Die Cutting Presses / Finding the Right Press

Written By Mark Batson Baril

Developing The Best Cutting Method For A High Volume Product - A Case Study

An engineering team for a large automotive subcontractor needs to develop a best method to produce parts in large quantities. Here are the details of that project and hopefully some answers that will swing them in the right direction.

• Total yearly volume - ramping from 2 million now to 16.5 million parts per year within 2 ½ years.
• 20 different but similar images to be cut. Possibly going to 40 within three + years.
• Images range from 12" x 14" (305mm x 356mm) at their smallest to 17" x 20" (432mm x 508mm) at the largest. Images are rectangular with radius corners. Some of the images have 1 - 3 simple interior cutouts that must be stripped. Tolerances are ±.060" (±1.53mm).
• Material is .015" (.381mm) Polyurethane
• Material typically comes 60" (1,524mm) Wide X .015"(.381mm) X long rolls.
• Raw material and cut parts have an unlimited shelf life.
• Cutting and finishing operations will take place in Mexico.

A numbers break-down looks like this:

16,500,000 parts per year / 20 images = 825,000 parts per image per year.

That's 68,750 parts per month of each image.

If the factory works 20 days per month they will need to cut a total of 68,750 parts per day.

Some of the basic needs include:

The process must be fast

Easy change-overs / set-ups

Material yields must be excellent for as little waste as possible.

Cut quality must be good. The final assembly process, after the cut, is forgiving of some quality issues arising out of production compromises.

And here's where the fun starts! What's the best method to cut these parts? Given the large quantity of parts, the large number of different images, and the material to cut, we should look at three different cutting methods. Flatbed diecutting, rotary diecutting and digital diecutting. The production and pricing numbers are ballpark estimates but are close enough to make a good comparison. For the sake of comparison I have also used an average sized part of 16" x 16" (406mm x 406mm) with an 18" (457mm) repeat in both directions.

Flatbed Diecutting:
Because of the roll goods, the very wide material width, the quick change over needs, plus the quantity of images and the easy cutting material, I automatically lean toward a steel rule die type set-up cutting against a hard plastic cutting surface. A belt drive system would allow this fairly floppy and stretchable material to feed well into the press and would allow the final parts to flow off the machine for stripping and/or packaging. Multiple layer feeding and cutting are possible, especially against a specialty belt material. By using a CNC controlled belt feed and cutting head system on some type of wide beam press, fantastic yields can be achieved through the use of the entire 60" (1,524mm) width of material. A production rate of 50 impressions per minute is a conservative enough number to use for comparison. A multiple up tool or full bed beam press may improve the numbers. At this rate one machine would need to run three shifts per day twenty days per month in order to keep up with the volume (50 per minute x 60 minutes per hour x 3 shifts or 24hrs = 72,000 parts). Some pressure could be taken off by adding a second machine or experimenting with multiple layer cutting. My best estimate is that this material could be fed and cut in layers of at least 3 deep, reducing the cutting to one machine on only one shift per day.

CNC / Steel Rule Die / Belt Feed Diecutting: Finished part tolerances ±.015" (±.381mm) ; Typical tooling cost for a one or two on die that moves with the head $200 - $400 USD; Capital expense for press/feeds $125,000 USD; Belt maintenance/etc…. $10,000 USD yearly; Tool life 100,000 + impressions; Tooling change-over time is 20 minutes.

Rotary Diecutting:
Because of the large quantity of parts to cut and the possibility of larger volumes after the initial three years, we must take into consideration the fast process of rotary diecutting.

Both Soft anvil and Hard Anvil cutting are options. In soft anvil you cut against a hard plastic blanket somewhat the same as the belt talked about in the flatbed diecutting above. In soft anvil cutting an inexpensive steel rule type rotary die is used. In hard anvil cutting you cut against a steel cutting cylinder with a solid steel machined rotary die. Both methods achieve very fast running times ranging from 75 to 150 feet per minute on a project/material like this. The major differences are trade-offs between quality and costs of tooling and machinery. Soft anvil cutting will typically produce a less accurate part than hard anvil. Soft anvil will typically be the least expensive route to take.

Given all the parameters of this project the following information is applicable.

Soft Anvil Diecutting: Finished part tolerances ±.060"(±1.52mm) ; Typical tooling cost for 4 - 6 on full width rotary die $1,500.00 USD; Capital expense for press/feeds $125,000 USD; Belt maintenance/etc…. $10,000 USD yearly; Tool life 100,000 + impressions; Tooling change-over time is 20 minutes.

Hard Anvil Diecutting: Finished part tolerances ±.010"(±.254mm) ; Typical tooling cost $25,000.00 USD in 60" width - approx $3,000 - $6,000 USD in 18" width; Capital expense for press/feeds $300,000 USD in 60" width - $75,000 USD in 18" width; Tool life 1,000,000 + impressions; Tooling change-over time is 20 minutes.

For both types of rotary cutting, a production rate of 100 feet per minute should be a conservative enough number to use. At this rate, on a 60" wide machine, one machine would need to run one shift per day fourteen days per month in order to keep up with the volume (assuming a 16" x 16" part running three across the web at 100 feet per minute x 60 minutes per hour x 1 shift or 8hrs = 96,000 parts). More pressure could be taken off the machine schedule by experimenting with multiple layer cutting.

In the hard anvil cutting the 60" width becomes a major hang-up due to the cost of the tooling and the cost of the capital equipment. The tool handling also becomes a factor as these monster tools are heavy! The web width could be reduced in both the machinery and tooling as well as the material but some yield compromises would have to be made and the production volume would be cut proportionally. If you had 16,000,000 of the same part this method would certainly be more attractive.

Digital Diecutting:
For this project I am including laser cutting, waterjet cutting and knife cutting within the digital diecutting areas of production. All are quite capable of doing a nice job on this material in the material width, within tight tolerances with excellent edge cutting results. With our average part having 70" of cutting (16" x 4 plus an internal cutout), our yearly volume of 16,500,000 parts would have total cutting inches of 1,155,000,000 (yes that's billion) inches. At 200 inches per minute, a good average for digital cutting, there is 96,250 hours of cutting. One year for one shift is 2,080 hours so we would need 16 machines running 3 shifts to keep up with the volume. Cut at four times that speed with a multiple head machine , or common cut as much as possible and you still need alot of machine time to get through the year. The advantage is that there are no tooling costs and set-up time is just about zero. The disadvantage is that the process can't keep up and be cost effective. Once again we run into the large volume fact that nothing beats diecutting for speed!

All In All:
Given all the factors discussed, the best route to take is to pursue both the flatbed diecutting and the soft anvil rotary diecutting. There are several manufacturers that would be willing to run real tests on real machinery in order to qualify the processing speeds and the quality of the cut. The other major factor to be tested on press is stripping of the internal waste pieces. Depending on the size and location of these cutouts, the rotary press may have the advantage over the flatbed process. The total cost for this type of testing should be limited to your supplied material, the applicable tooling, and an agreed upon fair hourly rate.

Calculating Die Cutting Tonnage Continued...

Let’s Get Really Technical:
 
A couple of us have actually talked about developing an on-line tonnage calculating website. It would be comprised of a database that held values and asked questions like; Strength values (Tensile) for most common materials (A), Shear strength values for several processes/rule types/ejection, etc…(B), Number of inches being cut (C), Thickness of the material (D). A X B X C X D = Tons 2,000

In fact, this is exactly how many software stress analysis programs work. They take a set of very obvious variables and make a simple calculation based on these (and other) numbers. It gives you a very consistent way of looking at every project you take on. Right or wrong, the answer is a base number to start with, and that is what we have gathered is the trick to determining proper starting tonnage numbers. Once you have this standard formula in place and trust that it will give you that base number, you can then depend on it and translate it to work in different machinery on your shop floor. Perhaps you have a string of ten punch presses and they all cut a little different. One is hydraulic, one is pneumatic, one is mechanical off a simple small cam while another throws off a giant flywheel that was welded back together by Uncle Joe a few years back. They all cut differently but they all have a factor you can use as a multiplier against that base number we just calculated out. It’s beautifully simple really, it just takes some time to develop and work out in your own shop, on your own equipment. Once you have that number, everyone can plan around the equipment you have vs. the projects you have with more confidence.

So then the formula may look like this;
(A X B X C X D) F = Tons (Where F is a press factor based on experience and/or a manufacturers guidelines.) 2,000 Putting this into a real life situation may look something like this; I have a ten up steel rule die cutting and creasing .018” paperboard. There are 1,000 total inches of cutting, creasing, stripping, support knives etc… I am using modern ejection materials. I am cutting on a platen style press.
17000(A) X 1(B) X 1,000(C) X .018(D) X 1(F) = 153 Tons. 2,000
Simple Formulas from above; C/6.5 = Tons (1,000 / 6.5 = 153.8 Tons) or (C X 400) / 2,000 = Tons (1,000 X 400) / 2,000 = 200 Tons

Both formulas work and give us a range that is safe and a good starting point.

Now Let’s Get Really Simple:
 
What seems to happen with all this fancy calculating in real world situations is that the base theory gets boiled down to simple formulas that work for similar situations. Most of us deal in very similar tooling and materials everyday and having a very fast and simple way of coming up with a safe base number is natural. If you are always working in paperboard in about the same caliper, taking the total periphery and dividing by a single proven number is a fantastic way to approach tonnage calculating. The same goes for plastic, steel, leather, or anything else you cut on a regular basis.

So, this article is not going to give a catch-all formula for determining tonnage for all materials on all press types, with all tools, because there are too many factors involved and nobody would ever use it in real life. What we can do is offer a base calculation where you plug in your own numbers based on experience. Your own situation will provide the best formula for you.

That base calculation would look like this;
Total Periphery to Convert X Material Factor / 2,000 = Tons Needed

Developing a living chart of MATERIAL FACTORS will then be the key to making this work in your business. We’ve been using paperboard a good deal in our discussions and it seems that a starting point for a folding carton manufacturer on a Flatbed style press would be a MATERIAL FACTOR of 300. The heavier gauge the material is the bigger the Material Factor. (1,000 Inches X 300) / 2,000 = 150 Tons Keep in mind that if your cutting process changes, maybe it’s as simple as going to a harder rubber or steeper bevel rule, you will have to use a multiplier to compensate for this change.

There is no trick of the trade in calculating the tonnage you need for a project but as you develop a more and more sophisticated list of materials and how they process on your equipment, you will have an estimating and production tool that will help you predict with greater accuracy how well a job will run, where it should run, how many up it can run, and whether or not it will run at all. You will have a leg up on the competition that is still shooting from the hip and this will really put the pressure on them…..

We’d like to thank all of the operators out there that are trying to improve their production techniques and came to The TECHTEAM with their questions!