High-Pressure Die Casting
This casting process injects molten non-ferrous metal such as aluminum or zinc into the cavity of a mold – also known as a die – under very high pressure. The cavity, which will create the shape and details of the part, is produced using two hardened steel mold cavities which have been machined in the reverse image of the part.
The dies are prepared each cycle by spraying the mold cavity with lubricant, which helps control the temperature of the die and assists in the removal of the casting. The molten metal is then poured into a sleeve and injected into the mold with the use of a plunger connected to a hydraulic cylinder that operates at high speed and pressure. Once the mold is filled, the pressure is maintained until the casting solidifies. The dies are then opened, then the shot (which can be multiple castings) is ejected from the die. Since dies can be re-used multiple times, the process is repeated depending on how many final parts are needed. The type of metal being cast determines whether a hot-chamber or cold-chamber machine will be used. The cold chamber machine is normally used for aluminum alloys and hot chamber is used for zinc alloys.
The casting equipment and metal dies require large capital costs, so this process is most appropriate for higher-volume production. While the equipment requires a significant investment, manufacturing of parts using this process is relatively simple and the die can last hundreds of thousands of cycles. This keeps the incremental cost per item low. High-pressure die casting is particularly well-suited for a large quantity of thin walled castings, from small-sized to large-sized, including parts which require greater detail and precision, near net final shape. This is why it’s the most used casting process. Die castings are characterized by a very good surface finish and dimensional consistency.
Sand has been around for millennia, so it’s not surprising that sand casting has an equally long history. It is a type of casting within the gravity casting process category, because the molten metal is poured from a ladle into a mold made from sand, using gravity rather than high-pressure injection to fill the die. The sand molds are non-reusable, although the sand can be recycled, and this process can produce both ferrous and non-ferrous materials to create products across a wide range of shapes and sizes. Different types of sand offer varying abilities to withstand heat, and allow gasses and steam to escape during the casting process.
The process begins by placing the pattern for the product into the sand mold. The pattern, gating system, and sand are placed in the mold to create a cavity in the sand, then the pattern is removed leaving the shape of the part and gate feed system in the sand. The molten metal is poured into the cavity, the metal cools and sets, then the sand mold is broken away in a “shakeout” process in order to remove the casting from the sand. Sand cores can also be placed in the mold before pouring to produce very intricate shapes and internal passages making this a preferred process for very complex castings.
Sand casting is relatively inexpensive, has the ability to produce castings in almost any material that can be melted and is suitable for both low-volume and medium-volume production, and is appropriate for parts of various sizes that may not need the precision, speed or volume of high-pressure die casting. Due to the gravity filling process, the wall thickness of a sand casting is much greater than a high-pressure die casting.
Permanent Mold Casting
This casting process shares similarities to both die casting and sand casting. As in sand casting, the molten metal is poured into a mold with a gravity process. The mold is clamped shut during the pouring process until the part cools and solidifies. Yet the mold is not destroyed after each cycle like you will find in sand casting. In contrast, permanent mold casting uses a metal mold that is typically made from steel or cast iron, and can be reused for several thousand cycles. This makes it much like die casting although you can also use sand cores to produce the complex internal shapes found in sand casting. This process is used with non-ferrous alloys due to the construction of the steel dies.
The process begins with mold preparation with heating and application of a ceramic coating to the surface of the mold that produces the part. The mold’s two halves are assembled, or clamped together with hydraulic press. The pouring phase is a gravity casting process of pouring molten metal at a slow rate from a ladle into the mold via a runner system. The molten metal is then allowed to cool and solidify. The two mold halves are opened, the casting is removed and any excess metal is trimmed away.
Permanent mold casting is typically used for medium-volume to high-volume production of small, simple metal parts with uniform wall thickness. The cost of tooling is lower compared to high-pressure die casting but more than a pattern in a sand cast process. Wall thickness of parts tends to be closer to sand casting due to the gravity pouring process.