Sand mold casting process design is the core task of mold design. No matter how good the material, surface quality and geometric size of the die are, if there is no sand casting process suitable for the casting, it can not meet the requirements of casting quality and mass production. Therefore, before mold manufacturing, we must fully understand the importance of sand casting process, so as to avoid that the designed and manufactured molds can not be put into sand casting production, or even scrapped. For sand casting process design, we should give more research and attention from the following aspects.
1. Drawing software
At present, UG and sol are the most widely used software in sand casting process design and mold design idworks、PRO.／ E、CI. mATRO. N and CATI A et al. For sand casting, as long as the method is correct, no matter which software is used, a more reasonable process can be designed. Therefore, the application method of at least one software should be fully mastered before sand mold casting process design and mold design.
2. Selection of parting surface
In sand casting, the parting surface is the contact surface of several parts of the mold set for the needs of modeling. Because most of the castings produced by sand casting have complex shapes, whether horizontal parting surface or vertical parting surface, one plane can not completely separate all its shapes. Therefore, the parting surface of sand casting is generally curved parting. This requires that the selected parting surface can not only ensure the casting quality, but also facilitate the molding process operation. For the sand casting process using automatic molding machine, the following aspects should be considered in the selection of parting surface:
(1) as the first mock exam, the casting should be placed in the same model as much as possible, so as to avoid excessive misfit and larger burrs and burrs.
(2) The plane shall be selected as the main parting surface as far as possible. On the premise of meeting the needs of sand casting, the number of parting surfaces should be as few as possible.
(3) The selected parting surface shall be convenient for demoulding, core lowering, box closing and other operations, so that the sand core and cold iron can be placed stably, and it is convenient to check the strength of sand mold and the condition of mold cavity.
(4) The selection of parting surface shall generally avoid the casting functional surface and minimize the use of sand core and cold iron.
(5) The selected parting surface shall be conducive to the smooth discharge of gas in the cavity during the mold filling process.
(6) The selected parting surface shall not increase the grinding amount of riser neck residue and ingate residue.
(7) The selected parting surface shall not affect the subsequent machining and assembly, and try not to affect the appearance of the casting.
3. Determination of scale
When metal changes from liquid to solid at room temperature, it will experience liquid Shrinkage from liquid to solid and solidification shrinkage before cooling from solid to room temperature. Generally, before cooling to room temperature, the size of sand castings will continue to shrink with the decrease of temperature, which is what we usually call “thermal expansion and cold shrinkage”. In order to avoid that the size of the final casting is not less than the requirements of the drawing, generally, a certain scale will be added to the original casting size when designing the mold. For sand casting, the selection of scale is mainly affected by molding equipment, alloy type, sand casting shape and molding sand quality. For nodular cast iron, the scale proportion offset by the increase of mold caused by graphitization expansion should also be considered. Therefore, the scale of nodular cast iron is generally 0.3% ~ 0.7%, and that of gray iron and vermicular cast iron is generally 0.4% ~ 0.9%. For non-ferrous alloys such as cast aluminum and cast copper, the scale is generally designed to be about 1%.
4. Design of gating system
Whether it is open, closed or semi open sand casting gating system, each has its own advantages. No gating system is the best. Only the gating system is relatively more suitable for the type of sand casting produced. The purpose of a well-designed gating system is to form a stable mold filling mode in the pouring process, reduce or avoid the involvement of gas and inclusions, reduce heat and kinetic energy consumption, and finally produce qualified sand castings that meet the quality requirements. The following factors should be fully considered in the design of sand casting gating system.
(1) Try to make the distance between the liquid metal from the inner gate to each cavity similar, so as to minimize the liquid metal filling process required for the whole cavity.
(2) Liquid metal should be smoothly filled in the gating system of sand casting to avoid a large amount of turbulence in the mold cavity, so as to effectively reduce air entrainment and sand filling.
(3) The turning times of sand casting gating system should be minimized to avoid excessive oxidation of molten metal, excessive pressure loss and excessive consumption of molten metal.
(4) The riser shall be set at the thicker part of sand casting as far as possible to avoid premature solidification of feeding channel; However, it can not be set at the particularly thick part of sand casting to avoid the occurrence of coarse graphite and shrinkage defects in local sand casting iron castings.
(5) The inner gate shall not be set in the mold cavity that blocks the flow of liquid metal, nor shall the liquid metal directly rush the mold wall and sand core, otherwise the liquid metal is easy to erode, collapse or flush away from the mold wall or sand core. The inner gate should not be set on the surface of sand castings with high surface roughness and no machining. Try to use a single inner gate. If multiple inner gates are required, prevent the metal liquid entering the cavity from impacting each other, resulting in eddy current, large amount of oxidation and air entrainment. At the same time, it is also necessary to avoid the formation of hot joints and shrinkage porosity at the inner gate.
(6) The cold iron shall be located at the position where the sand casting needs the most chilling. The cold iron shall not be set near the riser, inner gate and sprue. In the sand mold casting process, the use of cold iron should also avoid cementite, surface collapse or fold and other defects caused by excessive chilling of sand mold castings. If possible, cold iron should be made into cold iron core together with sand core in the core making process, or isolation coating should be brushed on the cold iron surface.
(7) The sequential filling mode shall be designed as far as possible to avoid premature blockage of the exhaust port by liquid metal during the filling process. Generally, sufficient number and area of exhaust channels shall be set on the parting surface to enable the gas in the cavity to be discharged smoothly.
(8) The purpose of all gating systems is to meet the needs of mold filling, feeding and exhaust of sand castings. During cleaning, it is necessary to remove it from the sand casting and finally become the recycled material. Therefore, the size of the designed gating system must be as small as possible to improve the utilization of metal and facilitate easy removal from sand castings.
In the actual design of the gating system on the mold, there are still many aspects not mentioned that need to be set according to work experience. In addition, the above details cannot be taken into account at the same time. The choice should be determined after full review, simulation and demonstration according to the characteristics of liquid metal and sand castings and customer standards, so as to avoid mechanically copying and self defeating.