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1、1,Fundamentals of Materials Forming Technology Casting,2004.10.11,倪东惠 Tungwai Ngai Phone：87111983 E-mail： 金属新材料制备与成形广东省重点实验室 阿里山路 维易达计算中心旁 South China University of Technology College of Mechanical Engineering,2,Please turn off your mobile phone! Thanks !,3,“Metallostatic” Force,“Metallostatic” forc
2、e is the force exerted by the molten metal in all the directions of the mold cavity. The force exerted in the upward direction may push the cope out of line. Fm = Aph g Fm = “metallostatic” force Ap = the projected area of the casting in the parting plane. It is the area on which the metal pressure
3、will be acting. h = height of the pouring basin to the parting line = density of the molten metal g = gravitational acceleration,4,Fundamentals of Materials Forming Chapter 11 Metal-Casting Processes For Students Major in Mechanical Engineering and Automation p. 264 - 281 in the textbook,South China
4、 University of Technology,5,Main Contents,South China University of Technology,11.1 Introduction 11.2 Sand Casting (砂型铸造) 11.3 Shell-Mold Casting (壳型铸造) 11.4 Expendable-Pattern Casting (Lost Foam) (一次模铸造) (消失泡沫) 11.5 Plaster-Mold Casting (石膏型铸造) 11.6 Ceramic-Mold Casting (陶瓷型铸造) 11.7 Investment Cast
5、ing (熔模铸造) 11.8 Vacuum Casting (真空铸造),6,Major classifications of casting processes Classification According to mold materials, molding processes, and methods of feeding the mold with metal: Expendable molds(一次型): After the casting has solidified, the mold is broken up to remove the cast, e.g. sand c
6、ast, plaster cast etc. Section11.2-11.8 Permanent molds(永久型): The molds are used repeatedly and are designed in such a way that the casting can be easily removed. e.g. die cast, squeeze cast, etc. Section11.9-11.14 Composite molds(复合型): Made of two or more different materials, combining the advantag
7、e of each material. e.g. sand, graphite and metal.,A summary of various casting processes General characteristics of casting processes,7,8,About 70-80% of the castings are made by sand casting. It is a traditional and widely-used casting technology,9,Sand casting consists of the following steps: pla
8、cing a pattern (模样) having the shape of the desired casting in sand to make an imprint; incorporating a gating system (浇注系统); filling the resulting cavity with molten metal; allowing the metal to cool until it solidifies; breaking away the sand mold; removing the casting.,11.2 Sand Casting (砂型铸造),10
9、,South China University of Technology,Fig. 11.4 Schematic illustration of a sand mold,Parting line分型线(面) Cope上模箱 Drag下模箱 Flask沙箱 Runner(横浇道, 内浇道) Sprue(直浇道) Core(型芯) Riser(冒口) Vent(排气孔) Pouring basin/pouring cup(浇口杯),11,South China University of Technology,Fig. 11.5 Typical production steps in sand
10、casting,12,South China University of Technology,Most sand casting operations use silica sand (SiO2). because it is Inexpensive and resistance to high temperature,11.2.1 Sands,There are two general types of sand: naturally bonded (bank sand) and synthetic (lake sand). Because its composition can be c
11、ontrolled more accurately, synthetic sand is preferred by most foundries.,Diameter: 0.05-3.35 mm Density: 2.65 g/cm3 Melting point: 1760C,13,Other sand for special uses: Zircon(ZrSiO4) 锆砂 Olivine(Mg2SiO4) 橄榄石砂 Iron silicate(Fe2SiO4) 铁橄榄石 Chromite(FeCr2O4) 铬铁矿砂,Better thermal resistance,Low thermal e
12、xpansion High heat transfer rate,几种常用型砂的熔点,特种砂通常具有耐火度高、导热性好、热膨胀小、抗熔渣浸蚀能力强等特点，主要用于合金钢和铸钢件，以及要求铸型尺寸稳定和铸件尺寸精度要求高的场合。其缺点是资源比较短缺，价格较贵。,14,Clay (粘土) e.g. mAl2O3nSiO2xH2O Used as a cohesive agent to bond sand particles, giving the sand strength,Sand having fine round grains can be closely packed and forms
13、a smooth mold surface. Although fine-grain sand enhances mold strength, the fine grains also lower mold permeability.,15,Properties of sand mold,Strength Permeability(透气性) allow gases evolved during casting to escape easily Collapsibility(溃散性,退让性) allow for the casting to shrink while cooling,Gas po
14、rosity,Hot tearing, Crack,The selection of sand involves certain tradeoff (折衷，权衡) with respect to properties.,16,碾轮式混砂机,Mulling machines(混砂机) are used to uniformly and thoroughly mix sand with additives.,17,In sand casting, re-usable, permanent patterns are used to make the sand moulds. The preparat
15、ion and the bonding of this sand mould are the critical step and very often are the rate-controlling step of this process. Two main routes are used for bonding the sand moulds: The “green sand“ consists of mixtures of sand, clay and moisture. The “dry sand“ consists of sand and synthetic binders cur
16、ed thermally or chemically.,18,Three basic types of sand molds: green-sand mold (湿沙型),cold-box mold (冷芯盒砂型), no-bake mold (自硬砂型)。 The most common mold material: green molding sand. Three methods to make a sand mold: Skin-dried method: the mold surface are dried by air or fire. Cold-box method: binders are blended into the sand to bond the grains chemically for greater strength. (3) No-bake method: a synthetic liquid resin
