Focus: Castings

Casting tends to generally be a labor intensive process lending it to economical production in China. Dies and molds are typically cost barriers for shorter runs in places like the US and Canada. By manufacturing with Riverstone, small and medium sized OEMs can take advantage of making the dies / molds which can cost as little as a third of the cost in North America and Europe. There are a wide variety of casting processes each with its own advantages and challenges. At Riverstone, we work primarily with four types of castings:

Investment Castings
Permanent Mold Castings
Die Castings
Sand Castings

Investment Castings

The investment-casting process, also called the lost-wax process or precision casting, uses a pattern made of wax or a plastic such as polystyrene.

Key Advantages

• Suitable for casting high-melting-point alloys with a good surface finish and close tolerances.
• Little or no finishing is required, which otherwise would add significantly to the total cost of the casting.
• High degree of design freedom. Complex shapes that would be too costly to machine can be produced quickly and economically as investment castings.

Disadvantages / Challenges

• The labor and materials involved make the lost-wax process relatively more costly

Investment Casting Process

The first production step is the injection of the wax pattern. This is done by melting the pattern wax and transferring it to one of the wax injection machines in a closed loop piping system. Once the proper temperature is reached, the wax is injected into the metal mold under precise conditions of pressure and flow rates. The metal mold remains closed until the wax solidifies.

After solidification is complete the die is opened and the wax pattern carefully removed. Individual wax patterns are then welded to a wax gating system to form a "tree". For the highest quality castings, it is critical that process design and control in the wax is monitored closely. See video as follows:

The next step is called "investing" where the wax is surrounded completely by a water based ceramic slurry as follows:

After the ceramic has set, the mold is moved to a steam chamber. In this chamber the heat of the steam melts the wax from the mold without changing the mold. The dewaxed molds are then placed in controlled ovens for programmed firing. After the molds are completely void of wax and moisture and the ceramic has been transformed to a phase that is no strong enough to withstand the stress of the metal casting process:

In the metal casting operation, the hot molds are taken from the firing ovens, loaded into a vacuum chamber along with properly prepared molten metal, and poured by remote control. The vacuum is released after pouring, castings are further densified and then passed on to ceramic removal. The castings are then cleaned to then move on to the finishing process.

Basic Design Notes and Constraints

Tolerances of 0.5 % of length are possible, and as low as 0.15 % is possible for small dimensions. Castings can weigh from a few grams to 35 kg (0.1 oz to 80 lb), although the normal size ranges from 200 g to about 8 kg (7 oz to 15 lb). Normal minimum wall thicknesses are about 1 mm to about 0.5 mm (0.040-0.020 in) for alloys that can be cast easily.

The types of materials that can be cast are Aluminum alloys, Bronzes, tool steels, stainless steels, Stellite, Hastelloys, and precious metals. Parts made with investment castings often do not require any further machining, because of the close tolerances that can be achieved.

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Permanent Mold Castings

Instead of using sand as the mold material, a metal is used as a mold. Typically cast iron or Meehanite (a dense cast iron) is used as the mold material and the cores are made from metal or sand. Cavity surfaces are coated with a thin layer of heat resistant material such as clay or sodium silicate.

The molds are pre-heated upto 200 ºC (392 ºF) before the metal is poured into the cavity. The cavity design for these molds do not follow the same rules for shrinkage as in sand casting molds, due to the fact that the metal molds heat up and expand during the pour, so the cavity do not need to be expanded as much as in the sand castings. However, care has to be taken to ensure proper thermal balance, by using external water cooling or appropriate radiation techniques.

At a production run of 1000 or more parts, permanent mold castings produce a lower piece cost part. Of course, the break-even point depends on the complexity of the part. More complex parts being favored by the use of permanent molds.

The usual considerations of minimum wall thicknesses (such as 3mm for lengths under 75 mm), radius (inside radius = nominal wall thickness, outside radius = 3 x nominal wall thickness), draft angles (1 to 3º on outside surfaces, 2 to 5º on inside surfaces) etc all apply. Typical tolerances are 2 % of linear dimensions. Surface finish ranges from 2.5 µm to 7.5 µm (100 µin to 250 µin).

Typical part sizes range from 50 g to 70 kg (1.5 ounces to 150 lb). Typical materials used are small and medium sized parts made from aluminum, magnesium and brass and their alloys. Typical parts include gears, splines, wheels, gear housings, pipefittings, fuel injection housings, and automotive engine pistons.

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Die Castings

Developed in the early 1900s, die casting is a varient of permanent-mold casting where metal is injected into the mold under high pressure of 10-210Mpa (1,450-30,500) psi. This results in a more uniform part, generally good surface finish and good dimensional accuracy, as good as 0.2 % of casting dimension. For many parts, post-machining can be totally eliminated, or very light machining may be required to bring dimensions to size. The weight of most casting ranges from less than 90 grams to about 20 kilograms.

Key Advantages

• High rate of production with high accuracy in sustaining dimensions part to part
• Smooth surface finish and able to incorporate cast-in details as holes, openings, slots, trademarks, numbers, reducing or eliminating need for post-machining
• Able to cast in inserts such as pins, studs, shafts, linings, bushings, fasteners, strengtheners, and heating elements
• Can produce thinner walls than those produced by other casting processes

Disadvantages / Challenges

• Dies are complicated and relatively more costly to construct so that high rates of usage are needed to justify the use of this process
• As the die is filled violently and solidification happens quickly, typically within half a second, air and die lubricant can be trapped in the cavity. This results in microporosity in the castings
• Generally, walls and other details perpendicular to the parting line can not be made since taper is needed to get the part out of the die. This problem can be solved for by use of expensive core slides

Basic Design Notes and Constraints

Minimum wall thicknesses and minimum draft angles for die casting are as follows:

Die-castings are typically limited from 20 kg (55 lb) max. for Magnesium, to 35 kg (77 lb) max. for Zinc. Large castings tend to have greater porosity problems, due to entrapped air, and the melt solidifying before it gets to the furthest extremities of the die-cast cavity. The porosity problem can be somewhat overcome by vacuum die casting.

From a design point of view, it is best to design parts with uniform wall thicknesses and cores of simple shapes. Heavy sections cause cooling problems, trapped gases causing porosity. All corners should be radiused generously to avoid stress concentration. Draft allowance should be provided to all for releasing the parts-these are typically 0.25º to 0.75º per side depending on the material.

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Sand Castings

Sand casting is used to make large parts (typically Iron, but also Bronze, Brass, Aluminum). Molten metal is poured into a mold cavity formed out of sand (natural or synthetic). From the initial engineering to monitoring the material quality and the production of the casting, Riverstone will help to ensure that your sand castings meet your requirements.

Key Advantages

• Intricate shapes with undercuts, reentrant angles, and complex contours, which would be very difficult to machine, can be cast using sand-mold methods.
• The product can be reassessed by the designer by conducting a stress analysis. Metal can be removed in areas of low stress and added in areas of high stress with relatively simple alterations to the pattern
• Almost any metal that can be melted can be cast.
• Cast components are usually stable, rigid, and strong compared with parts made by other processes.

Disadvantages / Challenges

• Sand-mold castings have irregular, grainy surfaces. Dimensional variations are expected.
• If the casting has a moving contact with other parts or if seals are required, then the casting should often be machined.


• Materials in China are susceptible to impurities and flaws which are critical to monitor, otherwise problems such as outgassing will make finishing considerably more difficult and expensive.

Basic Design Notes and Constraints

The natural shrinkage of cast metal as it cools and solidifies reduce the workpiece dimensions compared to the size of the mold cavity. It also causes induces stresses and distortion. Luckily, the amount of shrinkage of a given metal is predictable and can be compensated for by making patterns slightly oversized.

Potential Defects

Various defects can occur in manufacturing processes, depending on factors such as materials, part design, and processing techniques. While some defects affect only the appearance of parts, others can have major adverse effects on the structural integrity of parts made. As follows:

Metallic projections: fins, flash, or massive projections.
Cavities: Rounded or rough internal or exposed cavities, incl. blowholes, pinholes, and shrinkage cavities (see porosity below).
Discontinuities: Cracks, cold or hot tearing, and cold shuts. If the solidifying metal is constrained from shrinking freely, cracking and tearing can occur. Although many factors are involved in tearing, coarse grain size and the presence of low-melting segregates along the grain boundaries (intergranular) increase the tendency for hot tearing. Incomplete castings result from the molten metal being at too low a temperature or pouring the metal too slowly. Cold shut is an interface in a casting that lacks complete fusion because of the meeting of two streams of liquid metal from different gates.
Defective surface: Folds, laps, scars, adhering sand layers, outgassing, and oxide scale.
Incomplete casting: Misruns (due to premature solidification), insufficient volume of metal poured, and runout (due to loss of metal from mould after pouring).
Incorrect dimensions or shape: From factors such as improper shrinkage allowance, pattern mounting error, irregular contraction, deformed pattern, or warped casting.
Inclusions: Formed during melting, solidification, and moulding. Generally non-metallic, they are regarded as harmful because they act like stress raisers and reduce the strength of the casting. They can be filtered out during processing of the molten metal. Inclusions may form during melting because of reaction of the molten metal with the environment (usually oxygen) or the crucible material. Chemical reactions among components in the molten metal may produce inclusions; slags and other foreign material entrapped in the molten metal also become inclusions. Reactions between the metal and the mould material may produce inclusions. Spalling of the mould and core surfaces also produces inclusions, indicating the importance of the quality and maintenance of moulds

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