Minimizing "Sag" in holes in 3D printed parts...

[JeffB]

For those of you with more experience than I...

Any tips or "magic sauce" for minimizing or eliminating "sagging" in holes in printed parts.  Things like shaft holes, or axle holes or coned bearing surfaces in trucks.

I get a lot of sagging in the bearings and bores of such holes.  Admittedly, I've not experimented much with different resins (Elegoo Prime Grey and Siraya Tech Fast Grey are my current resins of choice), or exposure times.  It probably comes down to those two variables more than anything.

So...  Any tips you guys in the know could share would be appreciated.

Jeff

[Chris333]

I recently watched a video comparing resins that said there basically is not one resin better at detail than another. Not sure if a sag could be considered a detail. I have always reamed out holes to the correct size. Sometimes before curing while still a little bit soft.

[Lemosteam]

I do all of my supports in Lychee which has the ability to create thin wire supports, model surface to model surface, or model surface to support.  For anything larger than 2mm, I use a thin 0.4mm diameter support inside the hole, top to bottom, to keep the hole round (more if the cylinder is longer than 3mm).  I have also added a similar support in CAD before slicing.

Like Chris, I also ream out the hole to proper size prior to cleaning and curing.

[AlwaysSolutions]

All right - here's the secret sauce.  Some of it is geometry, some of it is viscosity, some of it is resin.  Start with geometry - imagine a 5mm thick "wall" that you want to bore a small hole through.  Good luck with that.  How about the same sized hole though a 1/2 mm thick "wall"?  Yes, please!  The concept of thin geometry between gaps helps for a lot of things.  Thinner walls allow your resin to slide off between layers as opposed to pooling on a "shelf".  Now combine that concept with viscosity and you'll realize that you want a runnier resin (warm) versus thick (cold) to facilitate that runoff.  Here is an illustration of how you can achieve a thin wall for your hole within a thick wall.

Front side - same dimensions visible for both objects - - the left one will have a "saggy" hole:
[ Guests cannot view attachments ]

Back side - you did some magic on one to thin out that wall where it matters:
[ Guests cannot view attachments ]

When I build a model I think of what's visible to the modeller, and what's hidden from the modeller.  It's in the hidden areas that you do stuff like this to manipulate the physics without interrupting the outside visual goal.

For your bearings, since it's already presumably a conical shape you may have better luck working the viscosity angle.  Try warming up your resin so it's nice and runny and see if that helps improve it?

Anyway, this is stuff that works for me.  I still encounter issues where the resin does something funky and then I have to figure out how to work around it but I'm having less and less issues now days.  I hope this is helpful.

Cheers -Mike

[JeffB]

Chris, John and Mike...

Thanks for your help guys!! 

I actually use a few of the suggested tricks.  Also ream out holes, but if it sags too much, the hole center can shift, which is more critical when you're printing a gearbox or something where center to center on holes makes a difference.

Going to try the warm resin trick very soon.  Getting set up in my garage to clean prints (changing over to Denatured alcohol for cleaning and it stinks too much to use in my workshop), will set up my Mini-4K out there too (it's been moved off the bench to make room for my Mini-8K) out there and do some experiments with it.  Being very warm out there (it's summer after all), it might make the difference.  But I do wonder if it's too warm, maybe not for the resin, but the stepper motor or electronics might object.

Thanks again...  Will report back on what I find.

Jeff

[Chris333]

https://www.youtube.com/@3dtvb

I'm sure warmer resin is better and pours better. But no idea how hot it needs to be.

[AlwaysSolutions]

https://www.youtube.com/@3dtvb

... But no idea how hot it needs to be.

Good point as I forgot to mention how warm... please definitely not boiling or anywhere near that hot.  Since I've never measured the temp I'd just say hot enough so that it sloshes around fluidly - it'll never flow like water but that's the idea.  I would guess around 100 deg F / 40 deg C.

-Mike

[GGNInNScale]

I have not seen sagging in holes, although I rarely do more than a few mm.  I do have the advantage of "runny" as garages in AZ are well over 100F all summer (maybe 115??).  So I do see that the resin has a lot lower viscosity than in the winter, so it flows "better".

[John]

In the winter time I use one of those coffee cup warmers to bring the resin up to about 70F . the printer does the rest as it's printing and will quickly raise the temperature

[Lemosteam]

All right - here's the secret sauce.  Some of it is geometry, some of it is viscosity, some of it is resin.  Start with geometry - imagine a 5mm thick "wall" that you want to bore a small hole through.  Good luck with that.  How about the same sized hole though a 1/2 mm thick "wall"?  Yes, please!  The concept of thin geometry between gaps helps for a lot of things.  Thinner walls allow your resin to slide off between layers as opposed to pooling on a "shelf".  Now combine that concept with viscosity and you'll realize that you want a runnier resin (warm) versus thick (cold) to facilitate that runoff.  Here is an illustration of how you can achieve a thin wall for your hole within a thick wall.

Front side - same dimensions visible for both objects - - the left one will have a "saggy" hole:
(Attachment Link)

Back side - you did some magic on one to thin out that wall where it matters:
(Attachment Link)

When I build a model I think of what's visible to the modeller, and what's hidden from the modeller.  It's in the hidden areas that you do stuff like this to manipulate the physics without interrupting the outside visual goal.

For your bearings, since it's already presumably a conical shape you may have better luck working the viscosity angle.  Try warming up your resin so it's nice and runny and see if that helps improve it?

Anyway, this is stuff that works for me.  I still encounter issues where the resin does something funky and then I have to figure out how to work around it but I'm having less and less issues now days.  I hope this is helpful.

Cheers -Mike

This might work well for sagging and visual holes, but if you are trying to design functional bearing areas say on a locomotive truck with gears, driveshaft support, etc., the thinner wall might not be enough for that.

@JeffB , yeah, the hole moving off center is an issue.  maybe a smaller hole with lead in chamfers on both sides of the cylinder to center the drill where you want it, and then gradually upsize the drill to the intended diameter.  I actually like to use spherical engraving bits instead of two flute twist drills to avoid breaking the part when the dill pierces through the other side,