Casting is a type of metal fabrication process in which molten metal is poured into the cavity of a specially designed mold and allowed to harden. After the metal workpiece hardens, it is removed from the die to undergo various finishing treatments or for use as a final product. Casting methods are typically used to create intricate solid shapes, and cast products are found in a wide range of applications, including automotive components, aerospace parts, electronics, mechanical devices, and construction supplies.
–>Interested in Die Casting Services? Try Xometry’s Instant Quoting Engine℠ to make your project come to life. Free shipping in the USA.
Although casting is one of the oldest known manufacturing techniques, modern advances in casting technology have led to a wide range of specialized casting methods. These methods include die casting, investment casting, plaster mold casting, and sand casting, each of which provides their unique fabrication benefits. This article looks at plaster mold casting specifically, explaining what it is how it is used.
What is Plaster Mold Casting?
Plaster mold casting, sometimes just called plaster casting, uses a mixture of gypsum, strengthening compounds, and water to create a mold. The plaster used is not pure plaster of Paris. Instead, it has additives that improve green strength, dry strength, permeability, and castability. For example, talc or magnesium oxide are often added to prevent cracking and reduce setting time. The plaster pattern is made of metal or sometimes rubber and is typically coated with an anti-adhesive compound to prevent it from becoming stuck against the mold. If the casting includes reentrant angles, an interior angle that is greater than 180°, or complex angular surfaces, then a rubber mold is flexible enough to be removed, unlike metal. These molds are also inexpensive, fast to create, reusable, more accurate than steel molds, and easy to change. Plaster fills in any gaps around the mold. Once the plaster material has been used to cast a part, it usually cracks or forms defects, requiring it to be replaced with fresh mold.
During plaster mold casting, the parting line is first determined. It is either a simple two-part or a more complex three or more part. Then, the plaster is mixed, and the pattern is sprayed with an anti-adhesive. The plaster is then poured into the pattern and shaken so that the plaster fills the pattern completely. The plaster sets in about 15 minutes and after this the pattern is removed and a mold formed. The mold is then baked to remove any excess water. Once the mold is dry, it is assembled, preheated, and the molten metal is poured into it. Finally, after the metal hardens, the plaster is broken from the cast part. It usually goes through finishing processes such as cleaning, grinding, heat treating, and blasting.
Plaster mold casting allows for the fabrication of more complicated parts. Since plaster has a low thermal conductivity and heat capacity, the metal in the molds cools slower than in a sand mold, which lets the metal to fill even thin cross-sections. The minimum possible cross-section is 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.
Plaster mold casting is a type of expendable mold process, meaning the molds are not generally reused. This process can only be used with non-ferrous materials: metals that don’t contain iron. It is used for creating castings as small as 30 g to as large as 45 kg. The plaster form typically takes less than a week to prepare, making it a faster means of production. Rates of one to ten units per hour can be reached with plaster molds.
This process tends to be more expensive than most sand casting operations, and the molds are less durable and may require frequent replacements material. However, the process is usually more effective and cost-efficient when the quality of the surface finish is a necessary requirement.
What are the Applications of Plaster Mold Casting?
Plaster casting fabricates products with a very smooth surface finish, allows for the casting of complex shapes with thin walls, can form large parts with less expense than other processes, such as investment casting, and has a higher degree of dimensional accuracy than that of sand casting.
The major disadvantage of plaster mold casting is that it can only be used with lower melting temperature non-ferrous materials, such as aluminum, zinc, magnesium, and sometimes copper alloys. The most common plaster cast material is aluminum. The maximum working temperature of plaster is 1,200 °C (2,200 °F), so if a metal needs to be heated at a higher temperature to melt, it cannot be plaster cast. The reason only non-ferrous metals can be used is that sulfur in the gypsum reacts with iron. Another disadvantage is that the longer cooling times can restrict production volume.
Plaster mold casting is often used to make lock components, gears, valves, ornaments, fittings, and toolings. It is also used for prototypes and short-run production of aluminum and zinc parts.
–>Interested in Die Casting Services? Try Xometry’s Instant Quoting Engine℠ to make your project come to life. Free shipping in the USA.
Plaster Mold Casting—Summary
This guide provides a basic understanding of plaster mold casting. For more information on related services, consult our other product guides or visit the Thomas Supplier Discovery Platform to locate potential sources or view details on specific products.
Other Casting Articles
- Shrinkage in Casting: Causes and Solutions
- Processes Involved with Casting
- Silicon in Electronics and Casting Applications
- How to Design a Die Casting Mold
- Alternatives to Casting
- Casting Need to Know Guide
- Die Casting Machines
- Different Types of Casting Processes used in Manufacturing
- Alloy Die Casting: A Look at Metal Die Cast Types of Alloys and Their Uses
- How Castings are Made
- Casting vs. Forging – What’s the Difference?
- Demand Planning Process: Key S&OP Process Steps Involved in Forecasting Components in a Supply Chain
- Die Casting Process Types
- Materials Used In Castings
- All About Investment Casting – What it is and How it works
- All About Sand Casting – What it is and How it Works
- Types of Casting Defects and How to Prevent Them
