Ultra-low speed die casting process

Ultra-low speed die casting uses laminar flow to fill the mold cavity with aluminum alloy liquid at a low injection speed. As a new die-casting process, it can well solve the problem of gas entrainment caused by traditional die-casting metal liquid injection, thereby improving mechanical properties.

Product types applicable to ultra-low speed Die Casting Process:

• Castings with low requirements for product appearance and surface sandblasting
• Airtight parts with high internal density requirements
• Castings with high T6 heat treatment strength requirements are required.

Such as shift forks, shock absorber arms, pistons, brake valve bodies, oil pump bodies, air filter housings and other castings that require high internal density.

During the die-casting process, the injection speed has an extremely important impact on the forming quality of the product. Under ultra-low-speed die-casting conditions, the gate speed is usually low, allowing the molten metal to fill the mold cavity in a laminar flow state, and the residual gas in the cavity can be smoothly discharged, so pore defects are greatly reduced. However, in the existing ultra-low-speed die-casting process, the molten metal stays in the pressure chamber for too long and loses a lot of heat. During the high-speed filling stage, due to the relatively low filling speed, the molten metal quickly cools and solidifies when it reaches the mold cavity, which reduces the mold filling capacity and easily causes defects such as cold shutoff and insufficient pouring. In this regard, by appropriately increasing the low-speed injection speed, greatly reducing the high-speed injection speed, and delaying the starting point of the high-speed injection, the filling capacity of the molten metal can be improved while ensuring ultra-low speed filling conditions.

In order to achieve ultra-low-speed die-casting process conditions and reduce the inner gate speed, it is usually necessary to thicken the inner gate. According to the structure of the aluminum alloy die-casting parts of the engine bracket and the characteristics of the gating system, multiple angles are used in conjunction with a certain angle of subduction to ensure balanced advancement of the molten metal. At the same time, the thickness of the inner gate is increased from the original 2.5 mm to 5.2 mm. To reduce pressure losses, the runner adopts a circular cross-section. The rounded corners are enlarged at the bifurcations and turns of the flow channel to prevent turbulence of the molten metal.

Since the filling capacity of molten metal in ultra-low speed die casting is lower than that of traditional die casting, appropriately increasing the pouring temperature will help increase the fluidity and filling capacity of the molten metal. However, an excessively high pouring temperature can easily dissolve too much hydrogen in the aluminum liquid, causing hydrogen gas to precipitate during the die-casting process, resulting in pores, excessive oxides, and oxidized slag inclusion defects. Through exploration, the appropriate pouring temperature selected in the ultra-low speed die-casting process is 680~690℃.

Since the ultra-low speed die-casting process has changed the filling method of molten metal in traditional die-casting, the original mold design no longer adapts to the requirements of the new process to a certain extent. Therefore, improvements need to be made on the basis of traditional die-casting molds, and the gating system, overflow system, cooling system, ejection system, etc. need to be appropriately adjusted.

When optimizing the mold structure such as overflow, exhaust, and slag discharge systems, the thickness of the slag collection bag mouth was increased from the original 1.1 mm to 2.3 mm. Change the structure of the slag bag and increase its feeding function. The overflow tank should be as large as possible and set at the final filling point and at a low mold temperature. This ensures that during ultra-low speed die casting, gas and cold material.

The smooth discharge avoids the turbulence of aluminum liquid. In the traditional die-casting process, the manufacturing tolerance of the ejector rod is usually 0~+0.04 mm, and the manufacturing tolerance of the mating hole is -0.03~-0.01mm. Since the mold temperature changes in the ultra-low-speed die-casting process are different from traditional die-casting, after a series of analysis of the expansion test data of the mold and ejector rod at simulated die-casting temperatures, and with the optimization of die-casting technologyand Mold Manufacturing processes, the fitting tolerance of the ejector rod was changed to 0~0.015 mm.

The temperature of the die-casting mold directly affects the quality of the die-casting parts. Since the wall of the gearbox bracket is thick, shrinkage holes are prone to occur. Therefore, when designing the mold, the principle is to enable the melt to solidify from the farthest point away from the gate, with the gate solidifying last, so that the shrinkage cavities can be eliminated by the pressure of the die-casting machine. The specific method is: during the injection stage, for parts that are far away from the gate, have a long flow of aluminum liquid, or are difficult to form, use partial sealing and reduce cooling flow to ensure the molding and strength needs of this area. During the solidification stage, X-rays are used to detect the size distribution and data of product shrinkage in each part, and the overall cooling layout and flow control are adjusted so that the thick-walled parts of the product are solidified first, and the parts close to the gate and the feeding slag bag are solidified last, thus solving the problem of feeding and shrinkage holes in the solidification stage.

• Select a bit gate with increased wall thickness
• Thicker gate thickness
• Choose a smaller punch (higher filling rate)
• The acceleration ratio is controlled within 10:1 (punch cross-sectional area and gate)
• The filling time is calculated between 0.05-0.1s