Back of head keeps cracking and splitting? (3d model ;)

Hey all, I sent a head model to a friend to print on the Form 2. First thing I noticed is the back of the head nearest the platform cracked. I then made the walls thinner in the head but the back still cracked. I would have thought this orientation would have been fine as I wouldn’t know what other position would be best for printing this. Anyone know why this is happening?

Here is the first print. Similar support setup but with less supports and no supports to the side. Walls were slightly thicker. The cracking looks much worse on this one.

I had a similar issue with cracks on bottom flat surfaces. Seems it was a lack of division, triangles, as large surfaces don’t have them. Does your back of head have a reasonable amount of triangles?

Hi Walter, yes there is a dense amount of triangles. I think some of her other prints cracked. Could it be something to do with the peeling process? I doubt this would have happened on the form 1 as I have printed similar heads in this position on the form 1 and no cracks at the back like this.

I would orient this model almost vertically, so the open end of the neck is parallel to the build plate. All the support touch points (with maybe the exception of one for each earlobe) would be on that bottom-most neck “flange”. It’ll take a bit longer to print because it’ll be more layers. But you won’t have nearly as much post-print cleanup after you remove the supports.

Hey Randy, yeah I’ve been thinking of doing that also. I’m not experienced in the Form 2 though with the Form 1 printing a head like this vertically would result in many peel lines. Do you think this will be th case with the F2? Still not sure why it’s cracking though. Could it be something to do with internal supports?

When models have hollow cavities like this, it’s important to add a drain hole for resin to escape. The drain hole should face the build platform so that resin always has a path to escape. Otherwise, there’s a suction cup effect between the tank and part which can put excessive force on the model during the peel movement and cause the cracking you’re seeing. Let us know if adding a drain hole corrects things for you!

The issue I see there is that your supports are all in a line which could possible cause a stability issue, try arranging them more spread out on that side to increase stability.

I think Frew is on to something here with the captured hollow. Looking at your section view there are times when you are definitely creating suction because you have no vent to the hollow cavity. You are also creating a cupping pressure when the build comes back down - it can also lead to troubles (thin walls flex out due to the increased pressure).

Frew, themedulla: Thats a good point. There isn’t a hole in the back of the head for direct drainage, though there is a large opening hole inside the top of the neck hollow and small hole at the top of the head though like you say it could be creating the suction as it takes a while to get to the hole during print. Also what Zac mentions about the line of supports (something preform did) maybe best to spread those out more. Ok great so will add a hole at the back on the head next time :wink: cheers all!

Looking at the way you’ve oriented the head, I don’t think venting is a problem. Doesn’t look to me like there is ever a time where the print will form a sealed “cup”. But if you stand the head up on the neck, since the neck is open and pointed towards the build plate, that will unquestionably address this concern.

What I look for when setting up a print is how much “swept” area there is on each layer. I scroll through each layer in PreForm and look at how much resin is being cured. If I find a region where the cured area is much greater than in other regions, I reorient the part to reduce the “peak” peeling force resulting from the larger area region.

If I apply that logic to your part, it looks to me like the layer surface area is minimized with the head completely vertical. You might want to increase the support touch point size to be sure the print doesn’t break free (which won’t matter since the touch points will be on the bottom “flange” of the neck), and maybe also reduce the height above base from 5mm to 3mm just to get those supports a little shorter/stiffer. But in my experience, anyway, I would not hesitate to orient the head “upright” on the build plate.

Yeah I think you’re right Randy. This was how I was originally going to print it, though my experience with Form 1+ told me it wouldn’t print properly (peel lines) though F2 new peel system may eliminate that and give me better results.

You might want to up the support density to something like 150%. It’ll make the bottom of the neck a mess, but you can hit it with a sanding block to clean it up after removal. And the additional support density will help prevent the print from moving around which is what causes layer lines…

Cheers Randy. Does this look ok? The eyebrows are coming out red, not sure why preform thinks those would be a problem but definitely not putting supports there. The bottom is packed out pretty dense.

Try leaning it back just a little and I bet the red eyebrows go away. You might also get rid of the supports on the chin and nose.

But yeah, this is what I was suggesting.

How hollow is the head?

ah yes that’s a good idea will try that. the head is pretty hollow and walls are thin, possibly 1.3 mm not sure so should be easy on the peeling… hopefully.

Looking at the model again, I hadn’t realized that the resin might drain out the neck - good catch. Looking forward to seeing how the vertical orientation works out @FormBeast. I might recommend a slight rotation, 20 degrees back or so to make the nose and eyes point up a bit. This will reduce the need for supports on the nose and eye sections and clean up your bottom surfaces a bit.

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