Knowledge

The gating system of castings is mainly responsible for transferring clean and slag-free molten steel from the ladle to the mold cavity without causing secondary oxidation or gas absorption. To some extent, all alloys are affected by slag formation, and some, such as aluminum-based and copper-based alloys, are particularly susceptible.

 

The most important aspect of pouring is to introduce the metal into the mold without turbulence, at the lowest possible speed, and with an appropriate filling rate. The optimal filling rate for a specific alloy cannot be regarded as a fixed value but depends on many factors, such as the weight of the casting, the thickness of the cross-section, and the shape of the casting. Excessive flow rates increase the possibility of turbulence, jetting, and oxidation at the front end of the metal, which can lead to a reduction in mechanical properties or even scrapping of the casting.

 

The necessary high filling rate conflicts with the low flow rate, especially for alloys that are prone to slag formation. This often results in an oversized gating system, far exceeding the usual area-based proportions of the gating system. For such alloys, it is ideal to pour using a "pressureless" gating system by removing the top surface of the runner. This ensures that the runner is always fully flowing. The distance between the sprue well and the first inner gate should be as large as possible to allow time for slag to float and be captured by the top surface of the runner. Inner gates should be placed as close to the bottom of the casting as possible to minimize turbulence in the mold cavity.

 

The Pouring Bush: The pouring cup should be used for all but the smallest castings. It should be designed to allow the pourer to quickly fill the sprue and maintain a constant head pressure during the pouring process. A biased design with a dam can achieve this. The shape of the pouring cup should be rectangular to facilitate the upward circulation during pouring, which helps remove slag. The outlet of the pouring cup should be arc-shaped and match the sprue. A plug is usually used in the pouring cup, allowing the pourer to fill it completely before removing the plug, giving time for slag to float.

 

Direct pouring into the sprue or using a conical liner to directly flow into the sprue is not recommended, as it not only causes the formation and introduction of air and slag into the gating system but also generates excessive turbulence in the gating system due to the high-speed metal flow.

 

The Sprue: The sprue controls the filling rate of the casting and is the most important part of the gating system. During production, the sprue should gradually decrease in size, with a smaller control area at the bottom, and other gating system components determined by the area of the sprue outlet.

 

There are many formulas and useful charts to determine the slope of the sprue. Starting from the control area, a 5-degree taper is sufficient. When the sprue height exceeds 300mm, it is recommended that the diameter (or side length) of the cross-section be increased by 50%. The cross-section of the sprue can be circular, square, or rectangular. There is evidence recommending the use of a rectangular cross-section because it has a tendency to reduce the formation of vortices and gas absorption. If there are no other reasons, square and rectangular cross-sections are easier to manufacture than circular ones.

 

The Sprue Base: Because the flow rate is at its maximum at the sprue outlet, it is important to buffer the liquid flow and allow it to change from a vertical to a horizontal direction with minimal turbulence. The recommended dimensions for the sprue well are: a diameter of 2 to 3 times the diameter of the sprue outlet and a depth of 2 times the depth of the runner.

 

Runner and Gates: As mentioned earlier, ideally, the casting should be made in a pressureless gating system (the runner in the lower mold and the inner gates in the upper mold). The area of the runner should be 2 to 4 times the area of the sprue well, and the total area of the inner gates should be at least equal to or greater than twice the area of the runner. This ensures the required filling rate at the lowest possible speed. Alloys that are particularly sensitive to slag require larger sprues and gates to ensure the minimum flow rate. The ideal cross-section of the sprue is rectangular, with a width-to-depth ratio of 2:1. The wide upper surface is designed to maximize the capture of slag and inclusions by the sprue. When the sprue has multiple gates, to ensure uniform flow rate at each gate, the area of the sprue decreases as it passes through each gate. It is also a good practice to set a slag pot at the end of the sprue to collect the severely oxidized metal that fills it first. The position where the gate connects to the cavity should be as low as possible to avoid turbulence caused by the drop of the liquid. Like the sprue, the cross-section of the gate should be rectangular rather than square to avoid the formation of "hot spots" at the contact point with the casting and subsequent shrinkage cavities. The exact ratio of width to thickness of the gate is determined by the solidification time of the casting. Based on experience, the thickness of the gate should be less than one-third of the thickness of the casting at the connection point.

 

Filtration: Metal filters have been in widespread use for several years. They come in various forms, ranging from simple filters and woven fabrics to different types of ceramic blocks. Ceramic filters mainly fall into two categories: one is extruded with straight and parallel holes; the other is foam ceramic, composed of a foam-like ceramic structure without a specific pore orientation. The benefits of using ceramic filter blocks are well documented. They are highly effective in removing inclusions and can improve mechanical properties. Foam ceramic filter plates have distinct advantages over extruded filter plates. When metal passes through, the initial metal flow is not divided, reducing the possibility of secondary oxidation at the filter plate outlet. The capacity of foam ceramic filter plates, or the total amount of metal that can pass through before being blocked, varies depending on the slag-forming tendency of the casting alloy. The capacity of the filter plate is also influenced by the upstream process of the filter plate's pouring system. For instance, if the metal reaching the filter plate contains a significant amount of slag, the capacity of the filter plate will decrease sharply. Filter plates should not be used carelessly and must be used in conjunction with a sound pouring system to achieve useful results.

 

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