Die casting is one of the main metalworking process for metal castings. It is operated by forcing molten metal under high pressure into a die cavity. Generally speaking, die casting molds are firstly precision machined to net shape or near net shape of finished die cast parts. Material of such molds for die casting is hardened tool steel. Manufacturing of die casting moulds will allow products to be made with perfect accuracy and repeatability. And, the die casting process will also produce fine details such as textured surfaces or names without requiring further processing.
Die casting is a wise selection for mass production products for its ability to produce high detailed and high accuracy parts. In our daily life, we can see lots of products made in die casting process. When you turn the faucet, you open the door knob, when you drive your car, almost every product or part of the product you use in your daily life is produced using this process.
The die casting process usually produce parts using primarily non-ferrous metals, such as zinc, copper, aluminum and magnesium. For steel and iron castings, investment casting and sand casting can be used.
1. Die making and assembly – Tool and die is the first step for starting die casting process, who is made up of two halves. Before die is ready for next part injection , each die half needs to be first cleaned from the previous injection and then lubricated. The lubrication will help to increase the size accuracy, as well as the number of cavities and side-cores. Also, depending on the material, lubrication may be required after 2 or 3 cycles, not each cycle. After lubrication, the two die halves, which are attached inside the die casting machine, are closed and securely clamped together. Sufficient force must be applied to the die to keep it securely closed while the metal is injected. The time required to close and clamp the die is dependent upon the machine – larger machines (those with greater clamping forces) will require more time. This time can be estimated from the dry cycle time of the machine.
2. Injection – Metal will be first molten and maintained at a set temperature in the furnace. Then transferred into a chamber where it can be injected into the die. Due to different kinds of die casting machines, whether hot chamber or cold chamber machine, we can decide the method of transferring the molten metal. Once molten metal is transferred, it will be injected at high pressures into the die, normally from 1,000 to 20,000 psi. Under the work force of high pressure, molten metal will get solidification. The amount of metal that is injected into the die is referred to as the shot. The injection time is decided when the molten metal fill all of the channels and cavities in the die. Typical injection time is very short, just less than 0.1 seconds, in order to prevent early solidification of any one part of the metal. Besides, The injection time can be affected by the thermodynamic properties of the material and the wall thickness of the part. Of course, larger wall thickness will cost longer injection time. For cold chamber die casting process, we still need to count into the time to manually ladle the molten metal into the shot chamber.
3. Cooling – After molten metal is injected into the die cavity, it will begin to cool and solidify. Finally shape of casting is formed when the entire cavity is filled and the molten metal solidifies. Only after the casting is solidified, can the die be opened. The cooling time can be estimated from several thermodynamic properties of the metal, the maximum wall thickness of the casting, and the complexity of the die. Longer cooling time will be required for greater thickness. Besides, for complex shape of die cavity, it may also require a longer cooling time due to the additional resistance to the flow of heat.
4. Ejection – When cooling time is enough, the die halves can be opened and the casting will be pushed out of the die cavity by an ejection mechanism. The time to open the die can be estimated from the dry cycle time of the machine and the ejection time is determined by the size of the casting’s envelope and should include time for the casting to fall free of the die. As the part shrinks and adheres to the die during cooling process, the ejection mechanism must apply some force. The die will be clamped shut for next injection after previous casting is ejected.
5. Trimming – During cooling, the material in the channels of the die will solidify attached to the casting. This excess material must be removed. The trimming way from the casting could be either manually cutting or sawing, or using a trimming press. The time required to trim the excess material is mainly determined by the size of the casting’s outline. After trimming, the scrap material can be recycled for other die casting production or discarded. For Recycled material, it may need to be reconditioned to the proper chemical composition before it can be combined with new metal for die casting process.
The die casting process can be typically divided into two different types:
Hot Chamber Die Casting is the process mainly for alloys like zinc, magnesium or copper. In hot chamber die casting, the injection system is immersed in pool of molten metal hence the name. The furnace is attached to the machine via a feeding system called a gooseneck. As the cycle begins the piston will retracts, which allows the molten metal to fill the “gooseneck” from a port in the injection cylinder. As the plunger move downwards, it seals the port and forces the molten metal through the gooseneck and nozzle into the die. Once the metal solidifies, the plunger will pull upwards. Afterwards, the die will open and the part is ejected. The advantage of this process is its short cycle time and it does not require metal to be transported from a separate furnace. Unfortunately, this die casting process is only suitable for alloys that do not attack the injection cylinder.
Cold Chamber Die Casting is the process primarily for manufacturing of aluminum die castings. In cold chamber die casting, it uses a ladle to transport the molten metal from the holding furnace into the unheated shot chamber or injection cylinder. This metal is then shot into the die by using a hydraulic piston. The main disadvantage of this process is that it is relatively slower compared to the hot chamber die casting process.
Freedom of size–With die casting process, we are able to parts with a wide range of shape and sizes. Unlike other manufacturing processes such as forging, the die casting process does not limit the shape of parts and in most cases will be the net shape of the parts.
Durability–Compared to plastic parts, die castings are stronger than their plastic counterparts. In most cases, even if thinner parts, die casting can still maintaining its strength for its application. Furthermore, die cast parts are able to withstand to a wide range of temperature, making it workable in harsh temperatures and working environments.
High production efficiency–Due to the ability to produce complex parts in net shape with little or no machining processes required, it will greatly reduce the production time. So die casting is considered as one of the most effective process to mass produce non-ferrous metal complex parts.
Green Manufacturing–How long does it takes for plastic to biodegrade? According to scientists, plastics takes at least a few hundreds of years to degrade fully. This means that obsolete plastic products are pilled up in the landfills, polluting our beaches, killing millions of sea animals and causing health hazards to the inhabitants in our neighborhoods. Unlike plastic, die casting parts are primarily made of recycled material. On average, approximately 95% of the metal is recycled from salvaged parts, making it a sustainable material that can be used over and over again without degradation. So die
casting is a green manufacturing technique.
Thinner Wall Castings–Compared with sand casting or permanent mold castings, due to the high pressure during the injection process, die casting is able to produce parts with thinner wall thickness. This allows lightweight construction as well as eliminate or reduce the need of secondary operations.
Casting with inserts–The die casting process allows inserts to be cast-in to form certain features such as threaded inserts, heating elements or high strength bearing surfaces.
The main disadvantage to die casting is the very high production cost. Compared to other casting process, both the casting equipments required and the dies and related components are all costly. Therefore, to make die casting cost-effective, a large production volume is preferred. Other disadvantages are that the process is limited to high-fluidity metals, and casting weights must be between 30 grams and 10kg. In the standard die casting process the final casting may have a small amount of porosity. This prevents any heat treating or welding, because the heat causes the gas in the pores to expand, which causes micro-cracks inside the part and exfoliation of the surface. Thus a related disadvantage of die casting is that it is only for parts in which softness is acceptable. Parts needing hardening (through hardening or case hardening) and tempering are not feasible for die casting.