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Basic Rubbering Techniques

by Roger A. Brown, Monroe Rubber & Plastic, Inc.

February-March, 2011
Recently we had someone inquire about what questions on rubbering a flat die are asked most frequently. That, I thought, is a very easy question. As a die ejection supplier to the diecutting and diemaking industry, the answer is, unfortunately, too easy. While we are sometimes asked “How ‘bout them Tigers?” or “Where are we going for lunch?,” we are very seldom asked questions about rubbering a die. More often the question is “Our die is not ejecting, can you help us?” or, “We over estimated the speed that we could run this job. Consequently, we are losing money on it. Can you help us obtain a faster run speed through improved die ejection?”

It never ceases to amaze me how a diecutter can invest seven figures in a very high-tolerance, high-speed diecutting machine and then treat the die as a commodity by insisting that the supplier of the lower tolerance, wooden-base, cutting die shave his price. In order for the diemaker to make a profit, he is forced to use lower quality products in that die. It’s no wonder that jobs don’t run well. It would be like trying to run a marathon in your bedroom slippers. While they are certainly a lot less expensive than a quality pair of running shoes, you won’t be able to run very fast or very long in them.

Like a marathon runner, a diecutter needs to have a high-quality die in order to run with the big boys at the front of the pack. High quality and inexpensive are inversely related, so if you buy the cheapest tooling, you know what you can expect...bedroom slipper results. With a “commodity mentality” then, it should be no surprise to anyone that suppliers of the tool are not even brought into the conversation of what can be done to improve the productivity of that expensive new machine. It is clear, however, that suppliers in our industry need to become more proactive as far as offering assistance to both diemakers and diecutters in order for them to improve the on-press performance of high-speed diecutting machines. Certainly the seminars and tradeshows offered by the IADD, particularly at the upcoming Odyssey, May 4 -6 in Nashville, are wonderful first steps in expanding one’s knowledge in this area.

But let’s get back to the premise that we are actually asked to make recommendations to improve the diecutting process. Here’s how we respond. For starters, we need to think about our common objective. That is, to get the current job on press quickly and soundly, start up the job with just a couple of test sheets (yes, a couple) and then run the job at a high level of productivity—at least 80 percent of the machine’s rated capacity.

Next, from a rubbering standpoint, we have felt for a long time that one cannot forge too far into higher technology without first understanding basic rubbering techniques.

What thickness should I use?
Since most all ejection materials take a compression set during a production run, it is important that the new rubber be slightly higher than the cutting rule – generally 2mm or 1/16". In addition, this surplus of rubber causes compression on the rubber when it is at knife height thus ensuring complete ejection off the knife. Remember, ejection materials must be compressed to have any energy or ejection force. Therefore the suggested rubber thickness to use is the sum of the exposed rule (the amount of rule projecting out of the die board) plus the 1/16" surplus.

What grade of rubber should I use?
When do I use soft rubber and when do I use the more firm materials? First take a look at the thickness of the substrate. Thicker substrates require softer ejection rubber and thinner substrates need the added ejection force of firmer materials. One might ask the question: why not just use the same firm grade on all our diecutting jobs? The answer is that firm grades won’t take a lot of compression without overloading the press and/or blowing out (rubber is destroyed). And by their very nature, thick substrates cause more compression on the rubber. Also, thicker substrates may be flat crushed or embossed with the impression of the rubber if a firm grade is used.

Balance ejection forces
Generally, everyone wants to use just one grade of rubber on the entire die. This is probably the worst thing you can do when rubbering a die for rapid ejection. Besides generating more resistance to diecutting, it causes nicks to break and more time to makeready, all things that are detrimental to rapid die ejection and low-cost production. Therefore, observe the die layout. Where there are tight areas between cutting knives, there is a tendency to trap the part or scrap and there is a need for more ejection force in those areas. Thus a firmer, more powerful ejection material should be used. In the more open areas of the die, where the required ejection force is less, use a softer grade of rubber. This simple technique of using two or more durometer grades of rubber on a die to bring the ejection forces more into balance is probably the one thing that is most overlooked yet is one of the most vital to smooth diecutting with rapid ejection.

Rubber placement
The ejection materials should work freely in an up and down direction without becoming caught against the rule. Therefore keep the rubber about 2mm or 1/16" away from the cutting rule and about 3/16" or 4mm away from creasing rule. While this suggestion is frequently violated, it is even more important that it be followed at the nicks. In the diecutting process the lower (or opposite platen) first makes contact with the substrate which is held firmly by the ejection rubber. If the rubber makes contact laterally with the cutting rule, it will pull the substrate away from the nicks in the rule, causing the tabs to break apart. This then will cause job interruptions, reduce the press speed and add hours to the overall job.

Waterjet cut ejection material
If the above general rules of rubbering a die are understood AND OBSERVED, then perhaps it would pay to move into a higher level of rubbering, and that is waterjet cut ejection materials. With complex, multiple-out dies run on high-speed platen presses, it is very important that the cutting die provide uniform lift throughout the die. That is where all identical cavities are ejected with precisely the same amount of ejection force. In fact, all parts of the die, if engineered properly, should eject at the same time with the same amount of force. That is what we call a balanced die. Providing such a die with hand-cut ejection materials is virtually impossible. A proper CAD software package, in addition to a knowledgeable CAD operator, coupled with a waterjet machine, can provide precut engineered ejection materials (utilizing the aforementioned general rules for rubbering) that can be placed on a cutting die – quickly and accurately. Everyone involved will be amazed at the improvement that can be made to the whole process when waterjet cut rubber is brought into the equation.

While there are other “rules of thumb” that can be mentioned and other techniques “tweaked” to further improve the process, these are the most basic. Have you ever slaved over a problem only to have a peer stop by your work area and make a suggestion that worked? What he or she brought to your problem was a fresh set of eyes that was not trapped by the closeness to your problem. Good suppliers can serve that function. As you forge that relationship, you will be rewarded many times over and your objective of process improvement will become a reality. After all, if you could obtain on average just a 10-percent improvement on all your jobs, imagine what that would do for the bottom line. n

Monroe Rubber & Plastic, Inc. (MRP) is a manufacturer of die ejection materials including waterjet services. To easily select the proper grade of rubber for well-balanced flat cutting dies, Monroe Rubber & Plastic has developed a “selector guide” in the fashion of a “slip stick” and it is available by requesting one from MRP. For more information, contact the company at 734.241.7101 or visit www.monroerubber.com.