Sunday 7 October 2007

Taking up the slack

I had a problem with my HDPE filament getting unwound from its reel. Because my extruder is attached to the z-axis, the filament gets pulled off the reel as the z-axis descends, but when it rises back to the home position there was nothing to take up the slack. Also the springiness of the HDPE makes it want to unwind. It needs a constant back tension to take up the slack and keep the filament on the reel.

My first idea was to attach a small DC motor to the roller to provide a backwards pull. As the motor would be permanently stalled I would have had to limit the current to something reasonable. After some thought I came up with a much simpler solution. I wound some picture cord around the roller and hung a weight from it. As the filament unwinds it lifts the weight. The weight is also tethered to the top of the machine, so once it gets to the maximum height it stops. The reel is only a friction fit on the roller so it starts to slip at that point. When the axis ascends again the weight falls and winds the reel backwards, taking up the slack. There is enough travel on the weight to cover the full z-axis travel, even when the filament has been used down to the inner diameter of the reel.

Thursday 4 October 2007

Sticking point

Over the last few days I have been working on getting my machine to lay down straight lines of HDPE filament. It was a lot harder than I imagined. Initially I could not get it to stick to anything. I knew Forrest, who has been pioneering the use of HDPE with Tommelise, had successfully used foam board as a base to extrude onto, and the RepRap design uses a sheet of MDF for CAPA. I didn't have any foam board to hand so I tried MDF and several other things with no success at all. In desperation I then tried slowing down the extrusion to 0.75mm per second and that did the trick. I found I could then extrude onto lots of things so I tried as many as I could think of to see the pros and cons. Today I got my hands on a piece of 5mm foam board as well.

This was 3mm thick cardboard, it didn't stick very well at the ends.



Blotting paper sticks better but the heat makes it wrinkle and it leaves residue when peeled.



Funky foam, my wife's contribution, sticks too well, it gets welded in and can't be separated cleanly.



A thin sheet of HDPE cut from a milk bottle. As expected it welds and cannot be separated. It could be a useful technique though, you would have to cut round the extruded object but it would be left with a strong smooth base.



Felt adheres very well and can be peeled off again but you would end up with a slightly hairy object!



MDF adheres well and peels easily but it does leave some residue fibres on the filament.



Anti-static foam from semiconductor packaging. This insulated the filament so well that it stayed molten too long causing the ends to stretch away. It sticks well but leaves a residue and a rough surface.


Foam board works very well despite having a glossy finish. That allows the filament to be peeled off cleanly and gives it a nice smooth surface. With this quick test there was no sign of damage to the board either but Forrest has reported the foam inside can melt.



This seemed to work so well I tried upping the speed to 4mm / second and that worked fine as well.



So I should have taken Forrest's word for it and saved myself some time, but it got me thinking why does it work so well? For the filament to stick, it must remain molten long enough to bind with the surface. That means something with low specific heat capacity and low thermal conductivity should work better. Paper has a specific heat capacity that is about the same as HDPE but that is only 0.2mm thick and then you have foam which is a good insulator. I had been trying things with some surface texture for the HDPE to bind to so I was surprised when something glossy worked. I don't know what makes the foam board surface glossy, maybe it is a thin layer of of plastic that binds with the HDPE by melting itself. Or maybe there is some molecular bonding going on, out of my depth here!

The next thing to do is to tidy up the line endings by adding a delay at the start and reduce the dwell at the end. Then I should be able to draw accurate outlines and fill them in.

I have started to think ahead to the next layer and what the requirements are to make it stick to the layer below. My mental model, which may be wrong, of how the heat flow works is to translate temperature into voltage, heat flow into current, specific heat capacity as distributed capacitance and thermal conduction as electrical conductivity. The extruded filament is then an infinite number of small capacitors, charged to 200V, linked by resistors. That will meet a bigger infinity of capacitors linked by resistors charged to 20V (room temperature). When the filament meets the already extruded layer the two surfaces behave like two capacitors charged to different voltages being connected in parallel. What happens in electronics is that the total charge is preserved so V(C1+C2) = C1V1 + C2V2, i.e. V = (C1V1 + C2V2) / (C1+C2) . If the capacitors are equal then V = (V1 + V2) / 2.

That means, if my analogy holds, that when two surfaces meet the temperature at the infinitely thin junction instantaneously becomes the average temperature, weighted by their specific heat capacities. In our case these are equal because it is HDPE at 200°C meeting HDPE at 20°C. It is my belief the junction will be at 110°C to start with. Heat will flow to it from the neighboring material on the hot side and away from it on the cold side. Since its temperature is half way between the two then these flows will be equal. The junction will stay at 110°C and this band of 110°C will start to spread to the neighboring material on each side. However, to form a weld the junction must reach the melting point of HDPE which is 135°C. The only way for this to happen is for the nozzle to stay around long enough to continue to supply heat. That puts a limit on how fast filament can be laid down and still bond.

To be free of this limitation the average of the temperature of the filament and the temperature of the workpiece must be higher than the melting point. If that is the case then it will weld instantly and there is no limit on extrusion speed. For HDPE and room temperature that would mean extruding at 250°C. Anything below that requires additional heat to flow from the nozzle to form the weld and hence sets a limit on how fast it can move away.

Tuesday 2 October 2007

Die swell revisited

My machine is back up and running again after replacing the thermistor and MSP430 micro. I added some high temperature insulation to keep the thermistor wires away from the heater wires in future! I salvaged it from the 10A shunt of an old multimeter that I scrapped.



I should really recalibrate the temperature measurement but after finding temperature is not too critical I can't be bothered at the moment. I might wait until I after I have blown it up again!

In my previous article Equations of Extrusion I put forward a theory for the significance of the Y axis crossing point on this graph, i.e. the minimum filament diameter at zero flow rate, was that it was the size of the hole in the nozzle.

My explanation was that perhaps I had inadvertently opened out the hole in the extruder nozzle by drilling too far from the back. While my extruder was off the machine for its new thermistor I had the opportunity to inspect the end of the nozzle.



As you can see the hole is still the correct size, 0.5mm, so my theory was wrong. My revised explanation for the minimum filament size is that HDPE is so viscous that there is a minimum pressure to make it flow through a small hole, below which it does not flow at all. The minimum diameter is then the hole size plus the die swell at that minimum pressure.

I used a fine stiff wire as a probe to try to get an idea of the depth of the hole. I think it is no more than 1mm, so I am at a loss to explain why I get variable die swell and other people do not. Perhaps my HDPE is different, I know mine is translucent whereas Forrest Higgs' is opaque white.