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The Process

Castings

Permanent Mold Castings

Permanent Mold vs. Other Processes

Pouring Permanent Mold Castings

Post-Casting Operations

Castings

When selecting a method of producing a part, various manufacturing techniques can be explored, such as forgings, machining, castings, weldments, extrusions, stampings, etc. Casting is a metal shaping process during which molten metal is poured into a mold.  Choosing a casting has many advantages. The most intricate of shapes, both external and internal, may be cast.  Because of their physical properties, some metals can only be cast to shape. Indeed, objects may be cast in a single piece which would otherwise require assembly of several pieces if made by other methods. Metal casting is a process highly adaptable to the requirements of mass production. Large numbers of a given casting may be produced very rapidly.

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Permanent Mold Castings

The Permanent Mold Casting Process (also referred to as Gravity Die Casting) is a molding method that incorporates gravity-induced pressure with rapid solidification to produce both aesthetically pleasing and fully functional castings. The Permanent Mold Casting Process consists of pouring molten metal into a permanent mold, usually created from iron or steel (as opposed to molds made of sand for the sand casting process). The molten metal enters the mould cavity under gravity pressure, flowing into the small crevices of the mold, and remains there until solidification to produce intricate and fully formed castings. This process is cost competitive with medium to large volumes of production.

Metal inserts can be used to produce various features during the casting process which will not need to be machined afterward, thus yielding some cost reduction. Sand cores (chemically bonded sand used to generate a desired shape inside a casting ) can also be used to hollow out the inner sections of a casting or to produce an area with undercuts – this variant is called Semi-Permanent Mold castings. Powercast manufactures its own sand cores.

Typical casting sizes range from 50 g to 70 kg (1.5 ounces to 150 lb). Typical materials used with this process include aluminium, magnesium and brass alloys. Powercast typically pours various aluminium alloys.

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Permanent Mold vs. Other Processes

When selecting a casting process, the permanent mold process is often compared to two others – namely the sand casting and pressure die casting processes. While each process has its advantages and disadvantages, the permanent mold process offers a middle-ground alternative: affordable tooling and the possibility of using cores (like sand castings), with excellent part accuracy and surface finish (similarly to pressure die casting parts). The fact that these castings use gravity as pressure ensures that they can achieve high levels of quality (as opposed to pressure that is forced – which may entrap air inside the casting). Moreover, due to the rapid heat transfer from the molten metal to the mold itself, permanent mold castings have finer grain structures and better strength properties than casts made by sand casting method. Bottom line: permanent mold castings often yield the optimal combination of high quality and competitive cost.

  SAND Castings PERMANENT MOLD Castings (a.k.a. GRAVITY DIE Castings) (PRESSURE) DIE Castings
PART COST ½
TOOLING COST ½
SURFACE FINISH ½
WALL THICKNESS ½
CORES/INSERTS N/A /
QUANTITIES (TYP.) Small Medium to Large Large
HEAT TREATING N/A
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Pouring Permanent Mold Castings

The type of furnaces used to melt aluminium varies per foundry; some prefer crucibles, other refractory-type, etc., and they all vary in sizes, depending on the foundry’s operations and plant layout. Some are gas-fired, while others are electric. Various aluminium alloys are poured, such as 319, 356, A356, 413, etc., depending on customer requirements. Alloys must be heated to the right temperature, with the right gas level.

Before and during the casting process, the molds are usually sprayed with one or more coatings: for instance one type is generally graphite-based and acts as a die-release agent, while another is silica-based and is used as a heat-preserving layer. The molds required to be pre-heated at the right temperature before the metal is poured into the cavity.

Permanent mold castings are generally either poured using a Static Pour or a Tilt Pour system. The Static Pour is the more traditional method of pouring – the aluminium is poured directly into the mold cavity, and the casting is removed after solidification. The molds are closed and set into the vertical position for pouring; thus, the parting line is in the vertical position. It is a flexible method of pouring and can accommodate various shapes and sizes of castings. The tilt Pour process involves closing and placing the mold in the horizontal position at which point molten metal is poured into a cup(s) attached to the mold. The mold is then tilted to the vertical position, allowing the molten metal to flow out of the cup(s) into the mold cavity. The tilt time is predetermined and programmed; hence part of the production process is automated. This partial automation helps control the metal flow into the mold cavity, thereby minimizing turbulence. Lesser turbulence generates better castings. Semi-Permanent Mold castings can be poured using either of the aforementioned pouring methods.

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Post-Casting Operations

Once the castings are poured, one can improve their mechanical properties by having them go through a Heat Treating process. This process consists of using a combination of heating, quenching and cooling in order to artificially age the castings quickly, thereby improving their hardness, conductivity, and strength. Once the parts are heat treated, they are also easier to machine. Powercast conducts its own heat treating. Permanent mold aluminium castings can be machined, anodized, painted, etc., and Powercast can take care of these additional operations for you. Most aluminium castings are used as is, while other are anodized or painted, depending on their functionality.

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