General Process Workflow in Additive Manufacturing

Abstract

In this chapter, general workflow in Additive Manufacturing process is shown, from preprocessing activities that include preparing appropriate CAD model, selecting required STL file resolution, up to setting processing parameters for AM process.

General underlying principle of additive manufacturing can be seen on Fig. 2.1. Main goal is to slice the desired product in layers along desired axis and then to build the product by stacking of each layer along the slice axis.

Fig. 2.1

(Source Ana Pilipović)

General additive manufacturing principle: a layer buildup, b finished product

With this type of product buildup “Stair step effect” is unavoidable, since each layer has a finite value of thickness. Most common axis layout is the one where layers are in X–Y plane and they are stacked along Z axis. It is important to remember that build accuracy and mechanical properties of finished products are not identical in all three axes. AM parts have a fixed value of accuracy in X–Y plane since this property is a limitation of AM machine hardware. Accuracy along Z axis can be altered by changing layer heights during slicing. By decreasing the layer height accuracy increases, but also the build time increases which is an important parameter to be considered. It is possible to slice a product in non-uniform layer heights which alleviates “Stair step effect” by decreasing layer height in areas where there is a substantial dimensional change between layers, and increasing layer heights in areas where layers are dimensionally similar, as shown on Fig. 2.2.

Fig. 2.2

(Source Tomislav Breški)

Product created with a thick layers b thin layer and c variable layers

Generally, process workflow in AM can be divided in two main activity groups: pre-processing for AM and build and post-processing in AM.

2.1 Pre-processing for Additive Manufacturing

All additive manufacturing processes begin with virtual representation of a desired product. Almost all commercially available 3D computer aided design (CAD) software can produce models which can be used for additive manufacturing. Models are supposed to be “watertight”, which means that there should not be any gaps in enclosing surfaces of a solid model, as shown on Fig. 2.3.

Fig. 2.3

(Source Tomislav Breški)

CAD model

In most feature-based CAD software models, which are represented as solid models in feature tree, generally do not have issues with gaps in enclosing surface. These issues can occur when models are created with 3D scanners or Computed tomography (CT) machines. Example of non-watertight model is shown on Fig. 2.4. Model of this cylinder was created with a 3D scanner which measuring volume was too big for this model and thus part of the inner cylinder was optically unreachable for both cameras of 3D scanner simultaneously, which is a requirement for a 3D scanner to adequately register a surface during 3D scanning.

Fig. 2.4

(Source Tomislav Breški)

Non-watertight model

There are many freeware software available (Meshmixer, GOM Inspect) which can do mesh alterations and closing of gaps in surfaces but it is important to keep in mind that by automatically closing large gaps in models, created geometry can substantially deviate from desired geometry.

2.1.1 File Formats Used in Additive Manufacturing

As already mentioned, first step in additive manufacturing is to design a product which will be produced. In most cases CAD models contain too much data, so they should be translated to a file format which contains only information about outer surface shell of a product. Most commonly used and oldest on the market file format of this type is STL (Standard Tesselation Language). STL files contain only data which describe surface geometry of a three-dimensional model (Fig. 2.5).

Fig. 2.5

Geometry approximation using STL model: a secant error of circular geometry with 4, 8 and 12 secants, b sphere approximation with varying number of elements (Courtesy of Andreas Gebhardt)

It is important to consider that not every STL file is manufacturable with additive manufacturing since STL files contain only data about the geometry. If a model is created only as a surface model but without thickness, STL conversion is achievable but additive manufacturing is not. Tesselation is a process of tiling an arbitrary surface with primitive geometric shapes (e.g. triangles, squares) without any gaps or overlaps. The model shown on Fig. 2.3 converted to STL file format is shown on Fig. 2.6.

Fig. 2.6

(Source Tomislav Breški)

STL model

By tessellating the model with planar triangles final representation is achieved which can be sliced in additive manufacturing software, in order to produce the desired part geometry. Planar surfaces of a model can be tessellated with large triangles, but non-planar surfaces of a model have to be tessellated with large number of small triangles which increases the number of triangles and thus increases the size of STL file. When manufacturing one item, file size is not an issue, but with emerging of powder bed fusion additive manufacturing technologies, which can produce large number of parts simultaneously, slicing can be substantiately shortened if parts are not tessellated with large numbers of triangles. Since non-planar surfaces are approximated with large number of small planar triangles dimensional difference between CAD and STL model is inevitable, but if deviations between models are smaller than additive manufacturing machine resolution and build accuracy these deviations are not translated to the final product.

Deviations between CAD model and STL model with 29,492 triangles are shown on Fig. 2.7. Largest deviations, as expected, are on non-planar surfaces, but there are also some unwanted artefacts on planar surfaces which were generated during conversion from CAD to STL file. As shown on histogram next to to the model representation, there is only a small percentage of geometry which deviates more than 50 µm from CAD file. Most deviations are in the range from −10 to 10 µm from CAD model, which can lead to the conclusion that conversion from CAD to STL file format is acceptable.

Fig. 2.7

(Source Tomislav Breški)

Deviations between CAD and STL model

With emerging of additive manufacturing technologies which can produce parts with varying colours and textures STL file format is becoming lacklustre in the quantity of data which is needed to be transferred to slicing software. Information about color, material, internal lattice structures and constellations of multiple parts need to be linked with the geometry of part to be built. Furthermore, STL files do not provide information about the dimensional scale of the product, so if the user unintentionally imports a model in a different unit environment (milimeters vs. inches) there is big possibility that the final product would not be dimensionally correct. Extensible Markup Language (XML) has been recognized as the best way to structure all forementioned data to be transferred to slicing software. Currently there are two file formats which can store sufficient amount of needed data in a single file; AMF and 3MF. Additive Manufacturing File Format (AMF) is being developed by ISO/ASTM subcommittees and many of the problems regarding STL file formats are solved, but unfortunately this file format is not well accepted on the market of additive manufacturing. One of the biggest improvements of AMF file format regarding geometry is the possibility of tessellation of surfaces with curved triangles which improves part accuracy after conversion from CAD model. 3D Manufacturing Format (3MF) is a new file format being developed by the 3MF Consortium which inherited most of the properties from AMF file format. One of the biggest advantages of this file format is the fact that it is being developed through collaboration of the biggest stakeholders in additive manufacturing currently in the market. Main features of this file format are as follows:

• all the necessary model, material and property information in a single archive

• human readable by use of XML structuring of data

• short and clear specification for ease in further development and validation

• extensible by leveraging XML namespaces for both public and private extensions while maintaining compatibility

• unambiguity by using clear language and conformance tests which ensures that the file is always consistent from digital to physical

• access to and implementation of the 3MF specification is and always will be free.

Most of the features implemented in 3MF file format are shown on Fig. 2.8 which shows a product which could not be manufactured without all the information included with 3MF file.

Fig. 2.8.

3D-printed part with variable colour (Photo by Tomislav Breški)

2.1.2 Part Placement in Machine Envelope, Slicing and Machine Setup

In order to generate machine commands for additive manufacturing, first step is to load of a given 3D model in appropriate file format. Most of the currently available AM machines have their own proprietary software for slicing and machine commands generation, but in this book, examples are given as screenshots from Autodesk Netfabb which has a large number of machine pre-sets (machine properties, build envelope, etc.) of commercially available AM machines. Part placement in build envelope is in most cases the defining factor for success in part manufacturing. Example of part placed in build envelope is shown on Fig. 2.9. Depending on the selected AM technology, with adequate part orientation, user can eliminate potential mechanical deficiencies due to mechanical properties anisotropy, increase production speed, optimize build material usage, etc.

Fig. 2.9

(Source Tomislav Breški)

Part placed in build envelope of FFF machine Ultimaker 2

Depending on the selected AM technology, part may or may not need support geometries for successful build. Furthermore, support geometries in polymeric AM technologies provide mechanical support for regions which are angled more than the specific value from vertical axis, while support geometries in metallic AM technologies in most cases are used for heat conduction and dissipation. Final part orientation with resulting support geometries for FFF technology is shown on Fig. 2.10.

Fig. 2.10

(Source Tomislav Breški)

Part (green) and support geometries (blue)

With the emerging of powder-based AM technologies simultaneous production of multiple parts in one print job is getting more accessible. But in order to maximize production of those technologies, the user has to orient parts in such a way that there is minimum distance possible between each part. With such part orientation, build height is minimized, which is good for materials usage since unused powder in a single layer must be recycled during post processing phase. Most parts produced with AM technologies generally have complex shapes, and such ideal part orientation is hard to achieve by manual orientation of single part in whole part group, so user has to use available optimization algorithms for optimal part orientation. Build envelope prepared for simultaneous production of 20 parts is shown on Fig. 2.11.

Fig. 2.11

(Source Tomislav Breški)

Part nested in build envelope of HP Jet Fusion 580 machine using Monte Carlo algorithm

Slicing of build envelopes is in most cases done automatically and is specific for each AM technology. With the selection of layer height, computer algorithms do the slicing of whole build volume from initial build platform position to final build platform position in vertical steps defined by layer height. Specific AM technologies have various machine parameters which have to be adjusted for each individual build layer and after completion, packed in a single file which will be executed on the machine during production. Example of a single slice for FFF machine is shown on Fig. 2.12. Since modern FFF machines can use multiple materials in single production cycle, support geometries can be built with soluble materials which result in shorter part post processing time. Slices by themselves are a visual representation for users to visualize AM process, but AM machines need specific commands which they can process and use for production. In most cases G-code is used, and it is generated automatically with slicing software, and includes all the necessary commands for all machine components which are utilized during production. FFF machines are generally the simplest of AM technologies, and have the least amount of movable components which are moved by stepper motors.

Fig. 2.12

(Source Tomislav Breški)

Slice of build envelope for FFF machine (build material in green, support material in blue)

Snippet of G code commands for build job is shown on Fig. 2.13.

Fig. 2.13

(Source Tomislav Breški)

G-code snippet for initial layer build on FFF machine

2.2 Build and Post-processing

With diversification of AM technologies, one aspect of AM manufacturing mutual with all of them, is postprocessing of parts after being built on AM machine. To achieve desired mechanical and aesthetic properties or geometrical accuracy, most of the AM parts need to be post processed in varying amounts. Polymeric AM technologies can include operations such as manual support removal as shown on Fig. 2.14, sanding, painting, etc. In extrusion-based AM technologies by utilizing of soluble support materials such as PVA, PVB, or HIPS mechanical support removal can be greatly alleviated and surface finish of supported surfaces has fewer geometrical artefacts which would need to be sanded.

Fig. 2.14

Manual support removal from FFF part (Photo by Tomislav Breški)

Example of a finished part made from ABS polymer on an FFF machine is shown on Fig. 2.15. This part has gone through the process of support removal, sanding and painting.

Fig. 2.15

Finished ABS part manufactured on an FFF machine (Photo by Tomislav Breški)

Metallic AM technologies can include all of the forementioned operations, but in most cases they also include heat treatment operations such as stress relief, hot isostatic pressing or case hardening. Binder jetting AM technologies with metallic powders are becoming leaders in high scale additive manufacturing, but due to the nature of the process, parts are printed with internal porosities which must be eliminated with heat treatment (i.e. sintering or infiltration) at specific temperatures.

If all aspects of AM process are taken into consideration, from AM technology and material selection to adequate steps in post processing stage, AM parts can replace conventionally produced parts. Example of such project is shown on Fig. 2.16 where a part which was discontinued and thus unavailable for procurement, redesigned for AM and printed using FFF technology.

Fig. 2.16

Replacement part made with FFF AM technology (Photo by Tomislav Breški)