Tire cord calender as process system: Operating for consistent product--part 1.

It is generally recognized that a tire cord calender is in reality a complex system, or perhaps more accurately, a "super system"--comprised of several complex subsystems. We can readily intuit that the success of the super system is dependent on the successful functioning of the various subsystems and the individual elements that comprise those subsystems. We also have an instinctive feel for the sensitivity of the process to variability in the functioning of those subsystems. For example, we know we can tolerate some variability in the temperature of the circulating oil without affecting the ability of the calender to function; contamination of the lubricating oil, while it will not cause an immediate failure, will, if uncorrected, lead to a bearing failure; a stoppage of the lubrication system will rapidly result in failure and a shutdown. In some instances, our knowledge of the sensitivity of the super system to performance variation in the subsystems and components is empirical, in some instances theoretical, and in some instances, we make decisions on how to deal with that variability based on a failure analysis.

What is less obvious is that the calender, with its various mechanical, hydraulic, electrical and electronic components, is itself a subsystem of the process system. Successful calendering depends on adequate control of all of the elements of the system. The overall system, of course, encompasses all of the materials and material preparation steps and equipment that come before the calender, and all those that come after it in the post calender train, up through delivery of calendered goods to the next step in the tire building process.

In the discussion that follows, we will deal primarily with the calender and the compound that is delivered to it. There is a number of variables and concerns relating to the quality, consistency and delivery of the cord or wire to the calender that are worthy of studies on their own merit. The same is true with the post calender train. The application of electron beam technology to the in-line post-calender partial pre-cure of tire ply has proliferated in recent years, with an accompanying increase in complexity and reliability concerns for the post-calender train.

Review

The simplest form of rubber calendering is the production of unsupported or unreinforced sheet. The operations necessary to produce an unsupported sheet include the following:

* Warming the feedstock--softening the stock and raising its temperature by mechanically working it, either on a mill, in a cold-feed extruder, or by a combination of extruder (s) and mill(s);

* delivering the warmed stock to the calender;

* accepting or feeding the stock in whatever form it is being delivered to the calender--distributing it over the width of the nip and delivering it to the pressure area ahead of the roll nip;

* metering the stock so that the mass flow through the nip is exactly equal to that needed to produce a sheet of the target thickness and untrimmed width;

* extrusion--forcing the stock through the gap between the two rolls;

* densification--rejection of any gases or air entrained in the feed bank, and compressing the stock sufficiently to produce a smooth, void-free surface with uniform elasticity and recovery over the full width of the sheet;

* gauging--bringing the material to a uniform thickness, in both the machine and cross-sheet directions.

All but the first two of these operations can be carried out in a single nip. If a second nip is employed, it is usually only to apply a bond-breaker or carrier film to the rubber sheet, and may take the form of an air-loaded laminating roll running against a calender roll.

Coating a fabric or substrate in the calender adds to the operations listed above:

* Transferring the rubber sheet from the calender roll to the substrate;

* forcing the rubber into the interstices of the substrate sufficiently to meet the requirements of the coating process.

There are three processes for accomplishing these operations. The first is laminating or skimming. In this instance, the substrate and the rubber are traveling at the same rate of speed, and the rubber is essentially pressed into or onto the substrate. The degree of penetration is dependent on the density of the weave, the viscosity of the stock and the pressure applied in the laminating nip. There is usually no material bank in the laminating nip.

The second is frictioning. In this case, the stock and the substrate are traveling at different speeds, and the rubber is scrubbed or frictioned into the substrate. In older three-roll single motor drive calenders, the prototype for the process, the stock is on the middle roll, and is traveling faster than the substrate. Typically, the frictioning nip runs with a small bank of material. The process is most frequently used for coating densely woven substrates with a relatively thin layer of rubber.

Both skimming and frictioning employ two nips--one to form and meter the sheet, the second to effect the transfer. The third process for transferring rubber to a sheet or substrate is bank coating. Bank coating may be carried out in one nip or two. It is similar to frictioning in that the roll carrying the substrate and the roll applying the rubber are moving at different speeds. The major differences are in the size of the bank, the thickness of the coating being applied to the substrate and surface finish imparted to the sheet. Bank coating is typically used where a relatively heavy layer of rubber is being applied to a substantial substrate, and the curing process will determine the surface finish. Conveyor belting is a classic example.

Traditional applications for calendering in the tire industry include coating cord and wire for tire plies and belts, coating fabric for chafer strip production, and the production of unsupported sheet for inner liner and tread components. Over the last few years, there have been reports in the trade press of smaller specialized calenders employed in conjunction with warm-up extruders to produce components on a continuous, as needed basis, for inline tire production. Details of these concepts, and the degree to which they have proved to be both workable and practical, thus far remain the proprietary knowledge of the developers. We acknowledge the development here only to underscore that changing methods and improving technologies are a constant in all industries. The principles of calendering and successful calender operation remain constant, even if the size of the machine on which they are applied, or the component being produced on it, should change.

While our discussion is directed at the tire cord calender, the principles are applicable to almost any elastomeric calendering operation.

Process and machine variables

Tire cord calendering is a coating process. In modern tire cord lines, it is a double-coating process, with two layers of rubber being produced separately, then simultaneously laminated to an array of cords or wires. Sufficient laminating force is applied to force the rubber to strike through the cord, fabric or wire, encapsulating the individual strands in rubber, and finishing with a rubber sheet containing reinforcing wire, cord or fabric.

Referring back to our introductory notes, the process is similar to skimming; in the laminating pass, the substrate and the stock are traveling at nominally the same speed, and there is no bank in the laminating nip. In practice, with modern calenders having individually driven rolls, lines are often run with a slight lead on the no. 2 or no. 3 roll, in effect running in a very light friction mode.

The objectives are as follows:

* Produce a web that is uniform in thickness from edge to edge and in the machine direction.

* Produce a web whose weight per unit area of rubber on the top and the bottom are within specification. (We usually think of equal weights on top and bottom--cord or wire centered in the web--but in certain applications, an unbalanced weight coat is the target.)

* Maintain a uniform wire or cord spacing in the web--uniform EPI across and down the web.

* Fully encapsulate each strand of reinforcement with a void-free matrix of rubber.

The process variables are:

* The work history of the feed stock or compound--the degree to which the viscosity of the rubber has been reduced as the result of the shearing action in the breakdown, mixing and warming processes;

* in-process storage, or aging--the degree of crosslinking that has occurred since the introduction of the cure system to the compound.

* formulation--what is in the polymer matrix - structural modifiers, fillers, extenders, internal lubricant, antioxidants, antiozonants, cure systems, etc.;

* stock temperature--the proximate cause of that part of the apparent viscosity variation that is independent of formulation, polymer breakdown and degree of crosslinking.

The machine, or system, variables include line speed; friction ratio in the forming nip; friction ratio in the laminating nip; roll temperature; roll gap--water and drive end; crown correction (roll bending pressure or degree of cross axis applied); and splice relief or laminating pressure (squeeze).

Broadly put, machine variables are manipulated to manage uncontrolled variation in the process variables. The act of calendering imposes a load on the calender roll. In response to that load, the roll deflects, or bends. The load is transferred to the calender frame through the roll bearings and the roll adjust system. Some portion of the load also causes compression in the roll adjust mechanism, and stretching in the calender frame. Building a robust calender minimizes the amount of deflection, compression and extension, but they still occur. If the load is constant, then we can adjust the calender, or manufacture it in such a way as to produce a flat sheet when it is subjected to a particular constant load.

The difficulty, of course, is that different jobs produce different loads: Thinner stocks produce a higher load on the calender than thick; stiffer, more viscous stocks higher loads than soft; faster line speeds higher loads than slow. The modern tire cord calender addresses this difficulty by using roll crossing or roll bending to compensate for the roll deflection. From the machine standpoint, the real complication ensues when the load imposed on the calender roll varies during the production of a particular product. Regardless of how robust the calender construction may be, deflection will occur, and the amount of that deflection will vary as the load varies.

This article will be continued in next month's Rubber World (August 2005), where overcoming and correcting for process variation will be explained, along with the effect of maintenance on operation and quality.

Lawrence R. Gooch

Gooch Engineering Associates