Question about the curing process

Excuse me, my English is not so good

I have a small problem with the curing of the epoxy resin.
I have a few geckos printed, these have a hole in the stomach where a magnet should go in. But more on that later.

The geckos were printed perfectly. Then I washed them in isopropanol and cured under UV light (self-built according to one of the instructions).

So far so good
Then I removed the supports and put in the magnets, then I put a drop of the resin on the magnets to seal them. Then again under UV light. But this drop of resin did not cure, it always remained a bit sticky. Now my question is: Either the resin needs the isopropanol to cure completely or my UV lamp is not strong enough.

Do you have experience with it

It could be the case that the light isn’t penetrating deeply enough into the part for it to cure the resin. Heat is needed to cure thicker components and that’s often missed out on with DIY cure boxes. Would cyanoacrylate based super glues work for your application? That tends to be the second best adhesive compared to curing resin.

You say you re using an UV lamp, not a laser ? A UV bulb usually has a very low power per unit of surface as opposed to a laser, so you want to either leave the resin to cure for an hour or more and only create thin layers or resin… or buy a cheap 405nm laser that has a power of 100mW or more, this will cure the resin in <5 seconds.

Another point is that UV bulbs quite often are 365nm, I am no expert in Formlabs’ resins but I think that while they are more sensitive to ~400nm than ~360nm.

EDIT : I also had great success with CA glue :wink:

Thank you for the Answer

It was just a drop of resin (transparent), since the light should be enough to harden it. In addition, I left it 12h under the light.
Of course other epoxy resin would go too, just this solution was the fastest and easiest at the moment. In addition, the adhesive dan has the same refractive index and thus does not disturb.

Thank you too for the Answer

I used 25 watt UV LEDs.
A test in the laboratory has shown that about 20% of the light is emitted in 405nm. That would be about 5 watts of light output.

The translucent epoxy resin cures allegedly at 405nm.

The geckos were 12h in the light and still they were a bit sticky (of course, only the gluing of the magnet). But I then washed the Gekos with isopropanol and allowed to harden for another hour. After that everything was hard and no longer sticky. So my guess is that isopropanol does matter.

Either as a catalyst or it wicks away components that do not harden.

Although in my experience resin cured manually with a >100mW laser isn’t sticky even after less than 5 seconds of exposure, it is also known that oxygen prevents polymerization (thus the post-curing in water for resins more sensitive to this effect) and it maybe what is happening to you. I think in this case the IPA is diluting away the uncured resin.

Keep in mind that your LED emits a rather diffuse light, and that only a small fraction the emitted power gets to the uncured resin. I would bet that the power per unit area for a small 100mW laser is much higher than a 5W LED at even a few centimeter from the object.

Can you set it in the sun? I do almost all of my curing setting on the window sill outside in the sun.

Hi JohnHue

First of all, I wish you a Merry Christmas

My LEDs are in a mirrored box which is 30 * 30 * 30cm. This will send the 5 watts (We ignore the mirrors) to an area of 30 * 30cm. That makes 0.0055 watts / cm2 the laser has about 1mm2 so that makes 10 watts / cm2.

Now is the time important:
5 seconds
10 watts / cm2 * 5 seconds = 50 joules / cm2

12 hours = 43200 seconds
0.0055 watts / cm2 * 43200 seconds = 240 joules / cm2

That’s almost 5 times as much energy and yet it was not hardened.
But that was the physical part, I’ll still look if I can buy a lamp with more energy and 405nm.

Then a question, how do you cure the resin?

The sun is unfortunately very rare in this season, we usually fog in Switzerland.

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