Saturday, May 13, 2017

Swaged vs. Standard Cartridge Heater Design and Heater Life

Cartridge heaters provide localized heat to restricted work areas requiring close thermal control. Dies, platens and a variety of other types of processing equipment are efficiently heated. Closely controlled work temperatures up to 1400° F (760° C) are obtained by a combination of heater location and proper wattage output.

Heater Life

Cartridge heater life is determined by how efficiently the heat generated in the resistance wire can be conducted away from the wire and into the part being heated.  The efficiency of heat transfer is generally controlled by three factors:
  1. Resistance wire watt density
  2. Density of insulating material around the wire
  3. Fit of the heater into the heated part
There are two basic designs of cartridge heaters - swaged and standard. Although both type heaters look identical, the internal construction is very different.

Standard Cartridge Heater Design

Standard Cartridge Heater Design
Standard Cartridge Heater Design
Nichrome wire heating coils are inserted in holes formed in ceramic tubes. Pure magnesium oxide filler is vibrated into the holes housing the heating coils to allow maximum heat transfer to the stainless steel sheath. The heater then has a Heliarc welded end cap inserted on the bottom of the heater and insulated leads are installed. The MGO powder is not compacted and heat transfer is a function of the grain-to-grain thermal conductivity to the heater sheath, and then into the heated part. Because of this, the heater wire watt densities must be kept in the low to medium range.

Swaged Cartridge Heater Design
Swaged Cartridge Heater Design

Swaged Cartridge Heater Design

Swaged cartridge heaters wind Nichrome wire around a precision ceramic core and the carefully position the resistance wire and ceramic core uniformly inside the the heater sheath. Pure magnesium oxide (MgO) powder is then vibrated in and the heater is swaged to a specific diameter. Swaging is a process that mechanically forces the heater through a confining die to reduce its diameter and thus compact the powdered MgO to rock-like consistency for greater thermal conductivity. This compressed MGO transfers the heat from the resistance wire much more efficiently. The improved heat transfer allows for higher wire watt densities allowing swaged cartridge heaters to operate at higher temperatures.

Cartridge Heater Fit

The most common cause of cartridge heater failure is an improper fit in the hole into which it is inserted. If the heater is surrounded by air, an excellent thermal insulator, it cannot dissipate it's heat into the part with optimum efficiency. The result is much higher temperatures on the Nichrome wire and failure.  The goal to longer life with cartridge heaters is to accommodate the tightest fit practical for a given application.

In summary, if you want the longest life cartridge heater, choose a swaged heater and make sure you provide a machined, close tolerance fit between the outside diameter of the heater and the inside diameter of the hole.