Wear and tear

My extruder's heater went open circuit so I removed the heat shield to have a look at it. I have actually run it for many hours now and have extruded quite a lot of HDPE. I have about 200g of extruded test objects and scrap which represents about 13 hours operation. I only recently started saving my scrap so I must have extruded a lot more. The 2.5Kg reel of HDPE is noticeably smaller.

The heater has also run for a lot longer than the extruder has been extruding. I got fed up of waiting for it to warm up at the start of each run so my host software leaves it on. I keep meaning to put a timeout in the firmware to turn it off when there hasn't been any Ethernet messages for a while as I have left it on for long periods a few times.

The extruder is starting to show some signs of aging. The plastic shield which keeps the fan draft away from the nozzle looked like this when I made it :-



But now it looks like this :-



The nozzle itself now looks like this :-



The JBWeld that surrounds the heater wire has gone very dark and has several cracks in it. One of the heater connections broke off in a previous accident so I dug it out and joined a piece of copper wire by squeezing it tight and soldering it. There is now no sign of the solder which is why it has gone open circuit.

The black stuff which looks like bitumen must be slow cooked HDPE. I am surprised that long term heating to 240°C causes it to decompose. I don't know if the white surface on the shield is just due to its surface melting a bit or whether something boiled off the nozzle and condensed onto it or reacted with it.

Even the high temp insulation over the thermistor wires is starting to look a bit sad!

I also noticed that the steel wire that forms the flexible drive coupling is starting to break up. A couple of strands have snapped and there is a pile of black dust on top of the pump shell.



The heater connection should be easy to fix. I have a few planned improvements to make to the extruder but I will wait till parts wear out before replacing them with better ones to get the most use of it.

To raft or not to raft?

When extruding HDPE onto foam board a raft needs to be laid down first to increase the anchorage at the corners to reduce curling. It becomes part of the object and has to be trimmed back to its outline with scissors or a knife. Now that I am extruding onto polypropylene cutting board I wondered if it was still necessary.

The temperature at which I lay down HDPE onto the cutting board is important. At 180°C it does not stick. At 200°C it sticks well but can be peeled off with the help of a penknife. Higher temperatures make it harder to remove and do more damage to the board.

Here are a couple of 15mm test cubes made directly onto the PP board without a raft :-



The one on the left had the first layer extruded at 200°C and subsequent layers at 240°C. As you can see it curled badly, particularly at one corner. The one on the right had its first layer extruded at 220°C. It looked promising but when I tried my standard warp test block the result was not good!



So it looks like the raft is here to stay. Here is an example :-



I lay down the raft at 4mm/s with a notional filament diameter of 1.1mm with the extruder head 1.3mm above the board. This is to get the filament as round as possible so that it doesn't form a solid weld. In actual fact, gravity causes it to slump to about 0.9mm high and spread to 1.3mm wide. The oval area calculation would give 1.34mm and a pitch of 1.3mm is sufficient to get adjacent filaments to stick together. My rationale for making the raft as thick as possible in one layer was to make it strong without taking too much time. It probably does not need to be as strong now that it binds to the PP.

I put the raft down at 200°C, then I do the first layer of the object at 240°C with the fan off to ensure it welds to the raft and then subsequent layers at 240°C with the fan on.

I calculate the amount the raft overlaps the object with this completely arbitrary function :-

def overlap(x):
  return x + 10 + 10.0 * (x - 20) / 80

I halved the overlap when I went from foam board to polypropylene.

Cutting corners

When making solid blocks with 0.5mm HDPE filament I noticed that the corners are not very accurate. The right hand edge of the 20mm cube below shows this effect at its worst :-



The problem is that, although the machine makes a perfect right angle, the filament appears to have a minimum bend radius and so cuts the corner. The amount it cuts the corner seems to vary from layer to layer giving rise to the rough edge.

I think the variation is due to the fact that my extruder spindle is a bit off centre. This causes the torque to go up and down as it rotates, which causes the flexible drive cable to wind up and run down again. This causes speed variations despite the fact that the motor speed is well regulated. At some point I will get rid of the flexible drive.

I expect the fact that I am stretching the filament doesn't help with the corner cutting. I improved it a lot by slowing down the drawing of the outline to 4mm/s and leaving the infill at 16mm/s. Here is the result :-



Still not perfect, another thing to try would be to recognise that there is a minimum corner radius and make the nozzle follow an arc of that radius around the corner. At least that way it might be more uniform.

Here is a close up of the top face taken with a scanner:-



As it goes round the corner the filament has an external radius of about 1.5mm and an internal radius of 0.9mm. As it is 0.6mm wide that is probably not bad. You can also see that the zigzag infill sometimes ends a bit short of the edge, probably also due to corner cutting.

To get sharper corners I expect I need to use a nozzle with a smaller hole, so that the filament can be fine without having to be stretched, but that has the disadvantage of slowing down the extrusion rate for a given pressure.