Development of sand casting process drawing

The first step of casting production is to determine the casting process plan and draw the casting process diagram according to the structural characteristics, technical requirements, production batch and production conditions of the parts. Casting process drawing is a drawing that uses various process symbols and colors to draw the information needed for manufacturing model and mold directly on the part drawing. The drawing shall include: casting position, parting surface, number, shape, size and fixing method of core, machining allowance, draft angle and shrinkage, size and position of gate, riser and cold iron, etc.

Selection principle of pouring position:

The pouring position of the casting refers to the position of the casting in the mold during pouring, which has a great influence on the quality of the casting. The following principles should be considered when selecting the pouring position: put the important parts of the casting and the parts prone to defects in the most favorable position.

(1) The important working surface or main working surface of the casting should face down or to the side, because the defects (such as sand holes, air holes, slag inclusions, etc.) on the upper surface of the casting are usually more than those on the lower part, and the structure is not as dense as that on the lower part. If these planes are difficult to face down, try to position them on the side. When there are several important machining surfaces of castings, the larger surface should be facing down, and the method of increasing machining allowance should be adopted for the surface facing up to ensure the quality of castings.

Casting position plan of valve body casting. As the flange face and sealing face are the key surfaces, no surface defects are allowed, and the structure is required to be uniform and dense. Therefore, the best way is to pour these faces downward, but it is impossible for the valve body, so we have to use the scheme as shown in the figure and place them on the side.

Pouring position plan of flange. Because of the high quality requirement of flange circumference surface, casting defects are not allowed. If horizontal pouring is adopted, it is convenient to adopt two box model and close the box, but the quality of the upper peripheral surface is difficult to guarantee. If the vertical pouring scheme shown in the figure is adopted, although the workload of modeling and box closing is increased, all the peripheral surfaces of the flange are on the side, the quality is uniform, and it is easy to obtain qualified castings.

(2) The large plane of the casting should face down. This is because the high temperature liquid metal has strong thermal radiation to the upper surface of the mold cavity during the pouring process. Sometimes the sand on the upper surface of the mold cavity is arched or cracked due to the rapid thermal expansion, resulting in the sand inclusion defect on the surface of the casting. Obviously, the thicker the horizontal plane is, the more likely the sand inclusion will occur on the upper surface. For this reason, for flat casting, the large plane should be downward.

(3) In order to prevent the defects of insufficient pouring or cold insulation of large area of thin-walled part on the casting, it is necessary to place the large area of thin-walled part on the lower part of the mold or vertically and obliquely, which is particularly important for the alloy with poor fluidity. The reasonable pouring plan of the oil pan casting, at this time, the pressure of the liquid metal in the plane is much higher than that in the upper part of the plane, and the liquid metal is easy to fill the mold.

(4) The hot spot part of the casting that is easy to form shrinkage cavity shall be set at the upper part or side near the parting surface at the pouring position, so as to directly place the riser at the thickness of the casting, so as to make it solidify and supplement from bottom to top in order to prevent shrinkage cavity. If the flange casting mentioned above, it is reasonable to place the thick end on the upper part; otherwise, if the thick end is on the lower part, it is difficult to supplement.

(5) It shall be able to reduce the number of cores and facilitate the fixation and exhaust of cores.