SLA 3D Printer Project Log 3: Salt and Spot-GP

So I finally got around to conducting an experiment in how much salt is needed per unit volume of water to buoy the 3D resin.  Overall results were:  Flotation, Frustration and Fiery throat...  well not quite, but 3D resin does get it's vapours in there and make it sore.

I filled a measuring jug with 500ml of water and an unspecified quantity of Spot-GP (around 75ml I believe, which in a 14cm x 9cm basin is around 6mm of resin), As before the resin sank to the bottom of the jug:

That's not another menacing fluid in that beer glass, that's Robinson's Orange & Mango.

I then proceeded to add salt in 10g increments, observing the water as it took on a misty opacity and more blobs of Spot-GP found their way up:

Sadly at this point, the blobs would still choose gravity as their mistress...

When I got to 60g of salt, I chose to adopt the more efficient method of weighing the salt bottle, dumping salt in, weighing it again and finding the difference; at 75g the resin was buoyed but over time it took on a form that would doubtless terrify any sea creatures below:

I suspect Archimedes will come in useful when ascertaining salt content for the actual tank.

Once 120g was reached this behaviour ceased, but the resin was still too heavy for my liking, so I added 30g to make 150g and that was my final amount - 1 Litre of water to 300g of salt:

Still have no idea why that step was there...

That isn't a final number - I couldn't test the planned 20mm of resin due to the constraints of the jug and Archimedes states that the force uplifting the less dense fluid is equal to the mass of the denser fluid displaced, thus it becomes easy to calculate the salinity required for a 20mm layer of resin in a 14cm x 9cm basin (with some cheeky help from here):

• 140mm x 90mm x 20mm = 252,000mm³

• Water is at 50C to account for exothermic heat from resin curing and summertime conditions.

• The density of Spot-GP is around 1050kg/m³.

• Hence the mass of the fluid is 0.2646kg.

• For the same volume of Saline Water to buoy this resin effectively we need a density of 1200kg/m³.

• At 50C worst case scenario therefore we need 270g of salt per litre.

• I plan to allow for up to 150mm of Z travel in my design, so basin depth will likely be 200mm with 180mm of Saline Water, giving a volume of 2,268,000mm³ which requires 612g of salt.

While this amount may be alarming, it shouldn't hit the wallet too hard since salt costs around 30p for 750g, which is staggering considering 500g of Sylgard 184 costs £50 and to fill the whole tank with Spot-GP will cost a humbling £230 just to fill the tank.

Other observations of note:

• Cured resin tends to float in the absence of a build plate, which should be good for build quality but presents a challenge for finding a suitable build plate material.
• The resin stuck well to a stainless steel teaspoon I had on hand (I used a cigarette lighter blue LED to do the curing, 3 cheers for professional equipment), and the part did not loosen even when stirred vigourously through the fluid.
• Some bubbles of water were present in the resin even after salt was added, so a through mixing may be useful before printing, perhaps with an electric whisk.
• Some of the salt came to rest at the bottom of the fluid even after stirring, so take that into account when mixing the Saline Solution.
• Natural UV light pollution from the room cured some of the resin around the edges of the jug (image below), so a "Dark-Box" will be necessary and the printer cannot be open-frame.

Quite a day today, especially since I did some science for once!  To cap it off, here are two ancillary images from the experiment:

A clear ring of resin is to be seen, there's another one at the bottom if you look closely.

Two of the test "Prints" done via blue LED.