Sunday, 12 September 2010

Some corners like it hot

Large objects with sharp corners, such as the Mendel z-leadscrew-base, produce enough stress to form a blister in the PET tape on my heated bed. These can only be flattened again by pricking the tape. I can't understand how air gets in and cannot get out again, but that is what seems to happen.

The blisters leave a small indentation in the object's base. It is only an aesthetic problem because the base remains flat, i.e. it doesn't rock on a flat surface.

Sometimes the blister allows the corner to peel from the bed towards the end of a build, allowing the corner to curl upwards a little. Generally I can avoid that by cleaning the bed with acetone before problem builds. I also use hexagonal infill on those parts and only two solid layers rather than three in an attempt to reduce the stress. When I design my own parts I round the corners, where possible, to prevent such problems.

A solution may be to use a sheet of PET rather than PET tape, but then you need to find a way of holding it down. One thing I have noticed though is that when I build a bed with four of the z-brackets closely packed the corners on the inside don't blister or lift. That must be because the air around them is hotter. As an experiment I added some little plastic walls to the build to act as baffles to keep the heat in as the bed moves through cooler air.


These have a 5mm thick base to help keep the tape flat and are 1mm away from the edge of the object. They work well and stop the blisters forming at the corners. They are very similar to Forrest's apron technique but their primary function is thermal rather than mechanical. A more general technique would be to build a thin wall all the way around the perimeter of the objects to cocoon them. I expect that would only need to be one filament thick and perhaps might give a similar effect to having a heated build chamber.

17 comments:

  1. thats what i was thinking just a thin wall on the out side to hold in the heat better, i think it would only need to be 1 line thick but i think it might help with some abjects.

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  2. Wonder if the reducton in warping would be enough to allow builds on a continuous conveyor. There is currently an issue with usign such a technique due to the warping lifting the bed/belt.

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  3. Maybe tiny little bubbles of air are getting in when the PET tap is first put onto the bed, and with excess heat they grow? What about after applying fresh PET tape, could you use a book press to push it really hard onto the bed and expel the bubbles?

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  4. I make sure I remove any visible air bubbles when I put the tape on and these blisters only form under corners of big objects. They don't reduce significantly when the bed cools.

    I doubt the tape is gas permeable though, so the only explanation I can come up with is that the silicone adhesive layer traps microscopic bubbles of air and these congregate at the stress points due to the vacuum produced by lifting the tape.

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  5. I was thinking of bubbles on the microscopic scale. Then again if they are that small possibly no amount of pressure from a book press will get rid of them.

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  6. I have the exact same problem on IceBot One. I use a 10mm thick heated aluminum build base with Kapton tape and on large parts the corners curl up and form bubbles too. I use a hobby knife and a putty knife to get rid of them but would love a different way. I'll have to try your technique.

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  7. What we need is a metal sheet with a PET coating, rather than relying on silicone adhesive which is inherently stretchy.

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  8. Is this the stuff? http://tinyurl.com/2796ey7

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  9. Could be. Also Dibond FR has "modified polyester lacquer system", so perhaps ABS will stick to the surface of that.

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  10. Nophead, why do you think the tap isn't gas permeable at elevated temperatures? It's my understanding that it is much rarer and more expensive to get things that are not gas permeable at elevated temps than the other way around.

    Also, the protective layer around objects is something I've seen commercial machines do even with a heated build chamber...specifically the Dimension machines do this.

    Demented

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  11. Hi Demented,
    I assumed PET was not very permeable at room temp because it is used to make fizzy drink containers. I hadn't realised plastic gets more porous when hot.

    Also I didn't realise commercial machines put a cocoon around the object. I haven't see that, but I only have a few videos to go by. Thanks.

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  12. Everything is permeable to gasses, how permeable depends on the pressure gradient.
    I worked with ultrahigh vacuum machines (10^-11 bar). At that low pressures gas molecules can even go trough the massive steel wall of the vacuum chamber (+_1 cm thick)

    As the plastic cools down, it shrinks and pulls the plastic, causing the pressure beneath the PET tape to be very low. I don't know how low the pressure becomes, but since the PET tape is very thin, I think that gas molecules could get through the tape, causing the bubbles.

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  13. A lot of the machines that need to have a lye bath after printing to remove the support material put cocooning around the printed object. I talked to a service tech about it and he said it was to stabilize temp gradiants across the object to reduce warpage and uneven cooling. Makes sense.

    Demented

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  14. Could it be that the solvent component of the tape's adhesive is volatile at elevated temperature?

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  15. Maybe, but I am not sure if silicone adhesive has a solvent. You don't get bubbles forming where there isn't a lot of tension.

    One thing I have found is that once the tape has blistered it doesn't stick down as well and will tend to blister in the same place, even with the shields. So after adding the shields I have to move the object to a new position on the bed to get a complete cure.

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  16. This post was pure gold. I'm running a Makerbot with polyimide tape as the bed surface. I am now printing flat and raftless for all mendel parts. I do get bubbles, but they always occur near the edges of the tape, where it meets the next bit of tape.

    One thought I have yet to try. Lazer copier transparency sheets are usually made of PET (I think) and one coule stick a complete sheet down using adheasive transfer tape. I'm going to order some tape and give it a try.

    Also, I have put a skirt on my printer. It traps heat that is rising from the heated bed, and has made all the difference in flatness of my prints. This means that I dont have to have all my motors in a heated environment, so they cool a bit more effectively

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