Definition Plaster mould casting is a manufacturing process having a similar technique to sand casting. Plaster of Paris is used to form the mould for the casting, instead of sand. In industry parts such as valves, tooling, gears, and lock components may be manufactured by plaster mould casting.
Raw Material for Mold Making Gypsum Plaster (plaster of Paris) Plaster of Paris was first named for a gypsum deposit that was located in Paris, France. While gypsum is a hydrated salt, the plaster is the th e result of reformed refo rmed gypsum. It was eventually discovered that when mixed with water, the plaster become hard, durable and resistant to temperature and water. Today, it is used for construction, casts and pottery. Proper Propertie ties s of Gypsum Gypsum Plast Plaster er 1. Stre Streng ngth th When the gypsum powder is mixed with water, it creates a plaster. This plaster can be developed in softer and harder forms and is used today for setting casts on a fractured bone. To achieve better strength, the plaster must be allowed to dry fully before being manipulated. The crystal structure of the plaster determines how strong it will be. 2. Water Water Resis Resistan tance ce The water resistant nature of plaster of Paris makes it ideal for pottery making and medical casts. For pottery that will be used--such as bowls, cups, mugs or plates--the water resistance ensures that water and oils will not seep into the pores of the dish and cause it to crack. Water resistance may make the dish microwave and dishwasher safe, as well as washable. For medical casts, the water resistant property of plaster of Paris is highly important. It prevents additional moisture from getting in between the cast and skin, which can lead to staph infections. 3. Sealin Sealing g & Softne Softness ss Softer forms of plaster of Paris are used in construction. Once the joists are built and the drywall panels are installed, the plaster is put on the wall to level holes from nails and to create a seal between two pieces of drywall. The sealing nature of plaster of Paris helps keep the moisture of paint from getting into nail holes and between the drywall, which can lead to softening and the breakdown of it.
Advantages of Plaster Mold Casting
The ability ability to produce complex complex shapes with uniform grain structure The ability to produce castings with good with good surface finish The ability to produce thin section castings The ability to produce castings with high dimensionally accuracy The excellent replication of pattern pattern detail
Disadvantages of Plaster Mold Casting
The process is limited to non-ferrous metals only Poor productivity due to lengthy processing problems The need for multiple patterns to improve molding productivity The need for special procedures to overcome the problems of poor mould permeability The possibility of impaired mechanical properties arising as a result of slow cooling of the casting
Limitations of Plaster Mold Casting
Temperatures: Size limits: Thickness limits: Tolerances: Draft allowance: Surface finish: Wall Thickness
below 2,000° F 1 oz – 15 lb as thin as 0.025 in 0.005 in on the first 2 in; .002 in per additional inch ½ - 1° 50-125 µin Thin Wall: .030" - .060" & Average: 0.080" - 0.120"
Definition The manufacturing process of ceramic mold casting is like the process of plaster mold casting but can cast materials at much higher temperatures. Instead of using plaster to create the mold for the metal casting, ceramic casting uses refractory ceramics for a mold material. In industry parts such as machining cutters, dies for metalworking, metal molds, and impellers may be manufactured by this process.
Raw Material Refractory Materials A refractory material is one that retains its strength at high temperatures. ASTM C71 defines refractories as "non-metallic materials having those chemical and physical properties that make them applicable for structure, or as components of systems, that are exposed to environments above 1,000 °F (811 K; 538 °C)". Refractory materials are used in linings for furnaces , kilns , incinerators and reactors. They are also used to make crucibles and for making molds in ceramic mold casting.
Properties of Refractory Materials 1. Refractoriness Refractoriness is a property at which a refractory will deform under its own load. The refractoriness is indicated by PCE (Pyrometric cone equivalent). It should be higher than the application temperatures. 2. Porosity Porosity affects chemical attack by molten slag, metal and gases. Decrease in porosity increases strength and thermal Conductivity. 3. Strength It is the resistance of the refractory to compressive loads, tension and shear stresses. In taller furnaces, the refractory has to support a heavy load; hence strength under the combined effect of temperature and load, i.e. refractoriness under load is important. 4. Specific Gravity Specific gravity of the refractory is important to consider the weight of a brick. Cost of bricks of higher specific gravity is more that of lower specific gravity. But strength of bricks of higher specific gravity is greater than one with lower specific gravity. 5. Spalling Spalling relates to fracture of refractory brick which may occur due to the following reasons: A temperature gradient in the brick which is caused by sudden heating or cooling. Compression in a structure of refractory due to expansion Variation in coefficient of thermal expansion between the surface layer and the body of the brick Variation in coefficient of thermal expansion between the surface layer and the body of the brick is due to slag penetration or due to structural change.
Advantages of Ceramic Mold Casting
Intricate detail Close tolerances Smooth finish
Disadvantages of Ceramic Mold Casting
Ceramic is not reusable Ceramic mold casting is relatively expensive The long preparation time of the mold makes slow manufacturing production rates
Limitations of Ceramic Mold Casting
Size limits: Thickness limits: Tolerances: Draft allowance: Surface finish:
several ounces to several tons as thin as 0.05 in, no maximum 0.005 in on the first inch; 0.003 in per additional inch 1° 75-150 µin
Definition Investment casting is a manufacturing process in which a wax pattern is coated with a refractory ceramic material. Once the ceramic material is hardened its internal geometry takes the shape of the casting. The wax is melted out and molten metal is poured into the cavity where the wax pattern was. The metal solidifies within the ceramic mold and then the metal casting is broken out. This manufacturing technique is also known as the lost wax process. Investment casting was developed over 5500 years ago and can trace its roots back to both ancient Egypt and China. Parts manufactured in industry by this process include dental fixtures, gears, cams, ratchets, jewelry, turbine blades, machinery components and other parts of complex geometry.
Raw Material Wax Materials Wax is the oldest thermoplastic material known to man. Beeswax was utilized in the lost wax process by craftsmen in the ancient civilizations of China & Egypt. Today the name ‘wax ‘applies to any substance having wax-like properties. Investment casting waxes are complex formulations containing many components like paraffin wax, microcrystalline wax, resins (strong, viscous, and brittle), polymers –ductile, fillers etc.
Categories of Investment Casting Wax
Runner wax Reclaim & Reconstituted wax Water Soluble wax Other Special wax –dipping/patching/adhesive
Pattern wax –straight (unfilled) –emulsified –filled
Wax Selection Wax selection is determined by the particular application and the process criteria are: 1. Injection –state/setting rate/rheology/geometry/equipment/surface finish 2. Removal, Handling, Assembly –strength/hardness/setting rate/stability/ability to make joins 3. Dimensional Control –thermal expansion/contraction/cavitation/distortion 4. Mold Making –strength/wet ability/resistance to binders and solvents 5. Dewax & Burnout –melting point/viscosity/thermal expansion/thermal diffusivity/ash content 6. Other –cost/availability/recycling/toxicity/environmental
Advantages of Investment Casting
Complex shapes which are difficult by any other method are possible Very close tolerances and excellence surface finish can be obtained Ready for use with little or no machining required Almost any metal can be cast ; ferrous & non-ferrous metals No flash or parting lines Nearly unlimited intricacy
Disadvantages of Investment Casting
Time consuming as shell moulds cannot be reused Expensive as many labor involved in the preparation of the wax patterns & shell moulds; Occasional minute defects
Limitations of Investment Casting
Size limits: Thickness limits: Tolerances: Draft allowance: Surface finish:
as small as 1/10 oz; usually less than 10 lb as thin as 0.025 in, less than 3 in 0.005 in on the first inch; 0.002 in per additional inch not required 50-125 µin
