Additive Manufacturing is defined as fusion of materials to build parts depending on the CAD data in contradistinction to subtractive and formative manufacturing methodologies due to ISO/ASTM 52900:2015.

INTRODUCTION

Engineering design process is a challenging process that begins with the definition of the problems and ends up with the manufacturing of the product. The available subtractive, formative and additive manufacturing processes were illustrated in Fig. 1.

Classification of shaping processes

Knowing that the design process is an iterative process, some steps can be repeated until the desired performance is achieved. For this reason, both release date of the product into the market is delayed and the cost of the product increases. Furthermore, the designed product may even become outdated until a company releases it.

Hence when the companies develop new products, they must present their products to the market in a short time. To survive in this competitive global environment, it has become compulsory for some companies to move from mass production to flexible manufacturing systems such as additive manufacturing (AM).

Additive Manufacturing is defined as fusion of materials to build parts depending on the CAD data in contradistinction to subtractive and formative manufacturing methodologies due to ISO/ASTM 52900:2015.

Importance of Additive Manufacturing

Main application areas of Additive Manufacturing technology can be briefly separated into rapid prototyping (RP) and rapid manufacturing (RM). In RP area Additive Manufacturing facilitates understanding and evaluation of the prototype. Thus, essential modifications can be made on the concept design (prototype) according to judgements from different departments in the facility before the release date of the product.

In RM area Additive Manufacturing can be used to produce end user product, finish parts that require assembly or for producing mold (Rapid Tooling). Also printed objects can be used as master models. Secondary processes like vacuum casting, silicon rubber molding etc. is applied to master model to produce mold or prototype part. These methods are called as indirect tooling and indirect prototyping processes respectively.

Besides AM’s contribution to mold and prototype production, it can be used to repair damaged or worn region of the any component with minimal heat effect.

Manufacturing of the complex geometries is limited in conventional manufacturing methods. Therefore, performance of some developed solutions to new generation problems are lower than Additive Manufacturing technology. For instance, cylinder head both designed by conventional and Additive Manufacturing were given in Fig. 1.2. Additively manufactured cylinder head both is lighter and can work in higher speed with improved cooling surface area. So, Additive Manufacturing makes companies more competitive in the market.

Performance evaluation of the traditionally and additively manufactured cylinder head

The more complexity means the higher cost and lead time to market in conventional manufacturing methods in contrast to Additive Manufacturing technology.

Since the part can be built just in time in Additive Manufacturing , both stocking and transportation (logistic) costs decrease. This is especially useful for the critic operations such as military etc. There is no need to wait for spare parts which cause delay in operation for days.

Moreover, investment in tooling equipment (jigs and fixtures etc.) decreases so that no longer need to large inventory.

Despite the advantages of Additive Manufacturing described above, it has following drawbacks:

  • Diversity of printable materials are restricted when it is compared with conventional systems.
  • Production time is high cause of the layered manufacturing.
  • Poor dimensional accuracy and surface roughness according to conventional processes.
  • Low level of accuracy in repeatability

But both academia and companies continue to work to overcome these problems.

Additive Manufacturing working principle

To build or print a part (3D object), most AM processes involve the following 5 steps as shown in Fig. 1.3.

Generalized Additive Manufacturing process flow chart

1.CAD model: The CAD file that describes the geometry of the part to be printedor build is required.

2.Conversion of CAD data to STL format: Using most computer-aided designprogram, CAD file is transformed to STL file format that involves data about the surface geometry of a three-dimensional model by the unit normal and vertices of the triangles according to the three-dimensional Cartesian coordinate system without any CAD model features such as color, texture etc.

3.Preprocessing: This step can be divided into four subsections;

  • Part orientation is the rotation of the part due to axes of the machine’scoordinate system in the build volume andthe orientation is adjusted in the build volume via software of the Additive Manufacturing machine. Orientation of the part before printing is crucial since properorientation improves dimensional accuracy, reduces the build time andthe support structure needed for building the model. Thus, cost of theprinting process will decrease.
  • Support structures are fine scaffold structures which are used for heatdissipation and fixation of the part in the build volume, especially for supporting horizontally oriented and overhanging surfaces. After determining orientation of the part to be printed, if it is required, support structure from the build material (for single nozzle extrusion Additive Manufacturing systems etc.) or soluble material (for multi-nozzle extrusion Additive Manufacturing systems etc.) must be employed to form complex geometries such as undercuts, and printed part assemblies with moving components etc. Proper support structure placement is critical since it affects the build time, material consumption and quality of the printed part.
  • Build parameters or printing parameters are adjusted by the user in the software of the Additive Manufacturing machine so that the machine operates according to these set values. Setting these parameters is very important since it affects the quality of the build object.
  • Slicing process: In this step, parallel planes are formed based on the orientation of the digital model then each two parallel planes are intersected with STL file of the model. Intersection of each plane with triangles of STL file gives set of lines / points which are used to create contour of the one slice or layer.

4. Printing of part is commenced due to the adjusted settings by the operator.

5. Post processing: Due to used Additive Manufacturing technique and material, proper post processing methods such as surface or heat treatment processes etc. can be applied to the printed part to improve its surface roughness, dimensional accuracy and mechanical properties.

Additive Manufacturing Processes

American Society for Testing and Materials (ASTM) group classified the current and future Additive Manufacturing methods in 7 categories. The categories and technologies covered are given in Table 1.1.

Additive manufacturing technologies

Vat Photopolymerization:

The UV laser beam is reflected to the liquid photopolymer material in thetank (vat) according to CAD data. Hence photopolymer material is cured by laser beam. After one layer is obtained, build platform moves downward by adjusted layer thickness. Then process continues until the desired geometry is obtained. Schematic representation of the process was given in Fig. 1.4.

Vat photopolymerization process

Material Extrusion:

In extrusion-based systems, polymer-based filament is heated above its glass transition temperature or melting point then it flows to the appropriate place on the build platform via controlled motion of the both drive wheels and extruder head depending on the CAD data. After one layer is deposited, build platform moves downward with set layer thickness. The process continues till the part is built completely. Typical material extrusion-based process was given in Fig. 1.5.

Fused deposition modelling process

Material Jetting

Material jetting is similar to inkjet printers. Droplets of suspension that includes either support or build nano particles, are jetted to the build platform selectively due to CAD data via continuous or drop on demand (DOD) method from thousands of nozzles. Solidification is achieved by ultra violet light for resin or temperature control for metal and ceramic materials. Temperature controlled environment (for metal or ceramic) evaporates the binder in suspension to form one layer.

After one layer is deposited, the process is repeated until the whole part is printed. Post processing is required to strengthen the part and remove support structure. Material jetting method is shown in Fig. 1.6.

Nano particle jetting process

Binder Jetting

The main material is in powder form at this type of systems. Process commences with spreading of the powder material by a roller as seen on Fig. 1.7. Binder that has special formula selectively deposited onto powder layer according to CAD data. Then the build platform lowers one-layer thickness and process continues until the whole geometry is achieved. Post processing is required to strengthen the printed object.

Binder jetting process

Powder Bed Fusion

Powder bed fusion processes requires the spreading of the powder material like in binder jetting. Laser or electron beam is used for the selectively formation of bond between powder particles. Mechanical properties of the build part are close to traditionally manufactured part. Post processing is performed to remove support structures, release residual stresses, improve metallurgical structure, and surface roughness. The powder bed fusion system is shown in Fig. 1.8.

Powder bed fusion process

Direct Energy Deposition

Working principle of the system is similar to FDM. But nozzle can move in multiple direction since it is mounted onto 4, 5 axis machines. Used material form can be wire or powder in these systems. When the material is deposited from a nozzle to existing surface of the object, in the same time plasma arc, electron beam or a laser is applied to melt the material. Also, the system can be used for repairing or creating new feature on the existing object. Some companies adapted these systems to their CNC machines and called as hybrid systems. Direct energy deposition process is shown in Fig. 1.9.

Direct energy deposition process

Sheet Lamination

Form of the raw material is sheet form in LOM. Feeding of the sheets onto build platform is supplied by rolls as it is seen from Fig. 1.10. Laser or knife is used to cut the sheet in desired shape. Bond formation between the layers occurs when heated roller applies pressure to adhesive coated sheet material and activate heat sensitive adhesive.

Sheet lamination process representation

 

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