Wooden overcoat

I can make raft-less ABS objects on a heated bed pretty reliably, but when I try to do a whole bed full I get corners lifting on the objects near the edge of the bed. I think the reason is that the air around them is not as hot. If you think about it, with a moving bed machine like HydraRaptor, if you have a single object in the centre of the bed then you have a buffer of hot air around it. The bed only has to move by the dimensions of the object, so if the object is less than half the size of the bed then it remains inside that buffer. When you make objects near the edge of the bed the buffer is smaller and the bed moves further, so you get a double effect. To mitigate this effect I have halved the size of my biggest build trays. This is less convenient as a full bed is about 8 hours giving three shifts a day.

For example, with natural ABS on Kapton I was able to get away with a full bed like this: -



... but with white ABS on PET tape I would always get the odd part around the edge lifting, so I have to do it as two builds now. I bought a new reel of natural ABS from MakerBot but I can't make it work with HydraRaptor. I made a couple of objects but then it started to always jam after a few layers. The reason seams to be that because it is undersized at about 2.8mm, and my barrel has an internal diameter of 3.6mm, molten plastic back-flows up the barrel as far as the cold zone, where it freezes and makes the filament hard to push. This causes the filament to buckle and jam. I don't understand what has changed. The last reel I got from MakerBot a long time ago was the same diameter and I only finished using it recently and had no problems in the same extruder with the same settings. I switched back to white ABS and it works reliably again, so it must be something to do with the plastic.

Another problem with ABS is the fumes. My Mendel extruder seems to give off more fumes than HydraRaptor's does, perhaps because the melt zone is much bigger, and the white ABS seems to smell more acrid than natural ABS. I did a build with a window open to get rid of the fumes but most of the parts then warped, presumably because there was no longer a buffer of warm air around them, but a cool breeze.

In an attempt to tackle both of these problems I built an MDF box around my Mendel.



The front of the box is held on by magnetic door catches. It is sealed by door draft extruder strips and has a window made from plastic from a picture frame. This is glued on with silicone sealant.

The box is tall enough to allow the filament to enter through a single hole in the middle of the roof with a felt gasket that catches the dust.



The fumes are extracted by a tiny fan mounted in a chimney in the roof and piped out through a window vent.






I made a little pipe with a flange that fits into a slot in the vent and taped up the other slots with PET tape. I have another vent in the other window for fresh air in.

This fan is controlled by a spare output on my extruder controller and I have a thermistor to sense the air temperature in the middle of the chamber at the height of the top of the Mendel frame. Together with a small fan to cool the extruder heatsink and a large fan to cool the bed that uses up all the free outputs of my extruder controller, but not for the uses I originally envisaged.



I set the target chamber temperature to 40°C because that is as high as I dare to run the electronics and power supply. With the front closed the small fan cannot hold the temperature down and I have seen it go as high as 50°C without any ill effects. The extruder stepper was then too hot to touch though. Note there is no chamber heater. All the heat comes from the uninsulated bed, extruder and the motors and electronics, so I have actually reduced the total power consumption slightly and gained a heated chamber. To maintain 40°C I have to leave the front open at the bottom. I will add some vents at the bottom of the sides to allow cool air in and perhaps use a bigger fan.

Even with a gap at the bottom of the door I cannot smell any fumes. Since using the chamber nothing has warped provided the first layer outline went down properly as discussed in my last post. It also makes the machine very quiet although it was already much quieter than HydraRaptor.

Black and White

I bought some new ABS filament from reprapsource.com 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.

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.