Your DLP type SLA printer will need to manage 2 basic tasks in order to print. The first is to adjust the Z-axis for each layer and the second is to send the appropriate slice image to the projector. This is pretty simple stuff, but unfortunately it is beyond the ability of a simple Arduino. It is within the realm of possibility for a RaspberryPi (they come with HDMI ports), but no one has written the software yet. So at this point the best solution is a Windows machine running Creation Workshop.
The Windows machine USB’s to an arduino based board (I use a ramps 1.4, but any will do) to interpret the gcode and control the z-axis stepper. If you have an old Windows box, or an old laptop lying around you may already have what you need, but if you don’t there is a perfect solution. Finally a good use for a Windows tablet! Perhaps because no one has found another good use they are practically giving Windows Tablets away. You need one which has an HDMI output and will drive two displays. You also need a usb port to connect to the ramps board. I also recommend you get one with 2gb of ram. I started with a Winbook TW700 which worked well except that if you had a large or complex model it tended to crash while printing. This wouldn’t be such a big deal except that it failed with the projector blasting pure white. If you weren’t right there when it happened this tended to cook off all the resin in the build vat.
It made an interesting brick like print, not to mention an awful smell. I only did this a couple of times before I upgraded to the Winbook TW802 which has double the memory and hasn’t crashed yet. This one cost $159, the TW700 was only $59, less than a retail license of Win8.
With all the hype Carbon3d has gotten lately about how it has revolutionized 3d printing with its continuous printing system, I started to ask myself what was stopping me from doing the same thing?
The standard layer cycle for top down machines is to expose the layer, lower the print into the vat, let the resin flow over the print, raise the print back up to the position for the next layer, wait for the resin to flow out evenly, expose the next layer and repeat. This may be faster than the peel cycle of a bottom up printer, but it still isn’t as fast as it could be. With the right resin and a good DLP projector the exposure time can be as little as a second or two, yet the whole cycle can take as long as 8-10 seconds. Now we are only talking seconds here, but an 8 second layer cycle takes 4 times as long to print as a 2 second layer cycle. That’s the difference between a 1 hour print and a 4 hour print. What if we could trim out the extra time so we are exposing resin for the entire layer cycle? It turns out it isn’t that hard to do. Creation Workshop gives you enough control over the program that it is quite easy to make your machine print continuous layers. You set the Z-lift distance to 0mm and the lift and sequence time to 0sec. The machine exposes a layer then drops to the next layer and exposes it with only a brief flicker of the projector as it changes layer. You will probably have to tweak the exposure time and layer height to optimize things, but it is pretty easy.
There are a few problems with the continuous printing system. First the initial layers on the build plate sometimes have trouble because there isn’t enough resin on the surface until the build plate is well submerged. A perforated build plate helps, but isn’t a perfect solution. The second problem is that the resin doesn’t flow out as quickly as we would like. The perfect resin would be water thin and would level out instantly. It doesn’t exist (yet). As print slowly submerges in the vat it sometimes takes too long for the resign to flow in and be exposed. This results in defects and sometimes entirely failed prints. My next project is to heat my resin vat to try to reduce its viscosity.
As you can see, even the most basic top down SLA printer is capable of making ridiculously detailed prints.
The top down system has advantages and disadvantages compared to the more popular bottom up system. On the advantage side, it requires no peeling. It needs no fancy vat coating or tilt or twist peeling mechanism. Because it doesn’t need to peel each layer it can run amazingly fast. Carbon3d’s secret sauce is a fancy technology to eliminate peeling on a bottom up machine. Top down machines are all just born that way.
A disadvantage of the top down system is that it is difficult to calibrate to make precise parts. The resin contains an inhibitor which prevents it from curing in the presence of oxygen so the layers are actually cured somewhere below the top surface. How far below depends on the projector and the pigmentation of the resin. The surface level of the resin is difficult to control which adds another layer of uncertainty. Plus the resin is viscous and forms a meniscus on the top surface (see your middle school science notes if you don’t remember these terms). Combine these factors with the spreading of the projector beam and you can see why it is difficult to make a finely calibrated print. It is difficult to know exactly how large the projected image will be at exactly the location where the cure takes place. The bottom up machines solve this problem by curing layers on the bottom of the build vat which is a readily defined and easily repeatable location.
Another disadvantage of the top down system is that it requires a build vat that can hold the entire finished print submerged. This requires a lot of resin unless you resort to trickery. The simplest solution (the one I use) is to fill most of the build vat with a saturated solution of salt brine. Mix salt into boiling water until no more dissolves. Once it cools to room temperature and the excess salt settles to the bottom it can be poured into the build vat. The saturated brine has a high enough specific gravity that the resin will float on top of if (think Dead Sea). This works for the most part if you keep a fairly thick layer of resin on top. If the layer gets too thin you can get drops of water where the resin should be which causes defects in the print.
The debut of Carbon3d’s layerless continuous SLA system certainly caused a splash in the 3d printing world followed quickly by the less well funded, but equally amazing Gizmo3d. These innovations made me yearn to tinker with SLA printing, so I set out to build my own SLA printer using primarily 3d printed parts and parts left over from previous printer builds.
A top down SLA printer is ridiculously simple to make. In its simplest form it consists of nothing more than a video projector and a z axis. All the magic is done by the video projector, and you just buy that part. If you buy the right one (I use an Acer H6510BD) you don’t need to modify the projector at all so it still works to watch movies. The software is the other part of the package. Creation Workshop provides a very simple easy to use solution. The package is free to use for individuals, but they license it to printer manufacturers, and make custom versions for different machines. The software is rather basic, sort of the RepG of the SLA world, but it works well enough and can be easily customized to run almost any configuration of SLA printer you can come up with.
In its simplest form the printer projects a slice then lowers the z axis and projects the next image. Unlike bottom up printers, top down machines require no peeling, so they don’t require special vat coatings or tilt mechanisms. The part is simply lowered into the vat in preparation for the next layer. Creation Workshop allows you to inject G-code at almost any time in the printing sequence which makes it easy to activate any sort of peeling mechanism or re-coating blade depending on your machine’s design.