Experiment (2) Metal Casting and Foundry 1. Objective: 2. Background: 3. Theory: Classification of molding and castings processes.

Transcription

1 Experiment (2) Metal Casting and Foundry Dr. Mohammad Al-tahat Department of Industrial Engineering. University of Jordan. Lab. Of Manufaturing Proesses. Course No: Objetive: The main objetive of this experiment is to enhane the pratial knowledge of the students in the field of metal asting tehnology and to review the basi prinipals for the design of asting patterns, feeding systems and gating systems, in addition to the investigation of the main fators affeting the funtion of suh asting elements. 2. Bakground: For more information about the subjet of the experiments, it is reommended for the student to review hapter five of the text. 3. Theory: It is a popular mean by whih a material is onverted into a final useful shape; a solid is melted, heated to a proper temperature, and sometimes treated to modify its hemial analysis. The molten material, generally metal, is then poured into a mold avity, whih ontains it in proper shape during solidifiation. The resulting produt an have virtually any onfiguration the designers want (patterns). In addition the resistane to working stresses an be optimized, properties an be ontrolled (heat treatment), and a good appearane an be produed (fettling and finishing). In general the following sub-proesses are involved in asting proess: 1. Casting design. 2. Melting proess. 3. Tapping and pouring tehniques. 4. Mold avity. 5. Solidifiation proess. 6. Shakeout proess. 7. Shot blasting proess 8. Fettling and finishing 9. Heat treatment 10. Painting of final astings (produts). Classifiation of molding and astings proesses. Proesses, tehniques and operations that being used for produing molds and astings an be lassified as follow: 1. Expendable (single-use) Molds with multiple-use pattern. Sand-mold asting Shell-mold astings V-proess (Vauum Molding) Casting Plaster -mold astings 15 of 1Page

2 The Shaw proess-astings. Graphite-mold astings. Rubber-mold astings. 2. Expendable (single-use) Molds with single-use pattern. Investment asting 3. Permanent Molds (Multiple-use-mold) asting proesses. Slush astings Corthias astings Low-pressure permanent-mold astings Vauum permanent-mold astings Die astings Squeeze astings (or liquid-metal forging) Centrifugal Casting Semi entrifugal Casting Ingots and Continuous Castings. Eletromagneti astings. Casting terminology. Figure 3 illustrates the ross setion of a typial two-part asting mold and inorporates many features of the proess. Parting line, ope, drag, mold avity, riser, gating system, sprue, pouring up, and many other features are shown. 2 Figure 3: Some features of sand mold asting 15 of 2Page

3 3 Types of sand molds. There are three basi types of sand molds: Green molding sand: Green sand molding is a mixture of sand, lay and water, it is the heapest molding method. Cold-box molds: Various organi and nonorgani binders are blended into the sand to bond the grains hemially for greater strength. This method is more expensive than green sand molding but more aurate astings an be produed. No-bake molds: Liquid resin is mixed with the sand, and the mixture hardens at room temperature. In the last two types of molding methods, bonding takes plae without heat so they are all old-setting proess. Charaterization of basi foundry sand. The most important speifiations of basi sand are: moister ontent, fineness (lay ontent), grain shape, refratoriness, hemial omposition and grain size distribution (sieve analysis). 1. Moister ontent: Sand should be dry, speedy moister tester. 2. Grain shape: Splintered, sub-angular, angular and round shapes. 3. Fineness: partiles less than 0.02 mm in diameter onsidered as fines partile. In siliate bonded sands fineness should be less than 2%. In resin bonded sand should be less than 0.05%. 4. Thermal expansion: eah sand type has its own thermal expansion oeffiient. 5. Refratoriness of sand or sand mixture is defined as the temperature at whih most of the sand grains are sintered. Fators affeting the refratoriness of sand mould are mainly: 1. Type of basi sand and its hemial omposition. 2. Inreasing alkali ontent in sand mixture. 3. Presene of metal inlusions, and 4. Inreasing the binder ontent. Sand Binders. Bentonite, Cement Sodium siliate, and Furan resin and hardener 15 of 3Page

4 Design of Casting proess The proess for making a asting is designed in a definite sequene, whih inludes the following: A. the drawing of the part to be ast, suh a proess is the main proess doument.the drawing is define all the features of the asting proess and is the basis for the design and manufature of the moulds and patterns and seletion of other applianes, whih are needed for the manufaturing of the asting mold (flask,template,et.).the elements of foundry tehnology indiated on the drawing should speify the following: 1. The best parting plane for the mold and pattern. 2. The positions of the mold for pouring whih is depending on the shape of asting, kind of metal, gating system geometry, speifiations of ast metal density, surfae finish and many others The mahining allowenes of the asting (thikness of metals to be removed after asting). 4. Draft alloawnes of the asting. The following figure illustrates the manner in whih taper (draft), and mahining allowanes are inluded in the pattern for a simple shape asting. Sine allowanes tend to be removed by mahining, efforts made to redue the allowanes will be well reeived. Fators affeting the draft allowanes are;molding method(hand or mahine);pattern material(wood, metal et.);pattern height; molding material (sand mixture, rubber, et.); material to be asted; and finally the parting plane loation. Conversly fators affeting the mahining allowanes onsidered as; nominal weight of asting and lass of auray; asting size and nominal dimension of the detail to be mahined; surfae position while asting; molding method(hand or mahine); and material to be asted. 15 of 4Page

5 5 Original Outline of Part to be asted Original Outline with shrinkage Mahining allowanes V-Slot to be mahined after asting Pattern or draft allowanes. 5. the number of ores to form the internal avities in the astings or some shaped-portions at its extention. The ores are numbared in the order they will be set in the mold. B. The assemembled mould of the asting with all its measurements, is reresented in a drawing or sketh, the drawing should outline the loation of ores, Gating system elemnts, hils, setion drawing of the mould are made so that the molder ould assemble the mold without refering to the ating drawing. Risering System Design Risers are elements of the gating and feeding system, whih are intended for displaing shrinkage avity and porosity outside of the asting. By the priniple of direted solidifiation. The thinner setions of a asting should preferably be loated at the 15 of 5Page

6 bottom and the thiker ones, at the top. The latter ommuniate with risers above them. If this is impossible, side risers are provided for the hot spots. Using internal and external hills an also bring about direted solidifiation. Drawings of astings are heked for the probability of formation of shrinkage defets by the method of insribed irle (Figure 1 a), whih should freely roll out, as it were, from lower setions of a asting into the upper ones and further into the riser. For the asting shown in Figure 1 a, this ondition is not satisfied (R 1 > R 2 ), and therefore, shrinkage avity 1 is likely to appear in the hot spot. After marking the mahining allowanes, draft 2 and fillet 3 in the drawing (Figure 1 b), the insribed irles will roll out freely (R 1 < R 2 ) from the bottom of the asting upwards into the riser, whih will ensure the direted solidifiation, and therefore, the absene of shrinkage avity in the asting. 6 However, the basi requirement of a riser is that is should: 1. Be the last portion to solidify; 2. Be effetive in establishing a pronouned temperature gradient within the asting to promote diretional solidifiation towards the risers; 3. Have suffiient volume to ompensate for shrinkage in the asting; 4. Completely over the asting setion that is to be fed; 5. Ensure the maximum yield possible. Apropos maximum yield, one has to know the different shapes of risers in ommon use - spherial, hemispherial, elliptial, ylindrial, square, retangular and that, for a partiular volume of the riser, the one having the minimum surfae area is the most effetive. Insribed irle method for Riser Calulations. Heuvers was developed this method. The riser diameter is obtained by multiplying the diameter of the largest irle (hot spot) that an be insribed in the setion to be fed by an arbitrary fator whih normally ranges from 1.5 to 3, i.e. riser diameter D 1.5 TO 3 times diameter of the hot spot. Though this method is empirial, it is still very muh in use beause of its simpliity. Modulus Method V Volumeof Casting The Modulus of a asting M is given by: M A Surfae Area of the Casting C 15 of 6Page

7 In view of the fat that the shrinkage avity of a riser an amount to a maximum of 14% of its original volume, the modulus of the riser must be at least 1.2 times the modulus of the asting. To ensure that the riser solidifies later than the asting, (theoretially the solidifiation of the riser will be about 1.44 times that of the asting) after obtaining the modulus, the size of the riser an be alulated by assuming a suitable height to diameter ratio. M r 1.2 M Determination of the numbers of Risers. Number of required Risers an be alulated using the following formula: L( mm) nf df ( mm) + FDT. ( mm) Where: n F : Risers numbers required. L : Casting length or mean irumferene. d F : Is the riser diameter. T :IS the thinnest asting setion through whih to feed. FD : Feeding distane fator, whih is (4-5 for steel), (5 for malleable iron), (10 for AL), (5-6 Al alloys), et. Example The asting shown in the figure below weighs 4300 Kg. The asting an be divided into the entral hub portion and a ring of outside diameter 108, inside diameter 90 and height 11 ½. The risers that will be kept on this portion will feed the projetion and the arms of 7Page

8 Solution. By hot spot irle method: Out side Ring. Hot spot diameter (108-99)/ Riser diameter D 1.5 TO 3 times diameter of the hot spot, Riser diameter (2.5)(4.5) 11.2 say 12. Riser Height 1.5 D (1.5)(12) 18. Number of risers 8 n F d F L( mm) ( mm) + FD. T ( mm) Cirumferene ( π.108) 339 nf (4)(11.5) 58 Central portion. Hot spot diameter (16-11)/ Riser diameter D 1.5 TO 3 times diameter of the hot spot, Riser diameter (1.5)(2.5) This is an annular riser with external diameter of (3.75)(2)+(11) 18.5 Riser Height 1.5 D (1.5)(3.75) 5.6. But the height will onsidered as shown in the figure same as height of the out side ring riser. By modulus method. M for the rim, V 2500, A M M r V A M 1545 Vr (1.2)(1.62) 1.94 A 2 π.d (1.5)D 4 1.5D D 8 π 2 + πd D say H n n F F d F L( mm) ( mm) + FD. T ( mm) Cirumferene ( π.108) 11+ (4)(11.5) r of 8Page M For Central Portion, V M M r π /4 π 4 V A M 2 2 ( D 11 ) 2 2 ( D 11 ) D 17 ( D 11) ( D 11) ( D 11).4 (0.5D )1.2 H Vr (1.2)(1.05) 1.26 A.16.5 ( D + 11) π , A 1247 r

9 Design of Gating System. The ideal optimum gating system should: 1. Fill the mold quikly. 2. Fill a mold with a minimum of turbulene. 3. Establish thermal gradients, whih promote soundness. 4. Avoid reoxidation of metal in the gating system. 5. Remove slag and dross from the metal as it flows through the gating system. 6. Not distort the asting during solidifiation. 7. Maximize asting yield. 8. Be eonomial to remove. 9. Be ompatible with the pouring system used. 9 Gating systems with various shemes of metal feeding showed below: The members of the gating system. In the following, the individual members of gating systems and of their assembly will be briefly presented: Pouring basin. Pouring basins that ontain a well deeper than their depth at the sprue juntion to effetively absorb the impat of the arriving stream, and flow veloity will be governed by sprue height only. Another advantage of this design is that pouring may start out slowly without iron entering the sprue. One the proper loation of the ladles lip has been established, fast pour and sprue filling begins with minimum slag entry. 15 of 9Page

10 10 Dam shaped basin of tunnel type with skimming plate. Sprue. Cirular ross setions are being used most ommonly. Tapering the sprue downwards is always a good pratie. Straight or nearly straight sprues may be used in all pressurized systems. Choked at the bottom (or sprue basin) of the sprue must be used in a nonpressurized gating systems. H φ sprue φ d 5 Runner. A straight runner is the best hoie of spae permits it. If bending the runner is unavoidable, it should be done with as large radius as spae permits, beause urvatures introdue additional turbulene. A minimum distane of 4 inh between the end point of the runner and the next gate us reommended. The ross setion of the runner is almost always retangular with thikness to height ratio of 1:2 in a pressurized system. 15 of 10Page

11 11 Sprue runner juntion. The first rule in shaping the sprue-runner juntion is that it must not loally derease the alulated sprue bottom ross-setion area. If then, the sprue ross setion is largely in any dimension than the horizontal setion of the runner, the sprue bottom should extend to the bottom of the runner, see the previous figure. Gates. Gates are the most deliate members of the system, Gates should be thin and orrespondingly wide, and should be easy to removed. The optimum gate ross setion is retangular with a little draft as ondition permit. Runner-gate Juntion. A gate must never be plaed in straight ontinuation of the runner. Gates must branh off the side(s) of the runner at near right angles. Gate-Casting juntion The gates need to join the thinnest setions of the asting as muh as layout limitations permit. The aim is to equalize ooling rate between the different segments of the asting. If deliate ores or soft mold wall would be damaged by the impat of entering stream of iron, gates may be flared out or their ross setion inreased nearing the asting. Suh preaution is seldom used beause it inreases leaning room ost, and the redution in linear veloity is not signifiant. Classifiations of Gating Systems. Aording to the hydrodynamis of flow of metal Gating system are divided into open and losed. Closed or pressurized gating system are haraterized by gradually dereeing rosssetional areas of the sprue, slag traps and runners: SA > SA > SA sprue runner 15 of 11Page gate

12 Better separation of slag, the metal enters the mold avity with a high linear veloity, whih an lead to splashing and oxidation of the molten metal, apture of air, and washout of the mould walls. Closed gating systems are espeially popular in the manufaturing of iron astings. Open or non-pressurized gating system are haraterized by gradually inreasing rosssetional areas of the sprue, slag traps and runners: SA < SA < SA sprue runner Open gating systems are used in asting of steels, aluminum, magnesium and other easily oxidable alloys and are oming into use in iron asting. Calulation of gating system. Example 8.1 The asting shown in the figure below weighs 4300 Kg. As we find from the last setion, at the ring portion, number of needed risers are 7 eah has 12 unit diameter and 18 unit height. At the entral (hub) one hollow riser of external diameter, eternal diameter and height of 11.0, 18.5, 18 unit respetively. It is required to design a proper gating system to ast this part. gate of 12Page

13 Solution. Assuming the density of ast metal is kg/inh 3 (beause all dimension of this example in inhes). Total feedable weight G t weight of asting G +weight of all risers G r. G t [( *7)+3127].[0.097] 5988Kg. The surfae area of the needed gate(s) AS gate, ould be found by the following equation: G t ASgate 55 Inh 2 2 If we reommend 7 gates then 55 2 ASgate 7.8 Inh 7 weightofgate( s) Kg Geometry of eah gate reommended being as in the figure bellow. 13 Considered a losed gating system (pressurized) with gating ratio as below: SA > SA SA SA sprue runner > sprue :SA runner 1.2 :1.1 :1.0 Total surfae area of the runner will be (1.1).(55) 60.5 If the sprue in the middle of the runner then we an say that there are two runners eah have (60.5/ Inh 2 ) surfae area, WEIGHT 983Kg. 5.6 :SA gate gate L of 13Page

14 The sprue surfae area will be (1.2)(55) 66 Inh 2 2 π D 66 4 D 9.2 Inh φ sprue 9.2 Inh weight Sprue Kg 14 Total asting weight sprue weight+ runner weight+ gate(s) weight+ G t Kg Casting yieldg / / % 4. Materials: Aluminum srap, green sand molding mixture, wooden pattern, Isolating material (dust), and some other materials are required. 5. Equipments: The following equipments and tools are neessary to perform a sand asting proess by the use of the above materials: Furnae Flask measuring tools Ladle (ruible) Clamps rammers Handling equipments Shrinkage rule different asting tools 6. Proedures: 1. Make an engineering drawing for a given produt, based on the drawing design a wooden pattern with reasonable gating and risering system. 2. Prepare the sand molding mixture and prepare all the needed tools and equipment. 3. Prepare the furnae, the harging material, the refratory materials and the handling equipments and tools. 4. Prepare the asting mold, in the mean while start melting the harge in the furnae. 15 of 14Page

15 5. Pouring of the molten metal in the mold avity. 6. Wait a proper time tell the poured metal is solidify then shake out the mold to get the final asting. 7. Requirements: 1. Summarize the proedures of the experiment? 2. Draw the part to be ast, and then indiate on the drawing the parting line, shrinkage rate, mahining allowanes (if there is mahining), and draft allowanes, ores et. 3. Calulate a proper risering system (use sribed irle method) and draw the risers? 4. Calulate a proper gating system; spree, runner(s) and gate(s), draw skethes? 5. Draw (skethes) the final asting with risers, and gating system and find the total weight, then find the yield of the asting? 6. Draw (Skethes) the used sand mold and alulate the used sand weight. Then alulate the sand to metal ratio? 7. Indiate the melting, pouring and tapping temperatures? 8. Disuss the defets of the final asing of 15Page