A Rapid Prototyping and .STL Informative Guide

November 25, 2009 in 3D, Articles, AutoDesk 3DSMax, Featured by admin

A Rapid Prototyping and .STL Informative Guide
(Autodesk 3dsmax 2004 And Up)

Welcome to STL and Rapid Prototyping 101. Here we will be discussing the modeling preparation and use of a few applications as well as how to bring your model through the rapid prototyping or 3d printing process. At the end of this article we will output a boxer from a Stratasys Dimension SST machine in an ABS Plastic material.

Rapid Prototyping and STL Guide - InstaTuts

The first question to ask would be, what is Rapid Prototyping? Rapid Prototyping can be described most accurately by calling it a collection of technologies which are driven by CAD Data to fabricate physical models through an additive process. This process can be stated to be the opposite of machining, since it is not subtracting the material, it is instead adding it. There are a few other names for Rapid Prototyping, such as Additive Manufacturing, 3D Printing, Free Form Fabrication, Solid Imaging and Direct Digital Manufacturing are some of the names it has been given.

The second question to be asked would be why is Rapid Prototyping the choice for output of a boxer model. This question needs to be asked so that we can decide what Rapid Prototyping should be used for, and what it should not be used for. Rapid Prototyping should be used for detailed, compound shapes with undercuts or complex curves. If your model is simple such as a cylinder or gear, you should go with a subtractive method such as a CNC milling machine. In this case, it is Rapid Prototyping that we need.

Now that we know that we definitely want to go with Rapid Prototyping we need to identify the limitations of such a process. Some limitations of Rapid Prototyping is the inevitable fact that you will get stair stepping since each object is built layer by layer in this process. When choosing a machine or company to produce a physical model by means of Rapid Prototyping you should look at the layer thickness and the tolerance that the machines produce. A thinner layer will obviously produce less stair stepping than one that produces thicker layers.

We have identified the process and we will now continue onto the file format in which 3D Printers use as the standard. The standard file format for Rapid Prototyping is the STL file format. In the next section we will take a look at the file format and how to properly set a file up for RP.

*Note* The physical model was produced in mid 1995. Newer machines are available today with greater tolerances and detail management.

Author: Ryan W. Knope

Ryan W. Knope is a freelance 3D Artist / Consultant with 13 years experience in the 3D and 2D industry. His main expertise lies with interior and exterior rendering. Ryan is also the architectural / interior voice for 3D Artist Magazine’s Question and Answer Panel. He lives with his wife in sunny Denver, Colorado.

 

Tutorial Details

  • Programs: Autodesk 3dsmax 5 and Up
  • Difficulty For Session 1: Beginner to Intermediate (Prior Knowledge of Software is a plus.)

The STL File Format

The STL (Stereo Lithography) File format is used to describe the geometry of the object that is to be created by the 3D Printer or Rapid Prototyping machine.

In more detail, the STL file is a triangular representation of a three dimensional object. The object is tessellated into a series of facets or small triangles. Each triangle has a perpendicular direction and three vertices to describe the corners and size. A unit normal is also identified for each facet. This normal lies perpendicular to the facet and has a length of 1.0. An STL file can be in many ways described as a list of the vertex and normal coordinates. There are 12 numbers stored for each facet or triangle. The units of measurement in a STL file are arbitrary, the file does not store any size information.

 

Common Rules Of Model Creation / Export

There are a few rules for model creation for STL export.

1.) The model must be “Water Tight.” This means that there can be no “holes” in the model. If there are holes, then the machine will not understand what to do and the physical model cannot be created.

2.) The Vertex to Vertex Rule: What this means is that each triangle of the mesh must share 2 vertices with its adjacent triangles. For example one triangles vertex cannot lie on the side of a triangle, it must be attached to another vertex of the triangle. For a visual of this Rule please refer to the image below.

3.) There cannot be duplicate nodes or overlapping triangles when creating a model for STL export.

 

Rapid-Prototyping-STL-Guide-Instatuts-f1To the left you will see a yellow X. The yellow X represents the violation to the vertex to vertex rule.
On the right is the correct example of the vertex to vertex rule.

 

The Modeling Process

In this article I have decided to use 3DSMax and Z brush for the model construction. The model originated in 3DSMax as a low poly model built by Enrique Caballero for the PC Game “Online Championship Boxing.”

Below, you can see the low poly mesh before exporting to Z brush. The model was then exported to a .obj format and imported into Pixologic’s Z brush using “Import Tool”. In Z brush the model was subdivided and detail was added to the model. Detail such as muscles, veins, boot laces, boot soles, hair and other small details were added as to test the detail of the machine output. Once finished in Zbrush, the model was exported out of Zbrush in a dxf file format and imported into 3DSMax.

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Once the model was imported into 3DSMax it had to be checked for errors using the STL Check Modifier. This modifier investigates the model for any errors that would make the 3d model unusable in the STL format. Below is a side by side comparison of the wireframe and shaded model, settings are also shown for 3dsmax’s STL Check Modifier.

 

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Throughout the modeling process the STL Check Modifier was used several times to try to catch the errors as they would occur. After importing from Zbrush several errors were found.

3DSMax’s STL Check Modifier will check for the following:

Spikes: Spikes are isolated faces that only share 1 edge with the object.
Open Edges: Open Edges are simply holes in the mesh.
Multiple Edges: Multiple Edges refers to faces that share more than one edge.
Double Faces: Double Faces refers to a face or faces that share the same 3D Space.

If you find that you have errors you can do a few things. You could use the “Cap Holes” Modifier, which in my situation had fixed my errors. To apply the “Cap Holes Modifier” go to the modifiers tab and select “Cap Holes.” After using the Cap Holes Modifier make sure the use the STL Check Modifier again to ensure that it has capped the holes and fixed the errors.

You can also manually fix the errors if the Cap Holes Modifier does not fix them. You may have to weld the vertices if there are any holes that the Cap Holes Modifier does not fix. It is good practice to use weld with a certain tolerance while prepping your model. The tolerance would be specific to each model and the scale.

Note: 3DSMax’s STL Check Modifier is not 100% guarantee to find all errors. There were still a few errors that had to be cleaned up by a technicians more advanced software.

Exporting To The STL Format

Now that we have confirmed that our model should be water tight and not have any errors we can prepare ourselves for exporting the final STL file to send in for Rapid Prototyping. This part is simple. In 3DSMax you go to Export, pick the Stereo-Litho format, name your file and press save. Once you press save the option dialogue will pop up that is shown below. Now you will notice a option for binary or ascii.

Most of the time you will use the Binary format due to the fact that it is much smaller in file size, which makes it more practical for general use. Generally an ASCII STL file is 6 times larger than the Binary STL file. The ASCII format is used mainly for testing CAD interfaces. After exporting the boxer model with binary, the file size jumped to 50mb.

There is also an option for “selected only”. Use this option if you have multiple objects in your scene and only want to export the selected objects for STL export.

Rapid-Prototyping-STL-Guide-Instatuts-f-stl-export

The Dimension SST 3D Printer

Here is a bit about the machine that created the boxer model. The Dimension SST 3D printer is manufactured by the Dimension 3D Printing Group, which is a business unit of Stratasys, Inc. Below the Dimension SST 3D Printer is shown.

This machine has automatic operation, which imports the STL file, slices the file, and builds support structures for the file if they are necessary, then builds the ABS model.

The machine is run on a Microsoft Windows platform (NT, 2000, XP) and has a maximum build size of 8” x 8” x 12” (203 x 203 x 305 mm). The ABS plastic standard colors are white, blue, yellow, red, green or black. Other custom colors are available upon request. Layer thickness is between .010” and .013” (.245mm and .33mm) of ABS Plastic material. Layer thickness refers the the vertical dimension of each slice. The more minute the vertical dimension is, the more smooth the surface will be, although this will usually concur a longer build time.

The rate at which machine operation is charged is between $40.00 – $50.00 USD an hour. The Dimension SST 3D Printer’s starting price is $24,900 USD. You can view more information on this 3D Printer at www.dimensionprinting.com

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The Final ABS Plastic Boxer Model

Figure 7 shows the physical model fabricated from ABS Plastic side by side with a render of the boxer model from 3DSMax. The machines build time took a total of 20 hours and 15 minutes for the model’s fabrication.

The basic structure of the model was nearly perfect. All muscle groups were formed excellently with a minimal loss of detail.

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Loss Of Detail And Stepping

There was minor stair stepping which I plan to resolve by sanding the surface lightly. You can view the stair stepping on the shoulder of the boxer model in the image below. Some detail was lost, such as in the eyes, ears, veins and boot laces. The underside of the boxer’s trunks (shorts) came out perfect. The veins were slightly visible. The model will need some work to gain back the details but all in all the project was a success.

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