During the operation of the sand core machine, the sand core plays a major role in forming the inner hole and cavity of the casting. Sand cores can be used to form some shape parts that hinder mold removal and are not easy to produce sand.
The working conditions of the sand core are relatively harsh, so the requirements for the sand core:
- 1) Have sufficient strength and rigidity;
- 2) Good exhaust performance;
- 3) Good compromise;
- 4) Small contraction resistance;
- 5) Good collapsibility, easy to produce sand. Sand core design includes: determining the number of sand cores, the shape and size of each sand core; the number, shape and size of core heads; core support, core bone; exhaust method; core sand type and core making method, etc.
Determination of the number of sand cores
The number of sand cores required for a casting mainly depends on the structure of the casting and the casting process plan. The principle of determining the number of sand cores is to reduce the number of sand cores as much as possible to reduce the cost of core boxes and core-making man-hours, and reduce the cost of castings.
- When the depth-to-diameter ratio (ratio of height to diameter or height to width) of the inner cavity or hole is not very large, the self-contained sand core should be used. The ratio of height to width of the self-contained sand core should not be too large, otherwise it will be easily damaged during drafting. Comes with a sand core size look-up table.
- Sand core and segmented sand core Sand core manufactured as a whole is easy to ensure the accuracy of castings, the number of tooling is small, and the strength and rigidity of the sand core are better. However, for sand cores that are too large in size and complex in shape, integral sand cores are still used, which is inconvenient to operate and should be manufactured by dividing them into two or several sand cores.
The block of the sand core was originally:
① The sand filling surface should be spacious;
② The supporting surface of the sand core should preferably be flat for easy placement and drying;
③ The surface of the box should be as consistent as possible with the parting surface.
④ Blocking should be convenient for core setting, box closing and inspection, so as to ensure the accuracy of castings.
⑤ The parts that require high dimensional accuracy should be formed with the same sand core as much as possible;
⑥ Oversized sand cores can be divided into several small sand cores in order to facilitate core making and core setting to solve the problem of insufficient lifting capacity in the workshop. Each small sand core needs to have sufficient strength and rigidity.
Core head The core head is an important part of the sand core.
The role of the core head is positioning, support and exhaust. In the case of ensuring reliable positioning, firm support, and unobstructed exhaust, the number of core heads should be as small as possible.
Vertical core There are three types of vertical core
- a) The upper and lower cores are made with accurate positioning, reliable support and smooth exhaust. This form is commonly used. Especially suitable for sand cores whose height is greater than the diameter;
- b) It is only used as the lower core head, not the upper core head, so it is convenient to close the box. Suitable for sand cores with large cross-sectional area and small height;
- c) No upper and lower core heads are made, which can reduce the height of the sand box and facilitate the adjustment of the position of the sand core. Suitable for relatively stable large sand cores. When the ratio of L to D is ≥ 5 times, an enlarged lower core head is adopted: D2=(1.5~2.0)D.
Some castings can only be used as upper cores, but not lower cores. At this time, the following measures can be taken:
- 1) To bury the core head. Take the core head (upper big and lower small) as a part of the pattern, shape it together with the pattern, and bury the core head in the sand mold. This method is suitable for small cores with little weight.
- 2) Lifting the core, using iron wire or bolts to hang the sand core on the upper box, the operation of the lifting core is troublesome, and it is only suitable for single-piece and small-batch production;
- 3) Cover the sand core and expand the core head. It is placed in the lower box, which is easy to operate, can ensure the precision of castings, and is beneficial to organize flow production.
- 4) Use core braces. The piece is large, complex in shape, and has many sand cores, so it cannot be hoisted and can only be supported by core supports.
Horizontal sand cores generally have two core heads, and the positioning is reliable. Sometimes there is only one core head (cantilever core), and the positioning is not reliable; or although there are two core heads, they may tilt or rotate. At this time, you can take:
- 1) Combined core head (also known as pick-up core head). Joint sand cores are commonly used for small elbow joints.
- 2) Increase or lengthen the core head so that the center of gravity of the sand core moves into the support surface (that is, into the sand mold).
- 3) Place the core support to increase the support point and support area of the sand core to make the sand core stable.
- 4) Adding process holes In order to stabilize the sand core and facilitate exhaust and cleaning, process holes can be added at appropriate positions to facilitate the setting of core heads. After casting, it can be plugged or welded with a screw plug.
After the core is set by the positioning mechanism of the core head, if the sand core is required not to move along the axis of the core head or to rotate around the axis of the core head. Or in order to make it difficult to mistake the position of the core, the positioning core head can be used.
Determination of core size The size of the cross-section of the core is generally determined by the hole size of the corresponding part of the casting. In order to facilitate core setting and box closing, the core should have a certain slope, and a certain gap should be left between the core and the core seat. The gap, the gap between the core head and the core seat, is usually achieved by making the core box a nominal size and enlarging the shape. Core head size includes core head height, slope, gap, core head slope and gap, which are set for the convenience of placing cores in boxes, and the core head size is generally determined by checking the table.
The checking calculation of the pressure-bearing area of the core head is general, and the size of the core head is determined according to empirical data, and no checking calculation is performed. However, for cores with a large weight or cores subject to a large buoyancy of the metal liquid, in order to ensure the quality of the casting, the size of the core head needs to be checked. checking method.
- Calculate the core weight (self weight)
- Calculate the buoyancy force on the core.
- Calculate the maximum pressure on each core head
- Calculate the pressure bearing area required for the cartridge.
- Calculate the core size according to the bearing area. The actual pressure-bearing area of the core head must be greater than the calculated value, otherwise measures to increase the pressure-bearing capacity must be taken. Such as improving the strength of the core base (place iron sheets or refractory bricks on the core base, etc.), place the core support, increase the length of the horizontal core head, add process holes, increase the core head, etc. In order to prevent the core head from crushing the core seat and causing the core to be eccentric and the casting to be scrapped.
Core support and core bone
Core support The core support is used to fix the sand core in the sand mold mainly by the core head. When the core head cannot be installed, or the core head alone is not sure, the core support must be used to play the role of auxiliary support. Requirements and principles of use:
- The surface of the core support is best galvanized and lead-plated. This is to prevent the surface of the core support from rusting and cannot be reconciled with molten iron. The surface of the core support should be clean and smooth. When in use, the surface of the core support should be free of rust, oil, moisture, and dirt; at the same time, after the core support is placed in the mold, it should be poured as soon as possible, especially the wet type, so as to avoid the condensation of water vapor on the surface of the core support and produce pores or Poor fusion.
- The melting point of the core support material should be higher than the melting point of the casting material, at least the same, to prevent premature melting and loss of support. (Use mild steel or cast iron core supports for iron castings; use the same alloy as the core support for non-ferrous alloy castings).
- The weight of the core support should be appropriate, not too large or too small. If it is too small, it is easy to melt, and if it is too large, it cannot be welded well with the casting.
- Core supports cannot be used on important surfaces, and should be set on non-processed surfaces or unimportant surfaces. 5. The core support should be well welded to the casting during the solidification process of the casting. If the welding is not good, it will cause bleeding and cannot withstand pressure. Thin-walled castings with a wall thickness of less than 8 mm do not use core supports as much as possible.
Core bone The skeleton of the sand core is referred to as core bone. The function of the core bone is to increase the strength and rigidity of the sand core. To ensure that the sand core is not deformed or broken during drying, transportation, core setting and pouring, especially for large cores, cores with complex shapes, and cores with thin cross-sections, the core bone is particularly important. Core bone has linear shape, surface shape and body shape.
The requirements for the core bone are:
- The core bone should be as simple as possible and easy to manufacture under the premise of ensuring that the sand core has sufficient strength and rigidity.
- Iron wire mandrels are often used for small sand cores (annealed in advance to eliminate elasticity), and cast iron mandrels are commonly used for medium and large sand cores.
- The bone should not hinder the venting of the sand core and the shrinkage of the casting. Therefore, an appropriate distance must be left between the core bone and the surface of the sand core, and the amount of sand eaten must be sufficient.
- When cleaning the sand, it is best to take out the core bone completely for reuse and reduce costs.
- Some mandrels need to be convenient for the lifting, merging and fixing of sand cores, so structures such as lifting rings should be made.