10 Things to Consider When Buying investment casting

Investment Casting : 7 Considerations When Choosing a Process.

Casting one of the oldest known manufacturing techniques is a process in which liquid material (e.g. molten metal) is poured into a mold cavity and hardened. After removing the piece from the mold, various finishing treatments can be applied to create a dazzling final product. This process is used primarily to manufacture complex solid and hollow designs for a wide range of industries, from aerospace and automotive to electronics.

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Although casting is a tried-and-true relic of the manufacturing world, technological advances have created specialized casting varieties appropriate for different applications. Below we'll take a look at investment casting and die casting advantages and disadvantages so you'll be armed with the necessary information to choose which process is best suited for your upcoming metal project.

How Investment Casting Works

Investment casting (also called 'lost wax' or 'precision' casting) is a manufacturing process in which a wax pattern is created, gated onto a sprue and repeatedly dipped into a liquid ceramic slurry. Once the ceramic material hardens, its internal geometry takes the shape of the casting. The wax is melted out, and molten metal is poured into the cavity where the wax pattern was. The metal solidifies within the ceramic mold, and then the metal casting is broken out (source).

How Die Casting Works

Die casting is a manufacturing process for producing metal parts by forcing molten metal under high pressure into a die cavity. These die or mold cavities are typically created with hardened tool steel that has been previously machined to the net shape of the die cast parts (source).

Which Process is Right for My Project?

Rather than pit these two processes against one another, we'll simply run through some key considerations when it comes to settling on a casting process. Keep in mind that there isn't a one-size-fits-all solution. Each product, project and company are different. Review the 7 considerations below to decide whether investment casting (IC) or die casting (DC) fits the bill.

1. Design Complexity

How complex is your design geometry? This will play a major part in selecting the right process. IC offers great design flexibility since you can cast intricate shapes and easily incorporate design features, such as logos and other information, into the component. You can also achieve precise dimensional results, complex geometries and thin-walled parts. DC offers good dimensional results but cannot produce the level of intricacy that IC can.

2. Material Selection

A wide range of alloys (including both ferrous and non-ferrous metals) can be used in IC, offering greater material options than DC. This allows for casting alloys that might be challenging to machine. Most DC are made from non-ferrous metals like zinc, copper, aluminium, magnesium, lead, pewter and tin-based alloys.

3. Annual Usage

One of the biggest misconceptions about IC is that it only makes sense for large order quantities. While you can opt for IC for smaller production runs, the final call usually comes down to tooling costs. Start by deciding your desired payback period for the tool and crunch some numbers to see if IC is actually the best option. DC is ideal for large production runs and high-volume projects since it produces excellent consistency and repeat-ability but comes with a higher tooling price tag.

4. Part Size

IC can accommodate parts from an ounce up to about 200 pounds. There is some limitation to the size of parts that can be investment cast simply because the wax pattern must be securely gated to a sprue for repeated dipping in the ceramic slurry. DC also comes with its own size limitations, but they tend to be less restrictive than IC; however, the larger the part, the larger the tool, the larger the tooling cost.

5. Tolerance

IC can really deliver on tight tolerances, while DC produces good tolerances. As a general rule, the smaller the casting, the greater the dimensional accuracy. Very large investment castings might lose some dimensional accuracy, so DC could be a better option for large-scale pieces.

6. Cost

IC ordinarily costs more than DC because it's a highly manual process that produces superior dimensional and excellent surface finishes. But the final cost truly comes down to tooling. IC can be designed for minimal machining, reducing both time and cost. DC comes with higher tooling costs and typically requires at least some secondary machining to properly finish the product. For these reasons, DC is most cost-efficient for high-volume runs.

7. Finish Requirements

The surface finish of IC is superior to other casting methods, reducing the need for excessive secondary machining. A 125 micro finish is standard, and better finishes can be achieved with the help of other finishing techniques like polishing or blasting. While DC produces good surface finish, more machining is usually needed to get the product to its final state.

Final Thoughts

Hopefully this information will assist you in choosing the right casting process for your upcoming metal project. If you're still unsure which direction to go, turn to your contract manufacturer for guidance. Here's a quick recap for your reference:

Investment Casting

• Excellent precision, ideal for complex geometries.
• Can meet tight tolerance requirements.
• Superior surface finish, little additional machining required.
• Higher total cost than other casting processes.
• Lower tooling costs.
• Suitable for both ferrous and non-ferrous metals.
• Some product size restrictions.

Die Casting

Metal casting is a complex process that requires numerous considerations when purchasing, including casting design, the metal casting process to be used, and quality control factors.  Ultimately, there are many considerations in metal casting and there is no 'one size fits all' solution.  Therefore, we developed an outline of the various considerations when a customer selects a metal casting solution.  Before reviewing important considerations in metal casting, let us review MetalTek's metal casting processes.

Materials Used by Metal Casting Process

Sand Casting Materials

Thesand casting process produces metal components of all sizes and shapes with exterior detail and inner passages using cores (if needed).  Almost any alloy can be sand cast.  In sand casting the 'purchased tool' is a pattern that replicates the part to be cast.  Patterns are usually hardwoods, polyurethane, or other durable material.  The non-reusable sand mold is created by inserting the pattern into a bed of sand, compressing the sand, and removing the pattern.  This leaves a cavity in the shape of the part. 

Molten metal is poured into this cavity by a series of hollow pathways called risers and runners.  After metal solidification, all previously hollow areas (risers, runners, and the cavity made from the pattern) are solid metal.  The risers and runners are removed, leaving a metal part resembling the pattern.

Investment Casting Materials

In investment casting, the tooling is a die with an inner cavity that resembles the part.  Wax is injected into this cavity to create a wax pattern.  The wax pattern is assembled onto a 'tree' with other patterns, which is dipped into a ceramic material to create a shell.  The wax pattern is melted away, resulting in a hollow shell into which molten metal is poured. 

After solidification, the shell and tree are removed.  Relative to other metal casting processes, the final part can have a smoother surface, higher dimensional precision, and thinner walls.  This results in cost savings due to lower part weight and less post-cast processing.  With investment casting, there is a large selection of alloys during the material selection process.

Centrifugal Casting Materials

With centrifugal casting, the basic tooling is typically a steel die into which molten metal is poured.  Dies are rotated around a vertical axis (for 'ring' shapes) or a horizontal axis (for 'tube' shapes).  This metal casting process uses centrifugal force to cause heavier material (metal) to move toward the outside diameter (OD) of the ring, and lighter material (oxides and other impurities) to migrate toward the inside diameter (ID). 

The metal will solidify from the OD towards the ID, leaving the purest metal on the outside of the metal casting and pushing the less pure elements toward the center.  The ID is machined to remove impurities, leaving a solid ring or tube of the highest casting quality.

Important Considerations When Selecting a Casting Process

Choosing The Right Alloy for the Operating Environment

There are important considerations in casting that customers need to be aware of before choosing the specific metal casting type (centrifugal casting, investment casting, or sand casting).  For example, what is the operating environment?  Will the component experience high heat?  High wear?  Both?  Will the component need to stand up to corrosion from saltwater?

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Once the specific operating environment is defined then the appropriatealloy can be chosen.  Alloy selection mirrors environmental factors, as various alloys hold up better in extreme temperature conditions, under high wear, or in corrosive environments.  MetalTekmetallurgists help customers choose the best alloys for their applications.

Optimizing the Manufacturing Process

Any engineer designing a part has a primary objective for the function of the part in question.  This objective is subject to a variety of competing constraints.  We have already discussed the operating environment, which helps determine a minimum baseline for the material/metal used. 

Optimizing the manufacturing process involves identifying and prioritizing all competing constraints.  It is imperative to recognize that these competing constraints typically require the customer and metal casting supplier to determine the optimal balance of cost, quality requirements, and speed to market.  The next section provides a deeper look into the primary considerations for each major metal casting process.

Balancing Cost, Quality Requirements, and Speed to Market

Presuming that the geometry, metallurgical requirements, and total quantity of parts validate the use of metal castings, here are some general considerations among MetalTek's three major metal casting processes:

Sand casting

Sand casting benefits:

• Lower piece price, tool cost, and lead time
• Broad range of geometries, including holes/passages not machinable
• Near-net shape to reduce machining costs
• Minimal size constraints
• Custom metallurgical compositions

Sand casting limitations:

• Dimensional tolerances (per ISO CT10 to CT12) not typically as tight as investment casting
• Cast thickness of 1/4' to 3/8' for limited distances; typically minimum of 1/2' for larger parts
• Machining required to improve as-cast surface finish (250+ RMS)
• Air-melted alloys

Sand casting best fits:

• Low (single piece) to very high production volume
• Need to eliminate machining and/or fabrication
• Wide range of metallurgical compositions (heat, corrosion, and wear-resistant alloys)
• Use of additive manufacturing/3D printing for sand molds or tooling to reduce cost
• Custom and/or complex geometries

Investment casting

Investment casting benefits:

• Higher metallurgical quality
• Tight dimensional control
• Smoother surface (80-125 RMS)
• Minimal metal waste in casting
• Little or no machining
• Can use with vacuum-melted alloys and air-melted alloys
• Broader range of geometries, including tighter dimensions and tolerances

Investment casting limitations:

• Higher piece price, tool cost, and lead time
• Size limited to 40' cube

Investment casting best fits:

• Medium to very high volume
• Expensive or exotic alloys
• Part weight restrictions (e.g., aerospace applications)
• Extreme heat in operating environment
• Many working surfaces or flow paths

Centrifugal casting

Centrifugal casting benefits:

• Directional solidification provides the highest metallurgical quality and superior machinability
• Minimal or no tooling costs for most tube or ring geometries
• Can cast tubes and machine into multiple ring shapes
• Lower costs and lead times than rolled-ring forgings
• Could utilize sand, investment, or other material molds to increase quality in those geometries

Centrifugal casting limitations:

• Geometries are somewhat limited to rings and tubes (which can limit production lot sizes)
• Used primarily with air-melt alloys, with limited size availability for vacuum alloys.
• Must machine ID (it is the riser for the casting)
• OD usually requires machining to remove 'orange-peel' surface.

Centrifugal casting best fits:

• Ring or tube geometries competing against forgings
• Some shaped geometries (extra tooling) where higher metallurgical quality is mandatory

How Does MetalTek Meet Quality Requirements?

MetalTek carefully reviews quality requirements from drawings and specifications prior to accepting an order.  We comply with all industry standards for allowable tolerances in the various metal casting processes, documented by all required/agreed-upon certifications.  In process, we go one step further to ensure compliance with metallurgical requirements.

Each MetalTek division uses a solidification modeling program to simulate the flow of molten metal into a mold and analyze the predicted solidification as the mold cools.  This model will help in the tool design, including locations for gates and risers, to ensure a metallurgically compliant part.

MetalTek's Advantages

MetalTek strives to build a collaborative relationship with every customer, to best meet both technical and commercial requirements.  The customer's engineering personnel are the experts in the design of their part, and MetalTek is the expert in alloys and metal casting processes. 

We can leverage our wealth of technical expertise and vast industry experience to help your company choose the optimal product design and manufacturing process to meet your objectives. 

Contact us and let us help you with your next project.

About the Authors

Tim Lulling is the Western Regional Sales Manager for MetalTek International. He joined MetalTek in and served in a series of sales roles of increasing responsibility before being named Regional Sales Manager in .  Tim holds a BS in Business and an MBA in Materials Management from Arizona State University.