Induction Heated Nozzle

Induction Heated Nozzle

Quote
jbayless
Dear Fluxtrol,

I am a senior engineering student at the university of British Columbia, and leading a student design team to develop an improved 3D printer. I'd like to use inductive heating, but I haven't learned about it at university, and I'm in need of some expert advice. My question is probably a bit unusual, but maybe you will find it an interesting problem.

So in our 3D printer, we build a model up one layer at a time by melting a plastic rod and extruding it through a fine nozzle. Normally the plastic is pushed into a resistively-heated metal tube, with a fine hole drilled at the end. However, it is important that the plastic transitions from solid to liquid over as short a distance as possible, and so a metal tube is not ideal - its high temperature conductivity makes it hard to make a steep thermal gradient.

I am investigating the use of a glass tube instead. We can easily draw it to a fine nozzle at one end, and take advantage of the low thermal conductivity of glass to achieve a steep temperature gradient. The problem is the resistive heater: It is difficult to transfer heat from the outside of the tube to the inside, because of the thermal resistance. A high heat flow is needed, so the temperature of the heating element must become exceptionally high - wrapping a heating element around the outside poses a problem. The thermal resistivity of the glass therefore both an attraction and a problem: it's good for maintaining a steep temperature gradient axially along the tube, but terrible for allowing heat to flow radially into the tube center from the outside.

That's why I would like to use an inductive heating system. I picture slipping a thin ring of resistive metal, such as stainless steel, into the glass nozzle. Then it can be wirelessly heated by a copper coil wound onto the glass nozzle. The ring will in direct contact with the plastic, and the thermal resistance of the glass will now only be an advantage - insulating the heater radially and axially.

The plastic only needs to be heated to 260 degrees Celsius to melt, which will require four to eight watts, depending on the flow rate. It's a 3 mm diameter plastic filament, so the metal ring should have an outer diameter of no more than 3.2 mm and as small a wall thickness as possible. We would like to drive the coil from a 12 volt AC power source, and the frequency should not be so high that it requires expensive electronics.

Beyond that, I am not sure about the details required. If I want a stainless ring of only 0.1 to 0.2 mm wall thickness, do I need to be concerned about the skin depth? Will I have a very low "K" factor unless I use a high frequency? Do you have any concerns about my design that I should be aware of?

Thank you very much for your help!

Jacob

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