21st Century Manufacturing Episode 1 Part 8 ppt

Finally, the rapidly decreasing costs of microprocessors during the early 1980s paved the way for the tessellation routines during CAD and the control of the lasers in the actual Formed

Fipre 4.2 Stereolithography (SLA), based on thecommercially published brochures of 3D Systems Inc The helium-cadmium laser in the SLA-250 cures and fuses successive layers of resin These descend on top of each other on the elevator until the whole part is formed at which point the object is lifted from the vat.

TABLE 4.1 History of SLA

19705 A Herbert

1970s H.Kodama

1970:; C.Hull

andRFreed

November 3D Systems

1987

Nagoya Prefecture Research, Japan R&D UItTaViolet Products,California R&D 3D SystemsInc., formed from Patent secured UltraViolet Products, California

SLA-l (which became the SLA_2S0) at the Autofact show in Detroit

leg, forming a solid shape Then, the use of a helium-cadmium laser created more

energy and more focused solidification patterns than a simple VV arc lamp Finally,

the rapidly decreasing costs of microprocessors during the early 1980s paved the way for the tessellation routines during CAD and the control of the lasers in the actual

Formed

object

X-¥movabt,

laser

Blade

liquid Liquid surface

One can also imagine the excitement these first inventors felt, as they saw the first layer of SLA material solidifying on the surface of a vat of resin: viewed at an angle it resembles the first layers of ice solidifying on a pond in early winter

In production, once this first layer is cured, the elevator type stage lowers by 50

to 200 microns (0.002 to 0.008 inch) depending on the desired accuracy, and further layers are cured and connected by self-fusing to the previous ones.At the end of the process, the elevator rises and the component is lifted out and cured in its entirety Postcuring is needed, probably overnight, before the prototype is ready for use Hand sanding may be required to mitigate thestair-stepping effect described later Note that the object in Figure 4.2 has overhanging areas about halfway down its height dimension During the actual process these need to be supported by slender sacrificial columns Without these, the horizontal part of the component sags Additional hand finishing is needed to snap out these slender sacrificial columns and sand any small stubs away from the surface

4.2.2 StereoUthography Details: The" STL"File Format

Introduced by 3D Systems Inc in 1987, the ".STL" file format has become the de facto standard, even though other "direct slice" methods have been tried The" STL" tagons on the surface of a soccer ball

The ".STL" file is (a) a header, (b) the number of triangles, and (c) a list of the triangle description by vertices and the normal vector to the triangle Table 4.2 shows the layout The size of the ".STL" file is (50 x number of triangles) +84 Thus a 1O,OOO-triangleobject needs 500,084 bytes

TABLE4.2 TheN.5TlNFile Format

The header

The number of triangles

For each tessellation triangle (50 bytesofinformation)

Normal vectorI

Normalvector J

Normal vector K

First vertexX

First vertex Y

Fint vertexZ

Second vertex X

Second vertex Y

Second vertex Z

Third vertex X

Third vertex Y

Third vertex Z

80 bytes Unsigned long integer(4bytes) See below

Floating point integer (4 bytes)

Floating point integer (4 bytes)

Ploanng polnt integer te bytes) Flontingpo;otinteger {4 bytes) Floating point integer (4 bytes)

Floating point integer (4 bytes)

FLoating point integer (4 bytes)

Two rules govern the triangle descriptions (Figures 4.3 and 4.4).

1 The right-hand counterclockwise rule, or "ccw rule," is a corkscrew acting

out-wardon the soccer ball, to order the vertices and the normal vector

2 The vertex-to-vertex rule, which insists that the vertices on an adjacent triangle link to the neighbor and that no vertices meet a neighboring edge 4.2.3 Stereoli1hography Details: C-Slice Processing

When the" STL" file arrives at the rapid prototyping bureau, the slicing begins as follows:

• Sort the" STL" triangles into "z values" (this establishes the layers)

• Find the boundary segments (gives contiguous internal and external pocket/shape contours)

• Create boundary polylines

• Apply edge compensations (based on operator's knowledge of laser physics)

• Compare with adjacent layers to minimize stair-stepping on chamfered sides

• Smooth boundaries

Fipre4.3 Rules for tessellation

r

,

,"

F'iJtlre4.4 Vertex-to-vertex rule means that a vertex cannot join to a random point somewhere

un an "ull'" Each v"rt"x hllllto meet another vertex 00 the neighboring triangle Normal

• Output boundary data.

• Treat next cross section

4.2.4 Stereolithography Details: The Resin

The photocurable liquid was developed for printing and for furniture lacquer/sealant Lasers provide more direct energy and allowed the invention of SLA once com-process compared with SLS (using a CO2laser)

Photopolymerization is defined as linking small molecules (monomers) into larger molecules (polymers) comprised of many monomer units Vinyl monomers have a carbon-carbon (C=C) double bond attached to complex groups donated by

"R." In the original resin, the monomer groups are only weakly connected to their break The broken monomer groups connect to each other, forming long chains (see Table 4.3)

TABLE 4.3 Polymerization

Weak: van derwaalsbonds

between the adjacent chains StrongcovalentbondsalOllgchains

The bonding between such chains then creates three key effects:

• The liquid gels into a solid

• The density increases

• The shear strength increases

Although the original vinyl monomers are already cross-linked, they get much more strength from the formation of the covalent bonds in the long chains 4.2.5 Stereolithography Details: The SLA Manufacturing

Process

To create any individual layer, the laser traces out the boundaries of a layer first This

is called bordering; imagine a large elastic band or loop lying on the surface Second, filled in, causing the final gelling and solidification (Figure 4.5)

After each layer is formed, the laser scanning moves to the next layer

How-HlC=CH

H,C ~T"

R

I 1/ Figure 4.5 Establishing the border, then hatching and filling (filling is shown on just one square)

Note: These steps are for an SLA-500 machine at the time a/writing. The details for the SLA-250 are slightly different, and, in addition, new refinements are constantly taking place

4.2.5.1 Step I Preparation afthe Script

The partbuilding needs instructions on the desired accuracyTypically, a layer thick-ness of100microns(0.004 inch) is the average build layer However,itmay range from50to200 microns (0.002 to 0.008 inch)depending on the desired accuracy Also the Zephyr bladesweeping times, and the" z-wait" times, need to be programmed in These are described later

4.2.5.2 Step2.Leveling and Laser Calibration

SLA resins undergo 5% to 7%total volume shrinkage, and of this amount, 50% to 70%, occurs in the vat during polymerization (Jacobs, 1992) Since the liquid level is alwaysshrinking down, a sensor mustbe installed to follow the level If the vat is not

fluid level by fluid displacement Also, it is crucial to adjust the laser position with tion occurs just before each layer is done

4.2.5.3 Step 3 Making the Initial Supports

The first few runs with the laser are not for the part itself but for small supports that those on a heavy sofa or piano: they are needed on the bottom of the part to lift the needed:

• So that the Zephyr blade will not hit the platform

• To compensate for platform distortion

• So that it is easier to remove the finished part

• Internal supports are also needed for any "overhanging" structures When making the supports, after the first laser cured layer is formed, the stage needs to be pulled down about 12 mm (0.5 inch) for the SLA·5oo(Jacobs, 1992) This the first layer of the supports The elevator then rises up to be positioned 100 microns (0.004 inch) below the surface It is usual to wait about 5 seconds and then do the laser curing again This creates the second layer -but still, this is concerned with the supports, not the part itself This procedure repeats until the supporting stubs are large enough The operator usually makes these decisions

4.2.5.4 Step4.Creating the Actual Parr

The procedure to make the actual part (not the supports) is somewhat different Once the supports are finalized, the first bottom surface of the part is generated by the "bordering + hatching + filling" described earlier

The elevator descends by 100 microns (0.004 inch) and then waits typically for

45 seconds This time is programmed in by the operator It is a recommendation from Note that although the laser has begun the polymerization process, it still takes up to

45 seconds for the full effect of polymerization to occur and to harden the layer layer is hardened enough for the Zephyr blade to sweep over the surface and pre~ cisely set the 100 micron (0.004 inch) layer of liquid for the second polymerization

4.2.5.5 Step 5 Sweeping Using the Zephyr Blade

At first glance, the Zephyr blade looks like a "hard squeegee" used to clean a car cavity is under the influence of a slight vacuum pump This draws SLA liquid into the bottom of the blade Thus, as the blade sweeps over the surface, it is "charged" with

honeylike SLA fluid is very viscous, anditneeds the distribution of the vacuumized Zephyr blade to get an even surface

As the Zephyr blade traverses the whole vat, it removes excess resin in some areas, and yet because it is "charged" with resin, it distributes and fills any areas that lack resin The sweep takes about 5 seconds (Jacobs, 1992) unless a hollowlike part is being made where the viscous fluid inside the hollow takes longer to follow the

is a tendency for resin to adhere to the blade, followed by separation and a "bulge" just downstream from the part's leading edge

4.2.5.6 Step 6 "Z-Wait" of about 15 Seconds

Even after all the adjustments and sweeping, a "crease" exists around the edge of the part at the solid-liquid interface The "z-wait" allows a relaxation of this effect to a flatter, smoother resin surface

4.2.5.7 Step7.Extra Skin Filling

At the very end of the process, more intense hatching may be desirable on the top surface of the part Very closely spaced line vectors cause more intense solidification structures on the up-facing surfaces It is likely that similar patterns would have been done on the down-facing outer skin in Step 4

4.2.5.8 Step8.Final Steps

The final steps include:

• Draining excess resin from any inner or depressed cavities

• Cleaning and rinsing with solvents

• Snapping out bridgeworks

• Hand sanding and polishing

• Postcuring in a broad spectrum UV light source

and Prototyping

During stereolithography, selective laser sintering, or any laser-based process, many ification and the accuracy that can be achieved First consider penetration depth Note that the bottom of each SLA layer has to adhere to the previous layer, and so more energy (i.e., are able to cause more "polymerization by irradlence") than reg-ular arc lamps But as they travel down through the resin or powder they do never-theless decay exponentially by the Beer-Lambert exponential Jaw of absorption:

A critical exposureH(c)is needed to "gel" the resin.Dpis a resin constant defined

by the depth of a particular resin that results in a reduction of irtadiance level to lie

x

Radius

F1gure4.6 Gaussian decay of laser across the surface

irradiance is -37% of Ro For the SLA-250, typical values are given by Jacobs (1992)

as follows, and it is of interest to relate laser behavior to resin solidification:

Nominal laser power =(P L)=15 milliwatts

Central spot size=(2Wo)=0.25 millimeters

For the whole spotthe Gaussian irradiance curve controls the basic physics, and like any point source of light it decays fromthe center

Across the surface, as opposed to downthrough the surface (Equation 4.1), the laser decays as follows:

( (2)")

where Wo is the ~GaUSSian half width (Figure 4.6)

Thus, at r=Wo,

(4.2)

H=Hoe- 2 =O.135Ro

It can also be shown that

(4.3)

If the scan speed is 200 mm per second, the scanning laser's exposure timeon

a given area is

The laser exposure's average energy density is then

The analysis now proceeds to calculate the polymerization ability from the laser's

(photon, ~ 11 ~ 3.25 X 10-5 em

E(pho,on<) = 6.1 X 10-19Joules per photon (4.8)

A is the laser wavelength

c is the speed of light

his Planck's constant Denoting that Nph =number of photons per square centimeter hitting the resin surface:

(4.9) This flux of photons penetrates into the resin and acts on the polymer chains to cause polymerization Even if the photochemical efficiency is only 50%, the {C=C! bonds will polymerize to{C-c q.

4.2.7 Selective Laser Sintering (SLS)

Another very popular method is selective laser sintertng (SLS), commercialized by the DTM Corporation In many respects SLS is similar to SLA except that the laser

is used to sinter and fuse powder rather than phorocure a polymeric liquid The first step is to prepare the ".sTUSLI" files as described earlier Inside the SLS machine, a thin layer of fusible powder is laid down and heated to just below its melting point by infrared heating panels at the side of the chamber Then a laser sin-ters and fuses the desired pattern of the first slice of the object in the powder Next, process repeats (Figure 4.7)

In comparison with SLA, this process can rely on the supporting strength of the unfused powder around the partially fused object Therefore, support columns for any overhanging parts of the component are not needed This allows the creation of rather delicate, lacelike objects Nevertheless hand finishing is still needed to improve the inevitable stair-stepping Also, SLS parts have a rough, grainy appear-ance from the sintering process, and it is often preferable to hand smooth the sur-faces Another difficulty is maintaining the temperature of the powder at a few degrees below melting This is done with the infrared panels, but maintaining an even temperature over a large mass of powder requires long periods of stabilization

JIIpre 4.7 Selective laser sintering (SLS), based on commercially published brochures from the DTM Corporation

4.2.8 Leminated Object Modeling (LOM!

Laminated object modeling (LOM) was developed by Helisys Inc., and like SLA and laser is used to cut the top slice of a stack of paper that is progressively glued together After each profile has been cut by the laser (shown at the bottom right of Figure 4.8), the roll of paper is advanced, a new layer is glued onto the stack, and the needed For larger components, especially in the automobile industry, LOM is often preferred over the SLA or SLS processes

4.2.9Fused Deposition Modeling IFDM)

Fused deposition modeling (FDM) was developed by Stratasys Inc and is executed

on machines called the FDM 1650,2()(M),or8()(M)series Figure 4.9 shows that the material is supplied as a filament from spool The overall geometry and system are head and emerges as a thin ribbon through an exit nozzle The nozzle is guided

I