Firstly congrats to the Makerbot guys on the MK2 Frostruder...

[blog.makerbot.com]

But looking at it... am I the only one that sees a potential for an extruder that could use granular plastics?

if it was a metal melt chamber, with a large air void at the top (and maybe a clever method for adding more plastic) where all the plastic was brought to melting temp... and then forced out in exactly the same way as in the Frostruder MK2.

If it works for viscous fluids at room temp, why not at higher temps? It will need a beefier heater, but thats the only real problem I can see... (and I'm already mulling ideas of how to print a pressure vessel, mmmmm dangerous!)

And yes, I'm ignoring the added complexity of swapping 1 stepper motor for a compressed air system and the mess that makes of the vitamin concept

a condensed gas being allowed to expand in the chamber would cool the melted plastic.

That's not a problem, because the only time it expands, is when you want to stop the flow.
Also, when you restart, you're compressing again, which heats it up.

Greetings,

have also been considering producing a granular pellet extruder, rather than a filament extruder for reprap.

You may want to consider a more elegant variation on theme.....(also has advantage that it is continuous and can be refilled whilst in operation)

In industry hot melt glue gun applicators for sealing cardboard boxes on production lines do pretty well what the extruder does (eg Nordson, Robatech etc)...That is provide a controlled feed of molten material.

The guns used on these can exhibit very fast on/off control ....just think how much time you have to squirt glue onto a small carton zipping past at 45-50 cartons per minute.

They are actually quite simple beasts - a heated tank, a pressurising pump, a heated delivery hose and a heated air solenoid activated glue head (gun) fitted with jet.

The bead dimensions these throw on smallest nozzle size is not significantly different to that of an extruder head, however they typically throw across gaps of mm's to cm's by jetting.

On reprap style extruder one doesn't need the heated hoses, and by combining the tank and gun head one should be able to limit heater control to a single element.

The pressurised portion is not the tank, only the molten material in the gun chain, so pressurised volume can be kept very low (safety!)

That leaves the pump and valve.

The pump on Nordson's for example is a piston pump driven by air. Delivery head is not extreme, and the real requirement is the ability to handle the heat. Typical pump speeds on Nordons in use are very slow.......this makes it likely that a motor driven piston or similar would be up to the task as one can gear down to provide torque easy enough. For safety you need to ensure that piston bypass ends up in the tank and not squirting all over your lovely reprap or worse your eyeballs.
VERY IMPORTANT HINT. Always where safety glasses no matter what, when playing with pressurised hot melted plastics. I've seen glasses strafed at 90deg to where a jet is pointed by partial jet blockage.

A simple alternative to a piston may be to manufacture a disc pump (Tesla turbine). These should work really well on viscous substances and are extremely simple in principle....a volute and a few discs with gaps between them.

That then leaves the valve This has to act directly before the jet port to prevent overrun- On industrial glue guns these are actually quite simple but internally they are pretty tight in tolerance and materials used.

In principle however they are simply air solenoids which means the possibility therefore exists to possibly use an electric solenoid driving a pin needle seat to act as control mechanism.

For jet, read small hole......pulling a nichrome wire through silver solder or similar should do the trick.

Why arent you using a propeller like screw that transports continously the granules into the heater chamber and at the same time acts as a seal for the chamber and by turning will also compress the melted granules nd force them out of the tip.

why not invent a machine that you dump plastic of any kind into, and it melts it into filament?

If you pressurise with just a screw you are likely to get overruns, unless you feed very slowly.

Screw is a nice simple idea for the pump though.

This idea is exactly that - pour any non thermo-set plastic in, and create a liquid phase filament.....cooling etc just makes the thing bigger and more cumbersome.

A "Jiffy mold" design is the simplest. I always hesitate to pile on complexity until I have a working prototype. I'm wary of the temptation to bog down projects with complexity until I've gotten that far.

Another simple, positive displacement pump is two gears intermeshing inside a figure 8 shaped cavity. Where the gear teeth intermesh, they expel the working 'fluid', and on the two outside tracks the gaps between the teeth push the material along. I believe that automobile engines use this technique for the oil pump, which generates considerable pressure.

This would have the advantage of pushing the granules, or melt, under considerable pressure, but also control the flow rate by how fast the gears turn. And since it is a positive displacement pump, when you stop the gears, it should hold the pressure for some time.

Wiki Gear Pump

Mike

hey guys dont forget any polymer has a solvent, if you choose the solvent that it evaporates fast enough and make it a viscous enogh solution starting from granules, then you have it already as is without a very minor modification, you need a glas syringe and a a PTFE stopper because the solvent will also dissolve your syringe and stopper.
Peter