White resin overcuring


#1

Has anyone experienced odd overcuring issues with the white resin?

It was fine three prints ago, now suddenly, negative features are ending up very noticably overcured.
For example, a 1mm dia hole oriented about 45 degrees to the tank plane, is ending up as an indent rather than a through hole.

The only thing that changed in between prints is that the parts that appear overcured are very near the farthest extreme of the tank from the peel pivot. I’m also using almost the whole volume of the Form2 (the object itself doesn’t have the full volume, but it’s an LCD bezel that spans very nearly the whole printer volume diagonally).

Laser calibration / focus suddenly gone bad?
Resin not mixed well and most of the photoactivator ending up on the bottom?
Any other ideas?

Edit: Actually, i was wrong, i’ve just checked. I had the holes on the pivot side in one print, and away from it the other. Same thing happened both times.


PoisedWhale reporting for duty
#2

@Ante_Vukorepa Yes I have found white to be the worst overcuring culprit. I’ve never had any accurate prints with white. I haven’t even used 1L it was so bad on the first few prints the cartridge went back on the shelf unfinished. That was version 3 - I tried again with version 4 and it was no better.

Anyway - that’s not helpful really since I haven’t noticed any change with white, but I’m very interested in the overcuring thing overall - did you see my thread on the subject?

Previously I put overcuring variation down to different firmware versions, or the resin type, or individual machine variation - like yourself I also wondered about some sort of machine deterioration.

But then I started using GreyV4 with good results and just put the overcuring question aside, until recently when I started getting overcuring with a GreyV4 cartridge, that had previously been perfect.

Unlike previous overcuring issues, I had changed nothing - firmware/preform/resin were all the same - however I’d had a 2 month gap without printing, and the resin had been sitting in the print tray in the summer heat.

I had 3mm sockets that were coming out about 2.6mm/2.7mm so I increased the socket size to compensate for the overcuring - but then as the cartridge emptied over the next few days and 600ml of prints - and replaced the tray resin, it looks as though the overcuring effect lessened and even disappeared, and now the sockets are too loose.

What was the time period between your previous good prints and the most recent 3 prints?


#3

White is normally a difficult colour, the pigment is usually titanium dioxide, which is fantastic at stopping the UV light penetrating the resin and thus curing (It acts as a light blocker) Normally the resin requires more power to cure for a given depth.

Its very likely that the white pigment is settling during the life time of the cartridge and thus giving different curing properties over a period of time. If the white settles then the resin will cure through less thickness (higher percentage of pigment at the cure layer) but the area of cure is more diffused (resulting in over cured “soft” rather than crisp edges. The increased white titanium dioxide pigment scatters the UV energy rather than allowing it to penetrate cleanly.

Its a common problem for 2D UV cured printers. (And relatively easy to cure by an experienced print chemist - who can stop the settling)


#4

About a week.

Yeah, that occurred to me too - although i’d expect UNDERcuring for the resin from the bottom of the cartridge (where the sludge of TiO is thickest), not OVERcuring :slight_smile:


#5

The issue is that when the white is in a higher concentrate it simply seems to stop the UV energy penetrating and diffuses the energy sidewards,

What we have found is that is what causes the dimensional overgrowth. Lightblockers tend to “alter” the energy (by changing the effective wavelength of the energy source through fluorescence, which changes the wavelength of the UV energy to one which is less effective at curing the resin)

Titanium dioxide (the main part of the white pigment) simply deflects the light sideways (imagine a mirror) hence the dimensional inaccuracy. showing through overgrowth. The more Titanium dioxide the worse the effect and the greater the error.


#6

Doesn’t titanium dioxide fluoresce as well? It’s fairly visible (as lower frequency blue light) in the curing chamber. Even with the light diffusion, that should greatly reduce the amount of curing…

Anyway, i’m now at the end of the cartridge (last of the resin emptied out) and the most recent print ended up with some of the layers near the end ripped off the supports (where they worked just fine previously). It looks like you’re right and the “bottom of the barrel” resin really might be the culprit.

It’s simply all over the place.


#7

Since i’m down to whatever resin’s left in the tank, i’m gonna do one more print, this time with slightly thicker support contact points and see where that gets me.

Interestingly, this last print had way less overcure - the 1mm holes ended up more like 0.75 or even 0.5, but they’re at least holes.


#8

Titanium dioxides do fluoresce. With a 405nm light source, it shows as a deep purple, but the vast amount of UV energy seems blocked from penetrating through a Titanium dioxide suspension, through reflection from the particles (effect increases or decreases in magnitude dependent on the TiO2 particle size) This effect is actually used to make UV cured resins / photoinitiators more reactive to the the UV energy. In that application the Titanium oxides are known as “photo sensitisers’”

To obtain a consistent cure rate the particles need to be held in suspension (prevented from settling) thats possible by some clever chemistry, without it you get variable cure rates as the pigment settles in the resin.


#9

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