3D Object Data Formats

1 Introduction

When dealing with 3D models for the use of manufacturing there are many different data formats. Some of the data formats are better for traditional manufacturing where it is more accurate and not an approximation. Others are better for additive manufacturing. There are also uses of 3D object data files in gaming, 3D graphics [1], virtual reality (VR), augmented reality (AR) [14], and simulation. Some data formats are better for different applications. There are different 3D modelling software and some of this software have their own data format. Blender for example uses their own file format of .blend [2].

This cannot be used with other software. When transferring files between different mediums there needs to be a common neutral file format used [7]. There are many different neutral file formats that can be used across platforms [9]. STL is a popular one used among the additive manufacturing industry. OBJ is another common neutral file format that is used in AM, it works similar to STL. Since there are so many different software that use their own file format, there needs to be a common neutral file format that is widely used enough to be useful across platforms. The STL format seems to be the industry wide neutral format to use. There are limitations with STL though, it only has the shape of the object in triangulation mesh, it does not have other input options like color or material. Some printable technologies can have color or material printed during the process. 3D Systems and Stratasys are industrial technologies that offer different material and color printing capabilities [8].

Other data formats like VRML, 3MF, AMF and COLLADA have these types of inputs, so they are a good standard for printing options that need color and material type. This is an important feature that will need to be implemented in the standard neutral data format for the future. A new data format called 3D Manufacturing Format (3MF) is a new open-source format that can have material and color in the data [16]. Transitioning from STL to data formats such as 3MF will be necessary to be used as the standard neutral format to accommodate the advancing technology in the field of AM.

How an STL file works will be explained going over the triangulation method to create a solid shape. This shows the inaccuracy of the STL file, and some algorithms are introduced to compensate for these inaccuracies. Using 3D object data files for other types of manufacturing will be explained next. A list of different 3D object data files will then be showed. The new data 2 formats available that can allow for multi color printing is next explained while discussing multicolor/material 3D printing

2 Material & Method

2.1 How an STL FIle Works

STL format uses a meshing system with triangles to create the 3D shape. This is done on a cartesian coordinate system using two set of parameters. The first parameter is the points of the shape in a vector of X, Y and Z. The second parameter is the normal vector to each surface. This is an important part to know the desired direction of the surface. This is a great way to display a 3D object in a 2D plane. One thing to note about this method is that it approximates the shape.

There can be a course meshing with less triangles which would be less accurate, but it would not be as much data and could be used and processed quickly. If the meshing and triangles gets higher the accuracy goes up, but the size of the file goes up with that which can cause slower the interface of viewing and processing [8].

Figure 1 Triangulation meshing with different number of meshing showing the course and fine models [6]

In figure 1 there is a range of different meshing accuracy from course in (a) until a really fine meshing in (f). There is an intuitive understanding why model (f) would require more processing power than (a). And that (f) is more accurate for application use [6]. Some ways to deal with inaccuracies of STL is discussed in the next section.

2.2 Optimizing the Accuracy of an STL File

When using an STL it can never be 100% accurate with the meshing method. When printing there are algorithms to compensate for this to print a better resolution shape than the raw STL file would have printed [3] [9].

Figure 2 Applying an algorithm to improve the resolution of an STL [3]

In figure 2 you can see the original STL file in (a) there are imperfections indicated in red and (b) and © are separate fixes by other software’s and the (d) is from the algorithm in this paper creating a higher resolution output [3]. These sorts of algorithms are a great way to advance in the technology. Since there are limitations in parts of the process, we can enhance these limitations with the help of algorithms without too much trouble in redesigning how the initial software works.

There are hundreds of different 3D object data formats. Some have a more accurate way of representing the 3D shape. If the data format is not accurate enough it has to be properly accommodated for 3D printing. So, it all depends on what is being done. In figure 3 another example is shown of triangulation of a 3D object that has inaccuracies to it and shows again the meshing. Some different industries that use 3D object data files is discussed in the next section.

Figure 3 Meshing of a 3D shape [10]

2.3 Uses of 3D Data Formats

SolidWorks for example uses .sldprt for the part that it creates in its software. When drafting up a SolidWorks part you create a new part in the interface and start drafting it up. This is saved and worked on again in the SolidWorks interface. This can be then put into an assembly into SolidWorks, which creates an assembly of different parts created using SolidWorks. If this part needs to be transferred to the manufacturing process, then it can be exported as a different file format. If one wants to transfer a part made in SolidWorks to a CNC machine, they could export it into a neutral format such as STEP or IGES [5].

Importing an STL into a slicing software is easily done and well accessible. Slicers will import the STL easily. The different drafting software will export an STL really easily as well. This allows for a well universal data format that is capable of cross platform. The limitation of STL though is that it does not have color and material type embedded in the data format of the file.

A common use for 3D object files is to 3D print them into the real world. Additive manufacturing is a leading industry and having a digital 3D object to work with is important. Any kind of manufacturing having a 3D model is important. When designing an engine for example a designer drafts it up in SolidWorks to create a virtual replica of the engine being designed. The different parts of the engine can be manufactured in different ways.

The engine block might be sand casting for example, where the pistons might be machine in a lathe. Some components could be manufactured with AM depending on the material that is needed. If it is a 5 plastic part, then certainly it could be 3D printed. All of these parts need a detailed representation of what needs to be designed. So, it can go to the manufacturing process with a clear set of instructions on how to manufacture it. Using a 3D object data file parts can be translated from the engineer’s design platform to the machinist lathe.

The different data files used, have different applications throughout the design to real part streamline. Upstream design could design using a SolidWorks file while drafting it up; then export as STL to transfer to a slicing software which then will create the G-code for the 3D printer.

3D objects are also useful in simulation. A solid model can be created in SolidWorks and imported in to Ansys to apply finite element method (FEM) simulation on it. Ansys has a solid modelling interface, but it is more common to design the part in a different platform and import it into Ansys. The solid file in Ansys is .scdoc file. In figure 4 a solid model is shown made in Ansys that is showing the deflection from when a simulated force applied.

Figure 4 3D model in Ansys showing deflection from a simulated force applied [17]

Another example is using AutoCAD’s file format of DXF one could create a design in AutoCAD then input this into a waterjet to create the desired part with that data format.

When 3D printing with multi-color/material then an STL file is not good enough there needs to be more data inputs. 3MF is an example of a data format that has color and material saved in its file to be able to output to a 3D printer that might need those parameters.

As the additive manufacturing technologies continue to advance there will be a greater need for printing in colors and materials mid print. This allows for more in production uses of their product can be the multi-color/material the end product needs to be. This need will create a higher demand in more detailed data formats than the standard STL. Using a file format like 3MF can create these inputs in the data format to allow for color and material in the print. A few common data formats are showcased next.

2.4 Common 3D object data formats

Here are some common 3D object data formats that are used in different industries. Some are for traditional manufacturing, for additive manufacturing and for computer graphics/gaming/VR. OBJ is another common neutral format used in AM similar to STL. Most slicers can take OBJ or STL and they are approximated and meshed in a similar way. 3MF, AMF and COLLADA are XML based formats. XML is a markup language that is very powerful in manipulating the data to create 3D objects. It is simple but powerful and can be edited easily through coding instead of a 3D modeling software which has its advantages [13].

Table 1 List of Common 3D object file formats [2]

2.5 Multi-Color/Material 3D Printing

A company based out of Toronto; Canada called Mosaic are in the multi-color/multi material printing industry for FDM. They have an attachment piece that can be added to an existing FDM printer. They also have an industrial 8 material FDM printer. They also have a smaller printer that is 8 material printing capability that is more suitable in smaller use applications. They have a special slicing software called Canvas where you can input your colors and materials in the model and the data for this will be transferred to the G-code to the printer.

The special slicing 7 software takes an STL file and add the color and material using Canvas. You can paint the model in the Canvas interface to the desired color. Using extrusion printing for multi-color/material typically it is done with multiple STL files that each have the different color information. It might be possible for an 3MF file were created with material and color inputs and then loaded in Canvas you would not need to input the colors. This is not yet implemented with the Canvas software, but it could be a good direction for them to go. This would save time which would save in production cost. They were a big starter in multi-color FDM printing so they might like the idea of implementing these capabilities in their software. For FDM the best way to do multicolor/material seems to be through Mosaic’s hardware and software [14].

In figure 3 is an image of a multi-color FDM print with Mosaic’s Palette 3 attachment. This shows the possibilities of multi-color FDM 3D printing. There are many different ways to achieve the multi-color/material 3D print, in FDM and other forms of AM. Other data formats such as VRML that have color and material embedded in them are used for multi-color/material printing in industrial printers such as Stratasys [15]. For FDM it is a more difficult to achieve.

Figure 5 Mosaic’s Palette 3 with a multi color FDM 3D print [14]

The demand for multi color goes up as the technology gains ground. It is definitely more useful in many applications than having the constraints of one color. When only able to print with one color one might have to paint it after

3 Discussion

3MF data format is a new open-source format that can have color and material properties in the 3D object file. This is important as the technology leads towards more demands in multicolor/material printing. There are many advantages to multi-color/material 3D printing. The main advantage of having these features is the capability to have production ready products being printed instead of limiting prototyping. Having more printable parts ready for production can decrease the cost of the overall manufacturing process, instead of printing a prototype then needing to create the real part with injection molding. Another disadvantage to needing to use injection molding for the production part is that you do not get the benefits of the 3D printing technology while manufacturing a unique shape. The final shape is then restricted by the injection molding manufacturing constraints. So, being able to design your part knowing the manufacturing capabilities of 3D printing and being able to use it in production is very valuable.

The importance of multi-color/material 3D printing will only increase as the technology advances. Technologies that are not able to print multi-color/material will quickly fall behind in the industry.

4 Conclusion

There are many different data formats for dealing with 3D objects. Some are better for traditional manufacturing while others are for additive manufacturing. There are also needs for gaming, virtual reality and simulation. A lot of the software used to create and use each object and then using it have proprietary data formats. To be able to cross platform data transfer a neutral data format is needed which there are many different ones. There are also benefits to different ones depending on the end application use. For additive manufacturing the STL has been the common one used but with leading technology going towards multi-color/material printing there needs to be a new data format that can accommodate these changes. 3MF data format is a new and leading open-source technology that has color and material capabilities that can lead the way for the future of additive manufacturing.

References

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[17] C. Hisey, 3D model made with Ansys and showing FEM simulation of deflection of the model, 11 July 2021.

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Cory Hisey

Cory Hisey

I am a Mechanical Engineer graduate and I am currently studying masters of Mechatronics Engineering.