Diecutting Plastics

We are a small custom diecut house, about to enter our 67th year in business. Through the years, we have diecut parts for many industries including defense, pharmaceuticals, electronics, automobiles, recreation, point of purchase, and OEMs. The main focus of our diecutting now is in the display industry, specifically point of purchase. We specialize in short run and quick turn around for this industry. Being the diecutter, which is typically the last step before packing, most of our jobs are already late even before we receive the material. This tends to force you in a position of quick turnarounds. Our diecutting is accomplished on platen diecutters, using the steel to steel method, or "onto" diecutting, 99% of the time. We currently do not have any heaters on our diecutters. We also have punch presses using male and female tooling and also combination dies.

This article will cover diecutting plastics, specifically plastics from .060 to .125 and thicker. I am going to discuss some diecutting theory, specific types of plastics and how they react to being diecut, die construction, problems and some solutions we have used.

Platen diecutting thicker materials is all about displacement. Let us look at the difference between shearing and punching, versus diecutting. In shearing and punching, the tooling and the slug removed are identical in size. [Diagrams 1A & 1B]



Diagram 1A



Diagram 1B

There is no lateral stress on the material being punched or sheared. This lack of lateral stress allows the material cut to have straight sides with little or no bevel. The bottom edge will also be very clean. Accuracy is also attained due to the lack of lateral stress. If one wants to punch a quarter inch hole, one would use a quarter inch punch. If money and time were not an issue, this would be the preferred way to diecut parts. As we all know, hard tooling is very expensive and requires a long lead time.

Our option then is steel rule diecutting. Let me read you a statement which I got from a recent article in Kevin Carey's publication "The Diemaker's Resource":

"Platen die cutting is a combination of vertical pinching pressure which requires less than 30% of the total pressure expended, and the displacement action, or the lateral push from the bevels on each side of the knife edge, which requires more than 70% of the total pressure expended."

Pinching pressure and displacement action—As you attempt to diecut thicker and thicker material, this displacement action becomes greater and greater. [Diagram 2, p. 21] Attempting to control this lateral displacement force is the challenge of platen diecutting thicker materials.

I have categorized some plastics into two groups: those plastics that are easily diecut, and those materials that resist diecutting.

Vinyls, low and high density polyethylenes, delrin, polycarbonates (lexan) and expanded PVC (sintra, celtec or trovecel). These materials tend to let the knife edge reach the cutting plate. The material stretches away from the knife edge. These materials tend to be "soft" or pliable. This quality allows for the ease of diecutting.

This pliability is not as prevalent as we move to more rigid plastics. These would include ABS, Kydex and high impact styrene. These materials tend to let the knife edge enter only two thirds of the way into the material. At this point, the lateral force built up by the rule penetrating the material overcomes the strength of the material, and the bottom third of the material bursts apart.

This bursting gives us a rough bottom edge, and can make the diecut parts jump out of the sheet.

In diecutting the softer materials, the diecutting environment tends to be relatively calm. The knife penetrates the material and is easily retracted due to the material displacing the width of the knife. These materials, the vinyls and the high density polyethylenes tend to allow a great deal of control during the diecutting process.

Rigid materials do not react in this calm environment. The bursting of the bottom third of the sheet is a violent reaction. This bursting is not a consistent event, which makes controlling the sheet to be diecut more difficult.

The objective in diecutting thicker rigid plastics is to successfully attempt to control the lateral forces caused by the thickness of the rule. If one can reduce this violent environment to one that is calmer, one can achieve a controllable environment which will lead to a desirable result.

DIE CONSTRUCTION

Wood

In die cutting material up to .090 thick, the conventional die construction can be used. We typically will use 5/8" wood and .937 rule. As you attempt to die cut material thicker than .100, you must allow more clearance for the material and the ejection rubber. You can accomplish this two different ways. One way is to use 9/16" wood and .937 rule. The other way is to use the regular 5/8" wood, but use 1" high rule. Both of these methods will allow an additional 1/16" clearance. If you attempt to diecut material thicker than .125, say .150 or .188, it would be wise to use 5/8" wood and 1-1/8" to 1-1/4" rule.

As you attempt to diecut thicker and thicker material, you need more clearance between the knife edge and the top of the dieboard. [Diagram 3] A note of caution: as you increase the exposed portion of the knife, you begin to compromise the stability of the rule.



Diagram 3

Rule and Spacing

Because the displacement of the rule width becomes such a large factor, attempting to minimize this force is the goal. Two point rule can be used successfully up to .060 thick. If a die has numerous curves or a large radius, two point could be used up to .080 thick.

Dimensional stability is compromised when 2 point rule is used to diecut material thicker than .080. The 2 point rule tends to flex so much that all of the joints begin to open, or the rule breaks or bends. Once your material is thicker than .080, 3 point rule should be used. Using three point rule with a long bevel also helps to reduce the lateral displacement. Long side bevel is a good choice when attempting to reduce the beveled edge on the finished part. It also helps to keep the joint tight as the lateral force holds the rule in place. [Diagram 4]




Diagram 4

Spacing

Proper spacing of multiple cavity parts is very important. We typically like to allow at least 1/2" on the perimeter of parts over .080 thick. [Diagram 5, p. 23]




Diagram 5

In multiple cavity dies, we also try to maintain this 1/2" between parts. The reason for this is the stability of the sheet. If the perimeter is very narrow, it tends to break at the corners. This is very common when side bevel rule is used. All of the lateral force is taken by the frame, and it snaps at the corners. Similarly, if the web between the parts is too narrow, the parts do not remain in the sheet. The parts will then fall in the press as the platen begins to open. The rubbering of the die can help in some of these instances.

Ejection Rubber

The rubbering of the die is as important as the proper rule selection. The ability for a part to run or not, may lie in the ejection rubber. The ejection rubber is what controls the environment when the plastic is diecut. One may only think of ejection rubber as a means to eject the part after it is diecut. In diecutting thicker plastics, the ejection rubber is used to "hold" the part before and during the diecutting process. [Diagram 6]



Diagram 6

This holding of the material helps to control the bursting effect. The rubber also helps to keep the diecut part in the sheet. We have found that rubbering a die with small pieces is NOT an effective way to diecut thicker plastics. [Diagram 7]




Diagram 7

What works for us is full strips of rubber along the entire knife edge. A medium to high density rubber tends to hold the sheet better. We like to use cork/rubber most of the time. We will also full rubber the dies if the parts are small, or if there is multiple parts on one sheet.

One difference between high speed diecutting and slow speed platen diecutting, is the position of the rubber next to the knife. In high speed diecutting, there should be a gap between the rubber and the knife. This gap allows the rubber to compress and release more quickly. In platen diecutting thicker plastics, the rubber should be touching the knife. This close proximity of knife and rubber helps to "hold" the plastic as mentioned previously. Sometimes different densities of rubber can be used in the same die. A denser rubber may be used to hold the part in the sheet, while a softer rubber may be used to let the stress go in an area of waste.

We have been talking about lateral force or stress. As the rule penetrates the material, all of this displacement has to go somewhere. The object is to control this stress so it affects the waste, and not the finished piece. This stress usually shows up as a crease in the finished piece. With the proper rubbering, one can direct these forces to the areas desired. The stress will tend to go where the resistance is less. Use a softer rubber to attract this stress. [Diagram 8]



Diagram 8

Punches

Most times the desired hole size will determine the bevel on the punch. On the 1/16th will be center bevel, off the 1/16th will be inside or outside bevel. Center bevel punches distribute the force of the punch equally between the hole and the finished piece. Outside bevel punches distribute all of the lateral force away from the hole and onto the finished piece. This may cause a raised rim around the hole, which may or may not be acceptable. An inside bevel punch distributes all of the lateral force onto the slug. This on occasion with thicker plastics, traps the slug inside the punch. If the slug is trapped, and you attempt to diecut another piece, you will break the punch. We have found that spring ejectors in punches work well up to .060 thick. Above .060, plugging the bottom with wood and using cork or dense rubber works well. Sometimes on a large punch, a ring of neoprene rubber can be used to help the ejection rubber. One caution—too much rubber can cause a punch to prematurely fatigue. Side clean out punches are a very good option if you have enough clearance for the slugs to fall out.

Jig Versus Laser Dies

This next section will cause the diemakers to start booing or throwing things at me. The use of laser dies is increasing due to the demand for close tolerances, and the ability to make the cavities in multi-up dies all exactly the same. The problem with laser dies is that they do not hold the rule tight enough in the die board. The charred sides of the slots tend to let the rule pull out when attempting to diecut thicker plastics. Pulsed lasers do not seem to be much of an advantage. When you look at a slot from a pulsed laser, only the points are touching the rule. If you have to rerule the die, the points then become even less effective.

Jig dies seem to work the best. The roughness caused by the saw blade gives plenty of grip to the rule. My advice for all of the diemakers is to make the slots narrow so that the rule is tight!

PROBLEMS AND SOLUTIONS

Tolerances

Tolerances can be a problem when diecutting thicker plastics. The bevel on the perimeter tends to make the finished parts larger. The die can be made exactly 12", but when you cut a piece of .060 polycarbonate, the part will measure around 12.030. Similarly a hole made with a .250 punch will measure around .220.

As a general rule of thumb, a diecut plastic part using center bevel rule will increase in size half of the thickness of the material. A diecut hole using center bevel rule will decrease in size half of the thickness of the material.

When diecutting lexan or polycarbonate panels for equipment, the tolerances become very important. You should adjust the measurements on the die before you jig them.

Buckling

Some diecut parts will have stress creases in them. If one tries to rubber a die with small pieces, this will most likely occur. Rubbering with strips or full rubber can control this. A multi-density rubbering technique may be needed.

Rough Bottom Edge

This rough edge is caused by the sheet bursting before the knife cuts all of the way through. Sometimes holding the sheet with dense rubber is enough to give you an acceptable edge. Another trick to use is to add another piece of rule to the outside of the perimeter. [Diagram 9, p. 25] This piece of rule should be lower in height than the rest of the die. This holds the frame from bursting out, allowing the knife to penetrate further.



Diagram 9

Styrene

Styrene seems to be the preferred material for displays. It is strong, holds up well, silk screens well, and can be used inside or outside. The problem is that it only allows the knife to penetrate 2/3rds of the way through the material. A rough bottom edge is the normal effect.

The standard styrene sheet is called HIPS, or high impact polystyrene. This typical sheet has between 7 and 8 percent rubber in it. If one buys EHIPS, or extra high impact polystyrene, the rubber content is increased to 10 percent. This increased rubber content allows the knife to penetrate almost all of the way through the sheet. We have found using extra high impact polystyrene a very effective way to improve our edge quality on thick styrene parts.

Expanded PVC (Sintra)

The first time we cut sintra, we were experiencing a summer like the one this year in Texas. We had 30 some continuous days above 90 degrees in Philadelphia. We received 4 x 8 sheets of sintra for a new display. We guillotined them into press size pieces with no problem. We put the job on the diecutter, and it was like diecutting butter. This was the neatest material I had seen in years. It was thick, rigid, flat and allowed the knife to cut all of the way to the plate. A miracle material, I thought. Six months later in January, we got a reorder of the same part. We went to cut press size pieces in the guillotine. To my great disbelief, the sheet shattered like glass. Whoa, they must have sent the wrong material. Wrong. After calling our supplier, and then the manufacturer, we discovered that the optimum diecutting temperature is 90 degrees. So only cut sintra on days above 90 degrees! Actually heating this material slightly in winter radically improves the bottom edge quality. So be aware of the temperature when diecutting this material.

CONCLUSIONS

Diecutting thicker plastics is a viable option. One must remember the increased lateral forces built up by the rule. One must construct the die with these forces in mind:

1. Allow enough room for the material and ejection rubber.
   
   
2. Choose a dense rubber or use of multi-density.
   
   
3. Plug punches and use ejection rubber.
   
   
4. Jig the wood so that the rule is tight.
   
   

This material was adapted from a presentation at the IADD Diecutting Solutions seminar held in August 1998.