Hot Glue Gun NTC Heating Element?

PTC heating elements often have a negative temperature coefficient at low temperature.

Here are some resistance vs temperature curves for TDK Series B59102 PTC heating elements, which are designed for use in home appliances such as hair curlers and glue guns:-

When powered on the NTC region at low temperature causes accelerated temperature rise as the reducing resistance increases power consumption, until it reaches the PTC region which reduces power as it stabilizes the element temperature.

The transition from NTC to PTC may only take a few seconds and is very temperature sensitive, so measuring the resistance with a multimeter and correlating it to actual element temperature is difficult.

I tested a 230V glue gun that measured 1014Ω at 23°C. After being powered on for only 2 seconds the resistance dropped to ~500Ω (it also generated a dc voltage of 10mV, which made the reading ~10% high or low depending on meter polarity). After a 10 minute cooling down period the outside of the gun was almost back to room temperature, but resistance had only risen to ~900Ω - indicating a higher internal temperature.

It's possible that thermal EMFs are screwing up your readings, so try reading the resistance in both directions and averaging the two, or (better but potentially more dangerous) measure the actual current draw.

I tried a cheapie (dollar store) hot glue gun and the current started out in excess of 200mA (at about 120VAC) and rapidly dropped to less than 80mA and was <60mA after less than 5 minutes. It's marked 10-15W and has an authentic-looking UL file number (didn't look it up).

With it hot, I measured the EMF at the terminals maybe 15 seconds after power was removed and it was more than 2mV.