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Improving calendering operations.

Improving calendering operations

Calendering is basically a continuous process of extruding or sizing rubber between a pair of rolls. Calendering differs from the extruder method in that an extruder has a discharge that is fixed in size by the die configuration. To alter the size requires a considerable amount of design work and time, and may require an entire die change. With the calendering method, a simple adjustment of the roll position makes an instant change.

A very critical function in a calendering operation is the feeding. Most calendering operations use one of two methods. One is used by most smaller companies and is the least desirable method from the standpoint of a good quality product. It involves the placing of rolled rubber, called pigs, into the nip, or bite of the calender.

When the "pig" system of feeding the calender is used, the only way it can function properly is when the pigs are kept to a maximum of four inches in diameter, and are maintained at a warm temperature sufficient to allow easy distribution across the width of the nip.

The second method is the feeding of the nip with a warm strip directly from a mill that is dedicated to warming calender stock. This method allows the calender to do what it was designed to do, calender the stock. However, in the pig feed method, unless the stock is kept very warm the calender has to do the function the warm-up mill was intended to be used for, to warm stock to the point it will flow smoothly.

I have observed, while visiting many of the rubber manufacturing companies in the United States that use the pig feeding method, that the calenders are being improperly fed. The pigs are usually too large and there have been too many rolled up ahead of the time they are needed, thus the warmed stock is no longer at the elevated temperature that is required to produce the quality of stock desired from a calender.

Rolled pigs that have been rolled for a fifteen minute period will have up to a fifteen degree difference in temperature between the outside and the center of the pig, forcing the calender to finish the function of the warm-up mill. This has two major effects. First, the calender is subjected to extreme separating forces and roll deflection, and as important, causes undue strain and wear on the calender while using more horsepower than necessary. Secondly, and most critical, the product is affected adversely - crows feet, or cold flow marks, will show up on the finished product. These are caused by improper feeding, or from incorrect temperatures.

There are other factors that may cause improper quality of stock finish, namely improper roll temperatures, incorrect friction ratios, worn or improper gearing, and incorrect horsepower. Worn gearing can usually be detected in the finished product, as the surface will show chatter marks usually in the pattern of the gear teeth spacing.

Although the strip feed method is the best, most of the companies allow this method of feeding to be misused also, in that they allow the operators to over fill the nip with a bank of rubber for the operators' convenience. When the bank size is controlled properly it will not exceed six inches in diameter, and smaller is even more desirable.

In most cases that the pig feed system is used, the warm-up mill is near the calender so converting from the pig feed system to the ribbon feed system is not a major expense. A simple narrow conveyor can feed the calender a strip from the mill, and this can be improved on by the addition of a pneumatic cylinder that moves the conveyor back and forth across the face of the calender roll allowing even distribution of the stock to the nip.

Some of the companies that have a three roll vertical calender feel they cannot feed a ribbon as it would have no way of staying in the nip, as gravity would cause it to fall. This is very easily remedied by the addition of a simple bankboard that is installed at a slight incline the full length of the nip. The gravity effect that was a problem without a bankboard now works for you in continually allowing the rubber to slide into the nip.

If blisters are seen in the calendered product they can be eliminated normally with the addition of an idler roll, or lift roll that raises the stock off the roll surface allowing trapped air to be released. With some particular stocks the blisters may have to be eliminated by the use of a pricker roll. This is nothing more than a roll with a number of pin like projections that when contacting the rubber releases the entrapped air.

If you are skim coating, or laminating in your calender operation and you observe separation of incomplete coating, one of the causes may be from roll deflection caused by separating forces. This may be accentuated by improper feeding. While proper feeding will help, the roll deflection can be minimized. The thinner the product, the greater the separating forces and the resultant roll deflection. The deflection is of course greatest in the center of the nip where the rolls are weakest due to the distance from the bearing supports. Some methods of compensating for the deflection are roll honing, roll crossing or roll bending.

If your company has a calender with roll bending or roll crossing then it is a simple matter to compensate for the roll deflection. Roll crossing is depicted in figure 1. This is accomplished by moving the rolls in the pattern shown thus the nip remains constant when the rolls deflect. The amount of crossing is determined by a check of the product gauge across the entire width of the product. The crossing adjustment is continued until the finished calendered product meets your gauge requirement. If you are fortunate enough to have a calender with roll bending (figure 2), then by varying the bending cylinder's pressure the roll deflects to form the opposite effect to the product that the deflection causes, resulting in the proper product gauge accuracy.

Most calenders do not afford the advantage of having roll bending or roll crossing as these are expensive add on options. Since this article is directed more toward the companies that have a standard calender with no deflection correcting option, the next best corrective function that can be done is roll honing (sometimes called roll grinding). This operation is done in your plant with the calender intact and no roll removal is required. The operation should be done by professionals that do roll honing service continually. In this service, they mount a set of honing blocks to a holding device that attaches to the calender frames and with the calender revolving the honing stones are drawn across the surface, refinishing the roll surface.

The deflection when the product is run, is the pattern the honing person duplicates in reverse on the roll surface. The result is, as the roll deflects the contoured roll deflects, but with the compensating honing pattern it produces a flat sheet. This is not quite as simple as it may sound as the operation has to be done several times with only a minimal removal of roll surface each time and then the product has to be run through to check for the proper contour. The operation is repeated to obtain the final result desired.

The problem with this method is the separating force causing the roll deflection varies with each product, so unless the calender is dedicated to a particular product for a reasonable length of time, the honing may not be advantageous. It should be noted that the proper feed of the calender is critical as the separating forces exerted on the rolls varies as the feed or the temperature of the feed changes. You could defeat the honing operation if proper manufacturing methods are not strictly adhered to. This of course means that management has to have the control over the calendering function, not an operator that is normally going to take the easiest method for himself, not necessarily the best for the product quality. Too many companies allow the calender operator to have the control because he has several years experience and therefore is considered an expert. The plant engineer has the responsibility for the product but not the authority to control, so he has a difficult time convincing management that some changes in the calendering operation should be made. In calculating separating forces on the calender it is good to remember that except for a few products, a separating force of 2,400 pounds per lineal inch can be incurred with a product gauge of .010".

The quality of the finished product can also be affected by the type of rolls the calender has. Calender rolls are usually chilled cast iron, or forged steel with the majority being chilled cast iron. I have observed that a good number of people when questioned do not seem to be sure if they have cored rolls, bored rolls or drilled rolls so I will add a little information on the chilled cast iron rolls (figure 3).

A chilled cast iron roll is manufactured by pouring molten iron of a specific quality into a steel jacketed form or mold that is positioned vertically. The molten metal is introduced into the mold at the lower point at a 45 degree angle to the surface thus causing a circular motion as the molten metal fills the mold and this forces the impurities and foreign particles upward so when the mold is filled, the impurities are easily removed from the surface. This method is why there is such good quality in the finished chilled cast iron rolls. As the molten metal comes in contact with the colder temperature of the mold, the outer diameter of the roll chills faster causing a chilled hard surface, usually 65 to 72 Rockwell "C".

The types of chilled iron rolls supplied to the industry are cored, bored and drilled. The cored roll is the least expensive but also the least effective. The cored roll is formed during the previously described pouring process by the insertion of a form in the mold. This causes a hollow center and when the core is removed, the cored roll is formed having a wall thickness of about 6 inches. Since the wall is six inches thick the heating or cooling media has to try to control the surface temperature through the entire wall, thus making the cored roll the least effective because there is the possibility of the core shifting slightly causing a temperature difference from one side of the roll to the other. The bored type roll has the core bored out so the wall thickness is the same around the entire roll allowing the more even temperature control and response time is minimized.

The drilled roll is by far the best type as the roll is manufactured to look like a round platen. The drill pattern is just below the surface of the face of the roll and response time to temperature control systems and the desired temperature action to the product are at the optimum. The roll will generally have a triple pass style drilling. The fluid enters from the one end of the roll through a rotary joint and is conveyed to the hole just under the surface where it travels the face of the roll, flows to the next hole and returns to the opposite end where it flows to the center of the roll and is siphoned out. This method permits the fluid to be subjected to only three passes across the face. The entire face has a minimal difference of temperature at any point on the surface, allowing for better product temperature control.

If you have drilled rolls and your are having a problem with a difference in temperature over the face of the roll, it is most likely the drill passages are partially blocked and they should be acid etched to remove the scale or the rust that has formed.

In conclusion, the calendered products you now manufacture can be improved in most cases with only minimal changes and with correct maintenance and proper operation. [Figures 1-3 Omitted]
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Copyright 1989, Gale Group. All rights reserved. Gale Group is a Thomson Corporation Company.

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Author:Bauman, Don
Publication:Rubber World
Date:Jul 1, 1989
Previous Article:Injection molding thermoplastic elastomers.
Next Article:Compounding requirements and techniques for rubber covered rolls.

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