| 词条 | Plaster mold casting |
| 释义 |
Parts that are typically made by plaster casting are lock components, gears, valves, fittings, tooling, and ornaments.[2] DetailsThe 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.[1] The ratio of ingredients is 70–80% gypsum and 20–30% additives.[2] The pattern is usually made from metal, however rubber molds may be used for complex geometry; these molds are called {{Visible anchor|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.[1] These molds are also inexpensive, reusable, more accurate than steel molds, fast to produce, and easy to change.{{Citation needed|date=June 2010}} Typical tolerances are {{convert|0.1|mm|in|abbr=on}} for the first {{convert|50|mm|in|abbr=on}} 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).[1] ProcessFirst, 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 {{convert|120|C|F|abbr=on}} and {{convert|260|C|F|abbr=on}}, 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 cannot be reused.[1][2] Advantages and disadvantagesPlaster 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 {{convert|0.6|mm|in|abbr=on}}. This results in a near net shape casting, which can be a cost advantage on complex parts.[1] It also produces minimal scrap material.[3] 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 {{convert|1200|C|F|abbr=on|sigfig=2}}, 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.[1][2] Another disadvantage is that its long cooling times restrict production volume.[1] 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.[4] Notes1. ^1 2 3 4 5 6 7 {{harvnb|Degarmo|Black|Kohser|2003|p=315}}. 2. ^1 2 3 4 {{harvnb|Kalpakjian|Schmid|2006}}. 3. ^{{harvnb|Todd|Allen|Alting|1994|p=283}}. 4. ^{{Citation| inventor-last = Ravins | inventor-first = Thomas L. | inventor2-last = Greve | inventor2-first = Dale R. | issue-date = 1989-04-04 | title = Fiber reinforced plaster molds for metal casting | country-code = US | patent-number = 4818287}}. References
1 : Casting (manufacturing) |
| 随便看 |
|
开放百科全书收录14589846条英语、德语、日语等多语种百科知识，基本涵盖了大多数领域的百科知识，是一部内容自由、开放的电子版国际百科全书。