FOR STUDENT REFERENCE METAL CASTING COURSE DJD 52023 MANUFACTURING PROCESS SUYANI ARIFIN, SARAH NADIAH MOHD GHAZALI, ANIZA MD LATIFF
For Student Reference METAL CASTING Course DJD 52023 Manufacturing Process
Suyani Arifin Sarah Nadiah Mohd Ghazali Aniza Md Latiff … All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, electronic, mechanical, photocopying, recording or otherwise, without the prior permission of the Publisher. © Politeknik Muadzam Shah Publisher Politeknik Muadzam Shah, Lebuhraya Tun Razak, 26700 Muadzam Shah, Pahang. 2022
i Abstract The first edition of Metal Casting is written for polytechnic students’ as a reference in their course study. The contents in this ebook are relevant to the course Manufacturing Process taken by semester 5 students in Department of Mechanical Engineering, Politeknik Muadzam Shah. It’s also can be used as a reference book for all undergraduate students in mechanical engineering. This book consist of 10 topics as a content of Metal Casting that consist of Fundamental Of Metal Casting, Patterns, Sand Casting and other type of metal casting. Hopefully the students get benefits from this Metal Casting ebook and succeed in their life.
ii ACKNOWLEDGEMENT First and foremost, thanks to Allah S.W.T for the guidance and mercy. The completion of this Metal Casting ebook could not have been possible without the support and encouragement from colleagues. Thank you to colleagues in developing the ebook and people who willing to help out with their abilities. Its has been a great honour and privilege to undergo this process of creating ebook that help gaining experience as a educator to share knowledge. Thank you very much.
TABLE OF CONTENTS ABSRTACT i ACKNOWLEDGMENT ii 1.0 FUNDAMENTAL OF METAL CASTING 1 1.1 Categories in Metal Casting 2 2.0 PATTERNS 4 2.1 Type Of Pattern 4 3.0 SAND CASTING 7 3.1 Pouring process 8 3.2 Possible Defect for Sand Casting 11 3.3 Design Rules for Sand Casting 12 4.0 SHELL MOULD CASTING 14 4.1 Shell Mould Casting Process 14 4.2 Advantages & Disadvantages 16 5.0 LOST-FOAM /EXPENDABLE PATTERN CASTING 17 6.0 PLASTER-MOLD CASTING 18 6.1 Features of Plaster Mold Casting 18 6.2 Application of Plaster Mold Casting 19 7.0 CERAMIC MOLD CASTING 21 7.1 Features Of Ceramic Mold Casting 22 7.2 Application of Ceramic Mold Casting 22 8.0 INVESTMENT CASTING 25 8.1 Step Investment Casting 25 8.3 Application of Investment Casting 29
9.0 DIE CASTING 30 9.1 Types of Die Casting Process 31 a. Hot chamber die casting 31 b. Cold Chamber die casting 33 c. Low pressure die casting process 36 d. Vacuum die casting process 36 e. Squeeze die casting process 37 f. Semi solid die casting process 38 9.2 Die Casting Advantages 38 10.0 CENTRIFUGAL CASTING 39 10.1 Centrifugal Casting Parts 39 10.2 Centrifugal Casting Process 40 10.3 Centrifugal Casting Application 40 EXERCISE 42 REFERENCES 43
1 1.0 FUNDAMENTAL OF METAL CASTING Metal casting is a process basically pouring molten metal into mould that contains a hollow cavity of a desired geometrical shape and allowed to cool down to form a solidified part. Casting first used around 4000 B.C. to make ornaments, copper arrowheads and various other objects. This process basically used to make complex or large part which is difficult and expensive to manufacture by using other process. Metal casting are most often selected process in manufacturing industries because Casting can produce complex shapes and with internal cavities or hollow selection Very large parts can be produced in one piece Casting can utilize materials that are difficult or un economical to process by other means The casting process is competitive with other manufacturing process Understanding of the fundamentals is essential for the production of good quality and economical castings and for establishing proper techniques for mold design and casting practice. Important considerations in casting operation: Flow of the molten metal into the mold cavity Solidification and cooling of the metal in the mold Influence of the type of mold material Table 1.0: Flow of the molten metal into mould cavity CHARACTERISTIC INFLUECE TO THE FLOW OF THE MOLTEN METAL VISCOSITY Sensitive to temperature increase, fluidity decrease SURFACE TENSION High surface tension of liquid metal reduce fluidity INCLUSION insoluble MOLD DESIGN Design and dimensions of the sprue, runners and riser DEGREE OF SUPERHEAT Increment of temperature alloy above melting point improve fluidity by delay solidification
2 SOLIDIFICATION PATTERN OF ALLOY fluidity is inversely proportional to the freezing range MOLD MATERIAL AND SURFACE CHARACTERISTIC The higher thermal conductivity of the mold and the rougher its surface, the lower fluidity of the molten metal RATE OF POURING The slower the rate of pouring molten metal into the mold, the lower fluidity because of the higher rate of cooling when poured slowly HEAT TRANSFER This factor directly affects the viscosity of liquid metal 1.1 Categories in Metal Casting Figure 1.0: Major Categories in Metal Casting METAL CASTING EXPANDABLE MOLD SAND SHELL MOLD PLASTER CERAMIC INVESTMENT PERMANENT MOLD SLUSH PRESSURE DIE CENTRIFUGAL SEMISOLID COMPOSITE MOLD SINGLE CRYSTAL FOR MICROELETRONIC SINGLE CRYSTAL FOR TURBINE BLADES DIRECTIONAL SOLIDIFICATION
3 • Typically are made of sand, plaster, ceramics and similar materials and generally are mixed with various binders (bonding agents) for improved properties • Consist 90% sand, 7% clay and 3% water •After casting solidified, the mold is broken up to remove casting Expandable Mold • Made of metals that maintain their strength at high temperature • Casting can be removed easily and the mold can use for the next casting • Metal molds are better heat conductors than expendable nonmetallics molds Permanent Mold - Made of two or more different materials (sand, graphite and metal) - These mold have a permanent and expandable portion are used in various casting process to improve mold strength, control the cooling rates and optimized the overall economics of casting process Composite Mold
4 2.0 PATTERNS The main tooling for sand casting is the pattern that is used to create the mold cavity. The pattern is a full-size model of the part that makes an impression in the sand mold. However, some internal surfaces may not be included in the pattern, as they will be created by separate cores. The pattern is actually made to be slightly larger than the part because the casting will shrink inside the mold cavity. Also, several identical patterns may be used to create multiple impressions in the sand mold, thus creating multiple cavities that will produce as many parts in one casting. 2.1 Type Of Pattern A pattern for a part can be made many different ways, which are classified into the following four types: 1. Solid Pattern / One-Piece Pattern 2. Split Pattern 3. Match-Plate Pattern 4. Cope And Drag Pattern
5 A solid pattern is a model of the part as a single piece. It is the easiest to fabricate, but can cause some difficulties in making the mold. The parting line and runner system must be determined separately. Solid patterns are typically used for geometrically simple parts that are produced in low quantities. A split pattern models the part as two separate pieces that meet along the parting line of the mold. Using two separate pieces allows the mold cavities in the cope and drag to be made separately and the parting line is already determined. Split patterns are typically used for parts that are geometrically complex and are produced in moderate quantities. SOLID PATTERN / ONE-PIECE PATTERN SPLIT PATTERN
6 A match-plate pattern is similar to a split pattern, except that each half of the pattern is attached to opposite sides of a single plate. The plate is usually made from wood or metal. This pattern design ensures proper alignment of the mold cavities in the cope and drag and the runner system can be included on the match plate. Match-plate patterns are used for larger production quantities and are often used when the process is automated. A cope and drag pattern is similar to a match plate pattern, except that each half of the pattern is attached to a separate plate and the mold halves are made independently. Just as with a match plate pattern, the plates ensure proper alignment of the mold cavities in the cope and drag and the runner system can be included on the plates. Cope and drag patterns are often desirable for larger castings, where a matchplate pattern would be too heavy and cumbersome. They are also used for larger production quantities and are often used when the process is automated. MATCH-PLATE PATTERN COPE AND DRAG PATTERN
7 3.0 SAND CASTING The sand casting process involves the use of a furnace, metal, pattern, and sand mold (sand, water, clay and additive). Sand casting is used to make large parts using Iron, but also Bronze, Brass, Aluminum. Castings produced using sand mould is known to have peculiar microstructures depending on average size, distribution and shape of the moulding sand grains and the chemical composition of the alloy where a study by Wasiu (2012) and state that these affect the surface finish, permeability and refractoriness of all the castings. The sand mixture for sand casting is the ratio between sand, clay and water. The ratio between sand, clay and water is important because it affects the mechanical properties of a material. Percentage of mold sand mixture is silica sand 100%, bentonic 4 - 5%, water 2%. Some smaller sand cast parts include components as gears, pulleys, crankshafts, connecting rods, and propellers. Sand casting is also common in producing automobile components, such as engine blocks, engine manifolds, cylinder heads, and transmission cases. Molten metal is poured into a mold cavity formed out of sand (natural or synthetic). The processes of sand casting are discussed in this section, include patterns, sprues and runners, design considerations, and casting allowance. Figure 3.0 show the mould section and casting nomenclature. Figure 3.0: Mould Section and casting nomenclature (Dr. A. K. Sharma, Dr. Pradeep Kumar, 2018)
8 3.1 Pouring process During the pouring process, the metal is got up to the required melting temperature in furnace before being poured into the mould. This temperature is dependent on the select of metal and on the required density of the casting. Figure 3.1 showed complete pouring process. Figure 3.2 showed mould section and figure 3.3 showed gating system in sand casting. Figure 3.1: Pouring process complete Figure 3.2: Mould Section for Pouring process
9 Figure 3.3: Gating System This is the procedure for pouring process. 1. FLASK which supports the mold itself. Two-piece molds consist of a cope on top and drag on the bottom. The seam between them is the PARTING LINE. When more than two pieces are used in sand mold, the additional parts called CHEEKS. 2. A pouring basin or POURING CUP, into which the molten metal is poured. 3. A SPRUE, through which molten metal flows downward. 4. The RUNNER system, which has the channels that carry the molten metal form the sprue to the mold cavity. 5. GATES are the inlets into the mold cavity. 6. RISERS which supply additional molten metal to the casting as it shrinks during solidification. There are 2 types, blind riser and open riser. 7. CORES which are insert made from sand. There are placed in the mold to form hollow regions or otherwise define the interior surface of the casting. Cores also use on the outside of the casting to form features such as lettering on the surface of a casting or deep external pocket.
10 8. VENTS, which are placed in mold to carry off gases produced when the molten metal comes into contact with the sand in the mold and the core. Vents also exhaust air from the mold cavity as the molten metal flows into mold. Sand Casting Process Sand casting is a casting-based manufacturing process that involves the use of a sand mold. This is the six primary steps of sand casting. Figure 3.4 showed picture six primary step. 1. Placing a pattern (having the shape of desired casting) in sand to make an imprint 2. Incorporating a gating system 3. Removing the pattern and filling the mold cavity with molten metal 4. Allowing the metal to cool until it solidifies 5. Breaking away the sand mold 6. Removing the casting Figure 3.4: Procedure Casting Process
11 3.2 Possible Defect for Sand Casting A metal casting defect is an imperfect casting condition compared to the requirements that must be repaired, removed or rejected. Some sand casting defects are small which can be adopted; some can be repaired and machined easily. There are so many that there is no other way but to reject. Table 3.0 showed defect and causes from sand casting. Table 3.0: Defect and causes from sand casting Defect Causes Unfilled Section • Insufficient materials • Low pouring temperature Porosity • Melt temperature is too high • Non –uniform cooling rate • Sand has low permeability Hot Tearing • Non –uniform cooling rate Surface projections • Erosion of sand mold interior • A crack in the sand mold • Mold halves shif BUCKLES • Due to a weak wet layer in the mold, the sand can buckle and form a deep groove on the casting surface. BURN-ON—This defect usually can be removed during shot blasting. • Clay-bonded sand • Lustrous carbon content too low • Proportion of low-melting-point substances too high Moulding plant • Uneven mould compaction Gating and pouring practice • Uneven distribution of inflowing metal with resultant over-heating • •Temperature of liquid metal too high BLOWS • A gas defect, it is denoted by large voids in the casting due to entrapped,soluble or reactive gas. PINHOLES • A gas defect, it is denoted by numerous
12 small holes in the casting due to entrapped, soluble or reactive gas. STICKERS • This defect results from the molding sand sticking to the pattern as it is drawn from the mold. EROSION • This defect is excess metal on the casting surface at places where high metal velocity exists, such as in front of a gate. RUN OUT • This defect occurs when metal leaks out of the mold through the parting line. 3.3 Design Rules for Sand Casting Maximum wall thickness Decrease the maximum wall thickness of a part to shorten the cycle time (cooling time specifically) and reduce the part volume. Figure 3.5 showed correct and incorrect wall thickness. Uniform wall thickness will ensure uniform cooling and reduce defects. A thick section, often referred to as a hot spot, causes uneven cooling and can result in shrinkage, porosity, or cracking. Figure 3.6 showed uniform and non – uniform wall thickness. Figure 3.5: Maximum wall thickness by part Figure 3.6: Maximum wall thickness for non uniform and uniform
13 Corners Round corners to reduce stress concentration and fracture. Inner radius should be at least the thickness of the walls. Figure 3.7 showed correct and incorrect corner Figure 3.7: correct and incorrect corner Draft Apply a draft angle of 2˚ - 3˚ to all walls parallel to the parting direction to facilitate removing the part from the mold. Figure 3.8 showed correct and incorrect draft angle. Machining allowance add 0.0625 – 0.25 in (0.16 – 0.64 mm) to part dimensions to allow for machining to obtain a smooth surface. Figure 3.8: correct and incorrect draft angle
14 4.0 SHELL MOULD CASTING Shell mold casting is a metal casting process similar to sand casting, in that molten metal is poured into an expendable mold. However, in shell mold casting, the mold is a thin-walled shell created from applying a sand-resin mixture around a pattern. The pattern, a metal piece in the shape of the desired part, is reused to form multiple shell molds. A reusable pattern allows for higher production rates, while the disposable molds enable complex geometries to be cast. Shell mold casting requires the use of a metal pattern, oven, sand-resin mixture, dump box, and molten metal. Shell mold casting allows the use of both ferrous and non-ferrous metals, most commonly using cast iron, carbon steel, alloy steel, stainless steel, aluminum alloys, and copper alloys. Typical parts are small-to-medium in size and require high accuracy, such as gear housings, cylinder heads, connecting rods, and lever arms. Figure 4.0 showed shell mould casting. Figure 4.0: Shell Mould Casting 4.1 Shell Mould Casting Process The shell moulding process is a precision sand casting process capable of producing castings with a greater surface finish and better dimensional accuracy than conventional sand castings. These qualities of precision can be found in a wider range of alloys and with greater flexibility in design than die-casting and at a lower cost than investment casting. The fundamental feature of the process is the use of fine-grained, high purity sand that contributes the attributes of a lower characteristic for surface roughness and dimensional accuracy to moulds cores and castings. In conventional sand moulding the use of such fine sand is precluded because it would dramatically reduce mould permeability (ATHI, 2022). Figure 4.1 showed step for shell casting.
15 1. Pattern creation - A two-piece metal pattern is created in the shape of the desired part, typically from iron or steel. Other materials are sometimes used, such as aluminum for low volume production or graphite for casting reactive materials. 2. Mold creation - First, each pattern half is heated to 175-370°C (350-700°F) and coated with a lubricant to facilitate removal. Next, the heated pattern is clamped to a dump box, which contains a mixture of sand and a resin binder. The dump box is inverted, allowing this sand-resin mixture to coat the pattern. The heated pattern partially cures the mixture, which now forms a shell around the pattern. Each pattern half and surrounding shell is cured to completion in an oven and then the shell is ejected from the pattern. 3. Mold assembly - The two shell halves are joined together and securely clamped to form the complete shell mold. If any cores are required, they are inserted prior to closing the mold. The shell mold is then placed into a flask and supported by a backing material. 4. Pouring - The mold is securely clamped together while the molten metal is poured from a ladle into the gating system and fills the mold cavity. 5. Cooling - After the mold has been filled, the molten metal is allowed to cool and solidify into the shape of the final casting. 6. Casting removal - After the molten metal has cooled, the mold can be broken and the casting removed. Trimming and cleaning processes are required to remove any excess metal from the feed system and any sand from the mold. Figure 4.1: The process of creating a shell mould (ATHI, 2022)
16 Figure 4.2: The process of creating a shell mold (ATHI, 2022) 4.2 Advantages & Disadvantages There are advantages as well as disadvantages in every casting process. Likewise for shell mould casting. The table 4.0 shows the advantages and disadvantages of shell mould casting. Table 4.0: Advantage and disadvantages for Shell Mould Casting Advantage Disadvantages Can form complex High equipment cost shapes and fine details Very good surface finish High production rate Low labor cost Low tooling cost Little scrap generated
17 5.0 LOST-FOAM /EXPENDABLE PATTERN CASTING Lost-foam casting (LFC) is a type of evaporative-pattern casting process that is similar to investment casting except foam is used for the pattern instead of wax. This process takes advantage of the low boiling point of foam to simplify the investment casting process by removing the need to melt the wax out of the mold. With LFC, the foam pattern is molded from polystyrene beads. LFC is differentiated from full mold by the use of unbounded sand (LFC) as opposed to bonded sand (full mold process). Here are the 9 step procedures for lost- foam casting and shown in figure 5.0. 1. Mold foam pattern sections. 2. Age pattern to allow dimensional shrinkage. 3. Assemble pattern if it is a multiple piece pattern. 4. Build cluster (multiple patterns per cluster). 5. Coat cluster. 6. Dry coating. 7. Compact cluster in flask. 8. Pour metal. 9. Extract cluster from flask. Figure 5.0: Step process for Lost-foam casting. (Chen, 2015)
18 6.0 PLASTER-MOLD CASTING In plaster mold casting, a plaster, usually gypsum or calcium sulfate, is mixed with talc, sand, asbestos, and sodium silicate and water to form a slurry. This slurry is sprayed on the polished surfaces of the pattern halves (usually brass). The slurry sets in less than 15 minutes to form the mold. The mold halves are extracted carefully from the pattern, and then dried in an oven. The mold halves are carefully assembled, along with the cores. The molten metal is poured in the molds. After the metals cools down, the plaster is broken and the cores washed out. Parts cast are usually small to medium size, ranging in weight from 30 g (1 oz) to 7 kg (15 lb). The section thickness can be as small as 0.6 mm (0.025 in) and tolerances are 0.2 % linear. The draft allowance is 0.5-1.0 degree. The surface finish is 1.25 µm to 3 µm (50 µin to 125 µin) rms. Low temperature melting materials such as aluminum, copper, magnesium and zinc can be cast using this process. This process is used to make quick prototype parts as well as limited production parts. Figure 6.0 showed mold and product for Plaster Mold Casting. Figure 6.0: Plaster Mold Casting (Su-Jin Kim, 2012) 6.1 Features of Plaster Mold Casting Casting size: small to medium ranging in weight from 30 g (1 oz) to 7 kg (15 lb) Section thickness: can be as small as 0.6 mm (0.025 in) . Tolerance:0.2 % linear. Draft allowance is 0.5-1.0 degree. Surface finish:1.25 µm to 3 µm (50 µin to 125 µin) rms.
19 6.2 Application of Plaster Mold Casting Materials with low melting points such as aluminum, copper, magnesium and zinc can be cast by this process. Can be used for making quick prototype parts as well as limited production parts. Plaster mold casting is just like the sand casting, the difference is that the plaster of Paris is used instead of sand in mold making. In plaster mold casting, plaster of pairs is mixed with a fixed quantity of water to make a mixture which became rock solid in few minutes. Steps involves and figure 6.1 showed in Plaster mold casting procedure. 1. The first step is the making of the mixture. Plaster of paris is mixed with water or some other binder to make a thick mixture which become solid in few minutes 2. Pattern is made of wood usually and sprayed with the thin layer of parting component in order to prevent the sticking of mixture with pattern 3. Mixture is then poured onto the pattern and make sure that the pattern is completely covered with mixture. 4. After the plaster is set. Patter is removed from the solid plaster 5. Mold is not yet perfectly dry that’s why it is heated at 120 C to remove the excess water 6. All the pieces of the mold are assembled and preheated and then molten metal is poured in the mold 7. After the metal solidifies mold is broken to remove the parts Figure 6.1: Step for Plaster Mold Casting Process
20 Advantage of Plaster Mold Casting Process Can produce complex shapes. Thin section casting. Exact copying of pattern detail. Dimensionally accurate castings. Good surface finish. Minimum residual stresses and distortion in castings. Limitations of Plaster Mold Casting Process Poor productivity as it has lengthy processing problems.Close monitoring of the production process.The problems of poor mold permeability. Impaired mechanical properties a possibility as a result of slow cooling of the casting.
21 7.0 CERAMIC MOLD CASTING Similar to plaster mold casting, the pattern used in ceramic mold casting is made of plaster, plastic, wood, metal or rubber. A slurry of ceramic is poured over the pattern. It hardens rapidly to the consistency of rubber. This can be peeled of the pattern, reassembled as a mold. The volatiles are removed using a flame torch or in a low temperature oven. It is then baked in a furnace at about 1000 °C (1832 °F) yielding a ceramic mold, capable of high temperature pours. Additionally, the pour can take place while the mold is until hot. This process is expensive, but can eliminate secondary machining operations. Typical parts made from this process include impellers made from stainless steel, bronze, complex cutting tools, plastic mold tooling. Step for Ceramic Mold Casting The concept of creating ceramic mold start with a precision pattern that creates the mold, which is covered with a ceramic slurry as in Figure 7.0. The mold is dried and baked. The molten metal is then transferred into the mold and continue to solidify (K.G. Swift, 2013). Figure 7.0: Process Ceramic Mold Casting. (K.G. Swift, 2013) 1 2 4 5 6 7 8 9
22 7.1 Features Of Ceramic Mold Casting Tolerances can be held to 0.4 %, surface finishes can be better than 2 - 4 µm (.075 - .15 µin). Add 0.3 mm (.012 in) for parting line tolerances. Wall thickness can be as small as 1.25 mm (.050 in), and the weights can range from 60 g (2oz) to a ton. Draft allowance of 1° is recommended. Figure 7.1: Ceramic mold (Liaoning Borui Machinery, 2022) Ceramic molds are naturally used in near-net shape investment casting processes of superalloy components because of their chemical inertness and high-temperature capabilities (Janos E.Kanyo, 2020). 7.2 Application of Ceramic Mold Casting Ceramic mold casting is very similar to sand or plaster mould casting, but the mold is made of refractory ceramic material. It is used to cast materials such as cast steels, cast irons and other high-temperature alloys. The ceramic material can endure high temperatures compared to the plaster mold (The Open University, 2017).
23 Using stainless steel and bronze, ceramic mold casting is best suited for casting a wide variety of products ranging from house hold goods to industrial tools. Some of casted products are kitchenware like kettles, industrial products like impellers, complex cutting tools, plastic mold tooling etc. All types of dies and molds for other casting and forming processes. Cutting tool blanks.Components for food handling machining, Pump impellers, Aerospace and atomic reactor components (K.G. Swift, 2013). Figure 7.2: Cavity drying for large casting part (Hebei Lanmin Trading, 2022). Advantage of Ceramic Mold Casting Ceramic hollow thin-walled shell has high strength, excessive refractoriness, good air absorbency, smooth inner wall and precise size. It can be used to create all kinds of large highend precision castings, such as: stainless steel and alloy steel castings with complex structure, castings with high internal and external quality requirements, large castings weighing dozens of kilograms to several tons, and several castings can be cast as a whole (Hebei Lanmin Trading, 2022).
24 High temperature pours possible therefore suitable for steels and other alloys. Creative complex designs can be made. Can be used for mass production. Casting with accurate dimensional accuracy possible. Little machining is required therefore difficult-to machine alloys can be cast. Supports both industry and home foundry operations. Complicate and innovative designs can be casted. Figure 7.3: EPS white mold, cavity and casting (Hebei Lanmin Trading, 2022)
25 8.0 INVESTMENT CASTING This process is commonly known as the lost wax casting process. It got its name because of the fact that ancient Egyptians used it to make gold jewellery hence the name Investment. Very intricate shapes with high accuracy can be made in this process. Additionally, metals which are hard to machine or fabricate can be cast with this process. Parts that cannot be produced otherwise by normal manufacturing processes like turbine blades with complex shapes, or airplane parts that needs to withstand high temperatures are examples of this process. 8.1 Step Investment Casting The process works first a mold is made by making a pattern. Wax or some other materials can be used that can be melted away. The wax pattern is dipped in refractory slurry, which coats the wax pattern and a skin is subsequently formed. It is then dried. Figure 8.0: Graphic representation of investment casting (PPCP, 2022)
26 The process used in the investment casting foundry is mainly lost wax casting. Thus, investment casting is also named lost wax investment casting, which contains pressing wax, repairing wax, assembling trees, dipping slurry, melting wax, casting molten metal post-processing. It includes coating a wax pattern with a liquid material. It is call investment casting because the mold used in the casting process will be set by the liquid refractory material (Susie, 2020). Wax Pattern The wax pattern can be made by a duplicating process using a stereo lithography or a similar model that has been fabricated by a computerized solid model master. Make a pattern by using wax or some other materials can be used that can be melted away. Make wax pattern tree for investment casting by assemble more than 1 single pattern. Figure 8.1: Creating wax pattern (Susie, 2020) The wax pattern assemble at a wax sprue for mass production. Using wax sprue, these investment casting manufacturers will have a combination of many wax patterns, also known as wax tree which will later provide voids for molten alloy, and the sprue also creates a flow path for the liquid molten alloy into the void (Susie, 2020). Figure 8.2: Wax tree assembly ( Niagara Investment Castings, 2021)
27 Slurry Coating The wax pattern is dipped in refractory slurry, which coats the wax pattern and a skin is subsequently formed. It is then dried. The process of dipping in the slurry and drying is continued till a firm thickness is achieved. The pattern is placed in an oven and the wax melted. This leads to a mold which can be easily filled with the molten metal. Slurry is a mixture of plaster of Paris, a binder and, a refractory material. Powdered silica is used for low temperature melts. For higher temperature melts, an alumina-silicate is used as the refractory material. While silica is used as a binder. Additional coatings of sillimanite and ethyl silicate may be applied to increase the quality of the finished product. Figure 8.3: Mixing of Slurry (The Open University, 2017) The process of dipping in the slurry and drying is continued till a firm thickness is achieved. The pattern is placed in an oven and the wax melted. This leads to become a mold which can be easily filled with the molten metal. Before the pouring operation, the mold is pre-heated at about 1000ºC (1832ºF) to remove traces of wax. Then, pouring the molten metal into investment casting. Pouring can be done in gravity, pressure or vacuum conditions. Then let the molten metal to be solidified. Break the mold to get the product. Finishing process needed for complete final product.
28 Wax Removing And Metal Pouring After mold is dry, it is heated in a de-waxing furnace to allow the wax to flow out of the shell, later it is collected and recycled for use as the sprue wax. There are two methods to finish the investment casting melting wax process (Susie, 2020): Steam-dewax autoclave: this kind of dewax machine removes the majority of the wax, this wax can be reused. Figure 8.4: Steam-dewax autoclave (Engineering Product design , 2022) Flashfire oven: burns off the rest wax and make the shell hard. Make a clear and tough shell for casting. Figure 8.5: Flashfire oven (PPCP, 2022) The wax is melted out of the pattern and molten metal is poured into the cavity. When the metal solidifies, the ceramic shell is broken off, leaving the shape of the chosen part (PPCP, 2022).
29 8.3 Application of Investment Casting The applications of investment casting mold are nearly in the field of all manufacturing industries, such as power generation, firearm, automotive, aerospace, military, gas and oil, food service, and energy industries (Susie, 2020). Typically materials that can be cast with this process are Aluminum alloys, Bronzes, Stainless steels, Stellite etc. Glass mold accessory castings, Valves and fittings, Gears, Levers and Splines are some of the popular usages. Figure 8.6: Applications of investment casting (Susie, 2020) Limitations Compared to other methods of metal casting, investment casting comprises complex steps making the process quite expensive. Nevertheless, some of the steps can be automated for certain products. It can be extra expensive than die casting or sand casting, but per-unit costs decrease with large volumes. Investment casting causing time consuming process and costly. Exceptional surface finish possible but minute lacuna can cause rejection of castings as a result scrap rates can be high.
30 9.0 DIE CASTING Die casting is a kind of metal casting that involves pushing molten metal into a mold chamber under high pressure. Operators machined the two hardened tool steel dies into form and operate in the same way as an injection mold throughout the operation to produce the mold cavity. Non-ferrous metals, such as zinc, copper, aluminum, magnesium, lead, pewter, and tin-based alloys, make most die castings. They employ a hot-chamber or cold-chamber machine depending on the kind of metal to cast. Die casting process First, The machine melts the inserted metals, the type of metals depends on the tool you will create. And then, the machine automatically pours the molten metal into the mold at high pressure. This pressure is generally in the range of roughly 10 to 175 MPa. When the machine pours the molten metal, the pressure is maintained until the casting hardens. Consequently, the machine opens the dies with the ejector pins and expels the shot (shots are distinct from castings since there may be several holes in a die, producing multiple castings per shot). After that, High-pressure injection results in a very rapid filling of the mold so that the molten metal may fill the whole mold before any portion hardens. In this manner, you may avoid the surface discontinuities even in thin-walled sections that are difficult to fill. After the Injection process, shakeout involves separating the junk, including gates, runners, sprues, and flash. The machine typically does this procedure by extruding the casting via a specific dressing die. Other sand falling techniques include sawing and grinding. Finally, faults may be verified once the shaking out procedure is finished. The most frequent faults are stagnation and chilly heading.
31 9.1 Types of Die Casting Process a. Hot chamber die casting In a hot chamber die casting, a puddle of molten metal fills the die under pressure. At the start of the cycle, the machine retracts the piston, enabling the molten metal to fill the gooseneck. A pneumatic or hydraulic piston squeezes the metal and fills it into the mold. This technique has all advantages of this technique for fast cycle rates (about 15 cycles per minute), easy automation, and the capacity to melt metal. Step 1 The injection mechanism of a hot chamber machine is immersed in the molten metal. The furnace is attached to the machine by a metal feed system called a gooseneck. DIE CASTING HOT CHAMBER COLD CHAMBER LOW PRESSURE SEMI SOLID SQUEEZE VACUUM
32 Step 2 The die is closed and the piston rises, opening the port, allowing molten metal to fill the cylinder. Step 3 Next, the plunger seals the port, pushing the molten metal through the gooseneck and nozzle into the die cavity where it is held under pressure until it solidifies. Step 4 The die opens and the cores, if any, retract. The casting remains in only one die half – the ejector side. The plunger then returns, allowing residual molten metal to flow back through the nozzle and gooseneck.
33 Step 5 Ejector pins push the casting out of the ejector die. As the plunger uncovers the filling hole, molten metal flows through the inlet to refill the gooseneck. Application Die casting companies can develop various interesting die casting applications that result in genuinely unique and highly functioning parts and components. A kind of die casting application may be more advantageous than another depending on your particular characteristics. b. Cold Chamber die casting In the cold chamber die casting, the metal melt in a separate crucible. Specific quantity of molten metal deliver to an unheated injection chamber or nozzle. And then, the machine pumps these metals into the mold by hydraulic or mechanical pressure. Due to the necessity to transport molten metal into the cold chamber, the greatest drawback of this process is the lengthy cycle of time. Cold chamber dies casting machines available in vertical and horizontal versions. Vertical die casting machines are typically tiny, whereas horizontal die casting machines are available in different types.
34 Step 1 Operating sequence of the cold chamber die casting process. Step 2 The die is closed and the molten metal is ladled into the cold chamber shot sleeve. Step 3 The plunger pushes the molten metal into the die cavity where it is held under pressure until it solidifies.
35 Step 4 The die opens and the plunger advances, to ensure the casting remains in the ejector die. Cores, if any, retract. Step 5 Ejector pins push the casting out of the ejector half of the die and the plunger returns to its original position. Application Cold chamber die casting machines have the casting set situated away from the melt. The machine fills the casting chamber with the alloy and pushes it into the die-cast mold to create a casting. And then, it hardens the casting under high pressure, and the dies are opened. This method suitable for material which has a higher melting point such as aluminum and copper. Example product are in portable gadgets, electrical components, and electric housings.
36 c. Low pressure die casting process In low pressure die casting, the die is filled with metal from a pressurised furnace, with pressures typically around 0.7 bar. The holding furnace is positioned in the lower part of the vertical die casting machine, with the molten metal injected upwards directly into the bottom of the mould. The pressure holds the metal in the die until it solidifies. One of the main advantages of this process is the precise control of die cavity filling. Molten metal flows quickly and smoothly through the feeding conduits, reducing oxide formation and preventing porosity. Figure 9.0 : Low pressure die casting process Application The automobile industry has depended upon low-pressure die casting for decades to produce robust, high-quality aluminum castings. However, owing to its rather lengthy casting process, its usage was mainly confined to the luxury segment of the automobile industry, where smaller volumes and greater prices are anticipated. d. Vacuum die casting process Vacuum Die Casting as an improved version of the conventional pressure die casting. It is a high-pressure die casting aided by a vacuum pump to remove the air contained inside the die cavity. Vacuum Die Casting is primarily used to minimize certain casting flaws in components that arise from air entrapment. So, die casting producers who wish to guarantee higher quality for the manufactured components would choose vacuum-assisted die casting equipment.
37 Figure 9.1 : Vacuum die casting process Application Vacuum die casting is widely popular in the following industries, automotive, aircraft, military, marine, construction, etc. These sectors require high-quality and durable components for their products. Traditional die casting may not be able to satisfy the strength and quality standards. So, more and more manufacturers are beginning to offer vacuum die casting solutions to consumers. e. Squeeze die casting process Squeeze casting is a mix of casting and forging techniques. The technique may result in the most significant mechanical characteristics possible in a cast product. The invention of the squeeze casting method may usher in the enormous potential for producing components of aluminum alloys, which the manufacturers didn’t fully market yet. It may also be helpful for the import replacement of critical parts. Figure 9.2 : Squeeze die casting process
38 Application The squeeze casting has been commercially successful in producing components, including an Aluminum dome, ductile Iron mortar shell, and Steel bevel gear. Aluminum automotive wheels and pistons, and gear blanks made of brass and bronze. Compared with the HPDC, the squeeze casting method with high applied pressure is a potential option for thick magnesium castings. Other components that have been Squeeze cast include stainless steel blades, super alloy discs. f. Semi solid die casting process Semi-solid metal casting (SSM) is a near-net form variation of die casting. Industries are utilizing the technique today with non-ferrous metals, such as aluminum, copper, and magnesium, but also may operate with higher temperature alloys for which no presently appropriate die materials are available. The technique combines the benefits of casting and forging. Application Usually, industries utilize semi-solid casting for high-end applications. For aluminum alloys, typical components include structural medical and aerospace parts, pressure containment parts, defense parts, engine mounts, air manifold sensor harnesses, engine blocks, and oil pump filter housings. 9.2 Die Casting Advantages 1. Die casting is fast - Die casting can be produced in seconds each part and quantities of hundreds to thousands of metal parts each day. 2. Near net shape - Die casting are produced “near net shapes” no matter how complex the shape are how tight the tolerances are. 3. Lighter weights - Die casting are stronger because of the material surface skin not the thickness of materials so parts can weigh less with thinner casting wall thicknesses. 4. Die casting is versatile - Many more part shapes and sizes can be produced using the die casting manufacturing process. 5. Die casting are durable - Die castings parts are metal and have a long service life. 6. Die castings are inexpensive - Die castings are fast to produce and useless material. Die casting are typically less expensive than most other metal parts manufacturing processes.
39 10.0 CENTRIFUGAL CASTING The Centrifugal casting process also known as Roto Casting uses rotating molds to feed the metal uniformly in the mold cavity. Directional solidification provides for clean, dense casting with physical properties that often are superior to static castings. Casting which can be made by this process are creativeness. The ideal casting is symmetrical with a hollow center. Casting of this type approach 100% yield. This means the centrifugal process is highly efficient and productive. Typical materials that can be cast with this process are iron, steel, stainless steels, glass, and alloys of aluminum, copper and nickel. Two materials can be cast together by introducing a second material during the process. Typical parts made by this process are pipes, boilers, pressure vessels, flywheels, cylinder liners and other parts that are axi-symmetric. It is notably used to cast cylinder liners and sleeve valves for piston engines, parts which could not be reliably manufactured otherwise. 10.1 Centrifugal Casting Parts Figure 10.0 : Centrifugal Casting
40 10.2 Centrifugal Casting Process 1. Mold preparation - Prepare mold in a cup-shaped component before pouring the molten metal 2. Pouring - Molten metal is poured directly into the rotating mold, without the use of runners or a gating system. The centrifugal force drives the material towards the mold walls as the mold fills. 3. Cooling - With all of the molten metal in the mold, the mold remains spinning as the metal cools. Cooling begins quickly at the mold walls and proceeds inwards. 4. Casting removal - After the casting has cooled and solidified, the rotation is stopped and the casting can be removed. 5. Finishing - While the centrifugal force drives the dense metal to the mold walls, any less dense impurities or bubbles flow to the inner surface of the casting. As a result, secondary processes such as machining, grinding, or sand-blasting, are required to clean and smooth the inner diameter of the part. 10.3 Centrifugal Casting Application Mold Preparation Pouring Cooling Process Removal of Casting Finishing Process To cast hollow cylindrical metal pipes Used in the automobile industry for various product manufacturing like pistons, cylinder and liners Aircraft industries for casting flanges, compressors, and rings Manufacture carriage wheels of railway and bearings Create any parts whose shapes are symmetrical about an axis
41 Figure 10.1 : Centrifugal Casting process Advantages •Has lower casting defects because of uniform solidification • The energy can be saved as it requires a lower pouring temperature •A thin wall cylinder can be easily manufactured • Produce components with intricate geometries at a lower cost •Used for mass production • Inclusions and impurities of the casting process are lighter Disadvantages • Temperature distribution and solidification time is difficult to determine • Loss in structural and purity benefit if manufacture other than cylindrical structure • The size of components we manufacture here is limited • It is not suitable for manufacturing any type of material. The material is limited • More maintenance and skilled operator is required
42 EXERCISE 1. Define metal casting process. [Jun 14] 2. Name major categories in metal casting process. [Dis 13, Jun 14] 3. Draw and explain the correct design rule for sand casting. [Dis 13] 4. List important consideration when selecting pattern material. 5. Give the difference between sand casting and shell mould casting. 6. Explain characteristics of molten metal that may influence fluidity. [Dis 13] 7. Describe features of plaster mould casting. 8. What are the advantages of the lost-foam casting process? 9. Draw and explain the process steps for sand casting from beginning until finish. [Dis 15] 10. Analyze the problem below and give solution base on the design rules of sand casting. [Jun 16] A sand casting company encounter a problem with one of its sand casting projects. The casting takes too long to cool down this effect the production cycle time. After the casting cool down the part extracted from the sand casting have multiple defect, shrinkage, porosity and cracking caused by Hot Spots. A visible facture line can be seen on the corners of the cast part.
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