Reprap Update

No pictures for this one I’m afraid but don’t worry, the next update on the Mobility Scooter will have a fair few.

Last time I wrote about the Reprap I was having a weird issue with the filament drooping down the side of the prints and just generally misbehaving as well as an issue with the Z axis going out of alignment at odd times.

Ive managed to narrow the Z axis alignment issue down to thermal expansion, i.e when the extruder is cold it is slightly shorter than when it is warm. Normally this wouldn’t be an issue but the tolerances between the extruder nozzle and the print bed need to be just right, to wide a gap and the first layer doesn’t extrude properly and to narrow and the first layer squishes out under the side of the nozzle, the nozzle drags in the print and things just go generally all wrong.

To remedy this firstly I got the bed completely level in relation to the extruder. Secondly instead of trying to get the Z end stop in exactly the right place I set it so that the end of the nozzle depresses the levelling spring in the home corner of the bed. This helps prevent the filament from oozing out of the nozzle as it heats up. I realise that this will result in a small section of the glass print bed heating up a lot more than the rest and accept the fact that at some stage it may crack but it’s a trivial matter to cut and fit another. In order to get the offset right I then adjust the Z offset in the slicer settings in increments of 0.10 of a mm until it’s correct with fine adjustments (which need to happen after the 1st print of a series of back to back prints due to the thermal expansion) being taken care of by tweaking the z axis by hand until it looks right.

The second issue took a bit more time to resolve, i.e droopy prints. Firstly it looked like the temperature was too high for the PLA however lowering the temperature until the plastic was being extruded correctly left me sitting at about 150 degrees which aside from being way too low resulted in a lot of stress being put on the connection between the extruder body and the thermal barrier as the viscosity of the PLA changed. I then started looking at the filament itself. In the previous post I mentioned a colour change in the extruded filament and whilst this does happen just after you change colours I was getting it after printing meters and meters of the same colour with the black going grey for short (<10mm) sections. When I took the hot end apart I found a sticky oozy substance that had seeped up between the brass outer and the PTFE liner which was a similar colour to the odd grey sections of print. Considering how tightly these two fit together it was obvious that whatever it was had a vastly lower viscosity than normal PLA does when it is heated and extruded, almost like coloured, sticky water. I was able to clean it all off and then reassembled the extruder. I did a bit of research on the internet and discovered that PLA is actually hygroscopic (it absorbs water) now this is mentioned on the Reprap website but I hadn’t been experiencing any of the listed symptoms (puffs of steam, plastic continuing to be extruded after the extruder stops (in excess to normal ooze)).

As a trial I borrowed a dehumidifier and set it up directly under the filament reels so that it would blow the dried air directly on to them. After sealing the room and removing about 20L of water from the air over a 24hr period I had another go at printing. I knew that I would have to put a fair amount of filament through the machine to ensure that the problem was resolved but I’m please to say that after 6 months of printing under the low humidity conditions the problem has yet to reappear. Considering that it used to pop up after a couple of days of heavy printing and only go away after not printing for a couple of weeks I’m pretty happy to consider this issue addressed. Looking at the way in which the issue appeared and disappeared I have to conclude that the PLA was absorbing water which was then ending up trapped in the extruder nozzle, separated from the PLA and not seeping out gradually but rather coming out in large batches after building up for a while. Letting the printer sit for a couple of weeks then let the PLA in the nozzle reabsorb the water which was then extruded all at once (before it had time to separate) whilst printing the skirt for the first printed part when I started the printer up again.

I’m still having the occasional issue with the extruder body stretching where it connects to the thermal barrier but this only happens every 4-5 months and just means that I need to keep a couple of printed bodies as spares. When they do need replacing it only takes about 30 minutes to swap everything onto a new body and provides a good opportunity to readjust everything and clean it all out.



Leave a Reply

Please log in using one of these methods to post your comment: Logo

You are commenting using your account. Log Out /  Change )

Google+ photo

You are commenting using your Google+ account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )


Connecting to %s