METAL CASTING PROCESSES
1. 2. 3. 4. 5. 6. Sand Casting Other Expendable Mold Casting Processes Permanent Mold Casting Processes Casting Quality Metals for Casting Product Design Considerations
Two Categories of Casting Processes
1. Expendable mold processes - mold is sacrificed to remove part Advantage: more complex shapes possible Disadvantage: production rates often limited by time to make mold rather than casting itself 2. Permanent mold processes - mold is made of metal and can be used to make many castings Advantage: higher production rates Disadvantage: geometries limited by need to open mold
Overview of Sand Casting
Most widely used casting process, accounting for a significant majority of total tonnage cast Nearly all alloys can be sand casted, including metals with high melting temperatures, such as steel, nickel, and titanium Castings range in size from small to very large Production quantities from one to millions 1. 2. 3. 4.
Steps in Sand Casting
Pour the molten metal into sand mold Allow time for metal to solidify Break up the mold to remove casting Clean and inspect casting Separate gating and riser system 5. Heat treatment of casting is sometimes required to improve metallurgical properties
Making the Sand Mold
The cavity in the sand mold is formed by packing sand around a pattern, then separating the mold into two halves and removing the pattern The mold must also contain gating and riser system If casting is to have internal surfaces, a core must be included in mold A new sand mold must be made for each part produced
Sand Casting Production Sequence
Figure 11.2 Steps in the production sequence in sand casting. The steps include not only the casting operation but also pattern-making and mold-making.
Sand Casting
The Pattern
A full-sized model of the part, slightly enlarged to account for shrinkage and machining allowances in the casting Pattern materials: Wood - common material because it is easy to work, but it warps Metal - more expensive to make, but lasts much longer Plastic - compromise between wood and metal
Types of Patterns
Figure 11.3 Types of patterns used in sand casting: (a) solid pattern (b) split pattern (c) match-plate pattern (d) cope and drag pattern
Core
Full-scale model of interior surfaces of part It is inserted into the mold cavity prior to pouring The molten metal flows and solidifies between the mold cavity and the core to form the casting's external and internal surfaces May require supports to hold it in position in the mold cavity during pouring, called chaplets
Core in Mold
Desirable Mold Properties
Strength - to maintain shape and resist erosion Permeability - to allow hot air and gases to pass through voids in sand Thermal stability - to resist cracking on contact with molten metal Collapsibility - ability to give way and allow casting to shrink without cracking the casting Reusability - can sand from broken mold be reused to make other molds?
Figure 11.4 (a) Core held in place in the mold cavity by chaplets, (b) possible chaplet design, (c) casting with internal cavity.
Foundry Sands
Silica (SiO2) or silica mixed with other minerals Good refractory properties - capacity to endure high temperatures Small grain size yields better surface finish on the cast part Large grain size is more permeable, allowing gases to escape during pouring Irregular grain shapes strengthen molds due to interlocking, compared to round grains Disadvantage: interlocking tends to reduce permeability
Binders Used with Foundry Sands
Sand is held together by a mixture of water and bonding clay Typical mix: 90% sand, 3% water, and 7% clay Other bonding agents also used in sand molds: Organic resins (e g , phenolic resins) Inorganic binders (e g , sodium silicate and phosphate) Additives are sometimes combined with the mixture to increase strength and/or permeability
Types of Sand Mold
Green-sand molds - mixture of sand, clay, and water; Green" means mold contains moisture at time of pouring Dry-sand mold - organic binders rather than clay And mold is baked to improve strength Skin-dried mold - drying mold cavity surface of a green-sand mold to a depth of 10 to 25 mm, using torches or heating lamps
Other Expendable Mold Processes
Shell Molding Vacuum Molding Expanded Polystyrene Process Investment Casting Plaster Mold and Ceramic Mold Casting
Shell Molding
Casting process in which the mold is a thin shell of sand held together by thermosetting resin binder
Shell Molding
Figure 11.5 Steps in shell-molding: (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;
Figure 11.5 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.
Shell Molding
Figure 11.5 Steps in shell-molding: (4) sand shell is heated in oven for several minutes to complete curing; (5) shell mold is stripped from the pattern;
Shell Molding
Figure 11.5 Steps in shell-molding: (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.
Advantages and Disadvantages
Advantages of shell molding:
Smoother cavity surface permits easier flow of molten metal and better surface finish Good dimensional accuracy - machining often not required Mold collapsibility minimizes cracks in casting Can be mechanized for mass production Disadvantages: More expensive metal pattern Difficult to justify for small quantities
Expanded Polystyrene Process
Uses a mold of sand packed around a polystyrene foam pattern which vaporizes when molten metal is poured into mold Other names: lost-foam process, lost pattern process, evaporative-foam process, and full-mold process Polystyrene foam pattern includes sprue, risers, gating system, and internal cores (if needed) Mold does not have to be opened into cope and drag sections
Expanded Polystyrene Process
Expanded Polystyrene Process
Figure 11.7 Expanded polystyrene casting process: (1) pattern of polystyrene is coated with refractory compound;
Figure 11.7 Expanded polystyrene casting process: (2) foam pattern is placed in mold box, and sand is compacted around the pattern;
Expanded Polystyrene Process
Advantages and Disadvantages
Advantages of expanded polystyrene process: Pattern need not be removed from the mold Simplifies and speeds mold-making, because two mold halves are not required as in a conventional green-sand mold Disadvantages: A new pattern is needed for every casting Economic justification of the process is highly dependent on cost of producing patterns
Figure 11.7 Expanded polystyrene casting process: (3) molten metal is poured into the portion of the pattern that forms the pouring cup and sprue. As the metal enters the mold, the polystyrene foam is vaporized ahead of the advancing liquid, thus the resulting mold cavity is filled.
Expanded Polystyrene Process
Applications: Mass production of castings for automobile engines Automated and integrated manufacturing systems are used to 1. Mold the polystyrene foam patterns and then 2. Feed them to the downstream casting operation
Investment Casting (Lost Wax Process)
A pattern made of wax is coated with a refractory material to make mold, after which wax is melted away prior to pouring molten metal "Investment" comes from a less familiar definition of "invest" - "to cover completely," which refers to coating of refractory material around wax pattern It is a precision casting process - capable of producing castings of high accuracy and intricate detail
Investment Casting
Investment Casting
Figure 11.8 Steps in investment casting: (1) wax patterns are produced, (2) several patterns are attached to a sprue to form a pattern tree
Figure 11.8 Steps in investment casting: (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 to make it rigid
Investment Casting
Investment Casting
Figure 11.8 Steps in investment casting: (5) the mold is held in an inverted position and heated to melt the wax and permit it to drip out of the cavity, (6) the mold is preheated to a high temperature, the molten metal is poured, and it solidifies
Figure 11.8 Steps in investment casting: (7) the mold is broken away from the finished casting and the parts are separated from the sprue
Advantages and Disadvantages
Advantages of investment casting: Parts of great complexity and intricacy can be cast Close dimensional control and good surface finish Wax can usually be recovered for reuse Additional machining is not normally required - this is a net shape process Disadvantages Many processing steps are required Relatively expensive process
Plaster Mold Casting
Similar to sand casting except mold is made of plaster of Paris (gypsum - CaSO4-2H2O) In mold-making, plaster and water mixture is poured over plastic or metal pattern and allowed to set Wood patterns not generally used due to extended contact with water Plaster mixture readily flows around pattern, capturing its fine details and good surface finish
Advantages and Disadvantages
Advantages of plaster mold casting: Good accuracy and surface finish Capability to make thin cross-sections Disadvantages: Mold must be baked to remove moisture, which can cause problems in casting Mold strength is lost if over-baked Plaster molds cannot stand high temperatures, so limited to lower melting point alloys
Ceramic Mold Casting
Similar to plaster mold casting except that mold is made of refractory ceramic material that can withstand higher temperatures than plaster Can be used to cast steels, cast irons, and other high-temperature alloys Applications similar to those of plaster mold casting except for the metals cast Advantages (good accuracy and finish) also similar
Permanent Mold Casting Processes
Economic disadvantage of expendable mold casting: a new mold is required for every casting In permanent mold casting, the mold is reused many times The processes include: Basic permanent mold casting Die casting Centrifugal casting
The Basic Permanent Mold Process
Uses a metal mold constructed of two sections designed for easy, precise opening and closing Molds used for casting lower melting point alloys are commonly made of steel or cast iron Molds used for casting steel must be made of refractory material, due to the very high pouring temperatures
Permanent Mold Casting
Permanent Mold Casting
Figure 11.10 Steps in permanent mold casting: (1) mold is preheated and coated
Figure 11.10 Steps in permanent mold casting: (2) cores (if used) are inserted and mold is closed, (3) molten metal is poured into the mold, where it solidifies.
Advantages and Limitations
Advantages of permanent mold casting: Good dimensional control and surface finish More rapid solidification caused by the cold metal mold results in a finer grain structure, so castings are stronger Limitations: Generally limited to metals of lower melting point Simpler part geometries compared to sand casting because of need to open the mold High cost of mold
Applications of Permanent Mold Casting
Due to high mold cost, process is best suited to high volume production and can be automated accordingly Typical parts: automotive pistons, pump bodies, and certain castings for aircraft and missiles Metals commonly cast: aluminum, magnesium, copper-base alloys, and cast iron
Die Casting
A permanent mold casting process in which molten metal is injected into mold cavity under high pressure Pressure is maintained during solidification, then mold is opened and part is removed Molds in this casting operation are called dies; hence the name die casting Use of high pressure to force metal into die cavity is what distinguishes this from other permanent mold processes
Die Casting Machines
Designed to hold and accurately close two mold halves and keep them closed while liquid metal is forced into cavity Two main types: 1. Hot-chamber machine 2. Cold-chamber machine
Hot-Chamber Die Casting
Metal is melted in a container, and a piston injects liquid metal under high pressure into the die High production rates - 500 parts per hour not uncommon Applications limited to low melting-point metals that do not chemically attack plunger and other mechanical components Casting metals: zinc, tin, lead, and magnesium
Hot-Chamber Die Casting
Figure 11.13 Cycle in hot-chamber casting: (1) with die closed and plunger withdrawn, molten metal flows into the chamber
Hot-Chamber Die Casting
Cold-Chamber Die Casting Machine
Molten metal is poured into unheated chamber from external melting container, and a piston injects metal under high pressure into die cavity High production but not usually as fast as hot-chamber machines because of pouring step Casting metals: aluminum, brass, and magnesium alloys Advantages of hot-chamber process favor its use on low melting-point alloys (zinc, tin, lead)
Figure 11.13 Cycle in hot-chamber casting: (2) plunger forces metal in chamber to flow into die, maintaining pressure during cooling and solidification.
Cold-Chamber Die Casting
Cold-Chamber Die Casting
Figure 11.14 Cycle in cold-chamber casting: (1) with die closed and ram withdrawn, molten metal is poured into the chamber
Figure 11.14 Cycle in cold-chamber casting: (2) ram forces metal to flow into die, maintaining pressure during cooling and solidification.
Molds for Die Casting
Usually made of tool steel, mold steel, or maraging steel Tungsten and molybdenum (good refractory qualities) used to die cast steel and cast iron Ejector pins required to remove part from die when it opens Lubricants must be sprayed into cavities to prevent sticking
Advantages and Limitations
Advantages of die casting: Economical for large production quantities Good accuracy and surface finish Thin sections are possible Rapid cooling provides small grain size and good strength to casting Disadvantages: Generally limited to metals with low metal points Part geometry must allow removal from die
Centrifugal Casting
A family of casting processes in which the mold is rotated at high speed so centrifugal force distributes molten metal to outer regions of die cavity The group includes: True centrifugal casting Semicentrifugal casting Centrifuge casting
True Centrifugal Casting
Molten metal is poured into rotating mold to produce a tubular part In some operations, mold rotation commences after pouring rather than before Parts: pipes, tubes, bushings, and rings Outside shape of casting can be round, octagonal, hexagonal, etc , but inside shape is (theoretically) perfectly round, due to radially symmetric forces
True Centrifugal Casting
Figure 11.15 Setup for true centrifugal casting.
Semicentrifugal Casting
Centrifugal force is used to produce solid castings rather than tubular parts Molds are designed with risers at center to supply feed metal Density of metal in final casting is greater in outer sections than at center of rotation Often used on parts in which center of casting is machined away, thus eliminating the portion where quality is lowest Examples: wheels and pulleys
Semicentrifugal Casting
Centrifuge Casting
Mold is designed with part cavities located away from axis of rotation, so that molten metal poured into mold is distributed to these cavities by centrifugal force Used for smaller parts Radial symmetry of part is not required as in other centrifugal casting methods
Centrifuge Casting
Additional Steps After Solidification
Trimming Removing the core Surface cleaning Inspection Repair, if required Heat treatment
Casting Quality
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: General defects common to all casting processes Defects related to sand casting process
General Defects: Misrun
A casting that has solidified before completely filling mold cavity
Figure 11.22 Some common defects in castings: (a) misrun
General Defects: Cold Shut
Two portions of metal flow together but there is a lack of fusion due to premature freezing
General Defects: Cold Shot
Metal splatters during pouring and solid globules form and become entrapped in casting
Figure 11.22 Some common defects in castings: (b) cold shut
Figure 11.22 Some common defects in castings: (c) cold shot
General Defects: Shrinkage Cavity
Depression in surface or internal void caused by solidification shrinkage that restricts amount of molten metal available in last region to freeze
Metals for Casting
Most commercial castings are made of alloys rather than pure metals Alloys are generally easier to cast, and properties of product are better Casting alloys can be classified as: Ferrous Nonferrous
Figure 11.22 Some common defects in castings: (d) shrinkage cavity
Product Design Considerations
Geometric simplicity: Although casting can be used to produce complex part geometries, simplifying the part design usually improves castability Avoiding unnecessary complexities: Simplifies mold-making Reduces the need for cores Improves the strength of the casting
Product Design Considerations
Corners on the casting: Sharp corners and angles should be avoided, since they are sources of stress concentrations and may cause hot tearing and cracks Generous fillets should be designed on inside corners and sharp edges should be blended
Product Design Considerations
Draft Guidelines: In expendable mold casting, draft facilitates removal of pattern from mold Draft = 1 for sand casting In permanent mold casting, purpose is to aid in removal of the part from the mold Draft = 2 to 3 for permanent mold processes Similar tapers should be allowed if solid cores are used
Draft
Minor changes in part design can reduce need for coring
Figure 11.25 Design change to eliminate the need for using a core: (a) original design, and (b) redesign.
Product Design Considerations
Dimensional Tolerances and Surface Finish: Significant differences in dimensional accuracies and finishes can be achieved in castings, depending on process: Poor dimensional accuracies and finish for sand casting Good dimensional accuracies and finish for die casting and investment casting
Product Design Considerations
Machining Allowances: Almost all sand castings must be machined to achieve the required dimensions and part features Additional material, called the machining allowance, is left on the casting in those surfaces where machining is necessary Typical machining allowances for sand castings are around 1.5 and 3 mm (1/16 and 1/4 in)
