Clint A. Medlock, Jr., Stafford Cutting Dies, Indian Trail, NC, USA
Having been a diemaker for over 18 years, the one thing that puzzles me the most is the lack of true knowledge in our industry. Even though I learn more about diecutting every day, I am amazed at the amount that is not understood by the end user. I blame diemakers, including myself, for this misfortune. We have not educated the diecutting industry on the capabilities and limitations of their diecutters and our materials.
There are many misconceptions about the accuracy of rotary diecutting in the feed direction. I have attended seminars, read many articles, spent time on the machine and questioned operators. Machine manufacturers, as well as most industry consultants, tell you how to control the length of the blank through the machine. It has become greatly apparent to me that they need to spend more time at the diecutter than in their theoretical world.
Some may talk about surface speed, anvil wear and other assorted reasons for short or long blanks. One may say adding rubber will help, while another may blame improper or worn feed rolls. While all of these things theoretically may be true, no one wants to hear them, let alone accept them. Everyone wants more accuracy without changing their parameters.
I spent over three years tracking every rotary die we made, trying to find a pattern of these inaccuracies. This was one of the most enlightening projects I have undertaken. During each job, I recorded all information I believed could possibly affect the accuracy. It soon became clear there were certain things that did effect the length of the blank.
Let's establish a few facts. If all dies were made with the same knife height, same wood thickness, same blank size and diecut on the same press, the following are proven to be true.
Notice I said nothing of anvil wear, wet board or deep knife penetration of the cutting rule. While all affect the accuracy, they are beyond my realm of control and will be addressed in a later article. By the way, each of these will cause the blank to shorten.
If you have any doubts about what I am saying, please stop reading and go to your diecutter. Pull a blank off the machine and check it against your spec sheet. Look for a die that has a large void area, fed with corrugation, preferably doublewall. It will be short. If you are running an item with perforating rule from front to back, it will be long.
If you did measure a few blanks off the diecutter, I am sure your next reaction was, "I know my people did not accept this die from our diemaker being that far out of spec. How could this happen? It certainly does not match the proof we received from them."
It happened because you forced your diemaker to give you a proof that matched your spec sheet. Yes, it did come straight off their proofing machine. What you do not know is what was done to the proofer and blank to make it that size. Yes, there may have been some slight adjustments made. Don't contact your diemaker and accusing him of misleading you. He would be a fool to admit it. An honest fool but, nevertheless he would lose your respect and probably your business.
I have visited dieshops in all parts of the country as well as Japan and Europe. I have trained diemakers from Israel and England as well. One of many issues I always address is controlling accuracy. To my dismay, I hear more stories about getting by the customer's quality program than actually solving the problem. I have employed several diemakers from national companies as well as local companies and to my disappointment, I found all were guilty of this. Well, enough about the problem, let's go on to the solution.
To understand what can be done, you need to understand a little about the concept of diemaking. Though all diemakers may not have the same processes to cut the lines in their desired location, we all achieve their position by using some type of circumferential dimension. This is based on the diameter of the die drum, the height of the knife (or score) and the amount of penetration required to achieved complete cutting.
To keep things as simple as possible, let's use a 66" (1.676 m) diecutter. The diameter of the drum is 19-3/16" (975 mm). Let's say we are using .970 (24.606 mm) cutting rule and hopefully the operator will only penetrate the mat by .045 (11.05 mm). This will give us a running circumference of 66-1/16" (2.65 m). Theoretically one revolution should equal that distance. Using the difference between the running circumference and the circumference on top of the wood we can derive an acceptable shrink. Theoretically all blanks will cut the same, according to some machine manufacturers and industry consultants. Not so!
What really happens is during the rotation, if there is not a concentration of rule, the blank moves closer to the wood, creating a shorter distance from lead to trail. This is why void areas tend to shorten the blank, while perforating rule tends to lengthen it. It is understandable how the rule will hold the blank out further from the wood causing a longer distance around the cylinder. Most people know adding rubber strips to the die will lengthen the blank, however it does not increase the blank as much as perforating or a heavy concentration of cut rule.
Often you will see certain areas having a concentration of cut with another area having significant void spots. If you measure a blank more closely, any area with a lot of cutting rule will be long, while the void area remains short. This could cause your overall blank size to be correct. This happens mostly in poultry style boxes, RSC style with small flaps and long depths fed with corrugation. The flaps tend to be long, while the body is short. Sound familiar?
While corrugated has to be compressed to cut, the thicker the product, the further down into the rule it is pushed, causing a shorter distance from lead to trail, thus leaving us with shorter blanks. Feeding blanks across the flutes tends to increase the blank length. Since the flutes are going horizontal across the press, the blank conforms more to the curve, enabling it to stay further away from the wood. Feeding with the flutes makes a stiffer substrate and more difficult to conform.
With this understanding, the diemaker and the boxmaker have a general idea of how the blank is going to react. Someone has to accept the responsibility to make the change and then determine the amount. Either the diemaker has to change the way he builds his die or the designer has to allow for these inconsistencies. I can only dream the latter to come true, thus creating a diemaker's nightmare.
We have to allow for void areas, concentration of cut, board thickness, feed direction and all combinations of any of the above. With all that information, we take an educated guess at the circumference dimension, then physically add or subtract from certain areas in the die. The only science we have here is the ability to define the variables, weigh the consequences and take appropriate measures (or guesses) based on our experience.
After completing my research, I wrote a computer program that compiles the information I collected. The diemaker is prompted to answer several questions about the diecutter and blank, then it computes a circumferential dimension as the starting point. Next we may make necessary adjustments in each panel to better achieve dimensional accuracy.
The next step of this project will have the CAD system evaluating the blank, assign different shrinks for different areas and scale according. If you ask me today how accurate can we make dies, I can honestly say "Better than yesterday and as accurate as the realistic need of the end user."