Cutting Polyesters and Polycarbonates with Steel Rule Dies

This article was derived from a presentation at the IADD technical seminar, "Solutions for Effectively Cutting & Trimming Different Materials." See the IADD Industry Calendar on page 40 for additional education opportunities.

Manufacturers have been pushing steel rule dies to their outer limits ever since the first printing rule "accidentally" cut through its first piece of paper nearly 100 years ago. Following the birth of the steel rule die and the diecutting process came the multitudes with their products and their questions "can we diecut this.... can we diecut that .... ?"

Rather quickly after the diecutting of paper and cardboard came the textile industries. The steel rule die led to the mass production of apparel and garments for the clothing industry.

On the "heels" of textiles came the leather and rubber industries, (no pun intended) as the shoe industry exploded in growth with the application of the steel rule die. With the "double-edged" die, you now had a better than even chance that your shoe for the left foot would be an identical mirror image of the shoe for your right foot.

And where would the first automobiles manufactured in the U.S.A. have been without steel rule dies to blank out the many gaskets found on the first "Model A"?

As labor rates began to skyrocket after the second world war, so too did the manufactured cost of male and female blanking tool sets. Yet this tooling was required to blank out the many billions of metal stamping parts produced annually.

Prior to World War II, people were buzzing about this "goopy" stuff turned to solid masses called "plastic." It came in different brilliant colors, smelled obnoxious when it burned, and seemed only suitable to make tiny toy cars for tots. A plastics company in Wilmington, DE. had the foresight to predict "better things for better living... through chemistry" and lo and behold, a multi-billion dollar industry was about to be born.

Any of you who have ever held an MSDS sheet in your hand have surely been exposed to the endless array of Acrylon, Porlon, Nylon, Ethylenes, Lucite, PVCs, Polyethylenes, Delrans, Captons, etc. The only thing more confusing was reading labels at your neighborhood drug store.

The production of plastic in sheet form opened many eyes to its converting capability. Not only could you mold it, but you could cut it with a hot wire, or band saw, or .... oh yes, the ultimate in fabrication .... you could DIECUT it.

Stability was a factor, strength was another. In the sixties, all the world became ecstatic over polyester. Not only did it make a darn comfortable pair of wrinkle-free trousers, but it made a heck of a sheet of plastic that was tough, durable, could be silk-screened upon....and could be diecut.

It wasn't long after DuPont introduced its Mylar, that G.E. followed with its "Lexan."

My firm was a party to helping G.E. develop diecutting capability of its Lexan product at the nearby Pittsfield, MA research facility. And the immediate question was "how do you expect a product that can stop a 35mm bullet to be pervious to a steel rule die?" That was left up to us. After all, it was WE who were the steel rule diemakers.

Today, Stan-Allen lives in a world of experience in diecutting polycarbonate, polyester and capton. We are helping tooling fabrications push the outer limits of steel rule die technology every day.

Little by little, these exotic plastics began replacing metal and fiber parts in critical component applications. And where hard tooling had been holding tolerances of better than a thousandth of an inch, we were now forced to produce dies with tolerances these fabricators could live with. They wanted these lighter, tougher plastics in their products; they wanted to lower their tooling costs, but they didn't want to give up their plus or minus one thousandth. Our job... to convince them they didn't need it, just so we could sell a steel rule die.

I remember exploring the possibility of substituting steel rule dies for hard tooling with a very demanding customer. He could not see how an image drawn on a block of wood, cut with a jig saw and steel rule bent with tools hardly more sophisticated than his SIGNODE strapping equipment could give him a die that would rival his $12,000 hard tool. I told him it couldn't, but when he got a look at our $300,000 laser, our CAD-CAM computers and our pneumatic bending equipment, he wanted to know .... WHY NOT? I said we were hard pressed to hold plus or minus 5 thousandths. He said he could drive an armored car through those kinds of tolerances. He wanted hard tooling tolerances at steel rule die prices. This man was from a vacuum cleaner company and he wanted the labels on his machines to have the tightest tolerances in the industry.

We are all faced with what is commonly called "over-engineering." Granted, there are applications where the part of the die is becoming an integral component in the medical, mechanical or aerospace product. That plus or minus a thou may indeed be very critical, and the decision must be made on just how effectively the steel rule die part is competing with the machined or hard tool blanked part. But where the function of the part may be merely decorative or informative, design engineers should take a second look at their requirement for critical tolerancing.

In just about every case, it is the part of the die and not the die itself that comes under intense scrutiny. This is as true with folding cartons as it is with a gasket or membrane switch. That's why we as diemakers have to be fully informed about the materials our diecutting customers are using in the process. In cartons, we know the integrity of the crease can very well determine how the carton will set up on the gluers or filling machines. Therefore SBS vs. recycled board may affect the type of creasing rule or the way we fabricate the counter in our construction of the die.

A fiber or rubber gasket material will suffer compression and distortion upon impact from the diecutting process and will be diecut in its distorted state. Once it is allowed to spring back to its original state, you may have a part that anything but resembles the steel rule die.

But most notorious are the plastics. Here are some considerations:

1. Close tolerance dimensioning comes more into play with plastics because they are most often replacing a hard tooled or machined part, so their application becomes more critical
   
   
2. The chemical composition of plastic sheeting is such that it is impossible to infinitely control all the variables when molding, and as a result the degree of stability can vary from sheet to sheet.
   
   
3. Plastics are often laminates, which mean the different layers of material react different against one another when exposed to temperature change or diecutting.
   
   
4. Plastics are often coated with various adhesives or abrasive inks that can greatly limit the life of the die and produce a "squishier" package to cut into.
   
   
5. As with most materials, the thicker the package, the more the diecutting problems begin to mount.

Stan-Allen has made a science of studying the effects diecutting plastics for over 25 years. We would like to think we have experienced just about every problem a plastics fabricator can encounter. Unfortunately, such is not the case. New problems crop up on a weekly basis.

We have actually seen instances where a part cut .015" oversize. So we compensated for that effect by building the die undersize. The customer experienced parts that were .015" undersize. That's a swing of .030" in our different experiences. Factors involved? The customer changed plastic sheet suppliers, not the actual material. He also has a different press than ours.

One thing is certain.... customers are interested in the final part, not the die. In many cases, we cut a diecut sample off the die that goes through our QC inspection procedure, along with the die for workmanship. We, as diemakers must face the reality that our responsibility may be forced to reach for coverage of the part, as well as the die. In other words, we may be asked to be responsible for factors and conditions well out of our control.

Plastics are tough. I mentioned one that can stop a bullet. That's a fact. G.E.'s Lexan has actually been used in bulletproof vests. They haven't asked us to build a die to cut material that thick YET, but .010 mil Lexan is commonly cut with steel rule dies. It's when you get up around 15 and 20 mil polycarbonate (which incidentally is the generic name for Lexan) that you start heading for BIG trouble with your diecutting process. Don't expect your part to look anything like your die when cutting these materials.

Let's look at a few of the tips we have learned over the years in building dies for these plastic materials. (See Figure 1)



Figure 1

First of all, polyester (vinyl) is equally as tough, if not tougher than polycarbonate. Polyester is more tinsel than other materials. It actually shatters like glass upon impact. That is why you can always tell when someone is cutting polyester because you will hear a "crack" upon impact. The die is merely scoring the surface. The die is stopped dead in its tracks due to the density of the polyester. But the surface impact has caused the rest of the "cut" to crack clean through, much in the fashion that you cut glass. The thicker the polyester, the more cracking that has to take place. As a result, the edge quality of polyester greater than .010 tends to degrade when examined under a glass, simply because it was "broken" away instead of diecut. Your tonnage is less in cutting polyester since you are scoring the surface. For lower quantities of pure polyester material, we recommend a shave edge cutting rule. However if there are any abrasive materials present, or if the number of parts is substantial, shaved edge rule will not stand up, and your edge quality of the cut will greatly diminish.

Let's look at polycarbonate, the bulletproof material. It's a much different substance. I'm not a chemist, so I can't tell you its composition, but I do know that both the G.E. engineering staff and our company were initially tearing our hair out over diecutting this stuff. Because of its silk-screenability, compatibility to various adhesives and other safety workability considerations, polycarbonate has become quite popular for producing a variety of components today.

Polycarbonate doesn't shatter as polyester will on impact. It will, however, also stop a steel rule die in its tracks. We have found chemical reaction actually takes place when you violate the material with a cutting or creasing rule. It actually becomes pliable and gummy for a micro-instant, reacting to the heat generated by the cutting action. As a result, you have to fully penetrate the material, unlike polyester. We often recommend a long bevel and a ground edge rule. Polycarbonate reacts to a saw tooth cut. Ground edge rule has tiny surface serrations, that tend to profuse as the rule breaks down. Edge quality actually improves after the die has been run several hundred impressions, and then may degrade after several thousand due to just plane dullness. The long bevel rule tends to overcome the resistance to penetration, making the "snow-plow" effect less a deterrent to the diecutting process.

While the steel cutting rule is attempting to push the polycarbonate aside during the cutting process, the polycarb is pushing back. Since the stuff is tough, it easily forces the rules resulting in out-of-spec and distorted parts. (See Figure 2A)



Figure 2A-2B

One method to address this problem is to do all your pushing toward the waste. This is accomplished by using side face rule with the bevel toward the waste. The tiny 3-7 thousandths bevel on the flush side usually produces an acceptable part with very little edge draft. We have found "flush bevel" (i.e. no bevel on the flush side) to be very ineffective due to the fragility of the cutting edge. Polycarb is so tough, it curls the edge over after very little use. If there is any abrasive materials present, forget it. (See Figure 2B)

Often when the two bevels oppose one another in the waste area, you have so much pressure created that you not only distort the rule in the die, but cause a "crimp" in the part. The bevels are forcing the material toward itself with no place to go but up... thus the crimp. Since the best approach is to hold the sheet as flat as possible during the diecutting process, we have instituted several techniques. One in to create a rigid ejector plate that sits up on top of the die and is spring loaded by utilizing ejection rubber underneath. The plate is often made of 1/8" acrylic and is carefully laser-cut to clear all of the knife edges of the die. The plate is the first thing to contact the polycarbonate sheet, and it holds it absolutely flat as the rule penetrates the material. This process greatly limits crimping. (See Figure 2C)

Wherever tiny ribs appear in the diecut part, they are susceptible to twisting, crimping and distorting. In this case we insert thin strips of flat bar stock, often upside-down creasing rule and spring load that with rubber ejectors also. The steel bars hold the polycarb in place during diecutting of the ribs. As a result, with no place for the crimp to occur, a greatly improved part is the outcome. (See Figure 3)



Figure 3

We have resisted welded joints. It draws out the temper, and there is often no way to get at the area to be welded. We will sometimes laser-cut a block area out on either side of the joint and insert a steel block to stabilize the joint area. We also may run a piece of flat stock low creasing rule, about 6 points wide, into the area at a perpendicular angle. We will "lock" that reinforcement rule from moving with "T" or "J" rules. (See Figure 4)



Figure 4

My final technique is one you are all quite familiar with the use of 7/8" dieboard instead of the traditional 5/8" or 3/4". This greatly limits the flexing that occurs when 3/16" or greater portion of the cutting rule is exposed; the knife has much less a chance to move while cutting the "tough stuff'. However you now have cut down your ejection power by allowing space for only 1/16" rubber to be installed. There are trade-offs in every approach.

The cutting of these exotic materials has brought our ingenuity to new heights. Although we have a basketful of techniques to employ, it seems customers will always come up with a new challenge, or a diecutting operation will defy all the rules by reacting entirely differently to a situation we thought we had down pat.

The bottom line is there are no rules. We will keep pushing the outer limits of steel rule die usage because, after all, it is one of the most economical approaches to fabrication since man invented the axe. I guess that's why we love our business. I guess that's why we love the challenge. And I guess that's why we positively respond to the customer who appears at our door with the wondering question: Can we cut this with a steel rule die?

Do you have questions about how to cut difficult materials? Call the IADD for technical assistance at 1-815-455-7519. One of your most valuable member benefits is your ability to get solutions to your diecutting problems.