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.

Plumbstruder

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.

Flash bang bed

As my MK3 heated bed on HydraRaptor has been working well I decided to scale it up for Mendel.

Buying aluminium that is flat seemed to be a hit and miss affair until a friend told me that what I need is tooling plate and put me in touch with a company that sells it. They recommended C250 cast machined tooling plate. It wasn't cheap (I got 5 pieces 200 × 200mm for ~ £140) but they are all flat.

I can't find a geometric definition of flatness. It is given as +/- 0.4mm for a 6mm sheet of C250 (I would have preferred 5mm to reduce the mass a bit but that is +/- 0.8mm). I take it to mean that all the points on the surface of a metre square plate will lie in a volume 0.8mm high. For a 200mm piece I expect the deviation to be about 1/5 of that, i.e. 0.16mm assuming it is a single curve rather than wavy. Since the bed can be levelled at the corners the deviation in the middle should be about half that again, 0.08mm, just about acceptable for raft-less printing.

When I tried levelling the bed I ran into a problem though. With my Dibond bed I could level each corner because it can flex a bit. With the rigid aluminium bed I can only level three out of the four corners at a time. When I move the nozzle to each corner in turn it behaves as if two diagonally opposite corners are lower than the other two. That would imply the plate is not flat, but I know it is when I put a straight edge across it. I think this means that the two y-axis bars are not quite level with each other at both ends, causing the bed to twist about the y-axis as it traverses it. I expect it could be corrected by adjusting the frame but I haven't got my head around what to adjust and in what direction yet.

Given that I am using 188W on a 150mm bed on HydraRaptor, to get a similar warm up time I would need 335W. That seems a lot to get from a PSU, so I decided to make it mains driven. I found that I could get 47Ω TO220 resistors cheaper than other values. Five in series across the mains gives about 250W, so I used two strings of five to give 500W. That gives a warm up time of about three minutes.

Equally spacing four or nine resistors on a square is easy but placing ten is an interesting problem. I used the solution to packing ten circles in a square that I found here. This is my layout with 16 magnets as well.



And here it is wired up: -



I used wire with PTFE insulation rated to 300°C. I have an earth connection of course. It would be a good idea to have a second earth in case the first one breaks due to the constant bed movement. I also fitted a 150°C thermal cut out that came out of a microwave oven. With 500W it would get very hot indeed if the control circuit failed.

I intended to mount the magnets the way I did before, by drilling holes not quite through, leaving a rim to retain them. I didn't tighten my drill stop enough and went all the way through so I decided to glue them in with JB-Weld.

I placed the bed onto a sheet of glass with some cling film on it. I then dropped in the magnets and glued them. When I turned it over the next day I found the magnets were sticking up from the surface. The glue must expand as it sets pushing the magnets down and lifting the plate!

I tapped them down with a punch but, unsurprisingly, they fell out the first time the bed was heated. In the end I jammed them in with PET tape. Drilling part way through is a much better solution.

I mounted the bed on top of the Dibond bed with nylon stand-offs.



Not an ideal solution as a lot of z-travel is lost, but the thermal cut-out is quite deep.

I used chocolate block connectors to wire up the mains. To make them safe and provide strain relief for the cables I RepRapped some plastic covers.



The lids just clip on with some tabs that fit into small slots. They didn't fit very tightly, I need to make the tabs bigger and a tighter fit. A boss and a screw hole would have been better I think.






For safety all the wires should be inside the cover as everything accessible should be double insulated. I will make it wider at my next attempt.



The bed worked well for the first few objects I made. Simple bang-bang control gave about 10°C overshoot initially but settles down before the object build starts so does not really matter. One thing I have realised is that the nylon pillars expand about 0.1mm when they warm up so I give them some time to do that otherwise the first layer has varying height.

I got some new ABS from reprapsource.com that turned out to be white, I was expecting natural as that is easier to work with. It seems to need higher temperatures to get it to stick to itself and the bed. I am extruding at 240°C with the bed at 140°C for the first layer and 110°C after that. I built one object like that and then disaster struck. The bed heated to 140°C and levelled off. While the extruder was heating I heard a few pops and crackles. When I looked at the temperature graph I saw the bed temperature soaring. Before I had time to think what was happening there was a loud bang and flash from underneath the bed and the 5A fuse in the plug blew.

What happened was one of the resistors developed a short between its tab and one of the connections. That caused a path to earth which increased the power on the remaining four in the chain. Several of those went short circuit as well in a chain reaction which ended up shorting the mains.What I couldn't explain at first was why the firmware did not turn it off and why the thermal cut-out did not cut the power. It turns out that I had swapped the live and neutral connections in the IEC connector, which meant that the solid state relay and the cut-out were in the neutral connection. As soon as the first resistor shorted it had bypassed all the control, not good!

I had originally chosen the resistors when I was making a bed for PLA at 60°C. Looking at the datasheet they have a maximum operating temperature of 155°C but they are de-rated to zero wattage at that temperature, so by putting 50W into them at 140°C I am grossly over loading them. I have abused AL clad and vitreous enamel resistors in this way and not had any problems but the TO220 seem far less robust. I don't know what they use for the tab insulation but I wouldn't be surprised if it was epoxy. The high voltage may also have been a factor as the ones on HydraRaptor have survived a similar overload so far. They have the same de-rating curve, but are made by a different company.

I rebuilt the bed and changed my firmware to stay inside the power curve by reducing the PWM ratio as the temperature increases. Unfortunately , I found I could only get to 130°C so I had to change the zero power point to 200°C to get to 140°C in a reasonable time. Even then it takes 400 seconds instead of 175.

So far it is holding up, but it is nowhere near as fast as I wanted. A shame because I had bought 50 of the 47Ω resistors, but I think I will have to scrap them and go back to AL clad. The smallest ones that I have used before are not rated for mains voltage so I will need some bigger ones. PCB or stick on silicone heaters are starting to look more attractive!