advanced rapid pro to typing
TRANSCRIPT
ME 402
Advanced Rapid Prototyping
Abby, Ihan, Yue, Ben
April 17 2012
Laser Sintering
Introduction
Laser sintering uses a high power laser, usually CO2 or Nd:YAG, to fuse small
particles of plastic, metal, glass, or ceramic powders into a three-dimensional part.
The laser beam, either in continuous or pulse mode, scans the first layer produced by
the stereolithography (STL) file to join the powder together then a second layer of
loose powder is deposited over it (usually by a roller) and that layer is then scanned.
The two major types of laser sintering processes are selective laser sintering (SLS)
and direct metal laser sintering (DMLS). Selective laser sintering uses nonmetals and
direct metal laser sintering uses metal powders. A pulsed laser is used in a laser
sintering machine because the finished part density depends on peak laser power, not
laser duration. The SLS machine preheats the bulk powder material somewhat below
its melting point to make it easier for the laser to raise the temperature of the selected
regions the rest of the way to its melting point. The layer thickness for both of these
laser sintering processes is around 20 micrometers.
Figure 1: Laser Sintering Process
Laser sintering takes place in a very short time interval, which is insufficient
time for binding to take place due to solid-state diffusion. Therefore, either melting
one of the low-melting-point components of the powder or completely melting the
whole mass causes the joining of the powders. There are other binding mechanisms
such as solid state sintering (SSS) and chemically induced binding, which are not used
as often. Sintering by melting part of the powders is most prevalent, because the
complete melting of the powders causes the problems of wavy surface and inaccurate
dimensions of the finished part. Sintering depends on the powder density and its
shape, size distribution, and flow rate. The density of metal powder layers, for
example, needs to be increased for better sintering which can be done by optimizing
particle shape and surface state. The sinterability of powders in SLS can be improved
by thermal pre-treatment. There are many parameters that vary in laser sintering such
as powder size, scan speed, scan size, scan spacing, power density, pulse frequency,
part-bed temperature, roller travel speed, and part volume. All of these factors can
affect part properties such as yield strength, hardness, surface finish, shrinkage,
porosity, and tensile strength.
Materials
Laser sintering is very versatile in terms of materials that can be used.
Selective laser sintering generally uses wax, ceramics, nylon/glass composite, and
polymers; usually these are two-component powders either coated powders or power
mixtures. Direct metal laser sintering uses polymer-metal, metals, alloys, and steel
powders; these are generally single-component powders.
Advantages and Disadvantages
Some advantages of using laser sintering are that it uses a variety of materials,
no post curing is required, build time is fairly fast, and there is a limited use of
support structures. Limitations of this process include rough surface finishes, material
changeover being more difficult compared to SLA and FDM, some post-processing is
required, and mechanical properties below those achieved in injection molding for the
same material. Defects found in laser sintered parts are balling or agglomeration of
the powders, tearing or stress cracking, poor cohesion, curling of layers, and
dimensional inaccuracy. There are post-processing operations such as electroless
nickel plating, polishing, annealing, coating, machining and others that help improve
structural integrity, surface smoothness, and decrease porosity.
Applications
Laser sintering has numerous applications due to its ability to use many
different materials. A very big area where laser sintering is being used is in the
medical field because it can build parts free of the traditional constraints imposed by
machining or molding. Laser sintering works very well with biocompatible materials
such as titanium, hydroxyapatite, and calcium phosphate. A benefit to using SLS or
DMLS in the medical field is because of the customization it offers. Nearly any bone
or joint can be made into an STL file by a 3D scanner, 3D MRI, or CT scan. Using
SLS for fabrication of bioceramics gives accurate construction of complete bone
structures. For example, a complete titanium lower jawbone was recently created for
an 83-year old woman using DMLS. Conventional surgery was too much of a risk for
her age, but laser sintering was an option; it weighed only one ounce more than an
actually jawbone. Titanium has also been used for dental implants because of its
strength-to-weight ratio, corrosion resistances, and affinity for binding with human
bone. These implants are beginning to be made using DMLS, and actually use a flaw
in the process to create a better product: the rough surface finish. The porous surface
characteristics create more surface area for the human bone to bind with which is hard
to obtain by traditional metal-finishing methods. Another area where this
customization is useful in the medical world is in knee replacement surgery. It
conventionally involves reusable measurement and drilling guides that are in
predetermined sizes. However, one medical device supplier is using laser sintering to
produce custom, disposable drill guides out of a biocompatible polyamide
thermoplastic. These guides result in smaller incisions, better-fitting implants, and
faster patient recovery. Another advantage using laser sintering is the ability to
control poor structure for biogenesis by controlling the content of the polymer. Also,
medical designers, for example, are exploring uses for a new high-performance
sterilizable PEEK thermoplastic for implants and cobalt-chrome knee implants. The
use for laser sintering in this field continues to grow and is very beneficial and
economical for the industry.
Electron Beam Melting (EBM)
Overview
Electron Beam Melting (EBM) is a popular method of rapid prototyping
method for metal parts. EBM process is a direct metal layered fabrication technique
commercialized by the company Arcam AB (www.arcam.com). EBM is a high end
rapid direct metal fabrication technique often utilized by industries that require high
precision parts. EBM may produce parts using a comprehensive set of metal materials
comparing to other prototyping methods. In order to perform the EBM process, it is
necessary to know the geometrical data of the component from a 3D CAD model. The
3D model is sliced into layers with a certain thickness in order to generate the layer
information. A typical EBM process begins by spreading a thin layer of desired metal
powder across the vertically adjustable platform. Each EBM machine is equipped
with a powerful electron beam with maximum power of 4.8 KW. After spreading the
metal powder, the layer is preheated by scanning at low beam power and high
velocity in order to hold the layer in place during the following melting at higher
beam power, and to reduce the temperature difference between layers. The preheated
layer is then melted by increasing the beam power and/or decreasing the scan speed.
The vertically adjustable platform will lower down one layer thickness as each new
layer of metal powder is spread on top of the previous ones. This spread-preheat-melt
process is repeated until the whole part is complete. For the purpose of maintaining
the chemical specification of the used material, it is important to make sure that the
entire building process is performed under vacuum condition which provides a base
pressure at about . Figure 1 shows the overview of EBM process.
Figure 1: EBM process
Materials
Companies have successfully used the EBM process to produce parts in the
following materials: Titanium Ti6Al4V, Titanium Ti6Al4V ELI, Titanium Grade 2,
Cobalt-Chrome, ASTM F75, Titanium aluminide, Inconel (625 & 718), Stainless steel
(e.g. 17-4), Tool steel (e.g. H13), Aluminium (e.g. 6061), Hard metals (e.g. NiWC),
Copper (e.g. GRCop-84), Beryllium (e.g. AlBeMet), Amorphous metals, Niobium,
and Invar.
Advantages and Disadvantages
There are many advantages using EBM process. EBM allows great
controllability such that it is able to produce finished parts with IT Grade 07
(International Tolerance Grade). EBM is highly energy efficient comparing to many
other manufacturing processes as some require up to 10 times more electricity. In
addition, EBM is able to work with materials that are highly transparent to laser
beams such as aluminium. Moreover, EBM produces void-free parts that are fully
dense and extremely strong. Furthermore, EBM process operates under vacuum
condition enables a certain degree of material purification. For example, it removes
gasses to avoid porosity that emerges in metal casting. In addition to expansive EBM
machines, the EBM operating pressure loses elements with high vapor pressure.
However, EBM cannot be utilized when dealing with nonmetallic materials.
Applications
Due to the high precision offered by EBM, aerospace and other highly
demanding mechanical applications are targeted. In addition, many use EBM to
produce medical components and anatomical models such as bone plate, knee implant
and hip stem, see Figures 2-4.
Fig. 2 Bone plate Fig. 3 Knee implant
Fig. 4 Hip stem
PolyJet
Overview
PolyJet is one of the more current rapid prototyping processes that appears in
today’s market. When considering its true functionality, the PolyJet process can be
categorize in the processes of three dimensional printing. Many considered it a cross
between selective hardening and drop deposition.
The most impressive feature is the printing head. Instead of traditional ink,
the printer head applies a liquid photo-monomer that will polymerize in the presence
of ultraviolet light. Due to being one of the latest technologies, PolyJet is considered
to be a good compromise between accuracy, detail, manufacturing speed, and surface
quality of the finished part. In addition, it has been seen that the mechanical
properties of the PolyJet parts are comparable to its injection molded counterpart. The
layer thickness on the PolyJet is about 16 micrometers. The application of the
individual layers is similar to common inkjet printers, in that the jetting head slides
back and forth along the x-axis depositing a single layer of the polymer onto the tray.
Bulbs alongside the jetting bridge emit ultraviolet light that immediately cure and
harden each layer (Figure 1). This eliminates the need for post cure applications as
seen with other molding applications. The only removal necessary is the supporting
material. Many times this is done through high pressure pumps and water. Although
not necessary, the finished product can be sand blasted, polished, painted or treated
otherwise. Many times the finished products are used for fabricating silicone molds,
and other molds for additional casting processes.
MaterialsFrom the basic principle of the process, two categories of materials are used.
The first one is what the model is effectively made from, and the second one is used for support. The material for supports is water soluble, which makes it simple for removal by pressurized water spraying. Today there are multiple types of materials used for the model. These photopolymers are made to simulate elastomeric, rigid thermoplastic, transparent thermoplastic and polypropylene materials. One of the most significant features is the fabrication of up to 50 different digital materials, with
up to 14 different materials within any single printed part. Digital materials are composite materials created by simultaneously jetting two different materials together. The two photopolymers are combined in a specific concentration and structure to provide enhanced mechanical properties. FullCure 700 is an example of a transparent material that has comparable toughness and tensile strength to commodity plastics (Table 1).
Table 1: Properties of FullCure 700
Advantages/Disadvantages
The PolyJet process enables on the fly fabrication of composite materials that
closely emulate the mechanical properties of products made from conventional
processes. PolyJet offers the same advantages as Sterolithography, but typically cost
less and produces a smoother surface finish. When compared to sterolithography,
PolyJet is more limited in the size of parts that can be created.
The main advantage of the PolyJet process is the fine layer thickness. Polyjet
products are suitable for highly complex shapes with fine details that retain its
structural and mechanical integrity. However, when comparing conventional methods
of production technology, PolyJet still has a very limited menu of raw materials. Kim
et al. compared the manufacturing speed of different rapid prototyping process. They
found through simulations that PolyJet offered the fastest capability of producing a
part from start to finish. However, bulk processing is not ideal, as the fabrication of
multiple parts is time consuming.
Applications
PolyJet has many applications including tooling, casting, medical imaging,
jewelry design, ect. Parts can be created for investment casting, direct tooling and
rapid, tool-free manufacturing of plastic parts. Also these designs can be used to
create silicon molding, aluminum epoxy molds, VLT molding and vacuum forming.
In the medical field, parts built by PolyJet have fine details and features which can
make the medical problems more visible for analysis and surgery simulation. In
addition, many hearing aid manufacturing utilizes PolyJet technology to deliver
perfectly shaped hearing aid molds with smooth, flawless surfaces. PolyJet
prototypes are best suited for applications where accuracy, detail and surface finish
are important and the part fits within a 5”x5”x5” build volume. While the maximum
PolyJet build volume is about 29”x15”x8”, other rapid prototype technologies may be
more cost-effective for larger parts.
Stereo lithography (SLA)
Overview
Stereo lithography is an additive manufacturing process which employs a vat
of liquid ultraviolet curable photopolymer "resin" and an ultraviolet laser to build
parts' layers one at a time. For each layer, the laser beam traces a cross-section of the
part pattern on the surface of the liquid resin. Exposure to the ultraviolet laser light
cures and solidifies the pattern traced on the resin and joins it to the layer below.
After the pattern has been traced, the SLA's elevator platform descends by a distance
equal to the thickness of a single layer, typically 0.05 mm to 0.15 mm (0.002" to
0.006"). Then, a resin-filled blade sweeps across the cross section of the part, re-
coating it with fresh material. On this new liquid surface, the subsequent layer pattern
is traced, joining the previous layer. A complete 3-D part is formed by this process.
After being built, parts are immersed in a chemical bath in order to be cleaned of
excess resin and are subsequently cured in an ultraviolet oven.
Stereo lithography requires the use of supporting structures which serve to
attach the part to the elevator platform, prevent deflection due to gravity and hold the
cross sections in place so that they resist lateral pressure from the re-coater blade.
Supports are generated automatically during the preparation of 3D Computer Aided
Design models for use on the stereo lithography machine, although they may be
manipulated manually. Supports must be removed from the finished product
manually, unlike in other, less costly, rapid prototyping technologies.
Materials
Most resins used in the SLA machines are photosensitive epoxy polymers.
Such like WaterShed 11122XC, the resin run in Viper si2 machines, is untinted and
can result in clear prototypes, given a Level 6 finish. The ProtoGen 18420 resin is can
be heat treated for higher heat deflection temperatures. The Accura 25 resin is a white
polypropylene-like resin designed for flexibility and snap fits. Accura 60 is a tough
general purpose resin which can make translucent parts.
Table 2: Details of resins
Advantages and Disadvantages
The advantages of SLA are separated in five sides. Firstly, automatic and can
be unattended until the process is completed which make the system very stable.
Secondly, good dimensional accuracy which make the process be able to maintain the
dimensional accuracy of the built parts to within +/-0.1mm. Thirdly, good surface
finish------glass-like finishing can be obtained on the top surfaces of the part although
stairs can be found on the side walls and curve surfaces between build. Fourthly, the
process is of high resolution and capable to build parts with rather complex details.
Furthermore, 3D Systems Inc. have developed a software called "Quickcast" for
building parts with hollow interior which can be used directly as wax pattern for
investment casting.
Some disadvantages are also existed. Such as curling and warping, because the
resin absorbs water as time goes by resulting curling and warping especially in the
relatively thin areas. As well as relatively high cost must be considered, however, it is
anticipated that the cost will be coming down shortly. Moreover, the material
available is only photo sensitive resin of which the physical property, in most of the
cases, cannot be used for durability and thermal testing. Then the post curing is also a
problem. The parts in most cases have not been fully cured by the laser inside the vat.
A post curing process is normally required. The last problem is high running and
maintenance cost, the cost of the resin and the laser gun are very expensive.
Furthermore, the optical sensor requires periodical fine tuning in order to maintain its
optimal operating condition which will be considerable expensive.
The SLA Process Overview
Pre-processing:
(1) Digital 3D Model
(2) Conversion to STL file formats
(3) STL verification
(4) Parts Placement and orientation
(5) Support generation and Editing
(6) Preparing and Slicing
Post-Processing:
(1) After the build process completed, the platform will rise automatically above the
resin surface. Let the platform stay for 5-10 mins. to drip excess resin.
(2) Place the platform with parts in a incline position inside the chamber to drain the
parts.
(3) Rinse the platform and the part thoroughly for 5 mins
(4) Remove the parts carefully. Start from the peripheral and work toward centre.
(5) Remove supports using hand tools.
(6) Wash the parts again to remove any support debris.
(7) The part will be cure fully by the UV light.
Applications
Direct Digital Manufacturing / Rapid Manufacturing: Medical and Healthcare;
Electronics; Packaging, Connectors; Homeland Security; Military Hardware.
Rapid Prototypes: Design Appearance Models; Proof of Concept Prototypes; Design
Evaluation Models (Form & Fit); Engineering Proving Models (Design Verification);
Wind-Tunnel Test Models.
Tooling and Patterns: Rapid Tooling (concept development & bridge tools); Injection
Mold Inserts; Tooling and Manufacturing Estimating Visual Aid; Investment Casting
Patterns; Jigs and Fixture.