Optimization and control of lost foam casting process
1. Control of dry sand molding process
Dry sand molding is to bury the model in the sand box and vibrate and compact it on the vibration table to ensure that the dry sand around the model is filled in place and obtains a certain degree of compactness, so that the molding sand has sufficient strength to resist the impact and pressure of the metal liquid.
The first step of dry sand molding is to add dry sand to the sand box. When adding sand, to ensure that the dry sand is filled in place, first add a certain thickness of bottom sand to the sand box and vibrate and compact it, then put in the model cluster, and then add a certain thickness of dry sand. The model cluster is buried to one-third to one-half, and then vibrate appropriately to promote the dry sand to fill the inner cavity of the model. Finally, fill the sand box and vibrate it. The vibration time should not be too long to ensure that the model is not damaged and deformed, and at the same time ensure that the coating layer does not fall off and crack.
The vibration parameters should be selected according to the structure of the casting and the form of the model cluster. For most castings, vertical unidirectional vibration should generally be used. For castings with more complex structures, unidirectional horizontal vibration or two-dimensional and three-dimensional vibration can be considered. The magnitude of vibration intensity has a great influence on dry sand molding, and vibration intensity is expressed by vibration acceleration. For castings and model clusters of general complexity, the vibration acceleration is between 10 and 20 m/s2. The amplitude is an important vibration parameter that affects the model to maintain a certain stiffness. The amplitude of lost foam casting is generally 0.5 to 1 mm. The selection of vibration time is relatively subtle and should be selected in combination with the structure of the casting and the model cluster. But in general, the vibration time should be controlled at about 1 to 5 minutes. At the same time, the vibration time when the bottom sand and the model cluster are half buried should be as short as possible, and 1 to 2 minutes can be selected. The vibration time after the model cluster is fully buried is generally controlled at 2 to 3 minutes.
2. Control of casting process
The lost foam casting process includes the design of the pouring and riser system, pouring temperature control, pouring operation control, negative pressure control, etc.
The pouring system plays a very important role in the lost foam casting process and is a key to the success or failure of casting production. When designing the pouring system, the particularity of this process should be taken into account. Due to the existence of the model cluster, the behavior of the molten metal after pouring is very different from that of sand casting. Therefore, the design of the gating system must be different from sand casting. When designing the cross-sectional dimensions of each part of the gating system, the resistance caused by the presence of the model when the lost foam casting metal liquid is poured should be taken into account, and the minimum flow resistance area should be slightly larger than that of sand casting.
Due to the wide variety and different shapes of castings, the specific production process of each casting has its own characteristics and varies greatly. These factors directly affect the accuracy of the gating system design results. For this reason, the castings can be classified in some way. For small and medium-sized castings, they can be classified according to the characteristics of the casting production process. The combination of model clusters can basically reflect the characteristics of the castings and the shrinkage compensation form of the castings. The cross-sectional dimensions of each part of the gating system are related to the size of the casting, the combination of model clusters, and the number of pieces per box. For this reason, when designing the process of new castings, targeted calculations should be made based on the characteristics of the castings and the characteristics of the gating system of similar castings.
Because of the existence of the model, the model gasification needs to absorb heat during the pouring process, so the pouring temperature of lost foam casting should be slightly higher than that of sand casting. For different alloy materials, compared with sand casting, the pouring temperature of lost foam casting is generally controlled at 30-50℃ higher than that of sand casting. The heat of the molten metal 30-50℃ higher can meet the heat required for model gasification. If the pouring temperature is too low, the casting is prone to defects such as insufficient pouring, cold shut, and wrinkled skin. If the pouring temperature is too high, the casting is prone to defects such as sticking sand.
The most taboo in the pouring operation of lost foam casting is intermittent pouring, which is easy to cause cold shut defects in the casting, that is, the temperature of the first poured molten metal is reduced, resulting in a cold shut between the later poured molten metal. In addition, the lost foam casting pouring system mostly adopts a closed pouring system to maintain the stability of pouring. In this regard, the form of the pouring cup is closely related to whether the pouring operation is stable. During pouring, the liquid level in the pouring cup should be kept stable to make the pouring dynamic pressure head stable.
Negative pressure is a necessary measure for lost foam casting of ferrous alloys. The role of negative pressure is an important guarantee measure to increase the strength and rigidity of the sand mold, and it is also the main measure to remove the gasification products of the model. The size and holding time of negative pressure are related to the casting material, model cluster structure and coating. For coatings with good air permeability and coating thickness less than 1mm, the negative pressure size for cast iron parts is generally 0.04~0.06MPa, and the upper limit is taken for cast steel parts. For cast aluminum parts, the negative pressure size is generally controlled at 0.02~0.03MPa. The negative pressure holding time depends on the model cluster structure. If there are a large number of model clusters in each box, the negative pressure holding time can be appropriately extended. Generally, the negative pressure can be removed when the solidified crust on the surface of the casting reaches a certain thickness. For thicker coatings and poorer air permeability of the coating, the negative pressure and holding time can be appropriately increased.
