Monday 24 May 2010

Black and White

I bought some new ABS filament from as it is a reasonable price, the postage from Germany is not too bad and being in the EU there are no customs charges, so it does not get held to ransom by Parcel Force for their ridiculous handling charge.

The advert does not state a colour so I assumed it would be natural, however when it came it wasn't like any ABS I had encountered before. Natural ABS is cream coloured and opaque. This was white and a bit translucent. At first I though it was HDPE, but when bent it bruised, which is a characteristic of ABS.

I ran it first in HydraRaptor. The only issue I had was that it didn't want to stick to the PET tape I was using until I raised the bed temperature to 140°C for the first layer and extruded at 250°C. For subsequent layers I revert to the bed at 110°C and filament at 240°C.

The objects produced look nice in white and seem to be harder than those made in natural. I don't think it is simply pigmented ABS, I think it is a different formulation.

My impressions of using PET tape instead of Kapton tape is that it doesn't seem to give as much grip as new Kapton, but it doesn't degrade. I can make most things on it with HydraRaptor without any warping at all, but Mendel bed springs tend to come unstuck. This is because they are relatively tall and have very little contact area with the bed. If the extruder hits a slight blob on a high layer it will snap the part off. Sometimes the loose part hits another part and starts a chain reaction where they all fall off.

When doing raft-less builds on PET or Kapton it is essential that the first layer outline sticks perfectly and has no gaps in it, especially at the corners. If the first layer is too high it obviously doesn't stick and takes short cuts across the corners. If it is too low it also lifts at the corners though. What happens is that the filament becomes squashed into a flat ribbon. When that tries to bend around a sharp corner the outside has to stretch but instead it lifts and folds over inwards. A difference in z-value of 0.05mm can make all the difference. Increasing the temperature also helps to make the plastic bend around corners. If a corner does not stick perfectly then after two or three layers it will curl up at an angle of about 45°. This effect is not like the corner warping you get on a cold bed. It is much more localised and extreme. Small objects tend to come off during the build if a corner lifts.

With the natural ABS I was using before on Kapton it was far less critical. Objects stuck so well I had to remove them with a hammer or use a flexible bed. With white ABS on PET tape the objects can be removed more easily. Sometimes they just come free when they are cooled.

When I tried the new ABS in my Mendel it took a lot more tweaking to get it to work. The first issue was that I had to increase the feed rate by about 18% relative to what I was using for PLA. My theory is that being softer it presses further into the threaded pulley and so sees a smaller pulley diameter. The hobbed M8 bolt has an internal radius of only about 5mm. The drive pulley on HydraRaptor is about twice that diameter and seems give more grip on softer plastics and doesn't need the 18% bodge factor when switching from PLA to ABS. I just tell it the filament diameter and it just works.

The next problem I had was that holes tended to shrink inwards and not meet the infill as you can see on this piece.

I also find PLA has a tendency to do this on my Mendel but not on HydraRaptor. For a sanity check I built the same object from the same g-code with black ABS.

Notice how much bigger the holes are.

When I was flushing the black out again with the white I noticed that the white had far more die swell and was coming out at about 0.7mm. The black was only about 0.55mm. This means that to extrude at 0.5mm the white is being stretched a lot more, which accounts for why the holes shrink inwards. To test this hypothesis I ran the same g-code again scaling up all the coordinates by 0.6/0.5. This produced a bigger object but the holes are much better.

I then re-sliced the object for 0.6mm filament and that also printed correctly.

So it seems that the white ABS has more die swell than natural or black. In that respect it also reminds me of HDPE. For some reason HydraRaptor is not affected and seems to have less die swell despite having a smaller nozzle, which normally gives more die swell in relative terms because the pressure is higher.

The other thing I discovered is that black ABS does not stick well to PET. It seems a bit greasy.

So with a 0.5mm nozzle if have to build objects at 0.6mm when using white ABS in my Mendel, but with a 0.4mm nozzle on HydraRaptor I can build at 0.375mm or 0.4375mm no problem and holes do not shrink excessively. I am not sure what the difference is, perhaps the length of the nozzle aperture.

Friday 14 May 2010

PLA on glass

A while ago Jordan Miller emailed me to say that PLA can be printed on hot glass. He had tried ABS but it did not stick at 90°C, which was the highest temperature his bed would go so I said I would try it at 140°C.

I found a piece of glass the same size as HydraRaptor's bed that was 5mm thick. It used to be the platform of a kitchen weighing scale. It has nice rounded corners, the only problem was that it had an aluminium boss glued to it. I tried to remove it first with a hammer, then I tried acetone and finally I tried a hot air gun. None of these methods worked so I put it in the oven at gas mark 6 for 10 minutes. It then just lifted off with a pair of tongs.

For a quick test I just taped it down with some Kapton tape. It holds firm as long as you do all four sides.

As you can see ABS does not stick to glass at 140°C.

Next I moved the glass onto my Mendel as it was set up for PLA at the time and I couldn't get PLA to stick to PET tape.

I printed a frame vertex on glass with the bed starting at 120°C for the first layer, dropping down to 45°C for the rest of the build.

That stuck well but came off easily when the bed was cooled. Next I tried a new piece of 4mm glass cut to the size of the bed.

That stuck so well that it took several blows with a hammer to to remove each object. One piece chipped when it hit the wall behind! For some reason the new glass seems to stick much better than the old.

The objects come off perfectly flat and glassy.

I dropped the bed temperature to 100°C, which makes them a little easier to remove, just a sharp tap with a hammer rather than a heavy blow! Any lower than that and I have trouble getting the outlines to stick. Jordan uses only 65°C and reports the objects are easy to remove, so I am not sure what I am doing wrong, different PLA perhaps. If I start with the head lower then the plastic rucks up during the first layer infill.

So glass looks like a good bed material for PLA as it comes completely flat and hopefully should not degrade. Jordan reports that finger prints prevent objects sticking but they can be removed with alcohol. Copper clad PCB material has the advantage that you can flex it to remove objects but doesn't give as good a finish.

Monday 3 May 2010


Brian Reifsnyder asked for volunteers to test his hybrid PEEK and PTFE insulator design, so I used it for the hot part of my Mendel extruder to start with. The drive mechanism is Wade's design.

It worked well at first, requiring little force to extrude PLA, but got harder and harder until eventually it completely jammed. This video below shows that even with the nozzle removed and starting with a completely empty barrel I couldn't push more than about 15mm of filament through it.

The reason was that the PTFE liner had slipped a little leaving a small gap between it and the end of the brass heater barrel.

This makes the extruder jam completely solid. The reason is that PLA goes rubbery above 50°C, so any pressure on it makes it expand width wise and grip the side of the tube. If there is a gap that it can expand into it locks the filament.

I stripped it down, cleaned it out and reassembled it with some washers to hold the PTFE down.

Brian has added a circlip to the design to solve the problem.

I haven't tested this version yet because I ran into another problem before it arrived. When I started using a heated bed for PLA the extruder jammed again. This time it was because the top end of the insulator got hotter than the glass transition of the PLA, so it swelled as it went into the insulator and jammed in the tapered entrance. There was also some leakage around the threads.

The reason it got too hot is a combination of the heated bed, the fact that I used an uninsulated heater with a large surface area, and the fact that the Mendel carriage traps the rising heat.

I decided to try out an idea I had a while ago, which is similar in intent to Brian's scheme. Instead of putting PTFE inside PEEK to stop it expanding I put it inside a 15mm copper pipe. This not only totally constrains it so it cannot swell, it also removes heat from it, shortening the transition zone. I am calling this one Plumbstruder. Here is a sketch of the layout: -

The end of the copper pipe is closed off by soldering an end cap on and then drilling it out to leave a lip to support a PEEK disk which the barrel screws into as well as into the PTFE. That means the PEEK supports the extrusion force, as in Brian's design, but I also use the thread in the PTFE as a seal rather than just having a compression joint.

The copper pipe gets hot so I coupled it to a big heatsink with a copper flange.

I turned this from a solid block of copper a friend gave me (thanks Paul). I soldered it onto the pipe and screwed it onto the heatsink.

I turned the one piece nozzle / barrel from hex stock so it has a nut shaped flange in the middle to make it easy to screw in and also gives the aluminium heater block something to tighten against.

I had to turn down the PTFE to be a tight fit inside the pipe. I was hoping to find a size where the ID of the pipe matched the OD of the PTFE. 22mm copper pipe has an ID of 20mm, so theoretically 20mm PTFE rod would fit. In practice I have found that PTFE rod is about +/- 0.5mm so, unless you were lucky, the fit would not be good enough.

Even with a big heatsink it was getting uncomfortably warm so I added a tiny fan.

I have been using this extruder on my Mendel for a few weeks and it is totally reliable, with no sign of leaking. I think that of all the extruders I have made, this one needs the least force to extrude. I can push plastic through by hand at high speed with ease. For an extruder to work I think the transition zone needs at least two of the following three attributes: short, slippery or tapered. Unfortunately a short transition zone seems to mean using a heatsink, which is not ideal for a moving head machine.

I also think a short melt zone improves the accuracy by reducing the start-stop time. In that respect this design is not ideal, although it is no worse than the standard design.