After understanding what is low pressure casting, you may also want to learn more about how is low pressure casting works and what is the process of low pressure casting. This article will give you a comprehensive answer.
How is low pressure casting works?
The principle of low-pressure casting: filling the sealed protective furnace with a certain amount of dry compressed air. The metal liquid in the holding furnace is punched into the mold cavity along the liquid riser under the air pressure so that the metal liquid is in the mold cavity (pressure Under the action) solidification to obtain a compact casting.
Low-pressure casting is a method to make liquid metal fill the cavity under pressure to form a casting. Due to the low pressure used, it is called low pressure casting. It is a casting method between pressure casting and gravity casting.
The specific performance is:
In a sealed container (melting furnace) filled with alloy liquid, dry compressed air (or inert gas) is introduced to act on the surface of the alloy liquid at a certain pouring temperature, resulting in a pressure difference between the sealed container and the mold cavity So that the alloy can be filled into the casting mold smoothly through the ascending channel and the casting gate under the inner hole of the rising pipe under the action of lower filling pressure (0.01 ～ 0.05MPa). After the liquid fills the cavity, increase the air pressure, so that the alloy liquid in the cavity is crystallized and solidified under the action of higher pressure, and then the pressure in the sealed container is released, so that the unsolidified alloy liquid in the riser and the runner depends on its own Gravity falls back into the crucible, and then the mold is opened to remove the casting.
Take the copper alloy casting products produced by low pressure casting machine as an example,
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The following are the process steps of low pressure casting:
1. Assemble the metal, riser tube and mold, and close the sealing cap.
2. Pass the dry compressed air (or inert gas) into the crucible that seals the metal liquid, so that the metal liquid enters the casting mold through the riser tube under the pressure, and solidifies under pressure.
3. Relieve the pressure and allow the unsolidified liquid metal in the riser and pouring system to flow back to the crucible.
4. Open the mold and remove the casting.
What is the low pressure die casting process?
The technological process of low pressure casting includes filling, boosting, holding pressure, releasing pressure, mold preheating temperature, pouring temperature, and mold coating.
(1) Filling and boosting
Lifting pressure refers to the pressure required when the metal level rises to the gate. The rising speed of the metal liquid in the riser pipe should be as slow as possible, so as to facilitate the discharge of the gas in the cavity, and at the same time, the metal liquid can not be splashed when entering the gate.
(2) Filling pressure and filling speed
The filling pressure Pa refers to the pressure required to raise the molten metal filling to the top of the mold. In the filling stage, the boosting speed on the liquid metal surface is the filling speed.
(3) Supercharging and supercharging speed
After the metal liquid fills the cavity, pressurization is continued, so that the crystallization and solidification of the casting are carried out under a certain amount of pressure. The pressure at this time is called the crystallization pressure. The greater the crystallization pressure, the better the shrinkage effect, and the denser the final casting structure. However, increasing the pressure by crystallization to improve the quality of castings is not applicable in any case.
(4) Holding time
After the cavity pressure is increased to the crystallization pressure, it is kept under the crystallization pressure for a period of time until the casting is completely solidified. The time required to hold the pressure is called the holding time. If the pressure holding time is not enough, the pressure is released before the casting is completely solidified, and the metal liquid in the cavity will flow back to the donation in whole or in part, causing the casting to be “emptied” and scrapped: if the pressure holding time is too long, the gate remains too long This not only reduces the yield of the process but also causes the gate to “freeze”, making castings difficult to mold, so it is necessary to choose a suitable holding time in production.
(5) Exhaust (pressure relief)
After the casting is solidified, the pressure in the crucible (ie, exhaust) is discharged, so that the liquid metal in the riser pipe and the runner that has not been solidified falls back into the crucible. After the casting is cooled, the casting can be taken out and opened.
(6) Mold temperature and pouring temperature
Various casting molds can be used for low-pressure casting. The working temperature of non-metallic molds is generally room temperature. Special requirements are required first, and there are certain requirements for the working temperature of metal molds. For example, when casting aluminum alloy at low pressure, the working temperature of the metal mold is generally controlled at 200 ~ 250 ° C, and when casting thin-walled complex parts, it can be as high as 300 ~ 350 ° C. Regarding the pouring temperature of the alloy, the practice has proved that under the premise of ensuring the casting molding, the lower the better.
In order to improve the service life and casting quality of metal mold low-pressure casting, paint must be applied; the coating should be uniform, and the thickness of the coating should be determined according to the surface finish of the casting and the structure of the casting.
Characteristics of the low pressure casting process
1. The liquid metal fills the cavity smoothly from bottom to top. The direction of liquid flow in the cavity is consistent with the direction of gas discharge, so it can avoid the scouring of the cavity and the core by the metal liquid, and at the same time reduce the liquid metal flow curl The possibility of secondary oxidation of gas and molten metal to prevent defects such as pores and non-metallic slag inclusions in the casting, and improve the quality of the casting.
2. The mold is filled under pressure and the fluidity is increased, which is beneficial to pouring more complicated thin-walled parts and make the outline of the casting clear; the solidification and shrinking process of the casting are carried out under external pressure, so the shrinking effect is good and the casting is dense High and good mechanical properties. Generally, the tensile strength and hardness of low-pressure castings can be increased by about 10% compared with gravity casting. This casting method is suitable for producing pressure-resistant and leak-proof castings.
3. The pressure and speed during pouring can be adjusted, so it can be applied to various casting molds (such as metal molds, sand molds, etc.), casting various alloys and castings of various sizes.
4. The bottom injection type filling is adopted, the metal liquid filling is stable, and there is no splash phenomenon, which can avoid the gas entrapment and the scouring of the mold wall and the core, and improve the casting pass rate.
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