SOLID FREEFORM FABRICATION (SFF)
4.1.2 The History of SFF Methods
During the late 19705,Mead and Conway (1980) created the groundwork for the fast prototyping of very large scale integrated (VLSI) circuits. Designers were encour-aged to think in terms of five two-dimensional (2-D) patterns. These patterns defined three stacked interconnection layers on a metal-oxide-semiconductor (MOS) wafer and their mutual connections through holes. The patterns described the actual geom-etry of the connection runs and via holes that one would see when looking down onto the circuit chip, regardless of the exact process and number of masking steps that were used to implement the chip (see MOSIS, 20(0).
Inspired by this success, beginning in the 19705, several companies tried to create layered manufacturing for mechanical parts. Also by the mid-1980s, several U.S. government studies analyzed the possibilities of a "mechanical MOSI$" (Man-ufacturing Studies Board, 1990; Bouldin, 1994; NSF Workshop I, 1994, and II, 1995).
The prospects for a mechanical MOSIS were thus frequently linked to the fab-rication processes in the lists mentioned (Ashley, 1991, 1998;Heller, 1991;Kruth, 1991;
Woo, 1992, 1993;Au and Wright, 1993; Kochan, 1993; Kai,I994; UCLA, 1994;Weiss
132 Solid Freeform Fabrication (SFF) and Rapid Prototyping Chap. 4
and Prinz, 1995;Cohen etal.,1995;Dutta, 1995;Jacobs, 1992, 1996; Beaman et al., 1997;
Kumar et al., 1998; Sachs et al., 2000).
The introduction of the first commercial SFF technology-stereolithography-was accompanied by the advent of the STereoLithography (.STL) representation of a CAD object. ".STL" is a modified CAD format that suits a subsequent slicing oper-ation and the "downstream" laser-scanning paths on a physical SLA, FDM, or SLS machine.
Is a soccer ball round? The answer depends on how carefully the ball is meas-ured. Nominally, it is a perfect sphere. However, on closer inspection, the leather is sewn together from about 20 little hexagonal patches and a few pentagonal patches to create the curvature. In reality it is an approximation to a sphere.
Likewise, the" .STL" format approximates the boundary surfaces of a CAD model by breaking it down into interconnected small triangles-a process called tes-sellation. Each triangle is represented by thex/y/z coordinates of each of its three ver-tices, enumerated by the right-hand rule-that is, counterclockwise (ccw) order as viewed from the outside of the body. The vector normal to the surface of each triangle is also specitied.This tessellated surface is stored as an".STLfile." This file, perhaps containing up to 200,000 triangles, is sent over the Internet to a prototyping shop.
As shown in Figure 4.1, this tessellated CAD model is then sliced like a stack of playing cards. For 3D Systems' machines this is known as the SLI or sliced file.
Other rapid prototyping machines use the slicing technique but have their own file creation details and names. Each slice for the imaginary soccer ball will thus be a circle. However, because of the tessellation procedure it will not be a perfect circle.
The slicing action cuts through the triangles on the boundary. Thus, each circular slice (or disc) will actually be a multisided polygon running inside the "bounding circle."
The number of sides on this inner polygon is of course related to how finely divided the original tessellation was made.
Inside the SLA machine, the laser first creates the outer boundary of each slice and then "weaves" across each slice in a hatching pattern to create the layer. The number of slices and the style of the weaving pattern are chosen by each rapid pro-totyping shop. Especially for SLA and SLS a certain amount of trial and error, or crattspersonship, begins to playa role at this stage. This is reviewed in more detail over the next few pages.
".STL" is now the standard exchange format for SFF processes. However, it is inadequate for many reasons. First, the files are large due to the tessellation method.
Second, there are redundancies in the" .STL" format. One example of redundancy is as follows: the triangles are represented by the "counterclockwise rule" so that it is clear in which direction the outer-surface normal acts. However, it has also become customary to specify the surface vector as well. Inconsistency can be introduced as a result of this redundancy, and no rules exist for resolving it.
McMains (1996) describes how ".STL" does not capture topology or connec-tivity, making it difficult to fix some of the common errors found infiles-c-such as cracks, penetrating or extraneous faces, and inconsistent surface normals-without resorting to guessing the designer's original intent. More general digital interchange formats have also been used with SFF.These inciudeACIS (1993) and IGES (Heller, 1991). However, as described in NSF (1995), problems arise with these formats, too.
4.2 Stereolithography: A General Overview '33
Materialaddition processes
CAD AUIOfllillicpl'OCtSSpilll'lllef Automatcd tabncauon rnachine
(hi
Fipre 4.1 An ~.STL" file is a tessellated object. The top figure shows a contact lens holder represented hy many surface patches. The ".STL" file is then sliced.
Laser motions then harden the part (courtesy of Lee Weiss).
One aspect of ongoing research is thus to improve this representation language (McMains et aI., 1998).
4.2 STEREOLITHOGRAPHY: A GENERAL OVERVIEW
4.2.1Background
Stereolithography (SLA) was launched commercially by 3D Systems Inc. in 1987 (see Jacobs, 1992, 1996). The process is shown in Figure 4.2.
The commercial launch followed from the studies of several independent pro-grams on the curing of photopolymers. Some of these are mentioned in Table 4.1.
Also of historical interest is that the photocurable liquid was first developed for the printing industry and for furniture lacquers or sealants. In the latter case, to avoid carcinogenic solvents, an ultraviolet (UV) curing process was developed for furni-ture sealants.
One can imagine how SLA grew out of these developments: the SLA inventors must have seen how layers of the photocurable liquids could be built up on a chair
3"D:;o]idmodd
le'X~l:ge
j fo.-mal
134 Solid Freeform Fabrication (SFF) and Rapid Prototyping Chap. 4
Fipre 4.2 Stereolithography (SLA), based on thecommercially published brochures of 3DSystems Inc. Thehelium-cadmium laserin theSLA-250 cures and fuses successive layers of resin. These descend on top of each other on the elevator untilthewhole partis formed. at which point theobject is lifted fromthevat.
TABLE 4.1 History of SLA
Date Person(s) Cnmpany and location Activity
19705 A. Herbert
1970s H.Kodama
1970:; C.Hull
1986 C.Hul!
andRFreed
November 3D Systems
1987
3M. Minneapolis R&D
Nagoya Prefecture Research, Japan R&D UItTaViolet Products,California R&D 3D SystemsInc., formed from Patent secured
UltraViolet Products, California
3D Systems Demonstration of the
SLA-l (which became theSLA_2S0) at the Autofact showin 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 SLA machine.
Formed object X-¥movabt, laser Blade
position Laser curable
liquid Liquid surface
4.2 Stereolithography: A GeneralOverview '35
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"
method tessellates the CAD model with triangles just like the hexagons and pen-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
Entity Described by
The header The number of triangles
For each tessellation triangle (50 bytesofinformation) Normal vectorI
Unsigned long integer(4bytes) See below
Floating point integer (4 bytes) Floating point integer (4 bytes) 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) Floating point integer (4 bytes) FLoating point integer (4 bytes) Floating point integer (4 bytes) Unsigned inleger (2 bytes)
13. Solid Freeform Fabrication (SFF) and Rapid Prototyping Chap.4
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.
6 2"
,
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
4.2 Stereolithography: A General Overview 137
• 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.
To avoid liquid that had carcinogenic solvents, the UV curing process was developed.
Lasers provide more direct energy and allowed the invention of SLA once com-puters were powerful enough to create a tessellation. SLA is a low-energy curing 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 neighbors by van der Waals bonds. As the laser acts on the bonds, the C=C bonds break. The broken monomer groups connect to each other, forming long chains (see Table 4.3).
TABLE4.3 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, a hatching or weaving pattern crosses the entire area. Third, the hatched areas are 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-ever, some careful process planning is needed to create the accuracy of only a few thousandths of an inch. Details that control accuracy are presented after Figure 4.5.
HlC=CH H,C ~
T"
R
138 Solid Freeform Fabrication (SFF) and Rapid Prototyping Chap. 4
'\
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 at the desired height for the beginning of the run, a plunger mechanism adjusts the
4.2 Stereolithography: A General Overview 139
fluid level by fluid displacement. Also, it is crucial to adjust the laser position with reflective "eyes" at the corners of the machine's stage. In fact, this check of laser posi-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 the actual part will rest upon. The supports can be viewed as small feet, rather like those on a heavy sofa or piano: they are needed on the bottom of the part to lift the lowest layer off the floor of the elevator platform. In particular, the supports are 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
"deep dip" allows the viscous, honeylike fluid to more easily flow over the surface of 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 the SLA fluid supplier as the time needed for the full curing to occur of a part layer.
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 enough to build subsequent layers on top of it. After the 45-second wait, the first 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 window. In fact, it has a long, hollow cavity between two adjacent blades, and this 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 liquid and more easily and uniformly deposits the next liquid layer onto the first. At the same time the sweeping blade distributes the SLA liquid evenly. Note that the
140 Solid Freeform Fabrication (SFF) and Rapid Prototyping Chap. 4
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 blade. The sweep gives a uniform thin layer, but given the viscosity of the fluid, there 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 4.2.6 Stereolithography Details: Laser-Based Manufacturing
and Prototyping
During stereolithography, selective laser sintering, or any laser-based process, many details of the "laser energy delivered" to the resin (or powder for SLS) control solid-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 the topic of main interest is the "energy at depth z" of the laser. Lasers give much 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:
H(x,y.d =H(x.y.OJexp(- ~) (4.1)
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 (=112.718)of the H, level on the surface (Figure 4.6).That is, at a depth ot r= Dpthe
4.2 Stereolithography: A General Overview
'4'
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 =(PL)=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)")
H(x,y,o) =H(r,o) =Roexp -
-wI
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)
=30.56 watts per cm2 (4.4)
If the scan speed is 200 mm per second, the scanning laser's exposure timeon
If the scan speed is 200 mm per second, the scanning laser's exposure timeon