Plaster mold casting - wikipedia Plaster mold casting Plaster mold casting is a metalworking casting process similar to sand casting except the molding material is plaster of Paris instead of sand. Like sand casting, plaster mold casting is an expendable mold process, however it can only be used with non-ferrous materials. It is used for castings as small as 30 g ( 1 oz ) to as large as 45 kg ( 99 lb ). Generally, the form takes less than a week to prepare. Production rates of 1 -- 10 units / hr can be achieved with plaster molds. Parts that are typically made by plaster casting are lock components, gears, valves, fittings, tooling, and ornaments. Contents 1 Details 1.1 Process 2 Advantages and disadvantages 3 Notes 4 References Details The plaster is not pure plaster of Paris, but rather has additives to improve green strength, dry strength, permeability, and castability. For instance, talc or magnesium oxide are added to prevent cracking and reduce setting time ; lime and cement limit expansion during baking ; glass fibers increase strength ; sand can be used as a filler. The ratio of ingredients is 70 -- 80 % gypsum and 20 -- 30 % additives. The pattern is usually made from metal, however rubber molds may be used for complex geometry ; these molds are called Rubber plaster mold s. For example, if the casting includes reentrant angles or complex angular surfaces then the rubber is flexible enough to be removed, unlike metal. These molds are also inexpensive, reusable, more accurate than steel molds, fast to produce, and easy to change. Typical tolerances are 0.1 mm ( 0.0039 in ) for the first 50 mm ( 2.0 in ) and 0.02 mm per additional centimeter ( 0.002 in per additional inch ). A draft of 0.5 to 1 degree is required. Standard surface finishes that are attainable are 1.3 to 4 micrometers ( 50 -- 125 μin ). Process First, the plaster is mixed and the pattern is sprayed with a thin film of parting compound to prevent the plaster from sticking to the pattern. The plaster is then poured over the pattern and the unit shaken so that the plaster fills any small features. The plaster sets, usually in about 15 minutes, and the pattern is removed. The mold is then baked, between 120 ° C ( 248 ° F ) and 260 ° C ( 500 ° F ), to remove any excess water. The dried mold is then assembled, preheated, and the metal poured. Finally, after the metal has solidified, the plaster is broken from the cast part. The used plaster can not be reused. Advantages and disadvantages Plaster mold casting is used when an excellent surface finish and good dimensional accuracy is required. Because the plaster has a low thermal conductivity and heat capacity, the metal cools more slowly than in a sand mold, which allows the metal to fill thin cross-sections ; the minimum possible cross-section is 0.6 mm ( 0.024 in ). This results in a near net shape casting, which can be a cost advantage on complex parts. It also produces minimal scrap material. The major disadvantage of the process is that it can only be used with lower melting temperature non-ferrous materials, such as aluminium, magnesium, and zinc. The most commonly used materials is aluminium. The maximum working temperature of plaster is 1,200 ° C ( 2,200 ° F ), so higher melting temperature materials would melt the plaster mold. Also, the sulfur in the gypsum reacts with iron, making it unsuitable for casting ferrous materials. Another disadvantage is that its long cooling times restrict production volume. Plaster is not as stable as sand, so it is dependent on several factors, including the consistency of the plaster composition, pouring procedures, and curing techniques. If these factors are not closely monitored the mold can be distorted, shrink upon drying, have a poor surface finish, or fail completely. Notes ^ Jump up to : Degarmo, Black & Kohser 2003, p. 315. ^ Jump up to : Kalpakjian & Schmid 2006. Jump up ^ Todd, Allen & Alting 1994, p. 283. Jump up ^ US 4818287, Ravins, Thomas L. & Dale R. Greve, `` Fiber reinforced plaster molds for metal casting '', issued 1989 - 04 - 04. References Degarmo, E. Paul ; Black, J T. ; Kohser, Ronald A. ( 2003 ), Materials and Processes in Manufacturing ( 9th ed. ), Wiley, ISBN 0 - 471 - 65653 - 4. Kalpakjian, Serope ; Schmid, Steven ( 2006 ), Manufacturing Engineering and Technology ( 5th ed. ), Pearson, ISBN 0 - 13 - 148965 - 8. Todd, Robert H. ; Allen, Dell K. ; Alting, Leo ( 1994 ), Manufacturing Processes Reference Guide, Industrial Press Inc, ISBN 0 - 8311 - 3049 - 0 Metalworking Casting Processes Centrifugal Continuous Die Evaporative - pattern Full mold Lost foam Investment ( Lost wax ) Permanent mold Plaster mold Sand Semi-solid metal Shaw process Shell molding Spin Components Chill Cope and drag Core Flask Molding sand Pattern Riser Sprue Terminology Casting defects Draft Dross Ingot Semi-finished casting products Slag Casting Fabrication Forming Jewellery Machining Metallurgy Smithing Tools and terminology Welding Retrieved from `` https://en.wikipedia.org/w/index.php?title=Plaster_mold_casting&oldid=864846554 '' Categories : Casting ( manufacturing ) Hidden categories : All articles with unsourced statements Articles with unsourced statements from June 2010 Talk Contents About Wikipedia Esperanto Edit links This page was last edited on 19 October 2018, at 21 : 33 ( UTC ). 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Question: what can you cast in a plaster mold
Answer: non-ferrous materials