Wednesday, November 21, 2018

Interesting 3D Printer Failures

I recently experienced a couple failures and almost failures that might be interesting to people who build 3D printers.

The first one was discovered when I started to rebuild the Y axis in Son of MegaMax (blog post will be made when the work is done), my bed flinger printer that lives at the Milwaukee Makerspace.  I took the bed plate off because I was going to make a new bed plate and convert the ball screw drive to belt drive.

Here's the bed plate about a year after it was put into use on the machine.  It has a self-adhesive 450W kapton heater.  I don't know what type of adhesive it had on it.

And this is what it looks like today, after 5 or 6 years of temperature cycling:

Notice the brown spots- there are air bubbles that formed between the heater and the bed plate under them.  Air is a great thermal insulator, so the aluminum bed can't take the heat away from the heater where there is a bubble and the result is hot spots.  It's probably safe to assume that almost any self-adhesive material is going to eventually let go this way, and the heater will eventually burn itself up.

The Keenovo silicone/fiberglass heaters, and probably a lot of others, come with 3M 468MP adhesive transfer sheet on them.  UMMD has had such a heater on it for a little over 2 years.  I recently noticed that the heater was starting to peel off the underside of the print bed.  Right now I have a piece of silicone foam wedged under it to keep the heater pressed against the bed plate, but sooner or later (probably sooner) the rest of it is going to start peeling off.  This is why it's a good idea to mount TCOs on the heater, as I should have done, instead of on the bed plate, as I did.  If the heater comes off the plate, having the TCO on the plate won't keep the heater from burning itself up, and maybe other things too.

Silicone foam used to keep the heater pressed against the bed when the adhesive started letting go, a little over 2 years after it was installed.

Finally, again on UMMD, the PEI print surface was mounted on the bed using 2" wide tape labeled 3M 200MP.  The standard stuff people use these days is 3M 468MP adhesive transfer tape, and if you look closely at the label on it, it says "200MP Adhesive" on it, so the two seem to be the same thing.

468MP adhesive transfer sheet commonly used to hold PEI and heaters on aluminum bed plates.

The 468MP transfer sheet that uses 200MP adhesive was letting go of the heater on the underside of UMMD's bed plate.  Then another odd thing happened just a couple days ago.  I started a largish ABS print on UMMD and went away and after an hour or so, the print failed.  It looked strange.  Like the edges of the print lifted, but closer inspection revealed that the PEI lifted up off the bed plate- the ABS was still stuck to the PEI.

Here's the print that failed.  Look at the edges of the bed plate and PEI...
Here it is from a lower angle.  The tape that was used to stick the PEI to the plate remains stuck to the plate, and is no longer sticky on the top side.  
The central area of the PEI was still stuck to the bed plate.  I removed the print from the PEI and found that the PEI laid back down flat on the bed surface and you'd never know anything was wrong just by looking at it. 

A couple years ago, when I noticed the PEI starting to lift at the edges of the bed on SoM, I removed it and retaped it to the bed plate.  One of several things that recently prompted me to start a rework of SoM's Y axis was that the PEI was starting to lift at the edges for the second time in about 5 years. 

I think there are a few things to learn from this: 

  1. 468MP adhesive transfer tape using 200MP adhesive has a limited life span when it is heat cycled regularly.  I've been getting about 2 years of use, but I print a lot of ABS.  If you print PLA or other lower temperature materials, you might get more than 2 years from it.  
  2. It's a good idea to inspect the bed frequently and pay special attention to the heater to avoid disasters.  When you're inspecting it, try lifting the edges of the PEI and the heater away from the plate to verify that the adhesive is still working.
  3. When you're assembling parts with 468MP/200MP, follow directions on 3M's web site to get maximum bond strength and lifetime.

Saturday, November 3, 2018

An Update to the Sand Table Mechanism

After the MakerFaire I looked at the sand table mechanism and found one problem.  The magnet carriage was tilted badly because the spring under the magnet was putting downward force on one side of the carriage.  The magnet carriage, made entirely of printed ABS, was wearing out quickly from sliding on the 16 mm square, powder coated X axis tube.  Apparently, when ABS slides on powder coat, ABS loses.

I decided I needed to do something about it or the whole thing would not be very reliable.  The UHMW bearings on the Y axis are holding up well, so I decided that I need to figure out a way to put UHMW bearings on the X axis, too.

After much thought and many CAD models, I came up with what seems to be a pretty good idea.  I used a piece of 1" square aluminum tube that fits over the 16 mm square X axis tube.  I added UHMW bearings to fill up the space between the two.  I milled some slots in the tube to allow easy attachment of the belts.  The belts pull on the aluminum, not printed plastic, so there are no worries about plastic breaking due to the stress.

The new magnet carriage design.  Belts will attach at the vertical slots in the aluminum body of the carriage, and the magnet holder will attach using zip-ties.
It turns out the belt attachments are almost exactly where they need to be to keep the belts parallel to the X axis guide tube.

Making the UHMW bearings was the hardest part of all this.  UHMW is a little tricky to machine because it's very soft.  When I first tried to fit it all together, I couldn't get the X axis guide tube to fit inside the magnet carriage tube with the bearings in place.  A few minutes with some very coarse sand paper fixed that.

Here's the first test video with the new magnet carriage in place:

New magnet carriage design for the sand table. from Mark Rehorst on Vimeo.

I intend to sand-blast the powder coating off the X axis guide tube which should reduce the friction.

Here's the new carriage with the magnet and spring in place.  When the sand box is on top of the mechanism, only a few mm of the magnet protrudes above the blue magnet box.  The white bits at the center are the UHMW bearings.

The UHMW bearings ride on all four sides of the X axis guide tube.  The trick to making them was to get the thickness right- too thin and the fit will be sloppy, too tight and friction will be too high.  

The spring fits in a circular hole in the magnet box, and the magnet is free to move up and down to follow the bottom surface of the sandbox.

Future updates may include switching to a Duet controller board and NEMA-17 motors to try to quiet things down a bit.  I think it will be a lot quieter if I run the motors at a lower speed, too, but then it won't be nearly as fun to watch it work.