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.