3D Printing and "Watertight" Digital Models

[fog37]

Hello,

I understand that CAD models (stl files) suitable for 3D printing must be "watertight" and not leaky in the polygon mesh. By analogy, if we filled the model with water, the water would not leak...This concept of watertightness means that there are no unintentional holes between the triangles forming the mesh or the connection between the triangles is not "good", correct? There are softwares that can check the health of the mesh...

But just to be clear, a 3D print model can be designed to have, for examples, holes or windows on its sides, correct? We are talking about undesired holes that can happen because of poor design or mesh complexity.

Also, another obvious requirement, is nonzero thickness walls for the model...

Thanks!

 

[FEAnalyst]

Imagine that you:
- put your model in water - it’s watertight if its volume doesn’t get filled with fluid
or:
- fill your model with water (like you said) - it’s watertight if the fluid can’t leak outside
You can have holes in the 3D model but they have to be bounded by surfaces.

 

[fog37]

make sense...Thank you. All clear on watertightness...

FEAnalyst, I have a segway question: when the CAD model is saved as .stl, the file only contains the coordinates of the three vertices+of each one of the tiny triangles making up the mesh. There is no information about the size of the model. However, when the stl file is opened in the slicer, the model appears with the same size it was designed in the CAD software. How is that possible?

Thanks!

 

[FEAnalyst]

STL file contains only surface normals and coordinates of vertices. These are defined in Cartesian coordinate system. Such information is sufficient to describe the model with its proper dimensions (since it consists of triangles with known vertex locations).

 

[fog37]

I see.

My dilemmas originated when I read the following on Wikipedia and other sources:

STL files describe only the surface geometry of a three-dimensional object without any representation of color, texture or other common CAD model attributes. The STL format specifies both ASCII and binary representations. ... STL files contain no scale information, and the units are arbitrary.

 

[bigfooted]

units are arbitrary.

When your model contains the triangle (0,0),(1,0),(0,1) then we do not know if the straight sides are 1 meter, one foot or 1 lightyear. A cad program can open and display it but it also does not know what units it has. It will just look at the minimum and maximum values in the file to show the entire geometry and show a scale. If you are going to print it, you have to tell the printer somehow how large '1' is.

 

[fog37]

Thanks bigfooted. I feel like I usually created a model in Tinkercad, save as stl file, open the file in Cura where it appears with the right dimensions, i.e. the dimensions it had in Tinkercad before I saved as .stl...

I don't recall setting any unit in the slicer...

 

[FEAnalyst]

CAD programs often use default unit system. Check if it’s the case for your software.

 

[Mason_Jays]

At first, I think it's better to understand about 3D modeling methods even very briefly
for Everyone who want to print the model from 3D Printer and who wants to draw models for 3D printer
you should understand about how STL format file works, and about other 'format'
Let's think about that you are designing piece of 10x10mm rectengular box in 3D Model.

You may consider which CAD or Modeling tool is better for Draw a Box?
In this case i will choose CAD Program for Modeling because it is a way easier to input dimension to BOX model.

Also CAD Tool is based on Solid Modeling.
Means if i draw a 10x10mm box and look inside of it is completely solid (watertight)
Now if i make 5 Pie hole's which going down for 5mm on top of the boxes
than still the box model is solid because it's solid model.
And you could even see solid surface with sectional view

Lets think about other Modeling tools like Sketchup
i often say sketchup is kind of surface modeling tools.

You can see it is complete same with Rectengular box model above
Only just outside of the box.
If you Cut right middle of the box you can see it is empty in side.
This is excellent demonstration of STL format and Solid type 3Dmodel format

The reason of 3D printer requires Water tight model is that.
If it is not watertight, the boundary of wall and inner volume of model is not clearified.
and this will be trouble when generating gcode,
So 3D printing Model need watertight and boundary clear model.

 

[fog37]

At first, I think it's better to understand about 3D modeling methods even very briefly
for Everyone who want to print the model from 3D Printer and who wants to draw models for 3D printer
you should understand about how STL format file works, and about other 'format'
Let's think about that you are designing piece of 10x10mm rectengular box in 3D Model.

You may consider which CAD or Modeling tool is better for Draw a Box?
In this case i will choose CAD Program for Modeling because it is a way easier to input dimension to BOX model.

View attachment 269098
Also CAD Tool is based on Solid Modeling.
Means if i draw a 10x10mm box and look inside of it is completely solid (watertight)
Now if i make 5 Pie hole's which going down for 5mm on top of the boxes
than still the box model is solid because it's solid model.
And you could even see solid surface with sectional view
View attachment 269103View attachment 269104Lets think about other Modeling tools like Sketchup
i often say sketchup is kind of surface modeling tools.View attachment 269105View attachment 269106
You can see it is complete same with Rectengular box model above
Only just outside of the box.
If you Cut right middle of the box you can see it is empty in side.
This is excellent demonstration of STL format and Solid type 3Dmodel format

The reason of 3D printer requires Water tight model is that.
If it is not watertight, the boundary of wall and inner volume of model is not clearified.
and this will be trouble when generating gcode,
So 3D printing Model need watertight and boundary clear model.

_

Wow! Thank you Mason_Jays!
On a similar note, I have a doubt about wall thickness in the digital file and printed wall thickness.
For example, I create a box in CAD (see picture below):

The wall of the box is set to have a certain digital thickness, say 2 mm. After importing the stl file i the slicer, we can set the number of shells/perimeters. What would happen if we chose 4 shells?

In general, I read that the overall wall thickness should be a multiple of the nozzle diameter. I guess it cannot physically be less than the width of a single layer which is ideally 1.2X the nozzle diameter.

Will the 3D printer print a 2 mm wall thickness for the box (what we designed in the CAD) or will it create a wall made by 4 shells for an overall thickness larger than 2 mm?

 

[Mason_Jays]

_

Wow! Thank you Mason_Jays!
On a similar note, I have a doubt about wall thickness in the digital file and printed wall thickness.
For example, I create a box in CAD (see picture below):

View attachment 269108
The wall of the box is set to have a certain digital thickness, say 2 mm. After importing the stl file i the slicer, we can set the number of shells/perimeters. What would happen if we chose 4 shells?

In general, I read that the overall wall thickness should be a multiple of the nozzle diameter. I guess it cannot physically be less than the width of a single layer which is ideally 1.2X the nozzle diameter.

Will the 3D printer print a 2 mm wall thickness for the box (what we designed in the CAD) or will it create a wall made by 4 shells for an overall thickness larger than 2 mm?

Since regular low cost 3D printer has poor precision quaility you may dissapoint on the overall quality .
The slice software (Cura,ETC...) will generate gcode which has same diemension with 3D model.
at this point even if wall thickness option has set to 4mm, slice software will recognize seperately from the orignial 3D model and it will not exceed from 2mm wall from orignal one.
However in the view of practical,precision use of 3d printing is limited because there is so many environment variable like nozzle hole diameter, air flow, temperature and humidity, even the materials status.
On my opinion wall thickness option is for big model, which need to be more hard, or less hard
so it will not affect anything for your models in process of printing