Edited by John R. Gyorki
Flexible Products Div.
Watlow Electric Mfg. Co.
St. Louis, Mo.
Silicone-rubber heaters can solve a wide variety of thermal problems that often don’t become apparent until after a product has been designed and manufactured. These heaters are most frequently used to reverse the adverse effects of low temperature or high moisture in diverse instruments such as blood analyzers, electrical enclosures, and equipment for silicon-wafer fabrication.
The heaters comprise two layers of flexible, reinforced silicone rubber. A wire-wound or etched-foil resistance element vulcanized between the two layers is patterned to best fit specific applications. Heaters may vary in size from several square inches to 3 X 10 ft and come in squares, disks, cylinders, cones, and boxes. The heaters typically generate temperatures from 40 to 450°F and can be regulated by thermostats, thermocouples, thermistors, RTDs, and thermal fuses.
Although the best time to design thermal heaters into a product is at the start of a project, many engineers discover a need for a heat source after the job is done. Whatever the problem, it’s usually necessary for the heater to fit within existing design parameters.
For example, frozen blood must be heated before administered in transfusions. If the blood is too warm or cold, the recipient can die. To fit within the dimensions of the equipment, engineers designed a warming device with silicone-rubber heaters that resembles a waffle iron. The top and bottom plates are individually monitored by thermistors and a two-channel digital controller connects to the heaters. The blood can then be safely transfused at 98.6°F.
Similarly, accurate testing of blood samples may call for an etched-foil silicone-rubber heater assembly to reliably maintain body temperature. Such an assembly was developed for a medical-equipment manufacturer. In this application, heaters are factory vulcanized to aluminum blocks which sandwich a precision hypo tube full of the sample.
Another application for a high-temperature (HT) foil heater is in heating a silicon wafer on a hot chuck for vacuum-vapor deposition at 300 to 500°C. The heater is only 0.030-in. thick and insulated with a mica sheet to provide fast temperature response. Etched-foil, zoned heating creates more uniform part temperatures.
An unusual application involves heating an underwater component. Heat activates a shape-memory alloy device used to keep large ships from rolling or listing in severe weather conditions when cargo is not balanced. A high-wattage, etched-foil unit is factory bonded to the alloy sleeve with a custom silicone insulation jacket molded around the assembly to provide a watertight package. When heated, the alloy sleeve expands with sufficient force to shear a bolt and release ballast, keeping the vessel level.
Silicone-rubber heaters can be custom formed into curious shapes. For example, one is a box shape to heat a valve assembly that controls SF6, a high dielectric gas used in high-voltage switching stations. An integral bimetal thermostat activates the heater only when ambient temperatures drop below freezing.
Most silicone-rubber heaters are custom designed for specific applications. And because quick turnaround is frequently imperative, the following is a number of questions that a heater specialist often asks product-design engineers when discussing an application:
• What type of material requires the heat?
• What is the temperature range?
• What is the ambient temperature?
• How quickly does the temperature need to rise?
• What are the part dimensions?
• Does the job require a thermostat or solid-state control with a thermocouple?
• How is the temperature controlled?
• What are the voltage and power requirements?
The versatility of silicone-rubber heaters lets designers heat flat or curved surfaces in most applications, and fit complex geometries to provide the precise amount of heat needed in hard-to-reach places. If required, holes, cutouts, and notches can be made to intimately fit the part. Furthermore, thin sheath material can be used to improve heat-transfer efficiency, and the units can be thermally insulated with various thicknesses of silicone sponge rubber. Special leads or terminals can be provided, and thermostats, thermocouples, thermistors, RTDs or thermal fuses can be positioned to sense either the heater-sheath temperature or the part temperature.