Detailed introduction of permanent mold casting process -3
Design of casting system
According to some characteristics of metal mold casting, the following points should be paid to when designing a gating system: The casting speed of metal is high, which is more than about 20% of the sand mold. Secondly, when the liquid metal is filled, the gas in the mold cavity should be removed smoothly, the flow direction should be as far as possible consistent with the direction of the liquid flow, and the gas should be smoothly squeezed to the riser or outlet, no Swirl, no impact on the wall or core, and no spatter.
The gating system of metal mould is generally divided into three types: top-pouring, bottom-pouring, and side-pouring
1) Top-pouring type, its heat distribution, is more reasonable, is advantageous to the sequence solidification, may reduce the metal liquid consumption. Still, the metal liquid flow is not stable, easy to enter the method when the casting is high, easy to impact the bottom of the molding glue, or the mold core. If used for casting aluminum alloy, generally only suitable for casting height less than 100 mm of simple pieces;
2) bottom injection type, the flow of liquid metal is more stable, which is favorable to exhaust gas, but the temperature distribution is unreasonable, which is unfavorable to the smooth solidification of the casting;
3) side injection type, both the advantages of the above two, metal liquid flow smooth, easy to slag collection, exhaust, but metal liquid consumption, gate cleaning workload
The structure of Metal Mould pouring system is similar to that of sand mould casting, but because the metal mould wall is not permeable and has strong heat conductivity, the structure of Metal Mould pouring system is required to reduce the velocity of liquid metal flow, smooth flow and reduce the erosion of mould wall. In addition to ensuring that there is sufficient time to remove the gas cavity, but also to ensure that in the filling process shall not produce a spray.
When a metal mold is used for pouring the Ferrous, because of the high cooling rate of the casting and the sharp increase of the viscosity of the liquid flow, the closed gate is usually used, and the sectional area ratio of each part is: F inside: F Transverse: F straight = 1:1.15:1.25
The riser of permanent mold casting has the same function as sand casting: feeding, slag collection, and exhaust. It is also designed in the same way as a sand riser. Because of the high cooling rate of the metal mould and the insulating coating or sand layer, the riser size of the metal mould is smaller than that of the sand mould.
Process parameters of permanent mold castings
The casting process parameters are slightly different from the sand mold casting due to the characteristics of the metal mold process. The linear shrinkage of permanent mold castings is not only related to the linear shrinkage of the alloy, but also the casting structure, the condition that the shrinkage of the castings is blocked in the permanent mold, the temperature at which the castings come out of the permanent mold, the expansion of the permanent mold after being heated and the change of its size, the value should also be considered in the test pouring process to leave room for size modification.
In order to take out the metal core and the casting, the casting slope should be appropriate in the direction of the core, and the casting slope should refer to the related manual.
The precision of permanent mold castings is generally higher than that of sand castings, so the machining allowance can be smaller, generally between 0.5 mm and 4 mm.
After determining the casting process parameters, the metal mold casting process diagram can be drawn, which is basically the same as the sand mold casting process diagram.