Many of the consumer products we use in our daily lives, such as mobile phones, faucets, razor handles, door knobs, eyeglass frames, power tools, football cleats and golf clubs, undergo the die casting process. A variety of other industries, including automotive, aerospace, computer, HVAC and many other sectors, rely on this increasingly popular fabrication process. Die casting, a term used to describe the finished part, refers to a manufacturing technique that allows for the mass production of metal parts by using high pressure to force molten metal into reusable die cavities.
Die casting, which has been around since the late 1830s, has a long history of success. In the past, the process developed a reputation for expensive machinery and dies, long set-up times and high scrap costs. Ranging from excellent to poor, the quality of castings depended on whether the manufacturer employed the same machine and the same operator. Hence, repeatable quality was also an issue.
Today, advances in technology, better material science, improved manufacturing processes and cleaner and more efficient machines have combined to make die casting more of an exact science. In addition, better die design and cutting-edge software enable faster die fabrication, enhanced performance and less waste. All of this, along with continuous computerized process-monitoring and automation, have improved the overall manufacturing process, lowered material waste and reduced the variation in quality.
Die casting customers can now expect stronger, more durable, and denser parts with exceptional repeatable quality, higher production rates, and lower costs.
Die Casting Machines and Processes
Manufacturers classify both hot chamber and cold chamber die casting machines by the tonnage of the clamp force they provide. The size of die casting machines ranges from 400 tons to 4,000 tons. Regardless of the size of the machine, or the die casting process, they all have the same basic purpose — to cast a part using injected molten metal. There are two types of processes:
- Hot Chamber Die Casting — This machine employs an injection system that is immersed in a molten metal pool. The furnace connects to the machine through a metal feeding system, or gooseneck. At the start of the cycle, the piston retracts, which allows the molten metal to fill the gooseneck from a port in the injection cylinder. The downward action of the plunger seals the port and forces the molten metal through the gooseneck and nozzle in the die. After solidification, the plunger pulls upward; the die opens, and the part system ejects the part.
Hot chamber die casting eliminates the need to transfer metal from a separate furnace. However, the process is feasible only for low melting point alloys that do not damage the injection cylinder, such as copper, zinc, lead and magnesium.
- Cold Chamber Die Casting — This machine utilizes a ladle to move the molten metal from a holding furnace into the unheated injection cylinder or shot chamber. The system relies on a hydraulic piston to deliver a shot into the die. Compared to the hot chamber casting method, cold chamber casting is a slower process. Nonetheless, it works great for manufacturing aluminum parts. However, molten aluminum alloys can attack and damage the metal cylinders, plungers and shorten the lifecycle of dies.
When selecting a die cast machine, the most important factors to consider are machine performance as it relates to clamping force. The following dimensional constraints of the machine are also important:
- Shot volume capacity
- Die opening stroke length
- Platen area
The projected area of the parts in the die and the pressure required for injection of the molten metal determine the required clamp force. Larger parts require a greater clamping force. The clamping force must be larger than the separating force of the molten metal on the die during injection in order to prevent the die halves from separating.
To determine the shot volume capacity of the machine, you must first calculate the total shot volume. You can find this by adding the volume of cavities, the volume of overflow wells and the volume of feed system together. Make sure the machine has a maximum die opening that is wide enough to allow for the extraction of the part without hindrance. The space between the corner bars in the platen area or clamp unit must also have adequate room for the required die.
Variations of the two casting processes include Low-Pressure Die Casting, which is typically employed to fabricate aluminum parts, like vehicle wheels that require symmetry around an axis of rotation. Squeeze Die Casting is used when the process involves metals and alloys that have low fluidity. Manufacturers use the Semi-Solid Die Casting method for parts where the specifications call for minimal porosity, maximum density and enhanced precision. Some products that entail post-casting treatment can benefit from a relatively new process called Vacuum Die Casting, which delivers improved strength and minimal porosity.
Types of Dies
Die makers categorize these tools as single cavity, multiple cavity, combination, and unit dies. Die makers fabricate die casting tooling from alloy tool steels. The die consists of a minimum of two sections — the fixed die (cover half) and the ejector die — which allow the removal of castings. Many modern dies have movable slides, cores or other sections, which produce threads, holes and other features in the castings. For zinc dies, the molten metal enters the die and fills the cavity through sprue holes in the fixed die. Typically, the ejector half of the die contains runners and gates that route the molten metal to the cavity. Dies include locking pins to clamp the two halves securely. The tool also has ejector pins to help remove the casting and venting for lubricant and coolant.
During the die casting process, the machine hydraulic pressure locks the two die halves in place. The surface area where the two halves of the die meet is called the “die parting line.” You can measure the total projected surface area of the casting from the die parting line. The pressure required of the machine to inject metal into the die cavity correlates with the clamping force of the machine.
Multiple cavity dies consist of several identical cavities. Sometimes referred to as a “family die,” a combination die has cavities of various shapes and can produce several parts for an assembly. Unit dies can create a number of parts for an assembly, or for different customers.
Runners transfer molten metal into the part cavity. Incorporated into the die, the channels vary for a cold chamber machine and a hot chamber machine. In the cold chamber machine, the material enters through an injection sleeve as compared to a sprue bushing in the hot chamber machine. After entering the die (in both machines), the molten metal flows through a series of runners and enters the cavity through gates.
Overflow wells within the cavities provide extra molten metal during the solidification. These wells give the extra material needed because the molten metal shrinks after the part cools. Small channels that run from the cavity to the exterior of the die function as venting holes, which allow air to escape the cavity. Another type of channel allows oil or water to flow through the die, which removes heat.
Customers will need to consider an array of factors when it comes to die design, including:
- Creating a die that allows the molten metal to flow easily into all the cavities
- Removal of solidified parts from the die (draft angle)
- Accommodation of complex design features like undercuts
Customers need to be aware that a reputable die cast plant only uses high quality tool steel that is certified and follows exacting specifications as set byt the North American Die Casting Association. Aluminum dies can last up to 150,000 shots depending on a number of factors such as quality criteria from the customer, part design, alloy specified, and whether surface finish is important. Die coatings are also available today at an added cost that can extend die life.
The Die Casting Process
Depending on the complexity of the product, the total cycle time can last between two seconds and one minute. The cycle process for die casting components consist of five primary stages: clamping, injection, cooling, ejection and trimming.
1. Clamping. The initial stage of the process involves the preparation and clamping of the two halves of the die. The operator first cleans the die of residual metal from the previous injection and then lubricates the tool to facilitate the injection of the next component. Based on the material used, lubrication may not be necessary after each cycle, but it is usually required after two or three cycles. The time it takes to lubricate the die varies depending on the size of the part, the number of cavities and side-cores. After undergoing lubrication, the two die parts, which are secured inside the die casting machine, are clamped together. The machine must apply sufficient pressure to keep the die securely closed during the injection of the metal. The larger the machine, the more time it requires to close and clamp the die.
2. Injection. Maintained at a set temperature in the furnace, the molten metal is transferred to the chamber and injected into the die. The technique used to transfer the molten metal depends on whether the operator uses a hot chamber or cold chamber machine. After the transfer, the molten metal — referred to as the shot — is injected into the die at pressures that range from 1000 to 20,000 psi. The pressure holds the molten metal in the dies until the material solidifies. The injection time denotes the period it takes for the molten metal to flow into all of the channels and cavities in the die. The extremely short time of less than 0.1 seconds prevents premature solidification of any one part of the metal. To determine the proper injection time, the manufacturer will need to take into consideration the thermodynamic properties of the material and the wall thickness of the casting. A thicker wall requires a longer injection time. For cold chamber die casting machines, the injection time must include the time required to manually ladle the molten metal into the chamber.
3. Cooling. Once the molten metal enters the die cavity, it begins to cool and solidify. After the molten metal fills the entire cavity, solidification occurs, and the casting takes on its final shape. The operator cannot open the die until the cooling time has elapsed, and the casting solidifies. The complexity of the die, the thermodynamic characteristics of the material and the wall thickness determine the cooling time.
4. Ejection. After the cooling period elapses, the operator can open the two die halves. An ejection mechanism pushes the casting out of the die cavity. The dry cycle time of the machine and the ejection time depends on the casting envelope. You should also add the time required for the casting to be removed from the die. During the cooling process, the component shrinks and adheres to the die, which requires adequate force to eject the casting. After the ejection, the operator can clamp the die shut and prepare for the next injection.
5. Trimming. During the cooling period, the metal material in the channel solidifies and adheres to the casting. Using a manual process (sawing or cutting) or a trimming press, the excess materials and flash must be trimmed from the casting. To determine the time required to trim the excess material, look at the size of the casting envelope. You can discard scrap from this task or recycle it for later use.
The most popular material for die casting consists of non-ferrous alloy, such as aluminum, copper, magnesium and zinc. Each material has unique properties as well as advantages and disadvantages, which the customer must carefully weigh. Factors you must consider when selecting your material include density, melting point, strength, corrosion resistance and costs.
Die Casting Vs. Other Processes
Modern die casting processes take place under high pressure and result in components that have superior surface finish and high integrity. Die casting provides a viable alternative for any production run above 10,000 pieces. Some of the manufacturing processes include:
- Plastics injection moldings. Die casting provide parts that are stronger, stiffer and dimensionally more stable than parts produced through plastic injection molding. Die casting also provides items that are more heat resistant and that exceed the performance of plastics on a properties/cost basis, and they have a high degree of permanence under load. These parts resist weathering, stress-cracking and ultra-violet rays. Although plastic parts may be less expensive on a unit-volume basis and have better inherent color properties that eliminate the need for finishing, the manufacturing cycle for parts fabricated via die casting is faster. Plastic parts are also temperature sensitive and are excellent electrical insulators.
- Forgings. Die castings offer more flexibility in the complexity of shapes, and they offer shapes that cannot be forged. Die casting can have thinner sections that are not possible under the forging process. Forging does offer to produce denser, stronger parts. Forgings also have the characteristics of wrought alloys. The process can use ferrous and other metals.
- Stampings. This process may require multiple parts compared to one die casting, which requires fewer assembly operations. Die casting offers complexity and more shape varieties; closer dimensional limits offer nearly any variation in section thickness and creates less waste. Conversely, stampings have similar properties to wrought-iron metals and allow fabrication from steel and alloys that are unsuitable for die castings. Simple stamping products typically weigh less and have shorter production cycles.
- Sand castings. Die castings have thinner walls that can be held to tighter dimensional limits, and smoother surfaces. They also have nearly all or most holes cored to size. Die castings offer faster cycle time for dies that make thousand of components without the need for replacement. They do not require new cores for each casting, and they have smoother surfaces and a lower labor cost per casting. You can manufacture sand castings from ferrous metals and from many non-ferrous alloys that may not be feasible for die casting. Sand casting also offers more possibilities for shapes and a greater maximum size. The machinery costs less and offers greater economy for small volume production. Although, the amount of machining on sand casting is always more than a die casting.
- Screw machine products. Die casting is faster than the screw machine process. It also creates less waste and offers more variation in shapes. Screw machine products, however, can be fabricated from steel and alloys that are not feasible with die casting. They have the characteristics of wrought metals, and the tooling costs less.
Overall, die casting offers manufacturers a versatile process that produces parts with complex shapes, smooth, or textured surfaces and a high degree of accuracy and repeatability. The process also allows for a wide variety of attractive and serviceable finishes.
Sourcing Die Castings Companies and Evaluating Quotes
Selection of the right die caster to produce your project can be quite challenging. Whether your project calls for aluminum, magnesium or zinc materials, you need to consider the cost of die cast tooling and production. During the initial engineering consultation and before commencing with building the tooling, the die caster should use the latest process simulation software and your CAD file to predict and optimize a number of important factors that are crucial to the die casting process, including:
- Metal flow
- Air entrapment
- Metal velocity
- Thermal balance
- Other elements
From start to finish, the die casting engineer must understand all of the variables involved with the project to avoid quality issues. The die caster should have experience and an unbiased opinion about the correct metal to use for your production run. If the product entails a post-finishing step, material selection becomes even more important because the alloy you choose must work with the type of finish you desire. A knowledgeable die caster will possess a complete understanding of finishes and can make the proper recommendations. The preplanning and analysis stage should include recommendations on cost-saving measures and ways to avoid expensive mistakes over the life of your project.
Prior to authorizing the die caster to build the die for your product design, make sure you receive a comprehensive description of the design and engineering consultation. A die casting engineer must possess a clear and complete understanding of your design concept and all components required to ensure the quality, functionality and performance requirements of the final product.
Naturally, you should seek the most competitive price, but consider what services the company includes in the quote. Keep in mind that a quote, which covers additional services, may cost you more upfront, but it may possibly save you thousands of dollars over the life of the product.
Premier Die Casting is a family-owned, business that has provided die casting services for a variety of industries for more than 70 years. We are based in the U.S., are members of the North American Die Casting Association, and are ISO 9001:2008 certified. We offer machining, metal finishing & assembly, and we specialize in aluminum die casting products. Contact our sales representative to learn more about our die casting services or to receive a quote.