The Basics of Die Casting
From engine blocks to door handles, die casting is a fast, accurate, and repeatable metal production technique for large or small parts. Die castings have an excellent surface finish, and the process is compatible with a wide range of nonferrous metals.
Due to the high startup costs of die casting, the process is often used for high-volume production, where the scale of manufacturing makes up for the high machinery and tooling costs. Die casting prototypes and small production runs are more difficult to obtain, as it is in the die casting company’s economic interest to work with customers for volume orders. This article takes an in-depth look at metal die casting, explaining suitable materials, surface treatments, and applications for the process.
1.What is die casting?
Die casting is a metal casting method that uses high pressure to force molten metal into a mold cavity formed by two molds. It shares characteristics with the plastic manufacturing process of injection molding. In the broader field of metal casting, die casting is one of the most popular techniques due to its accuracy, high quality, and finesse. The broader category of metal casting has been around for thousands of years and encompasses many different processes that use molds to form liquid metal. Historically, the process often involved pouring liquid metal into a mold with the help of gravity – many metal casting processes are still performed this way. However, die casting is a relatively new form of metal casting, introduced in the 19th century, which uses pressure rather than gravity to fill the die cavity.
Die casting is sometimes called high-pressure die casting because of the pressure (usually 10-140 MPa) required to force the metal into the die cavity. The related low-pressure die casting (LPDC) process is less common. Die casting is generally divided into two categories: hot chamber die casting and cold chamber die casting, each suitable for different types of metals. However, there are other more niche types of die casting, such as semi-solid metal casting (SSM).
2.How Die Casting Works
In simple terms, metal die casting works by using high pressure to force molten metal into a die cavity formed by two hardened steel dies. Once the die cavity is filled, the molten metal cools and solidifies, and the die is opened so that the part can be removed. In practice, however, the process has many steps and requires skilled engineers to operate the die casting equipment. Here we divide the die casting process into three stages:
- Mold manufacturing
- Casting
- Post-processing
3.How to make a die casting mold
A die casting mold consists of at least two halves: a cover side (mounted on a fixed plate) and an ejector side (mounted on a movable plate). Some molds also have other parts, such as slides and cores, for the production of more complex parts, such as parts with holes and threads.
Depending on the size of the manufactured part, a die casting mold may have multiple cavities in order to produce multiple parts per cycle. Such molds either have multiple identical cavities (multi-cavity molds) or a mix of different cavities to produce different parts (unit molds).
Die casting molds must be very strong and heat-resistant, and in addition must have good wear resistance and ductility. Therefore, they are made of high-performance hardened tool steel and are usually heat-treated to enable them to perform hundreds of casting cycles per hour and up to two million casting cycles throughout their service life. Die casting molds must maintain performance under very high clamping forces.
Making a die casting mold starts with the use of computer-aided design (CAD), combined with casting-specific design and simulation tools. Like injection molds, die casting molds must have gates, runners, and sprues to allow molten material to enter the cavity. Locking pins and ejector pins must also be incorporated to secure the mold and facilitate ejection. Digital design of the mold allows for the creation of complex shapes and tight tolerances.
CNC machining is widely used to make die casting molds. Typically, die casting mold manufacturing starts with rough machining of the mold shape, followed by heat treating the metal mold and finally a round of finishing. Prototype-level molds can also be made using rapid tooling, using CNC machining or other processes such as selective laser sintering (SLS).
4.How to Cast Metal Parts
Similar to injection molding, once the mold is made, the die casting can be made in a die casting machine. The die casting process consists of four main stages: preparation, filling, ejection, and sanding. However, the casting process differs slightly depending on whether a hot chamber or cold chamber is used. These two variations of the high-pressure die casting process have different advantages: one is suitable for high-speed casting, while the other is suitable for a wider range of casting materials.
①Hot Chamber Die Casting
In the hot chamber die casting process, the metal die casting machine contains the equipment necessary to heat the metal to a molten state. Since it is a self-contained system, it is much faster than other systems and has a short cycle time, although it is only suitable for some casting materials, including zinc, tin and lead alloys.
②Cold Chamber Die Casting
The cold chamber die casting process requires the use of a separate furnace to heat the metal. This naturally reduces productivity because the molten metal must be brought to the die casting machine by a ladle. However, since the separate furnace is more powerful than a hot chamber die casting machine, metals with high melting points can be cast. This method is suitable for aluminum casting. Whether using a hot chamber or cold chamber machine, the metal die casting process generally proceeds as follows:
- Die Preparation
- Filling
- Ejection
- Ejection
During the die preparation process, the inner surfaces of the two die halves are coated with lubricant to facilitate ejection after the casting is completed. The die halves can then be closed and secured with locking pins. Filling of the die is achieved by a pressure system. This system differs between hot chamber and cold chamber systems. In both systems, the end result is that the molten metal is forced into the die cavity through the gate by a plunger. High pressure (up to 35 MPa in the hot chamber and 140 MPa in the cold chamber) ensures rapid and complete filling, resulting in consistent cooling, preventing uneven shrinkage and consequent part distortion. Pressure is maintained during cooling.
The two mold halves are opened and the casting is removed using the ejector pins. Typically, the mold is immediately reclosed in preparation for the next shot. At the same time, the finished casting is ready for vibratome, which involves removing the waste material from the injection, such as gates, runners, and flash (material seepage at the parting line). Material can be removed using hand tools, tumbling, or hydraulic trimming dies.
③Post-machining
Many metal die castings require very little secondary machining. This is because die castings require very high pressures, which allow a high level of detail and good surface finish to be achieved. However, many net-shape and near-net-shape castings also require precision machining of holes, threads, and other features. Some cast metals are easier to machine than others: for example, magnesium and aluminum die castings are well suited to post-machining. A second benefit of post-machining of die castings is the ability to take advantage of the in-machine inspection capabilities of CNC machines, allowing machinists to verify the part.