(1.6)--Chapter 7 Sand casting process金属材料工艺基础.doc

Chapter 7 Sand Casting ProcessCasting is one of the earliest metal shaping methods known to human being. It generally means pouring molten metal into a refractory mould with a cavity of the shape to be made, and allowing it to solidify. When solidified, the desired metal object is taken out from the refractory mould either by breaking the mould or taking the mould apart. The solidified object is called casting. This process is also called founding.Sand casting is a mold based net shape manufacturing process in which metal parts are molded by pouring molten metal into a cavity. The mold cavity is created by withdrawing a pattern from sand that has been packed around it. Since the pattern imprint forms the cavity, the pattern creates the external shape of the cast part. If the part has undercuts or hollow internal regions, these can be formed by sand cores that are fabricated separately and then placed in the mold cavity. The cores are supported by core prints, and/or chaplets that allow the molten metal to flow between the core and the mold wall. In addition, cores may be necessary to produce a desired “zero” draft external surface, depending on the parting line selected. The parting line is formed by the interface between the cope (upper portion) and drag (lower portion) of the mold. The separate cope and drag are necessary to allow the pattern to be withdrawn from the sand and to allow the cores to be properly positioned within themold.Figure 7.1 Sand mold making procedure.7.1 Casting TermsIn the following chapters, the details of sand casting process which represents the basic process of casting would be seen. Before going into the details of the process, defining a number of casting vocabulary words would be appropriate. Reference may please be made to Fig. 7.1.Flask: A molding flask is one which holds the sand mould intact. Depending upon the position of the flask in the mould structure it is referred to by various names such as drag-lower molding flask cope-upper, molding flask and cheek-intermediate molding flask used in three-piece molding. It is made up of wood for temporary applications and more generally of metal for long-term use.Pattern: Pattern is a replica of the final object to be made with some modifications. The mould cavity is made with the help of the pattern.Parting line: This is the dividing line between the two molding flasks that makes up the sand mould. In split pattern it is also the dividing line between the two halves of the pattern.Bottom board: This is a board normally made of wood which is used at the start of the mould making. The pattern is first kept on the bottom board, sand is sprinkled on it and then the ramming is done in the drag.Facing sand: The small amount of carbonaceous material sprinkled on the inner surface of the molding cavity to give a better surface finish to the castings.Molding sand: It is the freshly prepared refractory material used for making the mould cavity. It is a mixture of silica, clay and moisture in appropriate proportions to get the desired results and it surrounds the pattern while making the mould.Backing sand: It is what constitutes most of the refractory material found in the mould. This is made up of used and burnt sand.Core: It is used for making hollow cavities in castings.Pouring basin: A small funnel shaped cavity at the top of the mould into which the molten metal is poured.Sprue: The passage through which the molten metal from the pouring basin reaches the mould cavity. In many cases it controls the flow of metal into the mould.Runner: The passageways in the parting plane through which molten metal flow is regulated before they reach the mould cavity.Gate: The actual entry point through which molten metal enters mould cavity.Chaplet: Chaplets are used to support cores inside the mould cavity to take care of its own weight and overcome the metallostatic forces.Chill: Chills are metallic objects which are placed in the mould to increase the cooling rate of castings to provide uniform or desired cooling rate.Riser: It is a reservoir of molten metal provided in the casting so that hot metal can flow back into the mould cavity when there is a reduction in volume of metal due to solidification.7.2 Basic Steps in Making Sand CastingsIn general, the sand casting process consists of five basic steps:1. Patternmaking (including core boxes);2. Core making;3. Molding;4. Melting and pouring;5. Cleaning.As shown in Figure 7.2, these steps are implemented in a series of foundry operations. Typically, the detail technical processes involved in each of these operations are the primary concern of the foundry.7.2.1 PatternmakingPatterns are required to make sand molds. Therefore, before a sand casting can be produced, necessary patterns must be fabricated in the pattern shop or by a toolmaker. The pattern, which is made of wood, metal, plastic or a variety of other materials, has the external shape of the cast part but is enlarged by a shrinkage allowance to compensate for contraction of liquid metal during solidification and cooling to room temperature. Also, an allowance, called taper or draft, is provided on the vertical sides of the pattern to facilitate removal from the sand. The pattern may also include core prints, gates and risers, although, in some cases these features can be cut into the mold sand by hand or can be made by placing loose pattern pieces in the mold. The gating system is a channel, or network of channels, through which the molten metal flows into the cavity. It is typically composed of a pouring cup, sprue, and runner. The riser is a reservoir in the mold that serves as a source of liquid metal to feed the casting and compensate for shrinkage during solidification.Figure 7.2 The manufacturing steps for the sand casting process.7.2.2 Core makingIf the casting is to be hollow, as in the case of a pipe fitting, additional patterns, referred to as core boxes (negative patterns), are used to form the sand cores that are used to create these cavities. Sand cores are molded in core boxes to the same standards of accuracy, and with the same problems of service life under abrasion, as patterns. After being molded in core boxes, the cores are placed in the mold cavity to form the interior surfaces of castings. Cores are ordinarily made separately from molds in a designated area of the foundry referred to as a core room. They are then transported to the molding area to be positioned in the sand molds. Like patterns, core boxes must be fabricated by the toolmaker or pattern shop prior to production of the sand casting.7.2.3 MoldingMolding consists of all of the operations necessary to prepare a mold for receiving molten metal. This typically involves packing sand or other molding aggregate around the pattern which is held within a supporting frame, or flask, withdrawing the pattern to form the mold cavity, setting the cores in the mold cavity, and assembling the cope and drag halves to close the mold. Once assembled, the mold is ready to receive the molten metal.7.2.4 Melting and PouringThe preparation of molten metal for casting is referred to simply as melting. Melting is usually done in a specifically designated area of the foundry, where melt chemistry, gas content, and other metallurgical factors are monitored and controlled before pouring. When the melt is ready, it is poured into the mold.7.2.5 CleaningCleaning includes all operations necessary to remove sand, scale, and excess metal from the casting. The casting is separated from the molding sand and transported to the cleaning department. Burned-on sand and scale are removed from the casting surface to improve the surface appearance of the casting. Excess metal, in the form of fins, wires, parting-line fins, and gates, are cut off by grinding. Defective castings may be salvaged by welding or other repair. Inspection of the casting for defects and general quality follows. The casting is then ready for shipment and/or further processing such as heat treatment, surface treatment, or machining.7.3 Mold Making ProcessesSand casting is an “expendable mold” casting process since the mold is destroyed during removal of the solidified casting. Therefore, a new mold must be made for each sand casting produced. From the point of view of dimensional accuracy, mold making is the key step in the sand casting process. Quality castings cannot be produced without quality molds.“Mold making process” refers to the method of making the mold. The differences among sand molding processes are primarily in the method of forming the mold and in bonding of the granular refractory. Exemplary mold making processes for sand casting include the following: Green-sand molding Dry-sand molding Core-sand or core molding Shell molding7.3.1 Green-sand moldingGreen-sand molding utilizes a mold made of compressed or compacted moist sand. The sand is called “green” because of the moisture present. The mold is not baked or dried. The mold materials consist of silica sand mixed with a suitable bonding agent (moist clay). To produce the mold, a metal or wood frame (flask) is placed over the pattern to produce a cavity representing one half of the casting. Compaction or ramming of the sand is achieved by either jolting or squeezing the mold. The opposite half of the mold is produced in a similar manner. The two flasks are positioned to form the complete mold. Figure 7.3 illustrates the physical process of the green-sand molding.Green sand molding is most adaptable to light, bench molding for medium-sized castings or for use with production molding machines. Figure 6.3 The green sand molding process(Courtesy: AFS, 1989)Advantages： Most metals can be cast by this method. Pattern cost and material cost are relatively low. The method is adaptable to large or small quantities. Disadvantages： There are practical limits to complexity of design. Machining is often required to achieve the finished product. Dimensional accuracy cannot be controlled as well as with other molding processes, although good standards are possible with quality pattern equipment, modern process controls, and high density molding.7.3.2 Dry-sand moldingDry-sand molding is similar to green sand molding in that moist sand is used to initially form the mold. The green sand mold is then dried by baking it at 400 to 600 (or 204 to 316) to develop good strength and other properties. Generally the mold cavity surface is coated or sprayed with a mixture that increases the hardness of the mold walls when dry. Some foundries use dry sand molds to produce intricate parts that are difficult to cast to exact size and dimensions. Castings of large or medium size, and of complex configuration such as frames, engine cylinders, rolls, large gears and housing are often made by utilizing the dry sand technique. Both ferrous and nonferrous metals can be cast using this type of process. Advantages： Dry sand molds are generally stronger than green sand molds and can therefore withstand additional handling. Castings produced using dry sand molds usually possess improved dimensional consistency compared to green sand. Also, the improved quality of the sand mixture due to the removal of moisture generally results in a smoother surface finish.Disadvantage： This type of molding is more expensive than green sand molding and is not well suited for high-production. Optimal baking (drying) time is essential.7.3.3 Core-sand moldingCore-sand molding is achieved by fitting cores together to form the mold. In this approach, core boxes are used in place of a pattern for making all parts of the mold. Core sands usually consist of mixture of sand grains and organic binders which develop great strength after baking. These strengths make it possible to cast metal around thin sand projections without having them break or erode because of the hot-metal erosion. Core sand molds are also sometimes made with dry molding cement-bonded sands.Figure 7.4 The Core-sand molding process7.4 Molding Materials7.4.1 Composition of Molding SandMolding sand is defined as a mixture of base sand or gravel with a suitable bond. The principal constituent of a molding mixture are base sand and bonding materials, but other substances are added to confer special qualities, for example improved resistance to specific defects.7.4.2 Properties of molding sands Fineness. This is a measure of the actual grain sizes of a sand mixture. It is made by passing a standard sample, usually 100 grams, through a series of graded sieves. About ten different sizes sues are used. As most sands are composed of a mixture of various size grains there is a distribution of sands remaining on the measuring sieves.Refractoriness. This is the ability of sand to withstand high temperatures without fusing or breaking down. From this we can deduce that a sand used for casting steel must be more refractory than one for brass or aluminum because of the greater pouring temperature involved. Also, a sand used to cast large heavy castings must be more refractory than one used for light thin castings of the same metal. Green Bond Strength. This is the strength of tempered sand expressed by its ability to hold a mold in shape. Sand molds are subjected to compressive, tensile, shearing, and transverse stresses. Which of these stresses is more important to the sands molding properties is a point of controversy.Tensile Strength. Tensile strength is the force that holds the sand up in the cope. And, as molding sands are many times stronger in compression than tensile strength, we must take the tensile strength into account. Mold failure is more apt to occur under tensile forces.Permeability. The ability of the mold materials to allow the steam or other gas pass through the mold wall. Inadequate permeability causes blowhole, Boiling, misrun, cold shut.7.5 PatternsA pattern is a shaped form of wood or metal around which sand is packed in the mold. When the pattern is removed the resulting cavity is the exact shape of the object to be cast. The pattern must be designed to be easily removed without damage to the mold. It must be accurately dimensioned and durable enough for the use intended. Either one time use or production runs.As has been defined earlier, a pattern is a replica of the object to be made by the casting process, with some modifications. The main modifications are:(a) The addition of pattern allowances,(b) The provision of core prints, and (c) Elimination of fine details which cannot be obtained by casting and hence are to be obtained by further processing.There are various types of patterns depending upon the complexity of the job, the number of castings required and the moulding procedure adopted.7.5.1 Single piece patternThese are inexpensive and the simplest type of patterns.