DIY UV cure source - is it enough light?


#1

I don’t have the printer yet, so I can’t just test it, but I’d like to have opinions about whether the DIY cure box I’m making is going to be worth the bother.

I’ve made my own PCB with 14 VLMU3100 UV LEDs. The power supply gives them each a bit more than 3 volts, and they’re annoying to look at. That’s about as much actual hard evidence as I have as to what they’re actually doing.

Their specs are a nominal 2.5 mW/sr each and they are arranged around the top of an 8x8x8 inch space that is internally reflective. The wavelength peak is 405 nm, which I selected from other recommendations here as being the good wavelength to use.


#2

You will be able to calculate the actual UV power at the surface of the print being cured from your figure of 2.5mw/sr. You must take into account the distance from the LED to the surface and must always assume some losses. (At least 30% being a reasonable starting point)

What is your reflector made from? UV reflection is very different to visible light reflection in terms of efficient surfaces for reflection (a glass mirror is poor compared to an aluminium reflector for example)

This link gives you the formulas to work out the power at the print surface. Aim for a minimum of 5mw/cm2 for a post cure in a reasonable time https://docs-apac.rs-online.com/webdocs/0321/0900766b803218da.pdf


#3

The 5 mW/cm^2 number is helpful. Thanks. I’m intending to use tin foil as a reflector (as was the case with the YouTube video Formlabs made of their cardboard budget cure box). Doing the math, I’d need to have the array within ~3 cm of the print to get that sort of intensity, but I think that assumes that there’s no contribution from reflections (that is, that whatever photons leave the LED and don’t travel straight to the destination are wasted), doesn’t it?


#4

Anything reflected should be treated as a bonus!! If you calculate raw figures then you will be in the right area


#5

I actually made a third one with 21 SM0603UV-405 LEDs.

I don’t have the printer yet, but I got a vat of white resin. It arrived today and I did a little experiment. 10 minutes of exposure was enough to make a little solid disk.


#6

Nice work - but I think you will find most resins will cure faster than that just being outside in indirect sunlight. Typically that size disk (at a guess perhaps 2 - 3mm thick) should through cure in 10 - 20 seconds, so if it took ten minutes then perhaps the UV power is a little low on your cure oven.

I have just cured a 3mm thick test sample of Formlabs white with 5mw/cm2 at 405nm in 14 seconds. A sample of resin configured for much lower power DLP (so its actually faster to cure - in fact far too fast for the Form 2) through cured in less than 2 seconds and was smoking!!


#7

Oh, I don’t know that it didn’t take less time than that. I was expecting it to take much much longer and that’s the first time I checked it.

I am thinking I may repeat the experiment closer to how the resin life cycle really works - expose raw resin for a minute or two, then wash the piece in IPA and then cure it for 20 minutes and see what happens.


#8

30 seconds wasn’t enough time, but two minutes was enough to make another little disk. I washed it in IPA and am now curing it for 20 minutes and we’ll see.