Feed Casting as It Colls and Shrinks

MANUFACTURING Engineering II CHAPTER 1. 2 SAND CASTING AAi. T Lecture By Dr Shanthakumar G C

The 10 Rules for good castings • Rule 1. Start with a good-quality melt Immediately prior to casting, the melt shall be prepared, checked, and treated, if necessary, to bring it into conformance with an acceptable minimum standard. Prepare and use so far as possible only near defect- free melt. • Rule 2. Avoid turbulent entrainment of the surface film on the liquid This is the requirement that the liquid metal front (the meniscus) should not go too fast. Maximum meniscus velocity is approximately 0. 5 m s– 1 for most liquid metals. This requirement also implies that the liquid metal must not be allowed to fall more than the critical height corresponding to the height of a sessile drop of the liquid metal. The maximum velocity may be raised to 1. 0 m s– 1 or even higher, and the critical fall height might be correspondingly raised to approximately 50 mm, in sufficiently constrained running systems or thin section castings. 6/11/2021 PRODUCTION ENGINEERING II 2

• Rule 3. Avoid laminar entrainment of the surface film on the liquid This is the requirement that no part of the liquid metal front should come to a stop prior to the complete filling of the mold cavity. The advancing liquid metal meniscus must be kept 'alive' (i. e. moving) and therefore free from thickened surface film that may be incorporated into the casting. This is achieved by the liquid front being designed to expand continuously. In practice this means progress only uphill in a continuous uninterrupted upward advance; i. e. in the case of gravity poured casting processes, from the base of the sprue onwards. This implies: • Only bottom gating is permissible. • No falling or sliding downhill of liquid metal is allowed. • No horizontal flow of significant extent. • No stopping of the advance of the front due to arrest of pouring or waterfall effects, etc. 6/11/2021 PRODUCTION ENGINEERING II 3

Rule 4. Avoid bubble entrainment No bubbles of air entrained by the filling system should pass through the liquid metal in the mold cavity. This may be achieved by: • Properly designed off-set step pouring basin; fast back-fill of properly designed sprue; preferred use of stopper; avoidance of the use of wells and all other volume-increasing features of filling systems (such as expanding channels sometimes known as 'diffusers'); small volume runner and/or use of ceramic filter close to sprue/runner junction; possible use of bubble traps. A naturally pressurized filling system fulfills most of these criteria. • No interruptions to pouring. Rule 5. Avoid core blows • No bubbles from the outgassing of cores or molds should pass through the liquid metal in the mold cavity. Cores to be demonstrated to be of sufficiently low gas content and/or adequately vented to prevent bubbles from core blows. 6/11/2021 PRODUCTION ENGINEERING II 4 • No use of impermeable clay-based core or mold repair paste.

Rule 6. Avoid shrinkage No feeding uphill in larger section thickness castings. Feeding against gravity is unreliable because of (i) adverse pressure gradient and (ii) complications introduced by convection. Demonstrate good feeding design by following all seven Feeding Rules, by an approved computer solidification model, and by test castings. Once good feeding is attained, fix the temperature regime by controlling (i) the level of flash at mold and core joints; (ii) mold coat thickness (if any), and (iii) temperatures of metal and mold. Rule 7. Avoid convection Assess the freezing time in relation to the time for convection to cause damage. Thin and thick section castings automatically avoid convection problems. For intermediate sections either (i) reduce the problem by avoiding convective loops in the geometry of the casting and rigging, (ii) avoid feeding uphill, or (iii) eliminate convection by roll-over after filling. 6/11/2021 PRODUCTION ENGINEERING II 5

Rule 8. Reduce segregation Predict segregation to be within limits of the specification, or agree outof-specification compositional regions with customer. Avoid channel segregation formation if possible. Rule 9. Reduce residual stress No quenching into water (cold or hot) following solution treatment of light alloys. (Polymer quenchant or forced air quench may be acceptable if casting stress can be shown to be acceptable. ) Rule 10. Provide location points All castings to be provided with location points for pick-up for dimensional checking and machining. Proposals to be agreed with quality auditor, machinist, etc. 6/11/2021 PRODUCTION ENGINEERING II 6

CASTING Working Design The general foundry work cycle design for a sand casting techniques: The first step is casting design and Definition of the (3 D) shape of the cast part. Obviously, some changes have to be made with respect to the final part. Casting geometry must comply with foundry constraints: All shapes may not be cast with each process, Holes with high length to diameter ratio, Flat perpendicular smooth surfaces etc. may give problems. In general, a subsequent machining is required for at least some part features. A correct casting design requires: The whole foundry cycle and, Changes in some of the part details to satisfy casting-related constraints. 6/11/2021 MANUFACTURING ENGINEERING II 7

CASTING Working Design 6/11/2021 MANUFACTURING ENGINEERING II 8

CASTING Working Design 6/11/2021 MANUFACTURING ENGINEERING II 9

CASTING Working Design First step is casting shape design, hence (in this order) pattern, risers & sprue design. At each step we need information from the previous ones. Shape changes can be made, but only if necessary & after part designer approval. Process design is circular: at each step, a feasibility control is made. If this check fails, modification of the previous results is required to continue. 6/11/2021 MANUFACTURING ENGINEERING II 10

Corners Angles and section thickness • Sharp corners, angles and filets should be avoided because they act as stress risers and may cause cracking and tearing of the metal(as well as die) during solidification. • Fillet radii should be selected to reduce stress concentrations and to ensure proper liquid-metal flow during the pouring process. • Fillet radii usually ranges from 3 mm to 25 mm. • If the fillet radii are too large , the volume of the material in those regions is also large and consequently, the rate of cooling is too low. 6/11/2021 PRODUCTION ENGINEERING II 11

FROM PRODUCT TO CASTING Casting design must deal with the following issues: Patterns must be extracted (drawn) from the mold (easily & without damaging it); There are some geometrical constraints, typical of the casting process ("deep holes", thickness distribution…); Casting tolerances are broad, some features require finishing by machining: Stock allowances must be left on some surfaces; Small fillet radii cannot be cast because of casting stresses; Cooling (solidification) speed should be kept under control, smooth & directional thickness change is extremely important; … etc. 6/11/2021 MANUFACTURING ENGINEERING II 12

Pattern • Draft : A pattern can be drawn satisfactorily from the sand good casting made only if its sides are tapered away from the parting line. In addition, projections such as bosses on the sides of the casting away from the parting line v. The allowance which facilitate easy withdrawal of pattern from mould walls v. Slight taper or draft is provided to vertical side of mould walls all around v. Drat allowance is additive: 1 – 1. 5 % of length 6/11/2021 PRODUCTION ENGINEERING II 13

FROM PRODUCT TO CASTING Stock allowance Casting accuracy is rather low. When higher precision is needed machining is required Almost all functional surfaces fall within precision A subsequent machining (that is, chip removal) is mandatory. The right amount of stock allowance to be left on the casting Need for extra material to compensate inaccuracy of the casting, Leaving at least some rough material to be removed; Need to reduce the amount of part left 6/11/2021 MANUFACTURING ENGINEERING II 14

FROM PRODUCT TO CASTING Stock allowance 6/11/2021 MANUFACTURING ENGINEERING II 15

Tolerance of plus or minus 1. 5 mm for dimensions up to about 300 mm are commercial practice for sand castings. Tolerances as close as plus or minus 0. 8 mm are held by some foundries but at extra cost that should not be incurred unless necessary. 6/11/2021 PRODUCTION ENGINEERING II 16

FROM PRODUCT TO CASTING Stock allowance thickness depends on both: Overall part size and Individual part dimensions. The effect of overall part size is due to: Mold deformation, under Metallostatic forces & Part weight. 6/11/2021 MANUFACTURING ENGINEERING II 17

1. When designing patterns for casting, patternmakers use special rulers that automatically incorporate solid shrinkage allowances into their designs. Therefore, a 12 -in. patternmaker's ruler is longer than a foot. How long should a patternmaker's ruler be for the making of patterns for (1) aluminum castings (2) malleable cast iron and (3) highmanganese steel? 6/11/2021 PRODUCTION ENGINEERING II 18

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Note that high-manganese steel and malleable cast iron were selected for this problem because they have extremely high and low shrinkage allowances, respectively. The aluminum ruler falls within the expected range, as do most other metals. 6/11/2021 PRODUCTION ENGINEERING II 20

2. The blank for the spool shown in the accompanying figure is to be sand cast out of A-319, an aluminum casting alloy. Make a sketch of the wooden pattern for this part. Include all necessary allowances for shrinkage and machining. 6/11/2021 PRODUCTION ENGINEERING II 21

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• The sketch for a typical green-sand casting pattern for the spool is shown below. A cross sectional view is also provided to clearly indicate shrinkage and machining allowances, as well as the draft angles. The important elements of this pattern are as follows (dimensions in inches): (a) Two-piece pattern. (b) Locating pins will be needed in the pattern plate to ensure that these features align properly. (c) Shrinkage allowance = 5/32 in. /ft. (d) Machining allowance = 1/16 in. (e) Draft = 30 6/11/2021 PRODUCTION ENGINEERING II 23

3. For the sketch shown , Starting from the finished part design of the mechanical part, evaluate the complete pattern design and the feeding system considering. 6/11/2021 PRODUCTION ENGINEERING II 24

• Stock allowance 3 mm • Linear shrinkage 1% • Draft angle 20 A. Compute the stock allowance, shrinkage allowance and draft angle for the product part drawing. B. Add the core print according to the dimension. C. Design the feeding system (Riser) of the steel rough sowed with the help of sketch. considering the geometry of the riser is square. D. The Number of riser and position E. Dimension for a riser F. Dimension for a riser neck 6/11/2021 PRODUCTION ENGINEERING II 25

No Nominal Dimensions Shrinkage Allowance 1% Stock shrinkage Pattern shape design 1 External φ140 140 x 0. 01 1. 4 +3 +3 147. 4 147 2 External φ100 100 x 0. 01 1. 0 +3 +3 107 3 Internal φ80 80 x 0. 01 0. 8 -3 -3 74. 8 75 4 Internal φ40 40 x 0. 01 0. 4 -3 -3 34. 4 34 5 Linear 20 20 x 0. 01 0. 3 +3 +3 26. 2 26 6 Linear 30 30 x 0. 01 0. 3 +3 -3 30 7 Linear 120 x 0. 01 1. 2 +3 +3 127. 2 127 6/11/2021 PRODUCTION ENGINEERING II 26

• For the Hole/Core No Nominal Dimension Shrinkage Allowance Stock Shrinkage 1% Pattern shape design 1 Internal φ80 80 x 0. 01 0. 8 -3 -3 74. 8 75 2 Internal φ40 40 x 0. 01 0. 4 -3 -3 34. 4 34 3 Linear 30 30 x 0. 01 0. 03 +3 -3 30 4 Linear 120 x 0. 01 1. 2 +3 +3 127. 2 127 6/11/2021 PRODUCTION ENGINEERING II 27

• • For the feeding system Calculate the volume and surface area Volume 1= [π(D 2 -d 2)h]/4= [π(1402 -802)20]/4=207345 m 3 Volume 2= [π(D 2 -d 2)h]/4= [π(1002 -802)10]/4=28274 m 3 Volume 3= [π(D 2 -d 2)h]/4= [π(1002 -402)90]/4=593761 m 3 Surface area 1= [{π(D 2 -d 2}h]/4+πDh+ πdh = = {π(1402 -802} 4+ [{π(1402 -1002}/4+πx 140 x 20+ πx 80 x 20]=31728 mm 2 Surface Area 2= πDh+ πdh= πx 100 x 10+ πx 80 x 20=5654 mm 2 Surface Area 3=[{π(D 2 -d 2}h]/4+πDh+ πdh = [{π(1002 -402}/4+ {π(802 -402}/4+πx 100 x 90+ πx 40 x 90= 49949 mm 2 Total Volume= V= 207345+28274+ 593761=829380 mm 3 Total Surface Area =TSA=31728+5654+49949=87331 mm 2 6/11/2021 PRODUCTION ENGINEERING II 28

• • • Modulus of Casting= (Volume of Casting)/Surface Area of Casting =829380 mm 3/87331 mm 2=9. 49 mm Now Taking Modulus of Riser=1. 2 Modulus of Casting Modulus of Riser= 1. 2 x 9. 49=11. 38 mm Now, Modulus of riser (shape of riser is square)=Vol. of Riser/SA Riser =H 3/5 H 2=H/5 = Height of Riser= Modulus of Riserx 5 Therefore, Height of Riser=5 x 11. 38=56. 94 mm=Riser Diameter Riser Neck Dimension=0. 4 x. H=0. 4 x 56. 94 mm= 22. 7 mm Riser Neck Length=0. 16 x. H =0. 16 x 56. 94 mm=9. 11 mm 6/11/2021 PRODUCTION ENGINEERING II 29

FROM PRODUCT TO CASTING Minimum thickness Some thin parts are difficult to obtain by sand casting: FROM PRODUCT TO CASTING –Holes Some holes are difficult to make in a casting: Slender holes (large H/D ratios): they would require a core that is too weak. Small holes (small D): core making techniques are unable to produce cores smaller than about 5 mm in diameter. Better made by drilling. 6/11/2021 MANUFACTURING ENGINEERING II 30

FROM PRODUCT TO CASTING –Holes As a "rule of thumb", we can use cores when: The hole diameter is at least once the wall thickness When the core diameter is larger than the wall thickness, the core has a positive effect on the casting, since it cools the wall till the end of solidification. On the contrary when the core diameter is considerably small, the core starts to cool the wall. However during the wall solidification the core is overheated. As a consequence the core reduces wall solidification, which at the end presents porosity & shrinkage. The same happens when the core height is considerably higher than the core diameter. 6/11/2021 MANUFACTURING ENGINEERING II 31

FROM PRODUCT TO CASTING Cope and Drag We always start from parting plane (parting line) selection; All other decisions will depend on this one; Try & avoid undercuts (easy drawing); Exploit symmetry, if present (for symmetric metallurgical structure); Place gate & risers in a suitable position (aesthetic of some as-cast surfaces); Use just one pattern half (cheaper molding); Reduce cavity depth (less extra material for draft). 6/11/2021 MANUFACTURING ENGINEERING II 32

FROM PRODUCT TO CASTING Cope and Drag 6/11/2021 MANUFACTURING ENGINEERING II 33

FROM PRODUCT TO CASTING –Patterns Pattern extraction is made possible by: 1) Correct positioning b/n cope & drag 2) Using more than two flasks: 3) Using sand inserts ("external cores"): 4) Dividing the pattern into more than two parts: GOAL: Minimize the number & complexity of mould elements to create the casting cavity closest to the desired geometry, & ensure good internal quality. 6/11/2021 MANUFACTURING ENGINEERING II 34

FROM PRODUCT TO CASTING –Draft Pattern drawing (stripping) is a critical operation. Pattern surfaces parallel to drawing direction are likely to damage the mold, requiring a manual mold repair. For this reason, these surfaces are tapered by a suitable draft angle for easy drawing, so that the drawing movement itself disconnects the pattern from the mold walls. Drafts cause extra material to be lie on some surfaces of the pattern, thus always increasing stock. 6/11/2021 MANUFACTURING ENGINEERING II 35

FROM PRODUCT TO CASTING – Draft angles are generally small. They depend on both surface properties of the pattern (i. e. pattern material) and Height of the part to be tapered. 6/11/2021 MANUFACTURING ENGINEERING II 36

FROM PRODUCT TO CASTING Fillets Sharp corners (both inner & outer) should be avoided on castings. Sharp corners should be replaced by fillets, because of: Unacceptable differences in solidification & cooling velocities Notch effect on castings (located in areas where casting stresses are remarkable) Unacceptable mold wear because of metal flow erosion during filling 6/11/2021 MANUFACTURING ENGINEERING II 37

FROM PRODUCT TO CASTING Fillets Fillet radii: - Outer fillet radii = Stock allowance - inner corners: Empirical equation or interpolation rule: § § § Divide a & b in the same number of segments Connect 1 with 1, 2 with 2 & so on Draw the curve tangent to the connection lines 6/11/2021 MANUFACTURING ENGINEERING II 38

FROM PRODUCT TO CASTING Stock allowance Sometimes, some extra stock is left for special purposes: a. Simplify core shape (economical reason); b. Allow directional solidification (padding action). 6/11/2021 MANUFACTURING ENGINEERING II 39

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FROM PRODUCT TO CASTING Thickness Ø Changes in casting thickness must be as smooth as possible, to control casting stresses (due to solid shrinkage). Ø Moreover, a regular change in thickness is beneficial, since a directional solidification contributes to the feeding action. 6/11/2021 MANUFACTURING ENGINEERING II 41

FROM PRODUCT TO CASTING Thickness Heuvers' circles may be used to evaluate thickness rate of variation. An Heuvers' circle is the biggest circle that can be inscribed in a casting cross-section. The difference b/n the diameter of any pair of adjoining circles (tangent to each other) should not exceed 15 -20%. 6/11/2021 MANUFACTURING ENGINEERING II 42

FROM PRODUCT TO CASTING Thickness 6/11/2021 MANUFACTURING ENGINEERING II 43

CASTING WORK CYCLE DESIGN Once casting shape is designed, it is possible to start pattern definition. 6/11/2021 MANUFACTURING ENGINEERING II 44

SHRINKAGE Undesired (yet unavoidable) phenomenon in casting processes. All foundry materials, during cooling, shrink. Shrinkage affects part accuracy & even its soundness. There is a liquid, solidification & solid shrinkage: 6/11/2021 MANUFACTURING ENGINEERING II 45

SHRINKAGE From cast to room temperature 3 (Three) different kinds of shrinkage take place: 1. Liquid shrinkage (VTC–VTL), not critical. 2. Solidification shrinkage (VTL–VTS), isothermal for pure metals & eutectic alloys. 3. Solid shrinkage (VTS–VTA), i. e. thermal expansion/contraction. 6/11/2021 MANUFACTURING ENGINEERING II 46

SHRINKAGE 6/11/2021 MANUFACTURING ENGINEERING II 47

SOLID SHRINKAGE Solid shrinkage must be controlled b/se of the following problems: Dimensions: at room temperature the cast part is smaller than mold cavity, this problem may be handled using a correct design; Casting stresses: due to inhomogeneous cooling velocity, they may lead to hot cracks. 6/11/2021 MANUFACTURING ENGINEERING II 48

SOLID SHRINKAGE –dimensions Linear solid shrinkage depends on cast material properties: 6/11/2021 MANUFACTURING ENGINEERING II 49

SOLID SHRINKAGE dimensions Example: the shrinkage of Carbon steel is about 1, 5%. Thus, cooling from melting Temp. a 1 -meter-long bar we get 980 mm. To get the correct final length we can oversize the mold cavity by the same amount. Therefore, to cast our one-meter-long bar we should use a cavity 1015 mm long. 6/11/2021 MANUFACTURING ENGINEERING II 50

FROM CASTING TO PATTERN Solid shrinkage It is worth noting that shrinkage is a linear contraction: Hollow parts shrink in the same way as solid parts (a hole reduces its diameter). As a consequence, during solid shrinkage, cores are "squeezed" by the casting. Core sand cannot be over-compacted b/se cores are not supposed to hinder contraction too much (it would cause extra stresses during cooling, moreover light compact design increases core permeability). 6/11/2021 MANUFACTURING ENGINEERING II 51

LIQUID & SOLIDIFICATION SHRINKAGE Liquid & solidification shrinkage (VTC–VTS) is controlled through the feeding system, composed of one or more risers, placed so that they supply liquid metal to compensate metal lacks. To work, they must be the last parts where solidification ends. 6/11/2021 MANUFACTURING ENGINEERING II 52

LIQUID & SOLIDIFICATION SHRINKAGE Volume contraction while solidification depends on cast material & on external pressure. NOTE: Gray iron has negative volume contraction (expansion). This is due to metallurgical transformation during phase change (formation of graphite, with lower density). When it happens, this phenomenon is generally helpful. 6/11/2021 MANUFACTURING ENGINEERING II 53

LIQUID & SOLIDIFICATION SHRINKAGE In an open mold, Solidification always starts near to the surface. Since volume decreases, liquid surface gets lower. As solidification goes on, a shrinkage cone is generated. We need an extra metal supply (riser), where solidification occurs later. 6/11/2021 MANUFACTURING ENGINEERING II 54

LIQUID & SOLIDIFICATION SHRINKAGE In a closed mold, shrinkage produces voids (i. e. shrinkage cavities), always located very far from the surface. Well designed risers (last in solidifying) can control this effect also in this case. 6/11/2021 MANUFACTURING ENGINEERING II 55

LIQUID & SOLIDIFICATION SHRINKAGE Example of a void: 6/11/2021 MANUFACTURING ENGINEERING II 56

SOLUBILITY OF GASES Some gases are far less soluble in solid metals than in their molten phase. Example: Hydrogen in Aluminum alloys. The excess of gas is released & it forms bubbles (gas porosity, pinhole porosity). Do not confuse with dendritic micro-voids. 6/11/2021 MANUFACTURING ENGINEERING II 57

SOLUBILITY OF GASES Hydrogen is especially harmful in castings: It comes mainly from dissociation of atmospheric humidity (at high temperatures water vapor splits into H & O). Gas content in molten metal (Example. Al) can be controlled by: Chemical degassing: specific compounds are added to molten metal to form insoluble compounds; these are removed with the slag (compound removal is often assisted by Argon fluxing). Vacuum melting, vacuum casting: very low pressure procedures to prevent contamination from air & to remove gases already dissolved in the metal. In die-casting, low pressure helps metal injection. Pressure casting: such as in injection molding, die casting, squeeze casting etc. ; pressure on the molten metal during die filling helps in reducing porosity. 6/11/2021 MANUFACTURING ENGINEERING II 58

Casting Pattern shape Design Starting from the finished part design of the mechanical part, evaluate the complete pattern design & the core design. Stock allowance 3 mm, Linear shrinkage of 1%, Draft angle 20 Compute the stock allowance, shrinkage allowance & the draft angle for the product drawing. Add the core print to the pattern according to the dimension. 6/11/2021 MANUFACTURING ENGINEERING II 59

Casting Pattern Design For the pattern negligible the draft allowance No Nominal Dimension Shrinkage allowance 1% Stock shrinkage Pattern shape design 1 External Ø 140*0. 01 1. 4 + 3 147. 4 147 2 External Ø 100*0. 01 1. 0 + 3 107 3 Internal Ø 80 80*0. 01 0. 8 -3 -3 74. 8 75 4 Internal Ø 40 40*0. 01 0. 4 -3 -3 34. 4 34 5 Linear 20 20*0. 01 0. 2 + 3 26. 2 26 6 Linear 30 30*0. 01 0. 3 + 3 -3 30 7 Linear 120*0. 01 1. 2 +3 +3 127. 2 127 The draft allowance for the hole / core No Nominal Dimension Shrinkage allowance 1% Stock shrinkage Core shape design 1 Internal Ø 80 80*0. 01 0. 8 -3 -3 74. 8 75 2 Internal Ø 40 40*0. 01 0. 4 -3 -3 34. 4 34 3 Linear 30 30*0. 01 0. 3 + 3 -3 30 4 Linear 120*0. 01 1. 2 +3 +3 127. 2 127 6/11/2021 MANUFACTURING ENGINEERING II 60

CASTING WORK CYCLE DESIGN Once the pattern is designed, it is possible to start designing the feeding systems. 6/11/2021 MANUFACTURING ENGINEERING II 61

FEEDING SYSTEM Feeding system design includes (in this order): Choice of riser(s) location; Choice of riser(s) number; Choice of riser type; Dimensioning of each riser neck (connection b/n the riser to the casting). 6/11/2021 MANUFACTURING ENGINEERING II 62

FEEDING SYSTEM Basing on experimental investigation, Chvorinov showed that solidification time Ts depends on casting geometry Namely, on solidification modulus M according to the following law: Where α & β depend on both casting & mold material. As an example, in sand casting of steel α = 0. 9 & β= 2, when Modulus M is expressed in mm & Ts in seconds. 6/11/2021 MANUFACTURING ENGINEERING II 63

FEEDING SYSTEM Let us evaluate the modules of elementary shapes: Thus, a sphere with radius R & a cube of side 2 R have the same modulus. NOTE: anyway, they do not have THE SAME VOLUME! 6/11/2021 MANUFACTURING ENGINEERING II 64

FEEDING SYSTEM For a plate: § Plate with thickness T: § For an infinitely broad plate (heat exchange on the sides is negligible) 6/11/2021 MANUFACTURING ENGINEERING II 65

FEEDING SYSTEM For other geometries: Long bars: Cylinder with radius R & height H: Only when BOTH base surfaces are effective (very often at least one is adiabatic …). 6/11/2021 MANUFACTURING ENGINEERING II 66

FEEDING SYSTEM As an example, let us consider a casting having this shape: Let us find a correct riser(s) position. 6/11/2021 MANUFACTURING ENGINEERING II 67

FEEDING SYSTEM Let us decompose the casting into three elementary parts A, B and C, then evaluate the modulus of each single part (take care of adiabatic surfaces). 6/11/2021 MANUFACTURING ENGINEERING II 68

FEEDING SYSTEM Assuming the following part dimensions, we have the following modules: L = 200 mm RA = RB = 50 mm HA = 400 mm HB = 150 mm Since: MA = 23. 5 mm MB = 35. 7 mm MC = 21. 4 mm MB > MA > MC we can conclude that solidification takes place first in C, then in A and last in B. 6/11/2021 MANUFACTURING ENGINEERING II 69

FEEDING SYSTEM Take care not to overestimate the accuracy of Chvorinov's law. Remember the assumption underlying this empirical rule: Constant heat flux on all surfaces (e. g. , In some cases surfaces exchanging towards cores are fictitiously reduced to take into account a lower heat flux, due to core overheating); Perfectly adiabatic (ideal) surfaces of contacting parts; The effect of chills (or othermally affecting devices) is neglected; Dendrite solidification (affecting metal flow) is neglected; … MA ≥ 1. 1*MB In practice, we can reasonably assume that part A will solidify after part B (for neighboring parts, this means that A will feed B during solidification) if its modulus is at least 10% larger: 6/11/2021 MANUFACTURING ENGINEERING II 70

FEEDING SYSTEM Thus, by dividing the casting into parts & evaluating part modulus, we can follow casting solidification. During solidification, "slower" parts can feed the neighboring "fast" parts, supplying molten metal & compensating solidification shrinkage, that is, they act as risers. The slowest casting part must be fed by the riser. To be sure in general, it is required that: M ≥ 1. 2*M Riser 6/11/2021 MANUFACTURING ENGINEERING II Part 71

FEEDING SYSTEM 6/11/2021 MANUFACTURING ENGINEERING II 72

FEEDING SYSTEM 6/11/2021 MANUFACTURING ENGINEERING II 73

FEEDING SYSTEM 6/11/2021 MANUFACTURING ENGINEERING II 74

FEEDING SYSTEM 6/11/2021 MANUFACTURING ENGINEERING II 75

FEEDING SYSTEM The "fineness "of casting division (number of sub-parts) for modulus evaluation should be enough to represent directional solidification, but not excessive because too many sub-parts would not increase the amount of information (while increasing the amount of computation). As a general rule, it is better to chose parts with simple (elementary) geometry. 6/11/2021 MANUFACTURING ENGINEERING II 76

FEEDING SYSTEM A directional solidification may be achieved by modifying the heat flux (rather than the geometry). It is possible: To increase heat flux where a fast cooling is desired (chills) To decrease heat flux where a slow cooling is desired (insulating pads and sleeves). This solution is more often used on risers than on castings. Chills may be of several kinds: Blocks & plates embedded in the mold sand (or in the core); Heat conductive blocks in contact with molten metal; 6/11/2021 MANUFACTURING ENGINEERING II 77

FEEDING SYSTEM Once riser(s) location is decided, we must check for range of effect. A riser has a well defined range of effect, beyond which the feeding effect vanishes. An explanation to this phenomenon is given by dendrite growth: Thus, when risers are too far away, Some part of the casting maybe porous. 6/11/2021 Some molten metal is "trapped" b/n dendrites. MANUFACTURING ENGINEERING II 78

FEEDING SYSTEM Directional solidification resulting from a gradual variation of thickness helps avoiding porosity. Thickness changes & resulting thermal gradient should be enough to prevent porosity, although not too high that solid shrinkage could cause problems. 6/11/2021 MANUFACTURING ENGINEERING II 79

Location of Risers • The thermal gradient, with in the casting, during the last stage of cooling is the most important factor. • The minimum allowable gradient depends on the shape and size of the cross section. • Normally, for a casting with low A/V ratio(e. g, . Cube, sphere), one central is able to feed the entire casting. • On the other hand, for a casting with high A/V ratio(e. g, . For a bar and plate)usually more than one riser is necessary. • In such a case, a proper location of riser has to be decide • For a plate of up to 100 mm thickness, one central is satisfactory if the maximum feeding distance is less than 4. 5 times the plate thickness. • The feeding distance should be measured from the edge of the riser in the figure. 6/11/2021 PRODUCTION ENGINEERING II 80

• It should be noted that, of the total distance 4. 5 t, the riser gradient prevails up to a distance 2 t, where as the wall gradient prevails in the remaining distance 2. 5 t. • Thus, the maximum distance between edges of two consecutive riser is 4 t and not 9 t. • A bar of square cross section with sides measuring 50 -200 mm can be fed satisfactorily from a single riser, up to a maximum distance of 30√s, where s is side of the square expressed in mm. • The maximum distance between the edges of two consecutive riser is found to be 1. 2 s • The presence of chill in the mould increases the feeding distance of the riser. This is achieved by providing a sharp thermal gradient with consequent decrease in feeding resistance. • It is obvious chill should be placed at the ends if a single riser is used. • For more than one riser, the chill should be placed midway between two risers. • Figure shows schematically the proper placements and chills. • Also the maximum permissible distances for various cases are also indicated in the figure. 6/11/2021 PRODUCTION ENGINEERING II 81

FEEDING SYSTEM There are several empirical rules to estimate the range of effect of a riser: Metals Handbook: plates having thickness T& width > 3 T 6/11/2021 MANUFACTURING ENGINEERING II 82

FEEDING SYSTEM Metals Handbook: bars (square cross section, side T) 6/11/2021 MANUFACTURING ENGINEERING II 83

FEEDING SYSTEM Giusti Santochi: with k= 3. 5 – 5 for steel k= 5 for iron k= 5 – 8 for bronze k= 5 – 7 for aluminum alloys 6/11/2021 MANUFACTURING ENGINEERING II 84

FEEDING SYSTEM To be able to use these empirical rules, we must decompose the casting into bars and plates. For other parts a high level of approximation cannot be avoided. 6/11/2021 MANUFACTURING ENGINEERING II 85

FEEDING SYSTEM 6/11/2021 MANUFACTURING ENGINEERING II 86

FEEDING SYSTEM Then, it is necessary to choose the riser geometry. They may be either cylindrical (more efficient) or rectangular (simpler), or sometimes with different shapes (semispherical, oval cross section etc. ) depending on the shape of the available room inside the mold. Like all cavity details, risers are made using a pattern. In general, standard patterns are employed. 6/11/2021 MANUFACTURING ENGINEERING II 87

FEEDING SYETEM Finally, each riser must be dimensioned. Depending on riser geometry, different dimensions must be defined: spherical and semispherical: radius; cylindrical (very common): diameter and height; oval, rectangular cross sections: two cross-sectional parameters and height. Definition of riser modulus gives us just one constraint. 6/11/2021 MANUFACTURING ENGINEERING II 88

FEEDING SYSTEM The method based on the shrinkage cone size is based on an experimental consideration. At the end of the solidification (if a sound part is produced), the material loss (volume Vl) in the riser (volume Vr) is found to be about: 14% for cylindrical and oval risers; 20% for spherical and semispherical risers. Vl = a. Vr It's also experimentally verify that: Vl = b. (Vr + Vp) where b is the volume contraction of casting metal and Vpis the volume of the piece. 6/11/2021 MANUFACTURING ENGINEERING II 89

FEEDING SYSTEM Solving the two equations of the previous screen, we get the riser volume as a function of the casting part volume(just the modulus of the casting part fed by the riser under design should be considered MX): This is the final equation expressing the volume constraint. Together with the modulus constraint: It is possible to design all riser dimensions. 6/11/2021 MANUFACTURING ENGINEERING II 90

CYLINDERICAL FEEDING SYSTEM 6/11/2021 MANUFACTURING ENGINEERING II 91

FEEDING SYSTEM Also riser insertion, that is the channel connecting riser and casting, needs to be dimensioned. For directional solidification we need: Depending on riser type, some results are available: 6/11/2021 MANUFACTURING ENGINEERING II 92

Casting feeding Design Starting from the finished part design of the mechanical part, evaluate the complete the feeding system. Considering the geometry of the riser is square. The number of riser & position, Dimension for a riser neck 6/11/2021 MANUFACTURING ENGINEERING II 93

For the feeding system 6/11/2021 MANUFACTURING ENGINEERING II 94

For the feeding system 6/11/2021 MANUFACTURING ENGINEERING II 95

CASTING WORKCYCLE DESIGN 6/11/2021 MANUFACTURING ENGINEERING II 96

GATING SYSTEM (SPRUE) The gating system connects the mold cavity to the "outside". Composed of: Ø Pouring basin: a cuplike cavity where molten metal is poured in, it may hold filters and skimmers to prevent dross (or slug) from entering the cavity; Ø sprue: vertical channel connecting the basin with the runner; Ø runner: a channel (generally horizontal) from sprue to gate(s), (runner box is a distribution box dividing molten metal into several streams before it enters the mold cavity); Ø gate(s): the portion of the runner through which molten metal enters the mold cavity. 6/11/2021 MANUFACTURING ENGINEERING II 97

GATING SYSTEM: TYPES Metal supply may be by top or bottom pouring, with several types of gate: Ø Horizontal Ø Vertical. 6/11/2021 MANUFACTURING ENGINEERING II 98

HORIZONTAL GATING SYSTEM: LOCATION 6/11/2021 MANUFACTURING ENGINEERING II 99

GATING SYSTEM The gating system needs accurate design, especially to select channel cross sectional area. The main design criterion is pouring time Tr, to fill completely the mold (or die) cavity. When Tr is: Ø short: fast flow, that may cause § mold erosion, due to excessive metal speed; § material waste, because all gating system is bigger. Ø long: some problem may arise § early solidification in thin and far parts; § mold damage by irradiation from the molten metal. 6/11/2021 MANUFACTURING ENGINEERING II 100

GATING SYSTEM 6/11/2021 MANUFACTURING ENGINEERING II 101

GATING SYSTEM –Choice of Tr 6/11/2021 MANUFACTURING ENGINEERING II 102

GATING SYSTEM 6/11/2021 MANUFACTURING ENGINEERING II 103

GATING SYSTEM For a steady-state flow of an ideal fluid, it is possible to compute the exit speed from an opening: where C= [0. 6 – 0. 7]. When dealing with filling, one must consider that: At the end of the filling, metal flow is slowed because of the pressure due to the metal already present in the cavity; Resistances to the flow are not negligible, both localized and distributed (this latter item is less important, since metal flow is laminar). 6/11/2021 MANUFACTURING ENGINEERING II 104

GATING SYSTEM Once the narrowest section SSTR is dimensioned, one must decide where it is located (on either Ssp: sprue, Srn: runner or Sgt: gate). The cross-sectional dimensions of the other sections have to be evaluated as well. In general, two (very different) kind of gating system are used: q With pressurization: SSTR is the gate section; q Without pressurization: SSTR is the sprue section. The choice between these gating depends on casting material, casting weight and molding procedure. For cast steels we use the pressurized one. 6/11/2021 MANUFACTURING ENGINEERING II 105

GATING SYSTEM With pressurization: ü SSTR= Sgt ü the flow at the gate section is more uniform; ü metal enter the mold with higher speed (risk of mold erosion). Without pressurization: ü SSTR= Ssp ü the speed of the molten metal in the gate is lower (less turbulence); ü they are advisable for metals with high tendency to form oxydes(Al, Mg, Zn); ü the flow at the gate sections is less uniform. 6/11/2021 MANUFACTURING ENGINEERING II 106

GATING SYSTEM The remaining cross-sectional areas are designed so that there is a constant ratio between these areas (standard values, from casting practice): For a pressurized system, it may be: While the non pressurized systems are often designed by fixing: 6/11/2021 MANUFACTURING ENGINEERING II 107

GATING SYSTEM The shape of gate cross-section is often rectangular or triangular (easier to draw). According to good design practice the crosssectional dimensions may be chosen as follows: 6/11/2021 MANUFACTURING ENGINEERING II 108

METALLOSTATIC LIFT Once the mold cavity is full of molten metal, this metal (like any other liquid) exchanges an hydrostatic pressure on the cavity walls. Such pressure may deform the molds, affecting dimensional accuracy. This phenomenon is controlled by using stiff flasks and lateral reinforcements for particularly high copes. Besides, mold deformation may be not uniform because of inhomogeneous sand compacting. The effect on part accuracy (both dimensional and geometric) is in this case more severe. 6/11/2021 MANUFACTURING ENGINEERING II 109

METALLOSTATIC LIFT In the physical sciences, Pascal's law or Pascal's principle states that the fluid pressure at all points in a connected body of an incompressible fluid at rest, which are at the same absolute height, is the same. p is the hydrostatic pressure (Pa); ρ is the liquid density (Kg/m 3); g is gravitational acceleration (m/s 2); h is the height of fluid above (m). 6/11/2021 MANUFACTURING ENGINEERING II 110

METALLOSTATIC LIFT The pressure of a fluid in a point p is: where γm is the fluid density and z is the depth of p(distance from p to the free surface). Thus, the force d. F acting on the small surface d. S is: And its vertical component (d. Fv) is: with α being the angle between the normal vector of d. S and the vertical direction. 6/11/2021 MANUFACTURING ENGINEERING II 111

METALLOSTATIC LIFT where γm is the fluid density and zis the depth of p(distance from p to the free surface). 6/11/2021 MANUFACTURING ENGINEERING II 112

METALLOSTATIC LIFT Evaluation Total metallostatic lift Fml on the cope may be evaluated by integrating over the contacting surface between metal and cavity (note: dealing with cores, not all the surface is in contact…): 6/11/2021 MANUFACTURING ENGINEERING II 113

METALLOSTATIC LIFT Evaluation As an example, for a horizontal cylinder having length L and radius R: 6/11/2021 MANUFACTURING ENGINEERING II 114

METALLOSTATIC LIFT Evaluation 6/11/2021 MANUFACTURING ENGINEERING II 115

METALLOSTATIC LIFT Evaluation The metallostatic lift can also be evaluated in another way, to avoid integration. Let us take into account the amount of metal present in the cope and write the equilibrium conditions along the vertical direction. On this volume, two kind of forces are acting: Ø volume force: the metal weight P(vertical) Ø surface forces: let us consider two actions by decomposing the total surface § on the cope cavity Fml (our goal) § on the parting plane Fps 6/11/2021 MANUFACTURING ENGINEERING II 116

METALLOSTATIC LIFT Evaluation By writing the vertical equilibrium on the volume: Where the evaluation of Fps is easy, since all points on the parting plane have same depth h: being Sps the projection of cope surface onto the parting plane. In conclusion: where Vcm is the volume of the cope cavity. 6/11/2021 MANUFACTURING ENGINEERING II 117

METALLOSTATIC LIFT Evaluation Moreover, the volume V': is the cope part just above the cope surface (see figure). The metallostatic lift is equal to the weight of the volume V', when filled with molten metal: In this way, a precise evaluation of metallostatic lift is easy. Do not forget to include other mold cavities(such as runners, blind risers…) into the computation. 6/11/2021 MANUFACTURING ENGINEERING II 118

BOUYANCY LAW (ARCHIMEDE LAW) A solid body immersed in a fluid will have an upward buoyant force acting on it equal to the weight of displaced fluid. This is due to the hydrostatic pressure in the fluid. When a rigid object is submerged in a fluid (completely or partially), there exits an upward force on the object that is equal to the weight of the fluid that is displaced by the object. 6/11/2021 MANUFACTURING ENGINEERING II 119

METALLOSTATIC LIFT Evaluation Since core sand is lighter* than molten metal, cores do "float" in the metal. As a consequence, cores experience a metallostatic lift that, when greater than core weight itself, is supported by core prints. Such lift may be evaluated by using Archimedes' principle: * Except for Mg alloys. where Vai is the volume of the core portion that is actually immersed in molten metal. If we subtract the weight of the core: where Va is the volume of the whole core, we get the (total) force Fa on the core prints, to be added to cope lift. 6/11/2021 MANUFACTURING ENGINEERING II 120

METALLOSTATIC LIFT Evaluation Thus, if the total force on the core prints is: If the core "sinks", giving no contribution to cope lift (in this case a small force would be supported by the drag). Thus 6/11/2021 MANUFACTURING ENGINEERING II 121

METALLOSTATIC LIFT Evaluation Vertical cores do not contribute to cope lift, because metal pressure on their surfaces is always horizontal. Vertical cores cannot be regarded as "immersed". 6/11/2021 MANUFACTURING ENGINEERING II 122

METALLOSTATIC LIFT Evaluation Total cope lift (Fcl) is therefore equal to the sum of metal pressure on its surface (Fml) and, if present, of the contribution of core prints (Fa): Cope lift takes place when Fcl is larger than the cope weight Cw (including the weight of the cope flask). Thus: When this value is less than zero, the cope weight is enough to support metallostatic lift. Extra weight may be added for safety, but overloading the cope may result in mold deformation and casting inaccuracy. 6/11/2021 MANUFACTURING ENGINEERING II 123

SOLID SHRINKAGE –casting stresses Solid shrinkage must be controlled because of the following problems: § dimension errors: at room temperature the cast part is smaller than mold cavity; § 6/11/2021 casting stresses: due to inhomogeneous cooling velocity, they may lead to hot cracks. MANUFACTURING ENGINEERING II 124

"DESIGN FOR MANUFACTURABILITY" "Design for manufacturability'' means taking particular care of production issues during the product design phase, to reach a more efficient (economical) production. As a general approach, some critical geometries are discussed, leading to processing problems. New part geometries are suggested (part redesign) to reduce production cost. Attention: when estimating production cost savings, the whole process plan should be taken into account (not only foundry operations). A case history is reposted as follows. 6/11/2021 MANUFACTURING ENGINEERING II 125

"DESIGN FOR MANUFACTURABILITY" Rule n. 1 Reduce undercuts. 6/11/2021 MANUFACTURING ENGINEERING II 126

"DESIGN FOR MANUFACTURABILITY" Rule n. 2 Keep parts thickness as constant as possible. Use particular care for contact/intersection between features. 6/11/2021 MANUFACTURING ENGINEERING II 127

"DESIGN FOR MANUFACTURABILITY" Rule n. 3 Control casting stresses by allowing relative displacement among features. Do not design "stiff" structures unless necessary. 6/11/2021 MANUFACTURING ENGINEERING II 128

"DESIGN FOR MANUFACTURABILITY" Rule n. 4 Take into account all possible parting planes (some placements may lead to undercuts). Remember that not all flasks are standard or easy to mold. 6/11/2021 MANUFACTURING ENGINEERING II 129

"DESIGN FOR MANUFACTURABILITY" Rule n. 5 Try and avoid mold protrusion beyond the parting plane. Molding will be easier. 6/11/2021 MANUFACTURING ENGINEERING II 130

"DESIGN FOR MANUFACTURABILITY" Rule n. 6 Design internal walls about 20% thinner than outer walls, to take into account a less efficient heat exchange towards cores. 6/11/2021 MANUFACTURING ENGINEERING II 131

"DESIGN FOR MANUFACTURABILITY" Rule n. 7 Make subsequent machining easier. When drilling is required, it is very helpful to provide surfaces normal to the hole axis. Surfaces for positioning the casting on a machine tool are useful as well. 6/11/2021 MANUFACTURING ENGINEERING II 132

"DESIGN FOR MANUFACTURABILITY" Rule n. 8 Provide reinforcement to the rim of holes made by casting. 6/11/2021 MANUFACTURING ENGINEERING II 133

COMPUTER AIDED CASTING DESIGN Recently, a new generation of FEM software codes was developed to assist casting design. These simulative packages are able to model: § cavity filling § solidification and cooling Such codes are able to predict the risk of potential defects, such as: § air holes § shrinkage cavities § porosity § casting stresses § hot cracks § … 6/11/2021 MANUFACTURING ENGINEERING II 134

COMPUTER AIDED CASTING DESIGN Cavity filling simulation is complex because of boundary modifications (the free surface of the molten metal is moving). Mathematical models of metal flow are based on: § Bernoulli's equation ("simpler" software) § Navier-Stokes' equation (more refined) Mold cooling (liquid, solidification and solid phenomena) exploits: § Fourier's equation of heat diffusion Basing on evaluated parameters (temperatures, stresses etc. ), defects may be predicted using: § physical laws; § empiric rules. The above mentioned prediction may be: § deterministic or § probabilistic. 6/11/2021 MANUFACTURING ENGINEERING II 135

COMPUTER AIDED CASTING DESIGN Solidification & cooling alone are simpler to model: the number of commercial codes able to perform such analysis is larger. 6/11/2021 MANUFACTURING ENGINEERING II 136

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