What is Die Casting?
A – Die casting is a manufacturing process for producing accurately dimensioned, sharply defined, smooth or textured-surface metal parts. Generally speaking, these die or mold cavities are created with hardened tool steel that have been previously machined to the net shape or near net shape of the die cast parts. This process allows products to be made with high degree of accuracy and repeatibility. The die casting process also produce fine details such as textured surfaces or names without requiring further processing.
As technology advances, the die casting process has become more and more efficient. New technologies allow greater pressure in the injection process, thus allowing us to produce parts that are closer to net shape and higher integrity than ever before.
Advantages of Die Casting Parts
One of the main advantages of die casting is the ability to produce parts and products with a wide range of shape and sizes. Unlike other manufacturing processes such as extrusion, the die casting process does not limit the shape of parts and in most cases will be the net shape of the parts.
HOW ARE DIE CASTINGS PRODUCED?
Type of die casting processes
The die casting process can be further divided into two different categories:
• Hot Chamber Die Casting
• Cold Chamber Die Casting
HOT CHAMBER MACHINES
Hot chamber machines are used primarily for zinc, and low melting point alloys which do not readily attack and erode metal pots, cylinders and plungers. Advanced technology and development of new, higher temperature materials has extended the use of this equipment for magnesium alloys.
In the hot chamber machine, the injection mechanism is immersed in molten metal in a furnace attached to the machine. As the plunger is raised, a port opens allowing molten metal to fill the cylinder. As the plunger moves downward sealing the port, it forces molten metal through the gooseneck and nozzle into the die. After the metal has solidified, the plunger is withdrawn, the die opens, and the resulting casting is ejected.
Hot chamber machines are rapid in operation. Cycle times vary from less than one second for small components weighing less than one ounce, to thirty seconds for a casting of several pounds. Dies are filled quickly (normally between five and forty milliseconds) and metal is injected at high pressures (1,500 to over 4,500 psi). Nevertheless, modern technology gives close control over these values, thus producing castings with fine detail, close tolerances and high strength.
Figure 1: Hot Chamber Machine. Diagram illustrates the plunger mechanism which is submerged in molten metal. Modern machines are hydraulically operated and equipped with automatic cycling controls and safety devices.
COLD CHAMBER MACHINES
Cold chamber machines (Fig. 2) differ from hot chamber machines primarily in one respect; the injection plunger and cylinder are not submerged in molten metal. The molten metal is poured into a “cold chamber” through a port or pouring slot by a hand or automatic ladle. A hydraulically operated plunger, advancing forward, seals the port forcing metal into the locked die at high pressures. Injection pressures range from 3,000 to over 10,000 psi for both aluminum and magnesium alloys, and from 6,000 to over 15,000 psi for copper-based alloys.
In a cold chamber machine, more molten metal is poured into the chamber than is needed to fill the die cavity. This helps sustain sufficient pressure to pack the cavity solidly with casting alloy. Excess metal is ejected along with the casting and is part of the complete shot.
Operation of a “cold chamber” machine is a little slower than a “hot chamber” machine because of the ladling operation. A cold chamber machine is used for high melting point casting alloys because plunger and cylinder assemblies are less subject to attack since they are not submerged in molten metal.
Figure 2: Cold Chamber Machine. Diagram illustrates die, cold chamber and horizontal ram or plunger (in charging position).
The complete cycle of the die casting process is by far the fastest known for producing precise non-ferrous metal parts. This is in marked contrast to sand casting which requires a new sand mold for each casting. While the permanent mold process uses iron or steel molds instead of sand, it is considerably slower, and not as precise as die casting.
CONTINUOUS COMPUTERIZED PROCESS MONITORING
Step 1 — Alarm monitoring — A computerized remote terminal mounted near the die casting machine continuously monitors the following: velocity, position, pressures at the accumulator, die lock-up cylinder, head and rod side of the injection cylinder, temperatures of hydraulic oil, metal (up to ten locations in the die), and strains on four tie-bars. Each of these variables is high/low limit checked every shot. An alarm is sounded or flashed if a casting variable goes out-of-limits.
Step 2 — Control — Automatic control valves and tie-bar adjusting motors are installed on the die casting machine and connected to the computerized remote terminal unit. The computer adjusts controls to maintain satisfactory die locking force, slow shot velocity, optimum fill time and proper intensifier timing. The computer may also be interfaced to a robot and/or a programmable controller.
Step 3 — Data acquisition — After installing computerized remote terminal units on each machine, a data acquisition system may be formed by interconnecting these units and communicating all information back to a master terminal computer in the office. The master computer prints out a management information report which summarizes the operation of the entire die casting shop, including which machines are running, when they stop, shots made (good and bad), which machines are out-of-limits and what to do. At the master station, the manager can analyze the process by studying shot profiles on a video monitor and statistical reports from a high speed printer.
Thinner Wall Castings:
Compared with sand and permanent mold castings, the die casting process is able to produce parts with thinner walls due to the high pressure during the injection process. 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.