Matrix: The Science of Creasing Fiber Board for Folding Cartons Joe McDowell, Channel Creasing Matrix, Inc./CCM Die Supply, Martinsburg, WV, USA In life we are often negative or antagonistic towards things we do not fully understand or things that present us with challenges. I think we can all agree there are few things more frustrating than troubleshooting an issue when you don t grasp the full picture of what s happening (or is supposed to be happening). About a year ago I wrote an article for The Cutting Edge regarding how versus why. We may understand how something works (the process), but we also need to understand why it works what the goal of the process is and why the process is designed exactly as it is. By understanding this we will then understand what is wrong when something does not work. On that note, I would like to talk about this subject as it relates to something we see daily: a better understanding of creasing and how and why creases form in various substrates. Every day we not only help people solve specific creasing issues, but we also teach them why things work the way they do, which often gives them a more positive outlook on their challenging situation and enables them to better approach future challenges. Although results will vary based on conditions and other factors, most people can achieve a consistent result if they simply follow proven formulas and communicate with each other. While substrates in today s market are always changing, we find there are still three basic ways to crease materials during the diecutting process: matrix, phenolic counter or steel counter plate. Which of these is the best way to crease your product? The answer is all of them when you use the appropriate method for your substrate s qualities and characteristics. Please note I am discussing this process only in flat diecutting; rotary diecutting is completely different. To simplify, we ll address substrates as three categories, which are as follows: 1. Fibrous material: This encompasses any material made up of paper fibers that are compressed and bonded together to form a sheeted material. These are generally known as SBS, CCNB, chip, Kraft back and recycled board, as well as some names I cannot write here. These are the materials you see every day in cartons, folders, mailers, etc. Diagram 1 6 The Cutting Edge December 2014
2. Fluted materials: Generally known to us as corrugated or, as people in the Stone Age used to call it, cardboard. This is usually a Kraft material, either found in brown box form or, as you will see more and more, in shelfready or POP displays. Many times you will find these fluted materials either printed on directly or laminated with a label or fiber board material. This can cause many problems depending on the material laminated. (There is another article to come on creasing this specific substrate; we have been doing research to find ways to accommodate these laminations and to address them with a formula, as we do non-laminated materials.) 3. Plastic: People are looking at this style of carton for a variety of reasons, one being the visualization of the contents, which are readily seen with the simplicity of the print. Another advantage is the virtual lack of moisture degradation that occurs if it gets wet. Creasing plastic is usually not done with a counter material, although it can be. Instead it is normally achieved by slitting or perforating the plastic. Fibrous material is generally used in the folding carton and printing industry (see Diagram 1). These materials are usually diecut using flat steel rule dies on either a platen or cylinder-style press. Roller presses are not set up to use matrix or any other type of counter material. Let s talk about deciding what to use to crease any of these fibrous products. There are many factors that need to be determined and discussed when creasing fiber board: The design of the finished product. Find the proximity of the creases to each other and the cutting rule. This information can lead to variations in the die manufacture and counter used. The substrate to be used. Different substrates have different characteristics and they will all have unique attributes that need to be handled accordingly. The type of press to be used. Is it a platen or cylinder-style press? Many people ask if it really makes a difference, and the answer is absolutely. We have found many ways around the differences, and they all entail a little work, but if not done correctly can cause many more problems on press that may not seem to be related to creasing, yet are. The preeminent problem is the lack of communication between a diemaker and diecutter in reference to what type of press is being used. The length of the run. Some counter materials are made for short run (15,000 or less), while others can run hundreds of thousands of impressions. If the above items are not discussed in more than a passing conversation, you are guaranteed to experience a problem during the diecutting process. Sometimes that problem is taken as status quo. For example, a long makeready may seem like standard practice, but the truth is it doesn t have to be. People who run cylinder presses have told me that dust is the nature of the beast, but after we make a few changes to the die they ve not just tamed the beast it has disappeared. I am not suggesting the problems are the fault of the diemaker; rather, I am saying the problem lies in the communication, or lack thereof, between the diemaker and the diecutter. If a diecutter will tell his diemaker some of the problems that are occurring, the diemaker may make some suggestions or People who run cylinder presses have told me that dust is the nature of the beast, but after we make a few changes to the die they ve not just tamed the beast it has disappeared. December 2014 The Cutting Edge 7
changes. On the other side of that coin, the diemaker needs to ask some of these questions to make sure they are making the best tooling possible. So let s look more closely at why fiber material folds the way it does. First we need to look at the makeup of the material. A typical SBS board looks like Diagram 2. As you can see, the fibers are bonded together and run in the same direction, thus the reason you can crease with grain (creasing in the same direction as the grain) or cross grain (where the crease runs perpendicular to the grain direction). The proper way to get a good crease is to break or delaminate these fibers while stretching the upper and lower liners of the material. To do this, we have studied how fibrous paper reacts under pressure and what causes the material to delaminate. Diagram 2 To be clear, the pressure I am speaking about is not only the vertical pressure of the crease rule, but also the lateral pressure. This lateral pressure comes from the material being forced into a channel that is narrower than you would think at first. In the old days, we used to use the following formula: 2 x paper thickness + crease rule width. If you think about it rationally, it makes sense; this formula would put a line in a piece of paper, allowing you to fold it. The problem came as the quality of print increased, the ways in which we printed and the drying of the inks evolved and the quality of the material changed with much of it being recycled. The newer methods of drying have also added some difficulty to the diecutting and creasing problems we have today. Getting back to the use of the 2 x paper thickness formula this theory sounded good and seemed logical. However, the physics involved in the proper folding of paper blows the logic out of the water. If you try to fold or bend this paper over without changing the dynamics of the paper (displacing materials), you will stretch the face or front liner over a greater surface area, which is created by the mass of material meeting at the crease area. When this happens you will find cracking all throughout the crease. This will come up later in this article as well, when we discuss the three basic cracks that occur, the reasons they happen and what to look for. The formula we use today is: 1.75 x material thickness + crease rule width This formula is based on the principle that proper creasing is created by delaminating the fibers. This is achieved when the channel you force the paper into is narrower than the original formula. That causes the material to pinch at the top of the channel, stretching the upper and lower liners slightly and breaking the bond between the fibers. Then, when the item is folded, the lower liner and the fibers that were delaminated move out of the way and the top liner stretches around a smaller surface area (see Diagram 3). Notice that as the fibers move out of the way the face or top liner is able to wrap Diagram 3 See MATRIX page 10 8 The Cutting Edge December 2014
MATRIX Continued from page 8 around the material without cracking. This is how a crease in fiber material should work and most times does. However, there are times when the coating was dried at a greater temperature to dry it faster, causing some really dry stock or dry print. This in turn may crack and will look like the board is cracking when in fact it is the ink. I am often asked what is the best way to run the crease. I would say from my experience that across grain will give you a more stable crease than with grain. The reason for this can be found when you look at the way in which delamination occurs. As you can see in Diagram 4, the material will delaminate in long lines when creasing across grain. However, when you crease in grain direction a different kind of delamination occurs. Imagine the fibers as a bunch of straws in a glass. Now put something in that glass and the straws will separate but remain in clusters. This is what happens when you crease in grain direction: the fibers have a tendency to cluster (see Diagram 5). Do you see how the delamination is a series of holes or pockets? If these pockets or clusters move too close in the same direction, you may get some cracking, even if the channel width is the same in both directions (with and cross grain). To eliminate this, you would narrow the channel to the next lower matrix size. Doing so will break the clusters down even more. This allows the material to move evenly when folded and not to cluster in the same place. Another thing I am often asked about is making the crease wider. You can do that using this formula just plug the thickness of the crease rule into the formula. However, I do caution people about doing this. Although it will give you a better-looking crease as far as delamination, it will also give you a ropy, wide crease which will not fold squarely. You will find that one side of the crease will become dominant over the other and this dominance will switch from side to side, not remaining constant. A good rule to follow is the channel width you use should be no greater than three to four times the thickness of the material being diecut. There may be times where four and half times the thickness is acceptable, but 10 The Cutting Edge December 2014
the general rule is three to four times. If you have to do a capacity fold or one that goes back onto itself, there are ways to do it besides just using a double crease, e.g. making two distinct creases. To explain would take too much time here, but I may write a separate article on how to achieve some of the folds people say you cannot produce. Crease rule height We have talked about channel width, which is imperative to getting the delamination correct, but it is not the only factor in obtaining the proper crease. We must also look at the height of the crease rule. The height is calculated by using the following formula: (cutting rule material being diecut) membrane thickness. The membrane is the material used to hold the matrix together and it is present in between the channels. If the material is metal, it is approximately 0.010" (0.254 mm) thick; if it is Mylar or plastic, it is usually 0.005" (0.127 mm) thick. Some people will say you do not need to subtract the height of the membrane, but I can tell you both from the physics of what we are doing when diecutting and from my experience, you must subtract the membrane! Before we go much further, let s look at our industry s acceptable tolerances for the materials that go into the diecutting process. The tolerance for cutting rule and crease rule is ± 0.001" (0.0254 mm); this is potentially a 0.002" (0.0508 mm) difference. For cutting plate, it is ±.0002" (0.0508 mm), potentially another 0.004" (0.1016 mm) difference from end to end. In essence, there could be a 0.003" (0.0762 mm) difference between the rule and the plate when you add the tolerances together in any given area. Over the length of the plate there can be a difference of 0.006" (0.1524 mm). Now the Diagram 5 Diagram 4 December 2014 The Cutting Edge 11
and leaving it a couple of thousandths of an inch (0.0508mm) short of full penetration (see Diagram 7). Diagram 6 Diagram 7 argument will be made that we use makeready for the low areas. This is true, but only for the cutting rules! If you do not take the membrane out when figuring the crease rule height, you can actually bottom out on the crease (see Diagram 6), leaving what we call stand-off in the cut areas near a crease. If you look at the sandwich you make, even without taking into consideration the tolerances, you will notice that with the cut rule minus just the material thickness (let s say it is Mylar-based), how do you account for the 0.005" (0.127 mm)? The membrane is still in the channel, so the crease rule would push the paper and the membrane still being intact would then act as a barrier, stopping the cutting rule from making a full cycle In this case, the diecutter might say the die is not right or the cutting rule is bad or stop the press and do makeready. Worse yet, they may add pressure and force the cut while the crease cracks. All of these could be avoided if the formula had been followed. When using a phenolic counter you must know how much material is left in the membrane of the channel. I have heard people say It s either 0.005" or 0.007" (0.127 or 0.1778 mm) it s only a 0.002" (0.0508 mm) difference. Yes, that is only a 0.002" (0.0508 mm) difference, but if the material is 0.016" (0.4064 mm), the difference is actually 12.5% and that can make a difference in the crease and the cut, as well as cause the problem of stand-off. It is my understanding that many of the diemakers today have settled at 0.006" (0.1524 mm). If the membrane is not taken into consideration, different problems can occur. First, you don t cut all the way through and the product doesn t strip at all. Second, the product cuts, but only in certain areas and you end up with a checking problem (where the bottom ink seems to strip off in pieces). Nicks can become stronger, making blanking or stripping a tougher task, and a major makeready delay. And, of course, the crease will be cracked either on the surface or under the ink, but will surface on the folder gluer, if not on delivery. Types of cracks Let s talk about the different types of cracks we see when we try to crease papers, and the results and causes. 12 The Cutting Edge December 2014
Cracking on the sides of the crease: Usually noticed on delivery before folding. Caused by the channel being too narrow. Cracking on the bottom of the crease: Usually noticed on delivery before folding. Caused by the crease rule being too high. Cracking on the folder gluer: Where the crease looks nice on delivery, but cracks when folded. Proper delamination has not occurred and too much material is in the way of the top liner, generally caused by a crease rule that is too low (see Diagram 8). Notice how much material is near the area that the top or face liner has to bend around. It is too great and the material ruptures. These problems can be avoided simply by following the formulas and with communication between the diemaker and diecutter. There are many other problems that occur because people do not follow formulas or take into account the collateral damage that can happen when the focus is on only the problem at hand. When it comes to creasing, some subtle changes can make major impacts on your profitability in your diecutting job. These changes and problems will be discussed in the next article. Thank you both of you for reading this. Joe McDowell is the Vice President of Channel Creasing Matrix, Inc./CCM Die Supply. He can be reached at 1-800-451-7373 or by email at jmcdowell@ccmdie.com. For more information, visit www.ccmdie.com. These problems can be avoided simply by following the formulas and with communication between the diemaker and diecutter. December 2014 The Cutting Edge 13