Thin wall bleeding - 0.1mm vs 0.05mm


#1

So I have a unique issue kind of pushing the boundaries of SLA. I have some thin wall features 0.34mm thick, spaced apart at 0.56mm. When I printed this (flat) with 0.05mm layer thickness, they came out fine. When I tried to print this at 0.1mm layer heights, there was partial resin curing in between the walls.

Does anyone have any idea why there is this discrepancy? I’m wondering if the increased laser power & exposure for 0.1mm causes extra bleeding/curing than 0.05mm.

Test


#2

When you cure a thicker layer of resin, the laser does not change- rather, the space between the prior layer and the bottom of the tank gets larger. That is, the platform comes down and stops shorter- trapping a thicker layer of resin between the tank floor and the prior layer of the build.
this thicker layer of liquid resin can cause the rim of the spot to diffract and diffuse more than it will in a thinner layer of resin.

that is- a given resin will diffract the incident light a specific amount- but as the layer height gets greater- the distance that bent light can diverge from the laser path grows greater.

Imagine a laser hitting a shallow dish of water. the angle of the light to the water surface will cause the laser path to bend- for sake of argument, let’s say it bends 45 degrees. if the dish of water is only 1/8" deep, the laser spot on the bottom of the dish will be 1/8" off. But if the dish is 1/2" deep- the spot on the bottom will be 1/2" off.

In the form 2 the diffraction and diffusion is pretty small- but thicker layers compounds it- as the laser penetrates the liquid resin.
The effect of this is to render the laser spot slightly larger in diameter and with more falloff around the rim when shining thru a thicker layer of liquid resin.
The laser path tries to compensate for this effect… but there is not much that can be done when you are getting close to the resolution of the laser spot itself.

at least- this is what I surmise from the way the machine functions.


#3

@Sculptingman interesting hypothesis. I would expect that either the laser power, or the exposure time may also have to change for curing a thicker layer of resin though, since you now have 2x the thickness of resin which needs to absorb a certain amount of light to solidify. Also, doesn’t light energy exponentially decrease with distance through a medium?

I would be curious to see if anyone from Formlabs can comment here.


#4

I don’t think the laser power changes at all.
Lasers usually emit a specific frequency range at a specific output. This stability of power output makes for a more reliable and long lived laser.
The depth of cure is set by the gap created between the tank floor and the prior layer of the model by the build plate- only allowing a very specific thickness of liquid resin in the laser path.

For more energy- the laser beam simply moves slower, or plays over the same area more than once.
The X/Y resolution of the laser is greater than the diameter of the laser spot simply because the software knows the diameter of the spot and can aim for the laser path to run the RIM of the spot at the edge of the profile being drawn- rather than the center of the spot.

I think diffusion of the beam is part of the reason the Form 2 doesn’t even try to offer build thicknesses greater than .1 mm- which would be faster and often sufficient for simpler parts,
The grey resin and others that are more opaque would significantly limit the penetration of the laser into the liquid… and since the resin is polymerizing AS the laser hits it…the polymerization itself is essentially getting in the way of the beam… either increasing it opacity or its diffusion. These parameters effectively limit how thick the laser can reasonably accurately polymerize the resin.

I have been using a laser pen to fuse multipart models together and so I am seeing the laser getting refracted or diffused when i shine it into too thick a puddle of resin. I have also run into the limit of how deep the pen laser can cure the resin. I can accidentally cure just the top of the resin if the resin I apply is too thick, and end up with a ‘scab’ of resin that can slide or peel off the still uncured resin underneath.

While laser power definitely fall off with distance, its not the standard inverse square law- because laser light is coherent. So the energy drop between .025 and .1 mm is not measurably significant.

Clearer resins allow the beam to penetrate pretty deep- but with consequent scattering and refraction that is best solved by printing thinner layers.

In the more opaque resins I would venture that the pigments are magnifying diffraction- but that the actual penetration of the lase has significant dropoff due to the pigments’ opacity.

Again- these are just how I think the thing works based upon my experience and basic education in physics and chemistry.


#5

Hm interesting. Yes by exposure time I meant either rastering slower, or going back over certain areas again, and assuming that the light scatters through the resin (which I believe people term “bleeding”), a longer exposure time will lead to increased energy absorption caused by scattering, which was originally my hypothesis for why there was some noticeable curing in between the thin walls at 0.1mm but not at 0.05mm.

This is a pretty interesting optics discussion. I admit I don’t have that strong of a knowledge in this space.


#6

Just wanted to hop into the conversation here! The bleeding you describe is absolutely the most likely culprit for losing some feature detail in a very small part like this. The beam of the laser is very small, but it still diffuses a tiny bit which can appear more readily in pieces like this.

One thing I’m actually asking our engineering team about is why this would appear more at .1 than .05mm. Typically we’d expect .1mm to produce the cleanest results simply because it’s just doing less passes. If I hear anything back I’ll post here!


#7

Great, thanks DKirch! My hypothesis is that the exposure time is less for each layer at 0.05mm, but it would be great if you got some internal feedback on this. Cheers.


#8

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