Continuous Printing Tests

After the outer ring took 4.5 hours to print, I decided to try some continuous prints that promise to be faster. My definition for a continuous print is where the layers aren't dipped, but the projector continuously displays the layer while the lift very slowly lowers. Since continuous printing does not have a dip phase, my layer time is cut in half! I've seen some videos of this happening, particularly from Carbon 3D. Continuous printing can work for top-down 3D printers because the oxygen in the atmosphere inhibits the top layer of resin, on the order of hundreds of micrometers, from curing. This inhibited layer allows resin to flow over the object during printing. However, the viscosity and thinness of the layer slow the flow of resin so that layers with large areas do not form correctly.

I wanted to know:

1. Does continuous printing work with my printer?
2. How fast is continuous printing exactly?
3. What is the maximum wall thickness before the resin cannot flow correctly?

I started with a tear-drop shape again, this time with 5 of various diameters: 10 mm, 5 mm, 3 mm, 2 mm, and 1 mm. I decided to start with a rate of 10 microns per second, which is similar to 10 seconds a 100 micron layer I normally use. I did slow the first 300 microns to get the resin to bond with the lift, using about 5 microns per second. After seeing the relative success of these tear-drops, I decided to print a dice with the same settings.

 Tear drops with a height of 5 mm and the 12 mm dice

The 10 mm drop had holes that came from the side and traveled into the center, like a bubble formed on the edge and slowly moved toward the center with the resin flowing from the top layer. The 12 mm dice exhibited the same problem. However, 5 mm down printed fine.

I decided to try slowing down the rate to about 6 microns per second, and print a second dice. Since the z-layer resolution only affects how long a layer is displayed, not the print time, I also improved the z-layer resolution to 10 microns instead of 100!

Below are photos comparing (left to right):

1. The best dice printed using the non-continuous method
2. The 6 micron per second dice
3. The 10 micron per second dice

 Continuous printing does not help bottom side bleed-through

 The 2 side is the roughest for continuous printing, yet smooth for the layer dice. Perhaps this is a mask difference, since I changed the projector mask between these prints.

 The 6-side tells the most information

The 6-side is the top side, with the dots mostly clearly showing the 100 micron layer size. The 6 micron per second dice does not show this due to a 10 micron layer size. However, both continuously printed dice are hollow, with holes that formed on the 1-side (the opposite side) going completely through the dice. Some of those holes filled at the very end for the 6 micron per second dice, as seen by a translucent top layer. The dots on the dice must have improved the resin flow at the very end.

I believe the holes are formed by fast moving resin. As the resin hardens, there are less places for the resin to flow to the top. With less places, the resin must flow faster through the remaining holes. However, fast moving resin does not get a chance to polymerize, leaving a channel that starts from the edge and ends toward the center, where the resin is most displaced. You might visualize the channel as a hole in each layer that slowly moves toward the center. The angle between the normal vector of the top plane and the channel is on the order of 15 degrees when starting from the edge, at least, for a 12 mm dice. Slowing the print speed reduces speed of the resin in the channels, and fills more holes (as seen between the 6 and 10 micron per second dice). Perhaps another day I will experiment with less speed, but I was already approaching my non-continuous print speeds and didn't care to go slower.

I have updated the git repository with pidishc.py, for continuous printing. I might use this for thin, shelled out components in the future, but ONLY for these. The most exciting feature of continuous printing is the removal of the time spent dipping, and print speed can be improved by orders of magnitude if I can improve the UV output of the projector. Prints would go from roughly 1 cm per hour to 7.2 cm per hour, but with an extreme limitation of the shape.

To answer the questions from above:

1. My printer can do continuous printing
2. The print speed is 6 microns per second
3. Print walls can be about 3-5 mm at their thickest before holes start to form.