Concept, need info on materials.

Hi all,

I am new here, great project! I am in the process of converting my homebrew rapid prototype machine into a RepRap, and I am looking for a little info on the extrusion temperatures of the various materials as used by the project.

The reason for this is the way my machine is set up. I am using a high temperature TEC (peltier) in reverse to melt the plastic for extrusion. The "cold" side is used to cool another copper ring to stop the heat traveling up the extruder assembly. The Peltiers I use are rated up to 200 degrees c max. Is this enough for all materials, or should I go with the Darwin extruder, and implement whatever it is you have come up with to date to stop the heat travel.

I have another extruder related question. You are using a screw thread to draw the material into the heating chamber. This seems like it would produce pretty low friction grip and slip at high motor torque. Is this the case?

My replicator uses a hopper at the moment, and I am not sure if it worth integrating the RepRap feeding mechanism principle in my design (I don't like the bendy cable from the drive motor, I would go with direct drive).

Last but not least...

I noticed one of the RepRap guys did an anti drip on the RepRap. Nice job. I am just in the process of implementing my take on this idea, but with no additional motors or solenoids needed. The solution I came up with involved a small mechanical linkage system on 4 toothed cam. When I throw the extrusion feed motor into reverse for at most 1/4 of a turn, I get a small vacuum to help contain any more drips from expansion of materials, and I also engage the anti drip pin.

How hard am I going to find coding that up? I assume it is pretty simple, although I will need some way of sensing a switch so I know when we engaged the pin.

Sorry for the brain dump. I look forward to your thoughts on the direction I should take.

Ginge

Hi Ginge!

RepRap has played with a variety of plastics, but as far as I can tell extrusion temps have stayed between 60 (CAPA) and 110 (HDPA) C. Forrest is the real pro on those matters. The Peltiers sound like a great idea given all the trouble we have been having with Nichrome wire, What do they cost, both as objects and in terms of current drain? Do you have a good supplier?

In general we have had very little trouble transferring force to the filament, slippage has generally been handled by tightening the extruder barrel to press the filament more firmly against the threads. Screw threads actually have tremendous mechanical advantage if they can bite into the material.

There has been a lot of talk here about granule/hopper feeders, what sort of arrangement are you using? In general the flex drive seems to have been an answer to a problem that never developed. A lot of people seem to favor moving away from it due to durability problems with the design.

I'm not a code monkey so I can't help with writing the anti-drip software. I don't know what your requirements are in terms of response time or how much lag would be in this system, but it seems that if you close the tip while retracting the filament, you are going to get a vacuum (of sorts) which is going to increase the filaments resistance to further retraction which will increase resistance to the motor which will increase the motors current drain which could be sensed by the motor controller and so relayed to the extruder control software. Or if you don't mind adding hardware and need really fast response, just insulate the valve wire except for the very tip and run a current down it to sense when it shorts out against the tip (which is to say blocks the hole). Just some ideas in black box format.

Good luck and keep us posted!

Hi Ginge,
The Peltier heater sounds very interesting. It should work for PCL and PLA but
unfortunately for HDPE and ABS you need to go a little higher to get good interlayer adhesion. Do they go up to higher temperatures?

These are the temperatures I use :-

HDPE.raft_temp = 200
HDPE.first_layer_temp = 240
HDPE.layer_temp = 220

PCL.raft_temp = 0 // no raft
PCL.first_layer_temp = 130
PCL.layer_temp = 120

ABS.raft_temp = 200
ABS.first_layer_temp = 215
ABS.layer_temp = 230

PLA.raft_temp = 0
PLA.first_layer_temp = 180
PLA.layer_temp = 160

The precise values depend on where the thermistor is mounted.

You can certainly extrude HDPE and ABS at 200C but I think you will hit problems with delamination.

I think the software and firmware already support reversing the extruder and adding a switch would not be difficult if you can find some spare I/O.

Adrian Bowyer is working on a hopper based extruder design so I am sure he would be very interested to see what you have done.

Edited 1 time(s). Last edit at 06/15/2008 04:30AM by nophead.

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[www.hydraraptor.blogspot.com]

Hi Guys, thanks for the feedback. Much appreciated.

BDolge Wrote:
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"The Peltiers sound like a great idea given all the trouble we have been having with Nichrome wire"

Just out of interest what trouble have you been having with Nichrome? I have a feeling I will need to switch to that. See my reply to nophead.

"What do they cost, both as objects and in terms of current drain? Do you have a good supplier?"

My peltier is 20mm x 20mm and cost in the region of about

Nichrome problems are predominantly:
insulation failures leading to shorted out sections,
expansion and contraction due to temp changes leading to flexing,strains and broken wires and
it's just fiddly, springy and fragile to work with

That doesn't sound too good.

Anyway, here is a link to the peltier I use. I just found it for sale at Greenweld (UK) for

Hi Ginge,

Please go all out on the granule extruder. Besides being cool, a common question I get about reprap is if it can recycle plastic. A couple of questions.

Rockwool insulation is supposed to be less irritating than fiberglass insulation, is that actually the case?

You mentioned that one motor drops the plastic beads across an air gap and the other turns the screw in the heated barrel. How do you prevent underflow or overflow of the barrel, is there some kind of feedback or does everything just work out?

By the way, great idea on sensing the motor reversal to activate the anti drip mechanism. That could free up I/O lines and/or simplify the firmware over the current anti drip setup.


Bdolge,

Thanks for pointing out the problems with nichrome. Has anyone considered putting insulated nichrome wire coil in a copper tube and wrapping the tube around the barrel? Similar to the candyfab:
www.candyfab.org/article.php/heater21/print

Only where they use a cartridge heater, reprap could use nichrome in a flexible copper tube. The nichrome would heat up, transfer heat to the copper tube, which would transfer heat to the barrel. The nichrome could expand and contract inside the copper tube, without strain. Kind of like how fiber optic lines are sometimes slack in a tube, so that the tube can be pulled and bent, without stressing the fragile fiber.

I have not had any problems with the nichrome itself, only the JB Weld insulation, which I have now replaced with Cerastil.

I would not describe Nichrome as fragile, it is a strong springy wire, even when very fine. You have to wind it under tension, otherwise it just unravels again. I use a small lathe, but you could use anything that you can rotate by with one hand while feeding the filament with the other.

I don't know how expansion and contraction are accommodated when it is encapsulated with JB Weld or Cerastil, but I haven't had it break. I think the coefficient of expansion is very small, compared to say copper, and it only expands 0.3% over 200C.

When it is used in toasters it does tend to wear out after a long time but that is getting red hot so expansion will be much greater. I think other parts of the machine will wear out long before the nichrome does.

I use bare nichrome and wind it on a layer of insulation but if you get the insulated stuff you can wind it in the screw threads which is much less fiddly.

Edited 1 time(s). Last edit at 06/16/2008 05:10AM by nophead.

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[www.hydraraptor.blogspot.com]

"Rockwool insulation is supposed to be less irritating than fiberglass insulation, is that actually the case?"

I have no idea. I don't even bother to insulate at the moment, as I don't use the "standard" setup

"You mentioned that one motor drops the plastic beads across an air gap and the other turns the screw in the heated barrel. How do you prevent underflow or overflow of the barrel, is there some kind of feedback or does everything just work out?"

Underflow is not a problem in this setup. I have them tuned up so that there is at least a little overflow. The overflow spills down a little runner into catch box.