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3D printing

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									                                    3D printing:
                           the world of rapid prototyping.

So just what is rapid prototyping?

Prototyping: the process of quickly assembling a model to test certain aspects of a
design, gather feedback and illustrate ideas and methods to be used on the finished
product. It is a very important part in the design process.

Rapid Prototyping: this is the production of physical objects using the process of
Solid freeform fabrication.

Solid freeform fabrication: a manufacturing technique whereby a delivery of energy
and/or material to a point in space is used to produce a solid object.

The printers can also be used to produce finished articles in some cases.




The CAD/CAM process

CAD – computer aided design

CAD: the use of computer tools to assist in the design process. 2d drafting or 3d
solid/surface modelling are the two major types of programs available for use in
design. The process involves design, modelling, development and optimization of a
product. With specialist software some testing can be carried out in the virtual CAD
state reducing costs later in the product development stage.

An object can be created to exact dimensions in virtual 3d form. It can be built up
from each of its constituent parts, with each of these in turn being completely
designed and built in the design software. Product designers and architects amongst
others use CAD drafting software to draw plans and sketches of projects, such as
building plans for a house.

Once a product has been designed in CAD the tools and machinery required for
manufacture can be chosen, and if necessary designed and built specifically for this
products manufacture.

A CAD drawing or design can be saved and edited with ease in future, replacing the
need to redraw a product if a change is made at any point. This not only reduces costs
but also the overall workload.

CAM – computer aided manufacturing

CAM: the use of software tools that assist in the manufacturing of a product. The
software is used to convert a 3D CAD model into a computer numerical code (CNC
code) that can then be fed to a Numerically Controlled machine which will then
manufacture the product. The code is often termed G-code.
CAM involves creating an interaction between at least two pieces of software. Often
the CAD and CAM software pieces are not linked, so the operator must export the
CAD file in some common form recognised by the CAM software, which will then
enable the code for the product to be created. The code is transferred to the machine
using a Direct Numerical Control (DNC) program.

At present it is still necessary in most environments for a machinist to select the tools
manually for the machining process, or adjust the G-code when it is created so the
machine selects the correct tool for its current task at the right tie in the overall
process.

Rapid Prototyping is a CAD/CAM process, a product is designed as normal and then
the code is generated for manufacture. The only difference is the manufacturing
process, and the different methods of manufacture are explained on this site.

Fused Deposition Modelling

A Plastic Modelling process

This machine is fed by either a hopper containing plastic pellets or a plastic filament
from a coil. The plastic is heated up to a molten state in the extrusion nozzle. This has
a flow control system to allow the process to be stopped or started at any time. The
nozzle itself is can be moved in all of the x-y-z directions, and thus it is the nozzle
that is the only moving part of the machine, the model is built on a fixed bed.

The model is created by manoeuvring the nozzle over the bed in the required
geometry; the nozzle deposits a thin bead of plastic to form each layer. For the next
layer the nozzle is moved up very slightly before it again deposits more plastic on top
of the previous layer, and gradually the model is formed. The plastic solidifies almost
immediately after it is ejected from the nozzle and it bonds to the previous layer. The
whole process takes place in a sealed chamber that it is maintained at a temperature
just below the melting point of the plastic. This means the process is much more
controllable and only a small amount of heat energy is required in the extrusion
nozzle to melt the plastic.

Materials
Materials commonly used in the FDM process include:

      ABS - – very strong
      Investment Casting Wax
      Polycarbonate
      Poly(phenyl)sulfone

The last two provide increased strength and a greater temperature range for the
method.

Laminated Object Manufacturing

Modelling using paper
Object profiles are cut from a roll of paper by a CO2 laser. The paper is fed from a
large roll, known as a web, through a heated roller and onto the stack of bonded, cut
paper to form the next profile layer of the model. The heated roller melts a laminated
plastic coating o the underside of the paper which enables it to be bonded to the rest
of the model. The laser mounted on an x-y rig, then traces the profile of that layer.
After the features are cut, all excess paper is cut away. This separates the profile from
the web; this excess is then wound onto a waste roller and the supporting platform
moves down in the z direction. This allows the next layer of paper to be drawn in on
top of the stack, the platform then moves back up so that it is 1 layer of paper below
its previous position. The process then continues.

Areas of the model that have to be removed once the process is complete are heavily
scored/cross-hatched by the laser to ease removal. However, this excess is useful as it
supports overhangs and undercuts on the model until the modelling is complete. Some
of this removal can be time consuming due to complex shapes making access
difficult.

The finished product looks and feels like wood and can be worked and finished to the
same standard. The finish and accuracy are not as good as other printing methods with
more ragged edges and occasionally a lack of continuity to the object surfaces. The
final object is laminated to prevent water absorption and thus spoiling of the product.

Laser engineered net shaping (laser fusing)

A method of Selective Laser Sintering

A unique process that can go from raw material direct to finished metal product often
without any further secondary operations.

A high powered laser is used to melt metal, supplied coaxially in powder form to the
beam focus through a deposition head. Usually the beam travels through the centre of
the deposition head and is focussed by one of more mirrors or focal devices. The x-y
table moves below the head to fabricate a layer, the head moves upwards in the z
direction as each layer is completed so the next one can be started.

The powder is delivered around the entire circumference of the head either by a
gravity feed or by using a pressurised inert gas to carry the powder, a carrier gas.
Even if a gas is not used as a carrier, an inert gas will be used to shield the metal melt-
pool in the head from atmospheric oxygen. This allows for better material property
control and better surface adhesion, as the lack of oxygen gives better surface wetting.

Some machines have been designed to use a fine metal wire as a feed, in this case the
wire is fed off-axis to the beam.

Materials

      Stainless Steel
      Inconel
      Copper
      Aluminium
      Titanium
      Composites (experimental)

The entire process takes place in a sealed unit. This isolates the process from ambient
surroundings. Also provides protection to the operators from the powders ad laser.
Laser power used is dependant primarily on feed-rate and material in use. The result
can be anywhere between a 100 Watts and 20+ KW.

Object fabricated are very close to the net shape but will in most cases require some
form of finishing. The dense well grained structure provides product properties
similar to, or better than intrinsic materials.

Selective Laser Sintering

An additive technique

A high power laser is used to fuse small powder particles of a material into a solid
mass which when finished represents a desired 3d object. Materials that are used in
the process are plastic, metal and ceramic.

A roller spreads powder onto the surface of the build cylinder, from a delivery hopper.
A scanner then traces the profile of the object over the tightly packed powder surface,
and the laser melts the powder where it strikes under the scanners guidance in the x-y
direction. Once a cross-section is completed, the build cylinder is lowered in the z
directionn by the thickness of one layer, and the roller spreads a new powder layer on
top of the previous one and the process repeats until the object concerned is
completed.

The powder is also held in a cylinder, and when more is required the cylinder moves
up by the thickness of one layer of powder, this layer is the spread onto the build
cylinder.

As is the case with other rapid prototyping processes, the build unit is sealed and a
temperature just below the melting point of the powder is maintained. So only a small
amount of energy is required from the laser. The atmosphere in the chamber is
nitrogen, as this will prevent explosions, which are very possible when handling large
volumes of powders.

Before the part can be removed after completion, it must be given a cooling period. It
can take up to two days for some large objects to cool down sufficiently. Once
removed a simple brush down removes excess material.

There is no problem with supporting overhangs etc in this process as the powder bed
provides support to the model; this reduces the complexity of the process and the
finishing time. No final curing is required in this process, but as the model is sintered
its surfaces are porous, so infiltration of another material can be used to cure this.
Surface finishes are of an acceptable standard but are not brilliant.

Many parts can be packed into one powder bed reducing costs of running repeated
individual processes, so in some cases this process is useful for batch manufacturing.
Stereolithography

A highly accurate process

A movable table is lowered I the z direction into a vat of UV reactive liquid
photopolymer. The whole process takes pace in a sealed unit to prevent escape of
fumes from the resin.

Photopolymer – when light of the correct colour strikes this liquid, it will change from
liquid to solid. Most react to UV light but some that react to visible light are also
available for use.

A laser is moved over the surface of the liquid by a scanner in the x-y direction. The
laser traces the profile and geometry of the object being created. Where it strikes the
liquid, it solidifies. Supports must be utilised in this printing process and they can be
either manually or automatically designed. Once a layer is completed, depending on
the machine in use, the table is either lowered into the vat a distance equal to the
thickness of a layer, or the table is lowered and the surface is scraped by a blade
depositing a fresh new layer. Typical layer thickness being: 0.002” to 0.006”. Pump
driven recoating systems are also being introduced to speed up this process. This
process is repeated until the object is completed and it sits immersed in the vat.

The completed object will be removed from the vat and drained. Swabs will then be
used if excess resin still remains. It will then be rinsed in a chemical bath and cured in
a UV oven called a PCA (post curing apparatus). This process is not always necessary
for all material/process types in stereolithography. The product is then ready to be
finished, so all supports can be removed and stubs sanded away. And any uneven
surfaces will be sanded and polished to produce a very neat, accurate and impressive
finished product.

3D Printing

A faster, more affordable Technology

An ink-jet type print head is used to bond the fine powder (often starch or plaster) to
create the model. The print head moves in the x-y direction above the bed of powder,
which moves in the z direction. The head deposits a liquid adhesive which bonds the
powder particles together in the shape of each cross-section of the object being made.
Once a layer is completed the bed is lowered by the thickness of one layer and fresh
new material is swept over the print surface. The process is then repeated until the
object is completed. This process is very similar to the SLS process. Once printing is
finished, the product is removed and brushed down to clean of any excess powder. In
most cases the object must be infiltrated with a hardener before it can be handled to
prevent any damage.

3D printing is the only technology that allows for a full colour model to be produced,
the ink-jet head is fed by different coloured adhesives which then dye the powder on
contact when they bond to create a layer of the object. The best printer available at
present features a 24-bit colour capability, produced by Z corps.

								
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