Fastener Placement and Its Effect on Heater Temperature Profile

The correct location of the fastener along the length of the heater is a feature that is often missed when designing affixed cartridge heaters. The location of the nut, flange, or threaded bushing influences both the mechanical safety and the temperature profile of the heater. A cold spot can form if a fastener is too close to the heated area. A fastener that is too far back can waste a lot of heated length. To get the positioning perfect, you need to know how the heater and the cavity work with heat.

The best place for a fastener is in the heater's cold zone. The cold zone is the part of the lead end that isn't heated and where the internal resistance wire doesn't make heat. The distance between the lead outlet to the start of the internal winding determines the length of the cold zone. A cold zone is usually between 15 and 30 millimetres long, but it can be larger for heaters with large diameters or for high-temperature applications. The sheath temperature is lower here because the active zone only heats it by conduction. Putting a fastener here won't change the temperature of the active length by much.

There are a few things that happen when the fastener has to be in the active zone. The fastener works like a heat sink, moving heat away from the heater body. The temperature at the fastener will drop by 10°C to 30°C, depending on the size and material of the fastening. Because of this localised cold region, the controller may give more power to the rest of the heater, which could cause other parts to get too hot. The heater's temperature is not consistent over its length.

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The fastener should be as small as possible to reduce the cold spot effect. A narrow nut or a low-profile flange doesn't hold as much heat as a thick one. The material used for the fastener should not conduct heat well. Titanium fasteners don't hold heat as well as stainless steel. Ceramic fasteners don't hold heat well, yet they are fragile. Adding a thermal barrier between the fastener and the heater is another way to do this. A thin mica washer or a ceramic spacer slows down the transfer of heat.

The distance between the fastener and the start of the active winding is also important. The cold area is more noticeable if the fastener is extremely close to the winding because the heat has to travel less distance to reach the fastening. There should be a space of at least 5 to 10 millimetres between the fastener and the active zone. The manufacturer can change the length of the cold zone to fit a certain fastener placement.

The ideal way to design the heater, based on what I've seen, is to put the fastener in the cold zone. This means that the cold zone needs to be long enough to fit the fastening. A cold zone of 20 to 25 millimetres is usually enough for a regular nut or flange. A cold zone of 30 to 40 millimetres may be needed for bigger flanges. The downside is that a longer cold zone makes the heated length shorter. If every millimetre of heated length is important, you may need to make a trade-off.

In short, where you put the fasteners has a big effect on the temperature profile of a cartridge heater. The best spot to put it is in the cold zone. If you have to put something in the active zone, make the fastener as small as possible, utilise materials that don't transfer heat well, provide thermal barriers, and keep a space between the winding and the fastener. The company that makes the heater can assist you figure out the best length for the cold zone for a certain fastening. A properly placed fastener keeps the heater in place without causing any problems with heat. If a fastener is put in the wrong spot, it might make a heater that works well become unstable.