SLA Biomodel
with Customised Implant
The model is built on to a platform within the resin tank. At the start
of the process the platform is positioned just below the surface of
the resin. The first layer of the model, the base, is drawn by the laser
to form a solid layer, typically 0.25mm thick. The platform then descends
in the bath to allow new liquid resin to cover the cured layer and the
next model slice is constructed above it. In this way the whole of the
model is built from the base up, as the platform descends in the tank.
Micromanufacturing
techniques / Vector-vector point curing
William O’Neil (Micromanufacturing Laboratory, Faculty of Engineering,
University of Liverpool) has documented a resolution of >5 microns
for SLA.
Process resolution for microstereolithography techniques in
terms of minimum voxel size is reported as 5x5x3µm (Ikuta and Hirowatari
1993 Proc IEEE MEMS Fort Lauderdale). In the 1990’s despite introduction
of a new laser with smaller spot size 0.08-0.1mm, point wise curing
and liquid levelling it became apparent that manufacture of true high-resolution
microparts were not possible with SLA, but was possible with other RP
systems which were able to produce a resolution down to < 1 micron.
MicroTec (Duisburg, Germany) has developed a process Rapid micro
Product Development using a split high resolution UV laser beams
which can cure in parallel layers down to 1 µm with a minimum component
size of 1x10x10 µm.
Laminated
Object Manufacturing - LOM (Figure 2)
In this system the model is formed from successive layers of heat bonded
sheet material. The sliced CAD data is used to control a laser, which
cuts the perimeter (only), of each slice in the sheet material. The
waste material around the slice is left in place to support the next
layer of the model but to assist with subsequent removal; it is scored
by the laser to form blocks. Once each layer is complete a new sheet
of material, typically adhesive coated paper, is heat bonded on top
of it and the laser cuts the next slice. The final model has the appearance
of soft wood.
Figure 2
LOM is used mainly for larger bulky parts, because the paper it uses is
relatively cheap compared with powders and resins used in other processes.
A contraction of 2-3% occurs in the z (build) direction following the
build and delamination may occur at elevated temperatures for a prolonged
time.
Laser Sintering
- LS (Figure 3)
This system operates in a similar way to stereolithography but in this
case the photosensitive resin is replaced by a fine heat fusible powder
(wax, thermoplastic etc.). A CO2 laser draws the CAD slices on the surface
of the powder, locally melting and fusing it together. A new layer of
powder is placed on top of the completed slice and then the next slice
is drawn. The unsintered powder around the slice is left in position to
provide support for the model and is removed once the building routine
is complete.
Figure 3
Sintered nylon can be used as direct patterns for sand casting. They
are more durable than SLA patterns but require more hand finishing,
and need to be sealed before use. Sintered sand is a quick reliable
route of producing prototype castings. Excellent internal shapes are
virtually guaranteed.
Sanders
Model Maker (Figures 4,5)
The ModelMaker II system produces solid 3D models with a far higher
level of accuracy than other rapid prototyping systems.
The following is from Sanders Prototype Inc web site:
Up to 20 times more precise than some other rapid prototyping systems.
Build envelope: 12 x 6 x 9 in. (30.48 x 15.24 x 22.86 cm)
Build layer: 0.0005 in. (0.013 mm) to 0.0030 in. (0.076 mm)
Achievable accuracy: +/- 0.001 in.
(0.025 mm) per inch in X, Y and Z dimensions
Surface finish: 32-63 micro-inches (RMS)
Minimum feature size: 0.008 in. (0.20 mm)
Automatic generation of model support structure. Figure 4
Point and click access to job status, remaining build time, and other statistics
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Figure 4
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EXCELLENT
CASTING QUALITIES
- Negligible co-efficient of thermal expansion (prevents
casting shell ruptures)
- Fast melt out in autoclave
- No ash or residue contamination
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Figure 5
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BUILD
PRECISION
With build layers as much as 20 times thinner, the ModelMaker II system
delivers higher precision models, prototypes, and patterns than other
rapid prototyping systems.
3-Dimensional
Printing (Figures 6,7)
To build a part, the machine spreads a single layer of powder onto the
movable bottom of the build box (steps 1,2,3). A binder is printed onto
each layer of powder to form the shape of the cross-section of the model
(4). Then the bottom of the build box is lowered by one layer thickness
(5) and a new layer of powder is spread. This process is repeated for
every layer or cross-section of the model. Upon completion, the build
box is filled with powder, some of which is bonded to form the part,
and some of which is loose.
Figure 6
Description
of Process:
1. Collect Powder
2. Spread Powder
3. Discharge Excess Powder
4. Print
5. Feed Piston Up, Build Piston - Repeat
Z-Corporation Z402 Machine (Figure 7)
In this 3D printer, to build a part, the machine spreads a single layer
of powder onto the movable bottom of the build box (steps 1-3).
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A binder is printed onto each layer of powder
to form the shape of the cross-section of the model (4). Then
the bottom of the build box is lowered by one layer thickness
(5) and a new layer of powder is spread. This process is repeated
for every layer or cross-section of the model. Upon completion,
the build box is filled with powder, some of which is bonded to
form the part, and some of which is loose.
Once a build is complete, the excess powder is vacuumed away and
the parts are lifted from the bed. Once removed, parts can be
finished in a variety of ways to suit your needs. For a quick
design review, parts can be left raw or "green". To quickly produce
a more robust model, parts can be dipped in wax. For a robust
model that can be sanded, finished and painted, the part can be
infiltrated with ZR10 resin. The machine prints between one and
two vertical inches an hour, and so in terms of speed is very
fast even in comparison with other rapid prototyping systems.
However, the accuracy level is compromised to achieve these speeds.
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Figure 7
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Fused Deposition
Modelling - FDM (Figure 8)
Figure 8
This process employs a system very similar to an X/Y plotter. In this
case the pen is replaced with an extruder head which draws each slice
of the model using a fine stream of plastic (or wax). The extruded material
cools very rapidly to form a solid layer. As with the other systems
each layer is built on top of the preceding layer, starting at the base
of the part.
Thermojet
Process (Figures 9,10)
Figure 9
| Wax parts produced in this system can be
used as sacrificial patterns for investment casting. The main
advantage is in the production of relatively complex castings
without the need for tooling. Cost effective complex metal parts
may be produced from CAD models in a relatively short period of
time. Wax patterns need to be finished to a high standard. One
problem with the system is the support system used which leaves
undulations on all downward facing surfaces of the pattern. The
supports have to be removed and surfaces cleaned by hand. This
process is best suited to small numbers of complex parts that
would otherwise require a significant amount of coring to accommodate
undercut features.
A major advantage with this approach is the designer does not
need to add draft angles to the geometry, although radii and machining
allowance may be required. Figure 10
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Figure 10 |
Solider
Solider (Figure 11) In this process the CAD data, again in the form of
slices, is used to generate a 'photocopy' of each slice on a piece of
glass; the cross section of the model is represented by clear glass, the
area around as black. Using the sheet of glass as a mask, a thin layer
of photosensitive resin is cured instantaneously by a burst of UV light.
Only the area of the model is solidified, any uncured resin is removed
and replaced with wax to provide support for the next layer of model.
In this way, the model is constructed from the base up, with the sheet
of glass being cleaned and the next slice copied for each layer. Once
the model has been completed it is heated to remove the wax.
Figure 11
Direct
Shell Production CastingTM - DSPC (Figure 12)
Direct Shell Production Casting TM is slihtly different from
the systems mentioned previously in that a cavity is produced to the
dimensions of the CAD model required. The shell is built in a ceramic
allowing the direct production of investment shells from CAD data. The
Massachusetts Institute of Technology developed 3D printing on which
the DSPC process is based. The system is licensed to Soligen Inc. USA
for use in metal casting.
A CAD file is used to define the required cavity, the system modifies
this file to suit the investment casting process by introducing fillets
and removing machined features such as holes. The number of castings
required are then entered into the computer controlling the system and
the model of the shell generated. At this point mould flow and simulation
can be run.
The process can loosely be described as a cross between SLS and FDM.
A layer of alumina powder is laid down onto a cylindrical bed.
A print jet projects a fine stream of colloidal silica in the required
plan of the slice onto the powder coating. This solidifies and also
adheres to the previous layer. Once the slice of shell has been drawn
the machine bed lowers and the process is repeated until the entire
shell has been formed. As with the SLS process the excess powder acts
as a support while the shell is being produced. On completion the shell
is removed from the bin and excess powder removed from within the cavity.
Figure 12
ENGINEERING ASSISTED SURGERY
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