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The Key Technical of the Casting Process for the Production of Large-Diameter Ductile Iron Pipe Fittings!

Sep. 02, 2020

Ductile iron pipes and fittings have three functions:

⑴ Realize small diameter and large flow for pipeline;

⑵ Realize high-pressure long-distance transportation to 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 casting ductile iron pipe fittings has the characteristics of short production cycle, advanced technology, and good surface quality (the surface roughness of the pipe fittings reaches 12.5μm). At the same time, lost foam casting has the characteristics of low investment, quick effect and wide application range. Lost foam casting is a casting process that has almost no machining allowance and can be accurately formed. It is easy to realize clean and mass production, especially for ductile iron pipe fittings because of its wide variety of specifications, short supply cycle, and short delivery time. It is more suitable for the production of lost foam casting process. Our company has successfully cast for four companies in Hebei, Shandong and Shanxi. Manage the overall design of the enterprise and provide all-round services. Casting ductile iron pipe fittings conforming to international and national standards for production enterprises. In this paper, the selection of foam raw materials for large-caliber ductile iron pipe fittings, foam molding control, foam pattern assembly, foam pattern coating, pouring system design, auxiliary support design, smelting pouring and other controls ensure that the product quality meets technical requirements in production practice .

1. The general process flow of lost foam casting of large diameter ductile iron pipe fittings

(1) Make foam white molds, combine pouring systems, brush and spray special coatings on the surface of gasification molds and dry them;

(2) Place the special sand box on the vibrating table, fill in the bottom sand (dry sand) to shake and level, put the dried foam pattern on the bottom sand, and fill it with dry sand (sand fill and tap );

(3) Cover with agricultural plastic film, put on the sprue cup, connect to the vacuum system to vacuum, dry sand is tightly formed, and then poured, the foam is vaporized, and the molten metal replaces its position;

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

(5) Falling sand, shot blasting, cutting and pouring riser;

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

(7) Anti-corrosion inside and outside;

(8) Packing and stacking.

2. Selection of lost foam casting process parameters for large-diameter ductile iron pipe fittings

2.1 Foam appearance

The foam pattern material adopts ordinary EPS foam molding. 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 large-diameter ductile iron pipe fittings generally consists of four parts (the socket part, the socket part, the pipe body part, and the flange part) , Bonded with cold glue. Taking into account the large size of the casting, the pattern will withstand greater force during coating and modeling. The density of the foam pattern is 0.022g/cm3-0.024g/cm3. The pouring system is made of 0.020g/cm3 foamed plastic (EPS) board. As shown in the figure, it looks like a large-diameter ductile iron pipe.

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2.2 Gating system

The castings can be placed in different positions in the sand box, and different casting methods such as bottom casting, side casting, top casting, and stepwise casting can be used. These casting methods have their own characteristics. The setting of the gating system should take into account the state of the pattern placed in the sand box. The principle is to facilitate the filling and compaction of the sand, forming a suitable solid method, in order to facilitate the filling of the molten metal and the smooth discharge of the pyrolysis products to prevent the mold from collapsing Defects such as collapsed box, sticky sand, deformation, etc., are conducive to filling molding sand during modeling. According to the structural characteristics of the 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, mid-filled or top-filled. The runner and the inner runner are on the inner wall of the pipe or the end face of the socket (or flange); the sprue is located at the intersection of the runner. Straight runner: horizontal runner: inner runner=1: (4-6): (1-2).

2.3 Coating

Lost foam casting paint is applied by dip coating method, which has the advantages of high production efficiency, saving paint and uniform coating. However, due to the low density of the foam pattern (a few tens of times the density of the paint), and its own low strength, it has high buoyancy during dipping, so it is only suitable for small and medium-sized pipes that can be immersed or semi-immersed in the paint. The foam plastic appearance of large-diameter pipe fittings can only be applied by spraying, brushing and curtain coating. For this reason, we install a mud pump at the bottom of the paint tank and connect it with a spray pipe, so that the paint can continuously flow out of the nozzle, with a spray function. When applying paint, spray the paint to all parts by turning over the pattern. The mud pump and its pipeline also play the role of circulating and stirring the paint in the tank. The coating thickness is controlled within 1.0-1.5mm. Because the foam pattern needs to be turned over during the application process, and many people are required to cooperate to complete it, the pouring system is easy to be knocked off or damaged. Therefore, we separate the pattern and the pouring system and apply the coating, dry them and then assemble them, 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 flask is a combination of bottom pumping and side pumping. The dry sand is made of sea sand, with a grain size of 20/40 mesh, and adopts a sand adding method that combines rain-drenching sand adding and flexible sand adding. In this way, it can avoid violent scouring of the model and damage to the foam pattern and paint layer. The sand adding method adopts the rain shower sand adding as the main method, and the flexible sand adding as a supplement. Use variable frequency three-dimensional vibrating table to shape and pack.

2.5  pipe fittings prevent deformation

Lost foam casting ductile iron pipe fittings are easy to deform. The main cause of deformation is in the process of molding, coating, and sand filling. As a result, the geometric dimensions of the pipe fittings do not meet the standard dimensions. For large diameter ductile iron pipe fittings, it is more prone to deformation. Therefore, the resin sand ring support method is used to solve the deformation or the combination of the internal support ring and the external support steel belt are used to prevent the deformation.

2.6 Melting and pouring

According to the ISO2531 standard; the material of ductile iron pipe fittings is generally selected as QT450-10, and the molten iron smelting and pre-furnace inoculation treatment process are completely controlled according to QT450-10. Taking into account the slow cooling of lost foam casting process castings in dry sand, we made proper alloying of molten iron. For lost foam casting, due to the foam pattern, a certain amount of heat will be consumed during the pouring process. Therefore, the pouring temperature of lost foam casting is generally higher than that of ordinary sand casting. Different pipe fitting specifications, different wall thicknesses, and different pouring temperatures. Due to the large-diameter ductile iron pipe fittings with larger outline dimensions, they belong to thin-walled castings and require high pouring temperature. However, under the action of negative pressure, high-temperature molten iron can easily penetrate into the molding sand and cause sticky sand on the surface of the casting to form sticky sand defects. We determine the pouring temperature according to the specific pipe fitting specifications; generally, the pouring temperature is controlled at 1420℃~1460℃. Ensure that the sprue is always full during pouring. The vacuum degree during pouring 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 within 20-25 minutes; the vacuum degree after pouring is controlled within -0.025~-0.030Mpa. Out-of-box time: 120~150 minutes. Figure 2 Large-diameter ductile iron pipe fittings.

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3. Conclusion

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

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luckykong@sjfr-machinery.cn 602589453