After deciding on your specifications, it's time to select the appropriate alloy to use. Alloy selection is based on many factors, including the part geometry, functional requirements, and cosmetic requirements. Other factors include your weight and strength requirements, possible corrosion resistance needs, surface finish or coatings, and the projected lifetime pieces required for your design.
The alloy choices Chicago White Metal offers are aluminum, magnesium, and zinc. Each metal provides unique benefits. Aluminum is a lightweight alloy and is used in projects that require corrosion protection, electrical conductivity, or dimensional stability. Magnesium offers the best strength-to-weight ratio, making it ideal for portable applications. Zinc alloys provide good impact strength, excellent surface for additional finishing, and it has the ability to be cast very thin while still maintaining its strength.
Depending on the metal being cast, a hot or cold chamber die casting machine is used. When making a casting out of zinc or magnesium, the hot chamber casting process is the most reliable option. This system is the most beneficial because the alloys are actually melted at the die casting machine itself and molten metal enters the "gooseneck" automatically. The hydraulic-powered vertical piston then forces the metal through the gooseneck and into the die. This provides for a very fast cycle with little to no operator involvement.
The cold chamber casting process is used to cast aluminum. Large quantities of aluminum are melted in a separate furnace and then transferred to the die casting machine where it is held in the machine's "holding furnace." When the cycle starts, a specific amount of aluminum is "ladled" into an unheated injection cylinder and a hydraulic piston pushes it into the die.
Following the solidification of the metal inside the die cavity, the machine's hydraulic clamping system releases the moving die half, allowing the ejection system to push the casting out of the mold. Castings then are transferred to the next stage where they are placed into a trim die. This operation removes the excess material from the part. This excess includes the gate, runners, overflows, and flash from the casting.
Conversely, where the die casting process is quick and efficient, the process of machining parts takes much longer. Although machining has low setup and tooling costs, long machining times may be required, making it unaffordable for larger quantities. As a result, machining is most often used for limited quantities. The process is usually reserved for fabrication of prototypes or custom tooling for other manufacturing processes because of time consumption and high cost.
I agree that converting machined parts to die casting usually makes sense. Because die casting tools can be expensive, it is important to first do a pay-back analysis and see if the volumes justify this change over.
In many cases we use both processes during the life of the product. When the initial design is likely to change and we need to enter the market quickly, we may start with a machined part. Then, as the design becomes stable and production volumes increase, we plan for a smooth cut-over to die cast tooling.
In most cases, casting a part versus machining it from bar stock is a no-brainer. In my career, I've only come across one part that made more sense as a screw-machined part than as a die casting. In that case, the geometry of the part made it extremely easy to screw machine. Also, screw machining allowed the part to be made out of a much stronger wrought alloy. It wound up being an 80% cost savings (from $4 to about 80¢), along with a more than 50% increase in strength.
But this is far from the norm, and as this article shows, casting is almost always much cheaper. A more interesting comparison would be between die casting and powder metallurgy. It would also be worthwhile to compare different casting processes (die casting, semi-solid processing, permanent mold, investment casting, lost foam, etc.). In addition to cost, these processes also vary in terms of the mechanical properties and dimensional accuracy that can be achieved.
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