Direct filament heating possibilities?

With all the talk of lasers for cuting or polymerising or selective sintering, I had another use for them :
Replacement of the heater barrel.

Would it be practical to feed the filament as in Darwin, into an insulator barrel with a window in the side just above the nozzle. Use a laser or similar energy source to DIRECTLY heat the polymer to melt.

It may even be possible to eliminate anti-ooze valves etc by making the extruder 'heat on demand'

THe same wattage will be put in (a few watts currently) as the hot brass system. In fact, less watts will be needed because there wont be any wast heat radiated from the outside of the heater & nozzle. Of course resistive heating is nigh on 100% efficient, and any laser etc isnt going to be, but as we only need a few watts, this wont be important.

The whole assembly could be much shorter because there wont be any heat soak back up the current insulator. and no variation in the melt point in the barrel. Ther will be very little thermal mass so the control electronics will need to tightly match the laser heat input to the required plastic flow. It wuld probably need a thermocouple mounted oppersite and just below the 'window' to feed back the melt temperature.

My thinking is : why heat a lump of brass in order to melt the filament when we could melt it directly and keep all that heat in the filament by making the barrel/nozzle in a good insulator material. (eg Duratec [uk.rs-online.com])


Edit : I just remembered my other heating idea -
Use a utrasonic technique in place of the laser&window. Ultrasonics are used to melt plastic mouldings during product assemly. Usually by 'staking' a small rod which is squashed flat like a rivet head to hold a PCB or case together.

Edit2 : Just thought of another idea - add metalic filings to the filament as it is made. Then use an inductive heating system to heat the filament directly in a non metallic barrel. (I say metallic, because eddy currents in aluminium will cause heating)

Edited 2 time(s). Last edit at 07/15/2008 10:01AM by Richard Benjamin.

Interesting concept, but it does seem rather more complex than the current system, and I don't see the big payout...

well, I was thinking along the lines of anti-ooze - freezing a plug of plastic in the nozzle as a valve, and sorta switched it round - drop on demand.

The whole dispense head will be smaller and lighter - higher acceleration & more precision from a lighter, cheaper chassis + motors.

The nozzle will be much closer to the XY assembly so thermal expansion in Z will be insignifcant (currently there is c0.5mm or so variation in Z from hot to cold as I understand it)

Edited 1 time(s). Last edit at 07/16/2008 06:22AM by Richard Benjamin.

How do you keep molten plastic from flowing out the window the laser shines in through? It seems like a lot of tiny fiddly bits to me... not to say that it couldn't be done.

Kyle - thats why it is a window and not a hole.

Made from material that is optically clear to the laser light in use but rigid enough to withstand the extrusion pressure.

My concern would be how to easily get all those little pieces to fit together nicely without interfering with flow or getting out of sync and letting some thermoplastic slip through. The extruder is a very cramped, hot, and high pressure. But if you can figure it out, more power to you!