A bit of a drag

One problem I have had, on and off, with Mendel is a tendency for the infill not to meet the outline. This was particularly bad with PLA. I have combated this by having some infill overlap and also extruding the plastic slightly faster than it should be, so that the solid layers are well stuffed. I don't have to do either of these things with HydraRaptor. I couldn't figure out what the difference was until I changed a reel of plastic recently. The first print on the new reel came out like this: -



The gap is always at one side like this, it is as if the infill is not centred within the outline. The reason, I have come to realise, is that the belt has some play in it because it is not infinitely taught. When the extruder pulls filament off a reel it exerts a force on the carriage, which displaces it slightly from where it would come to rest without any external force. Because the carriage moves, the filament only gets pulled from the reel at the local extremes of movement, the rest of the time it is slack. This causes small offsets in the filament paths. In particular, when it is doing zigzag infill it is using filament relatively quickly, so at the end of the zigzag furthest from the centre of the bed it is likely to give a little tug of the reel each time, causing the zigzag to stop short of the outline.

The conclusion is that the filament feed for a belt-driven moving-head machine needs to be very low drag. The hanging basket technique that I used on HydraRaptor is no good because it has to pull plastic out from under its own weight. Making the feed point high above the machine reduces the lateral drag on the carriage, but you can easily get enough vertical drag to deflect the x-bars upwards, or even lift the z-axis slightly because the backlash in the thread is only taken up by the weight of the x-axis.

The reason it was bad with PLA was because I was pulling it from a hanging basket and being very stiff, even a small coil needs a lot of tug.

The system I now use is a vertically mounted spool big enough to take 5kg coils, which last me a couple of weeks.



The bearing is just a stainless steel axle running in PLA bushes, lubricated with some lithium grease. It is low friction, but not as frictionless as a ball bearing. It needs a little friction to stop any in-balance in the coil causing the spool to spin to its low point. Also the faces of the spool need to be quite big to stop a loose loop of filament coming over the side. It pulls tight and jams if that happens.

I can take the spool apart to insert a new coil of plastic.



In general though I have to wind it all off and on again to get it tight and balanced enough to wind off smoothly. Since 5kg is about 800m it takes a long time to wind it onto a garden hose reel and then back on again. Someday I will get round to making a machine to do it for me. In the meantime I will make a second identical spool so that I can just mount the coil on one spool and wind it onto a second one to use.

Meltdown

While making its 18th child, my Mendel made a real pig's ear of laying down the first layer holes at the start of a build. So bad that the infill did not join to them and started curling upwards. I had to watch it a while before I realised what the problem was. The extruder had come loose and was bouncing up and down when the filament feed stopped and started.

I thought it was just that the bolts had worked loose but after I tightened them it was still moving and there were some worrying crunching sounds, so it was time to strip it down.


The bottom of the heatsink is covered with a sticky deposit. It is some volatile component that boils off the ABS and condenses on cold surfaces,



The main extruder bracket had broken and the carriage didn't look too good either: -



I stripped the carriage down as well and found that it was cracked and severely distorted.



The main problem I realised is my modified hot end. Normally the insulator is locked into the chunky part of the bracket by a couple of M3 bolts through it. I can't get those in because my heatsink is in the way, so I rely on the mounting bolts and the upper carriage to take the extrusion force. On reflection, not a good idea!



The lower carriage is less deformed because the extrusion force does not pass though it.



The heat rising from the bed and the extruder must be enough to soften the carriage and let it deform, but also it seems to have made the ABS weak and crumbly. Even the belt clamps have deformed.



This is after about 3 months of printing though so it isn't a big problem to replace them as long as you have spares. I had just printed a carriage before it failed so it was easy to replace but I had to print another extruder on HydraRaptor. You really need to have a full set of spares on hand, or have two machines.

I did various changes to make it more durable. The main thing is I fitted nuts under the heatsink so that no force goes through the carriage. I also put large penny washers on the top of the extruder bracket to reinforce the lugs. Ideally they should be a bit thicker but that would reduce the Z travel even further. The extruder motor clashes with the frame which reduces the height. Then my heatsink loses another 10mm or so and my heated bed loses 26mm. I am left with about 35mm which is only just enough to build the tallest Mendel part (the lower carriage).

I also used nyloc nuts in the captive positions in the carriage. The wiki advises against this as it may crack the plastic but it doesn't seem to be case with my ABS parts. Ordinary nuts don't stay tight because the plastic creeps.





I intend to fit some sort of heat shield to stop the heat rising from the bed reaching the carriage. In the mean time I have started fitting the front on my cabinet after the first layer is finished, when the bed temperature drops from 140°C to 110 °C. That can be up to 90 minutes into the build, so not convenient.

ABS on PC

My last heated bed ran for a long time but it finally went pop on Mendel print number 15. The TO220 resistors developed a short to earth about half way though an 8 hour overnight build. It took out a 5 Amp mains fuse and destroyed the 4 Amp solid state relay that was controlling it.

Clearly the cheap TO220 resistors are just not suitable for abusing as heating elements, so I went back to using aluminium clad resistors. The disadvantage is that they are higher profile and need two accurately drilled mounting holes, but they are a lot more robust and cheaper. The more expensive TO220 resistors I used on HydraRaptor are still going strong, but there is nothing to suggest that they are any better in their spec. It is the tab insulation that breaks down though, so it could be just the fact that the voltage is much lower on HydraRaptor.

I have used the Tyco THS10 series at temperatures up to 240°C and not had any fail yet. They are not rated for mains voltage though, so I moved up to THS15 series which are. They are slightly taller, which doesn't actually matter because I use 20mm stand-offs, so there is still sufficient gap. The mounting holes will take an M2.5 screw, but I didn't have any to hand, so I drilled them out for M3. There is just enough room for a screw head with an integral washer, a standard washer would not fit.

I have run the THS10 at about twice their rating so I did the same with these: 9 × 22Ω in series gives a total power of 290W at 240V. That gives a warm-up time of about 4 minutes to 140°C. My extruder takes longer to get to 255°C, so I set them both off together so that the bed has enough time at its steady state temperature for the nylon pillars to expand fully.



The white PTFE clamp is where I attach the thermocouple. The device wrapped in Kapton tape is a 190°C thermal cut-out to prevent melt down if the firmware crashes or the solid state relay goes short circuit. The mains wire has PTFE insulation to handle the temperature. Since the wiring is exposed it should really have an extra layer of insulation to be considered safe, but I am not about to stick my fingers under a hot bed so I didn't bother. If you have children or animals, or are completely risk averse, then you probably should.

I haven't put any magnets on this one yet as I haven't been making use of the ones on the last bed since I started using white ABS on PET tape. The objects mainly come loose when they cool down and are easily removed without having to remove the steel plate and bend it.

ABS on PET tape works well. The grip level seems to degrade much more slowly than Kapton does. After lots of use it becomes easier to remove objects, but then the amount of grip is not quite enough for some parts. I can make most of the Mendel parts time after time, but I have problems with a few. The outer corners lift slightly towards the end of the build of the large Z brackets when the PET is old and I am building more than one at a time.



Not easy to see, but the bottom right corner has lifted by about 0.5mm. It makes no difference to the function of the part but I like to get them completely flat.



At the opposite end of the scale I have problems with the bed springs and the X 360 Z bearing plates. These are very tall compared to their footprint, so as the nozzle bushes past the top of the objects they often ping off the bed due to the small contact area and the high leverage. When the PET is old I have about a 30% reject rate with these unless I do them one at a time.

I had a 5mm sheet of polycarbonate that I have been meaning to try as a bed material for some time. I think that is what is used on commercial machines. It has a high melting point (267°C), so will not melt when the hot filament lands on it. It also has a high glass transitions (150°C) so shouldn't soften on a heated bed.

I clamped it to the aluminium bed with some bulldog clips.



I tried it cold to start with but the ABS did not stick so I tried it at 140°C next. I made a test shape that I am using to research hole shrinkage. It stuck so well I broke it trying to get it off.



I had to use a chisel to get the rest off. Strangely, although the ABS is extruded below the melt point of the PC, so it can't form a diffusion weld, it forms a stronger bond with the PC than to itself.

I dropped the initial bed temperature to 50°C which seemed to be the lowest I could get the first layer outline to stick properly. After the first layer I set the bed temperature to 90°C to reduce the warping stress in the ABS. These are temperatures on the underside of the aluminium, so the top surface of the PC will be something like 15-20°C lower.



I made these tall objects that tend to come unstuck from PET. These held well, in fact, when I removed them, most of the springs and one of the bearing plates left their bottom layer behind. Not really a big problem, the bottom layer becomes a minimalist raft!

For general production I went back to PET tape. I covered a sheet of 1.5mm thick stainless steel and clamped it down with more bulldog clips. I can swap it with a sheet of glass if I need to do PLA. The steel seems to be strong enough to stay flat in the middle when clamped at the edge.

Broken bracket breakdown

Whilst printing a 16th set of Mendel parts, my Mendel printed a bed of brackets with bits missing: -



On investigation I found the idler bracket on the extruder had broken, so there wasn't any pressure on the pinch wheel.



It lasted a long time before it broke but clearly it wasn't strong enough. Wade made his in PLA, which is harder and I only use two of the four bolt holes, so mine is under more strain.

I made a stronger replacement. It is thicker and a little bit bigger in the other two dimensions. I also made the holes 4.5mm rather than 4mm so it slides on the bolts easier and I capped the ends of the axle holder as mine tended to slide sideways.



The files are on Thingiverse.