Understanding Watt Density: Why Your 28mm Cartridge Heater Runs Hot or Lasts Long

Have you ever puzzled why two heaters that look the same (same diameter, length, and wattage) last for quite different amounts of time in the same machine? Watt density is usually always the answer, but it isn't something you see on a simple specification sheet.

Watt density is just the amount of power that is focused on each square inch of the heater sheath's surface area. But this little measurement has a big effect on how hot the inside parts get and how long the whole thing lasts.


To find the watt density, divide the total wattage of a cartridge heater by the surface area of the part of the sheath that is heated. The surface area of a cylindrical heater is π times the diameter times the length of the heated region.

Think about a cartridge heater that is 28mm in diameter and has a heated length of 200mm. The surface area is about 17,584 square millimetres, or about 175.8 square centimetres. This is because 3.14 × 28mm × 200mm=17,584 square millimetres. If this heater is rated at 1000 watts, the watt density is about 5.7 W/cm², which equals 1000 ÷ 175.8.

That number, 5.7 W/cm², is perfect for tough industrial uses. Experience reveals that a cartridge heater that works between 5 and 7 W/cm² gives off a good amount of heat and lasts a long time. The heater operates cooler and lasts longer below this range, but it could not provide adequate heat for some uses. Temperatures inside rise quickly over this range, which speeds up degradation.


When a cartridge heater has too much watt density, the wire that connects the internal resistance makes more heat than the MgO insulation and sheath can move into the surrounding material. The temperature of the wire goes considerably over what it was designed to be.



The resistance wire oxidises faster, which makes it more resistant to electricity and creates hot spots.

The MgO insulation modifies its structure, which makes it less able to conduct electricity and heat.

The wire, MgO, and sheath all expand at different rates when they get hot or cold. This puts stress on the insulation, which finally causes it to split.

The sheath material could change colour, get scales, or even melt in the worst situations.

The result is that the service life is cut short, frequently in weeks or months instead of the years that were intended.

The 28mm Edge
This is when a heater with a bigger diameter really shines. A 28mm big diameter cartridge heater has a lot greater surface area than a smaller diameter heater that uses the same amount of power. At the same power level, a larger surface area indicates a lower watt density.

For example, a heater with a diameter of 16 mm and a heated length of 200 mm has a surface area of around 100.5 cm². A heater of size uses almost 10 W/cm², which is too much for most uses. In a 28mm heater, the same 1000W runs at 5.7 W/cm², which is completely safe.

In real life, this means that the 28mm size can manage more total watts while still maintaining inside the required watt density restrictions. The 28mm cartridge heater is sometimes the only viable choice for high-heat uses like large injection moulds or hot runner systems.


varying processes and materials need varying amounts of watt density.

When working with plastics and rubber, which are sensitive to degradation, a watt density of 10 to 30 W/in² (around 1.5 to 4.5 W/cm²) is best. When the density is too high, the plastic could burn at the mould surface.

For heating metals and other industrial materials, a medium to high density of 30 to 50 W/in² (4.5 to 7.5 W/cm²) is fine as long as the thermal contact is good. This range is perfect for the 28mm large diameter cartridge heater.

For specialised uses that need to heat up quickly, high densities of up to 100 W/in² (15.5 W/cm²) are achievable, but only if the fit is very tight and active cooling is used in some circumstances. The service life will be shorter no matter what.

For most industrial tasks, a cartridge heater that works in the 5–7 W/cm² range usually has the best blend of performance and longevity.


A lot of people who use machines think that getting a heater with more watts will just make the machine heat up faster. This is only true to a point. If the watt density is higher than what the application can safely handle, the excess power won't be able to get into the material as easily. The heater just becomes hotter on the inside, which burns it out without making it take much longer to heat up.

Based on experience, it's much better to choose a cartridge heater with enough watt density but not too much. Then, use correct temperature control to determine how quickly the heater heats up. Going above the recommended watt density limitations almost never gives you the benefits you expect.


When looking at a cartridge heater for a certain job, find out the watt density and see how it compares to the recommended range for the substance being heated. If the calculated figure for continuous operation is more than 7 W/cm², you should rethink the design. You can choose from:

The 28mm size is often the answer to this problem: making the heater bigger.








Watt density is not just a theory. It directly affects how hot the heater's internal parts get and how long the heater lasts. For demanding applications, staying in the 5–7 W/cm² range is a proven way to get the most out of your service life.

varying heating jobs need varying amounts of heat. A 28mm big diameter cartridge heater that is the right size for watt density will work better than any other heater that is not the right size. Taking the time to figure out and double-check watt density before placing an order will save you money on replacement parts and downtime.