There are many processes in the converting industry that require the use of a heated roller. While these processes often differ in many ways, there are common needs to be met and problems to solve when choosing a heat-transfer roll. For today's manufacturers, the need to increase line speeds and reduce scrap while also managing through shorter product runs and frequent changeovers are top challenges.

The typical roller selection for many decades has been the oil-heated roll. Hot oil from a remote reservoir is pumped in and out of a hollow or double-shell roller. To cause heat flow from the oil inside the roller to the roller surface where it contacts the web, the oil must be at a higher temperature than the intended roller-surface temperature. As the line speed increases, or the thermal load (heat transfer to the web) on the roller increases, the oil temperature will have to be increased to also increase the heat transfer to the roller shell.

Good maintenance and careful attention to the temperature profile is mandatory to ensure high-quality product runs. A temperature overshoot can cause process problems with the intended function of the roller until the temperature is reduced. Product run under those conditions will most likely go into the scrap or recycle bin.

There are a number of factors that can reduce temperature uniformity of the roller surface and thus affect product quality. The inside surface of the outer shell must be clean, uniform and free of contamination or corrosion. Small changes in the surface quality can have a dramatic effect on the rate of heat transfer to the outer shell. A very thin layer of corrosion can easily produce a change of several degrees in the roller-surface temperature.

Another major source of contamination of the roller's inner surface is decomposition products as the oil breaks down chemically, sometimes referred to as "carbonizing." Carbonized materials coat the inner surface of the roller shell and reduce the rate of heat transfer. The net effect of contamination and corrosion is that the roller has to be removed for internal cleaning. The cleaning process obviously stops production and can be somewhat hazardous.

One way to correct temperature-uniformity problems is with a high-performance, electrically-heated roller system. This typically consists of the roller core, which includes heat pipes, heaters, temperature sensors, and wiring; a rotary electrical connector (slip ring) and the control panel. A heat pipe is a passive device that requires no external power to make it work. It works continuously to make its own internal surface temperature uniform. It can detect and change temperature differences as small as 0.01 deg F.

The initial purchase price of a typical high-performance, electrically-heated roller is usually higher than an oil-heated system. However, electrically-heated rollers have several advantages compared to oil-heated rollers. For example, setpoint accuracies are +/-2 deg F, up to 650 deg F. Temperature uniformity will hold at +/-2 deg F under all load conditions and web widths. In addition, electrically-heated rollers can provide up to 100-percent greater power (heat output) than oil-heated rollers.

Total cost of ownership is something to consider when buying a heated-roll system. The questions facing all converters today: Is the return on investment for this technology present? Will performance or productivity benefit from the purchase?

Estimates of the costs for different types of heated-roller systems have been made. In the examples below, cost analysis made the initial purchase price of an electrically-heated system look insignificant. Remarkably, only small improvements need to be made to justify the expense of a higher performance roller.

Example: Roller 1 is a small, 8-in. diameter laminator, 30-in. long, which runs at 250 deg F and 100 fpm. The roller runs one shift per day, five days per week. If the electrically-heated roller is able to run only 5.0 percent faster with the same rate of scrap, the extra cost of the electrical roller would be covered in less than 12 months and an additional $19,000 would fall to the bottom-line annually—after this initial ROI period. If the production rate is increased 20 percent, that number increases to $59,000 annually. One customer with a similar setup was able to run 100-percent faster because of the higher heat-transfer rate to the roller surface.

Chart 1 (see page 42) shows potential dollar savings, with improved productivity from an electrically-heated roller. The percentages are combined savings based on increased performance and productivity enhancements witnessed by the use of electrically-heated systems.

The productivity increase measured in dollars is reflected in the third column of Chart 1 as $59,000 for a 20-percent increase. If this same customer was also able to reduce the scrapped product from 3 to 2 percent (one third less scrap), an additional $4,000 would be saved (see Chart 2 above).

An electrically-heated roller system benefits the user by having lower costs for electric power, maintenance and repair, downtime, scrap and rework, and operating costs. At the same time, such a system can provide higher production rates and more consistent product quality.

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