Howdy,
So which are the physics related to generate tubes on hollow objects to avoid model explosion?
how many tubes do you usually add and why? pointing from where to where?
Thanks
I think you are refering to holes. If the object is hollow, it will work like a cup on a flat surface, creating a vacuum inside, so you will need a breather hole to relieve the vacuum.
Depending on the complexity of the object, you might need more than one, but in general, the rule of thumb is you should have a breather hole for every large cavity. For a large single cavity object, one breather hole about 2-3mm in diameter is more than enough.
Once the object is printed, cleaned and the supports removed, I use a toothpick to pick up some resin to fill the hole, and cure it with a 405nm pen laser. The resin cures in a couple of seconds, and then I add some more until it’s flush with the surface, do a few passes with the laser to fully cure it, and sand it if necessary.
Oh, regarding the location of the breather hole. First look for an inconspicuous location, somewhere where it either won’t be seen, or where you can easily access it to fill and sand it after you plug the hole. Then orient your model on the print bed so the breather hole is facing the bottom (the print bed). After generating the supports, make sure no supports are plugging the hole, and manually edit the supports around or away from the hole.
When the printer prints the piece, and the part get’s pulled out of the liquid, the breather hole will always be at the top, and air will be able to escape. Since the resin always flows out of the cavity during print, very little if any resin will be trapped at the end, and it will probably be washed out as during the IPA bath.
I print a lot of figurines and I always hollow the models out to about 1.5-2mm wall thickness. Most of them have just one hole, some may have 2, but that’s about it. and all my prints have been successful when using this technique.
P.S. The models don’t “explode” rather, if they are hollow and there’s enough vacuum created between the model and the PDMS, they can tear during the peel process, which makes them look like they exploded.
Edit:
Here is an example. This bunny model from Meshmixer is hollow, and has a hole at the bottom. I oriented only slightly angled, so the ears will need minimal support. In the image on the left you can see the supports generated by Preform, on the right, you can see the breather hole at the bottom, as it’s being printed.
The rabbit won’t drain. The ears are the bottom during printing and there needs to be a hole at that part somewhere. Otherwise the piece will fill with resin and drain for days or weeks. I know, I’ve made the mistake.
Always remember that the Build Plate is the top, not the bottom.
I use to make “tubes” not just holes, that is: connecting both ends, one to let the air go, and one to let the resin go, and orient the holes so they’re perpendicuar to the build plate, is that the closest to ideal?
Actually that is not the case. As the hollow closes up, the most amount of resin it can trap inside, is the amount that can fill the hollow up to the level of the resin in the tank, in the case of the bunny, that is so minimal, that if you were to just let it drain, it will simply coat the inside wall and never actually leak out. When you submerge it in IPA, the IPA will wash it out. I’ve never needed any holes at the opposite end of the model.
BTW, if you fill the hole and cure it as I described in my post, you won’t have to worry about leakage at all. Whatever tiny amount of resin remains in there assuming it doesn’t get washed out by the IPA, will simply stay inside, and no, it will not affect the model’s stability over time.
When I refer to it as the bottom, I mean as you see it in PreForm. That is why when I said bottom I put “print bed” in parentheses for clarity. And yes, it is the physical top, which is why said “When the printer prints the piece, and the part get’s pulled out of the liquid, the breather hole will always be at the top, and air will be able to escape”
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The only reason to use tubes is to inter connect multiple hollows/cavities. If the model contains only one cavity, there’s no need to do that.
What do you use to hollow out your model? Do you do it directly in the CAD program, or afterward with something like Meshmixer?
Liquid resin will cling to the hollow inside and not drain out. When you turn the piece over it runs to the bottom where the hole is and pools then runs out. As I said, I’ve had this issue on a few items in the past.
As to tubes, I don’t see the purpose. The idea of the drain and vent holes is to allow for the maximum void inside the piece. Thus using far less resin.
When I wash my model, I always submerge it in the IPA upside down, so that IPA flows inside it and fills it out, then prior to removal, I slosh it around a bit, so the IPA cleans the inside then it drains out along with the dissolved resin. I then let it air dry for about 15-20 minutes, while still on it supports (with the hole at the bottom, so any remaining IPA/resin will drain out on a towel. I have not had any issues with this, and the only thing that drains out is IPA.
Once the model is removed from the supports, I fill out the hole as described above and it’s ready for finishing.
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My method as well good breakdown now I need to get that laser to fill
Yep I use meshmixer to generate “tubes”… In the end since the model is hollow it is “almost” useless, agreed, but I’m more confident on how to use the tubes rather than the “holes”
