March 2014. Why Bastion is a swear word. You'll see.
There is a point when you have to stop and think about what will come next. This was the point at which I decided to try some bigger models. The biggest will usually be a piece of terrain. As usual I like to start with something simple, so I decided on a Bastion, a Pentagon shaped bunker like building.
The model was finished shortly after I had been experimenting with FunToDo resin (left model). I had tried to use it with high resolution models and wasn't really content, but thought I give it a chance at 100µm. The model was ok, but not good enough, so I printed it with Spot-HT and everything was fine. But when I offered to print one for a friend, disaster struck. The prints all had holes like the model on the right.
I tried longer settle times, longer breathe times, slower open and close speeds, different rotation, I even modified my iPad software to allow for a post-cure-time. Nothing. Every print takes between 8 and 11 hours, so an early fail was still a success, because than I did not have to go through all those hours until I realized that the print failed.
8 tries later I remembered a thread in which somebody stated that protecting a "holy" side with a wall will prevent the holes. I tried and it looked good, except that the holes showed on the other side.
So I did a wall on the other side as well. It looked good until 10 hours 48 minutes into an 11 hour 3 minutes print and than the right wall broke free. I managed to get it out of the Vat, just to have the left wall break down 3 minutes later, finally ruining the print.
I was nearly at the end of my tether and began to run out of swear words.
Looking at the print showed it was only nearly flawless. Probably 98%. So I thought again what the reason for the holes could be. The current wisdom is some time of wave, which the walls are supposed to break.
But a wave should subside when the settle time is increased. But it didn't. Still it has to do something with the flow of resin, or the wall wouldn't make a change.
The main difference between this model an others is that it has an enclosed room in the middle and as it is mounted directly on the build platform the only way for resin and air to get in and out is through the bottom.
But what will happen if the bottom is below resin level all the time ?
There would be a vacuum and this will - I have no idea your guess is at good as mine. Wether the vacuum sucks the wall in or generates a steady flow of resin, that doesn't come to a rest for a long time, I don't know.
Probably there is resin streaming in while over the deep end of the Vat and when coming back over the shallow end it creates some pressure inside the model. Hpffffff.
I've stopped to care.
The solution would be to set some holes deliberately or put the model on supports allowing air to get in and out between them. And that is what I did.
It grows, if you look at the top of the model (in fact the bottom, but the model grows upside down, so it's at the top) you can see the supports.
To make the long story short, it didn't help. There was a tiny hole on one side and a lot of holes on a side that didn't show any up to than.
I have still one to three ideas. If they don't work out, I will break the model up and print it in separate pieces. Not what I want to do, but at some time you just have to play with the cards your dealt.
Not to waste all of the misprints - probably 50€ in resin wasted, I made a ruin from the most miserable of the prints.
On the left the first design, with the large door, big enough to allow an stalker to enter the Bastion. On the right side the newer design with the smaller doors for infantry only.
The perspective is a bit unlucky. The battlements reach just under the breast of an based model.
What finally did it is:
a strengthening of the Bastions walls from 2 to 4 mm
the post cure
two breaker walls
turning the model, so that the flat side is not closest to the Vat's walls.
I printed two Bastions in a row and they came out fine. I'm not willing to waste the resin and time to find out wether any one of those didn't contribute to the success.