A cartridge heater is a tube-shaped, heavy-duty, industrial joule heating

Joule heating

Joule heating, also known as ohmic heating and resistive heating, is the process by which the passage of an electric current through a conductor releases heat. It was first studied by James Prescott Joule in 1841. Joule immersed a length of wire in a fixed mass of water and measured the temperature...

 element (electrical resistance

Electrical resistance

The electrical resistance of an electrical element is the opposition to the passage of an electric current through that element; the inverse quantity is electrical conductance, the ease at which an electric current passes. Electrical resistance shares some conceptual parallels with the mechanical...

) used in the process heating industry, usually custom manufactured to a specific watt density, based on its intended application.

It may be used to heat any kind of substance or material, such as oil, air, or sugar. However, a common application are plastic pellets, which are melted in order to form molded plastic parts. An example of such a process is the "hot runner

Hot runner

A hot runner system is an assembly of heated components used in plastic injection molds that inject molten plastic into the cavities of the mold...

" process.

Construction

The cartridge heater may be divided in 7 main parts:

• Heating Coil
• Insulation
• Sheath
• Sealing
• Termination
• Lead Wire Type
• Watt Density

Heating coil

The heating coil is the actual resistance which is where the electrical load occurs. The most common type of metal alloy used for this purpose is a Nickel-Chromium mixture, also known as Nichrome

Nichrome

Nichrome is a non-magnetic alloy of nickel, chromium, and often iron, usually used as a resistance wire. Patented in 1905, it is the oldest documented form of resistance heating alloy. A common alloy is 80% nickel and 20% chromium, by mass, but there are many others to accommodate various...

. the Nicrome wire is winded around a ceramic core, and the number of spirals per inch vary according to the requested watt density. Potential from an alternating current source, which can either be 2 phase or 3 phase, flows through the coiled Nichrome wire, heating up the wire, which in turn, heats the cartridge heater sheath.

Insulation

Insulation is used to prevent the Nichrome coil to contact the sheath. If such event happened, it would ground the resistance and produce a catastrophic short-circuit, resulting in a melted sheath and a major equipment failure. To prevent the coil from touching the sheath, the coil is inserted into the sheath, and immediately filled with Magnesium Oxide

Magnesium oxide

Magnesium oxide , or magnesia, is a white hygroscopic solid mineral that occurs naturally as periclase and is a source of magnesium . It has an empirical formula of and consists of a lattice of Mg2+ ions and O2– ions held together by ionic bonds...

 (MgO). To ensure the MgO fills the empty space between the sheath and the coil, the cartridge heater is vibratory filled.

Sheath

The sheath is the part of the cartridge heater which makes contact with the material or substance to be heated. Several metal alloys are used, depending on the type of application, such as highly acidic or corrosive environments. The most common types of sheaths are 304 Stainless Steel

Stainless steel

In metallurgy, stainless steel, also known as inox steel or inox from French "inoxydable", is defined as a steel alloy with a minimum of 10.5 or 11% chromium content by mass....

, 316 Stainless Steel, and Incoloy 800. Incoloy has the highest temperature rating, it is considered a Superalloy

Superalloy

A superalloy, or high-performance alloy, is an alloy that exhibits excellent mechanical strength and creep resistance at high temperatures, good surface stability, and corrosion and oxidation resistance. Superalloys typically have a matrix with an austenitic face-centered cubic crystal structure. ...

.

Sealing

After the cartridge heater has been filled with MgO, a seal is applied to the open end of the cartridge heater (where the Nicrome coil was introduced), this prevents the coil and the MgO from coming out. Also, it prevents contamination, such as plastic debris, air, or moisture, to go inside the heater.

Termination

Since cartridge heaters are installed in a wide variety of machines, manufacturers must design the heaters to meet certain clearances. The cartridge heaters might be terminated with the leads coming out straight, or in a right angle. Also, manufacturer must be careful that the leads are not exposed to temperatures higher than the maximum rating for the lead wire. In order to prevent lead wire damage from temperature, movement or contamination, the lead wire can be protected with metal conduit or braided metal.

Lead wire type

Depending on the clearance and the design of the machine where the cartridge heater will be inserted, the type of wire used will vary. Fiberglass

Fiberglass

Glass fiber is a material consisting of numerous extremely fine fibers of glass.Glassmakers throughout history have experimented with glass fibers, but mass manufacture of glass fiber was only made possible with the invention of finer machine tooling...

 is the commonly used for cartridge heaters and other high temperature applications, such as automotive wiring harnesses and industrial equipment. Other variants used are silicone

Silicone

Silicones are inert, synthetic compounds with a variety of forms and uses. Typically heat-resistant and rubber-like, they are used in sealants, adhesives, lubricants, medical applications , cookware, and insulation....

 impregnated fiberglass and silicone rubber

Silicone rubber

Silicone rubber is an elastomer composed of silicone—itself a polymer—containing silicon together with carbon, hydrogen, and oxygen. Silicone rubbers are widely used in industry, and there are multiple formulations...

.