeed it is very common in plastics, foams, papers, etc… as well. It doesn’t seem like allot of material to be wasting when we take the last 20 feet (6 meters) of every 1,000 foot (304 meter) roll and toss it in the trash, but add it up at the end of the year and I wouldn’t want it coming out of my paycheck!
The basic problem occurs during the rewind operation at your material manufacturer. Typically the material is rewound while it is still warm right after processing. As it cools it keeps its wound shape. In some plastics this is not a problem, but in many the wrapped shape becomes permanent or close to it. The closer you are to the core, the tighter the curve is. Too tightly a wound material can result in the material being stretched around the core. This stretching (especially while warm) will result in one side of the material becoming longer than the other. A pretty darn close to permanent curved shape is the result of this stretching and can be very hard to remove with any technique.
There are a few, very straight forward, ways of dealing with the coil-set problem and here they are:
- Bigger Cores – The smaller the core, the better your chances are of having a few feet of material that are too curved to use. Work with the manufacturer of your material to develop the right size core for the material they are selling you. This is the most common, simple and effective way to get rid of that unwanted coil-set. It may not seem worth it to set-up your unwinds and rewinds with 6" or 9" core holders instead of the 3" now on the machine, but if you add up the material and time loss, the change should quickly tip the scales.
- Opposite Rewind– Try rewinding the material in the opposite direction to the way it came from the factory then let it sit long enough to try and reverse that coil set. This can be a very effective way of dealing with the problem but can be a time consuming one if you have to rewind your rolls before you use them.
- Roll Straighteners – Some people call these set-ups “coil-set removers.” Just like in the metal stamping industry, many plastics process well through a series of rollers (typically three – two on one side and one that does the squeezing on the other) that reverse and remove the coil set just before the cutting station. The adjustments can be tricky on these press add-ons and will usually have to be adjusted throughout the length of the roll. They can be worth the effort though and may work very well to flatten out that special curvy material you know so well. Check out your local metal stamping shop if you’ve never seen one of these in action.
- Heat Stations – If the coil set problems started with heat then perhaps they can end with heat as well. Some companies use heating ovens while others use heat lamps of one sort or another. Whatever the case may be, the technique involves warming the material to the point where it will be straightened out during the cut or even as it passes through the feeders just before the cutting station. Heating the material can also result in much easier, and better edge quality, cutting. Watch out, you may get addicted to it!
Those are the basics that seem to come up no matter who the “industry expert” is that you talk with. All very straight forward and all relatively inexpensive methods to try out. I hope this helps out in saving material and helping you to improve the way you do business. Good Luck……
Support for this condensed list of techniques and ponderings goes to Preco Industries, Crown Plastics, Southern Imperial, and Baril Die Co., Inc. Thanks Everyone!
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