A Primer on Permanent Mold Casting

An MCDP Staff Report
Click here to see this story as it appears in the November/December 2017 issue of Metal Casting Design & Purchasing

The permanent mold casting process produces engineered components with tight tolerances, good 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 metal mold applies chill characteristics to the metal during solidification for a finer grain structure, reduced porosity and higher mechanical properties of the solidified cast component.

In high volume production, permanent mold castings typically range in weight from 1 oz. to more than 100 lbs.; however, castings up to 400 lbs. are produced commercially. If a casting requires a core to form an internal cavity, either a reusable permanent metal core or a collapsible sand core can be used. This choice is based on casting design and complexity.

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.

Permanent mold casting uses zero to minimal levels of pressure or vacuum to pour metal into the mold while other processes use in excess of 15,000 psi. Generally suited for aluminum and copper-base component manufacturing, the type of permanent mold casting process selected depends upon the type of cast component being produced.

The four main types of permanent mold casting are:
•    Gravity.
•    Tilt pour.
•    Low-pressure.
•    Vacuum.

The selection of the proper permanent mold process for specific components depends on several factors, including quantity, size, cost restrictions and mechanical property requirements. In general, the more a permanent mold process involves pressures and/or vacuums, the greater the mechanical properties that can be achieved, as well as the higher the manufacturing costs. In addition, as the complexities of the casting process increase, so does the complexity and cost of the tooling.

Gravity pouring is the oldest, simplest and most traditional form of permanent mold casting. In the process, two metal mold halves are joined together to form the mold cavity.

Molten metal is poured down into the pouring basin (using gravity) and travels through the gating system into the mold cavity where it solidifies into the cast component.

Tilt pour permanent mold casting takes traditional gravity pour 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. 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. By reducing the turbulence, the mechanical properties of permanent mold castings are increased.

Low-pressure permanent mold casting 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 into the mold. The metal in the fill tube acts as the riser, feeding the casting during solidification, allowing this process to achieve good yield. In addition, by controlling the rate of mold fill with pressure, the metalcaster ensures the mold is filled smoothly, without trapping air and other inclusions in the casting—increasing casting quality and mechanical properties. This method also lends itself to automation, and usually runs at lower mold temperatures with shorter cycle times than conventional permanent mold. The lower temperature mold reduces casting solidification time, which increases casting mechanical properties.

Vacuum casting is similar to low-pressure casting except that 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 the case of vacuum and low-pressure, many innovations to the processes have allowed metalcasters to combine both vacuum and pressure during casting to better control mold fill. This is the key with the more advanced permanent mold casting manufacturing—controlling the flow of metal into the mold to ensure as tranquil a mold fill as possible in as short of a casting cycle as possible. The faster and less turbulent, the higher quality casting at a lower cost.

Metal Variations
The main metals cast in permanent molds are aluminum and copper-based alloys, but gray and ductile iron, magnesium and zinc also can be cast. Although the basic casting process doesn’t change between aluminum and copper-base casting, each metal, due to its individual physical and metallurgical properties, has unique concerns that must be addressed during casting.

Aluminum Alloys
The choice of aluminum alloys is based on the fluidity and mold filling characteristics of the metal. Due to the tendency for hot cracking of aluminum alloys, these alloys usually contain 5% or more silicon. Some aluminum alloys that are traditionally cast using the permanent mold process are:
319—This alloy and its variants are used when moderate mechanical properties without heat treatment are satisfactory.
333 and A333—These alloys have unusually good casting characteristics in permanent mold and develop better as-cast mechanical properties than 319-type alloys.
355 and 356—These heat-treatable alloys have good castability and are used when good tensile properties are required. 356 also has excellent corrosion resistance. By decreasing the impurities in these alloys to form C355 and A356, the mechanical properties of the alloys greatly improve.
443—This alloy and its variants are used when high ductility and corrosion-resistance are required but high strength isn’t important.
513—As a straight aluminum-magnesium alloy, with the addition of 2% zinc, alloy 513 can be used for simple castings in which outstanding corrosion resistance and good surface finish are required.

The metalcasting industry has been researching, trialing and proving the permanent mold casting of a variety of new alloys, such as A206. Always check with your casting supplier for other possible alloy options that could be a good choice.

Copper-Base Alloys
As with aluminum alloys, certain copper-base alloys are more readily cast in permanent mold. Because some of the alloy families’ limitations, it is important to consult the metalcaster to determine the ability to cast an alloy for a certain application. The following alloys are arranged by family with the unified numbering system five-digit code developed by the Copper Development Association:

Yellow Brasses (C8330-C89990)—These are copper alloys in which zinc is the alloying element. Although corrosion resistance in these alloys is lower than its counterparts, the high strength yellow brasses (C86200 and C86500) are used for mechanical products requiring good wear.

Silicon Bronzes/Brasses (C87300-C87900)—These are intermediate strength alloys. They exhibit good corrosion resistance in water, good casting characteristics and acceptable machinability. The silicon imparts the ability to cast fi ne detail and improves the casting’s surface finish.

Aluminum Bronzes (C95200-C95900)—These contain 3-12% aluminum, and iron, silicon, nickel and manganese are added singly or in combination for higher strength and corrosion resistance. These alloys form protective, alumina-rich corrosion product films, which provide exceptional oxidation and corrosion resistance. In addition, they exhibit moderate to high strengths and can be heat-treated to tensile strengths over 100 ksi.

SeBiLoy III (C89550)—This alloy is a selenium-bismuth containing yellow brass that was specifically developed for permanent mold casting. It is a lead-free, free-machining brass for potable water applications that exhibits improved mechanical properties, hot tearing and fluidity. The strength of the alloy also exceeds yellow brass.

Design Considerations
In order to achieve the required quality levels at the lowest cost, designers have many considerations to apply to any casting method. For example, sections should be as uniform as possible, without abrupt changes in thickness. Heavy sections should not be isolated, and tolerances should be no tighter than necessary.

In addition to the general rules, the following are a few specifics applicable to the low-cost production of permanent mold cast components. Every permanent mold casting facility is able to achieve different design standards, so it always best to confer with the casting supplier about limitations.
•     When possible, all locating points should be in the same half of the mold cavity. In addition, locating points should be kept away from gates, risers, parting lines and ejector pins.
•     The use of cored holes less than 0.25 in. in diameter should be avoided, even though cored holes of 0.125 in. are possible.
•     Draft angles in the direction of metal flow on outside surfaces may vary from 1 to more than 10 degrees and internal draft from slightly less than 2-20 degrees. However, using minimum draft increases casting difficulty and cost. Internal walls can be cast without draft if collapsible metal cores are used, but the practice increases cost.
•     Nuts, bushings, studs and other inserts often can be cast in place. The bond between casting and insert can be mechanical and/or metallurgical.
•     Under conditions of best control in small molds, allowance for machining stock can be less than 0.03 in. (0.8mm). However, maintaining machining allowances this low usually increases per piece costs. It is more practical to allow 0.03-0.06 in. (0.8-1.6 mm) of machining stock for castings up to 10 in. (250 mm) in major dimensions and to allow up to 0.125 in for larger castings.
•     The designer should not expect castings to have a surface finish of better than 100 micro inches (2.5 micro meters) under optimum conditions. Ordinarily, casting finish ranges from 125 to 300 μin. depending upon the metal being cast.

The production of a casting often can be improved by avoiding abrupt changes in section thickness. Heavy flanges adjacent to a thin wall are especially likely to cause non-uniform freezing and hot tears. In such cases, redesign of the casting may be necessary. The minimum section thickness producible at reasonable cost varies considerably with the size of the casting and the uniformity of wall thickness in the casting.

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.