Chapter 1 Sand Casting Processes

Transcription

1 Chapter 1 Sand Casting Processes 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 The separate cope and drag are necessary to allow the pattern to be withdrawn from the sand and to allow the cores to be properly positioned within the mold. 1.1 Basic Steps in Making Sand Castings In general, the sand casting process consists of five basic steps: 1. Patternmaking (including core boxes); 2. Coremaking; 3. Molding; 4. Melting and pouring; 5. Cleaning. As shown in Figure 1.1, 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. W. Wang et al., Rapid Tooling Guidelines For Sand Casting, Mechanical Engineering Series, DOI / _1, Springer Science+Business Media, LLC

2 2 Rapid Tooling Guidelines For Sand Casting Fabricate Core Boxes Fabricate Pattern Core Mat l Mold Core Sand Prepare Sand Build Mold Raw Metal Melting Metal Pour Casting Break Out Raw Casting Solidification and Cooling Degate, Clean & Inspection Finished Casting Figure 1.1 The manufacturing steps for the sand casting process Patternmaking Patterns 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

3 Chapter 1 Sand Casting Processes 3 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 Coremaking If 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 Molding Molding 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 Melting and Pouring The 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.

4 4 Rapid Tooling Guidelines For Sand Casting Cleaning Cleaning 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 heattreatment, surface treatment, or machining. 1.2 Mold Making Processes Sand 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 molding Green-sand molding Green-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

5 Chapter 1 Sand Casting Processes 5 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 1.2 illustrates the physical process of the green-sand molding. Iron casting to be produced. 1. Bottom half of the pattern is placed on the mold board and surrounded by the drag half (bottom) of the flask. 2. Molding sand is rammed around the pattern in multiple steps to provide uniform sand density. 3. After the bottom half of the mold is filled, it is rotated upright and the top half of the pattern and flask are put in place to complete the mold.

6 6 Rapid Tooling Guidelines For Sand Casting 4. This section view shows the competed mold with pattern still in place and the sprue hole formed for entrance of molten metal. 5. The cope and drag halves of the mold are separated so that the pattern can be removed. The gate channel is then cut from the sprue to the mold cavity. 6. The core is placed in core prints located in the drag half of the mold. The space between the core and mold wall form the internal passages of the casting. 7. After placing the core in the mold, the mold is closed and clamped to resist pressure exerted by the molten metal when it is poured in the mold. Figure 1.2 The green sand molding process (Courtesy: AFS, 1989)

7 Chapter 1 Sand Casting Processes 7 Green sand molding is most adaptable to light, bench molding for medium-sized castings or for use with production molding machines. 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 Dry-sand molding Dry-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 F (or 204 to C ) 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.

8 8 Rapid Tooling Guidelines For Sand Casting Core-sand molding Core-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 consists 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 Shell molding Shell molding is a semi-precision method for producing castings repetitively in high volume. It is probably used more for making cores than molds. The sand used for shell molding consists of fine dry sand grains and a phenolic resin. This is mixed dry or in the presence of alcohol. No water is used. The phenolic resin is a thermosetting plastic which is either mixed with the sand as a powder or applied as a liquid and then dried on the sand grains. To be suitable for molding, the mixture must be dry and free-flowing. The mold is formed by using a machined metal pattern, usually made of gray iron, aluminum, or brass. The pattern is heated to 450 to F (232 to C) and then the sand-resin mixture is dumped or blown on the surface of the pattern. Upon contact with the hot pattern, the resin melts, hardens, and strongly bonds the sand grains together. After a carefully planned dwell time (usually 20 to 30 seconds) the pattern and sand are inverted to remove excess unbonded sand from the layer or shell of bonded sand that adheres to the pattern. The thickness of the shell can be accurately controlled by the length of the dwell time, which is tailored to provide the required strength and rigidity for the weight of metal to be poured into the mold. The usual thickness is ¼ inch (6.35mm) or more. The uncured shell is then moved into an oven where it is cured for 15 to 120 seconds. After curing, the shell is rigid and can be stripped from the pattern by means of ejector pins. Once cured, the shell is readily stored and handled.

9 Chapter 1 Sand Casting Processes 9 The two halves of the mold are then assembled with the aid of alignment pins. The shells may also be joined together by pasting and curing and/or by clips, clamps, or other fastening techniques. In some cases, the shells are placed on end and are put in a flask with sand or shot around them. This backing resists the hydrostatic pressure of the metal, prevents distortion of the shell, and promotes close dimension control of the casting. If the casting has no heavy or thick wall sections, it is frequently possible to prepare the shell for pouring in the horizontal position by including a pouring basin in the mold. By controlling the dwell time and the thickness of the pattern, it is often possible to produce a shell of sufficient thickness to withstand the hydrostatic pressure without the use of backing. This greatly simplifies the process. Figure 1.3 further illustrates the shell molding process. In production, the shell-making operations take place within a machine or set of machines with very little attention from the operator except to remove the shells from the machine. The resulting automation contributes to superior dimensional consistency. Advantages. Molds may be stored for extended periods of time. Good casting detail and dimensional accuracy may be realized. Mold is light in weight. Disadvantages. High pattern cost, high pattern wear, high energy costs, higher materials cost compared to green sand molding.

10 10 Rapid Tooling Guidelines For Sand Casting Figure 1.3 Steps in the shell molding process. (1) Equipment for heating and measuring temperature of shell-molding pattern. (2) Dump box in position for dumping sand on pattern. (3) Shell on pattern. (4) Stripping shell from pattern. (5) Shell-mold halves. (Courtesy: Heine et al, Principles of Metal Casting, 1967, reproduced with permission of The McGraw-Hill Companies)