As an emerging technology, the advantages of 3D printing technology have been highlighted. It is almost unlimited in space during the printing process and can print parts with complex shapes. It easily solves the problem of some curved parts that are difficult to process or have high processing costs with traditional CNC machining centers.

There are a large number of small and curved oil passages inside the complex hydraulic valve body encountered during the casting process. These oil passages of different shapes require sand cores to be formed. Since the traditional core box mold has to consider mold ejection, these sand cores have to be divided into pieces. Manufacturing, and then assembling them together for pouring, requires several sets of metal core box molds to make sand cores, which requires a large investment in the early stage of sample development; in addition, when customers change the drawings during the new product development stage, modifications to the casting process are inevitable. If the development It is difficult to modify the fixed metal mold when changing the mold. If it cannot be repaired, the mold will be scrapped and the economic loss will be huge. At the same time, the development cycle of large-scale metal molds is long and it is impossible to quickly submit samples to customers. Based on the above reasons, we apply 3D printing new technology to print the sand core of the integral hydraulic multi-way valve casting, and verify the process and process parameters by pouring to provide empirical data support for subsequent large-scale mold opening, avoid scrap losses caused by large-scale modification of the mold in the later stage, and shorten the time Prototype development cycle.

1 Casting structure and sand core design

1.1 Structural analysis of plunger pump cover

The overall multi-way valve casting size is 368×334×211 mm. The internal oil passage body is divided into two layers and has 4 main valve holes. The weight of the casting is 130kg. The wall thickness of the casting is more than 100 mm. It is a large valve body among hydraulic valve bodies; Materials It is QT600-3. The internal oil passages are complex and the average wall thickness is more than 100 mm. It is a thick-walled casting. It is easy to produce shrinkage cavity casting defects inside. At the same time, the internal oil passages require a large amount of sand cores. The sand cores are made of binder resin during casting. The combustion gases are large and pore defects are easy to occur; however, the thickness of the small oil passage in the valve body is only 6 mm, and the wall thickness difference is large. Figure 1 is the overall multi-way valve casting diagram, and Figure 2 is the internal oil passage diagram.

Figure 1 Casting diagram after transformation

1.2 3D printing sand core design

Taking into account factors such as the size of the molding sand box, selection of parting surfaces, whether the sand core is firmly placed, and the position of the pouring system, the outer frame of the sand core is designed, the fine sand core is optimized, and special exhaust holes are set up.

1.2.1 The outer frame of the sand core is to reinforce the sand core and ensure easy grabbing during the transfer and dipping process of the sand core. The outer frame is designed with a wall thickness of 20mm (see Figure 3).

Figure 3 Sand core figure

1.2.2 Local optimization

When designing the sand core, we encountered a special-shaped independent oil channel sand core at the top of the sand core. Considering that when the casting is poured with molten iron, the sand core is located in the last area covered by the molten iron, and the molten iron flows into the casting mold during the entire filling process. Bake for a long time until the molten iron fills the entire cavity; in addition, the crystal form of the silica sand changes at about 575°C, causing the sand particles to expand, and the sand core is prone to fracture under the effects of expansion stress, thermal stress, own gravity and the buoyancy of the molten iron. or deformed. In response to this, we communicated with the customer to cast a Φ8 mm hole at the top, so as to add a support point on the sand core to prevent the oil passage sand core from breaking and deforming. The dark position in Figure 4 is the support sand core added to the independent oil channel sand core on the top surface.

1.2.3 Sand core internal exhaust design

Since the integral multi-way valve oil channel sand core is covered with molten iron after being poured, the resin component in the core sand burns to produce gas that needs to be discharged. Otherwise, the gas will break through the surface tension of the molten iron under pressure and invade into the molten iron and it will be difficult to escape. , it is easy to produce pore defects after solidification. Considering the wall thickness and strength of the oil channel sand core, multiple Φ4 mm vent holes are specially designed on the printed sand core to guide the oil channel sand core gas out of the casting. The circles in Figure 5 mark the sand core vents.

Figure 4 Additional supporting cores on separate oil passage top

Figure 5 Air vent on sand core

2 3D printing sand core control parameters

Since this is the first time that printed sand cores are used, based on the previous experience data of conventional coated sand cores, the sand core control parameters are proposed as follows:

(1) The tolerance level of printed sand core shall be CT7-CT8;

(2) Sand core strength requirements: tensile strength at normal temperature ≥ 2Mpa, bending resistance at normal temperature ≥ 2.5 Mpa;

(3) The amount of gas generated during pouring should be controlled at 12 to 15 mL;

(4) The printing core sand grain size adopts the 100/200 mesh recommended by the supplier;

(5) Use more mature inkjet sand printing technology.

3 3D printing sand core strength testing and dip coating

Through the above sand core design and the sand core parameters that need to be controlled, we submit the sand core digital model and control parameters to the outsourcing 3D printing service provider for customized printing. After the sand core is returned to the factory, we will clean and dip the sand core. We use water-based paint. The Baume degree is controlled between 30 and 34 Bo. The coating thickness is controlled between 0.2 and 0.25 mm. The drying temperature is 180 ℃. 1 hour. After the sand core was returned to the factory, the tensile strength at room temperature was rechecked with the printed “8” sample. The average value was 3.1 MPa and the gas generation volume was 10 mL, which met the strength standards proposed by us. Figure 6 shows the sand core printed as shown in the figure and the sand core after dip coating. Figure 7 shows the “8” sample of the sand core tensile strength at room temperature for re-examination.

Figure 6 Printed sand core and dip coated sand core

Figure 7 The sand core of “8” specimen with tensile strength at room

4 Multi-way valve casting casting verification and anatomical inspection

After the molding is completed, the casting is poured. After unpacking, grinding, and shot blasting, the casting is cut to detect the internal oil passages. The results show that the internal oil passage is complete and smooth. The inner wall of the casting is inspected by deep polishing and there are no defects such as shrinkage cavities, pores and slag holes. Figure 8 shows the casting after the shakeout is poured, Figure 9 shows the actual casting of the integral multi-way valve, Figure 10 shows the internal oil passage of the casting of the integral multi-way valve, and Figure 11 shows the depth anatomy of the casting of the integral multi-way valve.

Figure 8 Casting after pouring and sand fall

Figure 9 Integral multiway valve casting

Figure10 Oil passage in integral multiway valve

Figure 11 Anatomic diagram of integral multiplex valve

5 Conclusion

(1) By applying 3D printing technology to print a sand core and pouring it twice continuously, the development of the overall multi-way valve casting was successfully achieved.

(2) Use dissection to verify the size and surface quality of the oil passage, and perform dissection and section inspection. The oil passage size accuracy of the multi-way valve casting is stable and the surface quality is good. Internal sections verify that there are no defects such as casting holes, and the quality of the sample is qualified.

(3) Using 3D printed sand cores or sand molds is one of the ways to rapidly develop and apply complex and difficult castings. By introducing new technologies, prototypes can be developed at a lower cost, while providing empirical data support for subsequent large-scale mold openings, avoiding mold scrapping caused by large-scale modifications in the later period, and shortening the prototype development cycle.