Known for its ability to achieve tight tolerances, superior surface finishes and high mechanical properties, permanent mold is a quality option for many applications.
Fig. 1. This aluminum transmission main housing was cast via semi-permanent mold using 27 sand cores.
Unlike sand and plaster molding processes, where the mold is recycled after every part, the molds used in permanent mold casting can be used to produce thousands of components before they have to be repaired. The permanent mold casting process is known for its ability to achieve tight tolerances, superior surface finishes and high mechanical properties. Its ability to achieve these details is based on the fundamental principle of this process—the pouring of molten metal into reusable metal molds.
This intake manifold features cast bosses on the runners for a direct port nitrous system for competition applications.
The basic steps of the permanent mold process are as follows:
- A refractory or mold coating is applied by brushing, spraying or dipping the preheated metal mold to act as a release agent, which insulates the mold surfaces from thermal shock and prevents premature freezing of the molten metal.
- Sand or metal cores (if needed) are inserted to form internal cavities in the casting and the mold is closed either manually or mechanically.
- The alloy, heated above its melting temperature, is poured into the mold via gravity or counter gravity. The metal then travels through the mold’s filling/feeding system into the mold cavity.
- Once the casting has solidified, it is removed from the mold and follows the usual process steps for core knockout, cutoff and finishing.
Fig. 2. In tilt-pour permanent mold, molten metal is poured into cups outside of the mold. The mold is then tilted, and the metal enters the mold via gravity
The advantages of permanent mold include:
- Superior dimensional accuracy.
- The ability to achieve high-quality as-cast surface finishes of 100 root mean squared (RMS).
- Dimensional consistency from part to part with cored holes, bosses, pads and other points.
- The ability to cast - in ferrous and nonferrous inserts to combine properties for improved strength, wear resistance or fatigue life.
- A finer grain structure, less porosity and better mechanical properties due to the increased chill of the metal mold.
This cast component features six separate cored passages and chambers for oil and coolant, along with integral water pump geometry.
The four main types of permanent mold casting are:
- 1. Gravity, where two metal mold halves are joined together to form the mold cavity.
- 2. Tilt pour, which takes traditional gravity pour-ing and kicks it up a notch by turning/rotating the metal mold during or after the metal is poured into a pouring basin to fill the mold cavity (Fig. 2). The molds rotate up to 90 degrees during the tilting process, with the goal of reducing the turbulence the metal encounters as it travels from the pouring basin, through the gating system and into the mold cavity.
- 3. Low-pressure permanent mold casting, which turns the mold upside down and places it in a casting device above a sealed airtight chamber that contains a crucible holding molten metal. A fill tube extends from the mold down into the molten metal. The casting is made by pressurizing the chamber containing the molten metal and forcing the metal up into the mold.
- 4. Vacuum casting, where a vacuum is created within the mold cavity and the metal is pulled rather than pushed into the mold. Similar to low-pressure, excellent mechanical properties and high production rates are the norm with this process due to the low mold temperature. In addition, this process achieves similar casting yield results as low-pressure. However, this process is usually associated with smaller castings and requires specialized, complex mold designs to induce the vacuum properly.
In conclusion the article stated, "As with all cast component designs as well as manufacturing process choices, it is best to consult with your casting suppliers and allow them the opportunity to assist in process selection as well as design for manufacturing."
Fig. 3. This lightweight aluminum casting features 4-mm walls and 29 windows. It was produced via low-pressure permanent mold casting.
The thin and uniform wall thickness is helpful in optimizing weight, and low pressure permanent mold casting is a near net shaped process that provided excellent surface ﬁnish and dimensional tolerances.
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