Having a solenoid lift something floating both above and below the resin is what my video shows. By having it float the solenoid does not need much strength to change the height of the resin temporarily. However to avoid making waves a sheet of plastic must be placed between the object that displaces water and the actual print. This causes waves to reach the surface at small angles, which prevents surface waves for a reason I will explain here.

When making this demonstration I looked up information about how the surface waves are created. It happens when a wave runs into a lower index of refraction medium at an angle too great for the wave to pass through to the lower index medium. The energy of the wave is then dumped into the interface between the two mediums and a surface wave is generated there. If the angle is small enough then the wave will just carry on to the lower index of refraction medium and no surface wave will be created. The equation for the minimum angle to cause surface waves is angle = arcsin(n2/n1) where n1 is the higher index and n2 is the lower. Water and the resin have a much higher index of refraction than air. So in this case n1 is resin and n2 is air. Therefore if you disturb liquid then surface waves will result unless the waves travel in a direction pretty much normal (directly) to the air-liquid boundary. The sheet of plastic forces the waves generated from the moving floating object to be pretty normal to the air.

It is just like total internal reflection with a laser, except that instead of the wave reflecting it just dumps itself onto the surface causing waves. Here is a good video demonstration of laser light total internal reflection:



I think that it is pretty interesting that it works this way.