Design points and functions of casting process subsidies

1. The 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 the design points and functions:

Design key 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 and looseness, the subsidy thickness should be appropriately increased to ensure sufficient metal liquid for replenishment. For example, in cast steel parts, for parts with a wall thickness greater than 50mm, the process allowance thickness may be around 5-10mm. Shape: The shape of the subsidy should be adapted to the shape of the casting as much as possible to avoid the formation of obvious protrusions or depressions on the casting, which may affect the appearance and subsequent processing of the casting. For example, in the rim of wheel castings, the process subsidy can be designed as a concentric ring with the rim, uniformly increasing the thickness of the rim. Distribution: Reasonably distribute process subsidies based on the solidification characteristics of castings and possible defect locations. For areas where the cooling rate is slow during the solidification process, such as the inner corners and thick parts of castings, subsidies should be appropriately increased; For parts with faster cooling rates, such as thin-walled parts and outer corners of castings, subsidies can be reduced or not set. Promoting sequential solidification: By setting process subsidies in the thick and large parts of the casting to increase the amount of metal in that area, the casting can achieve sequential solidification from thin wall to thick wall, from far away from the riser to near the riser during the solidification process. This is beneficial for the metal liquid in the riser to supplement the casting, thereby reducing the occurrence of defects such as shrinkage and porosity. Improving filling conditions: In some complex shaped castings, process subsidies can improve the filling conditions of the molten metal, avoiding defects such as incomplete filling and cold shut. For example, adding subsidies in thin-walled areas or difficult to fill corners of castings can make it easier for molten metal to flow to these areas, ensuring the integrity of the castings. Improving 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 risers, thereby improving process yield and reducing production costs. Facilitating mold manufacturing and casting demolding: In some cases, process subsidies can simplify the structure of castings, making mold manufacturing and casting demolding easier. For example, adding subsidies in certain parts of the casting avoids overly complex shapes, making the parting surface of the mold simpler and demolding easier.

2. Do we need to remove the process subsidy for adding labor to castings during machining? The additional process subsidy for castings is usually removed during mechanical processing. The reasons are as follows:

Satisfy dimensional accuracy requirements: Process subsidies are set to ensure the smooth progress of the casting process and the quality of the castings, and are not part of the actual requirements of the parts. Parts have strict requirements for dimensional accuracy and tolerance during design. Only by removing process subsidies can the dimensions of the castings meet the design standards and meet assembly and usage requirements. Ensure surface quality: The surface quality of the process subsidy part is often inferior to that of the main body 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 mechanical processing to ensure that the flatness, roughness, and other indicators of the part surface meet the requirements. Meets the functional requirements of the component: The functionality of the component is typically achieved based on its precise design dimensions and shape. Process subsidies may affect the functional characteristics such as fitting accuracy, sealing performance, and motion accuracy of parts. Processing them can ensure that the parts can function properly.

3. Methods for removing process subsidies from castings

Mechanical machining turning: For rotary castings, such as cylindrical or disc-shaped castings with process subsidies, turning can be performed using a lathe. By using a turning tool to cut the outer circle or end face of the rotating casting, the cutting amount is precisely 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 operations such as flat milling and contour milling on the process subsidies on castings. For example, for flat castings with irregular shapes, milling can be used to remove process subsidies, ensuring the flatness and dimensional accuracy of the casting surface.

Grinding: When high requirements are placed on the surface quality and dimensional accuracy of castings, grinding is a commonly used method. Grinding can remove small residual amounts after mechanical processing, further improving the surface smoothness and dimensional accuracy of castings. For example, some mold castings with high requirements for surface roughness often use grinding technology to remove process subsidies and achieve the final surface quality requirements after turning or milling.

Gas cutting and plasma cutting: For thick cast steel parts, gas cutting is an effective method to remove process subsidies. It utilizes the high temperature generated by the mixing and combustion of oxygen and combustible gases, causing the metal to burn at high temperatures and be blown away by the oxygen flow, thereby achieving cutting. But after gas cutting, there will be a certain heat affected zone on the surface of the casting, which requires subsequent polishing and other treatments.

Plasma cutting: suitable for various metal castings, especially stainless steel, aluminum alloy and other materials. Plasma cutting uses high-temperature plasma arc to melt and blow away metal, with fast cutting speed, high precision, relatively smooth cutting surface, and small heat affected zone. However, for some high-precision castings, a small amount of mechanical processing may still be required after cutting to further improve accuracy.

For some small castings or situations with small process subsidies, manual trimming can be used. Use tools such as files and scrapers to manually file and scrape the process subsidies on the castings, gradually approaching the required size and shape. Although this method has low efficiency, it has high flexibility and can be finely adjusted for some special shapes or positions. Different castings and process subsidies require the selection of appropriate removal methods, and sometimes multiple methods may need to be combined to achieve the best removal effect and casting quality.