WO1989010222A1

WO1989010222A1 - Plastic mouldings for filtering hot liquids, in particular molten metals, and process for producing them

Info

Publication number: WO1989010222A1
Application number: PCT/HU1988/000021
Authority: WO
Inventors: György GYÖRÖK; Géza Kecskeméthy; István KOVÁCSAY; Zsolt SZABÓ; Árpád VÖRÖS
Original assignee: Müanyagipari Kutató Intézet; Csepel Müvek Vas- És Acélönto^"De
Priority date: 1988-04-18
Filing date: 1988-04-18
Publication date: 1989-11-02
Also published as: EP0365531A1

Abstract

The invention concerns specially shaped plastic mouldings for filtering liquids at elevated temperatures, in particular for filtering molten metals, and a process for producing said mouldings from compositions based on furfuryl alcohol/formaldehyde resins. The mouldings have filtration apertures (4) which expand from a cylinder to form a truncated pyramid or filtration apertures in the form of truncated pyramids which expand toward the base, the ratio of the cross-sections of the truncated pyramid and the cylinder or the ratio of the areas of the base and covering surface of the truncated pyramid lying between 20:1 and 2:1. The mouldings are also provided with ribs (2) and possibly with a plastic film (3) 0.2-0.5 mm thick over the filtration apertures (4). The mouldings are produced by mixing 28-55 % by mass of a furfuryl alcohol/formaldehyde resin with 40-60 % by mass of dry matter, 4.5-15 % by mass of a crosslinking catalyst, preferably mineral or organic acids in a concentration of 10-30 % by mass, 2-5 % by mass of cut glass fibres, 1-10 % by mass of aluminium flakes, and 20-40 % by mass of other additives, preferably graphite, allowing the mixture to stand for 1-24 hours, and then shaping the mouldings preferably by compression.

Description

Plastic moldings for screening hot liquids, in particular molten metals, and process for their

Production The invention relates to specially designed molded articles made of plastic which are suitable for sieving liquids at very high temperatures, in particular for sieving molten metals. The invention further relates to a method for producing these moldings from compositions based on furfuryl alcohol-formaldehyde resins. Molten cast iron mini must be sieved before it is poured into the mold. So far, only sieves made of ceramic have been used for this purpose. These screens are 8-15 mm thick, the holes have a diameter of 3-8 mm. The ceramic sieves do not meet the requirements in all of their own properties. For example, they can break when subjected to sudden thermal and mechanical loads, and ceramic sieves with small holes are also difficult to manufacture; with sieves that have larger holes, however, the desired effect is not achieved. In addition, the liquid cast iron reacts with the ceramic and in this way destroys the sieve.

An attempt has been made to use a reticulated sieve made of heat-resistant thin, flexible filaments to sieve cast iron. Such heat-resistant threads are, for example, fibers made from graphite or from pure silicon dioxide, ie from quartz. Quartz melts at 1300-1500 ° C. However, fabrics of the type described are firstly very expensive and secondly not sufficiently rigid. Because of this lack of rigidity, they can only be used for sieving cast iron in such a way that the edges of the sieve mesh cut to a suitable size are attached to the inlet opening of the casting mold. This is a cumbersome and unsafe solution because the dynamic pressure of the flowing cast iron can tear off the attachment. Sieving the cast iron is necessary for the following reasons. Especially at the beginning of the casting with the first jet of coarse slag ticks and non-metallic inclusions in the form. 90-95% of it is sand and also gets through the ceramic sieve. - In order to avoid eddies in the mold, the filling system must always be filled with liquid metal. The metal jet has to be broken down into many elementary jets so that no swirls occur due to the suddenly increasing flow velocity (Se: 2500 - 100 000). This can only be achieved with specially shaped Si. Sieving the molten metals has numerous other advantages, for example the strength of the finished cast article increases considerably because the graphite flakes in it take on a different modification, the size of the eutectic cells decreases and the dispersity of the pearlite also improves. Cast iron's tendency to form skin and its gas content also decrease, but the metal's liquid rises.

These advantages can only be achieved with a suitably designed, rigid sieve. Technologies were also developed where screening was not required. For example, the slag-retaining sprues were attached to the casting mold. However, these processes were all characterized by a higher cost of materials and more energy. For filtering lower melting metals, for

For example aluminum, a screen filter made of glass fibers and impregnated with phenolic resin is known (British Patent No. 1,228,298). However, this net-like Si eb is not completely suitable for sieving cast iron, because neither its heat resistance nor its strength are sufficient for this application.

Zi el of the invention was to produce a screen made of plastic, which has a sufficiently small hole diameter and sufficient rigidity and withstands a temperature of 1400-1500 ° C for the duration of the screening, generally 10-50 sec. The shape of the sieve must be chosen so that an effective sieving of the cast iron, which has a very high temperature, is ensured.

The sieve according to the invention is made from a composition which consists of inorganic and organic substances which, individually and at the specified temperature, do not have the corresponding mechanical properties. In the manufacture and use of the sieve according to the invention, however, structural changes occur which make the composition suitable for use at very high temperatures.

The composition from which the sieve is made consists of the following components: furan resin made from furf uryl alcohol and formaldehyde, which can be cross-linked with acidic catalysts; Borosilicate glass fibers with a fiber diameter of 6-13 microns; Additives.

The components mentioned are not individually heat-resistant at temperatures above 1000 ° C. The heat resistance of the furan resin ranges up to 300-400 ° C, at temperatures around 600-800 ° C it chars, and the resulting solid has practically no mechanical strength at all. The borosilicate glass fibers begin to soften at around 600 ° C and lose their strength.

However, the composition made from these fabrics can withstand temperatures up to 1400-1500 ° C for the following reasons.

The strength values of the charred furan resin are greatly improved if there are reinforcing fibers in it. Fibers that can withstand the above temperatures can be produced in the system in the following manner. Acidic catalysts are used to crosslink the furan resin, and cut borosilicate glass fibers are mixed into the composition. The following surprising phenomenon takes place. In acidic medium, K ⁺ , Na ⁺ , B ⁺⁺⁺ ions gradually emerge from the surface of the glass fibers. This The process is supported by the production and application of the

The prevailing high temperature favors. As a result of the migration of the ions mentioned, the glass fiber which originally contained these ions and which had a relatively low melting point gradually produces a pure silicon dioxide fiber whose melting point is between 1400 and 1500 ° C. Another effect of the migration is that the places of the migrated ions remain empty and this creates a fiber structure that is porous at the atomic level, which ensures excellent thermal insulation. In this way, the cheap glass fiber used creates a quartz fiber with high melting point that has special heat-insulating properties. This phenomenon can be verified with a simple experiment. A glass fiber intended to be mixed into the composition is held in the flame of the Bunsen burner: it melts at 600-800 ° C. If you pull a glass fiber out of the sieve residue after sieving the cast iron, it is completely intact and shows no trace of melting. If this fiber is held in the flame of the Bunsen burner, it does not melt and does not suffer any damage.

A further discovery according to the invention is that a known natural phenomenon, the Leidenfrost phenomenon, can be used as a heat-insulating effect by introducing suitable additives into the composition. The essence of this idea is that at high temperatures the material is steam-related or, respectively Gaseous substances emerge and form a vapor cushion of molecular size between the flowing hot, liquid metal and the solid substance of the sieve. This steam cushions, it is heat-insulating and protects the sieve. In order to produce such a gas cushion, substances are mixed into the composition which change to the gas state at higher temperatures, for example at 1000-2000 ° C., and thereby of the Leidenfrost phenomenon. As an additive, flaky aluminum is preferably used in an amount of 0.2-1 masses based on the mass of the sieve. The geometrical design is also important for increased heat resistance of the sieve. This can be seen from FIGS. 1-4. The wear of the screen is not only caused by the high thermal load, but also by strong erosion effects. The flowing iron gradually removes the charred layers from the surfaces that come into contact with it. With the sieve shaped according to the invention, the cross section of the sieve openings in the flow direction of the cast iron is constantly increasing. This means practically that the openings in the sieve are, for example, conical. The rigidity of the screen surface is ensured by ribs running between the openings. The screen openings are expediently arranged in a square network, and accordingly the ribs also follow this shape. It is preferred if the sieve openings have a prism shape, for example a pyramid shape.

In this case, the free cross section (flow cross section) per unit area is greatest. This embodiment has the advantage that the lower edges of the load-bearing ribs practically do not come into contact with the flowing thin liquid jet (cast iron). The ribs formed on the screen absorb the static and dynamic pressure of the cast iron, they are subjected to tension. The progressive wear of the sieve only reaches the ribs at the very end when the sieving process has already ended. In practice, sieving generally takes 10-40 seconds.

The shape of the sieve can be found in the accompanying drawings.

1 shows an embodiment of the sieve according to the invention in cross section. Fig. 2 shows the sieve shown in Fig. 1 in the

Top view. Fig. 3 shows a further embodiment of the sieve according to the invention in section, in Fig. 4 the embodiment shown in Fig. 3 is shown in plan view.

Fig. 5 shows a sieve, which is covered by a plastic film, and

Fig. 6 shows the flow conditions when using the screen.

The sieve according to the invention is preferably produced by pressing. According to a particularly preferred embodiment, a film 3 is formed on the inlet side of the screen openings from the same material from which the screen is made (FIG. 5). The thickness 5 of the film 3 is 0.2-0.5 mm. When using the ceramic sieves conventionally used for sieving the cast iron, a thin steel disc is usually placed on the top of the sieve. This holds the melt back for a few seconds; during this time, the slag floats upwards, and the gravity pressure of the metal that accumulates enables an even flow. The plastic film formed in the solution shown in FIG. 5 replaces the mentioned steel plate in that the thin film resists the pressure of the metal for a few seconds, then tears and allows the metal to flow in.

1 shows an embodiment of the sieve according to the invention, in which the sieve openings 1 widen downward (in the direction of flow) from a cylinder to a truncated pyramid. The cross-sectional ratio between the truncated pyramid and the cylinder is between 20: 1 and 2: 1. The base circle of the cylinder preferably has a diameter 6 of 1-2 mm, and the edge length 7 of the base of the pyramid is preferably 3-5 mm. The total thickness of the screen is preferably between 5 and 20 mm. The rigidity of the sieve is due to between the sieve Openings 1 running ends 2 guaranteed.

Another embodiment of the sieve according to the invention is shown in FIG. In this embodiment, there are eb openings 4 which have the shape of a truncated pyramid. The area ratio between the base and cover plate of the truncated pyramid is between 20: 1 and 2: 1. The edge length 8 of the cover plate of the truncated pyramid is preferably 1-2 mm, the edge length 9 of the base surface is preferably 3-5 mm. Between the sieve openings 4, the stiffness-providing ribs 2 are provided. Fig. 4 shows the same sieve as seen from above. In Fig. 5 an embodiment is shown with the film 3 e.

The correct design of the screen openings is extremely important. The flow cross-section should be as large as possible, and the sieve surface should have appropriate mechanical strength until the end of the sieve. For this purpose - as shown in FIG. 6 - it is necessary that the cast iron flowing through the sieve openings 4 does not touch the ribs 2, but that a small gap r is formed between the flowing beam s and the ribs 2. As a result of the already described Leidenfrost phenomenon, a thin vapor film is formed in this gap, which protects the material of the silicon.

When designing the sieve openings, it was assumed that the viscosity of the flowing cast iron was very low, but its surface tension was very high. With this in mind, the screen openings have been designed to gradually transition from a narrow section to a widening section; in this way the hot cast iron only comes into contact with the sieve opening at the entry point, the wall of the sieve opening is separated from the metal flowing further through the gap r which widens downwards. The sieve according to the invention is produced from a composition of a special composition, preferably by pressing. It is characteristic of the process according to the invention that the sieve is produced from a composition which consists of 28-55 mass% of a furfuryl alcohol-formaldehyde resin with 40-60 mass% of dry matter content, as well as 4.5-15 masses of a crosslinking catalyst, preferably mineral acids or organic acids with 10-30% concentration, 2-25 mass% cut glass fibers, 1-10 mass% aluminum flakes and 20-45 masses of other additives, preferably graphite. The mixture obtained by mixing the listed components is allowed to stand at room temperature for 1-24 hours, and then is preferably molded into articles by pressing at 140-200 ° C under a pressure of 40-1000 N / m ² within 2-5 minutes. Due to their special material composition and design, the sieves produced in this way are suitable for withstanding high temperatures and erosion loads. As mentioned earlier, it comes in manufacturing and

Use of the sieve according to the invention for a chemical conversion of the glass fiber. This conversion process begins under the action of the acid catalyst during the 1-24 hour stand-off period, continues with the pressing carried out at elevated temperature, and is carried out when the cast iron is sieved, i.e. at 1400-1500 ° C, complete; the result is the porous glass fiber. In the composition from which the molded body is produced, the proportion of acid catalyst is relatively high. This is because the catalyst has a dual function: on the one hand it serves to treat the glass fiber, on the other hand it acts as a crosslinking catalyst.

For the preparation of the composition, one can proceed, for example, by first mixing the resin with the glass fibers in a Z kneader, then the aluminum flakes and other additives and then adds the crosslinking catalyst.

The application of the sieve is extremely simple. The screen surface is placed as horizontally as possible on the opening of the pouring channel of the sand mold. It is advisable to form a nest in the sand mold for the plate. The shape and dimensions of the sieve can be different, it can be made as a circular disc, a square or a rectangle. The most practical is a circular disc with a diameter of 30-100 mm. The thickness of the sieve depends on its area dimensions and on the amount of cast iron to be sieved. In general, 3-8 mm thick screens are made.

The invention has the following advantages: - The self-supporting sieve for the first time made of plastic for the sieving of cast iron has greater strength than the ceramic sieves and can be produced with smaller sieve holes.

- The sieves according to the invention can be manufactured more easily and economically than the ceramic sieve.

- No mechanical fastenings are required to install the sieve, which simplifies the design of the sand mold and the process of sieving.

- Sieves, the inlet openings are closed with a predetermined breaking plate, can be produced in one operation.

The preparation of the sieve according to the invention is illustrated by the following examples. Example 1 100 parts (all parts are in a Z kneader)

Parts by mass) furfuryl alcohol-formaldehyde resin. The resin contains at most 5 masses of formaldehyde, its dry matter content is 60 mass%, its viscosity at most 2000 mPas. 25 parts of glass fibers, which were cut to a length of 2-5, are mixed into the furan resin. Then 50 parts of graphite and 10 parts of aluminum flakes (particle size 0.005 mm) are added. 10 parts of 10 parts perpetual sulfuric acid are added to the viscous mixture obtained. The mixture is left to stand for one hour and then pressed into shaped bodies in an appropriate pressing tool at 150 ° C. and 10 N / mm ² . The pressing time is 5 minutes, after which the finished piece is removed from the press. The pressing tool is designed in such a way that an approximately 0.1 mm thick film remains above the holes in the sieve, in this way the sieve according to FIG. 5 is produced.

In the case of iron casting, the sieve produced in the manner described is placed on the pouring opening of the mold, and the iron, which is at 1450 ° C., is let through. About 150 kg of iron can be sieved in 40-50 sec. During this time, the Si eb remains stable, is not destroyed and retains the pieces of slag in the iron. Example 2 100 parts of the furan resin according to Example 1 are mixed in a Z-kneader with 50 parts of cut glass fibers, 50 parts of graphite and 5 parts of aluminum flakes. The mixture is homogenized and finally mixed with 10 parts of 15% by mass hydrochloric acid. The mixture is left to stand for 3 hours and then pressed in the manner described in Example 1, using a tool designed according to the sieve shown in FIG. 1.

Example 3

In a Z-kneader, 100 parts of furan resin with 50 masses of dry substance are mixed with 10 parts of cut glass fibers, 100 parts of graphite and 20 parts of aluminum flakes, and finally 20 parts of 30% phosphoric acid are added. The mixture is left to stand for 16 hours and then processed into sieves of the form shown in FIG. 2 in the manner described in Example 1. Example 4

50 parts of cut glass fibers, 150 parts of graphite and 20 parts of aluminum flakes are added to 100 parts of the resin described in Example 1. 20 parts of 20% p-toluenesulfonic acid are added as catalyst. The mixture is left to stand for 2 hours and then processed in the manner described in Example 1.

Claims

1. Plastic molded body for sieving hot liquids, in particular molten metals, characterized in that a) the molded body with (in the direction of flow) downward from a cylinder to a truncated pyramid sieve openings (l) - the cross-sectional ratio of truncated pyramid and Cylinder is between 20: 1 and 2: 1 - and between the sieve openings (1) with ribs (2) and possibly over the sieve openings (l) with a

0.2-0.5 mm thick plastic cover film (3) is provided, or b) the shaped body with truncated pyramid-shaped sieves (4) in the direction of flow which widens downwards (the area ratio between the base surface of the truncated pyramid and its cover surface) is between 20: 1 and 2: 1 and the height of the pyramid is 5-20 mm - and between the sieve openings (4) with ribs (2) and, if necessary, over the sieve openings (4) with a 0.2-0, 5 mm thick plastic cover film (3) is provided. 2. A process for the production of plastic moldings for screening hot liquids, in particular molten metals, characterized in that 28-55 masses of a furfuryl alcohol-formaldehyde resin with 40-60 mass% dry matter content, 4.5-15 mass% of a crosslinking catalyst, preferably mineral acids or organic acids with a concentration of 10-30 masses,

2-25 masses of cut glass fibers, 1-10 masses of aluminum flakes and 20-45 masses of other additives, preferably graphite, are mixed with one another and the mixture is left to stand for 1-24 hours and then, preferably by pressing, molded into shaped bodies.

3. The method according to claim 2, characterized in that the composition is pressed at 140-200 ° C under a pressure of 40-100 N / cm for a pressing period of 2-5 minutes to give moldings.