RU2020027C1 - Method of vacuum-film moulding - Google Patents

Abstract

FIELD: metallurgy. SUBSTANCE: layer of sand which covers a model coated with a film is packed only in moulding. The remainder part one keeps unpacked. A layer (5-15 cm) of sand between model plate and level which is higher than top parts of the model is packed by vibration. In so doing sand density increases owing to decreasing pores between the sand grains. Then the mould is filled up with sand which is not packed and the mould is sealed with the film. As the result the sand layer adjacent to the model is denser, has high strength and low gas permeability. The remainder sand layer is porous and has high gas permeability. In casting the mould gases which sucking out by side walls are easily flow through the porous sand layer. The porous layer is used as an analogue of additional ribs-vacuum ducts for maintaining optimal pressure drop within the mould. EFFECT: enhanced quality of castings.

Description

 The invention relates to foundry, namely to the manufacture of molds by vacuum-film molding in frame supports with double walls, the inner surfaces of which are perforated, without additional vacuum wires blocking the flask, or with a minimum number of such vacuum wires.

 A known method of manufacturing molds by vacuum-film molding, including installing the flask on a model plate with a casting model, filling sand into the flask, vibration compaction of sand. When this method is molded in flasks without rib-vacuum lines, compacted sand in the central parts of the mold, 0.30-0.40 m away from the perforated walls, is at a reduced degree of rarefaction. The degree of rarefaction, which determines the strength of compacted sand, is further reduced during pouring and local depressurization of the mold due to the difficult filtration of gases through the gaps between the grains of sand, which reduces the pressure differential at the interface between the cavity and the mold wall. Reducing the pressure drop in places close to the geometric center of the mold, when pouring the mold, causes the sand to fall off the mold, especially in the ceiling places, which leads to the rejection of castings. Similar phenomena are observed in the places of the form, remote from the evacuated ribs by a specified distance.

 In addition, with insufficient pressure drop, "boiling" of the mold metal and the formation of surface gas shells of the casting are often encountered due to the difficult suction of gases released during the evaporation and burning of the film covering the mold cavity (especially in ceiling places).

 The aim of the invention is to improve the quality of castings.

 This goal is achieved in that only a layer of sand is sealed, covering the casting model lined with a film during molding, and the rest (between the model and the counter plate) part of the mold sand is kept uncompressed.

Thus, the sand layer from the model plate to a level higher than the upper parts of the model of not more than 5-15 mm is vibrocompressed using the optimal vibrocomposition mode, for example, as indicated in the article by V.K. Yarmolenko et al. Foundry, 1988, N 7, p. 20-22. In this case, the density of sand increases from 1490 to 1680 kg / m ³ or (1680-1490): 1490x100 = = 12.75% due to the reduction of pores between the grains of sand. Then they add sand to the level of the mold counter, leave it without vibration compaction and seal the mold counter with a film.

 At the same time, the sand layer around the model (excluding protrusions) is dense, of high strength and low gas permeability, while the rest of the sand layer remains "loose" with high gas permeability. Through the unconsolidated layer of sand, gases sucked by the side walls easily pass when filling the mold. The unconsolidated layer serves as an analogue of additional vacuum ribs - wires used to maintain the optimum pressure drop in the mold.

 The application of the proposed method improves ventilation of the mold, prevents sand from shedding from the surface of its cavity and prevents gas defects from affecting the casting of products of the evaporating film.

 The essence of the method is illustrated by the following example of its application. Cast iron billets of chill molds were cast with cast engraving of the working cavity in the flasks with the dimensions in the light of 1000x800x300 mm without ribs. Sizes of castings of bars (with cast engraving) 400x300x140 mm. A vacuum of 35-40 kPa of sand 1KO2A was used. When casting using traditional technology, local shedding of the mold was observed, which negatively affected the quality of the surface of the casting, and an increase in rarefaction caused a burnout.

 When applying the proposed method, sand 1KO2A was poured onto a model lined with a film, and with vibration acceleration of 6 ... 8g for 30 ... 50 s, it was vibro-compacted in the vibration mode, after which sand covered the model with a layer of a thickness of no more than 10 ± 5 mm. Then they added sand to the flask to the level of the counterattack, left it without vibration compaction, covered with a film, evacuated and carried out the broaching of the mold. Thus, the sand layer, spaced 10 ± 5 mm from the cavity, remained unconsolidated. The resulting castings did not show signs of shedding of the mold and were characterized by high surface quality on all sides of the workpiece. The outburst passing through an unconsolidated layer of sand had a mechanical burnout layer, but the outburst was then separated from the casting. When molding with a horizontal mold connector, and pouring with a vertical connector, the head and riser fall into the compacted sand zone and do not have a burnout.

 The proposed method allows the use of a relatively low level of rarefaction of forms, which saves the cost of evacuation. The use of a reduced level of rarefaction of a highly gas-permeable form prevents mechanical burning and allows not to apply non-stick paints, which reduces the consumption of materials and labor costs. The method can be used instead of using flasks with complex evacuating systems for VPF, repeating the configuration of casting, which makes casting more expensive.

Claims (1)

 METHOD OF VACUUM-FILM FORMING, including installing the flask on a model plate with a casting model, filling sand into the flask, vibration compaction of the sand, characterized in that, in order to improve the quality of the castings, part of the sand is first filled into the flask, vibro-compacting it, forming a layer with a thickness of 5 - 15 mm, then fill in the rest of the sand.