Specifying Mica Band Heaters
Important parameters to consider when specifying band heaters are: maximum operating or sheath temperature, AC voltage required, and watts. Electrical power is measured in watts. Maximum operating temperature is the maximum temperature that the sheath covering the heater may reach. The maximum sheath temperature does not represent the maximum temperature that a heated substance may reach. AC voltage required represents the minimum AC Voltage required to operate the heater. Watts represents the maximum wattage available from the heater. Important dimensions to consider for band heaters are the inside diameter of the band, the width of the band and the thickness of the band. The heater inside diameter should be the same size as the outside diameter of the heater cylinder to be heated.
Mica band heaters are manufactured by enclosing a precision wound nickel-chromium resistance ribbon inside a specially treated steel sheath. Mica is used to insulate the resistance ribbon from shorting out against the outer steel sheath.
Mica band heaters are designed with nickel-chrome resistance wire precisely wound around a mica sheet, which is then placed between two additional corrosive resistant outer mica steel sheets. Mica is used because it provides good thermal conductivity and dielectric strength. It can withstand voltage spikes, resist moisture, and is flexible enough to conform to the curvatures of plastic molding machine barrels. Since mica is also a heat insulator, only top grade mica, which is inorganic, has high temperature stability, and is “paper thin” is used. This “paper thin” characteristic enables a quick transfer of heat through the mica to the outside steel sheath, thus transferring the heat away from the red hot internal resistance ribbon winding.
The four basic parts or components of a heater are the resistance element, the insulation, the sheath and the termination. The resistance element is the “heart” of an electric heater. The resistance element is usually in the form of a small diameter, metal alloy wire or flat, thin ribbon. When a heater is connected to an electric power supply (Voltage) or electricity flows through the resistance wire. The wire resists the flow of electric current through it, and this converts electrical energy into heat energy, causing the resistance element to get very hot. That is how heat is generated in an electric heater.