Permanent Mold Casting Processes 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 Permanent Mold Casting Processes The processes include: Basic permanent mold casting Die casting Centrifugal casting Introduction In expendable mold casting, a separate mold is produced for each casting Low production rate for expendable mold casting If multiple-use molds are used, productivity can increase Most multiple-use molds are made from metal, so most molds are limited to low melting temperature metals and alloys Permanent-Mold Casting Also known as gravity die casting Mold can be made from a variety of different materials Gray cast iron, alloy cast iron, steel, bronze, or graphite Most molds are made in segments with hinges to allow rapid and accurate closing Molds are preheated to improve properties Liquid metal flows through the mold cavity by gravity flow Process can be repeated immediately because the mold is still warm from the previous casting Most frequently cast metals Aluminum, magnesium, zinc, lead, copper, and their alloys If steel or iron is to be used, a graphite mold must be used 1
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 Steps in (1) Mold is preheated and coated for lubrication and heat dissipation (2) Cores (if any are used) are inserted and mold is closed Steps in Steps in (3) Molten metal is poured into the mold, where it solidifies Advantages of Permanent-Mold Casting Near- net shapes Little finish machining Reusable molds Good surface finish Consistent dimensions Directional solidification Disadvantages of Permanent Mold Casting Limited to lower melting temperature alloys High mold costs Mold life is strongly tied to cost Mold life is dependent on the following Alloys being cast Mold material Pouring temperature Mold temperature Mold configuration High production runs can validate high mold costs Molds are not permeable Limited mold complexity 2
Applications and Metals for 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 Unsuited to steels because of very high pouring temperatures Applications and Metals for Low-Pressure and Vacuum Permanent-Mold Casting Schematic of the low-pressure permanent-mold process. Schematic illustration of vacuum permanent-mold casting. Note the similarities to the low-pressure process. Low Pressure Permanent-Mold Casting Tilt-pour permanent-mold casting Mold is rotated to force flow into the cavity Low pressure permanent-mold casting Mold is upside down and connected to a crucible that contains the molten metal Pressure difference induces upward flow Metals are exceptionally clean because it is fed directly into the mold Little or no turbulence during flow Typical metals cast using low pressure process Aluminum, magnesium, and copper Low Pressure Permanent-Mold Casting Vacuum Permanent-Mold Casting Atmospheric pressure in the chamber forces the metal upward after the vacuum is drawn Thin-walled castings can be made Excellent surface quality Cleaner metals than low pressure Lower dissolved gas content Better mechanical properties than low pressure casting 3
Die Casting Molten metal is forced into the mold under high pressure Held under high pressure during solidification Castings can have fine sections and complex details Long mold life Typical metals cast Zinc, copper, magnesium, aluminum, and their alloys 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 Basic Types of Die-Casting Hot chamber castings Fast cycling times No handling or transfer of molten metal Used with zinc, tin, and lead-based alloys Heated-manifold direct injection die casting Molten zinc is forced though a heated manifold Next through heated mini-nozzles directly into the die cavity Eliminates the need for sprues, gates and runners Basic Types of Die Casting Cold-chamber machines Used for materials not suitable for hot chamber machines Typical materials Aluminum, magnesium, copper, and high-aluminum zinc Longer operating cycle than hot-chamber High productivity Summary of Die Casting Dies fill so fast with metal that there is little time for the air in the runner and die to escape Molds offer no permeability Air can become trapped and cause defects Risers are not used because of the high pressures used Sand cores can not be used due to high pressures Cast-in inserts can be used High production rates Little post casting finishing necessary 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 4
Hot-Chamber Die Casting Hot-chamber die casting cycle: (1) with die closed and plunger withdrawn, molten metal flows into the chamber Hot-Chamber Die Casting (2) plunger forces metal in chamber to flow into die, maintaining pressure during cooling and solidification. Hot-Chamber Die Casting (3) Plunger is withdrawn, die is opened, and casting is ejected 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) Cold-Chamber Die Casting Cycle (1) With die closed and ram withdrawn, molten metal is poured into the chamber Cold-Chamber Die Casting Cycle (2) Ram forces metal to flow into die, maintaining pressure during cooling and solidification 5
Cold-Chamber Die Casting Cycle (3) Ram is withdrawn, die is opened, and part is ejected 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 onto cavity surfaces to prevent sticking Die Modifications and Die Life Die complexity can be improved through the use of Water cooled passages Retractable cores Moving pins to eject castings Die life Limited by erosion and usage temperature Surface cracking Heat checking Thermal fatigue Advantages of Die Casting High production rates Good strength Intricate shapes Dimensional precision Excellent surface qualities Small-medium sized castings Die Casting: Advantages Advantages: Economical for large production quantities Good accuracy and surface finish Thin sections possible Rapid cooling means small grain size and good strength in casting Die Casting: Limitations Disadvantages: Generally limited to metals with low metal points Part geometry must allow removal from die 6
Die-Casting Dies Die Cast Materials Various types of die-casting dies. Die Casting Machines Die Casting Products Squeeze Casting Combination of casting and forging in which a molten metal is poured into a preheated lower die, and the upper die is closed to create the mold cavity after solidification begins Differs from usual closed-mold casting processes in which die halves are closed before introduction of the molten metal Compared to conventional forging, pressures are less and finer surface details can be achieved Squeeze Casting 7
Semi-Solid Metal Casting Family of net-shape and near net-shape processes performed on metal alloys at temperatures between liquidus and solidus Thus, the alloy is a mixture of solid and molten metals during casting (mushy state) To flow properly, the mixture must consist of solid metal globules in a liquid Achieved by stirring the mixture to prevent dendrite formation Semi-Solid Metal Casting: Advantages Complex part geometries Thin part walls possible Close tolerances Zero or low porosity, resulting in high strength of the casting Squeeze Casting and Semisolid Casting Advantages High production Thin-walled parts Good surface finish Dimensional precision Good mechanical properties Squeeze Casting Large gate areas and slow metal velocities to avoid turbulence Solidification occurs under high pressure Intricate shapes with good mechanical properties Reduced gas and shrinkage porosity Semisolid Casting Semi-solid processing is typically classified into two major categories: Thixocasting and Rheocasting. Rheocasting Molten metal is cooled to semisolid Metal is stirred to break up dendrites Rheocasting Thixocasting Thixocasting refers to any process that starts with a specially prepared alloy that is reheated from ambient to the desired No handling of molten metal Metal is stirred as in rheocasting and produced into blocks or bars Metal is then reheated to semisolid and can be handled as a solid but processed as a liquid Injection system used is similar to the one used in plastic injection molding 8
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 Centrifugal Casting Methods True centrifugal casting Semicentrifugal casting Centrifuge casting True centrifugal casting Semicentrifugal casting Centrifuge 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 Setup for true centrifugal casting 9
Advantages: Pipe and similar products can be produced without using core. Fine structure can be obtained. (Better mechanical properties.) Fine finished surface. No scrap Filling time is low, therefore a lower pouring temperature can be choosen. Limitations: The shape of the parts is limited. High investment cost of the casting machines. In alloy elements, segragation can be seen due to different densities. Semicentrifugal Casting Centrifugal force is used to produce solid castings rather than tubular parts Molds use 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 10
Semicentrifugal Casting Semicentrifugal Casting Semicentrifugal Casting Centrifuge Casting Mold is designed with part cavities located away from axis of rotation, so 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 Centrifuge Casting 11