Advanced Printing Technics - Under Over Exposure
By: Bryce Bickham, October 10 2021
In this tutorial, we will be going over using over and under exposure in 3D microfabrication
- Penetration Depth
- Under Exposure
- Over Exposure
In this tutorial we are only going over a brief description of penetration depth and how we can use it to our advantage when 3D printing microfluidic devices. If you want to understand Penetration Depth a little more watch our Spectral Matching Tutorial.
Penetration Depth describes the distance the UV light penetrates into the 3D print, or resin, before it is too weak to affect the resin. This penetration depth is dependent on time, but at a normal exposure time, the penetration depth is generally several layers into the print.
When we look at an SLA system, we see UV light coming from a source. Once it hits the resin tray, the light will start to scatter throughout the resin. This causes some of the exposure to polymerize material transversely.
Let’s say you’ve got a small channel, maybe a 2 or 3 pixel wide channel and let’s say it’s 3 or 4 layers tall. It has already been formed in the print and now the printer is going to form the ceiling on the channel. Because the penetration depth of a normal exposure time penetrates into the print, the next few layers creating the ceiling will expose the channel underneath, either encroaching on it’s space or fully polymerizing it.
This is also why spectrally matching your resin to your 3D printer is important. In our Spectral Matching tutorial we go into how to select resins that will work with these advanced exposure technics, allowing you to print micro-scale features.
So, let’s say you are printing the same channel, let’s look at the effect of scatter. While exposing the pixels that make up the walls of the channel, the light scatters, and we start to penetrate into the sides of the channel. Again this will cause the channel to shrink albeit from the sides this time, and if the channel is small enough, the scatter is bad enough, and the exposure is long enough, it can polymerize the entire region. As you get above the channel, even the pixels on the corner of the channel will scatter and penetrate into it.
To attenuate these effects, we can decrease the exposure of the pixels and layers surrounding the channel to the minimum exposure needed to polymerize those surrounding regions. This will leave the material surrounding the channel weak, but the rest of the print will be strong to withstand pull-off. After the print is done and you have cleared the device, you can cure the device to strengthen the channel again.
A problem with DLP-SLA technology for microfabrication, is it’s inability to print outside pixels and layers. Well, we can use over-exposure as a way to flex these bounds to print any size we need.
Let’s say we want to print a 19μm channel on our EA Series Printer. It has a 7.6μm pixel pitch, so a channel this size would be roughly 2 ½ pixels. So we design the channel to be a 3 pixel channel. If we print the device using the under-exposure technique, we get a nice 3 pixel channel. To get our 2 ½ pixel channel what we can do is purposefull over-expose the pixels surrounding the channel to shrink it. You need to know your printer and resin to do this. If you are working on a small channel like this 2 ½ pixel channel, it might be best to not over-expose the pixel immediately surrounding the channel, but the pixels just off the channel, to minimize the scatter in the center of the channel.
If we use the penetration depth in our over-exposure technique we can accomplish something else pretty cool. Let’s say you are trying to print a super hydrophobic surface that looks like a “T” with these over hangs on the side. Normally, these overhangs wouldn’t be possible because there isn’t anything supporting them from below during the print process, but if you over-expose the edges of the “T-top” the light will penetrate past the top of the T and start forming the overhang. I’m sure you can think of other features that could use small overhangs like this.