Manufacturing Processes - Module 2: Casting Processes
Metal Casting
Metal casting is a versatile manufacturing process where molten metal is poured into a mold, which can be made of various materials (sand, metal, ceramic), to create a 3D metal part. This process accommodates intricate shapes and sizes, making it suitable for mass production and custom designs. The choice of metal and mold material depends on the casting's complexity, desired properties, and production volume.
Components of a Sand Casting Mold
A typical sand casting mold consists of several key components that ensure the successful creation of a casting:
Flask: A structure that holds the sand mold intact. It provides support and containment for the molding material.
Drag: The lower molding flask, which forms the base of the mold.
Cope: The upper molding flask, which is positioned on top of the drag to complete the mold.
Cheek: An intermediate molding flask used in three-piece molding, providing additional complexity for intricate shapes.
Pattern: A replica of the final object, used to create the mold cavity. Patterns can be made of wood, metal, plastic, or other materials, depending on the production volume and required accuracy.
Parting Line: The dividing line between the cope and drag flasks, and also between split pattern halves. It facilitates the removal of the pattern from the mold.
Bottom Board: A board (usually wood) used as a base during mold making. It supports the drag flask during the molding process.
Facing Sand: A carbonaceous material layer applied to the inner surface of the molding cavity to improve the casting's surface finish. It provides a smoother surface and prevents the molten metal from burning into the sand.
Molding Sand: The primary refractory material used to create the mold cavity. It is a mixture of silica, clay, and moisture, providing the necessary properties for mold formation.
Backing Sand: The bulk of the refractory material in the mold, typically composed of used or burnt sand. It provides support and insulation to the molding sand.
Core: Used to create hollow cavities within the casting. Cores are typically made of sand and are placed inside the mold cavity before pouring the molten metal.
Pouring Basin: A funnel-shaped cavity at the top of the mold where molten metal is poured. It directs the molten metal into the sprue.
Sprue: The passage through which molten metal flows from the pouring basin to the mold cavity, often controlling the metal flow rate. It connects the pouring basin to the runner system.
Runner: Passageways in the parting plane that regulate the flow of molten metal before it enters the mold cavity. It distributes the molten metal evenly to the gates.
Gate: The actual entry point where molten metal enters the mold cavity. It controls the flow rate and direction of the molten metal into the mold.
Chaplet: Supports cores inside the mold cavity, bearing their weight and resisting metallostatic forces. Chaplets are made of metal and are designed to melt into the casting.
Chill: Metallic objects placed in the mold to accelerate cooling and achieve uniform or desired cooling rates. They help to control the solidification process and improve the casting's mechanical properties.
Riser: A reservoir of molten metal that feeds back into the mold cavity to compensate for volume reduction during solidification. Risers ensure that the casting is free from shrinkage defects.
Vent: A small hole or passage in a mold to allow gases to escape. Vents prevent gas buildup, which can cause casting defects.
Sand Mold Making Procedure
The sand mold making procedure typically involves the following steps:
Preparing the drag (lower flask): Place the pattern on the bottom board and fill the drag with molding sand, ramming it to ensure uniform density.
Rolling over the drag: Invert the drag and bottom board, then position the cope on top of the drag.
Ramming the cope (upper flask): Fill the cope with molding sand, ramming it to ensure uniform density. Create the pouring basin, sprue, and runner system.
Assembling the mold for pouring: Carefully separate the cope and drag, remove the pattern, insert any cores, and reassemble the mold, ensuring proper alignment.
The process includes pattern making, core making using core boxes and core sand, creating the gating system, melting the metal in furnaces (such as cupola, electric arc, or induction furnaces), pouring the molten metal into the mold, solidification, shakeout and removal of the casting from the mold, cleaning to remove sand and excess metal, and inspection to ensure quality and dimensional accuracy.
Advantages of Casting
Casting can produce parts with complex geometries and internal cavities, allowing for intricate designs that are difficult to achieve with other manufacturing processes.
It can be used for both small (few hundred grams) and very large parts (thousands of kilograms), accommodating a wide range of casting sizes.
Any intricate shape can be produced, providing flexibility in design and manufacturing.
Any material can be cast (ferrous and non-ferrous), allowing for the selection of the most suitable material for the application.
It is economical with little waste, as extra metal is re-melted and re-used, reducing material costs.
Cast metal is isotropic, meaning it has the same physical and mechanical properties in all directions, providing consistent performance.
It is highly adaptable to mass production, making it suitable for high-volume manufacturing.
The tools required for casting are relatively cheap and simple, reducing initial investment costs.
For example, the automotive industry uses casting for mass production of engine blocks and transmission cases, taking advantage of the process's ability to produce complex shapes at high volumes.
Disadvantages of Casting
Poor surface finish, requiring post-solidification finishing operations such as grinding, machining, or polishing to achieve the desired surface quality.
Casting defects are common, including porosity, shrinkage, inclusions, and misruns, which can affect the casting's structural integrity and performance.
Lower fatigue strength compared to other manufacturing processes, limiting its use in high-stress applications.
Relatively poor dimensional consistency and accuracy, requiring tight control of process parameters and post-casting machining to meet specific tolerances.
High labor intensity, especially in sand casting, due to the manual nature of mold making and finishing operations.
Poor working environment due to high temperatures, noise, and dust, requiring proper ventilation and safety measures.
Generally limited to metals with lower melting points, although investment casting can handle high-temperature alloys.
Not suitable for low-volume production due to the high setup costs and time required for mold making.
Patterns
A pattern is a replica of the object to be cast and creates the mold cavity. The pattern's design and material influence the accuracy and quality of the final casting.
Types of Patterns
Solid or Single Piece Pattern: Simplest for simple shapes, such as flat surfaces like gear blanks and square blocks. It is Inconvenient and time-consuming, but used for large castings.
Split Pattern or Two-Piece Pattern: Split into two halves, one in the drag and one in the cope. Used for intricate shapes and large castings. Aligned with dowel pins and holes to ensure accurate mold assembly.
Match Plate Pattern: Split pattern where cope and drag areas are on opposite faces of a metallic plate. Requires little effort and gives high output. Gates and runners are included on the match plate, streamlining the molding process. Widely used for molding multiple products in a single molding box.
Cope and Drag Pattern: Two-part pattern molded into separate molding boxes. Used for casting heavy products like engine blocks, allowing for the creation of large and complex castings.
Shell Pattern: Used to make molds for curved or straight hollow parts like pipes. The pattern is divided along the center line. Outer shapes make the mold, and inner shapes make the core, enabling the production of hollow castings.
Casting Pattern Material Properties
Lower Cost and Less Weight: Balances costs and returns, making it economically viable for production.
Resistance of Water: Protects the pattern from rusting, increasing its lifespan and the quality of patterns produced.
Durable: Ensures a long pattern lifetime, reducing the need for frequent replacements.
Versatile: Suitable for various industries and easier to repair, providing adaptability for different casting applications.
Types of Pattern Allowances
Draft Allowance: Provided on surfaces perpendicular to the parting line to facilitate easy withdrawal of the pattern without damaging the mold cavity. Typically ranges from 1 to 3 degrees for wooden patterns. The draft angle allows the pattern to be removed cleanly without disturbing the sand mold.
Distortion Allowance: Compensates for distortions in the casting due to stresses during cooling. The pattern is initially distorted in the opposite direction. Useful for castings with shapes like U, V, T, or L, where distortion is likely to occur.
Machining Allowance: Adds extra material to the pattern to allow for machining of the casting. The amount depends on the size, shape, material, machining process, and required accuracy. This ensures that the final part meets dimensional requirements after machining.
Shrinkage Allowance: Compensates for the contraction of the metal during cooling. A shrinkage rule is used, which is longer than a standard rule (e.g., for cast iron, inch longer per foot). Different metals have different shrinkage rates, so the allowance must be adjusted accordingly.
Rapping/Shake Allowance: Enlarges the mold cavity slightly by rapping the pattern before withdrawal. The magnitude is highly dependent on foundry personnel practice. Can be ignored for small and medium-sized castings but is considered for large and precision castings.
Cores
Cores are used for creating internal cavities or passages in castings, such as those in automotive engine blocks or valve bodies. They are placed in the mold cavity to form interior surfaces and are removed during shakeout. Cores must possess strength, permeability, collapsibility, and heat resistance, so they are made of sand aggregates mixed with binders.
Types of Molding Sand
The various types of molding sand, each with unique properties for specific casting applications, are:
Green sand: Used for general-purpose casting.
Dry sand: Used for large castings requiring high strength.
Loam sand: Used to produce large castings with intricate shapes.
Parting sand: Used to prevent sticking of green sand to the pattern.
Facing sand: Used next to the pattern for better surface finish.
Backing sand: Used to back up facing sand, providing support and insulation.
System sand: Used in mechanical foundries for high strength and refractoriness.
Core sand: Used for making cores with high strength and collapsibility.
Sand, Silt, and Clay: Composition and Properties
Sand:
Loose granular material formed by the disintegration of rock.
Coarse and larger particles.
Size ranges from to mm.
Has no plasticity, providing stability to the mold.
Silt:
Dust-like sediment material transported and deposited by water, ice, and wind.
Ranges between sand and clay in particle size.
Size ranges from and mm.
Very low or no plasticity, contributing to the mold's overall texture.
Clay:
Extremely fine-grained natural soil material containing clay minerals.
Extremely fine particles.
Lower than mm.
High plasticity, providing binding properties to the molding sand.
Soil
Loose surface material that covers most land.
Consists of inorganic particles and organic matter.
Varies greatly in chemical and physical properties.
A mixture of minerals, gases, liquids, and organisms.
Consists of a solid phase of minerals and a porous phase that holds gases and water.
Types of Sand
Green Sand: Mixture of silica sand, to clay, and to water. Slightly wet and easily available, with low cost. Used for ferrous and non-ferrous castings, making it a versatile option.
Dry Sand: Green sand that is dried or baked. Suitable for large castings requiring high strength and stability. Same physical composition as green sand but with improved properties.
Loam Sand: Contains equal proportions of sand, silt, and clay. Used to produce large castings with intricate shapes, providing the necessary cohesion and plasticity.
Parting Sand: Used to prevent sticking of green sand to the pattern and helps cope and drag separate without clinging. Clean clay-free silica sand ensures clean separation of the mold halves.
Facing Sand: Used next to the pattern for better surface finish. High strength and refractoriness ensure a smooth casting surface. Made of clay and silica sand without used sand.
Backing Sand: Used to back up facing sand. Old and repeatedly used molding sand, providing support and insulation to the molding sand. Sometimes called black sand due to coal dust addition.
System Sand: Used sand cleaned and reactivated with water, binder, and additives. Used in mechanical foundries for high strength and refractoriness, ensuring consistent quality.
Core Sand: Also called oil sand. Mixture of silica sand and core oil (linseed oil, resin, light mineral oil, and binding materials). Used for making cores with high strength and collapsibility.
Melting Equipment
Crucible furnace: Suitable for small foundries.
Reverberatory or air furnace: Heat is passed over the metal.
Open hearth furnace: Uses the heat of combustion of gaseous or liquid fuels.
Electric furnace: Uses electric arcs to melt metal.
Cupola furnace: Vertical shaft furnace for melting metal.
Crucible Furnace
Most suited for small foundries, offering flexibility and ease of use.
Can be designed for melting any of the metals, making it versatile for different casting applications.
Two main types:
Pit type furnace: In the floor, natural draught, providing simple and economical operation.
Tilting type furnace: Above the floor, forced draught, allowing for precise control of the melting process.
Reverberatory or Air Furnace
Heat is passed over the hearth, which consists of the metal to be melted, providing efficient heat transfer.
Heat transfer is mainly through radiation from the refractory bricks, ensuring uniform heating of the metal.
Additional heating is supplied from burner; thus, providing precise temperature control.
The roof is arched to deflect the flame for reverberation, improving heat distribution.
Metal is heated until it melts, ensuring thorough melting.
Molten impure metal collects in the hearth, which is thick and resistant to slag disintegration, preventing contamination.
The process is repeated until the metal is ready for the foundry, ensuring consistent quality.
Open Hearth Furnace (OHF)
Uses the heat of combustion of gaseous or liquid fuels to melt metal, providing efficient and controllable heating.
High flame temperature is achieved by preheating combustion air and fuel, increasing energy efficiency.
Preheating is done in regenerators or checker chambers located beneath the furnace, maximizing heat recovery.
Checker bricks absorb heat from furnace off-gases, storing thermal energy.
Cycle reverses, with entry ports switching to exit ports, ensuring continuous heat regeneration.
Off-gases flow through a heat-recovery boiler and gas-cleaning system before being discharged, minimizing environmental impact.
Electric Furnace
Consists of a round, bowl-shaped carbon hearth with a dome-shaped roof, providing efficient heat containment.
Carbon electrodes pass current, striking arcs with the metal in the hearth, generating high temperatures for melting.
Can be stationary or tilting, offering flexibility in operation.
Capacity varies from 3 to 10 tones, accommodating different production volumes.
Best suited for laboratory work (few kg needed for research), providing precise control and flexibility.
Gives high melting rate, high pouring temperature, and excellent temperature control, ensuring quality and consistency.
Cupola Furnace
Cylindrical steel shell lined with heat-resisting fire bricks, providing insulation and containment.
Vertical shaft furnace where raw materials and fuel are charged at the top, utilizing gravity for efficient operation.
Air for combustion is introduced through tuyeres, providing the necessary oxygen for the combustion process.
Melts metal but does not reduce ores, focusing solely on melting.
Smaller than a blast furnace of the same output, making it more compact and efficient.
Casting Processes
Sand Casting: Versatile and widely used.
Gravity Die Casting: Utilizes gravity for metal flow.
Pressure Die Casting: Injects metal under high pressure.
Investment Casting: Uses a wax pattern for high precision.
Plaster Casting: Similar to sand casting but uses Plaster of Paris.
Centrifugal Casting: Uses centrifugal forces.
Lost-Foam Casting: Uses foam for the pattern.
Vacuum Casting: Conducted under vacuum pressure.
Squeezing Casting: Combines casting and forging.
Continuous Casting: Produces continuous metal profiles.
Shell Molding: Uses a hardened shell of sand.
1. Sand Casting
Versatile: can cast any metal alloy (ferrous or non-ferrous), offering broad material selection.
Widely used for mass production (automotive parts), making it suitable for high-volume manufacturing.
Uses molds made of silica-based materials (naturally-bonded or synthetic sand), providing cost-effective solutions.
Mold surface has two parts: cope (upper half) and drag (lower half), facilitating easy mold assembly and disassembly.
Molten metal is poured into the pattern using a pouring cup and left to solidify, allowing for the creation of complex shapes.
Trimming off extra metal finishes the product, ensuring dimensional accuracy and surface quality.
Advantages:
Relatively inexpensive, making it accessible for various industries.
Suitable for large components, also offering scalability.
Can cast both ferrous and non-ferrous alloys, giving flexibility in material choice.
Recycling ability supporting environmental sustainability.
Processes metals with high melting temperatures, expanding its application range.
Disadvantages:
Lower degree of accuracy, requiring post-processing.
Difficult for products with pre-determined size and weight specifications, making it challenging to achieve precise dimensions.
Rough surface finish, necessitating additional finishing operations.
2. Gravity Die Casting
Also known as Permanent Mold Casting is where a reusable mold is made of steel, graphite etc to fabricate metal and metal alloys. This type of metal casting can manufacture various parts like gears, gear housing, pipe fittings, wheels, engine pistons, etc.
Direct pouring of the molten metal into the mold cavity takes place under the effect of gravity, simplifying the pouring process.
The molten metal is allowed to be cooled and solidified within the mold to form products, ensuring consistent quality.
Materials like lead, zinc, aluminum, and magnesium alloys, certain bronzes, and cast iron are more common due to this process, making it suitable for a wide range of metals.
Advantages:
Better surface quality, reducing the need for post-processing.
High precision and tight tolerance, allowing for accurate dimensions.
Reusable molds save time and increase productivity, making it efficient for mass production.
Better mechanical properties, enhancing the casting's performance.
Good for thin-walled products, also expanding its design possibilities.
Disadvantages:
Difficult to cast complex objects, making it limited in shape complexity.
Higher manufacturing costs of molds, increasing initial investment.
Ejection mechanism may form a dent in the product, requiring careful handling.
3. Pressure Die Casting
Two types: low-pressure and high-pressure (high-pressure is more popular), providing options based on specific requirements.
Non-ferrous metals and alloys are injected into a reusable mold at high pressure, ensuring rapid and uniform filling.
High pressure maintained during the rapid injection process to avoid hardening, ensuring consistent material properties.
Extraction of casting and finishing to remove excess material, ensuring dimensional accuracy and surface quality.
Advantages:
High precision and dimensional tolerance, allowing for complex and accurate designs.
High efficiency and good product quality, increasing production rates.
Reduced need for post-casting machining, decreasing manufacturing costs.
Rapid cooling and faster production rates, optimizing production efficiency.
Can run for longer hours without replacing the die, reducing downtime.
Disadvantages:
Relatively high tool costs, increasing initial investment.
Limited to non-ferrous materials, limiting material choices.
Difficult to ensure mechanical properties, requiring tight process control.
Requires a large capital investment, which makes it less accessible for small-scale production.
4. Investment Casting
Also known as lost-wax casting, which is a precision casting method.
Wax pattern invested with refractory material and a binding agent to shape a ceramic mold, ensuring high accuracy.
Molten metal is poured into the mold to make metal castings, producing complex shapes with fine details.
Expensive and labor-intensive but is used for mass production or in complex castings, making it suitable for high-value components.
Advantages:
Can produce parts with thin walls, more complexity, and high surface quality, improving functionality and aesthetics.
Reduces the need for post-casting machining, lowering manufacturing costs.
Can cast hard-to-melt alloys, broadening material options.
Allows for castings with 90-degree angles, expanding design possibilities.
High dimensional accuracy, ensuring precise fit and function.
Disadvantages:
Requires labor, increasing operational costs.
Longer production cycle, reducing production speed.
Higher manufacturing costs of molds, increasing initial investment.
New die required for each casting cycle, adding to the cost.
5. Plaster Casting
Similar to sand casting, but mold is made of Plaster of Paris, resulting in a smoother surface.
Low thermal conductivity cools metal slowly, helping achieve high accuracy, but extends cooling times.
Not suitable for high-temperature ferrous materials, limiting material choices.
Can manufacture small (30 grams) and large castings (45 kilograms), accommodating various sizes.
Advantages:
Smooth surface finish, reducing post-processing.
Greater dimensional accuracy than sand casting, improving part precision.
Ability to cast complex shapes with thin walls, expanding design possibilities.
Disadvantages:
More expensive than most casting operations, increasing production costs.
Limited to aluminum and copper-based alloys, restricting material selection.
May require frequent replacements of the plaster molding material, increasing operational costs.
Not suitable for high melting materials, further limiting material choices.
Longer cooling times, effecting production rates, reducing efficiency.
Unstable material as compared to sand needing careful handling.
6. Centrifugal Casting
Also known as roto casting which is used for cylindrical parts.
Industrially manufactures cylindrical parts with centrifugal forces improving material density.
Uses a preheated spinning die where molten metal is poured creating uniform material distribution.
Centrifugal forces distribute the molten metal within the die at high pressure, reducing porosity.
Three variations exist: true, semi, and vertical centrifugal casting, providing options for different shapes and sizes.
Typically produces rotational shapes such as cylinders, limiting its use to specific geometries.
Advantages:
Improved process yields and reduced wastage, improving cost-effectiveness.
Casting has high density and almost no defects, enhancing performance.
Convenient for manufacturing barrel and sleeve composite metal castings, making it suitable for specific applications.
No requirement of gates and risers simplifying mold design.
Disadvantages:
Centrifugal casting requires high investments, increasing initial costs.
Requirement of skilled labor, heightening operational expenses.
Specific shapes production, limiting its versatility.
7. Lost-Foam Casting
Similar to investment casting, but uses foam for the pattern instead of wax, reducing costs.
Pattern is coated with a refractory ceramic, improving surface finish.
Molten metal is poured into the mold to form the desired product, creating complex shapes.
Used for materials like alloy steel, carbon steel, alloy cast iron, and ferrous alloy, providing material selection.
Advantages:
High precision casting, improving accuracy.
Allows flexible design, expanding design possibilities.
Clean production, supporting environmental sustainability.
Economic for high volume production, improving cost-effectiveness.
Disadvantages:
High pattern costs for low volume production, increasing initial investments.
Low strength causes distortion or damage of the pattern, requiring careful handling.
Many production processes and longer delivery time, reducing production speed.
8. Vacuum Casting
Production occurs under vacuum pressure of bar or less to exhaust gas from the mold cavity, reducing porosity.
Molten metal is poured into the mold cavity inside a vacuum chamber, preventing air entrapment.
Reduces entrapment of gases during metal injection, improving mechanical properties.
Metal is cured in a heating chamber and removed from the mold, ensuring dimensional accuracy.
Popular in automobiles, aerospace, electronics, marine, and telecommunication, making it suitable for various industries.
Advantages:
Reduce porosity, improve mechanical properties and surface quality of die casting, leading to enhanced performance.
Production of thin walled products expanding design possibilities.
Welding and heat treatment of products is possible improving material properties.
Suitable for low volume production, offering flexibility.
No requirement for expensive hard tool finishing which reduces manufacturing costs.
Diminishes air pockets and bubbles at early stages providing improved part quality.
Disadvantages:
High tooling cost, increasing initial investment.
The mold used in the process has a short life, raising operational costs.
Potential hollowness issues requiring process control.
9. Squeezing Casting
Liquid forging or squeeze casting is a hybrid metal forming process that merges permanent mold casting and die forging in a single step, improving material properties.
Specific amount of molten metal alloy is injected into a die, and pressure is applied to shape it ensuring complete filling of the mold.
Metal part is heated over melting temperature and extracted from the die, ensuring dimensional accuracy.
Potential casting process for safety-critical parts in automotive systems, due to high reliability.
Advantages:
Eliminates internal defects like pores, shrinkage holes and shrinkage porosity, and improving material density and strength.
Low surface roughness requiring minimal post-processing.
It can prevent casting cracks ensuring part integrity.
High strength components enabling use in demanding applications.
No wastage of material increasing cost-effectiveness.
Disadvantages:
Less flexibility in part geometry limiting the shapes that can be produced.
Lower productivity, needing optimized cycle times.
High machining requirements adding to production costs.
Requires accurate controlling slowing down the overall process.
10. Continuous Casting
Allows consistent mass production of metal profiles with a constant cross-section improving efficiency.
Molten metal poured at a calculated rate in a water-cooled, open-ended mold, ensuring proper solidification.
Surface of solid metal forms on the liquid metal in the center, controlling the process.
Strands of metal are continuously extracted from the mold facilitating continuous production.
Advantages:
Diverse size range of casting products, varying from a few millimeters thick strip to larger billets and slabs, supporting flexibility.
Lower costs due to continuous production improving affordability.
Lower material wastage increasing cost efficiency.
Disadvantages:
Requirement of continuous cooling of the molds, otherwise, center-line shrinkage develops requiring process control.
Casting of only simple shapes with a constant cross-section and limiting design possibilities.
Requires large ground space and high initial investment making it challenging to implement.
11. Shell Molding
Expendable mold casting process similar to sand casting offering increased accuracy and surface finish.
Hardened shell of sand forms the mold cavity instead of a flask of sand improving precision.
Finer sand mixed with a resin which improves surface finish.
Good surface finish and dimensional accuracy reduces post-processing requirements.
Advantages:
Casting of thin and complex parts allows the creation of intricate designs.
Semi-skilled labor reduces operational costs.
No further machining required minimizing production expenses.
Accounts for surface defects, which improves the integrity of parts.
Disadvantages:
Not suitable for small scale production which limits flexibility.
Limitations on size and weight requiring process adjustment.
Special metal pattern required which makes it expensive for large casting increasing initial costs.
Defects in Castings
Casting defects are imperfections or irregularities that compromise the quality specifications of a component and need to be addressed using quality control methods. They can result from material failure, equipment issues, or non-optimized procedures. They are typically grouped into four categories:
Metallurgical defects: relate to the metal's properties.
Defects due to heat: occur from temperature-related issues.
Mold material defects: come from the mold's properties.
Casting shape defects: result from the casting's geometry.
Metallurgical Defects
Porosity Defects: related to air entrapment or shrinkage.
Gas Porosity
Pinholes: small gas bubbles on the surface.
Blowholes: larger gas bubbles within the casting.
Open holes: gas bubbles that break the surface.
Shrinkage Porosity: from uneven cooling.
Open Shrinkage Defects: visible cavities on the surface.
Closed Shrinkage Defects: internal cavities due to shrinkage.
Warping: distortion of the casting shape.
Sinks: depressions on the casting surface.
Slag Inclusions: non-metallic materials in the casting.
Dross: oxidation impurities on the surface.
Soldering: molten metal sticking to the die.
Porosity Defects
Typically, porosities occur whenever air is trapped in the metal during the casting process resulting in structural resistance. There are two major porosity defects:
Gas Porosity
When the cast metal solidifies inside the mold, it cannot retain as much gas as when it was in liquid form and proper degassing techniques are needed. Therefore, the inability of the gas to pass through the mold easily leads to the trapping of bubbles inside the metal, appearing as Pinholes, Blowholes or even Open Holes. These defects can significantly weaken the structural integrity of the casting.
Causes:
Involved Gases in Metal Alloy Filling: Ambient gasses that get incorporated into filler material during the deposition process.
Turbulences often occur due to blind areas in the gating system, unreasonable casting parameters, and improper design of launders. These blind areas trap gases that becomes pockets when the material solidifies.
Released Hydrogen from the Molten Alloy: Hydrogen contamination results in porosity.
Gases from Mold Release Agents: Excessive use of the release agent can result in off-gas.
Remedies:
Ensure clean and dry metal alloy ingots are used to prevent hydrogen formation through quality control.
Use suitable casting parameters, including injection speed and pressure, to prevent turbulence. This balance ensures smooth filling and reduces gas entrapment.
Control the smelting temperature to prevent overheating through process optimization. Maintain optimal temperatures.
Employ sprue and runner with a sufficient length of greater than mm ensuring a stable flow of injected material. Thus, you will ensure a stable and adequate flow of gases by keeping the path clear of blockages.
The mold release agent should be of the highest quality and controlled quantity: High quality helps ensure a non-contaminated cast and the quantity will limit further issues. Less is more with mold release agents so keep usage to a minimum.
Shrinkage Porosity
Shrinkage porosity is different from the round, smooth surfaces of gas porosity; they occur as jagged, angular edges. The common shrinkage porosity defects are:
Open Shrinkage Defects such as caved surfaces and open holes: The air is drawn inside the mold when the metal alloy shrinks unevenly will create an open shrinkage. Molten material contracts as it cools.
Closed Shrinkage Defects appears as holes inside the casting where there's uneven heating of the molten metal. They can occur in micro or macro forms, appearing as networks of voids.
Warping: This shrinkage occurs during or after metal solidification, changing the component's shape and dimensions. They cause the metal to curve in flat or large sections.
Causes:
High concentration of the metal in specific areas of the mold: Areas of thicker metal take longer to cool, resulting in uneven contraction.
Too low injection pressure: Low pressure does not allow the material to fully fill to the edges while cooling.
Poor runner and gating design: Poor design leads to uneven filling. Implement simulations software to improve metal injection.
Uneven pattern of metal solidification: All metal should cool uniformly.
Temperature differences in different parts of the molten metal: Hotter arears cool slower!
Extremely high pouring temperature: Hotter materials will contract more leading to more defects.
Remedies:
Use simple casting geometries with improved runner and gating design: Geometries with uniform thickness will improve material distribution. Utilize simulation software to improve the runner.
Ensure optimum filling of cavities by improving runner using simulation software. Design a new runner system using all the tools available to simulate the process.
Increase the metal injection pressure: Increasing metal injection will ensure complete and through filling.
Insert cooling coils, ribs, or internal chills to ensure proper heat dissipation: Cooling methods as part of the design can alleviate many of the defects listed above.
Clean the metal surface to