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Extruder specifics

Posted by MotorAssist 
Extruder specifics
October 06, 2009 05:35AM

I see on the Extruder Nozzles page, about half way down, it says, "Get your temperature sensor as close to the barrel as you can without touching it, and get it as near the nozzle as possible while still retaining it strongly in the fire cement."

Why should the temperature sensor not touch the heated barrel? Surely it is the barrel temperature that is of interest?

Also, what is the advantage of using a large washer to support all the parts over the use of the copper pipe section? And is fire cement (or equivalent) still used in this arrangement? Is it there just to keep the Nichrome wire in place, or is it there to effect insulation, or both?

There seems to be a general move to a stepper motor for the extruder - why the change

I think the only thing I've decided upon is to use a pinch-wheel type drive.

Any other advice on extruder design? (Now there's a big topic.....)

Re: Extruder specifics
October 06, 2009 06:52AM
If it is a thermistor it can / should touch the barrel. Thermocouples need some electrical insulation to prevent them being shorted out unless you go for Wade's approach of using the barrel as part of the thermcouple.

The large washer in MakerBot like designs is to brace the PTFE insualtor against the thrust of the plastic. PTFE is too slippery to clamp so you something else to hold it. Adrian's design uses glue.

The washer also stops the PTFE elongating due thermal expansion, which is significant.

Kapton tape can be used instead of fire cement but you then need insulated nichrome which is not available in the UK.

DC motors need a shaft encoder to get accurate speed. They have brushes that wear out and also generate a lot of RFI, which needs a suppressor.

Stepper motors have none of these problems and can start, stop and reverse instantly. The down side is they are bigger and heavier unless you gear them down.

My advice would be to not use PTFE as it fails. I have moved to stainless steel tubing with a tapered outlet and heatsinking. I prefer resistors stuffed into aluminium blocks rather than Nichrome and fire cement.

Re: Extruder specifics
October 06, 2009 11:54AM
Thanks Nophead.
That's answered a lot of questions in one go! I have a thermistor, and a DC motor and its noise-suppressor circuit and a DC controller board, so those are what I shall be using. I'll put the thermistor on the heater barrel as I had thought.

I'm probably restricted to using a heating element wire.

Are you using a stainless steel barrel/nozzle because it is a poorer heat-conductor, or is there some other practical reason?

Re: Extruder specifics
October 06, 2009 01:39PM
I don't use it as a barrel or nozzle, I use it instead of the PTFE insulator. It is a poor conductor, but to use it as the thermal barrier it has to be very thin and needs a heatsink at the cold end.

If you have a GM3 style motor it will go too fast for a pinch wheel. One turn of a pinch wheel might be 20-40mm of feed, whereas one turn of a screw is either 0.8mm (M5) to 1.25mm (M8).

Re: Extruder specifics
October 06, 2009 02:27PM
From you blog I thought you were building an extruder controller. That does a stepper or DC motors I think.

Re: Extruder specifics
October 06, 2009 05:39PM
I bought a Solarbotics 224:1 GM3 to go with my extruder controller board 2.2. I thought it only suited DC motors, but either way, that is what I have.
I can use gears to reduce the travel of the plastic rod? I am not confined to a set framework, so I can put a pinchwheel on a gear offset from the motor spindle.

Sorry I was confused about your stainless steel thermal barrier - when you'd written you were planning a new extruder, and then I saw the ss reference, I put 2 and 2 together and got 5! Is it on any pictures on your blog?

Re: Extruder specifics
October 06, 2009 06:32PM
It was designed to drive 2 DC motors but Adrian Bowyer has used it to drive a stepper motor. [blog.reprap.org]

Yes you can gear down the GM3. You need to decide how fast your McWire will go. I would think about 4mm/s. If you are extruding 0.5mm filament from 3mm then you need to feed at 4 * (0.5/3) ^2 = 0.11mm/s. IIRC GM3 does about 40 RPM max. So to run it with some margin say 30 RPM = 0.5 RPS.

If your pinch wheel is say 10mm diameter then it will feed 31 mm / rev so the reduction would need to be 31 * 0.5 / 0.11 = 155:1. Too much for a single pair of spur gears.

Here is my extruder using stainless steel [hydraraptor.blogspot.com]. I am working on a better one at the moment, but it needs a lot of lathe work.

Edited 1 time(s). Last edit at 10/06/2009 07:10PM by nophead.

Re: Extruder specifics
October 07, 2009 02:08PM
I tried to reply earlier, but the system decided my entry was spam and refused to post it and deleted it all in one fell swoop!

Anyhow, the info I had was that the GM3 motor does 24 RPM - this was taken from the Solarbotics site. BUT EVERY other site says 38 RPM, and actually the Solarbotics maths doesn't add up, so it looks as if you are right.


So, wormdrive? Or am I better swapping the DC motor out for another one with low RPM such as the Kysan one Makerbot is selling (2 RPM)?
Even then, needing gearing, do you suppose the gears should be metal? I know the pinch wheel itself should be.
Re: Extruder specifics
October 07, 2009 05:12PM
If you want to stick with a GM3 I would use a screw drive and a shaft encoder. That is what HydraRaptor uses so I know it works reasonably well. In fact screw drive has a little more grip than pinch wheel when softer plastics are concerned. The main advantage of pinch wheel is speed but your McWire will not be able to keep up.

If you don't use a shaft encoder then you would need gears as on the version shown here: [dev.www.reprap.org]. Otherwise you can't slow the extruder down enough without stalling for McWire speeds. With a shaft encoder you can go as slow as you want but I don't know if there is firmware support for it in the official release. I know Wade made a firmware that supported it on older electronics.

You could use a worm drive, I plan to use a Meccano one on a NEMA17 stepper, but it is only 40:1 so you would need a big wheel and a small worm. Worm drives are less mechanically efficient than spur gears due to friction but that should not be a problem as pinch wheel is a lot more efficient than screw drive.

For pinch wheel I think it would be easier to use the Kysan than doing your own gearing. I am not sure how slow you can run it. You may not need any further gearing. Perhaps somebody with a MakerBot can tell you how slow they can extrude before stalling.

Yes I think the final gear should be metal, although I have made a pinch wheel extruder with a stepper and tiny plastic gears. [hydraraptor.blogspot.com] I have not run it long enough to know how it will last.

This stepper based pinch wheel extruder must be the easiest mechanical arrangement [reprap.org] and will be more accurate than an open loop DC motor.

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