# How can I increase heat transfer to a small surface area?

I am working on an electronics project in which I dispense melted wax at around 70 degrees through a syringe and metal veterinary needle as shown below. The syringe is heated via an aluminium block with embedded heater cartridge. The block sits around the syringe body tightly. The problem I am having is that everything heats up except the needle tip which I have drawn an arrow to in the image below. If i put my finger to the part just above the needle it is hot however the needle is not.

My question is how could I increase heat transfer to the needle tip apart from directly heating it with something like nichrome wire?

The part that You are heating is receiving heat from the heater and dissipating it into the surrounding medium (in this case air). Once the rates of receiving and dissipating heat reach an equilibrium, the part reaches its final temperature. Simplest way to increase the end temperature would be to alter involved temperatures by either:

• increasing the temperature of the heating element, thus increasing the rate of receiving heat;

• increasing the temperature of the medium, decreasing the rate of heat dissipation.

Alternatively You can change the heat transfer rate by changing the thermal conductivity:

• increasing the thermal conductivity of the heater by choosing a different heater or placing it closer to the heatable part;

• decreasing the thermal conductivity of the medium by choosing a different gas (with lower thermal conductivity), lowering pressure (vacuum has zero thermal conductivity) or by insulating it somehow.

Finally, as a heat gradient forms inside the heatable part, You could alter the respective gradient without changing surrounding conditions by:

• decreasing the length of the part;

• increasing the width of the base of the part and making it more cone-shaped;

• choosing a different material with higher thermal conductivity.

You could put some insulation around the needle, if this won't be a problem for your application. The problem is that the needle has a relatively large surface area and heat is lost throuht this surface. On the other hand the area of contact between the needle and the heater is very small. You cannot easily change these but you can reduce the heat lost throuh the needle walls with some insulation. At this low temperature won't be a problem to find some insulatig material. Even some cotton pads fixed with rubber bands.