Monday, 18 June 2012

Solving the problem of a sticky bottom

As the previous post explains, even if your part is not glued to the bottom of the vat it may still be stuck. I'm trying to find a way around this that allows for large area printing from the bottom (FTB).

To un-stick the part from the vat bottom we need to allow fluid to get between the part and the vat. The gap between the two is so small that there is no chance for it to just seep underneath and we cannot simply drill holes in the vat and pump it in as that resin would get cured as well.

My idea is to have a number of tunnels in the flexible PDMS (silicone) vat bottom containing a fluid that has the same refractive index as the PDMS, this means optically it is as if the fluid was not there. If a negative pressure if applied to this fluid then it will distort the PDMS and that could allow fluid to flow underneath.

Hopefully the above diagram makes sense. The vat bottom is glass and the projector shines up through it. On top of that is the clear PDMS layer and then a thin layer of the light cure resin. On the left we see a part with a large surface area being printed, a small layer of resin at the top surface of the PDMS has just been cured and although the part is not glued to the silicone it can still not be lifted off without significant force. Between the glass and the PDMS are the channels filled with the index matched fluid. On the right the fluid has been evacuated and the upper surface of the PDMS has become corrugated, with the correct design of channels it should be possible to get complete separation of the part and the PDMS. It make require a small amount of time while the fluid flows beneath, it will depend on the viscosity.

Here is one way the channels might be laid out. I've suggested a diaphragm based unit to provide suction actuated by a solenoid. A spring is added to make it more gradual.

And this is another way to configure the channels, this time with a syringe type suction device.

And a few ways the channels might be created depending on the manufacturing method.

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