Saturday, 17 October 2009

Extruder plans

MakerBot are sending me fresh capacitors (labelled, this time, I hope) and those parts that were missing from my PCB kits. These should be coming in the next week.

Meanwhile I've been reading up on the various designs and options for constructing an extruder. Looking at the heater details, I was faced with use of copper or stainless steel for the heater barrel and/or nozzle, separate or combined barrel and nozzle, PTFE to insulate or a heatsink block of metal to dissipate heat, uninsulated or insulated nichrome wire or resistors to provide the heating,....
The drive for the plastic rod was originally intended to be a screw-thread, but has been superceded in most cases by a pinch-wheel. There is also a move away from DC motors to a stepper motor.

I already have my DC motor and its RF noise-suppressor board and its controller board. I am planning to make a pinch-wheel extruder, but I am unsure of the rest of the design features to use.
As ever, I posted on the forum and Chris (Nophead) quickly replied explaining the main reasons for different choices.

Since I don't have machining capability for chunks of aluminium into which to push resistors, my heating element will be Nichrome wire. I shall order the insulated Nichrome from Makerbot, perhaps 2 feet, for $1 + P&P. I shall wrap this in Kapton (polyimide) tape, which I shall order from DealExtreme $2.92 (free shipping worldwide, apparently), following BodgeIt's tip. This one claims to work up to 300 oC, whereas the UK supplies state 260 oC. However, I did find reference to one rated up to 260 which stated it would withstand 300 oC briefly. The difference in price works out at about 50 p, so I'll try the one from the States' site, and see what the packaging says when it arrives. I wonder if the cheapest supplier for the UK hapens to be the same company anyway.... their sites are uncannily similar in many respects.

I shall try using my B&Q welding tip to make a combined nozzle/barrel. According to Nophead's work, the nozzle narrow should be around 2 mm or more in length to prevent the nozzle dribbling much. I would like to have a go at Frank's method (August 29 2009 entry on builders' blog) of using a brass nut to conduct heat away from the upper end to an aluminium heatsink. However, I may struggle to get the top (entry end) of the copper welding tip to be cool enough. The rule of thumb is that the heatsink should be not too hot to touch after a time in operation. From what I have read, with a long warm path for the plastic filament to travel before the zone where it melts, the plastic expands too much, which produces a lot of friction, ultimately making it difficult to drive the filament through the extruder.

I do not have a heatsink. However, I may have some aluminium I could use as a heatsink: the flat of the furniture leg I bought in order to attach a horizontal timber arm onto the MDF "vertical base". This would require the purchase of a second one just for heatsink purposes, but is probably the cheapest way of doing this, as it only cost around £6.
The flat has mass 164 g, and is 8x8x1 cm, ie 64 cm cubed. Density of aluminium is approx 2.5 to 2.9 g/cm cubed, therefore this piece of aluminium should weigh (say) 2.7x64 = 172 g. So at 164 g, it IS probably aluminium.
The surface area of the flat is 16000 mm square, so it should act as a good heatsink.

Further information from Nophead concerned the amount of gearing that would be needed to make my little GM3 motor rotate a pinchwheel slowly enough to provide a sensibly slow feedrate for the plastic filament. I don't think it is insurmountable. But I have been hunting a small gear, a large gear, and a tiny pinchwheel.
I have been doing a lot of maths with the various gear ratios, considering what is available and what I need to achieve.

Although this may not be adjustable in use, I have settled on a worm gear, matching large gear onto which to mount a small gear meshing onto another large gear, which will have the pinchwheel mounted on it. I know it'sa lot of gearing.

I shall have to come up with some way of mounting all of these, and I haven't yet decided on what to use as my pinchwheel.

Until I started looking into this, I had no idea there were different sorts of gears, and so many, all for different purposes: worm, bevel, ratchet, rack, spur, helical, pinion, contrate..... and a sprocket is something different!

My design will be a cobble-together, as is typical of repstraps and it may not work....
but I'll have learnt some more along the way, and I can always change components or features as necessary. If I don't try it, I'll never know....