After nearly half a century of vigorous promotion and rapid development, the lost foam casting process technology has gradually entered into a mature stage. Its products have evolved from simple castings to produce complex-shaped castings. The types of materials are also increasing, and the weight ranges from dozens of Grams to tens of tons. One of the characteristics of the lost foam casting process is the cooperation of the collective work team. The birth of a qualified blank must be achieved through several processes and the tacit cooperation of several workers. Therefore, the casting blank defects are accumulated, enlarged, extended step by step during the entire process process, and eventually lead to scrap casting blanks.
To fundamentally solve the problem of casting blank scrap, first of all, we must carefully analyze the reasons for the deformation, slag inclusion, iron-clad sand and cold insulation defects of the flywheel shell casting, and gradually improve and optimize through the process test to make the production crafts and production process reasonable and scientific. Scientific and reasonable technology is used as the implementation standard. And the stable quality of the product and high process yield and product qualification rate can be ensured by strict implementation, careful control and supervision.
2 Deformation defects and control
2.1 Reasons
Figure 2.1 shows three specifications of the flywheel housing, its material is HT250, the main wall thickness is 6mm. The flywheel shell casting is a thin-walled shell casting with a large internal cavity and a discontinuous casting structure. It is easy to deform during the process of forming, drying, assembly, dip coating and box vibration, and its scrap rate during the trial production Up to about 35%. The deformation of the casting is mainly due to the improper operation and inadequate control of the model in the processes of forming, drying, assembling, coating and vibrating boxing. For example, the operation control of the foam model at the time of mold release, the placement form during drying and the operation of coating application must have strict operation processes and control measures. The force of the molding sand applied to the foam model in different directions during the vibration packing is inconsistent.
2.2 Control
According to the analysis, it is considered that, firstly, the pre-foaming density and particle size of the foam pattern should be strictly controlled, and high-strength and low-density shot beads are required for molding; secondly, the coating preparation process is improved to increase the coating's foam pattern Deformation resistance and adhesion; The strength of the foam pattern is improved; at the same time, support ribs are added to the upper opening of the foam pattern shell. And the reaction force of the support ribs on the foam pattern is used to balance the force of the paint and molding sand on the model during dipping and vibration So as to prevent deformation.
3 Slag inclusion defects an control
3.1 Reasons
The scrap rate of slag inclusion defects in the initial flywheel shell casting is as high as 45%. On the one hand, the source of slag inclusion defects is when the casting is cast, the model produces a large amount of solid phase and liquid phase products. Inside the casting, slag inclusion defects that disappeared from the model will be formed. The greater the density of the model, the more solid and liquid products will be produced; On the other hand, during the pouring process, the molten iron penetrates into the model and the paint is washed into the liquid metal, and the slag inclusion defect is formed after the casting is solidified. The features are all distributed in black blocks. Figure 3.1 is the slag inclusion defect of the casting, and Figure 3.2 is the carbon defect diagram of the casting.
3.2 Control
It can be achieved by adopting low-density and high-strength foam patterns, setting reasonable pouring riser systems and correctly pouring operations, increasing pouring temperature, increasing the permeability of coatings and molding sand, and improving the surface quality of foam patterns to prevent paint from penetrating into foam patterns Prevent the occurrence of slag inclusion defects. However, if the negative pressure is increased, the pouring temperature is increased, and the permeability of the coating and the sand mold will cause other defects such as iron infiltration and sand sticking, so a breakthrough must be made in the density control of the foam pattern and the design of the riser system.
3.2.1 Use foam shape with low density and high strength foam shape
The smaller the model density, the less solid and liquid products are produced. The model density is controlled by the density of the pre-beads. The main wall thickness of the flywheel shell casting is 6 mm, which is a thin-walled part. Therefore, if the density of the pre-expanded beads is too small, it will inevitably affect the surface quality of the model. At the same time, the coating is more likely to penetrate into the foam pattern, but it increases the probability of slag inclusion. Through analysis, three kinds of bead pre-density of 24 ~ 26g / L, 26 ~ 28g / L, 28 ~ 30g / L were selected to compare the model quality and slag inclusion defects. After conducting small batch tests on various densities, it was found that when the pre-expanded density is 26-28g / L, its technical indicators are relatively good. By using 26-28 g / L pre-expanded beads, its scrap rate of slag inclusion is less than 5%.
3.2.2 Use reasonable pouring system
The pouring system is the key process of lost foam casting. After preliminary calculation, the cross-sectional area of each component of the pouring system is determined, and then it is improved and optimized through the process test to make it reasonable and correct. At present, most pouring systems are made of EPS foam board by manual cutting and bonding, and there are considerable limitations in processing and use, manifested in:
(1) Using resistance wire cutting. The cutting surface is rough, the raw material is foam board. The main packaging material used whose pre-foaming density is small (18 ~ 20g / L), resulting in a large bead gap during the molding process, and easy to penetrate into the coating ;
(2) The shape of the pouring system cut from the plate is a solid cube with edges and corners. When pouring, the molten iron is washed out greatly, and the high-temperature molten iron cannot directly enter the bottom of the sprue.
In view of the above reasons, sprue molding should be improved, switched to a hollow cylindrical sprue. Designing the mold and forming directly through the molding machine makes the manufacturing of the pouring system and the foam pattern have the same process, and the surface roughness is controlled by controlling the pre-foaming density. Secondly, to optimize the shape of the sprue, ensure the temperature of the molten iron and reduce the scouring of the coating by the molten iron, a cylindrical sprue is adopted in the shape. Furthermore, according to the principle of lost foam casting, high-temperature molten iron must gasify the EPS foam during the pouring process.This process will inevitably cause the loss of molten iron temperature.Therefore, in the case of ensuring strength, the less the EPS foam is used, the better. Hence, make it hollow.
The hollow cylindrical sprue has the following advantages: the cylindrical surface area is small and the molding is made. The surface is smooth and smooth, reducing the tendency of the casting to form slag inclusions; In the initial pouring, the molten iron directly enters the bottom of the runner, and the molten iron temperature loss is less, and the pouring time is shortened.
4 Analysis and control of iron-clad sand defects
The dead corner of the casting is a major problem in casting.Whether it is wet sand casting or lost foam casting, sand is not easy to fill this area. In lost foam casting, sand is not easy to flow into the dead corner area during molding, and the compactness of the sand is reduced. Molding sand support coating, at high temperature, high density and high flow rate of molten iron, can easily break through the coating and produce defects such as iron infiltration. Figure 4.1 shows the defects of casting iron-clad sand.
In response to this situation, the vibration frequency is adjusted according to experience, a three-dimensional vibration table is used, and the vibration acceleration range is 1-2g. The vibration time and frequency are adjusted according to different castings. When the vibration time is short and the vibration frequency is low, the vibration effect is not ideal; In turn, the molding sand that has been vibrationd is often loosened again. After the experiment, the vibration time was controlled at 20s, the vibration frequency was controlled at 45-50Hz, and the amplitude was in the range of 1 ~ 1.5mm. The method of burying the sand is to add bottom sand and then vibrate. Put the foam shape first, and then fill the sand twice. The height of the first sand filling is flat or slightly higher than the box body. Fill the solid sand with artificial auxiliary sand burial to ensure the tightness of the sand in the dead corner. The second sand filling is covering sand. The covering sand should have enough thickness, so as to ensure enough sand consumption. The compactness of molding sand is high, the problem of iron infiltration is basically solved, and the defects of iron-coated sand are controlled.
5 Analysis and control for cold insulation defect
The cold partition is another major defect of the thin-walled casting of the flywheel housing. When casting thin-walled shell castings, the molten iron flows faster than the thick
The castings are slow. The flow time is long. The temperature decreases quickly. At the place where the molten iron finally reaches, the temperature consumption is too large, and it can not be completely merged, forming a cold partition.
According to this defect, it can be solved by increasing the molten iron temperature, pouring temperature and pouring speed. The temperature of molten iron is increased to 1560 ℃, and the pouring temperature is not lower than 1480 ℃. At the same time, the operation principle of rapid pouring was worked out during the pouring process, so that the problem of cold insulation defects was solved.
Through a large number of production tests, it has been proved that the use of supporting ribs can prevent the deformation of the foam pattern. The use of a foam model with a pre-foaming density of 26-28 g / L and the use of a molded hollow cylindrical sprue effectively can solve the slag inclusion defect. Adjusting the vibration parameters and artificially assisting in burying the sand is the way to solve the iron-clad sand defects. Improving the molten iron discharge temperature, pouring temperature and reasonable and correct pouring operation to effectively can control the cold insulation defects.
