Mold casting is one of the best manufacturing processes for producing custom metal parts.
It involves pouring a liquid material, usually metal, into a mold that has the shape of the desired product.
The outcome of this is high-quality parts with good surface finish, dimensional accuracy, and mechanical properties.
Two main types of mold casting methods are popularly used today in foundries: expendable and non-expendable
Each one has different advantages and disadvantages, depending on the complexity, accuracy, and cost of the desired product.
In the following guide, we’ll explore the different types of expendable and non-expendable mold castings, the main comparisons between the two methods, and their different applications within various industries.
Armed with this information, you’ll be better equipped to choose the method that is best suited for your particular project.
In expendable mold casting, mold is made out of a non-reusable material to create a metal part.
The mold is typically made of materials that can be easily shaped and molded into the desired form (e.g. sand, plaster, or wax).
During the process, the metal is poured into the mold and solidified, after which it is then broken or burned away to release the part.
One major advantage of this process is that it is typically more suitable for making complex shapes than other manufacturing methods.
Some of the other major advantages of using expendable mold casting include:
There are also many types of expendable mold casting techniques, each with its advantages and disadvantages.
Three of the most common ones include:
Sand casting is one of the oldest and most widely used casting methods. The method involves packing sand around a pattern that resembles the shape of the part and then removing the pattern to create a cavity.
The sand is held together by a binder, such as clay, chemical, or oil.
Sand casting is prized for its ability to produce parts of various sizes and metals that have good dimensional accuracy and surface finish.
Also known as lost-wax casting, investment casting involves using a wax pattern that is coated with a ceramic slurry to form a shell mold.
During the process, the wax pattern is melted out of the shell, leaving a hollow cavity before it is poured into the cavity through a sprue, where the metal hardens and the ceramic shell is knocked out to reveal the part.
Investment casting is used primarily to produce parts with high dimensional accuracy, surface finish, and intricate details.
This method is similar to sand casting. The difference is that plaster of Parts is used instead of sand for molds.
The plaster is mixed with water and poured over a pattern that resembles the shape of the part.
During this casting process, the plaster sets and hardens around the pattern, which is then removed to create a cavity.
The molten metal is then poured into the cavity through a sprue where it is allowed to cool and harden. Finally, the plaster mold is broken apart to retrieve the part.
Plaster casting can produce parts with smooth surface finish and fine details.
The above expendable casting methods are popularly used in various industries to produce cost-effective metal parts with a high-quality surface finish.
Like expendable casting, non-expendable mold casting (a.k.a. permanent mold casting) involves pouring molten metal into a mold of the desired shape and then allowed to solidify.
However, unlike expendable mold casting, where the mold is destroyed after each production cycle, this metalworking process uses reusable molds that can be used multiple times without significant damage or deformation.
Reusable molds reduce the cost and time of making new molds for each casting.
Aside from its reusability feature, this casting method offers many other advantages to both metal manufacturers and businesses in need of custom metal parts.
Some of these include:
Non-expendable mold casting includes four different methods: permanent mold casting, die casting, centrifugal casting, and continuous casting.
Each of these methods has its advantages and disadvantages depending on the type of metal, shape, size, and quality of the casting required.
A brief description of these four non-expendable mold casting methods is given below.
This mold-casting process uses metal molds (e.g. steel and iron) to produce high-quality castings with good dimensional accuracy and surface finish.
Permanent mold casting is a cost-effective method for producing metal cast parts at high volumes.
While this method is ideal for casting large quantities of metal parts, it is limited to working with metals with lower melting points (eg. aluminum, zinc, copper, and magnesium).
During die casting, high pressure is used to force molten metal into a metal mold (a.k.a die).
The die is usually made of steel and has two or more sections that fit together to form the mold cavity.
The process involves injecting molten metal into the die through a nozzle or a sprue to fill the cavity rapidly. The pressure within the cavity is then maintained until the metal solidifies, after which the die is opened and the casting ejected.
Die casting can produce metal components with complex shapes, fine details, and excellent surface finish but requires high initial investment in equipment and dies.
This mold-casting process uses centrifugal force to distribute molten metal into a cylindrical mold.
The mold is usually made of steel, cast iron, or graphite and rotates at high speed around its axis.
In centrifugal casting, the molten metal is poured into the center of the mold where it flows outward due to the centrifugal force that is generated.
After the metal has been poured into the mold cavity, it is allowed to solidify against the inner wall of the mold forming a hollow cast. After the cast is formed, the force is stopped and the casting removed.
This casting method is ideal for producing castings with good mechanical properties and uniform wall thickness.
Continuous casting is used to produce long lengths of metal shapes without interruption.
The process allows for continuous and efficient production of metal cast parts with consistent quality and physical properties throughout the entire length of the cast.
During continuous casting, the molten metal is poured into a water-cooled metal mold (tundish) where it solidifies as it passes through and exits from the bottom as a continuous strand of semi-solid metal.
The strand is then cut into desired lengths or further processed through rolling and drawing methods.
Even though this non-expendable mold casting method can produce castings at a lower cost than other casting methods, it requires complex equipment and control systems to do so.
Each of the above non-expendable mold casting methods utilizes reusable molds, which not only helps reduce the associated costs and the time it takes to make metal cast parts but also delivers a higher degree of dimensional accuracy and better surface finish for those parts.
To get a better understanding of the advantages and disadvantages of each of these two mold casting methods, a comparison of the two based on the above factors is given below:
Expendable mold casting generally has lower initial costs than non-expendable mold casting.
The main reason for this is that expendable molds are usually made of cheaper materials such as sand, clay, plaster, and ceramic.
Expendable mold casting does, however, have higher recurring costs because new molds need to be made for each new casting.
The main reason why non-expendable mold casting tends to have higher initial costs than expendable mold casting is that the permanent molds that are used in this process are usually made of more expensive materials such as steel and iron.
However, since these types of molds can be reused over and over again, they tend to incur lower recurring costs than expendable casting methods do.
On average, expendable mold casting has a lower production speed and efficiency output than non-expendable mold casting due to the longer time and labor required to make the molds and remove the castings.
Expendable molds also have lower thermal conductivity than non-expendable molds, which means that they take longer to cool down and solidify the metal.
Non-expendable mold casting, on the other hand, has higher production speed and efficiency than expendable mold casting since the molds can be reused without much preparation and can also be quickly and easily separated from the castings.
Expendable molds usually have rougher surfaces and less dimensional accuracy than non-expendable molds and tend to produce more defects such as porosity, shrinkage, gas inclusion, and sand inclusion in the castings.
The process of cracking and removing the cast part in expendable casting increases the potential of defects in the casting.
Alternatively, non-expendable molds tend to have smoother surfaces and more dimensional accuracy than expendable molds due to the smoother material and precision tooling the process utilizes for its molds.
Permanent mold casting also tends to produce fewer defects in the castings because it employs controlled cooling measures for the molten metal, which helps maintain the shape and dimension of the cast part.
Expendable mold casting processes can accommodate a wider range of metals with different melting points, chemical compositions, and mechanical properties when compared to non-expendable methods.
Since expendable molds have a high heat tolerance they can work with both high melting point and low melting point metals.
Non-expendable mold casting methods have fewer material options than expendable mold casting because they are limited to lower melting point metals as the permanent molds would eventually crack or deform under high heat after repeated use.
Expendable mold casting generates more waste materials than permanent mold casting and sometimes also contains harmful substances such as binders, additives, and coatings that can pollute the air, water, and soil.
Non-expendable mold casting, on the other hand, generally has less of an environmental impact because the molds that are used in the process can be recycled or reused, and therefore generate less waste materials.
As can be seen from the above comparison, expendable and non-expendable mold casting have different advantages and disadvantages depending on the factors considered.
Therefore, the selection of which mold casting method to use will highly depend on which factors are the most relevant to the particular project requirements.
Given the many advantages of these two molding casting methods, it should be no surprise that many industries use each method in varying degrees to produce metal components for a variety of applications.
Metal castings that are made from expendable molds are used in the following industries:
Automotive: The automotive industry relies on expendable mold castings to make structural parts like engine blocks, cylinder heads, pistons, brake drums, manifolds, and other parts that require high strength and durability.
Aerospace: Expendable mold casting is used to make a variety of airplane and space components such as turbine blades, impellers, nozzles, housings, and other parts that require high precision and resistance to high temperatures and pressures.
Medical: Various applications within the medical industry like implants, prosthetics, surgical instruments, and other parts that require biocompatibility and corrosion resistance can all be made using expendable mold castings as the customization options the process offers are ideal for many patient-specific devices.
Jewelry: Rings, bracelets, necklaces, earrings, and other jewelry parts that require intricate details and fine finishes can be produced with expendable mold casting methods like lost-wax casting (investment casting).
Non-Expendable Mold Casting Applications
Non-expendable mold casting is widely used for producing large quantities of identical or similar parts that require high dimensional accuracy and surface quality.
Four major industries that use non-expendable mold casting are:
Electrical: Electronic components such as connectors, switches, relays, transformers, as well as other electrical parts that require electrical conductivity and insulation are typically made with non-expendable mold casting processes.
Consumer: Non-expendable mold casting is used to make toys, appliances, tools, furniture, and other parts within the consumer industry that require aesthetic appeal and functionality.
Industrial: Within the industrial industry, permanent molds are used for the production of valves, pumps, gears, bearings, pipes, fittings, as well as other parts that require mechanical strength and wear resistance.
Military: Defense applications such as shells, grenades, bullets, missiles, rockets, and other military components that require high performance and reliability are commonly made through non-expendable mold casting methods like aluminum die casting.
To recap, both expendable and non-expendable mold castings are used in various industries for different purposes.
In general, non-expendable mold castings are used for creating high-quality, accurate, and durable components, while expendable mold castings are ideal for manufacturing products with large parts, complex shapes, and intricate designs.
We hope you have learned the main differences between expendable and non-expendable mold casting methods, as well as some of the industries and applications that use them.
Each method has its pros and cons, depending on the type, size, shape, complexity, and application of the metal parts that need to be produced.
The decision between which one is better for a specific project does not only require a careful analysis of the above information but also the help of a reliable metal manufacturing company that has extensive experience and expertise in both expendable and non-expendable mold casting.
With their advice, the decision as to which mold casting method to use will be an easy one.
From a contract manufacturing firm, BuntyLLC evolved into a full service custom machined, forged and cast metal parts fabrication enterprise. We supply global solutions from our headquarters in Greenville, South Carolina.
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