WO1993025365A1

WO1993025365A1 - Procedure in association with production of plastic pieces

Info

Publication number: WO1993025365A1
Application number: PCT/FI1993/000244
Authority: WO
Inventors: Harri Sahari; Eerik TÖRMÄ; Hannu JYLHÄ
Original assignee: Harri Sahari; Toermae Eerik; Jylhae Hannu
Priority date: 1992-06-11
Filing date: 1993-06-07
Publication date: 1993-12-23
Also published as: FI922716A; FI922716A0; AU4073493A

Abstract

The present invention relates to a method in association with production of plastic pieces, particularly plastic pieces containing interior shapes, such as cavity spaces, counterdrafts and equivalent, essentially without any connecting joints. In the method, pieces to be made from a plastic material are produced upon a core, whereby the core is made from granular forming material and from a binding agent binding the forming material grains together, said core being removed after completing the piece. For the binding agent, water-soluble inorganic salt is used, said salt, being in the form of a binding agent solution dissolved in water, is mixed with a granular forming material and the chemical properties whereof being maintained unchanged in the forming incident and during the production process of the plastic piece. The combination of the binding agent and the forming material is so selected that the main minerals of the binding agent and the forming material are chemically unreacting with each other at the forming temperature and during the production process of the plastic pieces. In the forming incident the binding agent is crystallized by heating the core to be produced from the aqueous solution thereof for producing from the binding agent a fixed bridge binding the forming material grains together between the forming material grains. After the production process of the plastic pieces, the binding agent can be dissolved in water or in a non-saturated aqueous solution of the binding agent for breaking up the core and for removing it away from inside the finished piece.

Description

Procedure in association with production of plastic pieces

The present invention relates to a procedure in association with production of plastic pieces, particularly plastic pieces containing interior shapes, such as cavity spaces, counterdrafts and equivalent, essentially without any connecting joints, in which method the pieces made of plastic material are produced upon a core, by making the core from a granular forming material and from a binding agent binding the forming grains together, said core being removed after completing the piece.

Production of plastic pieces containing inside and closed shapes, such as various containers, bottles and equivalent, has been difficult and even impossible, particularly when a finished piece is required to have very precise dimensions or when the pieces are not allowed to include joints, such as glue or welding joints, flanges or equival¬ ent. The production of inner shapes containing counterdraft has also been extremely difficult. In plastic technology, pieces like these must in general be produced from a number of parts which are later joined to make one uniform piece. The production of long straight holes in manufacturing plastic pieces has also been highly difficult. The difficulties result mainly in that no such moderate-priced dissoluble cores have not been available in plastic industry which would enable a technology resembling the casting technology in producing plastics. Attention has already been paid for a longer time to said equivalent problems in metal production, and several different alternatives for solving the problems have been set forth. As the most sophisticated of said alternatives can be considered the method disclosed in the Finnish patent No. 78,247 (equivalent US. Patent No. 5,158,130), in which dissoluble moulds and cores are used in manufacturing metal pieces. In plastic industry no procedures similar to those have not been understood to adopt.

The fact that methods used in metal production have not been understood to adopt in producing plastic pieces is mainly due to the fact that the properties of metals and plastics are so radically different. In addition to said different properties, or because of them, also the manufacturing conditions of metal and plastic pieces are essentially different. As an example, the pressures in packing cores intended for producing plastic pieces are essentially greater, frequently several multiple units greater than those used the equivalent phase in producing core moulds intended for casting metals. Therefore, the cores intended for producing plastic pieces are required to possess a tens of times of more strength compared with the cores intended for casting metal pieces. This is because the surface of the core should be made as smooth and hard as possible in order to be non-penetratable by plastic. The pressures in processing plastics are 600 to 800 bar at greatest, so that the conditions compared with metal processing are completely different. Also removing cores from plastic casts are con¬ siderably more problematic than in metal casts because such core removal techniques cannot be used with plastics which are generally known and used in casting technology. Core removal techniques like these are for instance clattering, vibration, slinger blowing, thermal treatments, etc. A problem related to using cores in pro- ducing plastics is also that the plastics in molten state tend to penetrate into the core material.

Of the prior art procedures for instance molten core technique may be mentioned, in which for the core material a metal alloy with a low melting point is used. An essential drawback of the method is, however, that the core material used is extremely expensive so that the method can be used only in producing expensive special items. The core material also sets certain prerequisites for the production line, thus raising even more the manufacturing costs. In another prior art method dissoluble plastic core is used. In that method, mineral-reinforced mixed polymer of acrylic acid and acrylic ester is used for the core material. One of the problems of the method lies in the production of the core. Also this method is quite costs- involving.

As regards the state of art, reference is made to the references US-4,590,026, EP- 0,339,183, EP-0,313,923, and EP-0,471,459. Of the references, in the US. patent

No. 4,590,026 endeavours are made to produce on the surface of a plastic piece to be manufactured a reinforcing layer of a metal or ceramic and left within the plastic piece. In the method according to said US. patent the forming mass is heated so hot that the salt is in molten state. With the salt in molten state, the forming mass is poured into a core box where it allowed to cool and crystallize. In order to be able to pour the forming mass into the core box, there should be great quantities of the binding agent therein because the mass must be in dissolved form. In the method of the US. patent, the "step time" in making the core is very long, frequently several hours.

In the method according to EP application print No. 0,339,183, the materials used, such as fillers and equivalent, deviate from the invention. The process time of the method is very long, even tens of hours. Because of the long process time the method according to EP No. 0,339,183 cannot be industrially significant in practice.

In the method according to EP application print No. 0,313,923, the core is produced from a plastic dissoluble in an alkaline solution, on top of which plastic is then cast. In the method according to EP. No. 0,471,459, particularly the production conditions are special in character, because the production requires continuous maintenance of the vacuum. The materials are different from those used in the present invention.

On the basis of the above presented facts, with the methods of the state of art, the same advantageous design cannot be achieved as in the invention. The advantages and specific features of the invention are presented below in the detailed description of the invention.

The object of the present invention is to utilize the prior art method known in the FI patent No. 78,247 (US. patent No. 5,158,130), intended for casting metals in manu¬ facturing plastic pieces and to develop a method which is, in comparison with the methods used in producing various plastic pieces, substantially simpler and more economic. The aim of the invention is furthermore to provide a method which is appropriate for use in producing various products, such as injection moulding of thermoplastics and thermosetting plastics, rolling and lamination of thermosetting plastics, and other equivalent production methods. In order to implement the above object, the invention is mainly characterized in that for the binding agent water- soluble inorganic salt is used which as a binding agent solution dissolved in water is mixed in a granular forming material and the chemical properties whereof being unchanged in the forming incident and during the production process of the plastic piece, and that the combination of the binding agent and the forming material is so selected that the main minerals of the binding agent and the forming material are chemically non-reacting with each other at the forming temperature and in the course of the production process of the plastic pieces, so that the binding agent in the for¬ ming incident is crystallized by heating the core produced from the aqueous solution thereof for producing from the binding agent a fixed bridge binding the forming mat¬ erial grains with each other between the forming material grains, and prior to producing the plastic piece, the core is coated with a coating agent non-adhering to the plastic material, whereby the binding agent and coating agent can after the production process of the plastic pieces be dissolved in water or in non-saturated aqueous solution of the binding agent for breaking up and removing the core from inside the completed piece.

In the procedure of the present invention the moulds and cores are produced by binding the forming material grains together with an inorganic water-soluble salt, the melting point thereof being above the temperatures used in the production process of the plastic piece.

With the invention such advantages are achieved regarding the state of art, one of the most remarkable whereof being that the invention enables in an advantageous manner the production of plastic pieces which include complex inward shapes and which are without any connecting joints. The rest of the advantages of the invention are equivalent to those disclosed in the FI patent No. 78,247 (US. patent No. 5, 158, 139), of which let it be mentioned in the present context the simple breaking up of the mould and cores by dissolving the binding material of the forming mass off with water or with the non-saturated aqueous solution of the binding material, and that the forming material used in making the forming and the core can easily be regenerated for reuse in a wet way. The invention is described below more in detail by introducing the various phases of the method of the invention one by one.

(a) For the binding agent in the method of the invention an inorganic compound with a high melting point is used, particularly an inorganic water-soluble salt. An essential feature of the binding agent is that the melting point thereof is so high that it will not in general melt even at casting temperatures.

(b) As regards the properties of the binding agent, they are such that the agent will not react chemically at temperatures implemented in forming and the cast with the main minerals of the forming material grains, and thus, it will not form non-water- soluble compounds.

(c) From the binding agent according to items (a) and (b) an aqueous solution is first made which is mixed with the forming material grains.

(d) Owing to the surface stress of the binding agent solution, the binding agent solution forms a liquid bridge produced by liquid gathering in the contact points of the forming material grains.

(e) The binding agent solution has a high viscosity and it is provided with a great adhesion ability with the main mineral of the forming material grains, resulting in that the forming material grains are "glued" to each other and hold the forming mass in one piece and formable, although the binding agent in itself is still in solution form.

(f) After the water used for the solvent of the binding agent solution is removed from the forming mass in the mould or core, a fixed binding agent bridge is formed in place of the liquid bridge introduced at item (d), said bridge fixing the forming material grains firmly with each other. The physical state of said "fixed" binding agent bridge is partly crystallized, partly amorphous. The above-mentioned removal of the solvent of the binding agent solution from the forming mass can be ac¬ complished e.g. by evaporation, vaporization or boiling.

(g) Since the boiling agent is provided with the properties mentioned at (a) and (b), it will not melt, decompose or burn at temperatures prevailing in the casting incident, this resulting in no gas formation in the casting process, said gases causing otherwise pressure increase in the cores and in the mould parts, and further, as a result thereof, defects in casting.

(h) Breaking up the casting is carried out by dissolving the water-soluble binding agent off with water from the joints of the forming material grains and from the sur¬ faces of the grains.

(i) The forming material grains can be used again right after being washed and dried. The drying can be carried out e.g. by means of centrifugation.

(j) The amount of the binding agent of the dissolved agent used in the method is about 0.5 to 20 per cent by weight of the total amount of the forming material. The amount of the dissolved binding agent is preferably 1 to 5 per cent by weight of the forming material.

(k) An essential and a highly significant feature of the method of the invention lies therein that the compound produced by the binding agent and the forming grain material used in the method is so selected that not even at casting temperatures they do not react chemically with each other so that there would be any non-water-soluble reaction result.

(1) For a combination of the binding agent and the forming grain material according to item (k), e.g. the following combinations can be used:

(1) For binding agent, sodium aluminate NaAlO₂ or sodium metasilicate Na₂SiO₃, the molar ratio of each one being preferably 1: 1. (2) For forming grain, quartz (SiO₂), corundum (Al₂O₃) or other mineral grain, not reacting in process temperatures with the binding agents mentioned at (1).

(3) For combination of binding agent and forming grain material, any suitable combination can be used, in which the binding agent and the forming grain material comply to the principles presented above in (a), (b) and (k). The binding agent / forming grain material com¬ binations presented in (1) and (2) are merely preferred examples of potential alternatives.

An example is presented below of how a cast mould or a core is made utilizing the method of the present invention. The example is demonstrated by Figs 1 to 8 of the accompanying drawing, showing as a picture series the different phases of core production and use of the core in making a plastic piece.

Example

A requisite forming mass is produced first by mixing forming material grains and a binding agent solution at 20 to 120°C so that the binding agent solution soils all over the surfaces of the forming material grains. After mixing the forming mass, the forming of the mould and preparation of the core are carried out, still at 20 to 120°C. If proceeding in usual manner, the forming could be accomplished as fol¬ lows:

(1) by packing manually or by forming manually

(2) forming by sand stinger

(3) by shooting with a core cannon

(4) by vibrating and/or compressing (5) by some other method known in the art,

whereby a loosely coherent mould or core could be produced. According to the invention, however, the surface of the core should, as described above, be made as hard and even as possible, in order to be non-penetratable by the plastic. Therefore, e.g. the following procedure presented in Figs. 1 to 3 is adopted in the invention. At the beginning of the production process, demonstrated in Fig. 1, a hollow core support 3 is placed in a lower mould 2, an upper mould 1 being fitted on top of the lower mould 2, and requisite amount of core mass A is carried into the mould cavity 4.

In the next phase, as demonstrated in Fig. 2, a press 5 is disposed into the mould cavity 4 upon the core mass A. With the aid of said press 5 the core mass A is pressed, or packed into mould 1,2. The press 5 has been compressed into the mould cavity 4 with a very great force so that the pressure influencing the core mass A is considerable in the packing phase.

The "fresh" mould or core produced in the above-described manner is brought into a desired processing strength by drying it partly or completely. The drying can be carried out e.g. using the following alternative procedures.

(1) The drying of the mould and core can be carried out at conventional temperature e.g. from 130 to 200°C.

(2) Crystallizing the binding agent from the water solution thereof can be carried out so that the mould and/or core are/is brought into an electrical and/or magnetic field varying in direction, whereby the heating of the forming mass is carried out by means of the increasing kinetic energy of the electrically or magnetically polarized molecules or atom groupings. This can be implemented e.g. by heating the mould or core being produced in a microwave oven, that is, in high-frequency oven, so that because of the motion of the water dipoles contained in the forming mass, the forming mass becomes internally heated and in all parts simultaneously hardened. As regards the preferred manner of drying of the present invention, reference is made to Fig. 4 in the drawing, in the situation presented in which the core B packed in the final right shape has been taken off from the production mould, that is, from the core box, and inserted into the microwave oven 6 in which the core B is hardened with microwave radii r. The microwave radii r evaporate the water w off from the core B. On the other hand, the mould or the core can also be dried in a heating apparatus, in which it is subjected to variations of direction of an inductive or capa- citative field. Also in an apparatus like this the water dipoles in the forming mass are set in motion, because of which the forming mass heats internally and is in all parts hardened simultaneously, as in the microwave oven.

(3) A mould or a core heated according to any of the methods presented above in (1) or (2) can be set, in order to remove the dissolvent of the binding agent, like water, at a desired temperature, e.g. from 50 to 150°C, into partial vacuum, whereby by making use of the thermal energy of the binding agent and the forming material the dissolvent of the binding agent, like water, is evaporated or boiled off from the mould or the core, whereby the binding agent turns into crystalline and/or amorphous state owing to the physical phenomenon, but maintaining its chemical composition as it was.

(4) The drying of the mould or the core may also be accomplished using heating of the mould or core according to either (1) or (2), and the partial vacuum according to (3) simultaneously, whereby the forming mass need not be preheated.

A fixed mould or core produced as described above must in general be coated with different coating materials, depending on the plastic material. The coating is accomplished for the reason that no plastic should enter the plastic or core material. On the other hand, coating is used for preventing the erosion caused by the running of the plastics in the core and in the mould. Furthermore, with a coating the catching of the plastic onto the mould or the core is prevented. The coating material must be such which will not dissolve the binding agent of the forming mass. Of the other features of the coating let it be mentioned that it is expected to be removable with the same procedure as the core. Furthermore, the coating should be made of a material which does not catch the plastic material itself. Other requirements set for a coating are, for instance, that it must be possible to spread a coating on a core or a mould so that it will not increase the outer dimensions of the core, neither will it reduce the inner dimensions of the mould. The coating is applied on the surface of a core or a mould e.g. by dipping, spraying, brushing, or electrostatically as a powder. Fig. 5 demonstrates the coating of the core B, carried out for reinforcing and sealing the core B. Fig. 5 illustrates coating by spraying with a spray means 7. Most preferably a binding agent containing coating solution is used for the coating. The core B is then dried. The purpose of the coating is to provide the core B with a smooth, sealed and very hard surface a, which is non-penetratable by plastic. Other differences to moulds and cores used in casting metals in the method according to the invention lie therein that producing plastics requires that a core, respectively a mould, possesses a considerably greater strength, which can be provided using greater quantities of the binding agent. The grain size of the forming mass must also in general be selected to be finer than in metal castings.

Assembling moulds and cores for the production incident of plastic pieces can be carried out with prior art methods. Production of the piece itself can be ac¬ complished, as mentioned above, e.g. by injection moulding, rolling, laminating, or other equivalent methods used in producing plastic pieces. In Fig. 6 is presented a situation in which a coated and dried core B has been inserted in an die-casting mould consisting of two mould halves 11,12 and plastic p has been sprayed into the cavity of mould 11,12. As can be seen in Fig. 6, the hollow chaplet 3 is still in place. After the casting, the plastic piece 10 and the core B, together with the chaplet 3, are removed from the die-casting mould 11,12, as shown in Fig. 7.

After completing the piece, the breaking up of the moulds and cores can be carried out simply by dissolving the binding agent of the forming material off with water because the binding agent used therein is water-soluble. Dissolving the binding agent can be carried out with e.g. water spray, steam spray, or by immersing the piece into water.

After breaking up the mould or core in the above manner, the forming material grains are separated from the water solution mixture after the wash and drying treatment for reuse. Depending on the temperature, the water / binding agent solution is appropriate for the breaking up until the binding agent content of the sol¬ ution has grown into 30 to 50 per cent by weight. The sludge produced from the coating material can be removed by filtering from the breaking up solution. The binding agent can be separated from the breaking up solution in cold state by crystal¬ lizing or evaporating the solution until dry. The binding agent solution is highly alkaline, but, however, not making the waterways euthropic, whereby it will not cause environmental detriments.

Finally, Fig. 8 demonstrates the breaking up of the core and the further processing of the core mass. In the embodiment shown in Fig. 8 binding agent solution or water has been supplied through the hollow chaplet 3 for dissolving the core away from inside the plastic piece 10. After dissolving the core and after removing the chaplet 3, the plastic piece 10 is ready. From the core mass A obtained from inside the plastic piece 10 the extra binding agent and/or water is removed e.g. as shown in Fig. 8, by draining it through a net 14, a filter or equivalent into a draining vessel 13, so that moist mass A is obtained, from which a new core can be made. From the situation presented in Fig. 8 the core mass is returned to the phase shown in Fig. 1.

The method according to the invention is described above referring to the figures of the drawing and with the aid of an example, to which, however, the invention is not intended to be exclusively confined. The invention can be modified within the scope of the inventive idea defined by the claims below.

Claims

1. A method for producing plastic pieces, particularly those provided with interior shapes, such as cavities, counterdraft and equivalent, without any essential connecting joints, in which method the pieces of plastic material are prepared upon a core, whereby the core is made from granular forming material and from a binding agent binding the forming material grains together, said core being removed after produc¬ ing the piece, characterized in that for the binding agent, a water-soluble inorganic salt is used, which as a binding agent solution dissolved in water is mixed with granular forming material and the chemical properties of which are maintained in the forming incident and during the production process of the plastic piece unchanged, and that the main minerals of the binding agent and the forming material are chemically non-reacting with each other at the forming temperature and during the production process of plastic pieces, whereby in the forming incident, the binding agent is crystallized by heating the core prepared from the aqueous solution thereof for producing a fixed bridge to bind the forming material grains together from the binding agent between the forming material grains, and prior to producing a plastic piece, the core is coated with a coating agent non-adhering to the plastic material, so that the binding agent and the coating agent are after the production process of plastic pieces soluble in water or in a non-saturated aqueous solution of the binding agent for breaking up the core and for removing it from inside the completed piece.

2. Method according to claim 1, characterized in that for binding agent, sodium aluminate or sodium metasilicate is used.

3. Method according to claim 1 or 2, characterized in that for the forming grains, quartz, corundum or equivalent mineral grains are used, said grains being non- reacting with the binding agents at the process temperatures.

4. Method according to any one of the preceding claims, characterized in that the core is heated for crystallizing the binding agent from the aqueous solution thereof so that the water is removed by evaporation.

5. Method according to any one of the preceding claims, characterized in that production of a plastic piece upon a core is accomplished using an injection moulding method known in itself in the art.

6. Method according to any one of claims 1 to 4, characterized in that production of a plastic piece is accomplished using a rolling method known in itself in the art.

7. Method according to any one of claims 1 to 4, characterized in that production of a plastic piece is accomplished using a lamination method known in itself in the art.