Mejoras para el sistema SIMERT

There is a wide variety of ALM systems currently available, each of which has its

own advantages and disadvantages for different applications. They cover systems that can

produce components ranging from headlight assemblies and toilet seats to microfluidic flow

cells. All ALM systems can be further classified under one of three groups, based on their

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1.6.1 Solid Based Fabrication Methods

Solid based methods are those that use a pre-formed, solid, bulk material as the

primary method of fabrication. The two most notable methods solid-based systems are

laminated object manufacture (LOM) and fused deposition modelling (FDM).

LOM, or layered object manufacture (with other names such as Plastic Sheet Lamination

(PSL) and Paper Lamination Technology (PLT) given for more specific versions of the

technology), builds up the model layers by using laminar sheets of material. Each layer is

individually patterned using either a CO2 laser beam, steered using galvanic mirrors or a

blade. Before a layer is cut, adhesive is applied to the top of the previous layer so that the

layers of the model are bonded. In theory, any laminar sheet material can be used providing

it has a regular thickness and can be adhered. The excess material in the laminar sheets is not

removed during fabrication. This means that the method cannot be used to fabricate hollow

objects; however, the excess material is used as a support structure during fabrication. At the

end of the fabrication process the excess material needs to be manually removed. Typical

build resolutions for such systems are 100 µm in XY while the layer thickness is dependent

on the thickness of the laminate used; 100-200 µm is typical.

FDM systems create layers by vectoring the outline and infill space of the object using

an extruded thermoplastic material. A filament of the material (such as ABS, PLA

(polylactic acid), or PCL (polycaprolactone)) with greater diameter than the desired extruded

“string” is forced through a heated nozzle (usually between 150°C and 250°C depending on

the material), which is moved around the build area using an XY stage (sometimes the

extruder head is fixed and the build base is moved instead). Due to the means in which the

material is extruded they are also often colloquially called “toothpaste machines”. The

potential build envelope for FDM systems is only limited by the size of motion carriages that

hold the head, although dimensions on the order of 300 mm are typical. The resolution of

FDM systems is limited by the diameter of the extruded material; therefore, the resolution in

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difficult to generate solid layers, as such, components fabricated using FDM have a degree

of porosity.

1.6.2 Powder Based Fabrication Methods

Powder based fabrication methods are some of the most commonly used in industry

due to self supporting nature of the fabrication process being able to produce large

components with uneven, undulating surfaces – such as prototypes for car headlight

assemblies. Selective laser sintering (SLS) and 3D printing (3DP) are the most common

example of the technology.

In order to create layers, SLS begins by spreading a thin layer of powdered material

(equal to one layer thickness) on top of the previous layer. A laser is then used to selectively

sinter the defined areas within each layer. The non-sintered material is left in the build area

and can therefore be used as a support structure. Similar methods which are derivatives of

SLS are electron beam melting (EBM), which uses an electron beam in place of a laser,

selective laser melting (SLM), which fills the fabrication chamber with an inert gas whilst

building, and selective mask sintering (SMS) which uses a parallel irradiation method and

masks that are generated by patterning toner onto a glass plate. The resolution of SLS

systems is on the order of 0.5 mm.

In a similar manner to SLS, 3DP constructs each layer by spreading a layer of the

powdered material on top of the previous. However, instead of melting or sintering the

material, a print head (similar to those found in ink-jet printers) deposits a binding material

onto the powder surface. The binder can also include inks that allow full colour models to be

generated in one fabrication process. The resolution of such systems can be down to around

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1.6.3 Liquid Based Fabrication Methods

Liquid based fabrication methods are those that rely on the solidification of a

photosensitive liquid polymer. There are a number of systems that use this principle to

fabricate components, the most well-known of these is stereolithography apparatus (SLA).

SLA is generally considered as being one of the first, if not the first, method of ALM. The

earliest reference found was from a paper published in 1981 by Hideo Kodama [4] where the

process was first outlined. Six years later, in 1987, 3D Systems (USA) produced the first

commercial system [2, 5].

Traditional SLA systems operate by positioning a flat build platform in a vat of resin

material such that one layer thickness material remains between the free surface of the

material and the previously solidified layers. The layer is created by selectively manipulating

light in order to polymerise a photocurable liquid polymer resin into a solid polymer in the

desired pattern. This can achieved either by using galvanic mirrors to steer a laser beam (UV

[6] or IR [7]), by employing a dynamic mask system [8]. Once the material is cured, it

remains solid and will not re-dissolve in the monomer [9]. SL systems can achieve layer

thicknesses of 100 µm with XY resolutions of 50 µm. Microstereolithography (MSL or µSL)

is a form of SL that can be used to fabricate parts on the micron scale and therefore presents

a possible alternative to silicon based fabrication methods. Depending on the system, layer

thicknesses of 20 µm can be achieved with an XY resolution also down to 15 µm. The

process is discussed in greater detail in the following chapter.

There is a further liquid based method, rather confusingly also referred to as 3D printing.

This method again uses ink-jet technology to deposit measured amounts of material in the

desired 2D pattern for each layer. The material is then immediately cured using a UV light

source, which is attached to the print head. The systems employing this technology can have

large build envelopes - up to 490mm x 390mm x 200 mm for the Eden 500v (Objet Ltd,

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Table 1.1 shows a comparison of systems, a number of which show promise as a

potential solution to the issue of affordable fabrication of functional devices. However, it is

felt that stereolithography and in particular, microstereolithography, show particular promise

as an alternative method to fabricate functional micro-devices and components with micro-

structures. The use of its micro-scale manufacturing capabilities have already been

demonstrated through its use in the jewellery industry for creating moulds for the casting of

rings etc. However, commercial system manufacturers appear to be content with this, and

the current range materials and methods limit its use to purely structural or mechanical

components. Therefore there are a great number of materials covering these applications,

some including ceramics/glass particles for increased strength, others being wax based

aimed at casting applications.

The combination of advantages that MSL has over other ALM technologies,

specifically its resolution combined with the ability to use low material volumes and

incorporate secondary, possible functional components within the material (as demonstrated

by the commercially available ceramic materials), make it ideal as an alternative to silicon

processing. While some researchers have begun to demonstrate the potential uses of MSL in

more engineering related applications, such as basic air coupled ultrasonic transducers [10,

11] and micro-mechanical bellows [12], it is difficult to find any that present the material

itself providing functional significance in an application. One exception to this is where

researchers have begun to explore its uses in the medical field, for generation of bone and

tissue scaffolds [13-18]. As such, there is a need to expand the range of available materials

through the development of novel, functional materials. By increasing the range of materials

to include those with additional functional properties, introducing new ideas for methods of

fabricating existing components, and creating additional exemplar components using this

technology, it is felt that the advantages of MSL could become more clearly seen. It is felt

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an invaluable future technology as a possible alternative to more traditional techniques of

micro-manufacture.