A Virtual Prototyping System for Additive Manufacturing Process Development AMPT-libre

8/10/2019 A Virtual Prototyping System for Additive Manufacturing Process Development AMPT-libre

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A Virtual Prototyping Systemfor Additive Manufacturing Process Development

Okan Topu1, a,Yiit Tacolu2, b

1, 2

TOBB ETU, Mech. Eng. Dept., StzCad. No:43 Stz 06560 Ankara, [email protected],

[email protected]

Keywords:Multi-material, additive manufacturing, virtual prototyping, visualization.

Abstract.This paper describes a virtual prototyping (VP) system which is a part of an open source

software package for an additive manufacturing (AM) process under development. The VP system

facilitates the product development by uniting the AM process and virtual reality in order to produce

digital prototypes. Moreover, it combines particle based and layer based processes by including

powder-like particles as its basic material. These particles are used as color codes in the VP system.

This coding enables obtaining basic building blocks in homogeneous state or in heterogeneous stateby mixing with other particles. These blocks or bricks are collated side by side to obtain the

heterogeneous material property all over the solid body. The thin layers obtained by these bricks are

then subsequently stacked up to fabricate a virtual prototype. Construction of multiple material

prototypes is possible due to selective-additive nature of this process. The effectiveness of the

proposed system is demonstrated by processing a model of The Maidens Tower.

Introduction

Additive manufacturing, also called as rapid prototyping, is a technology which enables production

of a physical product directly from CAD models. It is preferred especially in the design and

preliminary product development stages. The prototypes are mainly used for testing the design ideasand verification of the form and functionality of the products. Currently, various commercial AM

machines are available and they can create single-material artifacts. However, multi-material

prototype production is becoming a demanding factor in the manufacturing community [1].

Multi-material artifacts, which are heterogeneous in material properties, are created from a group

of discrete materials or several materials with distinct properties [2]. Among the available AM

technologies, Fused Deposition Modeling (FDM) and Shape Deposition Manufacturing (SDM) have

the capability to fabricate multiple material objects, but this can only be achieved after a CAD

model or models are also represented as multi material object or objects [3]. In contrast, there is a

material information limitation in current CAD models. As a circumstance these models are not

suitable for the process without any modification or calculation. Consequently, there is anothergrowing demand for transferring extra kinds of data, which are mainly material information, for the

improvement of AM technology [4].

Virtual Prototyping. Virtual prototyping (VP) facilitates evaluation and analysis of specific

features of an artifact or a manufacturing process by integrating a digital prototype, in place of a

physical one. VP shortens production planning and provides manufacturing free product design and

early elimination of faulty fabrication without causing any costs and production efforts.

Choi and Chan proposed a VP system that exploits visualization to facilitate product

development [5]. Jayaram et al. carried out a virtual assembly design environment in VR [6].

Siddique and Rosen proposed a VP system which combines automated and interactive techniques to

generate complete disassembly process of a product design [7]. Bowyer et al. developed a virtual

manufacturing system which incorporates a milling machine and a virtual tool that could cut a

virtual block to create a workpiece with the preferred form [8]. Tseng et al. carried out a research


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with the intention of testing the balance between the process capabilities of a company and the

individual customization requirements [9]. In addition to industrial manufacturing, VP has also had

an intense impact on the medical field. It was used in creation of digital human organs, training of

medical students in a virtual environment and medical planning [10].

This paper presents a multi-material virtual prototyping (MMVP) system which is capable of

simulating digital fabrication of heterogeneous prototypes. In the future, this system will bemodified and combined with a material deposition instrument to produce a real multi-material

additive manufacturing (MMAM) machine.

The Developed Multi-Material Virtual Prototyping System

This section describes the methods and assumptions used in the development of the proposed

MMVP system.

The Slicer Module.The slicer consists of the slicing algorithm, detailed in [11], that slices a STL

model into a number of layers with desired thickness. In this study, the layer thickness is equal to

the height t of the honeycomb cells (Fig.1), which are used as building blocks.

Honeycomb Structure Generation. After generation of slice contours, honeycomb generation

process takes place. This process converts solid body boundary contours into honeycombs. The

dexel model [12, 13], is chosen to be the basis of the algorithm for honeycombing. The resolution of

the dexel model is defined by the dimensions of the honeycomb cell, which is the basic building

block of the MMVP system. Hence, all the system and the algorithm are both bounded by the size of

the honeycomb cell. Fig. 1 shows the relative dimensions of the used cell.

(a) (b)Figure 1.Dimensions (a) and 3D visualization (b) of a honeycomb cell.

A grid of rays is set inside the contours to obtain the new honeycomb boundaries. As seen in Fig.

2(a), a ray is cast from each starting grid location. The intersection points of each ray are used to

detect boundary points which reside inside the solid. At the next step, these points are stored and the

line segments defined by these interior points are classified as START and END points. In other

words, for honeycombing, the models boundary points are used to obtain honeycomb structures

starting and ending points. Fig. 2(a) depicts honeycombing of a circle which represents the solid

boundary of a model at a slice level. In Fig. 2(a), the cell centers which reside outside the contour

are marked, and the cells which are located in the boundary of the solid are color hatched.For every layer, the honeycomb is shifted by amount t along both axes to seal the connections of

the previous layer. Fig. 2(b) depicts the sealing effect of a covering cell when it is located over the


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existing cells. The effect is depicted by color hatching the covering cell and by bolding the

connections.

(a) (b)

Figure 2. (a) Honeycombing of a circle and (b) sealing of connections with the covering cell.

Representation of Multi-Material Artifacts by Color Codes.In order to create a multi-material

prototype, both geometric and material information should be provided in the required data format.

In the developed system, the additive color model, also known as RGB model, is used. In this

model, the additive primary colors red, green and blue are added together in various proportions to

reproduce a broad array of colors.

(a) (b)

Figure 3. (a) Schematic representation of three solid bodies with distinct material properties and

(b) a snapshot of the MMVP process.

For color coding multi-material artifacts that are made up of multiple single-material

components; firstly, each single-material component (i.e. STL model) is assigned a color codeunique to the material of the component. Then, the components are sliced and same level slices of

the independent components are merged to generate multi-material slice contours. At the sections

where two single-material bodies overlap, a new kind of material is also formed. The newly formed

material is represented by the mixture of the colors of the overlapping bodies, which is similarly

another color. The multi-material contours are stored in intermediate data files which are input to

the aforementioned honeycomb generation process. Without the material data, the honeycombing

process decides whether or not to place a honeycomb cell on the slice grid. With the addition of the

material data, the color codes of the honeycomb cells are also included in the final build data file.

Fig. 3 shows the schematic representation and the resultant output of three solid cylindrical bodies

merging under the explained conditions.


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Case Studies

The proposed MMVP system simulates a MMAM process to fabricate a digital multi-material

prototype. Subsequently, it provides dynamic 3D visualization of the build process and the

prototype for quality analysis and optimization of the MMAM process. In order to test the capability

of the system, two cases are tested.

Hollow Cube and Rhombic Dodecahedron.The hollow cube and the rhombic dodecahedron are

used to test and evaluate the STL slicer module in [11] as basic 3D solid bodies. Consequently,

these bodies are used for testing and evaluating the initial performance of the MMVP system. For

the sake of faster simulation and easy visualization, the merged bodies are sliced to ten layers and

hence the prototype is built with relatively large honeycomb cells. The snapshot shown in Fig. 4(d)

validates the viability of the proposed system.

(a) (b) (c) (d)

Figure 4. (a) A rhombic dodecahedron sliced at the center, (b) a hollow cube sliced at the center,

(c) center slice of the merged solid bodies and (d) a snapshot of the MMVP simulation.

The Maidens Tower. The model of The Maidens Tower in Fig. 5(a) was chosen as a more

challenging case study. The CAD model consists of 10 different components, each stored in a

separate STL file. Moreover, each component is assigned a different material (i.e. color code) and

the model is sliced to 1000 layers.

(a) (b)

Figure 5. (a) CAD model of The Maidens Tower and (b) the output of the MMVP system.

In this example, the total size of 10 STL files is 91 kBs. However, these files only include

triangulated geometries without any material data. The total size of the intermediate files containing

multi-material slice contour data is 492 kBs. However, after the honeycombing process, the entire

build data is contained in a single file of 155 kBs in size. This final file contains material data as

well as position data for the honeycomb cells. Fig. 5(b) displays the output for this case. This initial

study shows that the proposed MMVP system produces satisfactory performance.


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Conclusion

A MMVP system, which fabricates digital multi-material prototypes, is developed. Consequently, it

basically displays the 3D visualization of the resultant prototypes for visual inspection. The system

provides position and material information throughout the prototype which enables quality analysis,

optimization and further development of both the virtual and the physical system. Therefore, the

integration with a real material deposition capable AM machine to produce multi-material

prototypes is feasible.

A pattern for demonstrating multiple material objects based on the honeycomb representation is

described. The pattern uses the boundaries of the solid model to obtain the boundaries of the

honeycomb structure and it also uses a simple material list format to state the whole multi-material

object.

In order to obtain multi-material artifacts, an appropriate data for inputting material and

positional information to the corresponding machine must be obtained from the CAD model

representation of the artifact. The developed system uses STL data of the CAD model and the

manually assigned material properties to STL data to create an appropriate data for the MMVP

system. Because the building blocks have definite sizes, the data required for the representation ofthe multi material solid body reduces to the locations of the blocks. As a result of this,

comparatively small sized construction data is achieved.

References

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