The current urgent need to increase the output of ventilator has quickly attracted people's attention, that is, a flexible supply chain is needed to quickly respond to the growing demand. Products are becoming more and more complex, and more and more organic shapes are specified to achieve the required strength, while minimizing the use of materials. Additive manufacturing (AM) is involved in both trends.
People believe that products can be any shape we like and can be produced locally. In fact, AM has little room to replace traditional production in the foreseeable future. Issues such as material performance, raw material cost, machine cost and manufacturing speed mean that additive manufacturing will still be a relatively niche process.
This article details another highly automated manufacturing process that can produce complex shapes in high-strength alloys. Die casting may not be a new technology, but it is very suitable for many modern products. High pressure die casting is a highly automated process that can economically produce parts with very complex shapes. It is usually suitable for mass production. This article will study the process in detail, and study the tool requirements, breakeven amount, material properties and surface finish.
Die Casting Process Basics
Die casting is essentially the same process like injection molding. The die-casting process uses a permanent metal mold, or die. Molten metal is forced into the die cavity at a pressure of between 0.7 MPa and 700 MPa. The term injection molding refers to the production of plastic parts while die casting involves production in metals. Die casting is most suited to softer alloys. In the past, tin and lead were popular materials for the die-cast parts. Today, zinc, aluminum, and magnesium-based alloys are the most common. High-strength structural automotive and aerospace components are produced, as well as many consumer goods.
Each injection, is known as a shot. A shot may be made up of more than one casting or part, as well as the scrap material that is produced during the casting process. Scrap includes the sprue, where the material enters the die; the runners, which distribute material to multiple part cavities; and the gates, where the material flows into individual part cavities. Sprues, runners, and gates are found in other types of casting as well as in injection molding. If you’ve ever made an Airfix model, you will remember receiving the parts still attached to the shot. You may have also sometimes had to trim away flash, a thin layer of material that has leaked into the interface between the two halves of the mold. The parts are attached to the gates and joined together by the runners. In industrial casting processes, the individual parts must be separated from the scrap. This process is known as shakeout and may be carried out using a trim die in a press.
The die is made in two halves so that the shot can be removed. One half of the die is fixed and contains a hole through which the molten metal is injected. The other half is closed with a press that must be able to resist the pressure of the metal being injected. This pressure can be considerable, resulting in die casting machines being very large and heavy. Molten metal enters an injection cylinder, known as a shot chamber, and a piston then injects it through a nozzle into the die.
Die casting is normally categorized into two basic types, depending on whether the shot chamber is heated:
Die casting machines are rated by the clamping force used to keep the die closed, typically between 25 tons and 3,000 tons. Other machine specifications include the die size, piston stroke, and shot pressure. Higher pressures allow rapid cycle times, thin walls, and fine features.
Dies typically weigh 1,000 times the weight of the part being produced, so a 3kg part will require a 3,000kg die. Dies often include additional cooling channels, which must be machined and hardened. Ejector pins are also required to remove the part after it has been cast. Dies must be able to withstand the thermal shock of repeated thermal cycling and should not soften at the shot temperature. Hardened tool steel is normally used. These considerations mean that dies represent a considerable capital expense. Surfaces running in the direction of die separation must be designed with tapered faces, known as draft angles, to enable removal from the die. If overhangs are required, moveable cores or slides must be included in the die, which can greatly increase complexity and cost.
Despite their high initial cost, correctly operated dies can perform over 500,000 shots before showing significant signs of wear. Long die life, combined with a high level of automation, makes die-casting very economical for high-volume production.
Thin walls of just 0.4mm are possible. Thinner walls generally improve material properties as the more rapid cooling reduces the size of crystals in the solidifying metal, resulting in a fine grain structure. Dimensional accuracy and surface finish are also excellent for a casting method. For small parts, accuracies of 0.1mm are possible and surface roughness can be as low as one micrometer. It is also possible to include inserts such as steel bearing housings and threads during the casting process.