Heatwave

I made a start on my extruder controller, on breadboard, as it is a bit experimental. As you can see it's is a strange mixture of surface mount and though hole technologies!



The little PCB on the far right is a 3.3V regulator which I hacked out of a scrap PCB complete with all its decoupling caps. It had three handy vias for the three connections I had to make. The rest of the parts are the heater controller. The circuit could not be simpler. The heater is switched with a BTS134 protected MOSFET. Even with only a 3.3V gate drive its on resistance is so low it does not even get warm when switching about 1.5A.

The thermistor is just wired to a potential divider which gives 0.6V with an impedance of 100Ω. That makes a voltage that varies almost linearly with temperatures between 20°C and 200°C that can go straight into an analogue channel on the MSP430F2013. The micro can also measure its own supply voltage so that can be used to null out the supply tolerance.

Here is a graph of voltage on the input, inverted as it is an NTC thermistor, so it is roughly a graph of temperature :-



The heater was driven with a fixed 50% PWM drive. The temperature rises exponentially until it reaches equilibrium after about 12 minutes. I then turned on the motor and let it extrude some plastic. You can see the temperature drops significantly and rises again when I stopped the motor. This is because the hot plastic leaving the extruder carries heat away with it. Because plastic has a very high specific heat capacity this effect is significant. Finally the temperature falls exponentially when the heater is switched off.

The graph shows why closed loop control is necessary. The rise will be much faster because full power will be applied until the target temperature is met. That will reduce the warm up time considerably. I also hope to reduce the sag that happens when extruding which will make the extruded filament more consistent.

So a little bit of software now to close the loop.

All wound up

My years of hoarding junk is finally starting to pay dividends. I decided to address how I was going to feed the filament to my extruder. It only uses it slowly but when it runs out you have to strip down the extruder to start off a new piece. HDPE comes on a big 5 Kg reel like this :-



I thought it was asking a bit much for the extruder to rotate something that big and heavy so I started to look round for a smaller reel. I came up with this :-



It is a reel of 10000 4.7V zener diodes which I rescued from a skip. I removed the diodes, if anybody wants an envelope full just ask. It is about 270mm diameter, 70mm wide with an internal diameter of 70mm and a 30mm hole for a spindle. I wound some HDPE on to it and found that despite it being a lot smaller and lighter than the original reel it holds almost exactly half the plastic, i.e. 2.5Kg. The only problem I might have is that the plastic is quite tightly curled on the inside. Hopefully the extruder will have enough pull to straighten it.

So a plan was forming, I just needed an axle with descent bearings. Another piece of junk I had rescued from a skip was this aluminium roller:-



It was exactly the right diameter and was mounted inside a metal housing with ball bearings. I chopped up the housing to make two mounting brackets and moved the bearings around.



All that was left to do was screw it to the top of my machine. The roller is a bit long for an axle but it was easier to leave it full length than cut it and turn the end back down to fit the bearing. My lathe is nowhere near big enough for that. Here it is mounted up :-



I even managed to re-use the rubber 'O' rings on the roller to hold the reel in place. The bearings are so good that a quick twist will leave it spinning for more than 30 seconds so the extruder has no problem dragging the filament off.

Finally I replaced the knobs that I made with proper wing nuts as they are easier on the fingers.



The next task is to design the electronics to drive the extruder.

Sore thumbs

Well thumbs and fingers actually through stripping down and rebuilding my extruder a few times to solve teething problems. It has actually taken me a couple of days to get it working reliably. There were only two problems really :-

The first was that I was not tightening the springs enough. My springs are bigger diameter than the recommended ones and are too stiff to compress with ones fingers but even so I need to have them fully compressed for the extruder not to jam. What happens when they are not tight enough is that the screw thread slips against the filament and starts grinding it away instead of moving it.

The biggest problem was that my soldered joint between the steel cable and the drive screw kept breaking. The reason being that the drive screw is stainless steel which can not be soldered with normal flux. I tried cutting a cross in the end of the screw to give the solder something to hold onto. That did not work because the solder just forms a bead that does not penetrate the slots.

In the end I stuck it with JB Weld. For some reason it does not cure properly in the recommended 15 hours so I transfered it to the oven and baked it for 2 hours at gas mark 6. That seems to have done the trick.

I have found that running the extruder at different speeds gives different sized filaments.



The one on the left was extruded with the motor running from 4V and is about 0.8mm and the one on the right was extruded with the motor running from 10V and is 1.2mm. They are both extruded from a 0.5mm hole. I think what happens is that the plastic is compressed as it enters the hole and expands as it leaves it. The faster the motor runs the higher the pressure so the more it contracts and expands. The strange thing is that other people have not seen this effect. Possibly the hole in my nozzle is too deep or too shallow, I am not sure which.

I was surprised when I saw this piece emerge :-



But not when I examined the thermocouple I had used to measure the temperature of the molten plastic :-



It is supposed to work up to 250°C but it looks like the heatshrink sleeving they used is not up to the job.

My extruder occasionally produces swarf from the gap between the pump and the clamp. I am not sure of the exact mechanism for this is but it does not seem to affect its operation.



Here is a video of the extruder in operation and the filament produced showing self organising behaviour.