Top down wins...

[gmoneyphx]

Among those who have tried both bottom up and top down systems is there anyone who prefers bottom up? Seems no contest in the end. Here is my reasoning:

Top Down Disadvantages and counterpoints

Large expensive resin tank
Yes, you need more resin with top down systems but with the resins we are currently using nobody is printing large solid parts. Shrinkage/warpage just won't allow it. I would say that 8x8x8 is just about the maximum practical size for any of these systems with 5x5x5 being a good starting point. 5x5x5 vat = approx 2 liters of resin. Who is worried about $100 worth of resin??? Of course if your plan was to build a 24" x 24" x 24" volume printer the resin will cost a fortune. But the resin in the least of your problems in this case.

Viscosity / Leveling issues
The larger issue really is the limited selection of resins with really low viscosity. Absent a wiper you have to wait to allow the resin to settle to your approximate layer thickness between layers. Resins with viscosity of over 250 cps or so make this a slow painful prospect. Luckily we have SubG+ which gives a great compromise between shrinkage and viscosity. The really good DSM resins that have well under 1% shrinkage all have much higher viscosity. Not practical in a wiper free top down system.

Top Down Advantages

For me the ultimate, absolute, unimpeachable advantage - RELIABILITY
If you get good attachment in the first layer(s) to the build platform and you are printing a part that can actually be printed (correct supports etc...) - you can print unattended with confidence. There is close to nothing that can go wrong. Of course I am excluding systemic failures like stepper motor, stepper driver, dlp bulb etc... There is essentially ZERO stress on the part as it builds (system induced - shrinkage is always present). From extensive reading on the B9 and FormLabs forums it would seem that "fire and forget" printing on bottom up systems is a fantasy.

Second big advantage - SIMPLICITY
The vat tilt / release mechanism on bottom up is finicky thing. PDMS/FEP coating - try and find someone who thinks dealing with that is fun.

There will always be specific geometry you can pick that favors one system or the other but I think top down wins. Not trying to start a fight or anything

[johnrpm]

Commercial systems in the past were top down, they have a recoater blade, hollow with a vacuum to suck resin up,
they also have a vat raise and lower system to present the resin surface correctly, they tend to be laser based, so
not sure if with a clear resin dlp exposure might spread sideways ?.

Although bottom up has issues, top down would have had issues until they were addressed, I did think long and hard
about wich way to go, and decided on bottom up, knowing that if the problems with vat coatings etc could be solved
it would be a better and simpler, but if all else fails, can always change to top down.

laser based systems cure at the spot, therefore stress can be mimimised by dancing about, dlp must add stress
due to the blanket approach, what if we had some sort of dithering to spread the stress.

P.S many builds fail in top down, due to support issues, sorry for rushed reply.

[sitzme]

Thanks for taking the time to document the arguments. If a suitable liquid can be found to displace the resin and lower the amount of resin required, top down may take off. Mercury comes to mind. (not really!) So far I am not aware of anyone having great success though some have tried salt water with at least limited good results. The other option is to bite the bullet and just buy enough resin to fill the tank. It is basically a one time purchase just like the tilt mechanism. There would probably need to be a circulating pump and maybe a storage tank but there is always some fly in the ointment.

Build it and we will come!

[gmoneyphx]

I think usable resin at $50 or less per liter (as Makerjuice is) makes the resin problem go away. Again with a 5x5x5 build volume needing only $100 worth of resin it seems to be a non-issue.

I considered a salt water solution but I think there is a big problem. Even though the resin and saline will remain nicely separated I think the pigment will happily depart the resin into the saline solution. Maintaining a reasonably consistent level of pigment is an important part of the top down solution.

Anyways, the days of $300/liter resin are over (thankfully).

[gmoneyphx]

[Finhead]

Nice thanks for all the great information. Looking around on the web at Top down system (what little I could find) they seem to be a easier build.

Do you happen to have a webpage or blog on your build, care to share information on your setup?

Clinton

[AndersonTa]

Also in top down you have to keep the resin topped off at all times. So it's not just the build volume that you have to account for...its the volume and then the volume needed to maintain a topped of reservoir. But you maybe right, I think top down was written off in terms of cost, but now with affordable resins, that might not be such a big deal.

[gmoneyphx]

In terms of keeping the resin topped off...

I start all builds manually, with the platform slightly above the level of the resin (< .5mm) after dipping into the resin to get a thin layer for adhesion during the first layer (I use a scraper to remove excess resin from the platform and then let the remaining resin settle out to a thin layer). There is no practical, easy way to avoid this. To fully automate the process of resin level accurately enough is not worth the time or trouble. Takes just 3-5 minutes to get a build started.

I have not topped off resin during any of my builds. Yes, the resin is being consumed into the part, but the part is being lowered into the vat. The only true loss is the volumetric shrinkage factor. Have built parts as tall as 80mm without issue. Next week I will have it upgraded and able to do 250mm build height. We will see how that works...

[258cj7]

I would think that the level drop from resin shrinkage would be negligable (Unless you are printing something very large that nearly fills your build tank). One other thing to consider would be the displacement from the platform supports (not the platform that is already in the tank) as they lower further into the resin.

[gmoneyphx]

In my case the platform riser is a piece of CRS 3/16 x 1". So it seems to basically to offset the loss due to volumetric shrinkage as I build.