What should be paid attention to when designing the structure of castings?
Mar 17,2023
Due to the fact that castings cool and solidify faster in the metal mold than in the sand mold, and the metal mold has no tolerance, the casting stress generated in the metal mold casting is greater than that in the sand mold casting, and the tendency to crack is greater. It is also prone to defects such as insufficient pouring, cold shuts, and white spots (for castings). Usually:
(1) The following structural issues should be noted in preventing cracks in metal mold casting castings:
The requirements for uniform wall thickness, smooth transition and connection between walls, and appropriate rounded corners at corners should be stricter than those for sand casting castings;
B should change the vertically connected walls to inclined connections;
For structurally weak parts, ribs, bosses, etc. should be installed to strengthen them to prevent casting cracks;
D should minimize protruding parts such as bosses, ribs, and flanges that hinder the free shrinkage of the casting as much as possible;
When arranging reinforcing ribs on the casting, the impact on the shrinkage of the casting should also be considered.
(2) Attention should be paid to the following structural issues in preventing insufficient pouring and cold shuts in metal mold casting castings:
The wall thickness of casting A should be appropriate and not too thin, especially when the contour size of the casting is large;
B should avoid large horizontal planes, as they cause the metal liquid to rise slowly during casting, and the surface in contact with air is large and easy to oxidize. At the same time, due to the rapid heat dissipation of the metal mold, the metal liquid quickly loses its fluidity, which is prone to defects such as insufficient pouring, cold shuts, and slag inclusions;
The appearance of C castings should have a streamlined shape as much as possible to avoid sharp edges and sharp changes in connections, in order to facilitate the flow of molten metal.
When designing metal mold castings with complex shapes, if there are significant difficulties in the production process, the appearance of the castings should be as simple and the structure should be changed as possible without affecting the usage conditions of the castings, in order to facilitate the removal of the castings from the metal mold.
When designing the basic structural units and parameter selection of metal mold casting castings, attention should usually be paid to:
(1) Due to the fast heat dissipation of metal molds, the small wall thickness of metal mold castings should be larger than that of sand mold castings;
(2) The thickness of the inner wall and inner rib of the casting should generally be taken as 0.6~0.7 of the thickness of the connected outer wall. Otherwise, due to the slow cooling of the inner wall (rib), cracks may occur at the intersection of the inner and outer walls during casting shrinkage;
(3) In order to prevent the occurrence of white spots in gray cast iron castings, in addition to taking measures from a technical perspective, the wall thickness must not be too thin (some materials indicate that when the wall thickness is above 15mm, rounded corners must be used at the corners of metal mold castings, and for aluminum alloy and magnesium alloy metal mold castings, the casting rounded corners should not be less than 3-4 meters;
(4) Due to the non yielding nature of the metal mold and core, in order to facilitate the removal and extraction of the casting, the casting slope of the metal mold casting should be appropriately larger than that of the sand mold casting, generally 30% to 50% larger. It should be pointed out that the casting slope size is not only related to the alloy type and wall height, but also to the position of the casting surface. For castings with a tendency to detach from the metal mold surface during cooling shrinkage, a smaller slope can be designed, When the casting shrinks, the surface of the casting that tends to be pressed onto the metal mold should be given a larger slope.