print not sticking to build plate

Various topics related to Vat design, construction and coatings.
sitzme
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print not sticking to build plate

Postby sitzme » Thu Dec 05, 2013 1:52 am

I am having some difficulty with the print sticking to the Sylgard on the bottom of the vat instead of the build plate. The build plate is ground aluminum and was cleaned and deburred but no other treatment. This vat and build plate is large by current standards, 10 x 17 if I recall. The test print is a badge or coaster 30 layers thick and about a 3 x 5 oval. The first 15 layers are solid and then the image built on. The Sylgard cured for a couple weeks before use and it was heat cured during application. I stored the vat upside down so no cleaning was done before printing except blowing the dust out. The build plate is suspended on three studs so the initial layer thickness is set by homing z (down on this machine) with a sheet of paper laying on the vat bottom. The nuts are loose before starting and tightened after reaching bottom. Exposure times are long, 50 sec for base layers and 26 for the rest but the projector is 21" away and the image is scaled so one pixel = .01". I also have a smaller vat and have made a good gear print so the exposure settings should be OK I think. The build plate for the smaller vat is glass.

This printer uses an X axis for the sliding motion. My plan is to make the first move G1X7Z1F20 to break it loose and then return X while lifting Z. On the one print that would have worked tonight it sounded and felt good. Unfortunately I thought is had come loose and stopped the print. At F200 it sounded more like it was breaking and it would shake the machine when it came loose.

Should I be rinsing the vat with water to get the Sylgard to absorb some? Does the build plate need to be roughened? Longer exposure on base layers?

I did calculate the exposure time based on known good times from other users. This was followed by exposing a drop a piece of glass at the calculated time and it was pretty close.

One other thing is that the temperature was 60F but based on the random skin generated when the projector got turned off late, it doesn't appear to be an issue.

johnrpm
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Re: print not sticking to build plate

Postby johnrpm » Thu Dec 05, 2013 4:05 am

No sure if I can halp much, as I have not yet finished my build, sounds like you have
a large area to break free, would breaking up into segments help, to reduce contact area??,
any chance of some pictures,
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PacManFan
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Re: print not sticking to build plate

Postby PacManFan » Thu Dec 05, 2013 10:35 am

I ran into a lot of problems with prints not sticking to my build plates as well. I still suffer from it occasionally. I'm using a flat aluminum build plate, and my vat is coated with FEP sheet from CS Hyde. Having the build plate and vat bottom parallel to each other is critical for a successful build. When I start a build my build plate is essentially touching the bottom of the vat. I use a longer exposure time for the first 3 layers (12 seconds) and regular layer exposure time is 3 seconds. Every once in a while, I'll have the model come off the build plate halfway through a print. I'm considering roughing up my aluminum surface with rough sandpaper to increase the surface area.
I don't remove either my build plate or my vat, so I don't have to worry about constantly re-aligning them. I use a calibrated Z-Limit switch to know when the build plate is touching the bottom of the vat.

johnrpm
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Re: print not sticking to build plate

Postby johnrpm » Thu Dec 05, 2013 4:56 pm

would you say that the bond between part and vat base is chemical, vacuum, both.
is it worse for larger area parts, and less of a problem for low area parts, such as jewelery
for example.
For bonding between platform and part, is surface texture important?, material type or coating, I have seen perforated sheet used, and glass, also do different resins need different platforms, lots of questions.
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sitzme
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Re: print not sticking to build plate

Postby sitzme » Thu Dec 05, 2013 11:13 pm

On my first prints I was using a glass build plate. I had trouble until I put a brim on the part on try 3 or 4. The part was an extruder gear so was small. Removing it from the build plate was easy to do by sliding it. It broke free after a half inch to an inch. Then I read that many preferred aluminum so that was used for the big build plate. Some of the references talked about drilling a hole pattern to assist bonding to the build plate. I can't find those discussions again yet. I thought about cutting a pattern of lines or cross hatching but am trying a version scored with a utility knife first.

sitzme
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Re: print not sticking to build plate

Postby sitzme » Thu Dec 05, 2013 11:17 pm

Would it be a good rule of thumb that the first x layers need to have an area greater than any other layers? Addition of a brim or other features could provide this area.

johnrpm
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Re: print not sticking to build plate

Postby johnrpm » Fri Dec 06, 2013 7:11 am

I gave a lot of thought about weather to do a top down, or bottom up machine,
the advantages with bottom up are,

you don't need a vat full of resin.
you don't need a recoater to smooth miniscus.
easy to change resins.

but the problems mentioned above appear with bottom up

adhesion to platform base.
expensive vat coating.
part breaking away from platform.

please add to this list if you can think of anything.
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johnrpm
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Re: print not sticking to build plate

Postby johnrpm » Fri Dec 06, 2013 7:21 am

Would it be a good rule of thumb that the first x layers need to have an area greater than any other layers? Addition of a brim or other features could provide this area.

Like a thin flat plate, then supports, then part???.

if we have a base stl file that is placed under the part, with supports between?.
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PacManFan
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Re: print not sticking to build plate

Postby PacManFan » Fri Dec 06, 2013 12:13 pm

As far as top-down vs bottom-up goes: Several groups have recently been using the top down method, floating a layer of resin on top of a heavier liquid such as saline. This seems to be the best of both worlds, because you no longer have to fill the entire vat up with the more expensive resin. My next printer is going to be a top-down machine with resin floating on saline.

johnrpm
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Re: print not sticking to build plate

Postby johnrpm » Fri Dec 06, 2013 1:04 pm

I remember reading somewhere that the saline effects the resin after a time, turns it to mush
after a week or so, if this is so, it would mean emptying the vat if its not in use, but that has not been verified.
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