Among several metal parts manufacturing processes, metal casting is known for its capability of complex shapes, material versatility, and high-volume production. But what is casting exactly? It is the process of converting raw metal and alloys into desired functional parts by pouring their molten form into a pre-designed die cavity, followed by solidification. Meanwhile, the die cavity is the mirror of the intended part geometry.
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You can find many types of casting techniques, each with unique features and benefits; sand casting, die, and invest casting are a few. However, which methods to use depends on the raw material type and specifications of the designed parts. This article will discuss the casting process, techniques, compatible metals and alloys, advantages, and disadvantages to provide a comprehensive overview.
It refers to any metal and alloy part made when molten metal forms a geometry on solidification. The liquid metal takes up the geometry of the cavity containing it. After pouring, material flows across each corner inside the cavity (die or mold) and captures each detail. Then, the part is ejected for a die or mold cavity.
If we look into its history, casting can be traced back to seven thousand years (around BC) in ancient Mesopotamia and China to convert copper into different useful items. Then, sand casting became one of the most prominent fabrication ways of tools and cooking wares after 645 BC.
In the modern manufacturing era, the metal casting process is integrated into all industries as its capabilities develop continuously. It can make precise and complex components at low prices, especially in high-volume runs. The die, mold, or other forms of cavities can produce thousands to millions of identical parts. However, sand and investment molds are only usable for a single time, which is economical for prototyping projects. Additionally, the cast parts preserve the original mechanical and physical properties.
The casting or cast molding process involves two different types of approaches, using a pattern for cavity and precise die or mold that can withstand numerous cycles. The following are some terminologies related to its working;
Next, let&#;s elaborate step by step on how casting works.
A pattern is necessary to create the cavity since it acts as the body mass of the desired shape. The replica or pattern can be made with wood, plastic, or wax. Subsequently, some considerations are essential in pattern design and fabrication, such as size accommodation for shrinkage allowance, draft angles for easy removal, core prints for hollow sections, etc.
After creating the pattern, it is placed in a molding box surrounded by mold material like sand or ceramic. Then, it captures the shape and leaves the cavity while ejecting. However, the permanent mold cavities are made with advanced manufacturing techniques like CNC and EDM machining stainless steel or aluminum.
First, the material melting involves an electric arc, induction, or crucible, depending on the casting size. For example, a large-size casting requires large furnaces to melt the raw material and small ladles to transfer before pouring.
Pouring is done by manually handling the crucible or mechanical and hydraulic tipping. However, continuous casting machines can automatically pour molten material into pouring basing as molds move along a production line. The temperature varies based on the melting point of the casting material. For instance, the stainless steel casting reaches as high as &#;-&#;. Next, sprue and gating systems guide the liquid material to the cavity.
Once the liquid metal captures details of all cavities, it is left to cool and solidify for a specific time period. The cooling system might involve channels and passages to control the cooling rate. Meanwhile, the cooling time depends on the cast part size, intricacies, and material type.
During solidification, tiny solid particles called nuclei come together and start crystal growth, ultimately forming the grain structure of the respective material. This mechanism allows the regaining of the original properties.
Finally, the product is removed from the mold. If the mold is expandable( sand or investment casting), the operator or mechanical system breaks it to eject the final parts. On the other hand, ejection mechanisms remove the metal cast products in the case of reusable molds and die. Then, cleaning and post-processing ensure the dimensional accuracy and finish quality.
Now we know what is casting and its fundamental working mechanism, let&#;s discuss what the casting types are.
As industries continuously look for more precise and complex cast parts, metal-casting technologies are parallelly developing new methods and techniques. You can find numerous types of casting methods, such as sand or silica casting and die casting methods, each with some unique capabilities. The comparative analysis(process, benefits, and applications) of these techniques can help you decide which one is the best fit for your project.
The following are the common casting methods in manufacturing.
The sand mold casting method uses tightly packed fine silica-based materials or sand grains to make the mold. The mold involves two sections; cope and drag (upper and lower half). Meanwhile, the molten metal is poured into the cavity formed by these two sections. This mechanism also ensures precise alignment, easy handling, complex shapes, and efficient gating.
Benefits:
Applications: Pistons, bushings, electronic components, valves, bearings, engine crankcases, plant machinery, etc.
The investment mold casting process is relatively more complex than sand cast. It involves preparing a mold using wax patterns connected in a central wax sprue. Then, refractory material like ceramic surrounds this network. Next, heating the mold melts the wax patterns inside the refractory and leaves the cavity with a smooth finish, the reason why it is also known as lost wax castings. Then, liquid is poured into this cavity, and the mold is broken to remove the parts after solidification is complete.
Benefits:
Applications: Military and aircraft components like engine parts, industrial plant components, automotive engine and exhaust systems, medical implants and instruments, etc.
This casting uses permanent molds ( called die) made from strong materials like hardened tool steel. It is typically used for materials like zinc, aluminum, copper, and tin. The die casting process includes melting the raw material in a furnace, which might or might not be attached to the casting machine. Then, the hydraulic plunger or injection mechanism transfers the liquid into the die. Meanwhile, the pressure can reach up to psi.
Benefits:
Applications: It has a broad application spectrum; automotive parts, aircraft turbine blades and airframes, electrical housing, industrial items, machine tools, and household appliances are a few examples.
The centrifugal or rotatory cast method uses a cylindrical mold that rotates around its axis, and liquid metal is poured into this rotating mold. The centrifugal force pushes the poured metal against the mold walls, forming a uniform layer. Then, solidification gives the shape of the mold with a specific thickness.
Benefits:
Applications: This casting manufacturing is predominantly used to manufacture symmetrical items, such as hollow cylinders, shaft sleeves, pipes and tubes, pressure vessels, disk shapes, etc.
The pressure of the pouring or pressurized furnace attached to the casting machine typically consists of a pressure of 0.02 to 0.07 Mpa. The pouring furnace remains below the cast, forces the metal liquid above with the riser tube, and then transfers it into the cavity. The continuous pressure is necessary to fill the cavity. Once it is filled, the cooling channels cool the mold in a controlled manner until complete solidification is achieved.
Benefits:
Applications: Cylinder hub and frame, wheel hubs, kitchenwares, custom hollow and complex profiles, fittings, electronic appliance components, etc.
Pressure injection or plunger mechanism is unnecessary for the metal pouring during the gravity die-casting process. It involves transferring the material from the furnace or ladle into the mold using gravity. No other forces influence the flow until the filling is complete. Furthermore, it mainly accommodates non-ferrous alloys with low melting points, such as copper, zinc, aluminum, and magnesium.
Benefits:
Applications: Gravity die casting offers diverse applications across many industries, including automotive components, industrial machinery parts, aerospace engine and housing elements, decorative items, parts for household appliances, and more.
As the name suggests, the vacuum casting process molds the molten states of metal or plastics into the vacuum. Generally, a pump or similar mechanism removes all the air inside the mold during the process.
Silicone molds, like polyurethane, are used for rubber and plastic molding. However, other types of mold and dies are also applicable to vacuum-cast different materials. Additionally, this casting manufacturing mimics some characteristics of the injection molding technique. This vacuum casting strategy is also adaptable to standard die casting to minimize the risk of air trapping.
Benefits:
Applications: Functional plastic prototypes, medical implants and prosthetics, consumer goods, automotive body panels and other components, food processing components, etc.
This method creates the parts by pressing the liquid and semi-solid material inside the die. First, the liquid metal fills the heated open die, then the upper parts close it and give pressure through the attached hydraulic plate or other suitable mechanisms. The parts solidify under pressure and provide better mechanical property characteristics. Furthermore, it is also known as liquid-metal forging.
Two types of squeezing die casting exist: direct and indirect. In the direct process, the molten metal fills the die, and the upper half closes the die. On the other hand, indirect types first inject the molten metal into the cavity, followed by applying high pressure via a punch or plunger.
Benefits:
Applications: High-strength automotive components like chassis and engine brackets, aerospace parts, industrial gears and hydraulic components, medical device parts, etc.
The reason behind the name &#;lost foam&#; is that a polystyrene foam pattern of the desired part is responsible for producing the final shape. The pattern is placed inside the refractory materials, which melt away after pouring the liquid material and provide the shape after solidification. Then, breaking the mold reveals the part.
Benefits:
Applications: Rapid prototypes for testing, automotive cylinder heads and weldments, pipe fittings, valves, pump enclosures, etc.
The continual casting includes continuous pouring, solidification, and part removal in a production line. The setup and process involve a ladle or furnace, and material flows to the mold cavity, passing a control system. Subsequently, a cooling mechanism ( water channels) in the mold reduces the temperature quickly. So, while leaving the mold parts significantly cooler, further water spray brings them to room temperature.
Benefits:
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Applications: Parts with consistent profiles, such as beams, columns, rods, strips, tubes, etc.
Several ferrous and nonferrous alloys are compatible with the metal casting processes. The table below illustrates the casting materials, their properties, and grades in short.
Metal/AlloysCasting GradesTensile StrengthFatigue ResistanceHardnessWear ResistanceCorrosion ResistanceMagnesiumAZ91D, AM60, AM50ModerateGoodModerateModerateModerateAluminumA356, , ModerateExcellentModerateGoodExcellentIronGray Iron (G), Ductile Iron (65-45-12)HighHighHighHighLow to ModerateZincZamak 3, Zamak 5, ZA-8ModerateLowModerateModerateGoodSteelCarbon Steel (AISI ), Stainless Steel (304, 316)ExcellentExcellentExcellentExcellentModerate to excellentCopper AlloysBrass (C), Bronze (C)HighGoodModerateHighExcellentLead AlloysLead-Tin Alloy, Lead-Antimony AlloyLowLowLowLowHighThe casting process can create complex shapes and features with deatling. For example, internal channels and cavities, undercuts, thin walls, hollow sections, multi-part assemblies, asymmetric geometries, etc. Meanwhile, the reason behind this capability is that molten metal can flow inside any small cavity and sharp corners.
It can work with a variety of metals and alloys, ferrous and non-ferrous. Some examples include iron, steel, zinc, copper, aluminum, and lead alloys. Consequently, extensive material versatility gives the flexibility to choose the optimal material that fulfills the desired properties. Furthermore, casting the metals that are challenging to machining or forging is also easily achievable.
Sand and other expandable mold casting are cost-effective for small volumes or prototypes. On the other hand, permanent die or mold casting methods are adaptable for large-scale manufacturing as one-time tooling costs can cover up to millions of cycles. Overall, casting is a cost-effective manufacturing method.
Any size of parts is castable, from a few hundred grams to 200+ tons. For example, a small motorcycle engine block to large to large steam turbine housing and base for industrial equipment.
The controlled cooling process to solidify the mold ensures uniform grain structure and maintains the same mechanical properties (strength and toughness) across all directions. Furthermore, advanced techniques like heat treatment and alloying improve wear resistance, corrosion resistance, and overall durability.
The material waste from the sprue, intake gate, and runner can be reused in subsequent cycles or batches by re-melting them. Therefore, there is minimal material wastage in casting manufacturing, which lowers the production cost and is good for environmental sustainability.
The cost and time to set up the mold are more significant than the tooling cost of other manufacturing approaches, especially for permanent molds. It impacts the per-par production cost of small-scale production. The long setup and production cycles also impact the design-to-market time.
Cast parts often have a rough finish and inconsistency in dimensions due to the nature of the molding materials in processes with sand and refractory molds. As a result, casting products requires post-machining, such as trimming and grinding.
In the metal cast parts, defects might seen due to trapped gas, flow turbulence, ununiform cooling, complex quality control process, and other causes&#;for example, porosity, shrinkage, warping, and inclusion. These defects weaken the physical structure and properties.
Another casting disadvantage is the complexity of process control, and controlling many parameters like temperature, flow rate, pressure, cooling rate, and curing period makes it challenging to control the process accurately. Furthermore, the metallurgical properties considerations of molten material add complexity. As a result, the percentage of defective items and post-processing requirements might increase.
Moving on, let&#;s talk about what is casting in terms of applications across many sectors.
The diversity of casting applications in the automotive industry can be seen from the fact that &#; The die-cast method was initially invented to manufacture complex automotive zinc parts.&#; in the current scenario, automotive vehicles heavily depend on casting technology for producing lightweight yet durable parts, which contribute to fuel efficiency and performance, such as engine blocks, cylinder heads, and transmission components. The casting capabilities can precisely make these parts&#; intricacies and complex features using insert cores and a custom tooling approach. Meanwhile, aluminum die casting facilitates low-weight and durable parts.
The casting methods can process lightweight materials like nickel-based super alloys, aluminum alloys, and magnesium alloys into complex aerospace components. Meanwhile, the controlled cooling and solidification enhance their properties. So, it benefits the aerospace industry by producing lightweight and durable parts, which improve the fuel efficiency and performance of aircraft and spacecraft.
Subsequently, investment casting is the most common in the aerospace industry. However, die, lost foam, sand, and other casting methods are also prevalent. The casting process allows the manufacturing of aerospace parts with complex geometries and internal cooling channels, such as engine blocks, cylinder heads, and transmission components. Other applications include;
The ability to make complex designs such as ornate decor, detailed kitchen utensils, and durable cookware makes it suitable for many consumer goods. Techniques like investment, die, lost foam and sand casting manufacture these items. Since fine details and aesthetics are critical in these applications, casting methods also ensure that with the smooth surface of cavity walls.
Casting manufacturing maintains the original properties of raw materials and can even increase them by adding fillers and additives. Consequently, sand, die, and other casting molds can shape high-strength and corrosion-resistant alloys like stainless steel, carbon alloys, and aluminum. Additionally, casting is also compatible with large-size and weight designs. So, casting can produce strong, durable, wear & corrosion-resistant parts for heavy machinery and industrial plants. For example, pumps, gearboxes, valves, hydraulic cylinder components, crusher jaws, conveyor belt rollers, custom housings, etc.
The biocompatible materials chromium, titanium, zirconium, and titanium alloys can also be cast precisely using different techniques. The investment cast parts provide excellent finish, precision, and complex features that are mandatory for medical implants and surgical instruments to ensure patient safety. Meanwhile, sand, die, and other types of mold are also used to manufacture device housing, diagnostic equipment parts, and medical furniture.
Some other specific application examples are;
The casting of high-strength materials like stainless steel, magnesium, aluminum, and zinc alloys can create components and products for the energy sector that withstand high stress and harsh chemical and environmental conditions. Additionally, processes like investment and die-cast ensure precision and consistency with minimal defects, which is essential for the reliability of energy plants.
Fundamentally, the injection molding process also relies on the principle of solidifying the molten material in a pre-determined shape of the cavity. However, the detailed mechanism, capabilities, material compatibility, and other aspects are distinct from each other. Next, the individual elaboration of what is casting and what is injection molding shows the differences in depth.
It is a manufacturing technology that is predominantly used for the mass production of thermoplastic parts, except for a few thermosets and metals. The liquid plastic or metals are injected into the mold cavity at high pressure; the material flows inside the mold and takes the details of the cavity after solidification. Furthermore, ejector pins or other systems remove the part from the mold. Typically, injection molds are made with high-strength steel alloys.
High-strength automotive components, precision aerospace hardware, industrial plant machinery, reliable energy components, heavy equipment, medical implants, etc.
The casting, especially die casting tooling is relatively similar to injection molding; you can relate die to injection mold. However, there is a main difference in pressure application in die casting vs injection molding. The high pressure is maintained from injection time until the metal solidifies during the casting process. On the other hand, injection molding involves injecting molten plastic or polymer into a cavity under high pressure, which is then cooled to form a solid part.
Consumer items, medical devices, electronics parts and housing, industrial gear and bushings, sporting goods, home appliances, and storage containers.
We have discussed what is casting and other surrounding aspects, materials, types, pros, cons, and applications. Overall, it is a prominent manufacturing technology for complex metallic parts with high strength, structural integrity, wear and corrosion resistance, and close tolerances, suitable for numerous industrial applications. The diversity in casting methods, from sand to die and lost was castin, can accommodate countless manufacturing needs. However, choosing which casting technique or method suits your parts depends on the material type, design complexity, precision, production scale, budget, etc.
At RapidDirect, We offer comprehensive die casting and vacuum casting services with 50+ metal and alloy options. Our in-house die and mold-making facilities allow us to handle custom and unique designs. Additionally, our priority in real-time monitoring and quality control ensures precision and consistency in every part we cast. So you can trust our engineers and casting factory to help you with your project.