Building a basic SLA printer part2

As you can see, even the most basic top down SLA printer is capable of making ridiculously detailed prints.

tiny 3dpt penny

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.

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