Commonly known as the “Lost Wax” process, investment casting is one of the most precise methods used to cast near-net shape components. Process advantages are best utilized when thin walls, tight tolerances or complex geometry is required. The design freedom, investment casting offers, is unlimited. It allows you to combine multiple parts or even manufacturing processes into one component and offers a stronger and more efficient net-shape product. It also offers excellent dimensional stability, surface finish, and size range. When combining added value to the component, the process provides shorter lead-times and lower total costs.

The investment cast process consists of injecting wax into a tool/die and then attaching that wax positive onto a tree along with other positives of the same part. The tree is then dipped or “invested” into a bath of ceramic slurry and made to dry. This process is repeated between 5-7 times, causing a mold of ceramic to be built up around the tree. The mold is then fired in a kiln to melt and evacuate the wax while also curing the ceramic. What you are left with is a hollow and hardened ceramic shell ready for metal. Molten metal is then poured into the hollow mold, filling up the sprue and every cavity attached to it. The metal is then allowed to solidify and the shell is removed shortly thereafter. What remains is a solid metal tree with your parts attached to it. The components are then removed from the tree and processed.

Die Casting

Due to the fast cycle times and repeatability of this process, die casting is typically the preferred method used for “white metals” when medium to high volumes, thin walls and or smooth surface finishes are required. Individual part costs are typically the lowest in this process when compared to other casting methods. However, the per-part cost advantage must be weighed against the initial tooling investment. This can be considerable due to the complexity of the tool required to perform the job, which is typically why you do not see lower volume projects utilizing the die cast process.

Die casting basically consists of molten metal that is forced under pressure, into a steel die cavity to form a desired shape. Once in the cavity, the metal is allowed to solidify and slightly cool. The die is then opened up, the part is removed, the flash and gating are trimmed and the process repeats. Die casting tools are typically complex, having water lines for cooling and vents to allow for trapped air to evacuate. Standard tolerances can vary greatly depending on the metal being cast and the size and geometry of the part. Die life also varies considerably depending on the metal being cast.

Permanent Mold

Permanent mold offers an excellent alternative to die casting for “white metal” castings with low to medium volume requirements. It is also a good choice for large castings that have high volume requirements. Comparatively, a die cast tool for the same part would be extremely expensive. The permanent mold process is ideal for components that do not have the necessary production volumes to justify die casting, but need more process ability than what sand casting can provide.

Compared to aluminum sand castings, those done in permanent molds typically have better mechanical properties. This is due to quicker solidification causing the fill to be more laminar, thus creating less shrinkage, lowering gas porosity and providing finer dendrite arm spacing (DAS). Compared to die casting, those done in permanent molds are less likely to contain entrapped gas. The result is a denser, stronger, tighter aluminum casting that requires less finishing work. These qualities also remain more consistent from part to part, due to having less process variables to manage than in die casting or sand casting.

High quality as-cast surface finishes of 100-125 RMS can be achieved in the permanent mold process. Standard tolerances are typically expressed at +/- .015 or tighter if needed on critical features. Draft allowances of 1 to 3 degrees are recommended depending on the length and draw of the feature.

Permanent mold is similar to die casting in that they both use a steel die to form and solidify the molten metal once it is in the cavity. The primary difference is in how the metal is introduced into the cavity. In permanent mold, the metal is poured into the die, utilizing gravity to fill it. Gravity fed permanent mold yields a denser casting because air is allowed to evacuate while the cavity is being filled. In the case of die casting, the metal is pushed into the die using a pressurized injection system. This can create turbulence and may cause air to remain trapped in the cavity once the die is closed and pressurized.

Sand Casting

There are many hybrid methodologies that are all referred to as sand casting and rightfully so. They all use sand as the primary material to mold and solidify molten metal into a desired shape. However, the process capabilities of these different methods vary greatly, and so for convenience sake, I will simply refer to and describe one of the most common methods which is Green Sand – sand casting.

Sand casting is a good option when a component is needed that has a fairly loose overall tolerance or visual requirement. Tooling and per-part costs for sand casting are typically fair to reasonable. Typical surface finishes are much rougher compared to other processes and the tolerance capabilities are not as accurate. However, components that have minimal visual requirements or few critical dimensions can be a good fit because, like any other casting process, critical dimensions can be dialed in with secondary machining. It’s just a matter of how much machining you want to pay for to achieve your required net shape versus the cost advantages of the casting process.

Annual production volumes can range from low to high depending on the requirements and complexity of the part. With tooling being fairly inexpensive compared to other cast methods, sand casting is an excellent option for prototyping and low volume projects. Sand casting can also be used for projects requiring high volumes with the implementation of automated molding machinery such as a Disa, Hunter or B&P type machine. Automatic molding reduces labor and increases speed and efficiency, making the process competitive for higher volumes.

A typical sand mold is made by blowing or ramming prepared sand around a pattern, held in a flask. The patterns are then withdrawn, creating the mold cavity into which molten metal will be poured. Molds are made in two halves. The upper portion is called the cope, and the lower portion is called the drag. Once both halves of the mold have been made, they are joined together creating the complete mold cavity. The boundary between the cope and the drag is known as the parting line and can usually be seen on the casting. Sand castings typically have tolerance requirements of +/- .030 and 3 to 4 degrees of draft. Tighter tolerance capabilities and less draft can be achieved by some of the hybrid sand processes.

Custom Cold Heading

Custom cold heading is very similar to normal cold heading with the exception that components are manufactured to the customers’ individual requirements and specifications. Custom cold heading should be considered whenever there is a fastener requirement that cannot be satisfied with a standardized fastener or “off-the-shelf” product.

For companies currently using machined fasteners in their assemblies, the cold heading process should be considered as an economical alternative on each and every part if possible. Although some components may not lend themselves to cold heading, either due to low annual volumes or tolerances that are to tight to hold for the process, cold heading should be considered when a part is being designed. If a requirement can be satisfied by a standardized fastener part number, than it should be, but if the component requires any special tolerance or feature, then a custom or “special” fastener will be required.

Almost all of the standardized fasteners are manufactured overseas. With a standardized fastener design, a machine is set up and runs many millions of parts before the machine is turned over and another size or configuration is run. In the case of custom cold heading, a machine will be set up and parts will be run to your design and specifications. In addition, custom cold heading can, at that time, run expected quarterly or annual volume requirements. Parts are then released against the balance of that lot, to the customer as often as they require.

In the cold heading process, wire from a large bundle is fed into the cold heading machine where it is straightened and then into the die, where it is processed. The first stage or die is always set up to cut the desired length off of the wire, creating a “blank”” which is then moved and processed through the different stations. Upsetting (creating a bulge in the blank) or extrusion (reducing the diameter of the blank) is essential in controlling and reforming the raw blank into its desired final shape. Multi-stage dies make it possible to move the part through a progression of forming tools rather than trying to form the part in just a few blows at one stage. A finished part is ejected from the last die while a new blank is being sheared to its specific length at the first die. All of this is being accomplished at a few parts per minute to hundreds of parts per minute depending on the size of the part.

Standard industry tolerances are typically expressed at +/- .005. However, with highly accurate equipment coupled with excellent tooling, premium diameter tolerances of +/-.003 are achievable.