The Key Technical Points For The Production Of Large-diameter Ductile Iron Pipe Fittings Wigh The Lost Foam Casting Process

Abstract: The production process of lost foam cast ductile iron pipe fittings has the characteristics of short production cycle, advanced technology and good surface quality (the surface roughness of its pipe fittings reaches 12.5μm).

Ductile Iron Pipes And Fittings Have Three Functions:

⑴ Achieve a small caliber and large flow rate of the pipeline;

⑵ Realize high-pressure long-distance transportation of pipelines;

⑶ Prevent pipeline explosion; export ductile iron pipe fittings require high surface quality; strict geometric dimensions; single piece hydraulic test up to 2.5MPa;

The production process of lost foam cast ductile iron pipe fittings has the characteristics of short production cycle, advanced technology and good surface quality (the surface roughness of its pipe fittings reaches 12.5μm). At the same time, the investment in lost foam casting is low, the effect is fast, and the application range is wide. Lost foam casting is a casting process that has almost no machining allowance and can be accurately formed. It is easy to achieve clean and mass production, especially the ductile iron pipe fittings are more suitable for production by the lost foam casting process due to the large variety of specifications, short supply cycle and short delivery time. Manage the overall design of the enterprise and provide a full range of services. Ductile iron pipe fittings that meet international standards and national standards are cast for production enterprises.

In this paper, through the selection of foam raw materials for large-diameter ductile iron pipe fittings, foam molding control, foam pattern assembly, foam pattern coating, pouring system design, auxiliary support design, smelting pouring and other controls in production practice to ensure product production quality meets technical requirements .

  1. The General Technological Process of Large-Diameter Ductile Iron Pipe Fittings with Lost Foam CastingProcess

(1) Make foam plastic white mold, combined pouring system, brush the surface of the vaporization mold, spray special paint and dry;

(2) Place the special sand box on the vibrating workbench, fill it with bottom sand (dry sand) to vibrate, scrape it flat, place the dried foam pattern on the bottom sand, fill it with dry sand (fill while sanding and vibrate );

(3) Cover with agricultural plastic film, put on the gate cup, connect the vacuum system to evacuate, dry sand and fasten the molding, then pour, the foam pattern is vaporized, and the metal liquid takes its place;

(4) Continue to evacuate, turn over the box after the casting is condensed, and remove the casting from the loose dry sand;

(5) Sand falling, shot blasting, cutting pouring riser;

(6) Cleaning, grinding, processing, and water pressure inspection;

(7) Anti-corrosion inside and outside;

(8) Packing and stacking.

 

  1. Selection Of Lost Foam Casting Process Parameters For Large-diameter Ductile Iron Pipe Fittings

Ductile Iron Pipe

2.1 Foam Appearance

The foam pattern material is made of ordinary EPS foam. In order to ensure the dimensional accuracy and rigidity of the foam pattern and reduce the principle of error and deformation caused by multiple splicing, the foam pattern of the large-diameter ductile iron pipe fittings is generally composed of four parts (socket part, socket part, pipe body part, flange part) , Formed with cold glue. Considering the large size of the casting, the pattern will be subjected to a large force during the coating and molding. The density of the foam pattern is 0.022g/cm3-0.024g/cm3. The pouring system is made of 0.020g/cm3 foam plastic (EPS) sheet. As shown in the figure of a large diameter ductile iron pipe foam.

2.2 Pouring System

The castings can have different placement positions in the sand box, and different casting methods such as bottom injection type, side injection type, top injection type, and stepped type can be used. These casting methods have their own characteristics. The setting of the pouring system should take into account the state of the pattern placed in the sand box. The principle is to facilitate the compaction of the sand and form a suitable method of solidification. In order to facilitate the smooth discharge of the molten metal and the pyrolysis products, to prevent the mold from collapsing Defects such as collapsed boxes, sticky sand and deformation are beneficial for filling molding sand during molding. According to the structural characteristics of castings and years of on-site production experience, we have designed several pouring system solutions. We determine the pouring form according to the specific state of the pipe in the sand box: generally stepped, middle injection or top injection. The cross-runner and the inner runner are at the inner wall of the pipe fitting or the end face of the socket (or flange); the straight runner is located at the intersection of the cross-runner.

2.3 Painting

The lost foam casting paint is applied by dip coating, which has the advantages of high production efficiency, saving paint and even coating. However, due to the low density of the foam pattern (several dozen times different from the density of the coating), and its low strength and high buoyancy during dip coating, it is only suitable for small and medium-sized pipe fittings that can be immersed or semi-immersed in the coating. The foam plastic appearance of large-diameter pipe fittings can only be applied by spraying, brushing or showering. To this end, we install a mud pump at the bottom of the paint tank, which is connected to the spray pipe, so that the paint can flow out of the pipe mouth continuously, with a spray function. When applying paint, spray the paint to all parts by turning the pattern around. The mud pump and its pipeline also play a role in circulating and agitating the paint in the tank. The thickness of the coating is controlled at 1.0-1.5mm. Because the foam pattern needs to be turned over during the application process, and it needs to be completed by multiple people, the pouring system is easy to hit or damage. Therefore, we separately paint the pattern and pouring system, assemble them after drying, and carry out necessary repairs and drying for pouring. The drying temperature of the pattern is 45℃±5℃.

2.4 Modeling

The negative pressure pumping method of the sand box is a combination of bottom pumping and side pumping. The dry sand is made of sea sand with a particle size of 20/40 mesh, and a combination of rain sand addition and flexible sand addition. This can avoid strong erosion of the model and damage the foam pattern and paint layer. The sand addition method is mainly rain shower sand addition, and the flexible sand addition is supplemented. A three-dimensional vibrating table with frequency conversion is used for modeling and packing.

2.5 Anti-deformation Of Pipe Fittings

Lost foam casting ductile iron fittings are easily deformed. The deformation is mainly caused by the processes of mold making, coating, and sand filling modeling. As a result, the geometric dimensions of the pipe fittings do not meet the standard dimensions. Large-diameter ductile iron pipe fittings are more prone to deformation. Therefore, the resin sand ring support method is adopted to solve the deformation or the combined use of the internal support ring and the external combination of supporting steel bands to prevent it.

2.6 Melting And Pouring

According to the ISO2531 standard; the material of ductile iron pipe fittings is generally selected to be QT450-10. The hot metal smelting and pre-furnace inoculation treatment processes are completely controlled according to the QT450-10 material. Considering that the casting in the lost foam casting process is slower in dry sand, we have made proper alloying of the molten iron. For lost foam casting, due to the presence of foam patterns, a certain amount of heat will be consumed during casting, so the casting temperature of lost foam casting is generally higher than that of ordinary sand casting. Pipe fittings have different specifications, different wall thicknesses, and different pouring temperatures. Because the large-diameter ductile iron pipe fittings have a larger outline size and belong to thin-walled castings, the casting temperature is required to be high. However, under the effect of negative pressure, high-temperature molten iron easily penetrates into the molding sand, causing the sand on the surface of the casting to form sand sticking defects. We determine the pouring temperature according to the specific pipe specifications; the general pouring temperature is controlled at 1420℃~1460℃. When pouring, ensure that the sprue is always in a full state. During pouring, the degree of vacuum is controlled at -0.040MPa ~ -0.045MPa, and the pouring time is controlled at 45 ~ 60 seconds. The pressure holding time after pouring is controlled at 20-25 minutes; the vacuum degree after pouring is controlled at -0.025~-0.030Mpa. Out of box time: 120~150 minutes. As shown in Figure 2 large-diameter ductile iron pipe fittings.

Ductile Iron Pipe

  1. Conclusion

The large-diameter ductile iron pipe fittings produced according to the above process have a smooth appearance, a beautiful surface, and uniform wall thickness. At the same time, the casting size meets the requirements of ISO2531 and GB/T13295. After using the lost foam casting process, the product quality has been improved, the production cost has been reduced, and it has been recognized by customers.

 

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