21st Century Manufacturing Part 12 pdf

Itisalso important to look back on our "journey along the product ment path." As shown on the clock-face diagram in Figure 2.1, the various stepswere:develop-• Design of the product Chap

Trent, E M., and P K Wright 2000 Metal cuuing, 4th ed Boston and Oxford: Butterworths.Wagner, R., G Castanouo, and K Goldberg 1997 Fixture.Net: Interactive computer aided

design via the WWW.lnternationalJournalon Human-Computer Studies 46: 773-788.

Walczyk, D E and D E Hardt 1998 Design and analysisof reconfigurable discretedies for

sheet metal forming Journal of Manufacturing Systems 17 (6): 436-454

7.9 BIBLIOGRAPHY

Bamrnann, D 1 M L.Chiesa, and J C Johnson, 1995 Modeling large deformation anisotropy

in sheet metal forming In Simulation of materials processing: Theory, methods, and lions,657-660 edited by Shell and Dawson, Rotterdam: Balkema

apptica-Devries, W R.1992.Analysis afmaterial removal processes. New York: Springer-Verlag.Klamecki B E and K I Weinmann 1990 Fundamental issues in machining In Proceedings

of the Winter Annual Meeting of ASME in Dallas Texas, 43: New York: American Society ofMechanical Engineers

Kobayashi S., S-1 Oh and T Altan.1989 Metal forming and the finite element method. NewYork and Oxford: Oxford University Press

Komanduri R 1997 Tool materials In The Kirk-Othmer Encyclopedia of Chemical 110101;)" 4th ed 24 New York: John Wiley and Sons

Tech-OXley, P L B 1989 The mechanics ofmachining:An analytical approach to assessing ability New York: Halsted Press

machin-Pittman, IT., R D Wood,I M.Alexander, and 0 C Zienkiewicz.1982.Numerical methods in industrial forming operations. Swansea, u.K.: Pineridge Press

Shaw, M C 1991 Metal cutting principles. Oxford Series on Advanced Manufacturing, Vol 3.Oxford: Oxford SciencePublications, Clarendon Press

Stephenson, D A, and R Stevenson 1996 Marertats Issues in machining III and the physics of machining processes Ill, Warrendale, PA: TMS Press (Minerals, Metals, and MaterialsSociety)

Wang, C, H 1997 Manll!acturability-driven decomposition of sheet metal products. RoboticsInstitute TechnicalReport CMU-RI-TR-97-3S Pittsburgh, PA: Carnegie Mellon University

7.10 URLS OF INTEREST

A collection ofsites for machining planning and automation can be found at

<hUp:llkingkong.me.berkeley.edulhtmllcontactlmach_software.html> A site formetal products in general is <www.cemmerceene.cem»

7.11 INTERACTIVE FURTHER WORK " THE SHEAR PLANE ANGLE

Use Netscape with Java capability to access <http://cybercut.berkeley.edul chant> Dr Sandstrom of TheBoeing Company has built an interesting Javaapplet

mer-Complete the table for the following 12 cases:

o0.51.0o

0.5

1.0o

1.0

7.12 INTERACTIVE FURTHER WORK 2: "FIXTURENET"

Modular fixturing on the World Wide Web is by Dr Kenneth Goldberg and his dents The URL to use is <http://riotJeor.berkeley.edu>,andthenclickonFIxtnreNet.Brost, R., and K Goldberg 1996 A complete algorithm for designing modular fix-

stu-tures using modular components IEEE Transactions on Robotics and Automation

12(1)

Wagner, R., G Castanotto, and K Goldberg 1997 FixtureNet: Interactive

com-puter aided design via the WWW International Journal on Human-Comcom-puter Studies 46: 773-788.

A modular fixture consists of a metal lattice with holes spaced at even intervals(Figure 7.34a), three locators (Figure 7.34b), and a clamp (Figure 7.34c), whichmake four contacts and hold objects in "form closure." Figure 7.34d is a photograph

of their use

Figure 7.35 shows a part on the World Wide Web with three locators and clamp

in form closure The three locators fit into the fixed lattice and are positioned in such

a way that they are touching three edges of the part The clamp must also be placed

un the lattice so that damp motion is horizontal or vertical The clamp can be tioned to push against the object

posi-An admissible fixture is an arrangement of the three locators and clamp on thelattice that holds the part in form closure The conservative assumption is made thatthere is no friction TWo fixture arrangements are equivalent if one can be trans-RakellllglC'

(b) (0)

(d)Fipre 7.34(a) Modular lattice, (b) locator, (c) clamp and (d) physical setup

The general problem is:Given a polygonal part boundary, find aU admissiblefixtures (if any).The Algorithm is:

Step 1: Grow the part by the radius(r)of the locators, and shrink the locators to apoint Curved portions are eliminated because we assume that locators andcomers of delicate parts

Step ~ Labeleach edge 1.2,3, ,nin a counterclockwise fashion

Step 3: Consider all combinations of triplets in counterclockwise increasing for example,1,2,3 or 1,2,4 or 1,2,5 or 1,3,4or2,3,4.Foreach triplet,call

order-the edges a,b,c This will give us all possible arrangements of order-the three

loca-tors in contact with the three edges of the part

Step 4: Without the loss of generality, assume edgeais in contact with a locator at

Flgu~ 7.3$ Screen dump from the Web site.

late and then rotate the part while maintaining contact between edge a and

second locator (L2) can be placed, because the other three quadrantswould be reflections about the origin

Step 5: For each choice of L2, find all possible choices of L3 Given (a-L1, b-U),

solve for c Or (very fast) solve for (a-U,c-L3) an annulus and (b-L2,c-L3)another annulus Intersect to find all consistent choices for (c-L3).Step 6: For each triplet of locator-edge matches (two acceptable designs in thisexample), find all possible clamp arrangements Use the Reuleaux rotationcenter construction

Step 7: Repeat Steps 3 to 6 for each triplet of edges

Step 8: Output all possible solutions The time order of this algorithm is O(n5d5),

units

Question: Can all polygonal parts be fixtured?

Specific assignment: Use FixtureNet to design two modular alternative fixtures for a

7.13 REVIEW QUESTIONS

L In forming, forging, and extrusion operations, a popular technique for dicting approximate loads and metal flow patterns is the upper bound tech-nique (Johnson and Mellor, 1973; Rowe, 1977; Hill, 1956) The upper boundtechnique can also be used to make an estimate for the force necessary to formthe chip in metal cutting The analysis first enlarges the center section inFigure 7.36 and then considers the complete shear bandOD,which has a totallength of (s) Show that the final result for the forceFcis found as:

In this equation, kis the shear yield strength of the metal, Vis theincoming velocity,V~is the shear velocity along OD, and s is the length of OD.

Z The basic rolling operation creates a wide flat strip in a coil This strip is sold to

a secondary processor, who carries out the sheet-metal forming operation.Automobiles, washing machines, office furniture, filing cabinets, and the insidecasings that carry the PCBs in a computer all start as rolled product The sec-ondary processor takes the large coils that come off a rolling mill, shears theminto much smaller starting blanks, and then sheet forms them in a pressing dieshaped to the required geometry (Figure 7.37)

8.Show that the approximate roll loadP=w •Y Viidhwhere w is the width

of the strip, Y is the average yield strength of the material as it goes through

the roll gap,Ris the radius of the rolls, anddhis the reduction

b Figure 7.38 shows a strip being pushed from left to right and into the roll gap.The top edge of the strip(E) is shown meeting the rolt.Jt experiences twocounteracting forces: one that tries to push it out, and another, due to fric-tion, that tries to suck it in The conditions that allow the strip to go in andout component

F'lprft7.36 Stress element at the shear

Entry of strip, h1 E~tofstrip,h2

Figuft 7.37 Sheet rolling: material on the left enters the roll gap and is plasticallydeformed by an amount(h,-h, = dh)

Show that because of the balance between the friction that "pulls in" thesheet and the roll angle that "pushes out" the sheet, the maximum reduction inone pass is given by

(7.25)The basic physics of friction, and the roll radius, control the maximumreduction in one pass These mechanical analyses show why ultraexpensivemultiple stands are needed at the standard steel mill to produce flat rolled stripfor consumer products

Itisalso important to look back on our "journey along the product ment path." As shown on the clock-face diagram in Figure 2.1, the various stepswere:

develop-• Design of the product (Chapter 3)

• Prototyping of the product (Chapter 4)

• Making the inner brains (Chapter 5)

• Assembling the inner system (Chapter 6)

• Machining a mold (Chapter 7)

• Injection into this mold (Chapter 8)

As a result, plastic injection molding and product assembly can be seenas a mination of the processes and devices in all the previous chapters, arriving at the pro-duction of millions of units ready for the consumer At the same time it should berecalled from the casestudy in Chapter 2 on the fingerprint recognition device thatinjection molds can be machined from aluminum, allowing small batches of only 200unitsto be made for early customer testing or for evaluation kits

cut-8

8.2 PROPERTIES OF PLASTICS

Plastics, or polymers, have different properties than the metals presented in Chapter

7, and it is important to review these before moving on to injection molding or blownot all, of the part design and equipment design issues shown in subsequent figures

As in Chapter 7, it is assumed that the reader has enjoyed a freshman class inmaterial science and recalls that polymers fall into two broad classes:

• Thermosetting molding materials These include the melamine-formaldehydeused in hard plastic tableware and the epoxy resins used for glues and rein-forced cast products such as kayaks and tennis racket frames Thermosettingproducts are heated until they become a viscous liquid, poured or injected intoirreversible, infusible mass

• Thermoplastic molding materials These include polymers such as butadiene-styrene (ABS) and polycarbonate (Lexan is a common brand) usedfor toys, consumer electronic products, and more flexible kitchenware prod-molded, and cooled in a reversible, time-and-time-again manner As a result,they are perfect for the routine injection molding processes described later.They are therefore reviewed in more detail in the next section

acrylonitrile-8.2.1 Properties of Thermoplastics

Which particular polymer should be used for a given component? The answerdepends on how that polymer behaves at the operating temperature of the device.All thermoplastic polymers go through the generic transition described in Table 8.1for polystyrene, but they do so at different temperatures

At low temperatures, the polystyrene's structure is glassy and it has a high ness as measured by Young's modulus, E The stiffness can also be increased byincreasing the molecular weight of a polymer, by increasing the branching of thefolded against each other, and by adding elements that cross-link the chains.Speaking colloquially, the mechanical properties at low temperatures can be viewedperatures the molecular chains of the polystyrene slide over each other like cookedspaghetti

stiff-TABLE 8.1 General Characteristics of Thermoplastic Materials Related to Poly",tyrene

Bond stretching as in meta!sChain bending/uncoilingChain slipping

-c:

just overTg,rigid and

tough (polyethylene)

1 CryslallineslruclUraipolymer c-cTg(PMMA)

The stress-relaxation modulus is U1I;:ngiven by,

e,Typical results (McLoughlin and Tobolsky, 1952) are shown in Figure 8.l.These tests are for polymethyl methacrylate (PMMA), commonly called Plex-iglass (in the United States) or Perspex (in the United Kingdom).At 40°C,the mate-rial remains rigid for long periods, but with increasing temperature, the materialbecomes leathery above temperatures of 135°Cand eventually viscous.Another key concept is the glass transition temperature, at which a thermo-plastic transitions from its glassy to leathery behavior In Figure 8.2 the specificvolume of polyvinyl acetate is plotted against temperature The value of the glasstransition temperature is found by extrapolating the glassy region and the leatheryregion to the intersection point atTg=26°Cin this case

8.2.2The Influence of Properties on High-Level Design

From a design perspective the strategy is to pick a polymer that displays the desiredcharacteristics at the operating temperature of the product, most often room tem-perature Figure 8.3 shows this design strategy, which includes:

• Polymethyl methacrylate, which is a rigid-structured material at room

temper-ature, considerably below Tg•

• Polyethylene and acrylonitriie-butadiene-styrene (ABS), which are just over Tg

at room temperature but considerably below the melting point and thereforerigid and tough These are suitable for toys, car parts, and electronics packaging

• Polyvinyl chloride sheet, which is leathery at room temperature and suitablefor some forms of clothing and imitation leather products

This background sets the scene for the injection molding of ABS to createdevices like the fingerprint recognition unit and the InfoPad At the conceptual level,the ABS is heated into the highly viscous state, pumped into a die cavity, and thenallowed to cool into the desired product The details of the process, with some of its

8.3 PROCESSING OF PLASTICS I: THE INJECTION MOLDING

METHOD

8.3.1Overview

Injection molding is a key production method for the casings of consumer products

It is cheap, reliable, and reduces the device's weight compared with metals The STings (the case study in Chapter 6) were made in this manner A very wide array ofconsumer products ranging from toys to telephones to automobile parts is also madethrough this process

In Figure 8.4, some general features of the mold are shown A simple bucket orcuplike component could be made in the gap between the core and the cavity shown

at the bottom right In this case thepaning plane, shown on the bottom left, could be

at the lip of the bucket For other products the parting plane might be less ances where the parting plane has been located Further finishing by hand might bedesirable for such parts

conve-Hand finishing might also be needed (a) for the injection's "gate marks" thatmaybe visible as tiny "pimples" on the surface of a part and (b) for the "ejector-pin"marks that for a relatively small object like a cellular phone casing are usually about

6 mm (about 0.25 inch) in diameter The reader might be interested in picking up anyfamiliar plastic consumer product and searching for these inevitable markings How-ever, these are often located in noncosmetic areas of the part, because of the costinvolved in finishing operations Not surprisingly, they are often found on the bottom

or base of the object

Injection molding is shown in Figure 8.5 Pellets of the desired thermoplasticare loaded into the hopper on the right side and heated as they are pushed by a screw

1Partingdirection

zx~Yx-y ·"2Principaldirections

F1guJe 8.S Injection molding with reciprocating-screw machine

through heating zones This melted and mixed material is forced through a nozzlethe mold Once the two halves of the mold have been separated, the ejector pins

Nozzleinjectionmachine

Sprue bushFilled,Sprue halfImpression

Back-flowatou valve

Gate

Parting·

surface

8.3.2 The Reciprocating-Screw Machine

The reciprocating-screw machine is the most used machine in industry It isshown in the open position in Figure 8.5a Below the machine is a single-cavitymold and a multiple-cavity mold Note that the machine is horizontally constructedand operated Thus the mold is effectively laying on its side in comparison withFigure 8.4

In Figures 8.5b and 8.5e the cavity is on the right and the core is on the left.Molten plastic is shot into the mold from the nozzle of the reciprocating-screwmachine and into the sprue of the mold If there are multiple cavities (Figure 8.5d),the sprue feeds the runners and gates

The barrel of the machine is heated, and as the screw pushes the pellets ward, there is additional heating from the mechanical pulverizing effect In fact themachine is designed to operate in two distinct phases:

tor-• Step 1, plasticizing the thermoplastic: the combined action of the heaters andscrew feed creates a metered volume of liquid polymer that arrives at area Y,this point by a cold slug of plastic from the previous shot

• Step 2, injection into the mold: the screw action stops, and the whole stationaryscrew is now used as a ram to force the liquid out of the nozzle, through thesprue, and into the mold Immediately before the ram action, the mold hasbeen closed so that the liquid polymer fills the impressions shown as the darkareas in Figures 8.5b and 8.5L:

It should also be noted that a nonreturn valve just behind the Tam head vents the liquid from moving backward into the screw channels Following this two-

pre-a sufficient wpre-aiting period, the mold cpre-an be opened to eject the ppre-art To reduce thecooling period, the mold is actively cooled by water lines But during the cooldown,the screw can begin to turn again to collect its next shot of polymer pellets and moveback to create space in position Y for the next shot

8.3.3 Computer Aided Manufacturing

McCrum, Buckley, and Bucknall (1997) describe the research into equipment andpellets, compressing and melting them with help from the heater zones, and then,nozzle into the die The clearance between the barrel and the flat lands of the screwflights is only 10-2mm, demanding that the machine screw and the barrel be madefrom hardened steel with wear resistant coatings Internal pressures of 100 Mpa aretypical

CNC controllers are used to monitor various sensors in the system and toadjust the various parameters shown in Figure 8.6 The key features include:

• Thermocouples for the temperatures of the barrel, nozzle, and mold

Closed loop control of mold

pressure during packing

phase to ensure consistent

part density Measured mold

material in front of the

screw after the mold is

fiUed.This ensures that

the material in the mold

is subjected to the

proper pressure during

the packing phase and

a1sothat there is

sufficient material to

properly pack the parts

Velodty to pressure oontrolTransition from velocity program toram pressure control phase isautomatically activated usingmeasured machine or plasticselectable from mold pressure,hydraulic pressure, or ram position

RampressureooDb:oI

Initiated after thevelocity phase toprovide dosed looppressure during packhold and screw recoveryphases Maintainscontrolled pressureduring these criticalphases despite changes

in oil temperature,valve leaking, orsystem Loading

Proxnunmed injection(velocity control)Closed loop control ofbeing injected into themold Separatevelocity steps can beprogrammed to occuranywhere within theactive ram stroke

F1gufe tl6 Control of injection molding (courtesy of Barber-Colman DYNAProducts)

• Pressure sensors monitoring the pressure in the mold

• A screw position sensor (potentiometer or LVDT), which also measuresvelocity during the ramming step

The controller is used to optimize the cycle and to pack and hold the polymer in thedescribed in Section 8.3.5

8.3.4Behavior of the Polymer inside the Mold during the

walls must therefore be tapered to allow easier part ejection The volume tions of a polymer between its liquid temperature and room temperature are of themanufacturing of plastics because voids and sink marks would occur if the polymerwere allowed to shrink this much without special controls These voids can reducethe mechanical properties and cause cosmetic imperfections.

contrac-Figure 8.7 from McCrum and colleagues (1997) shows how this volumetricshrinking is addressed Linesa, b,and c in the diagram represent isobars of increasingpressure,withline c being the highest The details are explained in the next paragraph.

The cycle is as follows:

• At the highest temperature and specific volume (V A) the liquid plastic is firstpressurized, in the mold, to a pressure of approximately P=103atmospheres.This leads to a volume decrease of about 10% (i.e.,A taB between lines a and c).

In Equation 8.2 below, K is the bulk modulus, which is 1 GPa, typical of mostliquids:

• At point C, the gate freezes over, sealing the mold and preventing furtherpacking

• Between C and D, the plastic cools at constant volume (V D)under decreasingpressure until atmospheric pressure returns again at position D

ViscousTemperature

Figure 8.7 Liquid polymer follows the path

A to B as a pressure of approximately io'

atmospheres is applied The liquid is

"packed" betweenBand C At point C, the

gate freezes over Between C and D, the plastic cools at constant volume (VD)

(adapted from McCrum, Buckley, andLiquid

Packing