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theories on sticking/not sticking?

Posted by Buback 
theories on sticking/not sticking?
September 23, 2010 11:27PM
What forces make an extruded plastic stick to a build surface?

I'm guessing it's different factors for different plastics, but it would be good to spell out, for each plastic, what makes it stick to the best current build surface. I'm sure there are polymer experts in the forums that can answer the question.
Re: theories on sticking/not sticking?
September 24, 2010 04:42AM
When the surface is smooth, like Kapton film, then I think it is mainly van de Waals forces. If it forms a good seal with the bed then you get atmospheric pressure holding it down as well.

When the surface is rough, like painters tape, then I think there is some mechanical grip.

When the surface is something that melts then it forms a diffusion weld, for example when extruding HDPE onto a PE bed.

I think PLA sticks to more things because its monomer is an acid.


[www.hydraraptor.blogspot.com]
VDX
Re: theories on sticking/not sticking?
September 24, 2010 04:55AM
... Van-der-Waals- or electrostatic adhesion and mechanical 'grip' should be dependant on the molecular metrics too - smaller or longer molecules should stick better.

Another strong bonding parameter is the polarity/nonpolarity of the materials ...


Viktor
--------
Aufruf zum Projekt "Müll-freie Meere" - [reprap.org]
Call for the project "garbage-free seas" - [reprap.org]
Re: theories on sticking/not sticking?
September 27, 2010 12:29PM
Why does the plastic stick to a heated bed and not at cold bed? (like nophead's experiments with glass beds)

Another critical property is ease of release.

Has anybody tried a coated bed? I'm envisioning a spray on coating that adheres to the bed and plastic (perhaps with different forces), forming a strong, thin layer between the two, but can then be washed away to remove the parts.
Re: theories on sticking/not sticking?
September 27, 2010 01:21PM
For things like ABS on Kapton and PET, or PLA on glass you need the bed temperature to be higher than the melting point of the plastic when extruding the first layer, it can be less afterwards. I presume because if it is less than the melting point it freezes the filament instantly, before its molecules have had time to fuse with the bed surface. For things to bond the molecules need to come in close contact. That is hard to achieve when the material is solid.

I think I have heard of people spraying the bed with adhesive. A lot of adhesives don't work though because they melt when the hot plastic lands on them. If double sided carpet tape works, which I have read several times, then whatever adhesive that uses should work without the tape.

When I was extruding PLA onto glass I used isopropanol and a J-cloth to remove fingerprints. But that had the unexpected result of not only making it stick better when hot, but also they came off much easier when cold. When I looked carefully I realised the J-cloth had left a smear. Presumable something in it was dissolved by the alcohol and that acted as a release agent.


[www.hydraraptor.blogspot.com]
Re: theories on sticking/not sticking?
November 04, 2010 09:15AM
Just to add a little more to what nopehead was saying about the heated bed and adhesion; the hotter the plastic is when it comes into contact with the print bed, the lower its surface tension. This allows it to settle into more of the microscopic pores and cracks in the print bed surface. This is further helped if the print bed is hot as it allows more time for it to 'seep' in. This can dramatically increase the surface area in contact and so increases the number of secondary (van der walls) bonds formed.

From what I can gather with these very none polar plastics its all about maximizing the surface area in contact at the microscopic level. So on the one hand you can try and roughen the surface by sanding to increase the exposed surface area. However, due to the high surface tension of the molten plastic this can work against you as the plastic will not 'wet' the surface. In other words, at the microscopic level its a bit like trying to spread honey over the bristles of a stiff hair brush. You would have a lot of contact area area if every bristle was evenly covered, but in reality the honey (plastic) doesnt get past the surface. I hope that strange analogy makes sense eye rolling smiley

So if your like most people and printing onto a mental surface your best bet might be to find a way of quantifying the strength of adhesion and then trying different grades of sand paper to find the best one.

From there you can go into more depth on adhesion such as looking at the surface energy (think surface tension of a solid) of the print bed material and ways to treat it to create stronger types of bonds, such as silane treatments for glass. But just trying to increase the surface area in contact is probably a good start.

If you are interested in some more on the basics of adhesion I found some past lectures on the topic. You can find them here.

Password for the zip is 'mendel'.

pm me if the link becomes dead.

Edited 1 time(s). Last edit at 11/04/2010 09:17AM by Capo.


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capolight.wordpress.com
Re: theories on sticking/not sticking?
November 04, 2010 10:50PM
So for a rough surface, you need an intermediary that fills the cracks on one side, and bonds to the plastic on the other.
or fills the crack on one side and creates more compatible cracks on the other.

that's currently blue tape for PLA and kapton tape for ABS

I'm guessing that glass and kapton, even though they have less surface area than a rougher surface, have more surface contact, since the plastic isn't just touching the 'mountaintops' or bristle ends, so to speak.

But glass can be a problem when it isn't heated because the plastic cools off before it can spread out and form a bond. so it's also thermal properties of the bed that make a difference.
Re: theories on sticking/not sticking?
November 05, 2010 06:04AM
Spot on in all regards. I have always had it in my mind that kapton or blue painters tape is just a temporary (yet very effective) fix to the problem of printer bed adhesion. What's obviously needed is a surfaces thats as durable as glass but has the same adhesion effects as kapton.

So if it was me I would be starting with durable materials that have a high surface energy such as ceramics, anodized aluminium ect. Then, using a whole range of different sand paper grits, print bed temperatures or even acid etching times to test which combination provides the best result. But before you could do any of that you would need a way of consistently quantifying the force required to remove a printed object form the print bed. Then you can assign some some numerical values to your results so you know if your making any improvements.
Re: theories on sticking/not sticking?
November 05, 2010 03:10PM
I've always wondered "why blue tape?" isn't it just masking tape that's colored blue so that it's easy to see if you didn't line it up properly when painting around windows and doors?

Has anybody tried just masking tape? it's slightly cheaper (~$1)

--
Unfortunately there are a number of equally important considerations, such as cost, flatness. it's a juggling act.

we need some grad students to do this research.
Re: theories on sticking/not sticking?
November 05, 2010 07:53PM
Masking tape/blue painters tape is essentially just paper with an adhesive backing. Paper is made of compressed cellulose which is a natural polymer (a polysaccharide) which forms long chains like a plastics, with each chain attracted to the next by hydrogen bonding. If you have ever seen a picture of paper under a microscope it has a very rough texture giving it a very large surface area. This in combination with possible hydrogen bonding and London dispersion forces between the paper and an extruded plastic maybe what forms the strong bond. I'm guessing though that because these bonds are so good that every time you remove a printed object you take a few layers of the most protruding cellulose with it and so slowly reduce its surface area with time.

I'm not a chemist though, so this is all a stab in the dark really.

If your looking to do some quick testing with different materials you could try this: Take your selection of different preprepared test materials and use a paper mask to isolate an exact surface area, say 1cm^2. Then using a hot glue gun cover the exposed area and include a short length of string in the dollop. Then using an upright fish scale measure the force required to pull off the glue. You wouldn't have to worry about getting any glue on the paper mask as it wouldnt be attached to the surface and so should not influence the results. From this you could then narrow down your selections for testing on a reprap.
Re: theories on sticking/not sticking?
December 29, 2010 01:58PM
Ok so why are many reporting succes with protoboards/perfboards? Do those offer so much adhesion that heated beds are simply not necessary?
Re: theories on sticking/not sticking?
December 29, 2010 02:35PM
Who, where? I have never heard of anybody using perfboard.

In general, without a heated bed you need something that the plastic will form a weak weld with to hold it flat against the warping force. That only works up to a certain size of object and anything but the smallest objects need a raft.

With a heated bed you reduce the warping force massively, so you can use air pressure and van der Waals force to hold objects flat. Again that is only up to a certain size and then you need a heated chamber. I can make ABS objects the full size of my bed with a chamber at 45C but to make some shapes a chamber at 70C is probably needed. At that point you can no longer make the machine from ABS so it isn't self replicating.


[www.hydraraptor.blogspot.com]
I wasn't really sure where to put this, but since you guys are talking about the same thing it seems appropriate. For anyone who has an ABP and can't get the ABS to stick to it.
[printthat.wordpress.com]

Summary: The belts you tape together yourself have two surfaces, one
ABS will stick to rather well and one it won't stick to if you dangle
its children over a pool of croca-sharks. The way to tell which side
is good for printing is to mark the belt with a sharpie. The side ABS
won't stick to will also try to reject the ink. The other side, the
one that you can write on perfectly fine, is the one you want to print
on.

It's possible there is something going on there that could help you figure out a grand unification theory of build surfaces. At the moment, my theory is that part of the boPET (or just PET maybe) manufacturing process is to roll it really thin. The reason aluminum (or a-lu-minny-umm for you Brits) foil has a matte side and a gloss side is that two sheets are rolled together. The side against the roller gets smooth and the side against the other piece of aluminum gets what I assume are microscopic imperfections in it. If the plastic sheets Makerbot handed out for belts are made the same way then it could explain why one side is great for printing and the other is worthless.

Anyway, you should be able to test this really quickly. Just mark your
build surface with a sharpie and respond with:
1) have you been having adhesion problems?
2) does the top surface of your belt try to reject the ink?
3) or the other way around
Re: theories on sticking/not sticking?
January 06, 2011 03:01PM
Here: [www.thingiverse.com]

Perfboard was tried on a whim but it's apparently a huge success.
Re: theories on sticking/not sticking?
January 12, 2011 10:55PM
I think the 3M blue painters tape has a plastic coating. Not sure what kind it is but it's probably compatible with PLA but not ABS. From the wiki, PLA is actually a poly-ester even though it's made from polymerizing lactic acid. smiling smiley
raj
Re: theories on sticking/not sticking?
February 07, 2011 08:00AM
i am trying to stick polyester film ( 40 dyne) coated with adhesive ( vinyl base ) to a paper board what is the perfect surface tension to get fiber tear in the board.i do not know the surface tension of the paper board.
Re: theories on sticking/not sticking?
February 25, 2012 11:46PM
brnrd Wrote:
-------------------------------------------------------
> I think the 3M blue painters tape has a plastic
> coating. Not sure what kind it is but it's
> probably compatible with PLA but not ABS. From the
> wiki, PLA is actually a poly-ester even though
> it's made from polymerizing lactic acid. smiling smiley

This is the first time I've seen this written about 3M tape, but I agree entirely. I had been using cheap dollar store blue tape which held ABS beautifully, but using a nice roll of wide 3M and the ABS seemed like it was repelled! It wouldn't stick worth jack!

Now I find my PLA won't stick to the cheap tape, but I never tried the 3M again as I built a heated bed instead. But I'm intrigued - will the same coating that repelled ABS give me that sweet PLA adhesion everyone is talking about? Tune in again tomorrow to find out.

Thanks brnrd for the explanation!
Re: theories on sticking/not sticking?
March 20, 2012 07:12PM
Buback Wrote:
-------------------------------------------------------
> So for a rough surface, you need an intermediary
> that fills the cracks on one side, and bonds to
> the plastic on the other.


DING!

This is why abs/acetone solution on frosted/sandblasted glass works so well. The dissolved ABS bonds into all the nooks and crannies of the glass, and then when ABS is printed on it it has something perfect to stick to.


www.Fablicator.com
Re: theories on sticking/not sticking?
June 11, 2012 07:35PM
Buback Wrote:
-------------------------------------------------------
> Why does the plastic stick to a heated bed and not
> at cold bed? (like nophead's experiments with
> glass beds)
>
> Another critical property is ease of release.
>
> Has anybody tried a coated bed? I'm envisioning a
> spray on coating that adheres to the bed and
> plastic (perhaps with different forces), forming a
> strong, thin layer between the two, but can then
> be washed away to remove the parts.

It couldn't be washed away, but I used Scotch Super 77 spray glue to hold parts down to blue tape before I switched to a heated surface. It worked a bit too well.

Perhaps a solution of PVA in alcohol would work?
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