1

STL GENERATION Written by Greta DAngelo

In this section: An introduction on .stl files How to make good .stl files How to make .stl on different softwares (Solidworks, PTC Creo, Rhinoceros 5, Autodesk Inventor) Summary

INTRODUCTION TO STL STL is the short name for Standard Tassellation Language, and represent the standard file format used for

Additive Manufacturing production. It is a triangular representation of a 3D surface geometry. That means

when a CAD model is exported to STL format, the surface is be subdivided in a triangular mesh as shown in

Figure 1. Each triangular facet is described by its normal and three points representing its vertices. While the

aspect ratio and orientation of individual facets is governed by the surface curvature, the size of the facets is

driven by the tolerance controlling the quality of the surface representation in terms of the distance of the

facets from the surface. Namely, the outward normal is the vector indicating the boundaries of your object,

suggesting to the software which is the outer part of your geometry and which is the inner one. An explanatory

drawing of the orientation of a facet is shown in Figure 2. Vertices are listed following counter clockwise

direction.

FIGURE 1: EXAMPLE OF SURFACE TASSELLATION

FIGURE 2: STL FACET

As shown in Figure 1, the more triangles are applied to the surface, the higher is fidelity to the CAD model.

However having an incredibly high number of facets is not always synonymous of good quality STL. Indeed, a

2

higher tessellation leads also to a longer processing time of the file and in some cases, when the file is too big,

the software cannot process it at all. Therefore it is very important to find a good compromise between the

highest definition with the minimum file size possible. To consider as well, is the resolution of the machine.

Sometimes it can happen that the machine does not have the same high resolution that is given to the file, hence

it will not be able to perform it. Therefore having a high resolution file for a low resolution printer would be

worthless.

HOW TO MAKE A GOOD STL FILE Often the quality of the STL file is also determined by the quality of the CAD model. Hence there are few steps

that need to be taken care of while 3D modeling, and in the end few others while exporting. All these can be

summed up and listed in seven main settings that will determine the quality of the file.

1. Angle deviation and chord height

2. Surfaces

3. Wall thickness

4. Normals

5. Multiple shells or nested parts

6. Edges

7. Tabbed areas

Paying attention to these critical aspects before exporting to STL is very important since once the 3D CAD

model is converted to the new format it cannot be changed or modified anymore. This is also why it is highly

recommended to save a copy of the file AND the STL separately.

ANGLE DEVIATION AND CHORD HEIGHT When exporting to STL if the part is rougher or smoother than expected, it is possible to change the angle,

deviation and chord height to create the right surface finish. As said before, an excessive faceting is not

synonymous of good quality and it is of crucial importance that the machine resolution is taken into account

when doing this operation.

SURFACES Make sure that in the CAD geometry there are not any missing surfaces or holes of any kind. The solid geometry

has to be water tight when exported. Some of the 3D modeling software such as Inventor, Solid Works and

PTC Creo produces good solid geometries consistently. Some others software based on surfaces modeling,

such as Rhino, need to be used with more attention. Therefore the modeling should be done with an eye of

respect for naked edges especially.

WALL THICKNESS When modeling it should always be remembered that the STL only works for solid geometry and not for

surfaces models. Hence it is necessary that all the walls of the model have a relevant thickness, even if it is very

small. Also, the assigned wall thickness should match the resolution of the machine that is intended to be used

by verifying which one is the minimum feature size that the machine can handle.

NORMALS As said before, the triangular faceting of the surfaces is described also by normal. The normalsa are the

indicators of the direction of the object. With other words, according to the direction the normal vector is

3

pointing at, the inner side and the outer side of part is defined. Hence, in optimal situations all the normal are

pointing outwards. In some cases, especially when overlapping surfaces are present the normal are

mismatched. To verify this condition is necessary to scan the STL file with a software that will automatically

detect the error and eventually correct it.

MULTIPLE SHELLS, NESTED OR TABBED PARTS The production of multiple parts or parts for assembly can be sometimes problematic, therefore it is always a

good thing to save each individual part as a separate STL file. They will be then managed by a technician and

eventually put together on a same building plate afterwards, according to which machine is going to be used.

For example, if PTC Creo is used do not export the STL from the assembly mode. The main reason for this is that

the outcome will be overlapping surfaces, and some machines reject this option. For objects with embedded

mechanics it is recommended to consult the technician to define an optimal solution.

EDGES Especially in Rhino, when trimming an object, or when create a new one, some edges can be left behind in the

model. Be sure that you avoid them. They can create confusion during the processing of the file.

4

EXPORTING

SOLIDWORKS When starting modeling note that the orientation of the building platform is the XY plane corresponding to the

FRONT PLANE in Solidworks.

Always check Merge result on the left side bar is enabled when doing new features. If this option is disabled,

the new features created will be new bodies in the part. Before exporting to STL there should be only one

body in the file.

After the part is designed it is now possible to proceed with the exportation of the file to the STL file. However

we suggest to also save a copy on the original file format of the software in case you have to modify your part

later on. File > Save As > Select STL > Options.

5

Before proceeding with the saving operation, it is recommended to check the option panel first. In the dialogue box

make sure that the output is set on Binary, and the scale is Millimeters, or just the same scale used to create the model.

IMPORTANT FOR MAC USERS: Make sure that output is set on ASCII and not binary. Otherwise Cura will fail

during the slicing.

In the Resolution box the deviation should have a value between 0,02 and 0,05. The angle instead should be between

15o and 20

o.

When exporting from an assembly, make sure that the option Save all components of an assembly in a single file is

DISABLED, and all the components will be saved individually.

6

EXPORTING

PTC CREO (PRO/E) When start designing note that the TOP VIEW in the software correspond to the original orientation of build

platform of the machine.

The path for exporting the files is: File > Save AS > Save a Copy > select Stereolitography (*.stl) > OK

FIGURE 3: SAVING STL

7

After the OK button is selected, a dialogue window appears on the right side of the screen. There make sure that

the format is on Binary, and in the Deviation Control the Chord height is around 0,05 and the Angle control is

around 0,5. When clicking on Apply it is possible to see a preview of the triangulated mesh. Usually the default

values are acceptable for a good print. However optimum values depend on the geometry, surface and size of

the modeled object. Moreover as said before, also the resolution of the machine should be taken into account.

8

EXPORTING

RHINO5.0 One thing to check before start modeling with Rhino is the tolerance. That must be used to export the part to a

specific program. Tolerance settings in Rhino can be found in the Document Properties window in the Units tab.

Moreover when exporting make sure that:

The object does not have naked edges

The 3D model to export is not a bad object

To find more information in regard we suggest do download a more detailed guide at this link

http://www.design.upenn.edu/files/preparing-3D-print-files.pdf

When the object is ready for printing, from the top tool bar > File > Save As, then assign a name to the file and a

destination. Make sure STL is selected as final extension and then press ok. As always we recommend saving an

original file, plus the STL copy.

FIGURE 4: SAVING STL

If you are using Rhino and you are interested in knowing more about it, then you should have a look to the

interesting guide by downloading it from free at the following website:

http://design.umn.edu/current_students/leo/hall/documents/Rhino_3DP_prep.pdf

9

EXPORTING

AUTODESK INVENTOR PROFESSIONAL 2013 To export STL files from Autodesk Inventor Professional 2013 go on the top icon on the left and select >Save a

Copy.

Subsequently a dialog window will pop up, where it is possible to rename the file and select the right files

extension. Select type file: STL file (*.stl).

10

VERY IMPORTANT in this case is to select the Options button in the same dialogue window that will lead to

another dialogue window, where it is possible to change some of the saving settings. It is fundamental that

when exporting to make sure that the unit scale is the same used to make the CAD model. For example: in this

case, the drawing was made using a millimeter scale. Therefore, the exporting scale should be in millimeters as

well and NOT in centimeters. The negligence of this step will result in a model 10 times smaller, or in a

difference in size according to the difference of the units used.

In addition, from this dialogue box it is also possible to decide on the quality of the STL file. Once done, press Ok

and then Save.

11

SUMMARY

STL is the standard file format used in 3D printing An STL file appears as a triangular mesh of the CAD model and determines the resolution of the print It is possible to generate an STL file with nearly every CAD software In order to obtain a good STL file is important to observe the following conditions:

Angle deviation and chord height: adjust the size and the number of triangle to increase or decrease the resolution of the STL

Surfaces have to be water tight and geometries have to be solid. Do not forget holes or naked edges

Give a wall thickness to every feature. STL do not work for single surfaces Make sure there are not overlapping surfaces on the design. That leads to inversed normal

which is always good to avoid. (check and correct the inversed normals with Netfabb STL VALIDATION section)

Save every component of the design in a separate file. Do not print STL made from an assembly mode. If you have embedded mechanics consult a technician

The resolution of the STL file has to be made according to the resolution of the machine as well. Having a file with a higher resolution than the machine used is pointless

When modeling make sure that the part does not exceed the overall printing dimensions of the machine Always save a copy of the CAD file and a copy of the STL. Once made the STL cannot be modified.

Therefore if there are some mistake to correct you will have to build again the model from scratch. Mac users on Solidworks. Set the output on ASCII instead of Binary. Otherwise Cura will fail slicing. Always make sure that the units used for modeling are the same used for the exporting. This is critical

especially in Inventor.