Keeping PLA stuck to the bed

posted 19 Oct 2014, 05:17 by Colin Bell   [ updated 8 Nov 2014, 11:51 ]

The Huxley has a heated aluminium bed, which when covered in Kapton tape provides a very robust, flat and sticky surface for plastic to adhere to and I only had a minimum of warping with both ABS and PLA. By contrast, the PrintrBot Simple Metal does not come as standard with a heated bed, which means I need to get creative with my bed preparation in order to achieve successful prints. 

PB themselves recommend using blue painters tape which does work, however I got a lot of edge curling and warping even with the bed perfectly level, Z height optimised and auto-levelling enabled. 

As mentioned previously, I'd had a lot of success in the past using Kapton tape and a heated bed on the Huxley, although I did still get some edge curling even with that.

I've got a couple of PCB heaters which I'm going to modify for use with the PrintrBot, however until that happens, I need a solution for cold bed printing, especially with large objects. I'm not a big fan of rafts and brims, even though they will help of course - mainly this is because I want to do the minimum of finishing on most objects.

The first thing I tried was the blue painters tape I'd already bought (3M Scotch 2080EL) and covered the print area in a light coat of Pritt Stick Power glue - this worked very well on smaller objects and kept them stuck down nicely during prints. Larger prints still had a bit of curl on occasion, but this was mainly down to the painters tape itself lifting from the bed. Removal of finished objects was very easy - just lift the object with the tape attached to the bottom and run it under warm water to remove the tape.

I next tried just Kapton tape - this stays stuck well on the bed, but the PLA lifted from the cold bed a bit to easily, resulting in a couple of spoiled prints that got knocked off midway through the job.

So...logically the next step is a layer of my Pritt Power stick on the Kapton tape. Mission accomplished! It's absolutely rock solid - prints don't move, warp or curl - not even a little bit! The issue of course, is getting the bloody thing off the bed once it's finished.

Seriously, I never expected it to work quite this well, even on huge prints that use the entire bed area - I lost a couple of absolutely superb prints just trying to pry them loose - in a couple of cases the print de-laminated from it's bottom shell, and on one occasion I actually broke the damn thing in half...I was so happy after a 15 hour print as you can imagine!

So the solution to this is just to understand that your Kapton tape is sacrificial, which given it's price is a shame. However for me, this is worth the cost just to get the perfect print I want. Basically, in order to remove the object from the Pritt covered Kapton I either peel up the tape surrounding the object and gently tease the tape from the bed, or by carefully working my way around the edges of the model with a hobby knife, and can usually get it free enough to pop a larger putty knife under the model and pry it free. Once the object is off, the tape can be carefully peeled back without de-laminating. 

I have also tried hair spray on Scotch Painters tape without much success - I got the extra hold type, but even an extremely light cover just saturated the Scotch and made it literally fall off the bed. I haven't tried it on Kapton yet, but will at some point. I also picked up some low tack PVA glue to try as well.

So my pre-print checklist now includes the following items:

  1. Remove any tape that's previously been printed on (I move objects around on the bed in Slicing to maximise use of the bed material)
  2. Clean the bed with IPA Solvent
  3. Put the Kapton on
  4. Clean the tape with IPA Solvent to remove my fingerprints
  5. Slice the model and ensure that a skirt will be produced prior to the print
  6. Start the print and observe where the skirt outline is - kill the print
  7. Put a *THIN* layer of Pritt glue within the skirt boundaries so that the print has a sticky base
  8. Start the print for real
I've also found that letting the print cool for a good 20 minutes before removing it helps.