Sunday, 30 December 2007

Running repairs

No sooner than I had fixed my heater, the extruder motor failed!

I bodged the heater connection by putting some more solder on it. It's not a permanent solution because the solder is molten while the heater is on so it slowly oxidizes away. The last time bodged it that way it lasted six months though. It really needs a crimped connection.

The GM3 motor failed by running slowly, getting very hot and drawing lots of current. It eventually caused the protected MOSFET that is driving it to shut down. Opening it up soon revealed how it had failed :-

It has two pairs of copper brushes. Three of them have holes worn right through and the fourth has broken off. Its stub was touching the wrong side of the commutator, causing a short.

More expensive motors have carbon blocks on the end of arms which can wear down a lot further before they fail. Bigger motors have spring loaded carbon rods. The gearbox shows no sign of wear so it is let down by the cheap motor.

This motor is not really up to the job of driving the extruder. It is being severely abused by running it from 12V PWM when it is only rated at 6V. I anticipated it would not last long and ordered a spare when I bought it. I fitted that and HydraRaptor is up and running again. Curiously the second motor seems a lot quieter than the first.

At some point I think I will upgrade to a stepper motor. They are more expensive but, as long as you don't load the bearings, they last virtually forever. In the long run they probably work out cheaper and I can also dispense with the shaft encoder and the interference suppressor.

Friday, 28 December 2007

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.

Thursday, 27 December 2007

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.

Wednesday, 26 December 2007

Warped

Having got an idea of the HDPE warping for thin walled open boxes, I decided to start investigating solid shapes. I made a solid block 40 x 10 x 20mm to compare with the open boxes of the same dimensions.

Obviously there are many ways to fill the interior so I started with the simplest, just alternate layers of horizontal and vertical zigzags. HydraRaptor seems quite happy extruding 0.5mm diameter filament at 16 mm / second. If extruded into free air it would actually be 1mm at 4mm/s, but that is too course, so I move the head at 16mm/s which stretches it.

From trial and error I have found that a good layer height to use is 0.8 times the notional filament diameter. If it is more, then as the lower layers shrink, the nozzle rises faster than the object and a gap develops. Once that happens the filament squirms about and does not follow the path of the nozzle accurately.

So the extruded filament is constrained to 0.4mm high. Measurements show the width to be about 0.6mm. Incidentally, if it squashed to a perfect ellipse with a height of 0.4mm then it would be 0.625mm wide to have the same area as a 0.5mm circle. I extrude the zigzag with a pitch of 0.6mm so that adjacent filaments touch, but it means the object is not actually completely solid. The space occupied by each filament is a rectangular channel 0.4 x 0.6 = 0.24mm² but the cross sectional area of the plastic is π x 0.25² = 0.20mm², so about 18% is air. I confirmed this by weighing the block. It weighs 6.5g but if it was solid HDPE then 8ml would weight about 8g. It takes about 45 minutes to make the object including laying a raft.

Before I tried it, I always imagined the amount of plastic deposited would have to exactly match the volume of the extruded object otherwise it would sag or bulge. I could never understand how FDM worked reliably. Now I know that the volume can be a bit less and the difference is made up by air. That means the amount of plastic deposited is actually not that critical, which is why RepRap can get away with an open loop extruder.

I measured the warping with the three nail jig that I showed in the last post. The thin walled box is warped 0.44mm and the solid box has warped 0.87mm so that answers the question whether solid objects warp more or less. Note that the thin walled box is made with 1mm filament because 0.5mm filament is too thin to be self supporting.

I expect I can make a less warped block by extruding a thick base and then a less dense infill above that. Something else to try.

It is amazing how strong 10mm thick HDPE is. You don't often get to see plastics in that form. Most end products have optimised strength against cost by having thin walls and ribs etc.

Sunday, 16 December 2007

Chopping up chopping boards

Up until now I have been extruding HDPE onto foam board because it was the only thing that it sticks to well enough. However, it has a couple of failings: It is not strong enough to completely resist the warping caused by the HDPE and it is not reusable because the surface gets ripped off.

I have tried many other surfaces including various woods and metals (with and without primer), melamine and several other types of foam board but nothing worked. Obviously HDPE sticks to HDPE so I decided to investigate that further.

My first idea was to use a thin sheet of HDPE cut from a milk bottle. This makes a nice surface to extrude onto but the problem is holding it down. I first stuck it down with double sided tape but the heat melts the glue. Sticking it to a sheet of aluminium to take the heat away improved matters and I was able to get slightly less warping than with foam board.

To compare the warping on different base materials I made a test shape that is a 40mm x 10mm x 20mm open box with 1mm walls and measured how much the corners lift using a simple jig.

With foam board I was getting 0.83mm lift between corners and the middle. With HDPE stuck to aluminium I got 0.76mm. Not much better because the glue of the sticky tape stretches under the curling force.

I needed a thick HDPE base and I had heard that plastic kitchen chopping boards are made from HDPE. I bought a new one from ASDA which looks like this :-

It is 5mm thick, opaque and quite rigid. I realised it was very different from the other chopping boards we have which I think came from IKEA.

These are 10mm thick and made from a softer, more translucent plastic. To find out which was HDPE I used the flow chart on this website www.texloc.com/ztextonly/clplasticid.htm. I concluded the thin hard one from ASDA is HDPE and the thicker softer one from IKEA is PP. HDPE seems to stick equally well to both of them but the HDPE one warped a bit when it was only held down with masking tape, so I decided to go with the PP one. I cut it up and bolted it down to my XY-table. It was a bit curved due to years of dishwasher use but bolting it down pulled it flat.

Surprisingly, if I lay down a raft at 200°C it sticks well but can be easily prized off again with a penknife. The board is marked slightly but it can be reused over and over again.

I extrude the object at 240°C so that it welds to the raft and itself, and I turn the fan on after the first layer so that the object cools to room temp as fast as possible.

The board is strong enough to hold the object completely flat while it is attached but when it is removed it does still curl a bit. I measured 0.44mm on my jig so that is about half the curling I was getting with foam board. Other than extruding onto a convex surface, I think that is the best that can be achieved for that shape with HDPE at room temperature. Here are the three tests side by side :-

Next I will look at different solid shapes to see if they warp more or less.