Source
r/3Dprinting•
brainrobot

Why do Resin printers retract the print upside down from the liquid?

Discussion

Im in the process of designing a UV laser resin printer but ive noticed a trend.

Every DLP projector resin printer ive seen has always raised the printing surface and object upside down out of the bath into the air. Example: Upside down print

Why? What is the advantage over lowing a platform into the liquid with light projected onto the surface from above.

Example: Right side up print

Surely retracting the liquid would cause many technical problems, for example lowering the liquid levels therefor reducing height accuracy, especially in large prints. If they lowered the object into the liquid you would only be concerned with the shrinkage of that particular resin (1.5% or something). You could compensate in software, but its not something i think many prints do.

Additionally support material would be less necessary as lowering the object into a likewise density (minus 1.5% for shrinkage) liquid would be far better and you would need far less strong supports as you would not be fighting gravity anywhere as much.

An argument could be made for the fact you need more resin to print the same object as tall objects would need to be submerged, but when you are spending upwards of $3K for a printer, you can afford to use $40 of resin in a print and pour the remains back into the bottle.

Additionally, why is it that the light from the projector does not cure the resin in the vat as it passes through to the printing surface? surely all resin is partially cured as the UV light passes through the resin. If there is a reaction with the air, wouldn't that be another reason to print down?

Posted Aug 12, 2014, 11:34 AM UTC

311
calebkraftMake Editor -- Lulzbot Taz4•

Tons more liquid necessary in addition to the difficulty of keeping your top level perfect while having moving parts inside your resin chamber.

By lifting out of the resin you only need a fraction of the amount of resin and your vertical build space is much less limited.

10
[deleted]•

It's a larger bulkier design to print top down. Bottom up having a gap between the glass and the bed would produce better layer high accuracy imo and you don't need to worry about vibrations. Both printers exist though however bottom up would produce more consistent more accurate results imo.

Additionally, why is it that the light from the projector does not cure the resin in the vat as it passes through to the printing surface? surely all resin is partially cured as the UV light passes through the resin. If there is a reaction with the air, wouldn't that be another reason to print down?

I'm not really sure what you are asking here but the resin doesn't react with the air. The UV light crosslinks the monomers in the resin (curing).

5
brainrobotOP•

Thanks for the input.

The air part was in case the resin cures with UV light and materials in the air, but it sounds like that isn't the case.

The was I see it, if you can manage vibrations and leveling, and don't care about the high resin requirements, you can get some quality improvements.

1
TheWindeyMan•

The was I see it, if you can manage vibrations and leveling, and don't care about the high resin requirements, you can get some quality improvements

You're right, which is probably why all the big commercial SLA printers work that way around. Here's one printing a table.

Of course these printers cost between $100,000 and $500,000, and require huge resin tanks and an arm that can skim across the surface to spread resin out on the top of the model evenly.

Consumer printers generally go through the bottom to avoid having to have a very deep resin tank and a spreader on top. EDIT: Just to add as an example, the Form 1's build area is 125x125x165 mm^3 . To print the right way up it would need a 2.5l tank (around $375 worth of their own 1st party resin).

2
[deleted]•

It does offer some benefits. You don't have to mess around with borosilicate glass and coatings or the suction of the bed and the resin. You would have to make sure the resin levels out before printing the next layer so it might increase time. There are working printers using this method and they do make decent prints.

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lampar0prusa i3v•

I'm pretty sure that the diffusing light from the projector and the environment will eventually ruin the rest of the resin in the tub. Also, I'm not sure, but $40 seems like a low number for the cost of a tub full of resin.

5
brainrobotOP•

Maker resin stuff is about $40 per 500ml. The vat cost depends on your vat size. You would want to match your print to your vat size/shape.

1
curlrup•

With UV cure resins used in 3d printing oxygen stops the chemical reaction from getting out of hand. So some of the lifting action is peeling a cured layer off the bottom of the vat and some of it is getting oxygen to the fresh layer. SLA systems that build by dropping the part into the vat the reaction is on the surface ofbthe resin so there is plenty of oxygen to stop the reaction with that set up. Also on the large SLA systems vats of resin do thicken over time from uv scatter.

2
morgazmo99•

Fluid level would be displaced by volume of the print. Print might want to float. Maybe surface tension would interfere?

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TheWindeyMan•

The big commercial SLA printers work that way around so it's probably more optimal for quality and very large prints. Here's one printing a table.

Of course these machines are huge, cost $100,000-500,000 and need massive amounts of resin to fill their resin tank.

2
curlrup•

By building down you are not adding mass to the liquid that is already in the vat. So you are not displacing resin. Parts actually sink in the large SLA machines. Lost many a part to Davey Jones Locker.

1