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11 A - METAL CASTING , Expendable mold


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Metal casting
Processes of casting metals into expendable and permanent molds, expendable sand molds, other materials expendable molds, and permanent molds. >>> EXPANDABLE SAND MOLD MAKES A PERMANENT STEEL MOLD.
Sand casting
Casting a metal in a sand mold, an process that requires an addition of a few operations: making a pattern, making a core, preparing the sand, making the mold, all that in addition to melting and pouring and all the stuff that goes into making a sand casting >>> CHEAP SAND STEAL PRECIOUS TIME
Pattern types
Patterns can be grouped into four categories: [1] Solid pattern - a solid, elarged model which easy to make but hard to cast - the parting line is hard to locate, and the foundry workers must design the gaiting system - best for low production volumes. [2] Split pattern - a solid pattern cut to half which makes it easy to determine the parting line. [3] Match plate pattern - halves of the pattern are connected to a plate, making it easier to manufacture them independently. [3] Cope-and-drag pattern - a patter with the gating and riser system, allowing a fast creation of a mold. >>> SPLIT SOLIDS MAKE FAST MOLDS.
Pattern materials
PATTERNS ARE MADE OUT OF WOOD, PLASTIC, AND METALS, the metals offering the highest degree of durability and longevity, while wood patterns degrade over time. >>> STRONG STEEL BREAKS SILICA SAND
Sand mold materials
Sand used for sand casting is silica sand mixed with other minerals to hold things together - a bit of water and clay: 90% sand, 3% water, 7% clay, and some other binders such as resins and inorganic binders to hold the sand, since the sand processes good refractory properties, resisting high temperatures without melting and degrading, determining the part's surface finish by the grain size, although the smaller the grains are, the harder it is for air to escape - lower permeability, and if the grain size is irregular - it makes the mold stronger. >>> SMALL, BOUND SAND TRAPS AIR; RANDOM GRAIN SIZES INCREASE STRENGTH AND TRAP AIR.
Sand mold forming
Compacted around the pattern, sand makes a good mold inside a flask. The sand compacting can be done by hand or using machinery, and sometimes flaskless molding is used - the same flask is used for all the molds in a mechanized system of mold production. >>> COMPACTED SAND MAKES GOOD MOLD
Sand mold quality
Strength, Permeability, Thermal stability, Collapsibility, Re-usability --- parameters for accessing the sand mold quality: [1] Strength - ability to hold shape during casting. Depends on grain size and binders. [2] Permeability - ability of mold to release hot gases. [3] Thermal stability - resistance of sand at mold face to thermal cracks and buckling. [4] Collapsibility - how well the mold allows for casting to shrink without cracking the casting. [5] Reusability - ability to re-use the sand to make other molds. >>> SoPT CaR - STRENGTH, PERMIABILITY, THERMAL STABILITY, COLLAPSIBILITY, REUSABILITY
Sand mold types
[1] Green-sand molds - most widely used, it's a mixture of sand, water clay, they are called green cause they have water in it, and probably used to flour a bloom of green algae - moisture can cause defects . [2] Dry sand mold - baked sand with organic binder, has good surface finish, takes long to produce, fit to medium production quantities. [3] Skin dried molds - a green mold with baked surfaces: produces good surface finish, but needs special binders and time. [4] No-bake systems - molds with binders that harden them well enough to produce a good surface finish >>> GooD SuN: GREEN, DRY, SKIN, NO-BAKE
Other expendable mold processes
Although sand casting is very common, other methods of expendable mold casting exist: [1] Shell Molding, [2] Vacuum Molding, [3] Expanded Polystyrene Process, [4] Investment Casting, [5] Plaster-mold and Ceramic-mold casting. >>> SuV PIC: SHELL, VACUUM, POLYSTYRENE, INVESTMENT, CERAMIC
Shell molding
A molding process that was developed in Germany during the 40s and uses a thin, cured sand shell and produces very good surface finish. A part of the pattern is heated and coated with sand, facilitating the sand binding to the pattern and creating a thin half of a mold that is used, after curing it in the oven and combining with the other half inside of a sand or metal shot filled flask. The process is expensive for small quantities, economical for big quantities >>> CURED SHELLS CREATE A LEGION OF SHINING CAST SHELVES
Vacuum molding
A process that holds the sand in the flask by using vacuum, trapping the sand in the cope and the drag between two vacuum pressed plastic shits, allowing for a much better re-usability of the sand, saving on binders and mold preparation, and preventing moisture related defects, since the sand is dry, yet the process itself is slow and not readily adoptable to mechanization. Developed in Japan around the 70s, also called the V-process. >>> CLEAN VACUUM CREATES REUSABLE SAND
Lost pattern process
Cast metal replaces evaporated foam pattern covered in refractory compound, saving time by removing the need to have two parts of the mold and removing the pattern, using a disposable pattern with a gating system, saving time on mold preparation and sand recycling, yet the process depends on a disposable, single-serving, foam core that must be manufactured for every mold. >>> FOAMY PATTERN FORMS A FASTER PROCESS
Investment casting
A very accurate casting process, capable of producing net-shape parts by covering a wax pattern in ceramics, a pattern that is usually injection molded with consideration of wax and metal shrinkage, curing the ceramics, evaporating the wax, and pouring molten metal into the resultant mold, thus producing very accurate parts that require none to little processing to get to a net shape, a process used for surgical instruments, machine parts, aircraft engines. >>> HARD CERAMICS COATS SOFT WAX TO CREATE A STRONG BUTT-PLUG EXTRACTOR.
Plaster and ceramic mold casting
A process that produces very accurate parts using plaster or ceramics, although it's similar to sand casting, the process is quite accurate due to the liquid material penetrating to all the nooks and crannies, capturing the fine part details, yet the ceramics or plaster takes time to cure, must have a balance between dryness or wetness - too dry and the plaster will be brittle, too wet and the part will be damaged, and since the plaster/ceramics are not as permeable to air, foundrymen take measure to ensure that the gases can escape - removing the air before pouring, aerating the plaster slurry before pouring to create finely dispersed voids, or mixing the plaster with sand >>> SLOWLY CURING, NON PERMEABLE PLASTER CREATES SUPERB FINISH