Calculating Die Cutting Tonnage

Compiled & Written by The IADD TECHTEAM

Knowing How Much Pressure It Will Take To Convert Your Product Is One More Key To Success
 
Here on the TECHTEAM we sometimes see the same question more than once. The question of “how much tonnage will it take to cut this product?” is the most common, and also one of the hardest to answer because of the complexities involved in calculating a perfect number. In fact the question should probably be, “is it possible to calculate exactly the tonnage it will take to cut a specific product, in a specific material, on a specific press?” This article outlines the basic theory of calculating tonnage needed to cut and will give most readers a base starting point they can feel comfortable with for their business.

Here’s my problem with the idea of tonnage. It’s an intangible sort of a concept that really has no place at all in my business. It seems to change and move and act quite slippery and I really just don’t think I need to deal with it. That is true until I find out that the machine I just planned a job for in a very specific factory, on a very specific machine, on a very specific tool, in a very specific material, will not run because the machine doesn’t have the guts to get the job done. Then it becomes a factor that I should have paid allot more attention to right from the get go.

For most of us, the machines we plan around have all the tonnage we’ll ever need for 99% of the jobs we will ever see. I’d bet most estimators never even look at this factor when they plan a job. For many diecutters the advantage of knowing how to accurately calculate tonnage needed makes the biggest difference in faster make-readies, and that alone is well worth the effort of knowing how to at least get a good starting number. For some of us it will only make a difference when we are planning an usual job that needs to run perfectly the first time around, and that alone is also worth knowing exactly what you are talking about.

So, where on Earth do we start? How about a bit of theory? The tonnage needed to cut/emboss a material is strictly a function of the strength of the material, the shear power of the process being used to cut, the size of the image to cut, and other factors like flex in the machinery, ejection materials, air pressure build-up’s, etc… That being the case then we should be able to use a simple formula that says – This is the amount of image I have to cut/crease/emboss/etc. that has value A. This is how thick the material is and it has value B. This is the material that I am cutting and it has value C. Calculate them all out and I get an exact number that works every time. Answer D.

Straight from the from Machinery's Handbook 25th ED. P1924 “P = periphery x thickness x tensile strength(PSI) where P is cutting force in pounds.” That was easy! And in fact for many cases it may be a number that is good enough to get started. The problem lies in the fact that there are many different processes and techniques for cutting and each holds a different value that must be factored in, and there must be a starting value for the material you are cutting.

Over the past year we’ve been answering tonnage calculating questions and it’s really interesting to take a look at all the formulas being used out there. Here’s a typical and really great question. "Thanks for taking my call. Here is my question. I need to be able to figure if my die cutting press has enough tonnage for different jobs that we produce. We die cut anything from 10 pt to 100 pt. paper in different shapes and sizes. Sometimes they are 1 up, sometimes they are 100 up. Is there a formula that I can use to determine if I can run a job 2 up as oppose to 5 or 6 up? Something like 1 ton of press is equal to 6 in of cutting rule when cutting 18 pt. and 5 in of rule when cutting 24 pt. This would greatly help when trying to quote jobs knowing that I can't die cut as many up as I print. Thanks in advance for your help.”

Here’s a sampling of formulas and answers that worked for this and other applications;

• 6.5 inches of cut rule with ejection = 1 ton pressure This should be good for paper up to .030 thick for thickness .031 - .050 reduce the inch count by 10% for thickness .051 - .070 reduce the inch count by by 20% for thickness .071 - .100 reduce the inch count by by 30%
• Total inches x thickness x (some material strength factor you know to be correct) /2,000 is one way to get total tonnage needed.
• For each inch of rule, you will need 300 lbs of pressure and for each square inch of rubber; you will need approximately 50 lbs of pressure. So if a die has 1000 inches of cutting rule and is rubbered in strips of ½” wide on each side of the knife, it will calculate to 1000 x 300 and 1000 x 50 or 350,000 lbs of pressure or 175 tons.
• We use a factor of 400 pounds per linear inch of rule up to 500 pound per inch depending upon material being cut. Paper is 400 pounds. Ejection rubber can add up to 20% more pressure needed, depending upon amount of rubber in die.
• I have looked over the information you supplied me and I feel fairly confident your 50 ton press would be able to cut your material to the size and # up you are considering. The most basic formula many of us in the industry use is 6.5 inches of cut knife requires 1 ton of cutting pressure to cut and eject a paper sheet .018 thick. Considering other factors such as the die area, serrated rule and the fact that I feel that a press can be called upon from time to time to cut up to 110% of its tonnage rating. If I didn't know for sure that it did not have the ability to cut to this pressure I would take a chance and build this die and use it as a "benchmark" as what the press is capable of cutting. If you would like me to go into further detail please contact me.

Medical Device Tooling For Diecutting

Written By Mark Batson Baril

More than once in the last month, the question has been put forth as to how to produce a good steel rule type die that will be used to cut a disposable medical device. There are many different types of medical devices. The ones we are talking about here may be produced in clean room conditions but are more likely required to be produced in clean areas that have very little contamination allowed. These medical devices may be used internally and almost always come in contact with the body. The parts may be sterilized after they are produced but they are expected to pick up as few extra particles as possible during all phases of production. In some cases the tooling is used in production of bio medical devices/products where it is important to the product to be exposed to as little extra material as possible during processing.

The make-up of the tooling seems quite simple until you start to research the methods and materials needed to produce a die that will fit the following set of conditions:

-Will not rust even if exposed to alcohol, water, and other nasty chemicals (we consider water to be nasty because it causes rust)

-Will not flake or shed material, including steel, wood, rubber, plastic, etc…

-Will enable cleaning of the tool to remove glues, hydrogels, foam residues, etc…

-Will be accurate, reproducible, long lasting, fast to produce, and of course inexpensive

-Will not crack or loosen during production runs.

Considering all of the above - the tool should be made of some type of plastic base with stainless steel rule/punches and a non-shedding ejection material. Here's what we found in each case:

Base Materials:
Acrylic is the clear plastic base material of choice for many diemakers. The main reasons are that it is clear, readily available, and it cuts very well on a laser. The main drawbacks for medical are that it is not FDA*(US Food and Drug Administration) approved and it cracks easily under the stress of diecutting. This material is not a great choice for medical dies.

Polycarbonate (common trade name is Lexan) is another common choice. It is clear, easily found, and resists cracking very well. It is 30 times as strong as Acrylic. It's two main drawbacks for medical dies are that it cannot be cut on the laser (thin polycarbonate can be cut on the laser, while ½" to ¾" prove to be almost impossible) and it is not FDA approved. If your tools must be clear (see through) this is probably you best choice.

High Density Polyethylene, Low Density Polyethylene, ABS, PVC and PETG are also commonly available base materials that are tempting to use. None of them cut well on the laser and none of them are approved for use by the FDA. We see no advantage to considering any of them unless you need to think about electrical properties and static.

UHMW-PE (Ultra High Molecular Weight Polyethylene), Nylon, and Delrin are all readily available, and are FDA approved. None of them cut well on the laser but we highly recommend all of them for use in die bases. They are very impact resistant, chemical resistant, machine well, and come in White which really looks great when you are making a medical die. This is your best choice.

The problem with this best choice for base materials is that the laser is not a great way to work the material to the shape you want. The material can be jigged well and can be machined well which leaves us with quite a few production options. Most die shops will have a jig saw at their disposal and can produce their tool as it was done before lasers. Most also have some type of drill press or milling machine that will allow for simple shapes like holes to be cut to receive punches, washer sets, and specialty punches. All of us have at our reach the ability to outsource a specialty base like this to a machine shop equipped with CNC machining capabilities. The base can be machined in two pieces or more, in order to build just about any shape imagined. Offsets can be built in to receive rule or punches and the tool is built without bridges. Every project will be different, but there are very few limits that can be placed on a die when we combine the methods that are available to accurately machine plastics. Keep in mind one of the advantages of the old methods of producing steel rule dies (non-laser) is that the kerf is very consistent from top to bottom. There are typically no voids or pockets left to collect any of the fluids that the tooling may see for medical production clean-up and the top and bottom only grabbing we see from a laser kerf is replaced by a tight non-moving match of rule to base.

When it comes right down to it, if you can use a tool that has no base material, you will be best off. Forged tooling type dies are a great choice in this area.

Blade Material/Punches:
All the rules that we commonly use for steel rule dies will rust. They have coatings (usually oils) that stop them from rusting in the box, but once they're in the die and the oil wears off, they are going to rust, especially if you wash them with water. We have one customer who washes their tooling by putting it in a bucket of water (FDA Approved of course) and then scrubs it down. The other thing that we need to avoid in the medical field is delivering a tool that has oil on it to start with.

So there are a couple of choices to make. One is to produce the tool using a rust resistant steel, the other is to plate or cover the regular tool steel with a rust resistant coating.

Stainless Steel is the best thing to use for both rule and any type of punch. It is expensive, hard to find anyone who wants to work with, takes a long time to get through the machining process, and is hard to bend and work with once it is heat treated. 400 series stainless can be machined well, is heat treatable so it can be brought to the hardness needed for big volume diecutting, and it is still rust resistant. We accomplish everything we need except quick turn-around times for tooling and it is not cheap to make. 300 series is more common for diemakers to use in that it does not have to be heat treated, and is bendable. It is not as hard as a 400 series but it will withstand many impressions in many materials and is actually more rust resistant than the 400 series. 303 and 304 are more common than 316 and are use commonly for cutting medical products. The 316 series steel designation shows up as "the best" steel to use for medical devices and this is true for implanted devices that will be in the body for more than 30 days. For standard cutting the 303 and 304 work well.

Coatings are a great way to go if you have the right coating. The one major drawback that all coatings can have is possible flaking or wearing off. Especially in the medical field where non-contamination is key, the concerns with coating steel used for cutting is real. However, if you can create the correct coating the results can be no worse than the normal wearing off of steel you get from the typical steel rule, punch, or even stainless steel rule. We have found that Electroless Nickel Coating with an after coating heat treating, works well. This process adds .0002 (.00508mm) of rust resistant material that wears at close to the same rate as the steel it is covering. Relative to having the parts made in stainless it can be much less expensive, and it is a fairly common and fast process to get through. Make sure you deal with a company that can not only specify what they are doing for you, but can also provide a certificate that says that's what they did. Coating shops are a dime a dozen. Good ones are worth their weight in …. Electroless Nickel.

Ejection Materials:
Typical materials we use in the steel rule die industry for ejection will naturally wear and start to shed material after a certain number of impressions. That number will vary with every project and every press being used. Some of the tricks used for ejection for medical include the following:


-Don't use it! Yes, creating holes in the tool or some other way of ejecting the part is the best way to avoid contamination.

-Use Waterjet cutting to produce the rubber shape. This eliminates that first round of debris you may have from pressing the rubber into the tool or using another type of cutter to cut the rubber.

-Use a top coating or some type of sheet plastic material. This layer stays on top of the rubber and not only helps the top surface of the rubber last longer but also stops any debris from touching the product. I have seen regular old fiberglass reinforced packing tape work well for this.

-Use springs or even flat top ejection plates were you can within the tool. Make these out of stainless or have them coated.

-Consult with your rubber supplier on their best type of material that will give you the push you need and the lack of shedding that your customer requires.


Putting it All Together:

The one last key ingredient to make part of your system of making your medical dies work well is to train the final user to replace their tools on a regular basis. Base materials will wear and get contaminated. Steel used for cutting will wear, flake, and stop cutting well. Ejection will eventually stop ejecting and start to break down. Finding that breaking point in the tools productive life is probably best left to the operator. Telling the operator that a breaking point exists is up to the tool maker.

Tools for the medical industry can be tough to manufacture. I have met many die makers that tend to turn down this type of work. I have also met a few that like this type of work because it tends to pay very well and can be rewarding from a technical standpoint. I have found that it is very possible to meet all the parameters found in the first part of this article except - fast and inexpensive. I Hope this passes on a few tricks of the trade and helps you to develop new ideas on producing or buying better tools.

* The FDA recognizes certain materials as being OK to use for contact with food products during production. Although there probably is a special designation for base materials for cutting tools that the FDA sees as OK for producing medical devices, we have not been able to find it yet. We have always gone with the assumption that if it works for food it works for medical dies. Most medical manufacturers we have dealt with seem to run on the same assumption.

Please contact Cut Smart if you would like more information on this subject.