The application of AM Systems

Rapid prototyping (RP), rapid tooling (RT) and rapid manufacturing (RM) are three categories of AM application which have been introduced in manufacturing processes [Pham, 2001, Kruth et al, 2005b].

2.2.4.1 Rapid prototyping (RP)

Additive manufacturing (AM) was first used for developing prototypes of functional products. Functional prototypes are usually used for the visualisation of the concept to verify and justify design details; the verification and optimisation of the design of the proposed part and/or assemblies in order to meet the requirements of the form/fit/function; for design review and assessment; for marketing, promotion and advertising; and for communication with customers. By using rapid prototyping, these purposes can be fulfilled relatively rapidly, since no tooling is required.

2.2.4.2 Rapid tooling (RT)

In the product development cycle, tooling is a critical issue that must be considered. Mould-making for prototypes and production is a time-consuming, high-risk and expensive task due to requirements of geometrical complexity, high accuracy and tolerance [Dalgarno and Stewart, 2001]. As a result, tooling preparation may be the slowest and most costly phase in the product development cycle. The tool must be durable, wear-resistant, and produce a quality surface finish. As reported by Pham [2001], the introduction of rapid tooling has significantly reduced the manufacturing time of prototypes, pre-production and, in some cases, full production tooling.

Rapid tooling is categorised into direct and indirect methods. Indirect rapid tooling uses rapid prototype master patterns to produce a moulding before making a tool, whereas direct rapid tooling uses the rapid prototyping machine directly to manufacture the actual core and cavity mould inserts [Noorani, 2006]. Indirect methods are more commonly used than direct methods, but direct tooling offers more opportunities in time and cost savings. There are several problems with rapid tooling, for instance, accuracy, durability, mould finishing and hardness [Pham, 2001].

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2.2.4.3 Rapid manufacturing (RM)

Recently, additive manufacturing has begun to be applied to the manufacture of structural or functional end-use parts or components directly. This technology is continuously developing for a wide area of materials, including metals, polymers, and ceramics for many applications, such as in medical applications and other customised functional applications. This trend, known as rapid manufacturing (RM), is likely to be an attractive technological approach in the future, and is considered in detail in the next section.

2.2.5 Rapid manufacturing (RM)

2.2.5.1 Definition and basic process

Rapid manufacturing has been defined as “the manufacture of end-use products using additive manufacturing techniques (solid imaging)” [Levy et al, 2003]. Other sources describe rapid manufacturing as producing useable products or components directly by using additive fabrication technology [Castle Island‟s, 2010], or “a technique for manufacturing solid objects by the sequential delivery of energy and/or material to specified points in space to produce that solid. Current practice is to control the manufacturing process by computer using a mathematical model created with the aid of a computer” [Wikipedia, 2009a]. According to Hopkinson et al [2006], rapid manufacturing is defined as “the use of a computer-aided design (CAD)-based automated additive manufacturing process to construct parts that are used directly as finished products or components”.

2.2.5.2 Process chain of rapid manufacturing

Even though the basic process of rapid manufacturing (RM) is similar to that mentioned in section 2.2.1, in this section the process is described in more detail as a rapid manufacturing chain. Figure 2-8 illustrates the process chain of rapid manufacturing, showing the relationships between phases and which parts or steps of rapid manufacturing can be used to aid engineering design, in the design phase of product realisation.

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Moreover, the final step of the process chain, the post-processing phase, also shows that there is an opportunity to heat-treat the component to improve its material characteristics. This is one of the objectives developed in this research study.

Figure 2-8 Process chain of rapid manufacturing

The basic process chain of rapid manufacturing consists of the following steps [Cee Kai, 2003; Venuvinod, 2004;Noorani, 2006]:

1. Created of CAD model of the design:

First of all, a CAD solid model is created. Solid modelling software is commercially available, such as IDEAS, ProEngineer, SolidWorks,

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Unigraphics, CATIA, or many others, which can be used to create the 3D model part. In this phase, model design can be analysed and tested virtually, for instance regarding the failure analysis of the design prototype of the product.

2. Conversion of the design model to the stereolithography (STL) file format: CAD software uses various algorithms to represent solid objects, so that this conversion must be conducted since the STL format has been adopted as the standard for rapid manufacturing technology. The STL format represents a 3D surface as an assembly of planar triangles.

3. Slicing of the STL file into two-dimensional (2D) cross-sectional layers: In this phase, the STL file is made ready for construction. By using a pre- processing program, the user adjusts the orientation, location and size of the model. The pre-processing program is normally associated with a particular type of layer manufacture machine.

4. Manufacture of the part:

The machine constructs the part layer by layer automatically. Generally, the thickness of each layer, depending on the build technique, is in the range between 0.01 mm to 0.5 mm [Gibson and Shi, 1997].

5. Post-processing (clean and finish the model):

Post processing is the last phase. This work involves removing the part from the machine and detaching any supports that were applied. The component may need surface treatment and minor cleaning, depending on the process and application used. Furthermore, as a post-processing action, heat treatment to modify and characterise the material can also be used to improve its properties.

2.2.5.3 Issues in RM systems

As mentioned in the previous section, there are several reasons for the use of rapid manufacturing. However, one of the most interesting reasons for adopting this technology arises from its advantages in the area of design, rather than the manufacturing approach used [Hague, 2006].

During the last few decades it has been recognised that geometry is a critical factor in design analysis and the development of products, mainly due to cost factors.

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Within rapid manufacturing (RM), geometry is not a limiting factor, and freedom of design can be applied. It is claimed that this will reduce lead times and overall manufacturing costs [Hague, 2006].

The range of material, used in RM systems is still limited compared to conventional manufacturing. Since this technology has developed, the choice of possible materials and processes has grown, but not widely enough to support all materials-driven product design.

2.3 Selective Laser Sintering (SLS)

At the present time, SLS is a commonly used technique in rapid manufacturing for prototyping, tooling and manufacturing purposes [Levy et al, 2003] using a wide range of polymer, metal and ceramic materials [Kruth et al, 2003]. It was developed in the late 1980s as a technique for rapid prototyping (RP). In this section, SLS is described in more detail.