Casting

Casting is a process where liquid is poured into a mold, allowed to solidify or harden in the shape
of the mold, and then removed, leaving a fabricated object or part. The process ensures a timely production of parts that would otherwise have been manufactured using time-consuming and expensive machining operations. The four most common casting methods used today are: die casting, investment casting, sand casting and vacuum casting.

 

Die Casting

Die castings are produced by injecting melted metal into a hardened metal mold, called a die. Die castings have the advantage of being able to rapidly create thin wall complex shapes with high accuracy. However, they are limited to low melting point allows as high melting point alloys would damage the die. Once removed from the die, die castings require only very little secondary operations and while the process is expensive, cost savings can be attained if a large number of castings are produced at once. Die cast parts are generally between 1 – 20 pounds but sometimes are as heavy as 50 lbs. Examples include door handles, hood ornaments, and lamp bases.

Materials that are predominantly used in die-casting include aluminum, zinc, and copper. Pure aluminum is rarely cast by itself, however when combined with silicon and copper, the result increases melt fluidity and hardness and reduces machinability and ductility.  Due to its high melt fluidity, zinc can be made with thinner walls than aluminum. Aluminum of 4% is alloyed with zinc to add strength and hardness. Zinc alloys are mainly used for the production of parts such as gears, sprockets, and connector housings.

ADVANTAGES		DISADVANTAGES
• Complex Shapes Possible • Thin Wall Section Possible • High Production Rates • High Dimensional Accuracy • Minimum surface Treatment Prior to Plating		• Castings Generally Limited to 20 lbs. or less • Voids & Porosity is Possible in Complex Shapes • Expensive Machinery & Dies • Limited to Metals with Low Melting Points

 

Investment Casting

Investment casting produces near-net-shape configurations, offering designers and engineers freedom of design in a wide range of alloys. The process is capable of producing precise detail and dimensional accuracy in parts weighing many pounds or just a few ounces. Investment Castings use the “Lost Wax Process” to produce intricate castings with very tight tolerances. This method is used to mass produce parts that demand a high quality "as cast finish", which produces a visually attractive finish and also reduces machining cost. In this process a pattern is made by injecting wax or plastic into a metal mold. The pattern is then coated with slurry (a watery paste of silica and a binder) that will harden to produce a mold or shell. The mold is then turned upside down and the wax or plastic is melted out of the mold, revealing the final product.

The investment casting process is more expensive than the other casting methods, but because of the reduced need for secondary machining, it is more economical to use this process with complex parts. Investment casting is generally used for parts between 1 ounce and 10 lbs., however heavier parts are also occasionally produced. Typical materials that are used in investment casting include aluminum alloys, bronzes, tool steels, stainless steels, hastelloys, and precious metals.

ADVANTAGES		DISADVANTAGES
• Complex Shapes Possible • Thin Wall Section Possible • High Production Rates • High Dimensional Accuracy • Can be Used With Most Pourable Metals/Alloys • Minimum surface Treatment Prior to Plating		• Castings Generally Limited to 10 lbs. or less • Expensive Dies for Wax Pattern • Expensive Unit Cost, Labor Intensive

 

Sand Casting

The sand casting process involves a pattern (made out of wood, plastic, or metal) that is made in the shape of the desired part. Sand casting can produce simple designs (made in a single piece) or more complex ones (made in two parts, called split patterns). The split pattern casting has a top and bottom section, referred to as a cope (top) and a drag (bottom). The patterns are packed in sand with a binder, which hardens the sand into a semi-permanent shape. When the sand mold is cured, the pattern is removed and a hallow space in the sand is left in the shape of the desired part.

When the metal cools, the sand mold is removed and the metal part is ready for secondary operations, such as machining & plating. Sand casting is the least expensive of all of the casting processes. It is used to make larger parts than other casting methods. Metals that are commonly used in the sand casting process include iron (most popular), steel, bronze, brass, and aluminum alloy.

ADVANTAGES		DISADVANTAGES
• Least expensive casting process • Castings can be up to several tons • Can cast intricate shapes • Can be used with most pourable metals and alloys		• Can only cast basic parts shape • Castings require secondary machining • Rough surface finish

 

Vacuum Casting

Vacuum casting is used in the casting of jewelry or similar small metal parts that have fine detail. The process involves pouring a liquid (usually a polyurethane plastic) into a molded form that is pieced together along a parting line. It consists of three production phases: mold making, part pouring, and the curing process that includes the post cure finishing.

To produce the casting, the liquid mixture is poured into the mold cavity under a vacuum. This vacuum, created by the casting machine’s pump, eliminates air from both the liquid plastic and the mold cavity. Once fully mixed, the mixture is carried from a material chamber to mold chamber through tubing by gravity. Mixing time, mixing speed and temperature of the polyurethane are all factors in the cure time and shrink rate of the cure part.

Vacuum casting has the distinct advantage of low cost compared to traditional development processes. The urethane part used in vacuum casting is less expensive and has a quicker turn around and reworking flexibility.