Prepare your model for 3D printing with Alias

This tutorial is written and designed to provide 3D printing enthusiasts with accurate instructions for designing 3D files that are “3D Printable”

By the end of this tutorial you should be able to:

Alias is a complete professional surface 3D modeling software, which uses ‘NURBS’ type surfaces to produce closed shapes, instead of solids. This modeling approach provides you with complete control over every point on your model’s surface and makes the dynamic editing of its aesthetic appearance possible. This 3D CAD software—Alias—is generally used by professional designers and engineers operating in the industrial and automotive industries. Alias releases a free CAD software package or sample kit for students and its free packages come with the same exact features that can be found in its commercial packages..

A basic understanding of Alias is required in order to get the most out of this tutorial. Autodesk provides lots of great videos and tutorials for getting started with Alias.

The version used in this tutorial is Alias Automotive 2014: You can download the student edition of Alias Automotive for free here.

Designing a 3D printable model

When starting out with Alias, as in any other 3D modeling software, it’s essential to keep in mind that modeling characters for 3D printing is different from your average modeling project for other purposes such as rendering or animation. Therefore separate guidelines need to be respected throughout the whole modeling process in order to obtain a printable model.

In this tutorial, a model for a bow tie with moving parts will be created, for printing using Sculpteo’s Plastic materials. It is also important to note that the Selective Laser Sintering (SLS) technique will be used in printing the designed model which means this tutorial will be centered on modeling characters suitable for SLS printing technology. The design guidelines for this and Sculpteo’s other printing services can be found on our Materials page.

Understanding the rules from the above link while modeling difficult operations can reduce the difficulties that comes with troubleshooting designs and mesh fixing. The most common mistakes that occur with non-printable models are the absence of wall thickness and the presence of non-manifold edges or vertices on the mesh. Next, you will see the step by step process on how to create a correct manifold mesh during the modeling phase.

Model for correct, manifold mesh

The term manifold defines a mesh in which all its triangular edges are directly and individually connected to one another and on a surface modeling software like Alias this means that all the NURBS surfaces need to be in a relationship of continuity with one another.

This is quite obvious as can be seen from the example below of intersecting a series of extruded ( alias) surfaces in order to define or create a simple solid shape.

Also note that the excess areas on the left and right surfaces are cut away ( alias) because they are clearly not needed.

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The same principle of cutting away excess parts also needs to be applied when unnecessary surface portions which are not visible from the outside are located inside your model. This means that all unnecessary surface portions—visible or not—need to be cut off in order to prevent the creation of a non-manifold, invalid mesh.

The 2 surfaces in the example are cut at their intersection: in this way continuity between them is created, allowing them to be stitched in a later stage.

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An example of the difference that comes with modeling a character that will be 3D printed and one that will be rendered will be shown and discussed below. A model designed for rendering –with a non-continuous surface--will simply be reproduced by a rendering engine showcasing your mapped surfaces as they appear on your software viewer/workspace, but a 3d printer would recognize these non-continuous surfaces—even if they are located on the interior side of your model—as walls with no volume which are impossible to reproduce in a physical world.

In order to obtain the basic shape of the bow tie, you have to create a mirror model of the original model you created by using the mirror function: [edit>duplicate>mirror] which reproduces a mirrored image of the initial surface. The mirroring feature creates two identical parts and an overlapping surface in the middle which every 3d printer will see as an error. It’s better to avoid the creation of these kinds of surfaces while modeling or to delete them when detected, simply selecting the surface and pressing “Canc”.

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Creating a patterned hollow on your model

Now that the main body of the model has been created, you can now proceed to hollowing its interior for the 3D printer.

There are several reasons why digital models for 3d printing should be hollow but the main reason is the amount of material that will be used to produce the model. In 3d printing—unlike other production techniques— the cost of fabricating an object is not dictated by the complexity of its shape, but by the amount of material that is required to produce the design.

Therefore, making your object hollow will positively affect the manufacturing cost of 3D printing, by decreasing it to less than 60/70% of its initial cost. Another important reason to design a hollow model, is keeping your product lightweight if that’s a property you are aiming for.

But just hollowing your model doesn’t affect your product, if the hollow part is not connected to the outside through at least two holes. The reason is that “unprinted” material would be trapped inside your model if an escape passage isn’t designed.

The Sculpteo online uploader has recently been enriched with automated hollowing feature that allows you to handle the hollowing process online. This feature automatically generates the inner shell of your model letting you simply choose the required hole positions on your model. You can upload your 3D file and try it now.

In many examples of successful 3d printed models, we have seen designers take advantage of the production driven restrictions associated with creating hollows or cavity patterns along a model surface which increases its aesthetic beauty while reducing the amount of material and therefore the production cost.

Next, we shall see how a patterned hollow can be made with Alias.

To design a hollowed pattern, you need to start by drawing a series of parallel lines that will be projected ( alias) on the surfaces of your object, which will then be used to cut ( alias) the surfaces and create patterns on them.

You must also take care that the thickness of the stripes you create are not lower than the minimum wall thickness for the Plastic material, which has been fixed at 0.8 mm before designing your model. Also note that, having an accurate knowledge of your model’s dimensions before creating them eliminates future dimension problems that may occur while printing on sculpteo.com because reducing dimensions directly on sculpteo.com negatively affects the entire model.

Anyway, a stick made with this material is rigid once it’s approximately 2 mm in thickness (check about Sculpteo's plastic material page for more information).

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After this operation, your model will lose its volume, so you need to add extra thickness by offsetting ( alias) all its surfaces.

The feature ‘offset’, located under the ‘object edit’ tab of the palette window, allows you to choose the distance for the new surface that has been created from the original one. Next, you should set a thickness of 1.55mm that would ensure the appropriate rigidity of your model, (assuming that it will be printed using Sclupteo’s Plastic material). When all your surfaces are offset, you then need to close the volume by connecting them using ‘skins’ surfaces ( alias).

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Moving and interlocking parts

The final shape of the bow tie’s body has been defined and the next step is to enable it to spin on a pivot when attached to a shirt and in order to do that you must create an interlocked component. This leads to the concept of ‘clearance’ and its importance to modeling moving parts in 3D CAD software.

Clearance is the space between two facing component surfaces that are in a relationship of relative motion with one another. It is important to note that every material has a minimum clearance value that must be respected, and the more clearance you use the better it would be for your model.

Also note that if clearance isn’t taken into consideration, it will result in the two surfaces of the component ending up fused together after they have been printed. To know more about clearance and how they relate to printing materials, you can check our Materials page.

Note: the clearance of the plastic material to be used in this tutorial, is fixed at a minimum of 0.5 millimeters.

Next, you intersect ( alias) a cylinder with a base diameter of 3.5 mm in the center of your bow tie and then offset ( alias) , its tubular face with a distance of 0.5 millimeters, — which is the minimum value allowed by this material— to create the necessary clearance space between the cylinder and the bow tie. This means that a clearance space of 0.5mm has been created on each side of the cylinder.

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When designing a model for printing, it is important to have a final picture of how you want the size of the printed model to be in your mind’s eye. This image would then help you determine the minimum thickness and clearance space the final object requires to function. Any designer who plans to make use of Sculpteo’s marketplace, must also ensure that their object is designed to meet Sculpteo’s minimum size for the public.

Mesure elements and distances

To ensure the solidity of your model and eliminate weak pressure points, you need to take care while completing the last trimming operation to check if the holes you made in the middle of your model are not too close to one another on the surface pattern that was previously created.

To be sure that you are not generating a weak point on your model (i.e. thinner than minimum material thickness) you need measure the distance between the two holes to be sure about your surface thickness.

You should use a simple ‘keypoint curve’ ( alias) , snapped to the closest faces of the two holes, and read its length on the information window.

This quick and easy method can be used every time you need to be sure about a wall’s thickness, a clearance value or any other measure on your model. In this case the space checked is acceptable, so you can go ahead with the model.

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Producing correct shells

To complete the second component of the model, you need to add two elements to the extremities of the pivot previously created.

This component is now made up of 3 closed intersecting figures that will generate a valid, watertight, mesh for this configuration. Nevertheless, this configuration is still not suitable to be printed as it would require a subsequent Boolean unifying operation in another mesh fixing software in order to make the object printable. With Sculpteo, technical errors that occur are automatically corrected by its algorithm but if Sculpteo’s automated repair tools can’t solve the problems, a new page that also provides more explanation about the repair tools used as well as the repair options available to you will be displayed. You can then select the exact tools you need to eliminate the errors.

Sculpteo’s automated repairs may sometimes modify your model during the repair procedure in such a way that the final product does not meet your requirements, and also, there are some errors Sculpteo’s automated repair tools cannot correct automatically such as a file having the same object in it multiple times. Which confuses automated tools for they wouldn’t know which object to select. When this occurs it is advised that you troubleshoot every error individually to find the right solution to them.

Closed intersecting elements belonging to the same component are generally called shells and to simplify the model, it is best to have each component of the model made up of just one shell. To achieve this, all you have to do is intersect ( alias) and cut ( alias cut) the unnecessary areas on these surfaces.

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Embossing graphics or text

The shape of the object is now complete and what is left is embossing the Sculpteo logo on top of your model. What you need to do first in this case, is to import a vector file of the logo in a DWG or DXF format, then scale and position it as desired over the chosen surface.

Next, divide through using the ‘Cut’ feature ( alias cut) and offset ( alias offset) the shape of Sculpteo’s logo keeping in mind that the minimum height for a clear embossed detail—like a text or 3D letters— is 0.8 millimeters for a plastic material.

It is important to note that printing curved 3D surfaces can lead to inaccuracies due to resolution. So please check for more information about the plastic material on Materials page.

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Color and textures for fullcolor printing

Alias is a great and complete tool for surface modeling but lacks advanced tools for surface mapping and an obj+mtl exporter—designed for exporting 3D files in full color—, which makes it not really suitable for producing models that will be printed using Full Color material.

Surface mapping is a technique that allows you to apply textures and images to a mesh, and .obj plus .mtl is the file format that permits you to upload this kind of model, with their relative textures, on Sculpteo’s website. Note that Sculpteo also accepts wrml file formats (check all the formats Sculpteo accepts).

Surface mapping can be done with other powerful CAD software applications such as Autodesk 3d software, 3D Studio Max, and also with the free Blender software.

Finally, the shape of the object should meet your expectations or requirements and we should be ready to prepare a printable mesh of this model.

Produce a mesh

The model that has been created so far is not yet ready for a 3D printer, and the reason is that it speaks a language alien to a 3D printer. So far, you have used Nurbs to define shapes but a 3D printer needs polygons made of faces, vertices and edges in other to understand a shape. Therefore, before meshing your 3D model it is recommended that you stitch all the surfaces together by using the ‘shell stitch’ feature (( alias) , ( ) located in the ‘Surface Edit’ tab.

This will make the subsequent mesh orientation process way faster because with this command, you will be generating two shells with coherent face orientation instead of many surfaces with random orientation that will need to be updated one after the other. This will also be discussed when we orient the mesh.

To create a mesh of the bow tie and generate a model made of faces, verticals and edges for exportation as an ‘.stl’ file, simply move to the ‘Mesh’ tab and proceed by selecting the ‘Nurbs to Mesh’ ( alias nurbs to mesh) , feature and apply it to the whole model.

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It is important to understand that the surface quality of a printed product is determined by a number of factors which include:

  • The quality of the designed mesh for the model
  • The number of polygons in a model
  • The resolution of the 3D printer
When dealing with polygons and the quality of a product it is important to know that the number of polygons in a mesh directly affects the quality of the final product. While Sculpteo accepts models with up to 1,000,000 polygons, anything over 250,000 is generally unnecessary and will just make your model more difficult to work with. To learn more about resolution, simply check our Materials page.

On Alias, the density of your mesh can be controlled by the ‘Maxedge Length’ value found on the ‘Nurbs to Mesh’ window with a default setting of 5. For this model, the Maxedge length was moved to 0.5 in order to obtain a denser mesh. It is also advisable to tweak your settings to learn more about mesh densities.

Analyse and export a mesh

Once a good density compromise that depends on the resolution of the material to be used has been found, it’s important to analyze it in order to improve its quality in case you are not completely satisfied with it or to find problems that would make it not printable. These may occur in the case you haven’t paid enough attention when following the modeling guidelines just described.

Merge your meshes

Before proceeding to the analysis it’s better to unify the two meshes you have in this stage, which was generated from the two previous modeled shells. In this way we have to deal with just one mesh instead of two and we also will be later able to export it as a singular .stl file. This can be done with the command Mesh Merge ( alias) .

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Mesh orientation

The next step is to check the MESH ORIENTATION.

Although it might be obvious to you, 3D printers can’t guess by themselves which is the interior or exterior side of your model, so you must be explicit about the sides.

The feature ‘Reverse mesh orientation’—which is available on Sculpteo— ( alias) allows you to check your mesh orientation.

Note that the blue color means exterior visible side, yellow means interior and not visible side of the model. Then if you see some yellow sections just select it and choose ‘Reverse component’ on the bottom right, until your model is completely blue.

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Mesh structure

The next step is analyzing the structure and this can be done in the following ways.

By opening the information window of the Mesh repair command (Double click on Mesh Repair alias) , you will find 2 useful tabs called ‘Mesh Information’ and ‘Mesh Topology Checks’. When your mesh is selected, the details about the mesh appears on them.

The first two items, ‘Number of polygons’ and ‘Number of vertices’, define the mesh density. As mentioned before, any number between up to 1,000,000 is considered acceptable, depending on the size of your model. If that’s not the case for your project, consider re-meshing your model or use the ‘Reduce Mesh’ ( alias reduce) feature that will be described later to adjust the number of polygons.

The third item, ‘Number of boundaries’, should show 0 as value, if all the surfaces have been correctly stitched. If any boundary exists on your model, this needs to be fixed with an appropriate feature that will also be described later.

“Number of components” refers to the number of shells reported on your model. Even if you have merged your two meshes previously, the file will record them as two separate parts of the same model, so in this case, 2 components. In interlocked/moving 3D models like this one, every part should be a component (i.e. a shell), but it’s necessary not to have more than 1 component/shell in a singular part.

Then, in the second tab, we find 3 checks that can appear as ‘Pass’ or ‘Failed’. The first one, ‘Non-degenerate polygons’, refers to the presence of polygons in which boundaries coincide, and are therefore not visible. These polygons need to be deleted because they don’t occupy any area on the object’s surface, even if they are still mathematically present in the model, otherwise the machine will read this as an error.

The second one, ‘Manifold’ checks this already mentioned mesh feature, essential to have a correct printable model, because it defines a closed mesh where every edge is shared by exactly two polygons.

The third one, ‘Consistent polygon orientation’, checks that all the polygons are coherently oriented, meaning that the polygons belonging to the same shell also have the same orientation.

If any of these 3 checks appear as failed, you can - in some cases - repair them by just selecting the mesh and clicking on the ‘Repair All’ button on the bottom right of the viewer.

Otherwise, if no problem is reported, it means that you can proceed to export the mesh for 3d printing. It is also important to know that Sculpteo’s automated repair tools automatically fix these simple errors and you are also given the option of uploading the file through Sculpteo’s upload page to automatically handle such errors.

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Export your model

To export the model as a printable .stl file, select>Export>Rapid Prototype, select the mesh then click ‘Accept’ ad ‘Export’. The generated model is now a perfectly printable .stl file and can be uploaded to Sculpteo’s website and then you can 3d print it!

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Fixing common errors

Let’s now take a look at how to deal with incorrect meshes in Alias. It might happen that your mesh presents some mistakes that would make it unsuitable for 3d printing, these are normally due to common distractions or inaccuracy during the modeling phase.

At Sculpteo we have developed some great automatic mesh repairing algorithms that you can easily access online through our upload page. We can proudly say that these methods succeed in almost all cases in repairing common mesh problems, but as they are automated repairs, you might want to have better control on how your mesh topology should be modified following any repairs. Therefore when dealing with isolated and detectable errors, it is better to manually repair these errors yourself on your modeling software if it has the tools to do it, and Alias does have these tools.

The reason for manually troubleshooting errors is that no automated algorithm knows your mesh as accurately as you do.

Next, the tools Alias provides to solve the most common problems with invalid meshes will be discussed in the coming paragraphs. In this section, the common mistakes you can find in your mesh will be discussed and the repair process will be explained.

  • When you mesh is too big
  • If your mesh’s ‘Number of Polygons’ is more than a million, it will be quite difficult for Sculpteo to handle it which also means it contains unnecessary details. Therefore, you need to reduce this number through the ‘Mesh Reduce’ feature (Refer to ‘Mesh Reduce’ paragraph).

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  • Design is not orientable
  • This means that faces belonging to the same shell of your model might not have a coherent orientation, which means you then have to go back to the ‘Mesh Orientation’ paragraph and fix this issue using the ‘Reverse Mesh Orientation’ function (Refer to ‘Mesh Orientation’ paragraph).

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  • Flipped polygons
  • This means that faces belonging to the same shell of your model might not have a coherent orientation, which means you then have to go back to the ‘Mesh Orientation’ paragraph and fix this issue using the ‘Reverse Mesh Orientation’ function (Refer to ‘Mesh Subset’ and ‘Mesh Orientation’ paragraphs).

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  • Holes
  • Your mesh needs to be perfectly closed, or ‘airtight’, to be printable. The holes you might find are essentially of two types: holes defined by one edge or defined by two edges belonging to the same shell. In the first case, the feature ‘Mesh Hole Fill’ should be used to close them, while in the second case ‘Mesh Bridge’ should be used (Refer to ‘Filling and bridging a mesh hole’ paragraph ).

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  • Multi shell part
  • Every part that makes up your digital model needs to be made of just 1 shell, if this is not the case what you need to do is : separate these shells (With ‘Subset’), then intersect ( alias intersect) , cut ( alias cut) and unify again (With ‘Mesh Merge’) to create just one shell. (Refer to ‘Subset intersect and cutting to reduce shells’ paragraph ).

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  • Mesh with boundaries
  • If you see a boundary on your mesh that is not due to a hole or a non-manifold surface, it means that your model’s surfaces haven’t been stitched correctly. So simply go back to your surface model or stitch the mesh with the ‘Mesh Stitch’ feature (Refer to ‘Mesh Stitch’ paragraph).

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  • Non-manifold edges
  • If you detect an edge on your mesh belonging to a single face on your model, and it’s not defining a hole, it means that this is the reason why your mesh is not manifold and printable. The best thing you can do is to go back to the modeling phase, or you can try to cut the unnecessary parts with ‘Mesh Subset’ and then stitch the edge with ‘Mesh Stitch’ (Refer to ‘Mesh Subset’ and ‘Mesh Stitch’ paragraphs).

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  • Singular faces
  • One boundary can also define one or a group of unwanted polygons that for some reason belong to your mesh. You can easily delete these polygons with the ‘Mesh Subset’ command. (Refer to ‘Mesh Subset’ paragraph).

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Going back to the mesh analysis –by Double clicking on Mesh>Mesh Repair—, seen in the previous chapter, check if some of these mistakes have been detected.

Mesh reduce

As already mentioned, Sculpteo’s upload limit for meshes is fixed at 1,000,000 polygons, because meshes with more polygons would be difficult to handle. If in the analysis phase you see that your mesh is bigger that this limit, you are provided with two options for reducing its number of polygons:

  • By going back to your surface model and re-meshing it with a higher ‘Max Edge Length’ ( Suggested option)
  • Or if you are not in possession of the original modeled file, Alias provides you with a feature called ‘Mesh Reduce’ [Mesh>Mesh Reduce], double click to access its information window, choose ‘Fraction’ as the mode and apply the required percentage of reduction, then click the ‘Reduce’ button located at the bottom right.
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Mesh stitch

If your shells have not been properly stitched together during your modeling process, you might see some ‘Boundaries’ appearing on your ‘Mesh Repair’ window, which will be outlined by red lines on your model. These can be easily stitched together with the function ‘Mesh Stitch’ [Mesh>Mesh Stitch]. Double clicking on the boundary—which selects the area around it—and then click the ‘Stitch’ button on the bottom right corner.

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Filling and bridging a mesh hole

The presence of boundaries in your mesh analysis—outlined by the red lines on the model—can also imply the existence of holes in your mesh. You can visibly recognize the difference between two meshes that are not well connected –a boundary that needs to be stitched—and a hole in a mesh. In is this second case you can use the feature ‘Mesh Hole Fill’ [Mesh>Mesh Hole Fill] to manually close a hole.

Simply select your mesh and click on the detected red boundary around the hole, which will automatically be closed. To select a filling type, navigatæe to the information window of this feature and choose ‘Taut’ for planar filling or ‘Faired’ for tangent filling.

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When the hole detected in your mesh is defined by two closed boundaries instead of one, like in the next example, the appropriate filling tool is called the ‘Mesh Bridge’ [Mesh>Mesh Bridge].

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Select the mesh, and the hole boundaries will be highlighted, then proceed to select the edges of two opposite polygons which will automatically connect the holes. Repeat this operation for all the polygons’ edges that make up the boundary of the hole.

Subset intersect and cutting to reduce shells

If you see no boundaries on your model but your number of components is still higher than the singular solid parts in it, it means that some shells haven’t yet been unified. Once again, the best option is to go back to your surface model and stitch your surfaces correctly. If that’s not possible, then you have to do it on your mesh.

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When these shells have been found, in order to be able to intersect and cut them, you must first have to divide them using the feature ‘Mesh Subset’ [Mesh>Mesh Subset]. This feature uses black dots to select the areas in your mesh that need to be divided from the rest. You can then select with this tool one of the two shells that you want to intersect and press ‘Subset’.

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Now that you have divided the incorrect shells, you should be able to ‘Intersect’ them using [Mesh>Mesh Intersect]. Then choose ‘Cut’ [Mesh>Mesh Cut].

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Select one shell and the cutting line just created, select which part of this shell you want to delete—the one inside your model—and press ‘Delete’. Do the same for the second intersected shell and now you will have two correct continuous shells that just need to be stitched together with the ‘Mesh Stitch’ feature described in previous sections.

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If the tools and techniques just described do not fix your mesh, then trying Sculpteo’s awesome online printing automatic repair tools should be your best option. Simply export your model as .stl, as described above, upload it on Sculpteo’s website and let our repair tools do the rest for you!

Mesh subset

The feature ‘Mesh Subset’ allows you to select precise parts on your mesh. This feature can be useful when you need to delete unwanted parts of your already meshed model such as singular faces, or need to divide it into different parts that you need to keep separated.

For example, if you want to delete the embossed Sculpteo logo from the model. You can do this with the active command by selecting your mesh, and choosing a view that allows you to see all the parts of the mesh you want to subset—for the command resets when you move. Next, use a series of dots connected by lines, to define the area.

Some buttons will appear at the bottom right of the screen, choose ‘Select’ to pick the polygons—which become light blue when selected— and then click ‘Delete’ if you want to remove them permanently from your model, or ‘Subset’ to divide them from the rest of the polygons.

If—as in the provided example—you have deleted your polygons, a hole will then be created. Which can then be filled with the command ‘Hole Fill’

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