When Cartridge Heaters Fail: Root Causes and Practical Fixes

Learn why single head cartridge heaters break down and how to keep it from occurring to your equipment.


A cartridge heater stops working. The production stops. Parts that need to be replaced are ordered. The machine is not working. Costs go up every hour.

Everyone blames the heater for what happened. But in most cases, the heater was just a sign of a fault with the application or installation that could be fixed.

To stop single head cartridge heater failures, you need to know what really causes them. And most of the causes are things that the operator or maintenance staff can easily fix.

The first reason is dry-firing.
This is the most prevalent reason of cartridge heater failure, yet it is also the easiest to avoid.

When a cartridge heater is turned on before it is properly set in its mounting hole, it is said to be "dry-firing." The heater sheath temperature goes up a lot since the surrounding material doesn't have any thermal mass to soak up the heat. In just a few seconds, the internal temperatures can go above 1000°F, which burns off the resistance coil and damages the MgO insulation.

The solution is simple but must be followed: never, under any circumstances, turn on a cartridge heater that is not entirely inserted into a hole that has been correctly drilled.

Even for short tests, this rule still applies. Dry-firing for a few seconds can create irreversible damage that may not show up right away but will greatly limit the life of the gun.

Reason Two: Too Much Watt Density
Watt density, which is measured in watts per square centimetre (W/cm²) or watts per square inch (W/in²), shows how much power is concentrated on the surface of the heater sheath. The sheath gets too hot and the inside parts break when the watt density is higher than what the application can safely handle.

Different kinds of materials have different restrictions. For most industrial uses that need a lot of power, the best power density for a cartridge heater is between 5 and 7 W/cm². Going above this range makes failure happen faster. Staying much below it may make life longer, but it may take longer to heat up.

The 28mm big diameter single head cartridge heater has a lot of surface area, which helps keep watt density under control even when the overall wattages are very high. This is one reason why larger diameter heaters tend to last longer than smaller ones in high-power situations: they have greater surface area to lose heat.

Low-density cartridge heaters, on the other hand, usually work at 10 to 30 W/in², which is good for gently heating delicate materials. Heaters with a medium density range from 30 to 50 W/in², which is perfect for working with rubber and plastics. High-density heaters can go over 100 W/in², but they need very good thermal contact and usually don't last as long.

To choose the right watt density, you need to figure out the actual heat load of the application instead of just ordering the maximum wattage available.

Reason Three: The installation didn't fit well
If you put a cartridge heater in a hole that is too big, it will fail rapidly, even if it is well made.

It's simple: the bigger the space between the heater and the hole, the hotter the heater needs to be to provide the right amount of heat. A gap more than 0.05mm makes a thermal barrier that raises the sheath temperature a lot. At some point, the heater can't move heat fast enough to keep the inside from getting damaged.

The answer is to prepare the holes very carefully. For a single head cartridge heater with a 28mm big diameter, the mounting hole should be reamed so that there is a diametral clearance of 0.02mm to 0.05mm. The polish of the surface should be smooth, with a Ra of 1.6μm or better.

Some maintenance crews try to fix holes that are too big by wrapping the heater in aluminium foil or putting metal tape around it. This is a bad thing to do since it makes the problem worse instead of better by making touch uneven and creating hot spots.

Cause Four: Rust and contamination
A cartridge heater works in surroundings that can be rough at times. Moisture, oils, chemicals, and process vapours can all damage the heater sheath. More importantly, they can get through the lead exit seal and into the MgO insulation within, which is bad.

When moisture or conductive impurities get to the internal resistance wire, the insulation resistance lowers a lot. This could lead to short circuits, ground faults, and the heater breaking down.

To stop anything from happening, you can use three methods:

Choosing the right sheath material. Standard stainless steel can handle a lot of different settings, but it doesn't work well in places with a lot of chloride or acid. Incoloy is better at resisting corrosion at higher temperatures. When it comes to the most hostile chemical conditions, titanium is the best choice.

Terminations that are sealed. End seals made of epoxy or ceramic at the lead departure point keep moisture from getting in. For moist places, use terminations that are rated IP67.

How to store things. Before being installed, cartridge heaters should be kept in a dry, closed space. Hygroscopic MgO insulation can take in moisture from humid air, which lowers the initial insulating resistance.

Voltage Mismatch: Cause Five
Basic electrical testing can completely stop this from happening.

If you use a lower voltage than what is recommended on a cartridge heater, the power output will be diminished by the same amount. For example, a 240V heater that runs on 120V only produces a fifth of its rated wattage. The application might not get hot at all.

Using a greater voltage is far more risky. A 120V heater plugged into 240V uses four times as much electricity as it should, which causes it to overheat right away and in a very bad way. Before the internal resistance wire melts open, the sheath may turn crimson.

Before attaching power, always check that the supply voltage matches the heater nameplate. A multimeter check only takes a few seconds, but it can save you a lot of money.

Cause 6: Thermal Cycling Fatigue
The parts within a cartridge heater expand and contract every time it heats up and cools down. This movement can wear down the resistance wire, break the MgO insulation, or loosen internal connections over hundreds or thousands of cycles.

The harm keeps getting worse, but it can be controlled.

A PID temperature controller with soft-start functionality lowers thermal shock by slowly increasing power instead of quickly applying full voltage. Keeping operating temperatures consistent instead than letting them change a lot also lowers cycling stress.

If you need to use something that needs to cycle a lot, such sealing equipment that heats and cools with each cycle, you might want to choose a cartridge heater made for high-cycle operation. These are made with particular resistance wire alloys and fabrication methods that can handle repeated thermal expansion.

Identifying Failure Modes
Knowing how a cartridge heater fails will assist find the real problem when it does fail:

Open circuit: the resistance wire has broken. The measured resistance is infinite ohms. Some common reasons are dry-firing, thermal stress, or damage to the machine during installation.

Short to ground: The resistance wire has touched the sheath. The resistance between the terminal and the sheath is quite low. Moisture getting in, fractured MgO insulation, or damage to the sheath are all common reasons.

Lower insulating resistance: The resistance between the terminal and the sheath is low, but not zero. The most common cause is moisture getting into or contaminating MgO insulation.

Sheath discolouration or damage-signs of overheating that are easy to see. A common culprit is bad thermal contact or too much watt density.

The Maintenance Mindset
A cartridge heater is not something you can just put in and forget about. Regular checks and measurements can help things last longer and stop them from breaking down unexpectedly.

Use a megohmmeter to monitor the insulating resistance every so often. A lower reading means that moisture is being absorbed or that something is contaminating the area and needs to be looked into. Check the resistance across the heating element. If it goes up by 10% from the baseline, the coil is probably breaking down. Check sheaths for discolouration, pits, or swelling during regular maintenance.

Keep extra cartridge heaters on hand for important production lines. The cost of a spare part is quite little compared to the cost of having to wait for new parts and losing time.

Last Thoughts
Most cartridge heater failures can be avoided. People who specify, install, and operate the equipment are in charge of dry-firing, poor fit, contamination, voltage mismatch, and too much watt density.

A single head cartridge heater with a 28mm big diameter that is properly matched to its use and installed will work well for many years. Taking shortcuts to save time during installation or money on parts will always result in hours of unplanned downtime and money lost in production.

Different industrial settings have different problems; what works in one place might not work in another. To make sure that cartridge heaters work well, you need to know the individual risks of each application failing.