Friday, December 16, 2011

Solving a nagging question about print adhesion

Unlike most of you, I don't use an electrically heated print surface.  Some time ago I bought a Rapman 3.1, which used an acrylic 3 mm print table.  I soon discovered that 3 mm was far too thin and quickly warped beyond use.  Switching to 10 mm solved that problem.

After a long time of successful prints, I noticed that with winter causing colder temperatures in the print room I was having more and more trouble getting my prints to stick to the acrylic.  I tried cleaning it and sanding it with little avail.  Electrically heated print tables were just coming available but insofar as printing was concerned, I thought that things were already complicated enough without adding that sort of equipment to my Rapman.

I had an IR heat lamp in the lab, detritus of another experiment, and discovered that using it on a tripod to raise the temperature of the acrylic print table above 40 degrees Celsius measured with a non-contact IR thermometer gave me consistent adhesion.  I soon discovered that I could turn off the IR lamp after 4-5 print layers with no ill effects.  It was not needed for the rest of the print.

The rig looked a bit like this...

Note that the lamp is placed at a 45 degree angle to the acrylic print table.

I soon noticed that adhesion at the near side of the print table was much less firm than that at the back and less firm at the left side than the right.  I attributed this to various things, uneven heating being one of the possibilities.  While the left/right difference made sense the front/back difference didn't seeing as the IR lamp was aligned with the left/right axis.

Cranking the terminal heating temperature before starting a print to about 50 degrees solved most of the problem for the center of the table and I was able to print along the front/back axis with reliable success.  Unfortunately, the back side of the print area seemed to have the print pad melting into the acrylic while the front side would separate easily.

It made no sense.  I thought for a while that it had something to do with the acrylic plate and rotated it with no effect.  Swapping ends and sides always left the back side of the print table very firmly attached to the print pad.  While that wasn't a horrible situation it was annoying, because it meant that processing the printed objects after separation became more time consuming.

A few weeks ago, I purchased a FLIR E30 thermal imaging camera with the intention of learning more about what was happening with prints as they were being laid down, the ultimate goal being building in advanced heuristics into my Slice and Dice app which converts STL files into Gcode.  I also had hopes about eventually doing some research into what actually happens thermally with extruder hot ends with the notion that I might be able to design a better one.

Yesterday, the E30 arrived and I decided that a good beginning exercise might be to look at the distribution of heat on my acrylic print table when I used the IR lamp in its standard configuration to heat it.  The results were quite unexpected.


The lamp put down a marked hot spot at the upper right rather than at the right as I expected.  The upper right was exactly where I had the most trouble with print pad melting.  Obviously, the IR lamp did not give even heating when tilted but overheated in on the upper right.

This was nasty.  I had previously thought about using several smaller IR lamps at the corners of the Sampo printer that I have been developing.  If the smaller lamps behaved like my single, large one, however, this might not be a good idea at all.

I then got to thinking about how IR lamps are actually used in food heating cabinets.  They are almost always placed point straight down.  I rearranged my tripod to place the lamp almost vertically over the acrylic print table.

That sorted out the temperature distribution problem...