Polypropylene Cutting

Written By Mark Batson Baril

Prepare Yourself for a Mind Teaser!
This one has presented itself as a real technical dilemma, not only for the questioning company but for a large group of technical experts. It's a work in progress and may be for a while to come!

The Case Started with this question;

We are a manufacturer of a non-oriented polypropylene material. We have a fairly new product line that involves cutting holes in our raw material and delivering this finished product in roll form. We have had a flatbed machine built to perform the diecutting operation and are having problems delivering the material through the machine with zero waste left in the areas where the holes are cut. The material is being used extensively in the reinforcement of boat bodies and other hard shell consumer good products. Do you have any suggestions?

After a few phone calls and a visit to the facility, the following additional information started to gel:

• The raw material is produced, and final cut product will be delivered, in rolls. These rolls vary from 16" to 24" (406mm - 607mm) wide. The raw material is about the consistency of bathroom tissue and varies in thickness from .005" to .050" (.127mm - 1.27mm) thick. It has more pulling strength than bathroom tissue because of the micro-strands of polypropylene. These micro-strands are a major part of the cutting and waste hang problem.
• The pattern of cutting resembles that of a grid of holes that are approx. 3.00" (76.2mm) on center, cover the entire width and length of the material and range in size from 1/4" (6.35mm) to 1/2" (12.7mm) in diameter.
• The annual volume of goods is now approx. 700,000 lineal feet (214,000 meters) and is expected to get bigger.
• The press that was made specifically for this project is a flatbed style press with multiple, and individually air actuated, male/female stations. The separate feed and take up mechanisms are a constant feed and pull set of rollers. The male pins cut with a ball into a correct diameter female type set-up (very strange).
• The non-cleared reject rate is currently close to 40% but this converted material is used because this is the best available. There are currently no training or routine maintenance plans in action.

After checking out the process, a few thoughts on how to proceed to improve the process started to become obvious. Cut Smart answered the question this way.

Because of the rather large investment already made in equipment, I think it will make sense to continue to produce the parts/finished material in-house with most of the equipment already in place.

In order to do this you will have to make the following changes/improvements in this order of priority.

Try a new type of die punch male and female set-up. This is absolutely key! The punches we saw in operation and the ones given to me as samples are out of the ordinary and seem dysfunctional for the material to be cut. I don't think that this ball and cut mechanism is helping in the slightest and may really be throwing everything off. I would talk to one of the well known male/female punch experts to get their opinions on a reproducible and well thought out set-up for your press. By making this your first step, you may save a tremendous amount of time and effort in what other changes, if any, you make to your machinery.

SIDE NOTE - We discussed and talked to three of the big punch guys out there and nobody had seen anything like this nor did it look like these punches should work.

Change your feed mechanism from a constant pull to a pull and stop for each impression. No matter how fast your impression is made, the web is still moving and in theory you will get some of the pulling, tearing, and poor cutting results in the material just as you are getting now. This may be easier said than done especially in your thinner materials but is necessary for good cutting results in a flatbed operation like you are running. Your overall rate of production will probably be slowed by this stop and start but it will allow you to control the impression and eliminate a possible cause of your problem.

Connect both your unwind and rewind machines/systems to your diecutter. The fact that there are three units in operation and none of them are solidly connected, (plus the diecutter is on wheels) is not helping your consistency of set-ups or cutting. At least, any new feeder must be attached to the diecutter.

Once the above items are changed and can be proved to work individually and together, just in R & D or sampling, you should proceed with the following before you can expect consistently good results.

• Control the air temperature and humidity in the cutting area so they are both constant at all times.
• Develop a make-ready and changing punch procedure that is documented.
• Develop a system that tracks when in the life cycle the cutting dies are wearing out. Follow the answers from this documentation to replace or sharpen the cutting tools exactly when they need it. For a 100% perfect product, like you are hoping for, you will have to error on the side of caution when it comes to replacement.
• Train all your operators in these systems.

Putting all of these ideas into use, with a few more that I'm sure will be gathered along the way, will result in better and perhaps a perfect product using the same basic method you are using now.

The one possible problem I see with correcting these problems and spending the time, money, and effort to do so is that the flatbed cutting method may be too slow to make it cost effective in the long run. If this products' volumes will continue to increase, as they have been over the last year, you may find that rotary cutting is the fastest and cheapest alternative. By gathering some pricing not only for machines but for finished product via rotary converting you will probably find that rotary cutting is the first alternative to look at for your in-house operation.

Throwing good money after bad makes no sense - but finding a route to determining whether a method is doomed can be very frustrating. We hope this helps open up a new possibility or two or even a new line of thinking on this very difficult cutting problem. Good Luck!