PDMS - Cliff vs Waterfall

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mikej
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PDMS - Cliff vs Waterfall

Post#1 » Fri Apr 25, 2014 2:30 pm

The new vat will include a "Dam" that will allow us to pour a flat coating of PDMS without any slope and provide for a clean "cliff" separation. The waterfall method works, but applies some downward forces that try to suck the new layer over the waterfall and that can lead to part failure, most notably it increases the likelihood of ulcers in thin vertical walls.

The problem with simply using a flat vertical (removable) dam has always been the meniscus. As you can see in the image below, the dam design includes a "meniscus trap" that makes it easy to trim it off after the pdms has cured (and before the dam is removed).

Image




Steps to recoat:
1. Clean the vat and carefully remove the old coating.
2. Place the Dam about 5 mm from the edge of the glass (see the image above).
3. Mix up ~50 grams of Sylgard 184 (PDMS)
4. Set the vat on a level surface and pour the PDMS over the glass. The air bubbles in the PDMS will pop before it cures, do not be concerned about them.
5. It takes about 48 hours for the PDMS to cure at room temp. Or you can use a hot box at about 160 degrees F for a couple of hours.
6. After the cure is finished, carefully cut the pdms along the dam (see image above) and remove the dam along with the meniscus portion of the pdms.

The vat is ready to go again.
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akgold
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PDMS - Cliff vs Waterfall

Post#2 » Fri Apr 25, 2014 4:02 pm

I have always wondered about this. I assumed the glass was tilted ever so slightly.
Can we use a removable damn on our current vats?
How will this affect the flow back to cover the glass?
It looks like we would have to increase the depth of the resin
to allow it to flow back
I'm so far behind I think I'm First !!

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mikej
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PDMS - Cliff vs Waterfall

Post#3 » Fri Apr 25, 2014 4:09 pm

The removable dam idea would work on existing acrylic vats, but the pdms peels off very easily. I've been doing a second pour of pdms on the deep end to flow up against the first pour to "seal" it. (Using just enough to barely cover the floor.)
The resin level should always be deeper than the pdms level for quicker coverage. The new vat will hold more resin (and therefore requires more to reach the optimum fill level).
Measure Twice, Cut Once.

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Robert Howle
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PDMS - Cliff vs Waterfall

Post#4 » Fri Apr 25, 2014 4:48 pm

Will our new anodized Kirk vats work in the new HD machine?
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mikej
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PDMS - Cliff vs Waterfall

Post#5 » Fri Apr 25, 2014 6:16 pm

Yes, any vat that was designed for v1.1 should work in v1.2. The only issue is the v1.2 sweeper is wider, so you'll need a v1.1 sweeper to use the older style vats in v1.2. I plan to stock the v1.1 sweepers.
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benp
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PDMS - Cliff vs Waterfall

Post#6 » Fri Apr 25, 2014 7:10 pm

When will the new vats be available?

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mikej
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PDMS - Cliff vs Waterfall

Post#7 » Fri Apr 25, 2014 8:25 pm

I'd guess about 3-4 weeks.
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tim.yoshi
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PDMS - Cliff vs Waterfall

Post#8 » Sat Apr 26, 2014 3:34 am

Quote from mikej on April 25, 2014, 22:09
The removable dam idea would work on existing acrylic vats, but the pdms peels off very easily. I've been doing a second pour of pdms on the deep end to flow up against the first pour to "seal" it. (Using just enough to barely cover the floor.)
The resin level should always be deeper than the pdms level for quicker coverage. The new vat will hold more resin (and therefore requires more to reach the optimum fill level).


But if it's so hard to stick it to acrylic, then woh it sticks to the metal? Will this post no problems?
Image

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mikej
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PDMS - Cliff vs Waterfall

Post#9 » Sat Apr 26, 2014 4:27 am

It's working fine here in testing. You can always use the old methods if you prefer.
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Atso
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PDMS - Cliff vs Waterfall

Post#10 » Mon Apr 28, 2014 7:57 am

Having had a good run of some 20 prints whilst on the steepest part of the newbie learning curve, I finally had to replace the original as delivered PDMS. I cured it at room temperature and as a result most of the PDMS ended up in the deep area leaving a very thin layer where it matters and only a shallow waterfall. It lasted 10 minutes on the first print before tearing! The dam looks like a good idea that will ensure consistency. What are the dimensions for the meniscus trap? What volumes do you recommend for the first and second pours?

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mikej
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PDMS - Cliff vs Waterfall

Post#11 » Mon Apr 28, 2014 9:17 am

The lower edge of the trap should be about the same thickness of the window, ~1/8".
The height of the offset can be anything over 3/8".
I'd suggest around 30 grams and 15 grams or so for the pours.
Keep in mind that you may need to use some overlift with the v1.1 machines to ensure the table/part does not collide with the edge of the cliff during re positioning. The new software will have a backlash routine that should compensate for that, and the new v1.2 machine has no backlash so it's not an issue.
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Atso
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PDMS - Cliff vs Waterfall

Post#12 » Mon Apr 28, 2014 9:29 am

Thanks, Mike. I'll give it a go.

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akgold
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PDMS - Cliff vs Waterfall

Post#13 » Mon Apr 28, 2014 10:13 am

so the volume needed to fill the vat will continue to be determined by the build table?
I'm so far behind I think I'm First !!

vchmiele
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PDMS - Cliff vs Waterfall

Post#14 » Mon Apr 28, 2014 12:29 pm

Just wondering how over lift effects the build. Does having it for the entire build potentially introduce more Z axis artifacting due to inaccuracies when the lead screw changing directions?

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Robert Howle
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PDMS - Cliff vs Waterfall

Post#15 » Mon Apr 28, 2014 3:29 pm

I use overlift on every build and have had no problems.
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etienne
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PDMS - Cliff vs Waterfall

Post#16 » Tue Apr 29, 2014 2:39 am

I can confirm what Robert says: we use a 4mm overlift on every print, and additionally with longer settle and breathe times it has improved our print quality (and lets the PDMS last much much longer).

Atso
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PDMS - Cliff vs Waterfall

Post#17 » Tue Apr 29, 2014 7:56 am

Why 4mm overlift? Is there any science to this? I have increased settle times to ensure that the resin has stopped at least its major flow.

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Robert Howle
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PDMS - Cliff vs Waterfall

Post#18 » Tue Apr 29, 2014 2:53 pm

I use 1.270 mm overlift on everything. This is, if i am correct, is 2 of the smallest unit of rotaion/stepping (6.35 microns or .635 mm) of the z axis. Note: 31.75 Z is 5 of these units. I think this was pointed out by Justin. All of the Z axis slice thinknesses (in microns) are evenly divisable by 6.35 microns. I might be wrong, but the reasoning behind choosing overlift divisable by this number was for accuracy with repositioning of the build table. These are full steps with no partial stepping.

If I am incorrect someone please set me straight.

Robert Howle
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mikej
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PDMS - Cliff vs Waterfall

Post#19 » Tue Apr 29, 2014 4:08 pm

Actually, for overlift, it doesn't matter if you choose an increment of 0.00635 mm (6.35 microns) as we always return to the correct position for the next slice. It does matter (slightly) that you slice at a thickness that is evenly divisible by this increment if you want every layer to exactly the same thickness, but even if you don't, no layer will ever vary more than 0.00635mm from the sliced thickness.
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Robert Howle
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PDMS - Cliff vs Waterfall

Post#20 » Tue Apr 29, 2014 4:49 pm

Thanks Mike.
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