In order to understand the causes and related problems associated with heater failure we should first understand the construction of a heater.

(1) The sheath of a heater can be made from many types of metal tubing. The better heaters are constructed using a stainless steel sheath, which resists rust and corrosion.

(2) The core of the heater can be made from ceramic or highly-densified mag-nesium oxide. The better heaters are made from magnesium oxide because ceramic cannot be swaged.

(3) The internal thermocouple should be made from braided thermocouple wire. The most common thermocouple used in the hot runner manifold system is type J, (Fe-CuNi) Iron and Constantine. The white wire is positive (+) and the red is negative (-). If type J thermocouple wire colors are different than red and white,

or if the color has been changed because of heat, the positive (+) wire is magnetic and can be detected by the use of a magnet. The magnet will have no effect on the negative (-) wire. If thermocouple splicing is required, the splice will have to be made with type (J) wire.

If another type of wire is used, the temperature reading will not be correct or the controller may not work properly. The thermocouple may or may not be grounded.

(4) The heater wire used in most heaters is a Nickel-Chrome resistance wire – not much thicker than the hair on your head. This wire is tightly wound on the core material and spaced for proper resistance. Some heaters are progressively wound. This changes the watt density in selective areas of the heater, so it can produce more heat in an area where it may be required. These heater wires are connected to the heater leads by nickel terminals.

(5) The heater core is then placed inside a precut length of tubing, which has a cap welded or crimped to one end. The core is centered in the sheath using a fiber or cardboard insert. The assembly is then filled, using vibration, with Magnesium Oxide Powder. The end cap or seal is then attached.

(6) The inexpensive heaters are now ready for sale.

(7) The more expensive heaters are assembled in an oversized sheath and will go to the next step where they are swaged or reduced in diameter to the specified size. This process produces a superior heater because of the density increase of the insulation. Some are then centerless ground, and others have special coatings added all of which help to make a higher quality heater. These processes also affect the price of the heater, but the additional cost is justified with extended heater life.

(8) Magnesium Oxide is very hygroscopic. This means that the humidity, mold sweating or leaking, hydrocarbons and other forms of contamination in the molding environment will affect the heater’s life.