1. This article reviews a series of challenges in selecting the right material for covering a rubber roller.
2. It explains how the desired parameters can be listed to help establish selection criteria, and provides examples of how to go about that.
3. The paper also identifies questions to consider (such as what the roll is going to do, what it’s going to come in contact with, and how it needs to perform).
4. We then develop several triangle diagrams to present the material choices available, and to illustrate how different materials address often conflicting priorities.
5. By using these tools and this methodology, you will see how to improve the likelihood of selecting the right material for covering your roller.
A Customer’s Requirements for a Roll Covering
Sometime printers ask for roller properties which may conflict with each other.
So it is important for clients to understand that they can’t get everything they want. More specifically with rollers, sometimes you can’t optimize for all attributes and make it work.
Lets imagine having a dining room table, and for the purposes of our story it was the shape of a triangle.
Now further, I asked him to imagine each corner of the table representing a specific property he wanted his roller to have. Let’s say these criteria are:
- Chemical resistance
- Abrasion resistance, and
- Fatigue resistance
Now if the elastomer (defined below) we’re seeking is in this case a tablecloth and the tablecloth covered all three corners of the table, then it would be ideal, because it would do everything.
[An elastomer is a polymer. The term, which is derived from elastic polymer, is often used interchangeably with the term rubber. For additional information, see: http://en.wikipedia.org/wiki/Elastomer.]
The reality is that tablecloths are not ideal. Sometimes they can’t do everything we want. Sometimes they can cover two of the three corners completely, but the last corner not so well. So when you do a real good job of covering up the corners that are labeled chemical resistance and abrasion resistance, then the other corner (in this case fatigue resistance) isn’t fully covered. This means that attribute is not going to be maximized. So understanding the type of elastomer, and how important that attribute is to the customer’s process, we can see how small the tablecloth is in relation to the table.
Conflicting Requirements for a Roll Covering
The goal in creating the triangle diagram was to show graphically how the customer wanted a particular material, but he also wanted each of three different attributes to be maximized.
In this case, those attributes were resistance to a chemical, abrasion, and fatigue, and he had to make the roller work with a particular solvent, which the diagram below shows is isopropyl acetate.
So what I did was place in the diagram several different elastomers he had asked me about, and placed them where they would rank in terms of how they would handle each one of these challenges. Then the question became: “Which elastomer is going to be the most useful for covering a roller in his particular application?”
When we focused on the importance of chemical resistance, the diagram shows a green dot representing EPDM as having the highest value. But by selecting that material to cover his roller, its abrasion resistance and its fatigue resistance are nowhere near the other options available to him, towards the bottom of the diagram.
Then we got to discussing abrasion resistance, and its importance to him in achieving a long wearing surface. The problem was that a urethane, which would last the longest, purely from an abrasion standpoint, would not do well with the chemical resistance of the isopropyl acetate, and the customer was forced to use that solvent due to another parameter in his offset production process.
So what we then discussed was the merits of finding a covering for this roller that is going to be the best balance of all of these priorities.
Selecting the Right Covering for a Rubber Roller
Next we discuss the orange dot in the center of the triangle diagram, which describes NBR/PVC, which is nitrile PVC. It gives him the benefits of a nitrile, and the abrasion resistance of PVC. This material is approximately in the middle of the diagram. So it’s not the strongest in terms of abrasion, fatigue, or chemical resistance. But for the client’s application, it’s probably the best balance, and that turned out to be the best choice of material to cover a roller for what he was doing.
As the clients has many rubber rollers for different applications, a new triangle diagram has to be created based on properties required. Some are used for dampening or some are used for gluing / coating / uv application etc.,
We will have different diagrams to show the material options for covering a roller that would come in contact with MEK, methyl ethyl ketone, another solvent, and then the dots would be in different positions.
So depending on the solvent you’re using in your production line, you need to choose a rubber or an elastomer that has the appropriate balance of qualities.
Identifying Critical Attributes for a Roller Covering
At the root of making the right material selection is identifying what the critical attributes are. As discussed above, these could be considerations such as chemical resistance, abrasion resistance, fatigue resistance, or load-bearing ability. It’s critical to look at what a given roller actually has to do in the environment that it’s going to see, and if there’s no chemical resistance involved, maybe that is replaced with some other attribute.
Sometime the company is using the same elastomer on two different machines, and they get different results in terms of product quality and roller life, and they wanted to know why.
First, understand that they’re running a similar, or the same, product and one machine was designed and manufactured in-house, but still relatively sophisticated, and was built about ten to fifteen years ago. The second machine is newer. It was engineered and built by another company, and it has a lot more sophistication in terms of construction and other properties.
It’s better sit with the client and start selecting these materials from scratch, then let’s identify what we’re trying to accomplish, and divide them into three categories. So we created a diagram like the one below, with places to identify and sort out among things that were Must Have, Should Have, and Like to Have.
As a result of this conversation, we created a simple chart like the one below.
Basis for Recommending the Best Material for Covering a Roller
With agreement on direction prioritized that way in collaboration with the customer, I can then say which elastomers are consistent with what we put on the Must Have list. Sometimes it’ll happen that none of these may be good on the Should Have, but in this case, there was one that might be.
So the list we created together became the “success criteria” for evaluating the material we recommended for covering their roller.
And developing a simple list of priorities like this is a great way to start the process of finding the right material for a printer’s application.
But because we had identified together the list of Must Haves, Should Haves, and Like to Haves… they were able to understand that finding the answer was a matter of balancing multiple priorities, given the physical properties and performance characteristics of the available elastomers. Once we got that on the table, we were able to align ourselves and chart a course to take.
Now the team can discuss the pluses and minuses of each of these materials, or elastomers, to cover a roller.
Similar Methodology Applied to a Different Roller & Situation
A different situation would require an additional table, and could result in a different, more complex diagram. For example, let’s quickly consider someone working on a high-speed application, and the problem they’ve been having is rollers blowing up.
That phrase ‘rollers blowing up’ is often a euphemism for a problem with hysteresis. Hysteresis is the heat build-up within a roll covering from cyclic nip stresses (speed, force, rubber thickness and hardness).
This type of analysis would need a chart with more parameters than just the three corners of a triangle representing speed, load (force) and thickness.
Then we might say that the customer’s process demands a particular hardness. Perhaps the customer requires a 70-shore material.
And let’s say the customer wants to operate this machine at, let’s say, 450 RPM.
So what do they need as a covering material in order to get the roller to run at the designed speed of the machine?
Well, we would look at different dots, representing different elastomers, and place them in different locations inside the triangle.
But you would quickly see that at certain cover thicknesses, at that hardness, with a given material, it’s impossible to make that combination of variables work.
This might be a situation where all the dots go into one corner of the triangle, because no matter what you do, at that 70-shore, and at that 450 RPM, an elastomer can’t possibly survive the combination of those conditions.
In that case an option to consider is running at a reduced speed, below what the machine was designed to do. But that would lead to reduced output, and therefore lower productivity.
Another option is to choose a hardness and material that causes the hysteresis aspect – the blow-up characteristic – to be reduced.
It is in challenging situations, such as the conflicting priorities in this scenario, where the Imperial team can not only help customers choose the right material to cover their roller, but also help them look at what parameters might be varied, in order to have the best likelihood of achieving their production objectives.
We use rubber rollers in different applications , in different types of machine , in different environment , it contacts with different materials so the rubber performance will be different for all these. So it is always best to sit with operators, contact material suppliers , maintenance team, rubber roller manufacturers and list out the rubber properties required vs choice of rubber material available. This table will give you an exact idea what properties can be achieved and what has to be compromised. This will give a better cooperation between suppliers vs printers vs rubber roller manufacturers.
The above contents are compiled, edited to make it relevant and easy for printers to understand. This is only for reference and knowledge purpose.
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