Rotary Diecutting Accuracy Is it the Die or the Diecutter? Clint Medlock, Stafford Cutting Dies, Inc., Indian Trail, NC, USA

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1 Rotary Diecutting Accuracy Is it the Die or the Diecutter? Clint Medlock, Stafford Cutting Dies, Inc., Indian Trail, NC, USA With my 35 years of rotary corrugated diemaking experience, you would think I would have learned how to make an accurate rotary cutting die. How many times have you been told your rotary die is cutting inaccurately in the press direction? How many times have you made changes to those dies to satisfy your customer s demands, knowing full well it s not a die issue? How many times did you eat that cost? My guess is a lot more times than you would publicly admit. With my 35 years of rotary corrugated diemaking experience, you would think I would have learned how to make an accurate rotary cutting die. Yet I have never met a diemaker who can make every die cut the correct dimension. Simply put, we are trying to account for variables beyond our control. The reality is we should be measured on the accuracy of the die, not the accuracy of the finished product. If the die is correct, then why are we remaking dies to allow for machine variables? How do we know our die is correct? More importantly, how do we convince our customers? I think the real issue is that our field technicians lack an understanding of all the variables and do not have the confidence to explain them in terms anyone can understand. Correcting that is the purpose of this article. First, let s get some terminology and a few facts straight. Surface speed is the generally accepted term for the distance a sheet travels as it is transferred through a diecutter. Surface speed doesn t change with machine speed. If you have a large wheel and a small wheel driving a sheet, the smaller wheel will have to make more revolutions than a larger wheel to move the sheet the same distance. As you can imagine, all the wheels in a diecutter are different sizes and are controlled in most machines via servo motors. A servo motor is controlled by the amount of voltage provided. Wheel sizes change with wear. When I use the term wheel size, it can refer to anything circular moving in the diecutter such as feed wheels, pull rolls, transfer belts, die drum, anvil drum, take off belts, etc. You don t have to admit this out loud, but what are the chances you or someone in your company has altered the size of a diecut from your proofing machine by either pushing or tugging on the sheet as it is being diecut? You probably did this because your first proof was long or short and you know your customer will not accept the die that is not within their spec. If you push on the sheet from the feed side as it is being fed through, it will lengthen it. If you tug on it, it will make it shorter. Wouldn t you think the same things could happen on a real diecutter? I am not talking about you tugging or pushing a sheet in a real diecutter; I am talking about something in the machine that could cause the same affect. Let s look at some of the ways of working through troubleshooting these issues, starting with the most common issue (multiple ups, different size) beginning with die manufacturing. Cutting die manufacturing During the cutting die manufacturing process, the wooden shell is affixed to discs 6 The Cutting Edge July 2014

2 so the laser can burn the pattern into the die. If the wooden shell is not pulled down tightly to the disc, it can create dimensional inaccuracies. This can relate to a shorter or longer diecut depending on how the shell was mounted on the disc (see Diagrams 1 and 2). for multiple ups If you re working with a multiple up die, remove the die, spin the die around and remount. If dimension inaccuracy follows the die, then the die is incorrect. For example, the lead up was short by " ( mm) and after you spin the die around, the trail up is short " ( mm). If dimensional inaccuracy stays with the machine, then the die is correct. For example, the lead up was short by " ( mm) and after you spin the die around, the lead up remains short " ( mm). for single up Measure the product from the lead to any score or cutout during the first 16" (406.4 mm) of sheet (minimum of 10" or 254 mm). Remove the die, spin the die around, remount and measure the same dimension. If the dimensions are different, the die is correct. wooden shells are tight against each other. Occasionally, you may have broken bolts sticking up that hold the wooden shell off the cylinder, or the operator may have not bolted it down correctly. You should be able to visually see if the two wooden shells are pulled together tightly. If the dieboard is not pulled down tightly, it will cause the sheet to diecut shorter than expected. Looking at Diagram 2, one could see how a small gap between the dieboard and the cylinder could result in cutting a shorter dimension. Any raised area of the wooden shell after the die is manufactured will result in cutting a shorter dimension. The best way to verify the wooden shell is tight to the cylinder is to use a mallet to tap it in as many places as possible, listening for a change of tone. It will sound a bit hollow if it is not pulled down. Remove the die to look for broken bolts. If the cylinder is clean, then remount the die. If the die is incorrect, remake the cutting die. Cutting die mounting on diecut cylinder It is imperative to verify that the dieboard is pulled down tightly to the cylinder and any join lines of two Diagram 1 Diagram 2 July 2014 The Cutting Edge 7

3 Transfer section to diecutter When the sheet is transferred from the print section to the diecut section, both the transfer section and diecut section must travel at the same surface speed. If they do not, it can create a push or drag effect on the sheet as it passes through the nip. If it creates a drag, it will cause the diecut section to diecut shorter. If it pushes the sheet, it will cause the sheet to be diecut longer. Once the sheet is completely out of the transfer section, it is fully controlled by the diecutter; therefore, only being able to effect the dimensional accuracy of the sheet from the lead edge to the point where the sheet is released from the transfer section. If this happens, it could be the reason multiple ups are different sizes. In older diecutting equipment, pull rolls were used (see Diagram 3). As a pull roll wore down from sheet abrasive, the surface speed decreased. This would put a drag on the sheet as it entered the diecut section, thus causing the shortened dimension on the leading edge. This was common knowledge, and I have to assume that if it were true, then using a vacuum transfer could cause the same affect if it was not timed perfectly. Anvil surface speed The cutting anvils are urethane products that wear as they are penetrated by the knife. As it wears, some diecutters have a grinder to keep the surface parallel. Since the anvil becomes smaller as it is ground, the circumferential speed must be increased to keep the same surface speed. This can be accomplished by using a VSA (variable speed anvil) if you have that option on your diecutter. If the surface speed does not match the die, there will be a pull or push on the sheet that can alter the dimensions. Cutting die surface speed The surface speed of the cutting die is determined by the height of the crease or cutting rule. This must match the surface speed of the printing plates. The taller the rule results in greater surface speed. As the surface speed increases, the dimensional cuts will become longer. In contrast, the more penetration of the anvil resulting in shorter knives will reduce the surface speed and reduce the length of the diecut sheet. Skip feed a sheet larger than the size of the diecutter. At that time, print a single item so it repeats after it has made one revolution. At the same time, make sure the die is cutting completely and then measure the same cut after it repeats on the sheet. Both the print and cut repeat should be the same measurement. Increasing/decreasing the amount of die impression will vary the cut repeat. Increase or decrease the amount of impression. Consider raising or lowering cutting rule heights. Diagram 3 8 The Cutting Edge July 2014

4 Diagram 4 Transfer from diecut to take off section When the sheet is transferred from the diecut section to the take off section, both the take off and diecut sections must travel at the same surface speed. If they do not, it can create a drag or pull effect on the sheet as it passes through the nip. If the take off section is running slower (surface speed), it creates a drag and could cause the diecut to be shorter. If the take off section is running faster (surface speed), it pulls the sheet and could cause the sheet to be diecut longer. Until the sheet reaches the take off section, it is fully controlled by the diecutter; therefore this will not affect the dimensional accuracy of the sheet from the lead edge to the point where the sheet is in the nip, but it can affect the accuracy from that point to the end of the sheet (see Diagram 4). For example, if you have a 40" (1016 mm) sheet and the dimension between the nip and beater bar section is 14" (355.6 mm), only the trailing 26" (660.4 mm) could be possibly affected by this. If the take off section is running faster (surface speed) than the diecut, you may notice longer dimensions in the trailing part of the diecut since it will pull the sheet faster than being diecut. This would be more applicable when the diecut has minimum rule or rubber, but even with a lot of rule and rubber, it could change the dimension slightly. Push back the take off section so sheets will fall on floor. Diecut several sheets at normal press speed. July 2014 The Cutting Edge 9

5 If the dimensions are different from those with the take off section inline, change the belt speed to match the diecut section. Other possible issues Motor too small on anvil cylinder With the amount of rule and size of the dieboard, a lot of force is required when the lead edge hits the anvil. It is possible that the immediate blow or force slows down the anvil cylinder, causing the sheet to drag at the lead edge. This would cause it to be short in that area. Once enough knives penetrate into the anvil, the diecutting cylinder has enough torque to get it back to speed. Die lifting off cylinder due to not being held down by Serrapid mushroom locks With the amount of rule and size of the dieboard, a lot of force is required to hold the dieboard tight to the cylinder. It is possible that if the die lifts as the rule is being pulled from the anvil, it will cause inaccurate dimensions. Conclusion While I believe that all of the causes described above are very plausible, it may not be limited to just one of these. It could be one of the issues in some cases and a combination of them in other situations. My belief is that the main issue is the transferring of the sheet from the print to diecut. If this surface speed is too slow, it will create the shortness that we commonly see in the leading edge. I also believe that the wrinkling of sheets is related to these issues. If the push/pull of the sheet is dramatic, then you will wrinkle it. This is easier to do when feeding across the flutes since it is more flexible in that direction. With all the uncontrollable machine variables that we face, I am actually amazed at the level of accuracy we attain today, especially when you take into consideration the many variables I have not mentioned. These include amount of rule/perf/rubber on the die, feeding with versus across flutes, paper moisture content and, of course, human error. It is obvious that all the variables I mentioned in this article are being worked on by great engineers at the diecutter manufacturers, but until they are all resolved, we must continue to educate everyone. Once we fully understand the issue, someone will develop a solution for it. While we will always be judged by the final diecut product, we have to be able to troubleshoot all the machine variables before we agree to remake dies. I hope this knowledge allows you to be better prepared for that challenge. Clint Medlock is the President of Stafford Cutting Dies, Inc. He may be reached at or by at clintm@gostafford.com. For more information, visit 10 The Cutting Edge July 2014