METAL CASTING PROCESS

Transcription

1 METAL CASTING PROCESS 1. Permanent Pattern a. Sand casting b. Shell molding 2. Expandable pattern. a. Investment Casting b. Expanded polystyrene process (full-mold) c. Permanent mold casting process 3. Variation of permanent mold casting Figure 13.2 Steps in the production sequence in sand casting. The steps include not only the casting operation but also pattern making and mold making. Figure 13.3 Types of Pattern used in sand casting a) Solid Pattern b) Split Pattern c) Macth-Plate Pattern d) Cope and Drag Pattern Figure 13.4 a) Core held in place in the mold cavity by chaplets. b) Possible chaplet design c) Casting with internal cavity

2 Figure V8 engine block (Bottom Center) and the five drysand cores that are used in its construction. (Courtesy of Central Motors Corporation) Figure Four Methods of making a hole in a cast pulley Figure (Top) Typical Chaplets. (Bottom) Method of supporting cores by use of chaplets (relative size of the chaplets is exaggerated)

3 Figure 13.5 Steps in shell molding: 1) A match-plate or cope and drag metal pattern is heated and placed over a box containing sand mixed with thermosetting resin; 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) 4) 5) 6) Box is repositioned so that loose uncured particles drop away; Sand shell is heated in oven for several minutes to complete curing; Shell mold is stripped from the pattern; Two halves of the shell molds are assembled, supported by sand or metal shot in a box, and pouring is accomplished. 7) The finished casting with sprue removed is shown in 7 Figure (Top) Two halves of a shell-mold pattern. (Bottom) The two shells before clamping and the final shell-mold casting.

4 Figure 13.6 Steps in vacuum molding: 1) A thin sheet of preheated plastic is drawn over a matchplate or cope and drag pattern by vacuum; the pattern has small vent holes to facilitate vacuum forming; 2) A specially designed flask is placed over the pattern plate and filled with sand and sprue and pouring cup are formed in the sand. 3) Another thin plastics sheet placed over the flask and vacuum is drawn, which causes the sand grains to be held together forming a rigid mold 4) The vacuum on the mold pattern is released to permit to pattern to be stripped from the mold; 5) This mold is assembled with its matching half the form the cope and drag and with vacuum maintained a both halves, pouring is accomplished. The plastic sheet quickly burns away on contacting the molten metal. After solidification nearly all the sand can be recovered for reuse Figure 13.7 Expanded polystyrene casting process: 1) Pattern of polystyrene is coated with refractory compound; 2) Foam pattern is placed in mold box and sand is compacted around the pattern; 3) Mold metal is poured in the portion of the pattern that forms the poring cup and sprue. As the metal enters the mold, the polystyrene foam is vaporized ahead of the advancing liquid, thus allowing the resulting mold cavity to be filled

5 Figure 13.6 Steps in investment casting: 1) Wax pattern are produced 2) Several pattern are attached to a sprue to form a pattern tree; 3) The pattern tree is coated with a thin layer of refractory material; 4) The full mold is formed by covering the coated tree with sufficient refractory material; 5) The mold is held in an inverted position and heated to met the wax and permit it to drip out of the cavity; 6) The mold is preheated to high temperature, which Ensures that all contaminants are eliminated from the mold; it also permits the liquid metal to flow more easily into the detailed cavity; the molten metal is poured; it solidifies and 7) The mold is broken away from the finished casting. Parts are separated from the sprue Figure 13.6 Steps in Permanent Mold Casting: 1) Mold is preheated and coated; 2) Core (if used) are inserted and mold is closed; 3) Molten metal is poured into the mold; 4) Mold is opened. Finished parts is shown in 5

6 Figure Cycle in Hot Chamber Casting: 1) With die closed and plunger withdrawn, molten metals 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 and solidified part is ejected. Finished part is shown in 4.

7 Figure General configuration of a (cold-chamber) die-casting machine. Figure Low Pressure Casting. The Diagram shows how air pressure is used to force the molten metal in the ladle upward into the mold cavity. Pressure is maintained until Casting has solidifies. Figure Cycle in Cold Chamber Casting: 1) With die closed and ram withdrawn, molten metal is poured into chamber; 2) Ram forces to metal to flow into die, maintaining pressure during and solidification; 3) Ram is withdrawn, die is opened and part is ejected. (Gating system is simplified)

8 STING QUALITY There are numerous opportunities for things to go wrong in a casting operation, resulting in quality defects in the casting. In this Section we compile a list of the common defects that occur in casting and we indicate the inspection procedures to detect them. Casting Defects: Some defects are common to any and all process. These defects are illustrated in figure and briefly described in the following: a) Misruns: A Misruns is a casting that has solidified before completely filling the mold cavity. Typical causes include 1) Fluidity of the molten metal is insufficient, 2) Pouring Temperature is too low, 3) Pouring is done too slowly and/or 4) Cross section of the mold cavity is too thin. b) Cold Shut: A cold shut occurs when two portion of the metal flow together, but there is lack of fusion between them due to premature freezing, Its causes are similar to those of a Misruns. Figure Some common defects in castings: a) Misruns b) Cold Shut c) Cold Shot d) Shrinkage Cavity e) Microporosity f) Hot Tearing c) Cold Shots: When splattering occurs during pouring, solid globules of the metal are formed that become entrapped in the casting. Poring procedures and gating system designs that avoid splattering can prevent these defects.

9 d) Shrinkage Cavity: This defects is a depression in the surface or an internal void in the casting caused by solidification shrinkage that restricts the amount of the molten metal available in the last region to freeze. It often occurs near the top of the casting in which case it is referred to as a pipe (Figure 12.7). The problem can often be solved by proper riser design. e) Microporosity: This refers to a network of a small voids distributed throughout the casting caused by localized solidification shrinkage of the final molten metal in the dendritic structure. The defect is usually associated with alloys, because of the protracted manner in which freezing occurs in these metals. f) Hot Tearing: This defect, also called hot cracking, occurs when the casting is restrained or early stages of cooling after solidification. The defect is manifested as a separation of the metal (hence the terms tearing or cracking) at a point of high tensile stress caused by metal s inability to shrink naturally. In sand casting and other expandable mold processes, compounding the mold to be collapsible prevents it. In permanent mold processes, removing the part from the mold immediately after freezing reduces hot tearing. Some defects are related to the use of sand molds and therefore they occur only in sand castings. To a lesser degree, other expandable mold processes are also susceptible to these problems. Defects found primarily in sand castings are shown in figure13.23 and describe here: a) Sand Blow: This defect consists of a balloon-shaped gas cavity caused by release of mold gases during pouring. It occurs at or below the casting surface near the top of the casting. Low permeability, poor venting and high moisture content of the sand mold are the usual causes.

10 b) Pinholes: A defect similar to a sand blow involves the formation of many small gas cavities at or slightly below the surface of the casting. c) Sand Wash: A wash is an irregularity in the surface of the casting that results from erosion of the sand mold during pouring. The contour of the erosion is imprinted into surface of the final cast part. d) Scabs: This is a rough area of the casting due to encrustations of sand and metal. It is caused by portions of the mold surface flaking off during solidification and becoming embedded in the casting surface. e) Penetration: When the fluidity of the liquid metal is high, it may penetrate into the sand mold or sand core after freezing, the surface of the casting consists of a mixture of sand grins and metal. Harder packing of the sand molds helps to alleviate this condition. f) Mold Shift: This is manifested as a step in the cast product at the parting line caused by sidewise displacement of the cope with respect to the drag. g) Core Shift: A similar movement can happen with the core but the displacement is usually vertical. Core shift and mold shift are caused by buoyancy of the molten metal. (Figure ) h) Mold Crack: If mold strength is insufficient a crack may develop in to which liquid metal can seep to form a fin on the final casting. Inspection Methods: Foundry inspection procedures include; a. Visual Inspection to detect obvious defects, such as Misruns, cold shut and severe surface flaws; b. Dimensional measurements to ensure that tolerances have been met; c. Metallurgical, chemical, physical and other tests concerned with the inherent quality of the cast metal. Tests in category 3 include 1) Pressure testing to locate leaks in the casting 2) Radiographic methods, magnetic particle tests, the use of fluorescent penetrants and supersonic testing to detect either surface or internal defects in the casting; 3) Mechanical testing to determine properties such as tensile strength and hardness. If defects are discovered but are not too serious, it is often possible to save the casting by welding, grinding or other salvage methods to which the customer has agreed.