Gregory Zimmer, Zimmer Industries Inc., Hawthorne, NJ, USA
Ever since the advent of corrugated paper, converting it has created certain difficulties. In order to intelligently discuss approaches to dealing with these difficulties, we must obviously first define and examine those difficulties. In this article, we will primarily address the rotary conversion of corrugated, but many of the elements discussed here should have some crossover value to flat conversion also.
Challenges of Corrugated
There are many obvious and some not so obvious reasons for the inherent problems associated with converting corrugated. Those reasons include:
The constant and extreme changing character of corrugated based on its moisture content and temperature. Dry' cool corrugated is a totally different animal than wet, warm corrugated and each will ill act very differently in the converting process
Air--the very element that helps to create the unique properties of corrugated--also creates major problems in conversion. Cutting, perforating, or creasing a material that significantly consists of trapped air, automatically creates several difficulties, one of the most obvious being corrugated's propensity for crushing, which of course is due to necessity of material compression prior to cutting. Another problem related to the air in corrugated is how the air reacts differently to converting attempts based on whether cutting is being attempted with the flutes or against the flutes. Obviously. cutting against the flutes presents a more supported, easier to cut material.
Corrugated should not be cut in such a way that the resultant product edges are so sharp that they might produce paper cuts. This calls for a slight breaking of the product edges, which can be accomplished in a couple of ways we'll examine later.
Rotary diecutting has historically been less accurate than flat diecutting. One of the primary reasons for this is that most rotary cutting is done by cutting into a blanket. The distance of penetration into the blanket changes the size of the blank being cut. That distance can be affected by the condition of the blanket, the press operator and the amount and type of rule being employed. We will discuss how the right kinds of rule can potentially increase the life of the blanket and the accuracy of the diecutting further on in the presentation. Clint Medlock recently had a very interesting article in The Cutting Edge that deals with the problem of achieving maximum accuracy from rotary dies. I recommend you read it if you haven't already done so. It contains some very helpful and insightful information. Clint and I both seem to share the opinion that rotary dies can be made to achieve significantly greater accuracy than is now commonly being produced.
Weights and thickness of corrugated vary greatly, often requiring different approaches to conversion and presenting different problems to be dealt with.
Operator efficiency also plays a very large part in corrugated diecutting. Proper training and motivation of the operator is extremely important.
Stripping of the scrap is absolutely critical to effective conversion of corrugated. Many variables affect this aspect of diecutting.
There is a vast selection of rotary serrated rules available today, both foreign and domestic. Choosing the right ones for a specific application can be very confusing and difficult.
It has been proven that curved rules in rotary diecutting should be somewhere between .020" - .030" lower than the straight rules because they cut with much greater ease than the straight rules, due to the lesser number of teeth penetrating, at any one time
Effective nicking of corrugated has often proved to be problematic. There are new rule products which may aid in that area
Finally, there is the issue of the increased speeds of rotary diecutting and the methods of optimizing and maintaining the achievable speeds.
Characteristics of Rules
Now let's go over some general characteristics of most rotary rules. As many of you know, there are three primary manufacturers of rotary rule in the U.S.--National, Simonds, and Zimmer. All three companies buy their cubic steel from the same sources and the steel they buy is virtually identical. The usual choices of tooth patterns and bevels for rotary cutting rules include 8, 10, 12, and 20 teeth per inch, with occasional requests for 3, 4, 4.6, 5, 6, 13 and 16, though many of these are often used for non-corrugated applications.
The bevels usually employed include: centerface, sideface and off center, centerface.
In general the more teeth per inch, the more cutting pressure is required. Also in general, the more teeth per inch, the lower or shallower the gullets or valleys of the rule are. This fact may require less penetration in order to achieve the cut and in certain cases, may increase blanket life. As a rule, the more teeth per inch, the better the edge appearance and feel usually are.
All three manufacturers supply 4 point as the standard, but 2, 3, 6 & 8 point are also available for certain applications.
In flat diecutting of corrugated, a wave rule is sometimes used to break the sharp edges of the diecut product. This achieves a similar effect as the teeth do from the serrated rules used in rotary diecutting.
With any manufacturer's rule, the proper use of rubber is absolutely critical. Any of the major rubber suppliers will be happy to assist you in this arena, but let it be emphasized that if everything else about the die, the operator and the equipment is the absolute best, if the rubbering is wrong, you will not be successful--period.
Problems & Answers
Now let's see if we can put together the inherent problems of rotary corrugated diecutting with the general characteristics of rotary rules and find out if we can get some answers as to how to tackle some of those problems rotary diemakers and diecutters face every day.
Moisture
We spoke of the moisture content and temperature of corrugated being potentially problematic. As the diemaker, there isn't too much you can do to affect the moisture or temperature, other than possibly to caution and or advise the diecutter if you see evidence of this problem. However, if a particular plant very often runs wet board at higher than normal temperatures, there are ways to address the problems these conditions often present. Reduced cutting pressure would help in these circumstances because hot, wet board tends to almost tear when significant cutting pressure is needed.
As we discussed previously, the fewer teeth per inch, the lower the cutting pressure needed. Therefore, in a plant such as the one described above, a tooth pattern such as 10 or 8 would probably be preferable to a 12 tooth, everything else being equal. Some of the newer, more exotic tooth patterns such as Arrowhead, Ultra 10 and Supercut might also be good solutions here. Remember though that everything else being equal, coarser tooth patterns often require deeper penetration and the attendant possibility of decreased blanket life.
Air
Air was also mentioned as a potential problem. In general, coarser tooth patterns can also help in this area. Again, coarser teeth with reduced cutting pressure can help decrease the amount of crush that may occur. In the curved direction, cutting with the flutes, it is often important to have a sharp tooth one that sets or holds the material in place as it is being cut. A tooth that is not truly sharp may have a tendency to tear rather than cut, especially in the flute direction, cutting into air.
Ideas
Regarding the edges of diecut products, and making sure those edges don't promote paper-cuts, serrated rules and or wave rules could attend to that requirement. The fineness of the tooth patterns is directly related to how fine an edge will be produced on the product. For an obvious example in rotary serrated rules, a 12-tooth pattern will provide a less rough or coarse product edge than an 8-tooth rule. An even finer edge then could be achieved by a 20-tooth pattern. Remember though that the 20-tooth pattern would take more cutting pressure than the 12 or the 8.
Accuracy
As to the possibility of maximizing the dimensional tolerances or accuracy of rotary diecutting, there are several influencing factors:
The condition of the blanket--the more level and less eaten up--the better.
The skills and attention of the operator. A poor operator can pretty much wreck any chance of dimensional accuracy. Too much pressure, too little pressure, damaged dies. poor machine maintenance can all contribute to inaccuracy
Of course, the dimensional accuracy of the die and the rules used in the die are extremely important For example, height tolerance of the rule or inconsistencies of tooth depths or tooth forms can adversely affect accuracy. If everything is as it should be, penetration into the blanket can be kept to a minimum. This simple step can help blanket life, improve the product's dimensional accuracy and even enhance machine performance.
Again I would recommend you to Clint's articles for further discussion of improving accuracy.
In addition to what's been discussed here, there have been several improvements relative to blanket performance by companies such as Dicar who make the Equalizer, which helps to compensate for some of the inaccuracies normally associated with rotary diecutting. These kinds of improvements may often make it possible to use rotary diecutting, for accuracy that previously was achievable only with flat diecutting.
Varied Weights & Thicknesses
The various weights and thickness of corrugated create differing requirements of the rules employed Again, it is often a balancing act of trying to achieve a nice edge, minimal cutting pressure and a reasonable amount of penetration. In general, the heavier weight, thicker boards call for the coarser tooth patterns. There are a couple of rules that claim more of a "Universal" usage than conventional rules, and you may want to look into those also.
Stripping
Stripping, as mentioned, is absolutely critical to effective rotary diecutting and here is an area where the different bevels or cutting edges that are available may come into play. For many years it was thought that a sideface bevel created the perfect conditions for effective stripping. This type bevel tends to crush one side of the blank and leaves the other side basically uncrushed, providing an ease of stripping or scrap removal because the scrap is in a sense--directed. There are diemakers who still believe that sideface bevel is the best for rotary diecutting. The industry in general, though seems to be moving more toward off-center center bevels and true center bevels. The standard centerface bevels actually create a product edge that is basically identical on both sides of the cut and both these sides are better in appearance and crush than the "bad'' side of the sideface cuts. Neither side, though, is as good as the "good" side of the sideface cut. It is a compromise of sorts and provides a good solution for common cuts and the assurance that the rule cannot inadvertently be put in the die backward by the diemaker or the diecutter who may replace worn out rule. The off-center, center bevel is a bit of a hybrid, giving some of the benefits of a sideface bevel and some of the benefits of a standard centerface. Being off center, there is a possibility of putting the rule in backward. Returning to the issue of crush again; it is very important that the rule used minimizes crush to provide optimum stripping.
Nicking
A new product has recently come to the marketplace It is used to provide nicking of corrugated. It is a pre-nicked cutting rule primarily for light corrugated stock and may be used in flat or rotary applications. This is a very promising concept. Simply explained, it takes the obvious idea that many small nicks can provide the same or greater strength than a few large nicks. By making the nicks smaller, they become less offensive both to the eye and to the touch but because there are so many of them, you do not sacrifice the overall strength of the nicked areas and the risk of the material being inadvertently broken apart during processing. This product presents some interesting possibilities and might warrant your further investigation.
What to Look For
Here are some things to look for in any high quality rotary rule:
The three American rule manufacturers previously mentioned, Simonds, National and Zimmer enjoy international reputations as rotary rule suppliers and I'm sure that any of the three would be more than happy to assist you in the proper selection of the right rotary rule for you and your customers.