Some corners like it hot

Large objects with sharp corners, such as the Mendel z-leadscrew-base, produce enough stress to form a blister in the PET tape on my heated bed. These can only be flattened again by pricking the tape. I can't understand how air gets in and cannot get out again, but that is what seems to happen.

The blisters leave a small indentation in the object's base. It is only an aesthetic problem because the base remains flat, i.e. it doesn't rock on a flat surface.

Sometimes the blister allows the corner to peel from the bed towards the end of a build, allowing the corner to curl upwards a little. Generally I can avoid that by cleaning the bed with acetone before problem builds. I also use hexagonal infill on those parts and only two solid layers rather than three in an attempt to reduce the stress. When I design my own parts I round the corners, where possible, to prevent such problems.

A solution may be to use a sheet of PET rather than PET tape, but then you need to find a way of holding it down. One thing I have noticed though is that when I build a bed with four of the z-brackets closely packed the corners on the inside don't blister or lift. That must be because the air around them is hotter. As an experiment I added some little plastic walls to the build to act as baffles to keep the heat in as the bed moves through cooler air.

These have a 5mm thick base to help keep the tape flat and are 1mm away from the edge of the object. They work well and stop the blisters forming at the corners. They are very similar to Forrest's apron technique but their primary function is thermal rather than mechanical. A more general technique would be to build a thin wall all the way around the perimeter of the objects to cocoon them. I expect that would only need to be one filament thick and perhaps might give a similar effect to having a heated build chamber.

Freezing your bits off

Since I started cleaning my PET tape with acetone it can be hard to remove the parts from it sometimes. Somebody suggested trying freezer spray a while back, so I gave it a go.

I got this Arctic Spray, which is intended for freezing water pipes so that you can work on them without draining the water. I must admit I wouldn't fancy having a strict time limit if I was plumbing, you would have to be sure you had all the right tools and materials to start with. My occasional forays into plumbing rarely go to plan and usually involve a trip to B & Q in the middle.

I tried it first on an ABS part before the bed had cooled for any length of time, so it would be at about 100°C and the parts still soft. The part curled up at the edges and so came off easily. I thought I had ruined it by making it warp, but to my surprise it became flat again when it cooled. Still that seems a bit risky, and the spray isn't cheap, so now I cool the bed to 50°C with a fan and then spray any stubborn parts that I can't pull off. It works a treat but I don't know how long a can will last. It would have to be a lot of uses to make it worth the cost: £4.49 plus £2.20 on eBay. Hitting them with a block of wood and a hammer is a lot cheaper!

Friday the 13th

When I got up last Friday morning my PC had this on the screen: -

This is despite the fact that I had automatic updates set to ask me before installing. This was the result :-

The PC had stopped talking to my Mendel about half way through a seven hour build, so the axes had stopped moving but the extruder was left running for a few hours. The result was that the extruder was encased in a solid ball of ABS about the size of a tangerine. Thanks Microsoft! What I actually said at the time was less polite!

Of course I was planning to put some safeguards in the firmware and also run the machine from an SD card, but have never quite got round to it as I have been printing virtually non-stop for months.

I couldn't remove the extruder from the carriage because the blob was too big to go through the gap, so I had to dismantle the x-axis to release the carriage.

I have seen this happen to other people and it wrote off the extruder, but I thought this one should survive because it is mostly metal underneath the blob.

I tried using a loop of hot nichrome to slice bits of it off. The nichrome cut through OK, but the ABS closed up behind it, so it achieved nothing.

Next I tried a small circular saw attached to a Dremel. That worked OK, but threw off sawdust and bits of ABS hot enough to burn, even through light clothing. I got the bulk of it off that way but when I nicked one of the heater wires I decided to stop.

I got some more off by heating it with a hot air gun and pulling lumps off with a pair of pliers. That was OK but the whole extruder got too hot to hold and it was starting to soften the carriage.

I got the remainder off by running the heater up to 200°C and using a knife, wire cutters and pliers. It took me about 3 evenings in total to remove the blob and the machine was out of action for a week while I reassembled it and calibrated it again.

It now takes a bit longer to warm up, and extrudes more filament during the warm up process than it used to. I suspect therefore that the thermistor is reading low and so it is running hotter. It doesn't seem to cause a problem with the ABS that I am using. I had to increase the time I run the extruder to prime it after warm up though.

I also seem to have managed to bend one of my z lead-screws while sliding the x-axis bars in and out. It doesn't matter as the axis is constrained by the z-bars, but annoying as it rattles a bit.

All in all a bit of a disaster. It's running again now though, but I still haven't put a safeguard in my firmware. I will have to develop it on HydraRaptor and load it into Mendel between builds. I have disabled automatic updates!