This article contains everything you need to know about permanent mold casting.
You will learn:
- What are Permanent Mold Castings?
- How the Permanent Mold Casting Process Works
- Products Made from Permanent Mold Casting
- Uses for Permanent Mold Castings
- And more…
Chapter 1: What are Permanent Mold Castings?
Permanent molding casting is a casting process that uses strong durable reusable molds to produce parts and components using molten metals. The molds for permanent mold casting are made from steel or cast iron and are able to endure repetitive use to produce thousands of dimensionally accurate parts and components. Typically, aluminum, copper, and magnesium are melted and poured into permanent molds but the process can be used for any type of metal that can be melted to a molten state.
The popularity of permanent mold casting is due to the tight tolerances, surface finishes, and exceptional mechanical properties produced by the process. These characteristics are the result of pouring molten metal into a reusable mold, which is unlike sand casting and lost wax casting that have molds that are not reusable and are expendable. The surface of the permanent metal mold chills the metal being cast during solidification, which produces a finer grain structure and reduces porosity in the cast component.
Chapter 2: The Permanent Mold Casting Process
The molds for permanent mold casting are the aspects of the process that determine the exceptional tolerance of the cast product. Each of the processes must be performed in the correct order and carefully monitored to ensure the final product meets design parameters, standards, requirements, and tolerances.
The term permanent mold casting is a generic descriptor that refers to several processes that use permanent molds to cast parts. Regardless of which method is used, all types include a method for inserting molten metal into a mold. How the insertion is completed varies according to the process and desired outcome. The different types of permanent mold casting are gravity, low pressure, vacuum, and slush casting, each of which have variations to their process.
Creating the Mold
Molds for permanent mold casting are formed from fatigue resistant metals such as iron, copper-based alloys, bronze, graphite, or steel. The various metals are chosen for their ability to withstand the multiple times that molten metal is placed in them. Permanent casting molds can have two or more parts depending on the complexity and intricacy of the part being cast.
The manufacturing of the mold for permanent mold casting begins with the creation of the mold, which starts with a pattern for the piece to be produced. Included in the creation of the pattern are gates with sprues to allow for the flow of molten metal. Since permanent molds are produced in halves, the gates with sprues are strategically placed to ensure full and complete coverage of the molten metal in the mold and its smooth application.
The mold is a negative representation of the shape that will be cast and an exact duplication of the pattern from which the mold is made. All of the castings from the mold will be exact replicas of the original pattern. Permanent molds have several advantages over temporary molds made from other materials. Since the steel or iron material of the mold are solid and smooth, there is reduced porosity in the casting, which is essential to the strength of the casting. The wall thickness of the mold evenly distributes stress to help improve the strength of the casting.
As with many manufacturing processes, the amount of time it takes to produce a casting mold varies in accordance with the type of casting to me produced. Prior to the tooling of a mold, it has to pass through a set of approvals from which it is given to the mold maker who is a machinist. The production of molds varies from one week up to a month and is calculated into the lead time for the production of the final casting. Once a mold is created, it can be used tens of thousands of times during its useful mold life. Several factors affect mold such as temperature, mold material, and the types of metals being cast.
A part of mold design is the guide, alignment, or locating pins that position the halves of the mold properly. They are placed on one half of the mold and align with holes on the other half of the mold. For a casting to have the proper tolerance, the halves of the mold have to be aligned for accuracy and precision, which is the function of the locating pins.
Heating the Mold
Casting companies store their molds to be able to use them for other orders. A new mold, once it has been properly finished, is normally put to immediate use. The permanent molding process begins by heating the mold, which allows the molten metal to flow easily to every part of the mold without cooling and solidifying. Additionally, heating the mold eliminates the potential of thermal shock to the mold caused by suddenly introducing hot metal into a cold mold.
The halves of the mold are coated with a heat resistant coating and die casting agent to help the mold withstand the cooling and heating cycles of the casting process. The addition of the coatings enhances the appearance of the surface finish of the casting and makes it easier to remove it from the mold. The preheating temperature varies between 150°C and 300°C (302°F and 572°F). Permanent mold manufacturers have special preheating stations that are separate from the molding equipment and used to preheat molds. The various producers have methods and techniques for preheating molds each of which ensures the production of quality castings.
Choosing the Metal
The metals that are cast in permanent mold casting are various alloys with aluminum and copper based alloys being the most prevalent. Each of the various metals have physical and metallurgical characteristics that make them qualified for the casting. The characteristics of the chosen metal determine the mold temperature, the metal temperature, pouring rate, pressure, and cooling rate. Additionally, the complexity of the geometry of the casting can also influence the choice of metal since some alloys are unable to be shaped into complex and intricate shapes. At this stage of the process, designers, engineers, and customers have already determined which metal alloy to use.
|Advantage and Application of Metals
|Lightweight, excellent strength-to weight ratio, high thermal conductivity
|Aerospace, automotive, industrial, consumer goods, electronics, construction
|Lightweight, excellent strength-to weight ratio, good damping capacity
|Aerospace, automotive, electronics, sports equipment, consumer goods
|Good electrical and thermal conductivity, high corrosion resistance
|Electrical and electronic components, plumbing, HVAC systems, decorative items
|Good casting properties, excellence surface finish, high corrosion resistance
|Automotive, consumer goods, industrial, construction, decorative items
|Low melting point, good casting properties, high ductility
|Low-temperature applications decorative items
|Low melting point, excellent casting properties, good corrosion resistance
|Industrial, plumbing, radiation, shielding
Clamping the Mold
The mold clamp holds the halves of the mold together to prevent the halves from separating. In addition, the clamp keeps the halves from shaking, moving, or shifting during the casting process. The clamp force has to resist the separation force of the mold that is caused by the pouring of the molten metal into the mold. As the size of the mold increases, the amount of force necessary to keep the mold halves in place has to increase. Mold clamps come in different sizes, types, materials, and designs, each of which is designed to meet the needs of a particular permanent molding process. Although the other aspects of the process of permanent mold casting require skill and proficiency, the clamping process is labor intensive and requires strength. In many operations, different forms of automation are used that apply high pressure to the mold to tightly shut and seal the halves.
Pouring the Molten Metal
The pouring process is completed using one of three methods, which are static pour, tilt pour, and reverse tilt pour. The variations in the pouring method are partially dependent on the size of the mold since small molds can have the molten metal simply poured from a crucible while larger molds may require a team to move the metal from a furnace to a larger crucible. The pouring process has to be carefully regulated due to the nature of the process and the dangers of the molten metal.
The static pouring method or dump pour method is traditional and involves pouring directly into the mold. The pour speed and flow rate of the pour has to be monitored to prevent porosity in the casting due to turbulence and the pour speed. Static pouring is used for small parts produced by permanent molds but seldom used for other castings.
Tilt pouring is a very common method for pouring and is used to decrease turbulence to prevent porosity in the casting. Tilt pouring is machine controlled to provide a more even and consistent pour. The pouring machine has a crucible that is filled with molten metal. It slowly tilts to allow the metal to smoothly flow into the mold. During the pouring process as the mold fills with the molten metal, the mold moves from a horizontal position to a vertical position. The machine controls the flow of the molten metal and the timing of the tilt to control turbulence.
The reverse tilt pour method is a combination of the static and tilt pour methods. The casting is center fed with the molten metal being fed into the riser as the mold moves such that the parting line is parallel to the floor. The pouring method is like a static pour while the movement of the mold from an upright position is like a tilting pour. The target of a reverse tilt pour is the center section of the mold with the riser being the last section of the mold to solidify. Reverse tilt pouring is used in the production of large castings that have large sections, consistent geometry, and center symmetry.
Cooling and Solidifying
The solidification process has to be properly managed in order to reach the best quality and tolerance for the casting. The geometry, material, and process are major factors in the amount of time it takes for a casting to solidify with permanent mold casting being a heat extraction process with the rate at which the heat is extracted determining the production rate. The temperature of the mold is managed by different thermal tools that include inserts, water coolant, forced air, mold coatings, and adjustments to cycle times.
Permanent molds are air or water cooled with air cooling being the simpler of the two methods and produces less thermal shock, which leads to a longer mold life. With air cooling, an evaporator connected to a condenser removes heat from the mold while the condenser removes heat from the evaporator. The addition of cooling fins and compressed air can be added to speed up the rate of cooling.
Water cooled molds have passageways for the application of coolant. The drilled cooling lines are highly effective in removing heat from molds in bulk areas. The cooling lines are controlled to turn on and off depending on the requirements of the mold. The types of cooling lines are dependent on their length, diameter, design, and how clean they remain.
Part Removal and Ejection
If every stage of the permanent molding process has been completed correctly, the casting is ready to be removed from the mold. The shaping, molding, and forming of a casting is useless if it cannot be removed from the mold without being distorted or having a damaged surface.
The key to the ejection process is the draft angle of the mold, which is the amount of slope that the wall of the mold was tilted, makes the opening of the casting wider, and allows for efficient ejection. Any casting that has straight walls requires an angle of 3° to 5° for casting removal. Additionally, a proper draft angle helps in maintaining the quality of the casting.
If a coating has been applied to the mold during preheating, the casting will easily release without sticking to the surface of the mold. Also, the coating helps to enhance the surface finish of the casting and eliminates defects.
A common method for removing castings from a mold is ejector pins, which are mounted on a movable plate and extend through the mold. They are designed to push the casting out of the mold and leave small impressions on the casting that are removed during secondary processing. Ejector pins can be placed such that they do not put marks that would have to be removed during secondary processing. Once the ejector plate pushes forward to assist in removing the mold, they retract prior to the pouring of molten metal for the next casting.
Chapter 3: Types of Permanent Mold Casting
The concept of reusable molds can be applied to a variety of casting processes with four main types being gravity, slush, low pressure, and vacuum castings. Each of the four types vary in how the molten metal is fed into the permanent casting mold. The general factors of a permanent mold casting such as the mold, inserting of molten metal, and having a mold with two halves apply to all of the types.
Gravity Permanent Mold Casting
Gravity permanent mold casting is the oldest of the permanent mold casting processes. The aspect of gravity permanent die casting is how the molten metal enters the permanent mold. The molten metal is poured directly into the mold or as the mold is tilted. The cavity of the mold is filled as the molten metal moves through a gating system as it is poured into the sprues of the mold.
Slush Permanent Mold Casting
Slush permanent mold casting is typically used to cast hollow castings. With slush casting, the geometry and strength of a casting are not accurately controlled. The steps of slush casting follow those that are common for all forms of permanent mold casting including heating the mold, clamping the mold, and pouring the molten metal into the mold.
The principle of slush permanent mold casting is built on the idea that castings harden from the walls of the mold outward and will solidify along the walls of the mold. The interpretation is that the skin of a mold thickens and hardens before the rest of the mold. With slush permanent mold casting, during solidification and after a solid boundary has formed in the mold, any remaining molten metal is poured out of the mold leaving a solidified metal shell that has the shape and characteristics of the desired part. The wall thickness of the hollowed out part becomes thicker the longer the molten metal is allowed to solidify.
All of the other aspects of the process, including the removal of the part, follow the same procedures as are common to permanent mold casting. The use of slush permanent mold casting is ideal for metals with low melting points such as zinc, tin, or aluminum alloys. An essential part of the slush permanent mold casting is having the ability to mechanically and rapidly turn the mold over and have a crucible or container available to catch the molten metal.
Low Pressure Permanent Mold Casting
The low pressure permanent mold casting process relies on the use of low pressure at 3 psi to 15 psi, which is applied to a reservoir of molten metal to fill a permanent mold. This is unlike the gravity method where molten metal is poured into a tilted mold. With low pressure permanent molding, the molten metal is placed in a reservoir that is below the mold. Pressure is applied at slow increasing increments to force the molten metal upward through risers into the mold cavity.
The pressure is maintained on the molten metal until the casting solidifies. As the pressure is released, residual molten metal from the risers and the mold flows back into the holding furnace or reservoir. The slow application of pressure in the process makes it possible to minutely control the filling of the mold for a smooth and even distribution of the molten metal to reduce porosity, reduce the formation of oxides, and promote consistent filling from the top to the bottom of the mold. The outcome of the process is castings with exceptional density, excellent strength, and precision dimensional accuracy.
Low pressure permanent mold casting is commonly used for simple geometric castings and high volume production. It is well suited for the production of simple uncomplicated castings since it does not require complex machinery or advanced technology.
Vacuum Permanent Mold Casting
The process for vacuum permanent mold casting varies radically from the other methods of inserting the molten metal in the permanent mold. While slush and gravity casting use the pouring method and low pressure casting uses pressure, vacuum permanent mold casting uses the power of a vacuum to fill the mold cavity. In some ways, the vacuum method of permanent mold casting is similar to low pressure casting in that it uses a form of pressure to fill the mold cavity.
The structure of the mold for vacuum permanent mold casting is similar to that of low pressure permanent mold casting in that the molten metal is located below the mold and is connected to the mold by risers and a gating system. The molten metal flows upward, as it does in low pressure casting, as the mold is suspended over the molten metal reservoir. In many cases, the suspension method is controlled by a robotic arm that can precision control the stability of the mold. Pressure in the mold is reduced to the point that the molten metal is drawn upward through the risers and gate system into the mold cavity.
During solidification, the mold is moved away from the molten metal reservoir in order to eject the casting from the mold. The wide use of vacuum permanent mold casting is due to its ability to produce close dimensional accuracy, exceptional surface finishes, and superior mechanical properties. The process is ideal for castings that require thin walls, a feature that is difficult to achieve with other casting methods.
The process of vacuum permanent mold casting is very similar to low pressure permanent mold casting but has an advantage over low pressure casting in that gas defects are eliminated or reduced with vacuum casting. Like low pressure permanent mold casting, vacuum casting is a low cost and effective method for the production of high-volume parts and components.
Leading Manufacturers and Suppliers
Chapter 4: Permanent Mold Casting Applications
The permanent mold casting process is widely used for the production and manufacture of metal parts that have exceptionally tight tolerances, smooth even surface finishes, and no or reduced porosity. The flexibility of the process makes it possible to engineer and create components with intricate and complex features that have any type of wall thickness. Since the tooling and design of the process are simple and uncomplicated, the manufacture of parts and components can be completed rapidly with short lead times.
Permanent mold casting is used for high production rates and high-volume manufacturing. The strength and durability of the molds makes it possible to use them thousands of times before failing. This aspect of permanent mold casting makes it a cost-effective production method. Although the initial cost of the tooling of the mold is high due to the hardness and toughness of the metals used to fabricate the molds, the initial cost is amortized over the longevity and durability of the molds.
Industries that Rely on Permanent Mold Casting Components
The industrial use of permanent mold casting is due to the low cost of the process, which produces components and parts that are durable and long lasting with few flaws or deformities. Energy efficient and high tolerance parts can be produced to scale with a minimum of effort and cost.
The automobile industry relies heavily on permanent mold casting for the manufacturing of a wide range of auto parts. Engine blocks are cast from cast iron or aluminum, which are reliable and dependable. Additionally, automobile maintenance costs are kept to a minimum through the use of safe, clean, and efficient cast parts that require less energy and emit less harmful substances.
In the marine industry, parts are subjected to harmful, hazardous, and stressful conditions that test their durability. Extreme temperatures, torrential rain, and salt water test the stability and ability of a part to perform. Parts that are cast for the marine industry are made of highly durable materials including stainless steel, aluminum, and various alloys that contain copper and bronze. The different types of alloys prevent damage from marine life.
The construction industry demands that all components and parts have sufficient strength to endure the types of load bearing the industry requires. Permanent mold casting produces base and column foundations, manhole covers, gates, and piping. Additionally, components and parts for bridges, dams, and structures that have to endure stressful weather conditions have structural pieces manufactured by permanent mold casting.
Components for the aviation industry must be exceptionally durable but light enough for flight. Parts have to be precision manufactured with tight tolerance that are dimensional and size accurate. In the majority of cases, components for aviation are made from steel, titanium, and aluminum and include low density aluminum and steel alloys. Aside from mechanical parts, the interiors of aircraft include several permanent mold castings including seat belts, seats, doors, and cabin accessories.
Many of the permanent mold castings for the military are used for the production of firearms and front combat line applications due to the strength, durability, and toughness of permanent mold castings. Each component made for the military is required to follow a specific set of standards that have been established and tested by the army corp of engineers. Every detail of a part is outlined down to the smallest and most minute aspect of a component. As with other permanent mold castings, items for the military have to face harsh and demanding conditions that require the highest quality.
The mining industry makes extensive use of components that are produced by permanent mold casting due to the nature of mining processes. Permanent mold casting is used to produce wheel hubs and clamping rings for shakers, crushers, and piston shafts. Mining and casting are intertwined in that the metals from mining are the raw materials for the permanent casting process. Products from permanent mold casting guarantee that parts will have minimal wear without mechanical fatigue.
Chapter 5: Advantages of Permanent Mold Casting
The primary benefits of permanent mold casting are excellent surface finishes, dimensional precision, high volume and high production rates, and reusable molds. During solidification, permanent mold castings form microstructures that are superior and stronger than those formed by expendable mold castings.
For many years, parts with intricate shapes and complex formations were made by machining and stressing pieces of metal to force them into the proper dimensions. The process was labor intensive, required great skill, and took many hours to complete. With the advent of permanent mold casting, the same components that took several hours and days to make are now completed in less time with greater precision and accuracy.
Permanent mold casting has the tightest tolerances compared to any other casting process with a 0.02 tolerance for the first 6 in. and 0.002 for each inch after. These tolerances vary in accordance with the intricacy of the design, cooling rates, and the amount of control that is implemented in the process. Every casting in a casting batch has exactly the same tolerance without any measurable differences.
Speed of Production
Modern production and the modern market require the rapid manufacturing of components to meet consumer requirements. It is for this reason that permanent mold casting is so widely used in so many industries where time is crucial in getting a product to market. The resilience of permanent molds makes it possible to complete over 50,000 cycles with a mold with each cycle producing parts with identical tolerances and stability. The production rate for permanent mold casting varies between 5 to 50 pieces per hour.
A common problem with most casting methods, and one that engineers account for when designing a part, is shrinkage, which can be as high as 6% for certain melts. Shrinkage occurs in castings as they cool and can lead to defects in a component. Of the various casting methods, permanent mold casting has the lowest shrinkage due to the absence of gas porosity. With the elimination of porosity, the grain structure of permanent mold castings is more stable and uniform.
Aside from shrinkage, the porosity of other casting processes leads to the weakness and lack of strength of cast components. This is not a factor with permanent mold casting where porosity is minimal or eliminated due to the nature of the mold and precision control of the process.
Several of the metals that are used for permanent mold casting have exceptional surface finishes that could be damaged through the casting process. With permanent mold casting, the excellent surface finishes of metals are retained due to the smooth walls of the mold cavity, which also produces a finer grain structure for greater strength.
One of the aspects of permanent mold casting that is constantly emphasized is the durability of the molds, which can last through thousands of manufacturing cycles. The strength and reliability of permanent molds make it possible to quickly, efficiently, and continuously produce high quality components in rapid succession with every component having the same tight tolerances.
The benefits of long lasting molds lower costs and increase profits. Although the initial cost of producing a highly stable and strong mold may seem to be prohibitive, its long mold life decreases the cost per completed components.
Waste and Scrap
One of the problems with other methods of casting is the amount of scrap they produce, which can be as high as 88%, which is not the case with permanent mold casting where waste is minimal or non-existent. In processes such as slush permanent mold casting and low pressure permanent mold casting, leftover molten metal returns directly to the crucible to be processed for the next mold.
As with many of the factors of permanent mold casting, the absence of waste is an important part of lowering the cost of finished castings. Aside from the cost of disposing or repurposing waste material, the elimination of waste prevents the process from polluting the environment.
The permanent mold casting process produces castings with smooth even surfaces that can fit easily into various types of machinery. It is for this reason that the process is used to produce automobile engines and parts. The key features of permanent mold castings are their strength, toughness, durability, and ductility due to the mold walls rapid removal of heat during solidification. The rapid solidification rate generates a fine even grain structure that is not found in any other form of casting.
Chapter 6: Types of Metals Used for Permanent Mold Casting
There are several ductile metals that can be used for permanent mold casting. The selection of which metal to cast is dependent on the required characteristics of the completed component, which can vary by strength, durability, appearance, and intricacy or complexity. Design engineers and clients specify the properties and qualities that they require to ensure the proper selection of metal.
Of the different metals, aluminum is the most commonly chosen due to its many positive attributes. It is a low-cost machinable metal that is corrosion resistant. A wide range of aluminum alloys are used for permanent mold casting with the 300 series being one of the more common sets of alloys. Aluminum alloys with a high amount of silica, at 12%, are ideal for permanent mold casting due to the silica content lowering the melting temperature of the alloy.
|Quality of Aluminum 300 Series Alloys for Permanent Mold Casting
|Resistance to Hot Cracking
|Resistance to Corrosion
|Elevated Temperature Strength
As with aluminum, copper is seldom cast in its pure form due to its lack of strength. Several copper alloys are used for casting that have various elements added to enhance the strength of the base metal. In many cases, copper can be customized to fit the needs of a casting by changing the elements of its alloys. Colors, strength, corrosion resistance, and forming of copper can be changed by altering the alloys.
Several zinc alloys are used in the permanent mold casting process and include zamak 2, zamak 3, zamak 7, and ZA 8. Of the different zinc alloys, zamak 2, known as Kirksite, is the strongest and the hardest of the zamak family of alloys due to its copper content of 3%. The term zamak is an anagram for zinc, aluminum, magnesium, and Kupfer, the German word for copper.
The wide use of zamak 2 is due to its easy castability and resistance to creeping. Zamak 2 has a melting point of 2615°F to 2635°F (1435°C to 1446°C) and a Brinell hardness of 100. It is used for its structural integrity and strength for applications that have to endure high stress.
Zamak 3 is very castable and has excellent dimensional stability. Unlike other zinc alloys, zamak 3 can be coated, plated, and chromated. It has a Brinell hardness of 82 with tensile strength of 283 MPa and yield strength of 221 MPa. The strength and ductility of zamak 3 makes it an ideal metal for casting.
Nickel alloys are resistant to oxidation and corrosion, which makes them ideal for rugged and rough conditions and high temperature applications. There are three common trademark names for nickel alloys, which are Monel®, Hastalloy®, and Inconel®. Monel is 67% nickel and copper with small amounts of iron, manganese, carbon, and silicon. It is stronger than pure nickel and is corrosion resistant.
Hastalloy® is widely used in permanent mold casting due to its ease of castability. The molybdenum content of Hastalloy® makes it harder and stronger at high temperatures. The castings for Hastalloy® can be cold worked and welded.
Inconel is oxidation and corrosion resistant, which makes it ideal for extreme environments with high pressure and heat. When heated, Inconel forms a passivating oxide layer that protects its surface. The key to the use of Inconel is its strength and appearance at high temperatures after it has been solid solution strengthened.
- Permanent molding casting is a casting process that uses strong durable reusable molds to produce parts and components using molten metals. The molds for permanent mold casting are made from steel or cast iron and are able to endure repetitive use to produce dimensionally accurate thousands of parts and components.
- The popularity of permanent mold casting is due to the tight tolerances, surface finishes, and exceptional mechanical properties produced by the process.
- The molds for permanent mold casting are the aspects of the process that determine the exceptional tolerance of the cast product. Each of the steps of the permanent mold casting have to be performed in the correct order and are carefully monitored to ensure the final product meets the designed parameters and tolerances.
- The concept of reusable molds can be applied to a variety of casting processes with four main types being gravity, slush, low pressure, and vacuum castings. Each of the four types vary in how the molten metal is fed into the permanent casting mold.
- The permanent mold casting process is widely used for the production and manufacture of metal parts that have exceptionally tight tolerances, smooth even surface finishes, and no or reduced porosity.