The practice of sand casting involves the use of sand as a mold material to perform metal casting. The process is one of the most ancient methods used in manufacturing metal parts and items. Nowadays, sand casting remains one of the most widely employed casting techniques.
Several reasons account for the popularity of sand mold casting. Firstly, this form of casting typically involves very low overhead and permits the use of the lowest, most cost-effective tooling techniques. In addition, manufacturers can cast practically any metal employing sand mold casting. Also, the process allows for the production of virtually any size, shape and weight of metal part.
Although the process of sand casting has remained the same for several hundred years, recent innovations have enabled manufacturers to create expandable sand molds more quickly and efficiently. Currently, around 75% of all US copper alloys, stainless and aluminum alloys, as well as cast or ductile iron are being produced using the sand mold casting process.
Advantages of Sand Castings
Although individual production situations may differ, sand mold castings in general provide a number of benefits, including the following:
High efficiency: Sand mold casting typically does not involve a lot of waste, making it a very efficient process. Today, many manufacturers reclaim and eventually re-use a high percentage of the casting sand utilized during this form of casting. Recycling excess parts significantly reduces the cost of the overall process.
Producing parts with complex details: Sand casting permits the production of intricate metal parts and exterior surfaces. It also allows for the production of varying metal parts with complex details.
Generous tolerance ranges: Depending on the metal and available manufacturing technologies, sand mold casting may permit dimensional control and consistency within ±0.030 to ±0.125” (± 0.8 to 3.2 mm) tolerance range. Usually, designers require more generous tolerance ranges along parting lines.
The ability to utilize a variety of finishes: Sand mold castings generally utilize a variety of surface finishes, including but not limited to paint, powder coating, electroplating, shot blast finish and more. Usually, surface finish will range between approximately 300 and 500 gin (7.7 – 12.9 gm) rms. Also, minimum wall thickness amounts to.125”, whereas the desirable wall thickness equals .250”.
Relatively low tooling and part costs: Sand mold casting does not necessarily involve the use of expensive manufacturing tools or supplies. Consequently, manufacturers can often produce products comparatively inexpensively utilizing this process.
The ability to cast a wide variety of metals: Sand mold casting allows for the use of almost any metal or metal alloy. For example, manufacturers can utilize this process to create castings in cast iron, copper, copper alloys, carbon steel, steel alloys, gold, stainless steel, brass, silver and complex metal alloys.
High versatility: High quality sand mold casting techniques offer the benefit of versatility. In other words, the process can be used to create both simple and complex metal parts.
Production volume flexibility: Manufacturers apply sand mold casting for both low and high volume production runs. Therefore, this manufacturing technique may prove highly cost-effective in situations in which a company does not know how many units it will eventually need to produce.
The Sand Casting Process
Sand mold casting has achieved popularity in part because this manufacturing process requires only five essential components:
- metal (usually in the form of ingots or scrap metal)
- a metal-melting furnace
- a wood or metal pattern
- casting sand which typically includes a bonding agent
- a sand mold box, also referred to as a casting flask
Sand mold casting involves the following ten steps:
1. Initially, a prototype pattern in wood or metal is generated to accurately duplicate the structure and design of a metal part. The prototype typically includes an excess portion called a sprue gate to create a channel for the entry of molten metal in the final sand mold.
2. The pattern is then placed into a metal or wooden box. The prototype pattern must fit entirely within the enclosing sand mold casting flask.
3. Casting sand is firmly inserted around the prototype on both sections of the sand mold. The sand around the pattern is rammed in order to create the part cavity.
4. Next, the prototype pattern is withdrawn from the sand mold flask, leaving a hollow impression in the sand. This impression matches the prototype pattern in every respect when joined together along the parting line.
5. A furnace is then used to melt ingots or scrap metal. It may utilize electricity or gas in order to achieve a desired temperature.
6. The sprue channel is used to pour the molten liquid into the empty sand mold cavity.
7. The metal settles in the cavity and gradually solidifies again as it cools.
8. After the metal has cooled down sufficiently, sand is removed from the cast part either manually or through mechanical means.
9. A metal saw or band saw blade is employed to remove the sprue funnel and vent hole burrs from the final casting.
10. The final step in the manufacturing process involves cleaning the casting, blasting away any remaining sand and removing burrs as well as any other imperfections from the parting line.
Sand Casting Molds And Tooling
Today, sand mold casting often occurs in highly automated environments. Although the basic process remains simple, modern manufacturing techniques have added additional layers of complexity. Several factors influence the process of sand casting molds and performing tooling procedures. As a general matter, the following principles apply when performing industrial high quality sand mold casting:
Sand Casting Molds
Under most circumstances, sand casting molds are manufactured in two sections: an upper portion called the cope and a lower portion called the drag. These two distinct parts form a cavity when the cope is affixed over the drag. The line where the cope and the drag meet is usually referred to as the parting line and a cast part usually requires extra finishing in this region.
A funnel-like connection called a sprue provides a hole for injecting molten metal into the mold. Additionally, sand casting molds typically contain a riser or vent hole to permit the escape of gases and enable sufficient metal to fill the mold. Nowadays, most sand casting prototypes designate the location for this important feature. As the metal is poured into the mold’s cavity, gases will escape through the vent hole. The vent’s presence helps reduce the formation of bubbles and other defects within the cast part.
Three key variables affect the selection of tooling techniques: the number of parts in a production run, the foundry’s available sand molding equipment, and the overall size of each part. For very low production volumes, modern foundries often employ loose or split patterns. Yet in most situations, foundries today rely upon match plates. Workers mount the drag and the cope on both surfaces of an aluminum plate or a board. They may use multiple impressions on a single plate.
When manufacturers employ high speed molding equipment, they typically require the use of aluminum match plates. They may generate cores with the assistance of molds within core boxes. Costs may vary widely depending upon the type of process utilized to manufacture cores.