Tuesday, 1 January 2013

The futures Borosilicate...perhaps

Bath University April 2012 - The calm before the storm!
Amongst many of the comments passed whilst building Huxley #710 at Bath University. One stuck in my mind made when watching the guys print a parliament of Owls. We had seen filament being wiped by a sponge before entering the printer. But I also noticed glass on the table of one of the printers rather than the more usual Kapton tape. Whilst discussing the pros and cons, out popped the comment "Adrian Bowyer only prints on glass now". At the time we were reviewing his machine which had, amongst other things an interesting X carriage.

Now one of the things that sticks in my mind from physics at school, from a superb physics teacher (Mr J Jarzecki), was that heat and glass don't mix! To the amazement of the class and without safety net (although I do recall safety glasses, he was very advanced!) he applied heat from a bunsen burner to the center of a piece of glass to show that the glass expanded where heated and not away from the heat i.e. unevenly creating stresses in the material. Sure enough it did! Splitting, with rather a loud crack and if I recall the glass fell off the metal tripod it was positioned on with a smack.

When living in France, many years later, in the summer of 2003 (El Niño year), I asked for some windows to be painted to block the heat from the sun. I had expected them to be painted white, to my amazement they were painted black! Sure enough they cracked, probably as the steel frames were restricting the expansion and putting the glass under strain. Mr Jarzecki was right!

You can therefore imagine why I opted to reserve judgement on the glass table, until I had the opportunity to ask a few more questions and do some research.

Unfortunately my research left me no clearer as to an answer, simple and straight forward. On the one hand, some blogs seemed to indicate problems with PLA on glass. Then others seemed to say it was the greatest thing since - well, sliced bread.


Here are a few:
http://hydraraptor.blogspot.co.uk/2010/05/pla-on-glass.html
forums.reprap.org/read.php?1,35071,49047#msg-49047
forums.reprap.org/read.php?1,33327,51041#msg-51041
http://www.emakershop.com/forum?vasthtmlaction=viewtopic&t=592.0
http://www.emakershop.com/forum?vasthtmlaction=viewtopic&t=48.0
https://groups.google.com/forum/#!topic/bay-area-reprap/or_mVVH-y9g

And I am sure there must be quite a few more read.

Adrian Bowyer - Printing on glass

After hunting around for some glass ideally very cheap, if not free. I found some picture frames in IKEA with glass. These are getting rarer now as they seem to be moving to plastic sheet. Whilst thinner than the glass recommended by Adrian Bowyer (see video), I thought they should do. Additionally they had nice rounded edges. The thought of the Y table slicing my fingers off and the blood mucking up a print was vivid in my mind. I also didn't want the boys to slice themselves up!

As you might have guessed, no matter what I tried, I could not get PLA to stick to the stuff. It was OK in the middle but would peel from the edges readily and did not seem to have the adhesion that printing on Kapton tape gave. I tried all sorts of temperatures up to 100C in 5C increments. Cleaning the glass with nail varnish (Tesco brand no fancy additives). Measuring the temperature with traceable measurement equipment to ensure the required temperature were being reached on the surface of the glass and so on. Mr Jarzecki would be proud of the scientific approach, but all to no avail. Every print curled off the table from the edges, although the surface was superb. So I used the glass as an interchangeable table upon which I applied the Kapton tape; which was significantly easier that trying to place it on the Aluminum print bed directly!

I had almost given up on totally smooth surfaces like Nophead's, not fooling with Kapton tape for an hour to get all the bubbles out and the tape aligned nicely so there are no big gaps!

In the depths of my research on the topic I did however remember reading about Borosilicate glass (aka Pyrex) in a few locations and this seemed like a reasonable idea as it should at least withstand pretty poor thermal coupling and thermal cycling likely to arise from localized hotspots on the table and frequent use. Additionally I had not used the thickness recommended by Adrian, so perhaps some room for improvement.

With this in mind, I purchased some Borosilicate glass which was only available in 3.3mm thickness but that's twice the thickness of the IKEA picture frame glass. It was expensive, at least the 140mm by 140mm sheet I purchased was. Additionally the shop told me "it's a devil to cut" and the glass specialist told met they can not round the edges. So 25 UKP later I have some Borosilicate glass set up on Huxley #710.

The performance to date has been superb!

Borosilicate Glass Table Recipe
Table surfaceBorosilicate glass 3.3 mm
Table temperature60C
MaterialPLA - Faberdashery Arctic White
Extrusion temperature196C
Huxley #710 printing PLA on Borosilicate glass

Borosilicate Glass Table Technique
Heat the table to 60C, ensure its the top of the glass table at this temperature. I have a 20C thermal gradient between the indicated table temperature and my thermocouple bead on the top of the glass. Allow at least 5 minutes to thermally soak. Print the part, if using a fan I start it after the first layer has completed as the table will cool a little with the fan on. I leave the table heated throughout the print.
At the end of the print allow table to cool with fan. The part always seems to comes off readily by hand once the table has dropped below about 40C. Frequently there is a little crack as the tension of the plastic relieves once the table releases the part. If your fan is too power full it could blow the part off the table!

For PLA at least, no scraping, aligning tape or bashing parts to get them off! The futures Borosilicate for Huxley #710!






Adrian Bowyer video location
http://reprap.org/wiki/Mendel_User_Manual:_Host_Software#Heated_beds

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