Process Control of the Motor Shell Lost Molding

1 Original process and parameters

1.1 original process

The casting structure is shown in fig. 1, the material is HT200, and the top injection pouring system is adopted. the gate cup size is shown in fig. 2. the sprue is 40 × 50mm,8 cross runners (cross-sectional size is 15 × 10) are evenly distributed on the circumference, the inner gate size is 2 × 10mm (width is 2mm, length is 10mm), and the closed pouring system is used. 3 layers of paint brush with a thickness of 1.3~1.5mm, intermittent drying (boiler burning for 8h during the day, then drying by waste heat), pouring temperature about 1500 ℃, pouring negative pressure 0.04~0.05MPa, pressure holding for 6min.

1.2 results and analysis

Pouring is carried out according to the original process, and the pouring results of the motor shell castings are shown in Figure 1.


1a                                    1b                                 1c


1d                           1e                         1f

Fig. 1 Casting of the motor case according to the original process

The surface defects of castings are mainly sand inclusion, slag inclusion and cold insulation. The main reasons for such defects are as follows:

(1) The slag removal treatment of molten iron is not in place and the slag blocking effect is not good. The slag and scum agent in the molten iron enter the cavity with the molten iron. This problem can be prevented by beating slag many times, correctly placing slag-blocking rock wool and covering with iron sheets.

(2) There are gaps in the connecting parts such as gate cup and inner runner or the coating strength of the bonding parts is not enough, and the coating layer is peeled off and washed with sand due to molten iron washing coating. To prevent this problem, it can be controlled by increasing the number of coating layers in the gating system and increasing the coating strength. At the same time, attention should be paid to the moisture return of the coating. After taking it out from the drying room, the box should be buried immediately and pouring box.

(3) The density of the white mold is too large and the pouring temperature is too low, which leads to the incomplete gasification of the white mold. The glass body formed under the action of molten iron is subjected to sanding treatment to form slag eyes. The density of the white mold is controlled, the pouring temperature and speed are increased as much as possible, and a slag collecting riser is added at the support part of the motor housing to make impurities such as cooled molten iron and slag float up to the slag collecting riser.

(4) Pit-like defects appear on the surface of the casting, the continuity of the white mold is not good, the filling of the white mold is not real or the advance is not good, resulting in large surface roughness of the casting and pitting.

(5) casting shot blasting treatment, the use of 0.8mm grain size steel shot than 1.2mm grain size steel shot surface quality is much better, small grain size surface is more delicate, at the same time according to the casting surface quality requirements, can choose different shot blasting speed that is to choose different speed of the motor.

(6) the pouring temperature is 1530~1560 ℃, the temperature is too high, the strength of gray iron coating may not be enough, resulting in sand washing.

2 Process improvement and process control

2.1 product model

This process improvement takes 225 model motor case as an example. The main dimensions of the product are: 343mm in diameter, 420mm in height, 5mm in wall thickness, 60 fins in total, 5mm in thickness, and 40mm in depth between fins. The product structure is shown in Figure 4. This part is exported to ABB. The surface quality is very high and no defects are allowed.


Figure 2 Motor Shell Castings Produced by Improved Process

2.1.1 Foam Bead Advance

Jiachang B- 107 copolymer is used, because the casting wall thickness is thin and the cooling fins are dense, the advance density is controlled at 24~25g/L in order to control the variation of white mold. The advance process pays attention to the change of main pipe pressure and in-cylinder temperature, and intermittently measures the density to ensure the uniformity of bead density and size, thus laying a good foundation for the surface quality of white mold.

Note: The main parameters are to keep the pressure of the main steam pipeline at 0.08MPa, set the pressure in the cylinder at 0.02MPa, and set the heating time at 18s.

2.1.2 White mold making

This process improvement takes the 225 model motor case as an example, using a manual mold. fig. 3 shows the mold structure with 6 charging ports. due to the mold structure, some positions will be unfilled in different feeding sequences. Fig. 4 shows the defect of false filling of the white mold. after repeated tests, the charging sequence of the charging gun is determined. at the same time, the surface quality of the white mold changes greatly due to different charging methods, and this process requires higher operation level of the operator. During the white mold making process, pay attention to check the surface quality of the white mold. Figure 5 shows that there is a needle-like defect with poor surface continuity, and Figure 6 shows that the surface fabric is raw and unsatisfied with the appearance of bead unevenness.

The reasons for this defect are:(1) the water occupying space caused by the water on the surface of the mold is not blown, and the water droplets on the surface of the mold cavity can be blown before closing the mold;(2) The beads are placed for a long time (more than two days), the internal components of the beads volatilize, and the beads cannot completely expand when steam is introduced;(3) The main heating time is short and the pressure in the cavity is low, and the beads are not completely expanded;(4) The advance density is small, the particles are large, and the voids cannot be completely filled after foaming, etc.; After adjustment, the surface details of the high-quality white mold are shown in Figure 7.


Fig. 3 Structure of Mold Structure 4 Defects of White Mold Filling Fig. 5 Needle-like Defects with Poor Surface Continuity



Figure 6 The surface fabric is raw and unsatisfied, resulting in uneven beads Figure 7 After adjustment, a high-quality white mold is made.

2.1.3 White mold drying

The setting temperature of the drying room is generally 40~50 ℃, and the humidity is controlled below 15%. According to the characteristics of high humidity in the local environment (the air humidity reaches above 80%), the humidity is above 18% when the temperature is lower than 50 ℃, and the temperature is adjusted to 53 ℃, and the humidity can be controlled at 13%. The beaten white mold is put into the drying room as soon as possible for drying and shaping. The drying process is intermittently weighed. The drying is performed after the weight does not change. The drying time of the white mold is required to be not less than 3 days.

2.1.4 Group type

The top injection pouring system is adopted. The pouring system expands the size of the inner gate to 5 × 10 on the original basis. The group process is to control the circular ellipticity of the end face. Under the premise of using orthopedic tooling, the wood strips are bonded and the triangular bracket is bonded at the bottom as support, as shown in Figure 8.

Fig. 8 Group Type

2.1.5 Paint brushing and drying

The Baomei degree of the first coating is 1.6, the drying time is not less than 10 hours, the Baomei degree of the second coating is 1.7~1.8, the drying time is not less than 24 hours, and the total thickness of the two coatings is 1.0~1.2mm. During the whole coating operation process, the model is kept upright. During rotation, the stress point is the sprue and the end point of the triangular support frame at the bottom. The temperature of the drying room is set to 55 ℃ and the humidity is 15%. During the drying process, the weight of the yellow mold is measured intermittently. If the weight does not change for an interval of not less than 3 hours, it means drying, after repair, the placing time shall not be less than 8 hours before pouring (this parameter will vary according to different regions). The model to be poured should be placed separately and never in a drying room with the freshly brushed model to prevent moisture absorption.

2.1.6 Modeling of buried box

The dried yellow mold is stored in the drying room for continuous drying before the box is buried and taken with the box.

The bottom sand is 150mm thick, scraping and vibrating, placing a yellow mold, adding sand to the position of the gate cup, vibrating, covering the film and covering the sand, and treating the gate cup to ensure that sand does not splash into the gating system during the pouring process, as shown in Figure 9.

Fig. 9 No sand in the sealing pouring process of the pouring cup

2.1.7 Pouring

The pouring temperature is 1510 ℃, the negative pressure is 0.05MPa, the pressure is kept for 12min, the single weight of the casting is 75kg, the pouring process is continuously and rapidly completed, and the time is controlled within 10s. Prepare the pouring bag before discharging the furnace. The surface of the pouring bag shall not be attached with impurities such as slag, glaze and so on. There shall be no loose sand at the pouring bag opening and the bag shall be dense. After discharging the furnace, the slag shall be beaten. The rock wool shall be covered at the pouring bag opening and the iron plate shall be pressed to prevent the slag from entering the cavity.

3 Result analysis and process improvement

After the casting is shot blasted, as shown in fig. 10, the cooling fin of the casting has serious sand adhesion but can fall off, and there is no iron-clad sand. as shown in fig. 10, the regional surface slag pores in the middle and lower parts of the casting are shown.


Fig. 10 Slag porosity defects on the middle and lower regional surfaces of castings

First Process Analysis and Improvement of 3.1

The reason for sand sticking is that the coating is thin and the refractoriness is not enough. The occurrence of regional slag porosity is due to the low pouring temperature and the insufficient temperature of thin iron of castings to gasify the white mold. Process improvement plan: the coating between the heat sinks is brushed three times, and the pouring temperature is increased to 1530 ℃. After cleaning, the casting fin pieces are still sticky sand, lighter than before, and there are still slag porosity defects in the middle and lower parts of the casting, as shown in Figure 11.

Analysis and Improvement of the Second Process of 3.2

The sand sticking problem and casting slag porosity are better than before, but the pouring temperature and speed are within the required range. Please consult Zhao Gong. After his analysis and guidance, it is found that the sand sticking positions are evenly distributed on both sides of symmetry, that is, the possibility of coating problems is relatively low, which is probably caused by insufficient compactness of the molding sand, and the corresponding seismic platform direction of the sand sticking position is X direction when the box is buried; slag pores exist regionally and regularly, and there are no such defects in other positions. Combined with the box burying process, the yellow mold tilts during the vibration process, resulting in that the molten iron is not filled in the expected way during the pouring process, the molten iron at the higher position enters less, the temperature drops quickly, becomes pasty and solidified, and the tissue is not dense and loose.


Fig. 11 Sticking Sand in Heat Sink and Middle and Lower Slag Pores

Process improvement scheme: adjust the eccentric block of the vibrating motor in the X direction of the seismic table to make the coincidence degree to 90%; The bottom sand in the box burying process is required to be leveled with a scraper. The yellow mold cannot be tilted during the vibration process, or the sand box is padded with a gasket to reach the pouring position after tilting to keep the yellow mold as level as possible. The pouring process should be stable first and then fast. The results are shown in Figure 12 to completely solve the above problems.


Fig. 12 Process Improvement Final Sound Castings

4 Summary

For thin-walled castings of motor housings, when using the lost foam process, all links from white mold raw materials to finished castings need to be strictly controlled to ensure the implementation of the process. The main equipment such as seismic table, negative pressure system, pouring bag, etc. need to be adjusted to the best state (especially important and difficult for manufacturers who have just used the lost foam process). The process of using lost foam to produce motor shell has been relatively mature. In Fujian region, due to climate and environmental reasons, lost foam process has not been widely used. This time, high-quality castings have been successfully produced by using lost foam process on site in Fujian, which proves that all process links are strictly controlled and lost foam process can still be successfully used in this region.

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