3. Pat t er n, Mold and Cor e Des ign

Transcription

1 3. Pat t er n, Mold and Cor e Des gn The most mportant decson n pattern and mold desgn s about the partng lne. It affects and s affected by part orentaton, desgn of pattern and cores, number of cavtes n the mold, locaton of feeders, and channels for gatng, coolng and ventng. In ths chapter, we wll frst develop a scentfc defnton of partng lne, followed by the desgn of partng lne, pattern, mold cavtes and cores. 3.1 Partng Lne Desgn The partng or separaton between two or more segments of a mold s necessary to create the mold cavty (as n sand castng) and also to remove the manufactured part from the mold (as n decastng). For any gven castng geometry, a number of partng alternatves may exst; vsualzng and selectng the best alternatve s a non-trval task even for smple shapes. Varatons n customer requrements, qualty specfcatons, manufacturng facltes and economcal consderatons may lead to dfferent partng solutons for the same shape. For ntrcate parts, there s a hgh possblty of overlookng feasble alternatves and dffculty n assurng that the selected alternatve s the ndeed the best one. To evolve a scentfc approach to partng lne desgn and analyss, unambguous defntons of partng and related features are requred, vald for all types of toolng beng consdered. The followng defntons are proposed. Mold segment s a dstnct body, at least one face of whch s n contact wth the castng. Draw drecton of a mold segment s the drecton along whch t s wthdrawn from the adjacent mold segment. Partng surface s the surface of contact between any two segments of the mold. Partng lne s the contour of ntersecton of a mold partng surface wth castng surface. Partng surfaces may be classfed based on the type of mold segments at the nterface. Consderng three types of mold segments: cope, drag and cores, we have cope-drag, cope-core, drag-core and core-core partng surface. In practce, only cope-drag nterface s referred to as the partng surface. The cope-core and drag-core nterfaces correspond to the portons of mold that support a core. The core-core nterface s encountered n core assembles. The nterfaces between the segments of a three-part mold (cope, cheek and drag) can be treated smlar to those n a two-part mold. 23

2 Fg.3.1: Partng lne s the contour of ntersecton of mold partng surface (cope-drag or mold-core) wth part surface. Partng lnes may be classfed based on the number of planes n whch the dfferent segments of the partng lne le. A flat partng lne les entrely n a sngle plane. A stepped partng lne les n two planes, one of them normal to the draw drecton. The segments of a complex partng lne le n multple planes. Characterstcs of a partng lne nclude the followng: 1. Partng lne dvdes the part surface nto separate regons each produced by a dfferent mold segment. 2. Partng lne concdes wth the projected boundary of the castng when vewed along the draw drecton 3. The nternal angle at the partng lne s less than 180 degrees. 4. Partng lne concdes wth part of the boundng lne of a partng surface. 5. Flash appears along the partng lne. Partng lne dentfcaton: The frst step n partng lne desgn s to select an approprate draw drecton. One set of alternatves for the draw drecton s provded by the centerlnes of the mnmal boundng box of the castng. Other alternatves are gven by the drecton of normal to large faces n the castng. The crtera for selectng the most sutable draw drecton nclude (n decreasng order): 1. Mnmze undercuts (number and then volume of cores) 2. Mnmze the total draw dstance (for both halves) 3. Mnmze draft allowance (volume ncrement). The partng lne can be dentfed usng ts characterstc that t concdes wth the projected boundary of the castng when vewed along the draw drecton. The edges of the castng are frst projected on to a plane perpendcular to the draw drecton. The nner segments of the projected edges are gnored and the outermost permeter (slhouette 24

3 boundary) s determned. Ths s projected back to the castng and the correspondng landng ponts are determned. These ponts are connected n sequence to form the partng lne. If multple landng ponts occur (say, along edges parallel to the draw drecton) then multple alternatves of partng lnes are avalable for selecton. Dependng on the shape of the component, the above method can generate flat, stepped or complex partng lnes. Fg.3.2: Partng lne generaton by back-projecton of part slhouette. 3.2 Partng Lne Analys s When multple alternatves are avalable, then the most sutable partng lne s the one that optmzes a set of desgn crtera. The crtera are geometrcal functons of feature parameters related to partng. They are dmensonless and return a value between 0 and 1, a hgher value ndcatng better qualty, economy or productvty. Important crtera are descrbed below (refer to Table 3.1 for the correspondng equatons). Undercuts ncrease the number of elements n the mold; also the cost of a core for producng a partcular feature s sgnfcantly hgher than an ordnary mold of equal volume. Ideally, undercuts should be absent and the crteron evaluates to one. Flatness crteron measures the closeness of a gven partng to a flat plane by comparng the projected length of the partng lne on a plane perpendcular to the draw drecton 25

4 wth ts actual length. A flat partng evaluates to one and s consdered deal. A nonplanar partng ncreases the complexty of the toolng and should be avoded f possble. Draw dstance s the relatve movement between a mold segment and the pattern (or product) along the draw drecton untl they are clear of each other. It affects the manufacturng cycle tme and the unformty of compacton n the case of sand molds. To evaluate the draw dstance correspondng to a gven partng, t s compared to the deal mnmum, whch s half the smallest overall dmenson of the product. Draft appled to faces parallel to draw drecton to ease the wthdrawal of pattern or product from the mold results n ether ncreased weght of the product or cost of machnng the addtonal volume. A partng lne whch does not have large faces parallel to draw drecton s preferred. Flash crteron returns a hgher value f the partng lne s located along convex or sharp edges of the component, snce ths helps n trmmng the flash. All edges belongng to the partng lne are used n evaluatng ths crteron. Msmatch between mold segments affects faces whch le on both sdes of the partng lne. The areas of the two portons of such faces are compared wth the total area of the face to evaluate the msmatch crteron. It returns a hgh value f such faces le to one sde of the partng lne, ndcatng hgh dmensonal stablty. Mold cavty shape smlar to a rectangular pocket not only conserves mold materal but s also easer to manufacture. Ths aspect s measured by the volume rato of the sold correspondng to the cavty n a partcular mold segment and the boundng cubod. The sdes of the cubod are ether parallel or perpendcular to the draw drecton. Sub-surface qualty s mportant for crtcal or machned faces of a component. It s affected by the orentaton and locaton of such faces n the mold. The choce of draw drecton and the castng orentaton whch results n crtcal surfaces beng placed at the bottom of mold, yeldng hgh sub-surface qualty at these places, s preferred. Soldfcaton characterstcs of a castng as nfluenced by ts orentaton are assessed by ths crteron. It consders the rato of the largest to the smallest secton n the castng, and the heght of the largest secton from the mold bottom. Top heavy castngs are favored, snce ths ads n drectonal soldfcaton and facltates placement of feeders at the top. Sde thrust may occur n permanent molds flled under pressure f the partng s nonplanar and asymmetrc about a vertcal plane. The crteron for sde thrust ndcates the extent of force unbalance along one of the horzontal axes. The score S j of the j th partng alternatve s gven by: S j = w C j where, w = weght of th crteron and C j = assessment of th crteron for j th alternatve. partng 26

5 Table 3.1 Partng desgn crtera CRITERION FUNCTION PARAMETERS Undercut 1 1+ N C 1 V ( C ) V ( D) Flatness ( e sn ) Draw e dstance max ( ) Draft Flash 1 1 N C = number of cored undercuts V ( C ) = volume of core V (D) = volume of component θ e = edge of partng lne d è = angle between e and draw drecton 0.5d d mn = dstance of wthdrawal of mold segment d mn = smallest overall dmenson of part A( f ) A( D) 0.5(1 cosα ) e e snθ Msmatch 2 max [ A( f 1 ) ( ) ], A f ( f ) 1 A( f ) A ( f ) 2 A ( ) A ( f ) = area of face f draw drecton A (D) = surface area of component e = edge of partng lne á = nternal angle between faces at e θ = angle between e and draw drecton A = total area of face f f = area of f n frst mold segment = area of f n second mold segment Mold shape V ( M ) V ( M ) = volume of cavty n mold segment mn V ( B ) V ( B ) = volume of boundng cubod for M Surface qualty 0.5(1 + n. z) A( f 1 A( f ) ) h h m n = unt normal to crtcal face f z = vertcal drecton h = heght of f from mold bottom h m = vertcal dmenson of component Soldfcaton d mn d + mn 2h d = da of largest nscrbed sphere S 1 1 h = heght of center of S from mold bottom d d hm d mn = da of smallest nscrbed sphere h m = vertcal dmenson of component 1 e Sde thrust ( e s ). x e = edge of partng lne s = edge of partng surface connected to e x = x axs 27

6 3.3. Pattern Desgn A sand castng pattern s smlar n shape to the cast product (but not exactly the same). A sand mold cavty s a negatve replca of the pattern and s produced by packng sand around the pattern. Mathematcally, pattern desgn can be treated as a seres of transformatons startng from the product shape to fnally obtan the shape correspondng to the mold cavty. The transformatons are brefly descrbed here. [Pattern] = [T hole ] [T shrnkage ] [T machnng ] [T dstorton ] [T draft ] [T fllet ] [Product] Elmnatng holes: All holes that are produced by cores are removed from the product. Snce the pattern must also produce the pockets for seatng the cores, the core support geometry must be added to the product shape. Ths mples that the cored features must be dentfed, followed by the desgn of ther support n mold (core prnt). Then the volume correspondng to the entre core (ncludng ts prnt) s to be added to the part geometry to obtan the pattern shape. Shrnkage allowance: To compensate the sold phase contracton of the castng. The part dmensons are ncreased by a certan amount, dependng on the cast metal and type of mold. It ranges from 13 mm/m for alumnum alloys, 16 mm/m for copper alloys and 20 mm/m for grey ron. Note that the castng shrnks away from the mold wall, mplyng that whle external dmensons must be ncreased, nternal dmensons (ex. hole dameter) must be decreased. Machnng allowance: It s provded on surfaces that are machned later. It nvolves addng materal to part surface along the drecton of ts normal. The amount of addton depends on the dmensonal tolerance acheved by the process, sub-surface qualty, part sze and the type of machnng (manual or automatc). The allowance ranges from 1 mm for small alumnum decast parts to 20 mm or more for large grey ron sand cast parts. Draft: All faces of the product that are parallel to the draw drecton are provded a draft angle to facltate wthdrawal. The draft angle depends on the dstance of the face from the mold partng, length of the face along the draw drecton, type of face (external or nternal), mold surface roughness (surface fnsh and applcaton of lubrcants) and the type of moldng/castng process (manual or automatc). It ranges from 0.5 degree for small external faces close to partng lne n automated decastng machnes, to 3 degrees or more for large nternal faces n manual moldng for sand castng process. Fllets: All sharp corners must be rounded to facltate moldng and fllng. Whle the product desgners regularly provde fllets, these may not be adequate, especally n sand castng process. Too generous fllets are also not recommended, especally when only the nternal corner s flleted and the opposte external corner s sharp, snce ths may lead to local hot spots and shrnkage porosty defect. In general, a fllet radus of tmes the wall thckness s recommended. 28

7 The allowances can be combned n dfferent ways to mnmze the ncrease n castng volume (compared to the product volume). A fllet to a small rb along the draw drecton elmnated the need for draft. A vertcal face far from the partng plane, for whch ample draft has been appled, may requre less machnng allowance Core I dentfcaton Core s a separate entty placed n a mold to produce a correspondng cavty hole or undercut n the castng. Cores are also used for producng complex shaped pockets and specal features (for example, a vertcal face wthout draft) that cannot be produced usng a pattern or mold alone. Cores may be dspensable (n sand castng) or permanent (n de castng). In gravty decastng, ether permanent or dspensable cores may be used, usually decded by the core shape smple or complex, respectvely. A core conssts of two portons: the body of the core and one or more extensons (called prnts). The body of the core s surrounded by molten metal durng castng process. A core has to wthstand more heat and for a longer duraton than the mold. However, once the castng has cooled, a sand core must easly collapse to facltate ts cleanng out. The prnts are necessary to support the core n the mold. They also conduct the heat (and gases produced by a sand core) to the mold. Fg.3.4: Gravty decast compressor casng and ts core Cores for sand castng are manufactured by packng specally prepared sand n coreboxes. Core-makng processes nclude ol sand, hot box and cold box, whch are sutable for dfferent types of applcatons. The cavty n a corebox s a negatve replca of the correspondng part feature. The corebox s made n two segments (wth a partng) to enable removal of the core. Complex cores are prepared by assemblng or glung two or more cores of smpler shapes. The core-related actvtes: sand preparaton, core shootng, coatng/treatment and placement n mold, consume sgnfcant resources. Thus the number and volume of cores must be mnmzed to the extent possble, to reduce toolng cost and manufacturng tme. 29

8 Cored holes through and blnd, can be automatcally dentfed by geometrc reasonng. Undercuts can be dentfed based on the drecton of face normal wth respect to the draw drecton of mold segments. Ths s explaned n detal next. A smple yet robust feature recognton methodology can be developed based on Boundary Representaton of sold models. Let the part model be completely defned by a set of boundng facets, each facet by three edges and each edge by two vertces. Each facet s also assocated wth a unt normal vector that ponts from nteror (sold) to exteror (space). The rght-hand thumb rule apples to the face normal wth respect to the three vertces of the facet. The model conforms to Euler s equaton: F+V = E+2, where F, E and V are the number of facets, edges and vertces, respectvely. Edge Classfcaton: The edges of the part model can be classfed dependng on the nternal angle between the two adjacent faces that share the edge. If the angle s exactly 180 degrees, then t s a smooth edge, mplyng that the two adjacent facets are n the same plane. If the angle s less than 180 degree, then t s a convex or external edge; otherwse t s a concave or nternal edge. Hole Recognton: A depresson feature (hole or pocket) can be defned as a set of faces each of whch have at least one concave edge, and the openng of the feature comprsng a closed loop of convex edges. The number of openngs may be one (blnd hole), two (through hole) or more. The openngs are closed and the negatve space correspondng to the feature s converted to a postve space to create the body of the core. Undercut Recognton: Facets belongng to the nterface between a par of bodes beng wthdrawn from each other are tested for undercut condton. The bodes can be pattern and dspensable mold, part and permanent mold or permanent core and mold. If the angle between a facet normal and the draw drecton of the body contanng the facet s less than 90 degrees, then the face forms an undercut. The body of the core correspondng to the undercut feature can be created by extendng the faces adjacent to the undercut. CONCAVE EDGE Fg.3.5: From left to rght: part model wth holes, core body extracton, prnt desgn 30

9 3.5 Core Desgn and Analys s The prnt s an extenson of the core body, usually along ts axs. The prnt desgn depends on the drecton of core axs and the number of openngs. Each openng corresponds to a separate prnt for core support. Major consderatons n core prnt desgn are lsted below. 1. The prnt must balance the body, so that the core stays n place durng mold assembly. 2. The prnt must wthstand the buoyancy force of the metal and not get crushed. 3. The prnt must not shft durng mold fllng. 4. The prnt should mnmze the deflecton of the core. 5. The prnt should maxmze the heat transfer from the core to the mold. 6. The prnt should allow the nternal gases generated n the core to escape to the mold. 7. Unsymmetrcal holes should have foolproof prnts to prevent ncorrect assembly. 8. The prnts of adjacent cores may be combned nto one. The core prnt desgn depends on the type of core: 1. Horzontal smply supported core 2. Horzontal overhangng or sde core 3. Vertcal doubly supported core 4. Vertcal hangng core 5. Vertcal bottom core Let us analyze the forces on a horzontal smply supported core. Consder a cylndrcal core of dameter d, body length l, prnt length a and prnt dameter D. Let ρ metal and ρ core be the denstes of the molten metal and core materal, respectvely. Also, let σ comp be the compressve strength of the mold materal. Self weght of core body W B = π d 2 l ρ core / 4 Self weght of core prnts W P = π D 2 a ρ core / 4 Total weght of core W = W B + W P Applyng the frst desgn rule to balance the core durng placement n mold, we have W B WP (1) The buoyancy force B on the core B = π d 2 l ρ metal / 4 The net force on the core (upward) = B W = π d 2 l (ρ metal - ρ core ) / 4 The compressve stress on each core prnt σ prnt = 0.5 (B - W) / (a D) Applyng the second rule to prevent core falure by crushng due to buoyancy forces, σ prnt σ comp (2) 31

10 The above two equatons can be employed to desgn the dmensons of a horzontal smply supported core prnt. Smlar treatment can be developed for other cores. For vertcal cores, there are two addtonal consderatons. One s that the buoyancy forces transmtted by the core prnt may shear the top part of the mold. Ths s prevented by ensurng suffcent thckness of the mold wall above the core prnt. The second consderaton s that the core prnt must be tapered to facltate ts placement n mold. The draft angle ranges from 2-4 degrees. As mentoned earler, cored holes and undercuts lead to ncreased toolng cost and manufacturng cycle tme, and must be mnmzed to the extent possble. They also lead to qualty problems, whch may be prevented by satsfyng the followng desgn crtera. Core dameter: Very small sand cores, especally n thck sectons of a castng, are lkely to fuse wth the castng and are dffcult to remove and clean later. Ths s caused by hgh heat accumulaton n the core surrounded by molten metal. In decastng, the molten metal shrnks on to the metal core, grppng t tghtly, and t becomes dffcult to wthdraw the core later (ths can be reduced so some extent by applyng draft to the core body). A secondary, but mportant consderaton s that t may be more economcal to machne small holes than to produce them by cores. The mnmum recommended core sze depends on the part metal, castng process, thckness of the secton n whch the hole s located, and the length of the core (see the next crteron). In grey ron parts made by sand castng process, holes below 8 mm are usually dffcult to produce by cores. In alumnum alloy decast parts, the lmtng dameter s about 3 mm. Core aspect rato: Narrow cores those wth large length to dameter rato are lkely to dstort durng mold fllng, especally under pressure. Narrow sand cores may even break. The lmtng aspect rato depends on the type of core (horzontal or vertcal; smply supported or overhangng), core materal, cast metal and fllng pressure. For sand cast grey ron parts, the lmtng aspect rato ranges from 2 for overhangng cores to 4 for smply supported horzontal cores. Inter-core dstance: Several consderatons govern the lmtng dstance between two cores. If two holes are too close, leadng to a thn secton n between, the followng problems arse. Frst, the metal may not fll the secton completely. Secondly, The nbetween secton may become a hot spot because cores poorly transmt heat, leadng to shrnkage porosty n the secton. Thrd, even a slght shft n the poston of cores (durng mold assembly or durng mold fllng, especally under pressure) that leads to further reducton of wall thckness n between, further aggravates the above two problems. The lmtng dstance s thus a functon of the core dameter(s), core materal, part metal and castng process. In general, the thckness of the secton between the cores must be greater that the core dameter. In other words, the center dstance between the cored holes must be greater than twce the hole dameter. 32

11 3.6 Mold Cavty Layout In general, t s more economcal to produce several castngs n a sngle producton mold, because of materal, energy and labor savngs: 1. Less amount of mold materal utlzed per castng. 2. Common feeders and gatng system can be used, mprovng the yeld. 3. Reduced set up tme durng manufacture (for mold preparaton, pourng, shakeout and fettlng) per castng. The savngs must offset the hgher cost of toolng manufacture. Multple cavty molds are therefore preferred only when the castngs are very small compared to the smallest sze of producton molds and the producton quanttes are large. The mold cavty s usually taken up after decdng the castng orentaton and partng. The mnmum dstance between cavtes and from any cavty to the nearest edge of the mold must be set. The mnmum dstance must be suffcent to: (a) prevent damage to the mold, and (b) to allow adequate heat transfer so that local hot spots are not formed n the porton of a castng close to another cavty. The mnmum dstance ranges from 25 mm for small castngs to 50 mm for medum sze castngs. The number of cavtes n a mold has to be optmzed n terms of metal to sand rato: ñmetal (N c V c + V f + V g ) / ñsand (V mold (N c V c + V f + V g )) where, ñmetal and ñsand are the densty of castng and mold materals, N c s the number of castng cavtes, V c s the volume of a sngle cavty, V f and V g are the total volume of feeders and gatng, respectvely, and V mold s the overall volume of mold (based on ts dmensons). A low value of the rato ndcates poor utlzaton of mold materal. A hgh rato must also be avoded, snce ths may lead to poor heat transfer, n turn leadng to defects related to castng soldfcaton. The mnmum recommended value of metal to sand rato for ferrous castngs s 1:1. In practce, t ranges from 1:2 to 1:6. The optmal number of cavtes can be determned followng ths procedure: 1. Choose a mold (n terms of ts dmensons: length, wdth and heght). 2. Set the mnmum dstance (cavty-to-cavty, cavty-to-mold edge). 3. Determne the maxmum number of cavtes that can be accommodated. 4. Determne the value of metal to sand rato. The procedure s repeated for dfferent szes of mold, and the mold whch gves the hghest value of metal-to-sand rato (but more than the mnmum lmt) can be selected. In practce, the mold cavty layout wll also depend on the poston of feeders and gatng, especally f they are common to more than one castng. It s also possble to desgn the layout wth dfferent types of castngs n the same mold. For example, a large castng (but not large enough for a good metal to sand rato) can be combned wth one or more small castngs n the same mold. 33