Design points and functions of casting process subsidies
1. Design points and functions of casting process subsidies
Casting process subsidies are additional parts added to castings during casting process design to ensure casting quality and facilitate casting production. The following are its design points and functions:
Design points
Thickness: The thickness of the process subsidy is usually determined based on factors such as the structure, size, material and casting process of the casting. Generally speaking, for thick and large parts that are prone to shrinkage cavities and shrinkage, the thickness of the subsidy should be appropriately increased to ensure sufficient molten metal for shrinkage compensation. For example, in steel castings, for parts with a wall thickness greater than 50mm, the thickness of the process subsidy may be around 5-10mm.
Shape: The shape of the subsidy should be as consistent as possible with the shape of the casting to avoid forming obvious protrusions or depressions on the casting, affecting the appearance of the casting and subsequent processing. For example, in the rim of a wheel casting, the process subsidy can be designed as a ring concentric with the rim to evenly increase the thickness of the rim.
Distribution: The process subsidy should be reasonably distributed according to the solidification characteristics of the casting and the possible defect locations. For areas with slower cooling speed during solidification, such as inner corners and thick parts of castings, subsidies should be appropriately increased; while for areas with faster cooling speed, such as thin-walled areas and outer corners of castings, subsidies can be reduced or not set.
Function
Promote sequential solidification: By setting process subsidies in thick parts of castings, the amount of metal in these parts is increased, so that the castings can achieve sequential solidification from thin walls to thick walls and from parts far from the riser to parts close to the riser during solidification, which is conducive to the shrinkage compensation of the casting by the molten metal in the riser, thereby reducing the occurrence of defects such as shrinkage cavities and shrinkage.
Improve filling conditions: In some castings with complex shapes, process subsidies can improve the filling conditions of the molten metal and avoid defects such as incomplete filling and cold shut. For example, adding subsidies to thin-walled parts of castings or corners that are difficult to fill can make it easier for the molten metal to flow to these parts and ensure the integrity of the casting.
Improve process yield: Reasonable process subsidy design can make the solidification of castings more reasonable, reduce the scrap rate caused by defects, and also reduce the size and weight of the riser relatively, thereby improving the process yield and reducing production costs.
Facilitate mold manufacturing and casting demolding: In some cases, process subsidies can make the structure of castings simpler, facilitate mold manufacturing and casting demolding. For example, adding subsidies to certain parts of the casting avoids overly complex shapes, makes the parting surface of the mold simpler, and makes demolding easier.
2. Does the process subsidy part of the casting need to be removed during machining?
The process subsidy part added to the casting is usually machined off during machining. The reasons are as follows:
Meet dimensional accuracy requirements: The process subsidy is set to ensure the smooth progress of the casting process and the quality of the casting, and is not the actual part required by the part. Parts have strict dimensional accuracy and tolerance requirements when they are designed. Only by machining the process subsidy can the size of the casting meet the design standards and meet the assembly and use requirements.
Ensure surface quality: The surface quality of the process subsidy part is often not as good as the main part of the casting, and there may be defects such as sand holes and pores. In order to obtain good surface quality, it is necessary to remove it through machining to ensure that the flatness, roughness and other indicators of the part surface meet the requirements.
Meet the functional requirements of the part: The function of the part is usually achieved based on its precise design size and shape. The process subsidy may affect the functional characteristics of the part such as the matching accuracy, sealing performance, and motion accuracy. Machining it can ensure that the part can perform its function normally.
3. Methods for removing process subsidies on castings
Mechanical processing
Turning: For rotating castings, such as cylindrical or disc-shaped castings with process subsidies, lathes can be used for turning. The outer circle or end face of the rotating casting is cut by the turning tool, the cutting amount is accurately controlled, and the process subsidy is gradually removed to obtain the required size and surface accuracy.
Milling: Suitable for castings with various complex shapes. The milling cutter of the milling machine can be used to perform plane milling, contour milling and other operations on the process subsidies on the casting. For example, for flat castings with irregular shapes, milling can be used to remove process subsidies to ensure the flatness and dimensional accuracy of the casting surface.
Grinding: When the surface quality and dimensional accuracy of the casting are required to be high, grinding is a common method. Grinding can remove the tiny residual amount left after machining, and further improve the surface finish and dimensional accuracy of the casting. For example, some mold castings with high surface roughness requirements often use grinding technology to remove process subsidies and achieve the final surface quality requirements after turning or milling.
Gas cutting and plasma cutting
Gas cutting: For thicker steel castings, gas cutting is an effective method to remove process subsidies. It uses the high temperature generated by the mixed combustion of oxygen and combustible gas to make the metal burn at high temperature and be blown away by the oxygen flow, thereby achieving cutting. However, there will be a certain heat-affected zone on the surface of the casting after gas cutting, which requires subsequent grinding and other treatments.
Plasma cutting: It is suitable for various metal castings, especially stainless steel, aluminum alloy and other materials. Plasma cutting uses a high-temperature plasma arc to melt and blow away the metal. It has a fast cutting speed, high precision, relatively smooth cutting surface, and a small heat-affected zone. However, for some castings with high precision requirements, a small amount of machining may still be required after cutting to further improve the precision.
Manual finishing
For some small castings or small process subsidies, manual finishing can be used. Use tools such as files and scrapers to manually file and scrape the process subsidies on the castings to gradually approach the required size and shape. Although this method is less efficient, it is highly flexible and can be finely trimmed for process subsidies of some special shapes or positions.
Different castings and process subsidy characteristics require the selection of appropriate removal methods, and sometimes multiple methods may need to be used in combination to achieve the best removal effect and casting quality.
