Permanent Mold Casting: Definition, Importance, How It Works, Applications, and Advantages

What Is Permanent Mold Casting?

Permanent mold casting is a process that generally uses a clamped, two-part, reusable mold. This mold is generally cut from cast iron or steel (and historically stone or fired clay) to cast low- and moderate-melting-point metals. This method produces castings with higher-quality surface finish, dimensional accuracy, and mechanical properties than sand casting. 

Permanent mold casting is particularly suitable for producing parts with relatively simple geometries and no fine features, as these can be resistant to filling effectively.

Permanent mold casting originated earlier than the Bronze Age, using open stone molds to cast lead, gold, and copper. Examples of such casts are found in Mesopotamia, Egypt, and China. Greek and Roman metallurgy made use of processes that are considered predecessors to modern permanent mold casting. They employed somewhat reusable clay molds to cast lead, for example.

Historical records suggest that early metalworkers used materials such as clay and stone to create reusable molds for casting metal objects. Improved understanding of metallurgy, metalworking, and casting techniques has evolved into considerably more sophisticated methods.

Permanent mold casting developed early in the Industrial Revolution. In the 19th century, with the advent of better materials, techniques, and equipment, casting processes became more refined. Manufacturers began to recognize the benefits of using permanent molds made from durable materials. Such molds remove the effort required for one-off processes that destroy the tool. Permanent mold casting gained further traction in the early 20th century as industrialization and manufacturing processes rapidly advanced.

The Strengths of Permanent Mold Casting

Permanent mold casting stands out among other types of casting methods due to its particular combination of benefits and advantages. Permanent mold casting delivers components with a smoother surface finish, better dimensional accuracy, and tighter tolerances than equivalent sand cast parts. Permanent mold casting is ideal for medium to high production volumes. It strikes a balance between the low CAPEX of sand casting and the precision of high CAPEX pressure die casting. In particular, the ability to switch molds and handle intermittent bulk production is valuable across most industries.

The use of permanent molds allows for the creation of greater intricacy, thin walls, and internal features, so products suffer lower design limitation impact than sand casting. While the initial mold preparation costs are higher than sand casting, the reusability of the molds, low operational labor, and reduced post-casting machining result in overall cost savings, for medium to high production volumes. Low material/ancillary waste and reduced energy consumption costs are also significant in longer production runs. Finally, permanent mold casting offers better control of solidification than an equivalent sand cast and delivers improved mechanical properties.

The Permanent Mold Casting Process

1. Pre-Heat the Mold and Apply a Ceramic Coating To Enhance Metal Flow and Extend the Mold Life Span

Preheat the mold to an appropriate operational temperature before casting. The mold temperature is the primary method for controlling solidification, as a way to adjust finished part properties.

Coat the interior surfaces of the cavity with a refractory material or mold release agent to facilitate the release of the solidified casting. This coating is a per shot (release agent) or per production run (refractory coating) service item. Reduced tool wear can have a significant effect on quality and economics.

2. Insert and Clamp Cores if Needed

Assemble the split parts of the mold including any cores or inserts required for internal features. This can be a labor-intensive process. It often justifies some degree of automation when the volumes rise to a certain level defined by product economics and labor costs.

3. Pour Molten Metal Into the Mold Through a Sprue and Runner System

A molten charge is metered into the mold cavity through a gating system (sprue, runner, and risers). The gravity-filling of the mold can take place from the top, or the liquid metal may be introduced at the lowest point (using a more complex gating system) to ensure good flushing of air and a complete fill. This stage can include tilting the tool to encourage venting and complete filling. The need for this will depend heavily on component geometry.

4. Allow the Metal To Cool and Solidify in the Mold

As the molten metal comes into contact with the cooler mold walls, it begins to crystalize and reflect the shape of the mold cavity. This stage is highly dependent on the bulk of the part. It can be shortened by water-cooling galleries that add tooling complexity but reduce process times.

5. Open the Mold and Remove the Casting

Once the casting has cooled to the expected degree, the mold is opened. The solidified casting remains in one half of the mold, from which it is pulled either by hand or by an automatic process.

6. Remove Excess Material by Cutting Away Solidified Metal From the Runner System and Sprue

The casting will require post-processing steps that trim gating system material and perform surface finishing tasks, such as flash removal, and reducing gate disruption. The gate trimming can be performed either manually with clippers, manually in a custom-made tool that improves repeatability, or it can be fully automated in a more advanced setup.

Different Types of Permanent Mold Casting

1. Low-Pressure Permanent Mold Casting

This process gravity feeds molten metal into a cavity from a sprue at the top of the tool, via a feeder that runs to the bottom of the cavity. Bottom feeding—as opposed to the common system of top feeding—results in better air expulsion and improved fill. In some circumstances, the feeder can be configured to provide a compensation reservoir that helps in reducing the effects of shrinkage during cooling.

2. Vacuum Permanent Mold Casting

Vacuum-fed casting is similar to low-pressure or gravity die casting. Except, the feed is from a reservoir beneath the tool, using a vacuum source applied at the top of the tool to draw the charge into the cavity. Vacuum charging of the cavity ensures better and faster filling of the tool and greater uniformity/repeatability. In particular, the risk of air entrapment during the pour stage is removed, as the charge is lifted undisturbed to fill the cavity, with lower turbulence and no bubbles.

3. Slush Casting

Slush casting uses a flow of charge hitting a cold tool to form a skin that represents the cavity. Bulk material remains liquid and is flushed out of the cavity to leave a hollow body that reflects the tool cavity. Limited control of wall thickness is possible through charge temperature (melted to slush rather than low-viscosity liquid) and additional tool cooling. This process is generally only suited to low-strength applications. This is because the material properties are not optimized and wall thickness consistency can be hard to achieve. Slush casting is generally used for decorative objects and has a characteristic stippled and rough internal surface.

Metals Used in Permanent Mold Casting to Make Parts

1. Copper Alloys

A variety of copper alloys such as brass, bronze, gunmetal, Muntz metal, and nickel-aluminum bronze are all amenable to permanent mold casting. Some alloys tend towards better-quality results in this low-pressure process. In particular, low-grade brass is used in slush casting to make hollow decorative and low-stress objects with excellent external surface finish.

2. Zinc

Most zinc alloys are highly suitable for permanent mold casting. In general, alloys such as ZAMAC (formerly MAZAC) have low melting points, low melt viscosity, and low abrasion effect on tooling.

The combination of low melting point, low viscosity, and moderate strength make this a very widely utilized group of alloys for permanent mold casting.

3. Iron

Iron is not suitable for permanent mold casting, as its melt temperature exceeds the practical requirements of the process. However, sand casting and post-machining of permanent casting mold tool parts are commonly performed in iron, as it offers a relatively low cost for precise and reliable tooling. Tool life in cast iron can be lower than in hardened steel alloys, so this option should be carefully considered as a lower-cost, moderate-performance route to tooling.

4. Aluminum

Most aluminum alloys are suited to permanent mold casting applications in many moderate-accuracy and larger components. Pure aluminum, however, is not considered ideal for permanent mold casting.

Applications of Permanent Mold Casting

Permanent mold casting is used to produce heavier and thicker walled components such as engine blocks, cylinder heads, transmission housings, intake manifolds, and lower-stress suspension parts. This casting process is also widely used in manufacturing parts for machinery, pump and gearbox housings, and machine bases. Other applications of permanent mold casting are plumbing fittings, valves, components for HVAC systems, boat engine parts, propellers, structural components, and components for power generation systems, such as turbine parts, generator housings, and heat exchangers. 

Quality of Permanent Mold Casted Products

Permanent mold cast products are renowned for their high quality and consistent production performance and reliability. The quality of permanent mold casting products is driven by several factors that can be carefully moderated, such as surface finish, dimensional accuracy, reduced porosity, enhanced mechanical properties, and repeatability. Good dimensional accuracy and tight tolerances can be achieved in the process—as long as tooling is precise and process conditions well-controlled. The control of the cooling process also helps minimize porosity, resulting in components with improved mechanical properties and structural integrity, compared with sand casting. When well-operated, the method ensures consistent quality from one casting to the next across large batches, with uniformity and low defect rates.

Accuracy of Permanent Mold Casting

The accuracy of permanent mold casting can be excellent, compared with sand casting. This is particularly true with finer design elements (that are hard to fully reproduce in sand-cast parts) and in improved surface finishes that are typical of the process. The accuracy deliverable through permanent mold casting varies due to several factors such as: the complexity of the part, the quality of the mold, material shrinkage properties, and particularly the process control.

While permanent mold casting offers improved accuracy compared to sand casting, it is generally less accurate than either investment casting or pressure die casting. Permanent mold casting sits in the middle ground of performance, between precision and production efficiency. The level of accuracy achieved will depend on the establishment and process control of the production.

Advantages of Permanent Mold Casted Parts

1. Smoother and finer surface finish than sand casting, reducing the need for post-work fettling and machining.
2. Better dimensional accuracy and tighter tolerances than sand casting.
3. Minimized porosity compared with sand casting.
4. Improved mechanical properties such as strength, hardness, and crystalline structure.
5. Consistent quality is repeatable across and between long production runs.
6. Intricate shapes, thin walls, and internal features that are harder or impossible with sand casting.
7. Reusable molds and reduced post-cast fettling deliver overall cost savings, increasing as production volumes rise.
8. A wide range of metals and alloys, including aluminum, zinc, copper, and magnesium alloys are accommodated.
9. Production can begin and ramp rapidly—beneficial in meeting demand fluctuations and tight schedules.

Disadvantages of Permanent Mold Casting

The disadvantages of permanent mold casting are listed below:

1. Generally involves significant initial tooling costs compared to sand casting.
2. Extremely intricate and highly detailed geometries can be more challenging than in either investment or pressure die casting.
3. Very large or oversized components can be out of the practical range for normal equipment.
4. Mold surfaces wear due to repeated casting cycles. Regular maintenance or mold replacement is necessary to mitigate this issue. Mold life will, however, greatly exceed sand cast pattern life.
5. Offers slightly less precise control over cooling rates than some processes, potentially producing lower-quality microstructure.
6. Some alloys are not suitable for the process due to factors like melting temperature and mold abrasion.
7. Mold preparation, gating system cleanup, and surface fettling involve manual labor and skilled craftsmanship.
8. Design changes can be more challenging and costly compared to methods like sand casting. The tools are less adaptable and more expensive to replace.
9. Cycle times can be considerably longer than for pressure die casting.

Costs of Permanent Mold Casting

The cost of permanent mold casting varies widely based on a variety of factors such as: the complexity of the part, the size of the production runs, the material selected, the quality of the mold, and labor costs. The most influencing aspects include: mold preparation, material and labor costs, tooling and equipment, fettling, quality control, production volume, and tooling maintenance.

Permanent mold casting tends to be both lower cost to establish and have a higher cost per part than pressure die casting. This is a product of cycle time being longer and tooling costs being lower. This makes a relative cost comparison between the two processes difficult. Where volumes are high and precision requirements demanding, pressure die casting offers better levels of component quality and a lower cost per part, but at the expense of higher tooling and equipment costs.

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