Watt-Flex® fit for life
Good fit for starts with the correct bore size!
Fit is a critical issue for effective operation of cartridge heaters. Fit is the
difference between the diameter of the cartridge heater and the bore diameter.
Wall-to-wall contact provides conductive heat transfer, which is the most
effective way to move heat from the heater to the tooling host metal.
Conventional cartridge heaters can only make point contact along their profile,
as shown in the left illustration below. Radiant heating, sometimes referred to
as “open air” heating is the least effective manner to transfer heat due to air’s
very low thermal transfer coefficient. The layer of air around the conventional
heater and inefficient radiant heating limits effective heat transfer.
Conventional heaters, must therefore, depend on a tight fit for effective heat
transfer…the downside is that they often bow in the middle and wedge
themselves into the bore – seizing.
The Watt-Flex® heater is engineered with a 0.005” smaller diameter to
guarantee easy installation, shown in the right illustration above. Once inserted,
Watt-Flex® cartridge heaters expand when energized, as shown in the
center, to achieve conductive heating on both sides, doubling the contact for
superior heat transfer.
The general rule for fit is to allow 0.005” for the heater to expand. Using a ½”
heater as an example, the heater diameter is 0.495” +/- 0.002”. We determine
minimum bore diameter by adding 0.005” to the maximum tolerance of 0.497”
and obtain minimum bore of 0.502”. Since the bore drilling process has a
normal machine tolerance, the bore is typically specified as 0.502” – 0.505”.
The split sheath design expands, making contact with both walls of the bore,
producing effective conductive heat transfer and long life.
Factors that influence bore specification are:
- Long heaters are more tolerant of over-sized bores
- Short, large diameter heaters will not expand as longer ones
- Short, small diameter, higher watt density, heaters need tight fit bores
To get custom fit, contact your Dalton representative and Trust the Dalton
Difference to be fit for long life.