Plaster casting or plastic molding is another kind of metal casting process that is not command as well other casting methods such as; sand casting, investment casting, expandable mold casting, etc. Here, we explain the general characteristics of plaster and ceramic mold casting methods and some process parameters about them.
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What Are The Plaster And Ceramic Mold Casting Methods?
First of all, metal or plastic patterns must be prepared to obtain mold cavities. After this, the mold is prepared upon these patterns. Wood patterns are also available but, plaster is mixed with water before application to the pattern. The water consistency of wood can be harmful to successful mold creation.
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The plaster is known as ‘Paris’ which is a mixture of gypsum(CaSO4–2H2O). It has the same characteristics as sand casting, but plaster is used instead of sand. The advantage of plaster over sand is, the water inside the plaster leads to more fluidity and contact with the pattern which means, intricate sections and a very good surface finish can be obtained.
When the plaster is applied to the pattern, it is left to set. Setting or curing of plaster takes about 25 minutes and when the pattern is detached after curing of plaster, the plaster mold is left inside special Owens to adjust the moisture inside it.
An important drawback of plaster molds is that they can not withstand high temperatures as sand casting molds. So, this leads to a great restriction of metal type usage in plaster molds; such as copper alloys, aluminum alloys, and magnesium alloys.
The moisture content inside plaster is very important. If excessive moisture is left inside the plaster mold after heat application, strength problems can occur. Strength is very important in terms of the application of high density and high-temperature metal fluid. If the moisture is excessively taken from plaster, the mold can crack or demolish during the casting of metal. It is the hardest point of plaster mold casting processes.
Also, various additives are added inside plaster, such as talc, silica, etc… These additives adjust the curing time of plaster and also add strength and prevent cracks.
The most important disadvantage of plaster molds is the absence of porosities from the mold surface to the atmosphere to prevent gas porosities inside the mold. But this problem can be overcome with some measures;
- ‘Antioch process’ can be utilized to prevent the poreless structure of plaster. In this process, an additional 50% sand is made into plaster. This mixture is heated and cured inside the autoclave(superheated steam is applied at high pressure), then left drying. The obtained plaster-sand mold structure has very good pores for possible entrapped gases inside the plaster molds.
- Vacuuming the plaster mold cavity is another solution for this poreless structure of plaster mold.
- Aeration of liquid plaster to obtain fine pores for evacuation gases result from this temperature casting.
So, if we consider the all parameters, advantages, and disadvantages of plaster molding processes, some low melting temperature materials such as rubbers and plastics can be molded. Also, intricate parts such as turbine impellers and thin machinery parts like this can be cast by the plaster mold casting process.
Difference Of Ceramic Mold Casting Process
Consider all the advantages of the plaster molding process which are the intricate parts can be produced in the range of 20 grams to 100 kilograms parts. Ceramics are refractory materials that also metals that have high melting temperatures are molded. All the advantages of plaster mold casting are valid for ceramic mold casting with high temperatures.
These are the general properties and characteristics of plaster mold casting and ceramic mold casting processes. Do not forget to leave your comments and questions about, ceramic mold casting and plaster mold casting processes below.
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