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ME 402 Advanced Rapid Prototyping Abby, Ihan, Yue, Ben April 17 2012

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Page 1: Advanced Rapid Pro to Typing

ME 402

Advanced Rapid Prototyping

Abby, Ihan, Yue, Ben

April 17 2012

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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

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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

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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

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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

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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

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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

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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

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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.

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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:

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(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.

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