Plaster mold casting is a prototype manufacturing process for simulated die-castings. Although there are several variations of this process, it usually begins with a master shape of the die-casting.
It is not absolutely necessary to include draft in the walls at this stage, but it can help. A silicone rubber reversal is then molded over the master. A second silicone rubber is molded into the first. This provides a silicone rubber positive of the original model. Plaster is molded around the second silicone rubber positive to provide a plaster cavity. Metal is poured into the plaster cavity. After solidification of the metal, the plaster is broken away.
The rubber version of the master is required so that it can easily be withdrawn from the plaster mold. It is also possible to mold epoxy off of the master and pour plaster over this. The epoxy molds will have a greater life than those made from rubber.
Typical leadtimes from the manufacture of the master model to manufacture of 10 castings is about 8 days, and two weeks to produce 30 to 50 castings. However, three to four weeks is a more typical delivery time. The cost of prototyping with this process is about two to five percent of the cost of a production die, so it is considered to be a good insurance.
The advantages of the process are low mold cost and good surface detail. Also, it’s possible to produce reasonably large parts with this process. One disadvantages is lower cooling rates, which means poorer mechanical properties. This can lead to parts with a yield strength that is 20 percent lower than conventional die-casting. Another disadvantage is that you must produce a new mold for each casting. And finally, slightly different alloys are used for plaster casting compared to die-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.
it is Very rare that you still remember plaster mold