Microfluidic Resins - Resin Matching
By: Bryce Bickham, September 8 2021
3D Printing Exposure Per Layer
During your 3D print, the 3D printer exposes an image at a time printing your device layer by layer. (More on the printing process can be seen in our Basic Microfluidic Design Tutorial) Each one of these Layers is given a certain amount of UV exposure to polymerize. What we see here is a graph of the given exposure across a layer. You get the most exposure at the start of the layer and less exposure the further you get into the print. As you expose the next layer the exposure doesn’t just stop when it gets to the previous layer. The tail end of this exposure penetrates into it giving the 1st layer additional exposure. Again, on the next layers the tail end will penetrate into the previous layers. At some distance the tail is too weak to really polymerize anything. We call this the penetration depth.
A resin that is not designed for micro-scale features, or that is poorly matched with the printer will have a tail that doesn’t diminish quickly, exposing many layers into the print.
A good resin will have a steep tail to cut off the dose the previous layers receive quickly, avoiding extra polymerization in your important features.
How Resins Work – Resin Components
There are 3 main components of resin: The monomer, the initiator, and the absorber.
The Monomer
Here we have a monomer. The monomer is the major part of a resin. Generally at least 95% of a resin is the monomer.
When we introduce the right energy into the system, the monomer starts to link into strands like this.
The Initiator
The monomer needs a kick in the system to polymerize. Normally, that comes through an initiator.
Initiators are chosen because they can absorb some form of energy (in our system light) and pass that energy to the monomer. Once the energy passes from the initiator to the monomer, the monomers will start to bind together, forming the polymer.
The Absorber
Lastly, we have the absorber. Many resins don’t even have a separate absorber because the initiator absorbs a little, and because micro-features may not be important to them, but if you want to achieve micro-feature printing, an absorber is necessary.
The absorber takes in the energy just like the initiator, but instead of passing it on to the monomer, it dissipates it. This stops the energy from penetrating into your print and polymerizing regions meant to be void. The absorber is the key piece that allows us to print micro-feature devices.
Spectral Matching
There are a lot of resins out there; To get micro-feature prints, you need to spectrally match your resin to the LED in your printer.
Here I have the LED spectra of a 365nm and a 385nm 3D printer.
Let’s look at this green graph representing the absorbance of one of our 365nm resins. The absorbance spikes at about 365nm and then drops rapidly, and by 400nm it really isn’t absorbing much of anything. This absorber won’t be sufficient for the 385nm printer, because the tail end of the 385nm LED emission is past anything the absorber can handle. The absorber works ok for the 365nm printer, as it absorbs almost the entirety of the 365nm LED spectra, but looking at the tail of the emission spectrum of the 365nm LED the absorbance is cutting it pretty close. Our EA Series Printer fixes this by adding a patented filter to cut off the 365nm LED tail.
As another example we have an absorber represented by this orange absorbance spectrum (one of our 385nm resins). This new absorber peaks at around 370nm and drops off more gradually until by 430nm it really isn’t absorbing anything anymore. Because it absorbs the UV across both LED spectra, it will work for both printers.
Here are two photos of the resins. The first absorber, the one designed for the 365nm EA Series Printer, is clear, while the 2nd absorber, the one that would work for either system has a yellow tint. These are the colors of these absorbance spectra, not just these specific resins. Resins that work for the 385nm printers are going to have a similar yellow color. Because of this, I always recommend choosing a 365nm printer, that enables you to print micro-scale features with clear resin.
Summary
In this tutorial, we went over spectrally matching your resin to your 3D printer. Look out for future tutorials on resin development and penetration depth coming soon.
References
Gong, H; Beauchamp, M; Perry, S; Woolley, AT; Nordin, GP. RSC Adv. 2015, 5, 106621.