- August 01, 1995, Hahn, Mark; Hyllberg, Bruce
New heater roller technology is reportedly yielding reduced warmup and temperature-recovery times while reducing maintenance downtime and improving process control.
Converters using heated rollers to preheat, heat-set, heat-stretch or laminate a substrate typically employ oil, water or steam to heat the roller. These liquid systems have dominated the industry virtually since the first heated roller was used. Many require a remote reservoir to heat the liquid, a pump to transfer it to the roller and a complex power supply to handle the high-energy requirements.
An alternative to liquid-heated roller systems has been developed in response to industry requests for an easier-to-use, lower-maintenance system. The new heater roller combines a thermally conductive ceramic coating material and state-of-the-art electronics to generate and control resistive heat on the roller's surface.
With this technology, heat is produced in closer proximity to the substrate in order to provide rapid temperature recovery and reduce initial warmup times to maximize production. In addition, simplified components and direct-line voltage connection eliminate the need for complex power supplies to reduce maintenance items and downtime.
An overview of the characteristics of liquid heater roller systems, along with a look at how the new ceramic roller works, highlights differences in the operation and maintenance of each.
The heated roller used in liquid systems consists of a simple, single-shell design or, more commonly, a spiral-baffled, double-walled roll to maintain more consistent thermal transfer across and around the roller. In either case, the roller is heated from the inside out, so the surface reaches the desired setpoint temperature. Therefore, the whole core mass has to reach temperature and stabilize before the roller can be used.
To produce enough heat at the surface of the roller, internally heated systems may have to be set at temperatures of 2.5 [degrees] to 50 [degrees] F (or more) above the desired roller surface temperature. Liquid systems require a specific roller diameter and a large amount of heat-transfer fluid to maintain temperature at high speeds, which is reflected in energy usage.
Typical warmup times with liquid systems are 30-60 min., depending on the processing temperature required. Temperature is monitored at the tank, not the roll, which makes temperature adjustment at the roller more art than science.
The oil-heated system has been the workhorse of the converting industry for many years. Recently, however, concerns have risen about the disposal of carbonized oil. Through constant thermal cycling and high temperature, oil breaks down and carbonizes. At this point the waste oil requires special packaging and disposition at expensive, isolated toxic waste landfills. Also, carbonized oil restricts flow and coats the internal surfaces of the roller, hoses, pumps and filters. This negatively affects heat transfer and, therefore, maximum system output. Caustic properties of carbonized oil also make cleaning a challenge.
The vast majority of thermal laminating, heat-stretching and preheating applications use oil-heated systems because of the medium/high requirements of the process. Critical system parts include the heater element, remote oil reservoir, temperature-sensing device, pump, filter, seals, plumbing and rotary union. All are typically stocked by converters to minimize downtime due to component failure.
The most significant challenge converters experience with an oil system is maintenance. Routine cleaning and replacement of worn components is critical to sustain satisfactory performance. Older systems often require more maintenance and, thus, increased downtime.
Another relatively common occurrence is leaks from constant thermal attack, resulting in potential safety concerns over oil on the plant floor.
Approximately 20-25% of heated roller systems in use today are water or steam. Generally, water is used for low-heat applications, below 200 [degrees] F, while steam is typically used for medium/high applications that can go beyond the range of oil systems. In both systems encased heater elements are usually situated in the bottom of the remote reservoir to heat the water.
Internal and external corrosion and scale buildup on the roller are common occurrences in both water and steam systems. These phenomena limit the life of the roller and dictate that rollers be acid-washed on a regular basis to remove scale deposits.
A high-temperature steam system may require a gas-fired boiler to generate excessive heat, creating a pressure vessel that requires regular inspection and repair.
With water and steam systems, converters usually stock mechanical components and heater elements to minimize downtime associated with integral component failure.
The Ceramic Heater Roller
A new ceramic heater roller is designed for applications requiring heat from ambient to 450 [degrees] F. Heat is generated on the outside of the roller, so the surface is the first area to reach the desired setpoint temperature. With this configuration, it is not necessary for the entire core to reach temperature before production can begin, as is the case with liquid systems. This translates into faster warmup times (about 1/2 to 2/3 of the time required for an internally heated roller of the same wattage in the same application), quicker temperature recovery and more consistent temperature control.
The roller consists of a variety of ceramic materials thermally applied to the outer surface of the core, which, when combined, form a thin, homogenous heater element that covers the entire roller face and precisely generates radiant heat. Electrical connection is made to each end of the roller and brought to the controller via a rotary connector.
A noncontact, infrared temperature sensor is typically used to directly monitor roller surface temperature. The sensor signal is sent back to the solid-state electrical controller, which analyzes the information and determines how much power should be sent to the heater element.
Since heat is produced and measured near the outer core, the desired roller-surface temperature can be accurately monitored and maintained. The effect of the system's simple construction and control mechanisms is a significant reduction in maintenance downtime and improved process control compared to liquid systems.
Temperature recovery with the ceramic heater system is also superior to conventional systems due to the external heat source and solid-state electronics. Quite simply, the isolated ceramic heater element is close to the roller surface, and, therefore, less heat-transfer time is required to maintain consistent temperature.
In cases where dwell time isn't a critical factor, converters may be able to run their lines at faster speeds, increasing overall throughput. Also, it may be possible to utilize smaller-diameter rolls, because temperature-recovery time is greatly reduced.
The ceramic heater roller uses simple, standard core construction to provide adequate core strength without the excessive weight of the double-shell-type roller. While the initial investment may be equal to or slightly higher than that of liquid-heated rollers, the long-term energy savings, improved process controls and reduction in maintenance more than make up the difference in cost.
The general trend in heater roller technology is toward maximized efficiency in order to increase throughput and machine uptime. Converters want to streamline operations and reduce energy consumption within the confines of their existing systems.
Because of increased environmental demands, those companies with oil systems are replacing refined and synthetic oils with animal oils that are reported to produce less carbon. Those with water systems are tending to use treated water to reduce scale buildup. Other improvements have also been made in flow rate and roller design to reduce overall weight.
The ceramic heater roller system can be retrofitted in numerous operations because of its wide temperature capabilities and variety of covering materials, including abrasion-resistant, high-polished ceramic; conventional rubber; high-release formulations; nylon; and Teflon.
Mark Hahn is market development manager for the Arcotech Specialty Roller Div. of American Roller Co., as well as product manager for all advanced hard-coat rollers used in converting processes. He has served the graphic arts and converting industries for over 10 years and is an active member of the Flexographic Technical Assn.
Bruce Hyllberg is research specialist and senior chemist at American Roller's research and engineering facility. His expertise lies in the design and development of components for electrical roller products used for printing and converting applications.