CHAPTER 4: METAL CASTING PROCESS

Transcription

1 CHAPTER 4: METAL CASTING PROCESS

2 CHAPTER OUTLINE 4.1 INTRODUCTION 4.2 EXPANDABLE MOLD CASTING PROCESSES Sand Casting Shell Molding Plaster Mold Casting Ceramic Mold Casting Evaporative Pattern Casting Investment Casting 4.3 PERMANENT MOLD CASTING PROCESSES Die Casting Centrifugal Casting Vacuum Casting Low Pressure Casting

3 4.1 Introduction Basically involves (a) pouring molten metal into a mold patterned after the part to be manufactured, (b) allowing it to solidify, and (c) removing the part from the mold. General phenomenon in metal casting process are: - Solidification of metal - Fluid flow - Fluidity of molten metal - Shrinkage - Defects

4 Fluidity of molten metal The capability of the molten to fill mold cavities which consists of two basic factors: (1) characteristics of the molten metal and (2) casting parameters. Shrinkage Because of thermal expansion characteristics, metals usually shrink (contract) during solidifications and while cooling to room temperature. Three sequential events of shrinkage are: - contraction of the molten metal as it cools prior to its solidifications. - contraction of the metal during phase change from liquid to solid. - contraction of the solidified metal.

5 Defects The International Committee of Foundry Technical Associations has developed a standardized nomenclature, consisting seven basic categories of casting defects identified: - metallic projections - cavities - discontinuities - defective surface - incomplete casting - incorrect dimensions or shape - inclusions

6 Typical cast part Figure (a) Typical gray-iron castings used in automobiles, including the transmission valve body (left) and the hub rotor with disk-brake cylinder (front). Source: Courtesy of Central Foundry Division of General Motors Corporation. (b) A cast transmission housing. (c) The Polaroid PDC-2000 digital camera with a AZ191D die-cast high-purity magnesium case. (d) A two-piece Polaroid camera case made by the hotchamber die-casting process. Source: Courtesy of Polaroid Corporation and Chicago White Metal Casting, Inc.

7 4.1 Expandable Mold Casting Sand Casting The traditional method of casting metals is in sand molds. Most sand casting operations use silica sand (SiO2) as mold material. Sand is inexpensive and is suitable as mold material because of its high temperature characteristics and high melting point. There are two general types of sand: natural bonded (bank sand) and synthetic (lake sand). Outline of production steps in a typical sand casting operation

8 Pattern From the design, provided by an engineer or designer, a skilled pattern maker builds a pattern of the object to be produced, using wood, metal, or a plastic such as expanded polystyrene. Patterns are used to mold the sand mixture into the shape of the casting Selection of a pattern material depends on the 1. Size and shape of the casting 2. Dimensional accuracy 3. Quantity of castings required 4. Molding process

9 Types of pattern: (a) solid pattern, (b) split pattern, (c) match plate pattern, (d) cope and drag pattern

10 Sand mold Major features of molds in sand casting Flask Cope on top and a drag on the bottom Pouring basin / Pouring cup Sprue Runner system, gates Risers Cores Vents

11 Cores Cores are placed in the mold cavity to form the interior surfaces of the casting It is removed from the finished part during shakeout and further processing

12 Figure Schematic illustration of the sequence of operations for sand casting. (a) A mechanical drawing of the part is used to generate a design for the pattern. Considerations such as part shrinkage and draft must be built into the drawing. (b-c) Patterns have been mounted on plates equipped with pins for alignment. Note the presence of core prints designed to hold the core in place. (d-e) Core boxes produce core halves, which are pasted together. The cores will be used to produce the hollow area of the part shown in (a). (f) The cope half of the mold is assembled by securing the cope pattern plate to the flask with aligning pins and attaching inserts to form the sprue and risers. Continued on next slide.

13 (g) The flask is rammed with sand and rthe plate and inserts are removed. (h) The drag half is produced in a similar manner with the pattern inserted. A bottom board is placed below the drag and aligned with pins. (i) The pattern, flask, and bottom board are inverted; and the pattern is withdrawn, leaving the appropriate imprint. (j) The core is set in place within the drag cavity. (k) The mold is closed by placing the cope on top of the drag and securing the assembly with pins. The flasks the are subjected to pressure to counteract buoyant forces in the liquid, which might lift the cope. (l) After the metal solidifies, the casting is removed from the mold. (m) The sprue and risers are cut off and recycled, and the casting is cleaned, inspected, and heat treated (when necessary). Source: Courtesy of Steel Founder s Society of America.

14 Sand casting process (a) open mold, (b) close mold

15 Pouring process

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18 Sand Casting Defects There are numerous opportunities for things to go wrong in a casting operation, resulting in quality defects in the product The defects can be classified as follows: Defects common to all casting processes Defects related to sand casting process

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23 4.1.2 Shell Molding Pattern made of a ferrous metal or aluminum. Working process: - Pattern heated to a range of 175 C to 370 C. - Coated with a parting agent (such silicone). - Clamped to a dump box. - Pattern and dump box rotated to investment position. - Pattern and shell removed from dump box. - Join mold. - Pouring. - Break mold and finishing. - Inspection.

24 Shell molding can produce many types of castings with close dimensional tolerances and a good surface finish at low cost Applications include small mechanical parts requiring high precision such as gear housings Shell sand has lower permeability than sand used for green-sand molding Complex shapes can be produced with less labor since it can be automated easily

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26 (1) A match plate or cope and drag metal pattern is heated and placed over a box containing sand mixed with thermosetting resin.

27 (2) Box is inverted so that sand and resin fall onto the hot pattern, causing a layer of the mixture to partially cure on the surface to form a hard shell. (3) Box is repositioned so that loose uncured particles drop away.

28 (4) Sand and shell is heated in oven for several minutes to complete curing. (5) Shell mold is stripped from the pattern. (6) Two halves of the shell mold are assembled, supported by sand or metal shot in a box, and pouring is accomplished. (7) The finished casting with sprue removed.

29 4.1.3 Plaster Mold Casting Mold is made of plaster of paris (gypsum or calcium sulfate) with the addition of talc and silica flour to improve strength and to control the time required for the plaster to set. These component are mixed with water, and the resulting slurry is poured over the pattern. After the plaster sets (usually within 15 minutes) it is removed, and the mold is dried at a temperature range of 120 C to 260 C to remove the moisture.

30 4.1.4 Ceramic Mold Casting Similar to plaster mold process. The slurry is a mixture of fine grained zircon (ZrSiO4), aluminum oxide, and fused silica, which are mixed with bonding agents and poured over the pattern. The pattern may be made of wood or metal. After removed, mold dried, burned off to remove volatile matter and baked. Mold clamped and proceed with pouring process.

31 4.1.5 Evaporative Pattern Casting Usually pattern made from polystyrene. Pattern is coated with a water base refractory slurry, dried, and placed in flask. The flask then is filled with loose, fine sand, which surrounds and supports the pattern and may be dried or mixed with bonding agents to give it additional strength. The sand periodically is compacted, without removing the pattern. The molten metal is poured into the mold. The polystyrene immediately vaporizes and fills the mold cavity, completely replacing the space previously occupied by the polystyrene pattern.

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33 4.1.6 Investment Casting The pattern made of wax or of a plastic. Pattern is dipped into slurry of refractory material such as fine silica and binders, including water, ethyl silicate, and acids. After this initial coating has dried, the pattern is coated repeatedly to increase its thickness for better strength. When the mold is dried, heat applied at temperature 90 C 175 C in inverted position for about 12 hours to melt out the wax. Mold is then fired to 650 C 1050 C for about 4 hours drive off the moisture and to burn off any residual wax. After the metal has been poured and has solidified, the mold is broken up and the casting is removed.

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35 4.3 Permanent-Mold Casting Processes In permanent-mold casting, the mold is reused many times. Disadvantage of expandable mold processes is that a new mold is required for every casting. The basic Permanent-Mold Processes Use a metal mold constructed of two sections that are designed for easy, precise opening and closing. These molds are commonly made of steel or cast iron. Metal commonly cast in permanent molds include aluminum, magnesium, copper-base alloys, and cast iron. Cores can be used in permanent molds to form interior surfaces in the cast product. If withdrawal of a metal core would be difficult or impossible, sand cores can be used, in which case the casting process is often referred to as semi permanent-mold casting. Advantages is good surface finish and close dimensional control. Typical parts include pistons, pump bodies, and certain missiles.

36 Figure above: Steps in permanent mold casting: (1)mold is preheated and coated; (2)cores (if used) are inserted and mold is closed, (3) molten metal is poured into the mold, where it solidifies; (4) mold is opened; (5) finished part

37 4.3.1 Die casting The molten metal is injected into the mold cavity under high pressure. The pressure is maintained during solidification, after which the mold is opened and the part is removed. Modern die-casting machines are designed to hold and accurately close the two halves of the mold, and keep them closed while the liquid metal is forced into the cavity. There are two main types of die-casting machines; (1) hotchamber and (2) cold-chamber, differentiated by how the molten metal is injected into the cavity. Figure above : General configuration of a (cold-chamber) and (hot chamber) die-casting machines

38 In hot-chamber machines, the metal is melted in a container attached to the machine, and a piston is used to inject the liquid metal under high pressure into the die. Typical injection pressure are 7 to 35 MPa and production rates up to 500 parts per hour. Limited to applications to low-melting point metals that do not chemically attack the plunger and other mechanical components. The metals include zinc, tin, lead, and sometimes magnesium. In cold-chamber machines, molten metal is poured into an unheated chamber from an external melting container, and a piston is used to inject the metal under high pressure into die cavity. Injection pressure are 14 to 140 MPa and production rate are not usually as fast because of the need to ladle the liquid metal into the chamber from an external source. The metals include aluminum, brass, and magnesium alloys. Low-melting point alloys can also be cast on cold-chamber machines.

39 Figure above: Cycle in hot-chamber casting: (1) with die closed and plunger withdrawn, molten metal flows into the chamber; (2) plunger forces metal in chamber to flow into die, maintaining pressure during cooling and solidification; and (3) plunger is withdrawn, die is opened, solidified part is ejected; and (5) finished part

40 Figure above : Cycle in cold-chamber casting: (1) with die closed and ram withdrawn, molten metal is poured into the chamber; (2) ram forces metal to flow into die, maintaining pressure during cooling and solidification; and (3) ram is withdrawn, die is opened, and part is ejected

41 Molds use in die-casting operations are usually made of tool steel, mold steel, maraging steel, tungsten and molybdenum with good refractory are also being used. Ejector pins are required to remove the part from the die when it opens. Venting holes and passageways must be built into the dies at the parting line to evacuate the air and gases in the cavity. Advantages of die-casting include: ~ high production rate ~ economical for large production quantities ~ close tolerance for small parts ~ good surface finish ~ rapid cooling provides small grain size and good strength to the casting

42 4.3.2 Centrifugal Casting Refer to several casting methods in which the mold is rotated at high speed so that centrifugal force distributes the molten metal to the outer regions of the die cavity. The group includes (1) true centrifugal casting, (2) semicentrifugal casting, and (3) centrifuge casting. True centrifugal casting, molten metal is poured into a rotating mold to produce a tubular part. Typical of parts is made like pipes, tubes, bushings, and rings. Orientation of the axis of mold rotation can be either horizontal or vertical.

43 Figure above: Setup for true centrifugal casting Figure above: Semi-centrifugal casting

44 Semi-centrifugal casting, centrifugal force is used to produce solid casting, rather than tubular parts. The rotation speed in semi-centrifugal casting is usually set so that G-factors of around 15, and the molds are designed with risers at the center to supply feed metal. Typical parts is made like wheels and pulleys. Centrifuge casting, the mold is designed with part cavities located away from the axis of rotation, so that the molten metal poured into the mold is distributed to these cavities by centrifugal force. The process is used to smaller parts. Figure above: (a) centrifuge casting centrifugal force causes metal to flow to the mold cavities away from the axis of rotation; and (b) the casting part

45 4.3.3 Vacuum Casting Suitable particularly for thin walled (0.75mm) complex shape with uniform properties. The mold immersed partially into molten metal and held in an induction furnace. The vacuum reduces the air pressure inside the mold to about two third of atmospheric pressure, thus drawing the molten metal into the mold cavities through a gate in the bottom of the mold. The metal in furnace is at a temperature of usually 55 C above the liquidus temperature of the alloy. Consequently, it begins to solidify within a very short time. After the mold is filled, it is withdrawn from the molten metal.

46 4.3.4 Low Pressure Casting Liquid metal is forced into the cavity under low pressure approximately 0.1 MPa from beneath so that the flow is upward. The molten metal is forced upward by gas/air pressure into a graphite or metal mold. The pressure is maintained until the metal has solidified completely in the mold. Figure above: Low-pressure casting. The diagrams shows how air pressure is used to force the molten metal in the ladle upward into the mold cavity. Pressure is maintained until the casting has solidified

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