21st Century Manufacturing Part 6 ppt

This process is beingused to build the ceramic molds for metal castings and powder-metal toolingfor injection molding dies.. The SLA process uses photocured polymers that do not exhibit

/Platform

Part block

Optics X"Ypositioning

deviceLayer outlineand cross hatch

Several types of 3-D printing processes have been developed in recent years and areconstantly being updated at the time of this writing Some of these are aimed at theeducational CAD/CAM market where students are invited to obtain quick models of

an emerging design At the same time such machines might be useful in an industrialdesign studio, where artists might want to generate and regenerate a quick succession

of prototypes for the "look and feel" of an emerging design Examples include:

• 3-D printing of cornstarch, followed by layer-by-layer binder hardening, is thebasic principle behind the Z-eorporation machine The first step inFIgure 4.10a

is to spread a thin layer of powder of the desired material across the top of the

Step 1: collect powder

Step 3: spread powder

[a) '"'Step 4: deposit bind

~'

(b)

Step 5: source piston up,

build piston down

.:·:r~i ISteps 1-5 are repeated untilbuild is completeStep 6; build complete

~'~i

S~d"

I ~ II ~ till

USI layerpriflltd Finished part

Ftpp:e4.10 (a) 3-D printing (based on commercially published brochures of

Z-Step 2: spread powder

In!,mn(;'dinlestage

droplets of the binder stream are printed down through a continuous-jet nozzlecarried by the print head Since material is built up layer by layer in anx/y plane,tional word processing printer

• A more accurate 3-D printing process, developed by Sachs and colleagues atMIT, was the forerunner to this technology (Figure 4.10b) This process is beingused to build the ceramic molds for metal castings and powder-metal toolingfor injection molding dies Commercial applications of this process aregrowing (Smith 2000; Sachs et al., 1992, 2000)

4.2.11 Solid Ground Curing (SGCI

Solid ground curing was introduced by Cubital Inc A schematic diagram of thewith the operation at the "cross roads," where the mask is being used to photocurethe uppermost layer of liquid of the block Solid ground curing uses exactly the samephysical process as SLA to photocure the polymer liquid The key difference is thatSLA does itbyusing a laser point source, whereas SGC does it by exposing a com-

How-ever, two interlaced processes occur simultaneously On the left of the schematic thelayer, and the finished mask rotates into the exposure area On the right of themoves into the exposure area to be hardened Postprocessing steps on the right

F1pre 4.11Solid ground curing process, based oncommercially publiahed brocbures of

4 Remove uncured resin 5.Fillcav\!ieswithwax6.Chi!lan,

, Solid Freeform Fabrication (SFF) and Rapid Prototyping Chap.4hollow areas, cooling, and planarization before returning to the first position againfor more spreading of the photocurable liquid Meanwhile, as shown on the left, thefirst pattern on the mask plate is erased and the next pattern is applied.4.2.12 Shape Deposition Manufacturing 150M)

In some of the two processes described earlier, for example, 3~D printingbySandersand SGC by Cubital, a combination of material additive and material removal takesplace Shape deposition manufacturing (SDM) also exploits this paradigm (Weiss etfor SDM are to combine the advantages of SFF {i.e., easy to plan, does not requirespecial fixturing, arbitrarily complex shapes, and heterogeneous structures) with theadvantages of machining (i.e., high accuracy, good surface finish, and wide-scaleavailability of existing CNC machines and infrastructure)

In 80M, a CAD model is again sliced into 3-0 layered structures Layered ments are deposited as near-net shapes and then machined to net shapes before addi-tional material is deposited The sequence for depositing and shaping the primaryand support materials is dependent upon the local geometry (Figure 4.12) The idea

seg-be machined but are fonned by previously shaped segments 80M can use tive deposition sources from welding to extrusion Producing smooth surface transi-tions between layers, however, remains a challenge, due in part to the layer-by-layeraccumulation of residual stresses

alterna-SOM can therefore combine complex surfaces and high accuracy In the future

it also promises to fill a niche for creating "wearable computer" products with

mul-tiple materials and even with embedded electronics (8mailagic and 8iewiorek, 1993)

Remove

P~ll

material

4.3 COMPARISONS BETWEEN PROTOTYPING PROCESSES

4.3.1 Materials That Can Se Formed with the Various

Processe.

The SLA process uses photocured polymers that do not exhibit great strength orhas emerged as the industry standard for creating amaster pattern that might then beused as the basis for a casting or injection mold

On the other hand, if asingle prototype needs to be tested to destruction, or ried around for a while, it really has to be made from metal or a structural plastic suchchoice FDM can extrude ABS polymers and create prototypes that are between50% and 80% of full ABS strength For full strength plastic or metal prototypes, the

car-of geometric shapes that can be made by machining CNC machining is also the mostlikely prototyping process for thesmall batch manufacturing of 2 to 10 components

If CNC machining is out of the question because of geometric complexity, SLSmetal-powder parts might be the best choice Beyond batch sizes of 10, it is worthconsidering the use of small batch casting methods This decision will be influenced

by desired accuracy, machining being better than casting Some developments inshape deposition manufacturing and 3-D printing are leading to direct mold making(e.g., Weiss et al., 1990; Sachs et al., 2000)

4.3.2Accuracy

Accuracy is perhaps the next key feature that distinguishes the various prototypingprocesses The list that follows gives some very general values for a variety of SFFcomponents.1

• Hot, open die forging:+1- 1,250 microns (0.05 inch)

• Laminated object modeling:+1-250 microns (0.010 inch)

• Investment (lost-wax) casting:+1-75 microns (0.003 inch)

• Selective laser sintering: +1- 75 to 125 microns (+1- 0.003 to 0.005

inch)-depends on part geometry

• Stereolithography: +1- 25 to 125 microns (+ 1- 0.001 to 0.005 inch)-depends

on part geometry

• Plastic injection molding from a machined mold (prototyping version): +1- 50

lu 100 microns(+1-0.002 to 0.004 inch)

• Rough machining: +1- 50 microns (0.002 inch)

• Finish machining:+1-12.5 microns (0.0005 inch)

lThe rust entry corresponds to the age-old vtuege blacksmith's prototyping shop See Wright and

• Electrodischarge machining: +/-2.5 microns (0.0001 inch)

• Lapping and polishing:+1-0.25 microns (0.00001 inch)

When comparing the everyday prototyping methods, the most accurate remainsmachining, with easily achieved accuracies of +/- 25 microns (0.001 inch) and evenhalf this with a good craftsperson The next most accurate is prototypingbyplasticmolding from a machined mold, with an accuracy of +/- 50 microns (0.002 inch).After that the SLA and SLS processes are listed For a typical component,selective laser sintering and stereolithography average out at+/-50 to 125 microns(0.002 to 0.005 inch) This is different fromthe accuracies of 25 microns (0.001 inch)quoted bythe suppliers of SLA equipment, and confrontational e-mails will prob-ably be a result of the obvious differences used in this text However, these adver-

to make complicated computer casings and medical monitors where open shell tures "warp and shrink all over the place," to quote one user In some cases thiswarping and shrinking worsens the SLA accuracy to as much as +/- 375 microns(0.015 inch)

struc-For SFF, the other accuracy consideration is stair-stepping Mentally picture thesoccer ball again, but this time with perfectly smooth surfaces Now approximate thethe equator; the smallest slice is at the poles Figure 4.13 from Jacobs (1996) showsthat the approximation to the soccer ball becomes worse as the bounding curvecomes up around the object toward the poles In addition, the loss in accuracy/fidelity

is related to layer thickness Since SLA processes are improving all the time, layersdown to 25 to 50 microns (0.001 to 0.002 inch) are now possible, therefore givingbetter and better accuracies As with all manufacturing processes, the process thendoes take longer and more cost is involved

Investment (lost-wax) casting is listed at +/- 75 microns (0.003 inch) Thus ifthe casting process is used to make a short-run prototyping mold and then the partsome hand finishing and some cosmetic work on the mold will give as good a plasticpart as the cast mold

Large layer thickness Medium layer thickness Fine layer thickness

4.3.3 Lead TIme of Prototypes

With an in-bouse dedicated FDM machine, a part can be produced within a 24-hourperiod For an ongoing design activity where a design team needs a series of proto-types-for the look and fit of subcomponents and subassemblies-the FDM process

is ideal

An in-house integrated CAD/CAM system for machining can generate asimple part in a "morning's work," whereas more complex parts will take two tothree days An in-house stereolithography machine will also create the same parts intwo or three days, measuring the time from receiving the" STL" file to a fully curedproduct The curing time, incidentally, is an added time factor, often overlookedwhen rival companies develop their advertising literature and compare their partic-ular process with others

If an in-house machine is not available, it should be realized that the SLAservice bureaus are swamped with business in today's economy Unless a special cus-tomer relationship exists, turnaround time of one to three weeks is more probable.Given the need for some negotiation with a client, and the need to check incomingprototyping shops are selling "service and speed" rather than "fidelity."For small batches (10 to 5(0) of injection molded plastic parts, customers canexpect a three- to six-week turnaround time The steps might be (a) anSDRCflDEAS or Pro-Engineer CAD file is received from the Internet, (b) files arechecked, (c) an SLA master is made, (d) an aluminum mold is cast, and (e) the fin-ished batch of 10 to 100 is injection-molded in ABS plastic

4.3.4 Batch Size

Chapter 2 describes the influence of batch size For just one component, SFFprocesses-such as stereollthography.fused deposition modeling, and selective lasersintering-or machining is the obvious choice Small-batch casting in metal, or small-batch injection molding in plastic, is used for batch runs between 50 and 500

4.3.5 Cost

In general, cost increases with fidelity and accuracy needed, fur all the rapid more accuracy, the" STL" files will need to be of finer resolution, the slicing will alsoincrease Also, all prototyping processes (SFF or machining) require some hand fin-smoother surface finish In all prototyping processes there is also a relationshipbetween complexity, surface finish, accuracy, and cost For SFF, Figure 4.14 showsthat overhanging features require explicit support especially for SLA For thearrangement on the right of Figure 4.14, the support columns have to be broken off

proro-by hand after manufacture This usually leaves small stubs on the surface, which must

a Complementary support

JIIpre 4.14Supporting structures for SLS and SLA (courtesy of Lee Weiss)

b.Explicit support

Rapid prototyplng maclrine (RP~)

TABLE 4.4 Rapid Prototyping Machina Cost-Alao sea Section 4.3.6 forInstallation and the Like (aa of March 2000)

310,000

4.3.6 Ancillary Cosu

The costs shown in Table 4.4 are the base cost of the machine It should be sized that there are also additional miscellaneous costs of a warranty, installations.and so on For example, the Helisys 2030H LOM machine has a base price (as ofMarch 2000) of $275,500, which actually includes a first-year service warranty.Installation is estimated at $3,000; training at $2,000 Additional options include achamber heating module at $4,499 and an initial supply package at $5,995.Thus thetotal for the complete package is $292,494 This example is not meant to endorse

empha-or criticize the LOM machine; rather it shows the real cost of doing business Allbase price Some processes such as SLS also require a supplementary room forpowder preparation and venting Further data on cost comparisons (Table 4.4),materials (Table 4.5), part size (Table 4.6), and total part cost (Table 4.7) now

4.3.7 Commercial Comparisons of Cost and Capability

Rapid prototyping

TABLE4.5 Modeling Material Comparison

Liquid

photocurablepolymers

Sintered

='"

powder and

ViscousSheet Polymer solidifyingmaterials spool polymersStereolithography

Selectivelaser sintering

Laminated object modeling

Fused deposition modeling

Solid ground curing

Machine

TABLE4.6 Maximum Part Size Comparison (as of March 2000)

Company

Partsi:;ecapability (in.)SLA-250

Hellsys.IncCubiral America, Inc

DTMCorpDTMCorp

12x 15

15 X 13x16.7

TABLE4.7 Rapid Prototyping Process, Speed and CostCompariscn-c-Chrvsler

Benchmarking Test Reported In "Rapid Prototyping Report," Vol 1, No.6, June 1992 Note That This Comparison Was Done with 1992 Machines Such

as the 3D Modeler by Stratesvs Many Machines Such as the Sinterstatian

2500 P1u• Have Become Much Faster Since Then.

Total process time

(hr:min)

Stereolithography

Fused deposition modeling

Laminated object modeling

Solid ground curing

Selectlve laser simermg

SLA-250

3-D ModelerLOM-I0ISSolider 5600SinteTSlalioo2000

109.40

88.70'"

199,23

ComparisoD of Approxlnule AeeurllCY

of Rapid PwlutypinK PIon:_

4.4 CASTING METHODS FOR RAPID PROTOTVPING

4.4.1 Introduction

The classic manufacturing textsbyDeGarmo and associates (1997), Kalpakjian(1997), Schey (1999), and Groover (1999) are remarkably comprehensive in theircoverage of the casting process The several methods of casting include:

• Lost-wax investment casting

• Ceramic-mold investment casting

• Shell molding

• Conventional sand molding

• Die casting

Rather than duplicate the material found in other books, this section focuses

on casting as it is done by rapid prototyping companies Batch sizes from 50 to 500are typical The key market strategy is that casting is cheap and fast However, it maySLA·250(SLA)

wax processes to +/- 375 microns (0.015 inch) for standard sand castings (also seeChapter 2).

4.4.2 Lost-Wax Investment Casting

As mentioned in Chapter 1, the fundamentals of casting were invented by Korean

and Egyptian artists many centuries ago The following steps are known as the

lost-neering or art object is first carved from wax; (d-f) it is surrounded by a ceramicslurry that soon sets into solid around the wax; (g) the wax is melted out through ahole in the bottom, leaving a hollow cavity; (h) this hole is plugged, and liquid metalthe ceramic shell can be broken away to get the part;(j)some cleaning, deburring,and polishing are needed before the object is finished

The process was greatly improved and made more accurate during World War

II for aeroengine components Today it is used for products such as jet engine turbinefrom injection molds, assembled on treelike forms, and then treated with the slurry.Alternate layers of fine refractory slip (zircon flour at 250 sieve or mesh size)are applied, followed by a thicker stucco layer (sillimanite at 30 sieve or mesh size).and liquid acid hardener Drying takes place in ammonia gas.The next step is to elim-inate the wax in a steam autoclave at 150°C,fire the mold for 2 hours at 950 "C, thenpour in the liquid steel or aluminum

In summary, the modern lost-wax method has one of the best tolerances in thecasting family because the original wax patterns are made in nicely machined molds.Today,tolerances of+/-75 microns (0.003inch) are readily obtainable Also the as-castsurface is relatively smooth and usable for the same reason Other advantages include:

• No parting lines if the wax original is hand finished

• Waxes with surface texture can give direct features such as the dimples on agolf club

• Automation of the slurry dipping is possible using robots, thereby reducingcosts

• Products such as turbine blades can be unidirectionally solidified, giving goodmechanical properties in the growing direction

4.4.3Ceramic-Mold Investment Casting Procedures

The snag about the previous method is that the wax pattern is destroyed The

ceramic-mold investment casting technique therefore employs reusable submaster

patterns in place of the expendable wax patterns This version of investment castingfine care and expense that go into creating the original master positive in Step 1 Thesteps are as follows:

• Step L Positive: make an original master pattern with stereolithography or

Slurry coating Stucco coating Completed mold

Figure 4.1f The lost-wax investment casting process Upper diagrams (a) through (c) lead to

the tree of wax master patterns Middle diagrams show the slurry and stucco being appliedLower diagram shows the casting (adapted from literature of the Steel Founders' Society ofAmerica)

• Step 2 Negative: create a shell around the master with highly stable resin Anegative space is created around the original positive master pattern This shellcan be pulled apart to give a parting line.

• Step 3 Positive: create reusablesubmaster rubbery molds from the shells

• Step 4 Negative: create the destroyable slurry/ceramic molds

• Step 5 Positive: pour metal into the ceramic molds, which are then brokenapart to get the components, which must then be degated and deburred.SLA can be used to make the original master pattern, or a CNC machine can

be used to mill the master from brass, bronze, or steel Of course, the process can start

at Step 3, but this might damage the original master, especially if it is SLA Also, toget high productivity in the factory, it is preferable to have many molds at Step 3, all

of which can be made from the stable resin negative in Step 2

Prototyping companies like to use the hard resin to fabricate the negative inStep 2, because the resin has good dimensional stability Note that it is typical to havetwo resin molds, one for each side of the casting, separable by a parting line.Once the hard resin shells have set, they can be filled with a slurry gel that solid-ifies to a hard "rubbery positive" for Step 3 This intermediate submaster mold can

be stripped away from the resin shells while it is still "rubbery." The material is idealfor the rather rough handling environments of a foundry, and the rubbery propertiesmean that no draft angles are needed for stripping these submasters off the resinshells

The Step 4 negative mold is made from a graded aluminosilicate with a liquidbinder (ethyl silicate) and isopropyl alcohol This is poured around the subrnastersinner cavity, the slurry is fired at 950 ~C to give it strength, and the casting process,say with molten aluminum, can begin

After solidification, the component is broken out of the ceramic, cleaned up,and deburred The parting line can cause problems, but in general, good accuracy is

obtained: +f- 125 to 375 microns (+1-0.005 to 0.015 inch).

4.4.4Shell Molding

An alternative form of high-accuracy casting isshell molding. Metal pattern platesare first heated to 200°Cto 240°C A thin wall of sand,S to 15 millimeters (0.25 to0.75 inch) thick, is then sprayed over the plates The sand is resin-coated to ensureadhesion to the metal plate Phenolic resins, with hexamethylene-tetramine addi-tives, are combined with the silica to ensure rigid thermosetting of the sprayed sand.accurate for casting Once the excess sand is removed and casting is finished, accu-racies can be as low as +/- 75 microns (0.003 inch)

4.4.5 Conventional Sand Molding

The cruder, cheaper version of casting starting with wooden or plaster patterns is

risers for the poured metal This gives tolerances of +1- 375 microns (0.015 inch).

Newer developments include:

1 A high-pressure jolt-and-squeeze method: Here mechanical plungers push thesand against the mold at a jolt of 400 psi This gives a tighterfitof the sandagainst the pattern and hence better tolerances after casting

2 Carbon dioxide block molding: Here the interfacing between the sand and tbepattern is made up of a special material about 12 millimeters (0.5 inch) thick

It is a refractory mix of zircon or very fine silica, bonded with 6% sodium cate, which is then hardenedbythe passage of carbon dioxide

sili-4.4.6 Die Casting

Die casting is predominantly donebythe high-pressure injection of bot zinc into apermanent steel die Today, the die or mold for this type of casting is almost certain

to be milled on a three- or five-axis machine tool

Die costs are relatively high, but smooth components are produced with

accu-racies in the range of +1- 75 microns (0.003 inch) However, these high costs for the

permanent molds mean that die casting does not really fit into the rapid prototypingfamily It is mostly used for large-batch runs of small parts for automobiles or con-sumer products Since low melting point materials such as zinc alloys are used in theprocess, component strengths are relatively modest

Today, the injection molding of plastics (Chapter 8) is often preferred over zincdie casting

4.5 MACHINING METHODS FOR RAPID PROTOTYPING

4.5.1 Overview

Chapter 7 deals with the generalized machining operation including the mechanics

of the process This chapter focuses on advances in CAD/CAM software that allowCNC machining to be more of a "turnkey rapid prototyping" process One goal is tothe intensely hands-on craft operations (e.g., process planning and fixturing) thatdemand the services of a skilled machinist

CyberCut™ is an Internet-based experimental fabrication test bed for CNCmachining The service allows client designers on the Internet to create mechanicalcomponents and submit appropriate files to a remote server for process planning andfabrication on an open-architecture CNemachine tool Rapid tool-path planning,novel fixturing devices, and sensor-based precision machining techniques allow theoriginal designer to quickly obtain a high-strength, good-tolerance component(Smith and Wright, 1996)

4.5.2WebCAD: Design for Machining "onthe Internet" on

the Client Side

A key idea is to use a "process aware" CAD tool during the design of the part This

prototype system is called WebCAD (Kim et al., 1999) Sun Microsystems' Java™_