What is causing these stripes and tears?

[bbussey]

It depends on how high the build plate lifts in between layers, and how much resin is in the vat. It is unlikely, but possible.

[Erock482]

I didn't realize that air could get trapped inside the printed part. I thought it was immersed in resin.

Imagine you're printing the gondola flat, and remember that if you print it upside right, the last layers to be printed are the top ones.

So the layers after you print the solid floor, have 4 sides. as the build plate lowers, it creates an air bubble, which as the plate compresses down will allow some resin to compress the air, but the resin being a liquid is non compressible.

Now once you try to lift this after curing, it's just like trying to un stick a plunger. a bit of suction is created.

All of this causes the resin to move more then it normally would, between air trying to escape and resin trying not to be compressed. causing the resin to move and thus the weirdness in the layers. Or for the model to be ripped from the supports entirely because the suction is too high.

This is why file validators like UV tools have a suction cup detection mode. to find pockets that might create issues and introduce drain holes to let liquid resin or air escape or flow back inside during the movement of the building plate

Basically, for the layers in which you create a sealed cavity, so for the sloped version until the 4th wall disappears, and for the flat version the top of the gondola ends, there are all of these forces at play as the volume of air/resin inside the model changes and adjusts. In a way it's like mold flow analysis. you don't want to create traps for resin or air to get stuck.

[Chris333]

I understand what and why of what you're saying, but I have never put in a vent hole.

Printed flat:

[peteski]

Imagine you're printing the gondola flat, and remember that if you print it upside right, the last layers to be printed are the top ones.

So the layers after you print the solid floor, have 4 sides. as the build plate lowers, it creates an air bubble, which as the plate compresses down will allow some resin to compress the air, but the resin being a liquid is non compressible.
. .  .

I understand what you are saying, but the printing starts and continues fully immersed in a vat of liquid resin.  We are taking about a small N scale car. Where is the air coming from? Doesn't the vat have at least 1" of resin in it?  It is not like the model gets fully lifted out of the resin into ambient air after each layer gets printed.

[Erock482]

If the lift height was so that it was completely lifted out of the vat and resin, thats where your air would get in.

I have mixed results with gondolas, sometimes I get the artifacts like shown here, other times I don't like in Chris' example. I've seen this occur in other conditions as well.

The added vent hole helped my results, and I've seen similar methods used across other 3D printing forms combatting similar issues.

[peteski]

If the lift height was so that it was completely lifted out of the vat and resin, thats where your air would get in.

Now that makes sense. But I thought that for small shallow items like gondolas the printed object would stay fully immersed in liquid resin.  But at least now I understand where you coming from.

[Lemosteam]

@peteski, each light cycle requires the build plate to cycle up and away from the FEP (the membrane between the LCD and the build plate.  Each cycle, the build plate has to draw the pervious layer off the FEP (which acts like a drum) until the layer separates. this is one form of suction.  The slicing software allows the user to determine the distance and speed at which the plate lifts to do this.  Once the layer separates and pulls away in Z, the resin needs time to re-level itself in the vat. if the build plate lifts to little, the print will not pop off the FEP, or if it pulls away to quickly, it can tear the model (I suspect that here).  Also if the build plate returns too quickly, the resign may not have enough time to re-fill the area of the next slice within the vat, also possibly trapping air.  Ths is a factor of the lift off distance and the viscosity of the resin.  Ambient temp, and age of the resin comes into play as well.

The challenge with resin printing is to find the right balance with many variables, and if you switch resins you start all over.

These many variables quell the fun, but like playing golf (many variables), one good golf shot (print) can make your day, but the rest of the time it can be miserable. LOL.

[bbussey]

Once you have everything calibrated though for the various resins though, having found the "sweet spots", you can crank stuff out.  For example, Chris can pump models out with little to no trouble.  I'm getting close to that point with prints of new models.  The last new model I printed needed two attempts, but due to a design flaw rather than a print setup issue.

[Chris333]

If it helps at all here are my settings right now.

[Jesse6669]

Just curious if the printer is on a solid workbench, and you aren't doing jumping jacks adjacent while it's printing?  May be obvious, but I'm always paranoid even walking near the printer when it's exposing a layer.
Is the build plate the standard plate or one of the magnetic ones?  If that plate moves even a hair it will cause stuff like that.  OTOH I use a magnetic plate myself and haven't had any issues.
Jesse

[garethashenden]

An update. I've been trying to troubleshoot this on two different forums and I've received conflicting results, which is kinda why I asked. There is agreement that the models are too close to the buildplate and that I'm having suction issues. The solutions are where the disagreements are, which shows that there's more than one way of doing things. I did find that the screws that hold the buildplate on were a little more than finger tight, but not tight tight. They are now. I've only done one print since then and it is improved. I started with both models 10mm above the buildplate. For the cattle wagon I angled it up 60°. For the open wagon I put it at 135°, or upside down and then 45°. The sides look fantastic on both of these. Nice and smooth with good detail. Perfect! The ends have lots of steps. The open is worse, but its there on the cattle wagon too, particularly on the side that was facing the buildplate.



For my next attempt I will print them both flat and upright. I'm going to copy Chris's lifting speeds. That's not something I'd considered before, its always just worked on other printers. Here are my current settings:

[Chris333]

One of my machines had a bunch of loose bolts right out of the box so I sorta got into the routine to check them all. Plus I usually tear it apart anyways 

Yeah everyone prints differently and some try to get the speed down to the last millisecond. Me I'm just happy when my prints come out and leave it alone after that. I doubt my settings are perfect, but it's what my machine is at right now and it's working.

Most model railroad items will benefit from being printed flat or standing straight up I think. The tilt is good for rounded items. Tilting a square object will induce stepping unless you get the angle just perfect.

I print steam locos at an angle (23 degrees) so the boilers will come out smooth:



No boiler it goes flat:


Your gondola tilted like that will get a ripple at the layer where the lower end stops printing because the FEP tension will change. This is even if the printer is working perfectly. But as for all the other ripples and lines I really don't know for sure what is causing that.

[sd80mac]

Hi All,

Check out this video. It discusses print angle, and the math behind it. It really opened my eye to finding the proper orientation for our printed models

Cheers,
Donnell

[bbussey]

As I’ve stated upthread, you’re going to continue to have stepping until you set the build angle to the arctan based on your pixel height size and your slice layer height. Your raft is too thin as well based on the screen shot. The 15 on your bottom layer count determines how many layers are exposed at 40 seconds, not how thick the raft is.

[Erock482]

I've had very good luck with models printed flat to the build plate, about 6mm off the bottom. I try to avoid sloping prints for the same reason you've discovered, you compromise detail on whichever side is facing down.