Split-Sheath Cartridge Heaters

The efficiency and life expectancy of a cartridge heater depends mainly on the fit tolerance. Cartridge heaters are typically manufactured .003" to .005" smaller than the nominal diameter. Drilled holes are typically .001 to .003 larger than the nominal diameter. Inserting a cartridge heater with a nominal minus of .005" into a drilled hole with a nominal plus of .003" results in a fit tolerance of .008". Although a tighter fit is desirable to efficiently transfer heat and to get long heater life, a looser fit will help in installing and removing long heaters. We therefore recommend split-sheath cartridge heaters(.pdf).

A "loose" fit condition of .008" may be acceptable for:

1. low watt density at higher temperatures (example: 30 WSI @ 1400°F)
2. higher watt density at lower temperatures (example: 200 WSI @ 200°F)

Besides the heater, you need to consider also:

• Temperature sensor
• Temperature controller
• Electrical power switching

If the process requires higher temperature and/or higher watt densities , consider this:

1. Instead of drilling the hole, ream it to a tighter tolerance
2. Use split sheath cartridge heaters (see below)
3. Add more heaters to reduce watt density per cartridge heater

General considerations when ordering cartridge heaters:

1. Application
2. Total watts required
3. Lead configuration

The split sheath design has a continuous heating coil which eliminates the cold spots which can occur at core junctions in ceramic core heaters. Uniform sheath temperature is vital when molding heat-sensitive plastics where the temperature window between gate freeze-off and material degradation is very narrow. By varying the watt-density, the temperature profile may be customized to specific requirements. For example, applications requiring higher temperatures at the tip can be accomplished with split sheath heaters but not with conventional cartridge heaters.