A CORE BOX AND METHOD OF MANUFACTURING THE SAME

Abstract: A core box for the production of sand cores which contain water-glass and are subjected to thermal curing using microwave energy, said core box having a body of plastic material and an inner layer (8) of a mouldable plastic or rubber material with a surface (9) forming the contour (6) of the cavity (5) of the core box. According to the invention the body (7) has a tensile strength of at least 55 MPa, and a bending module of at least 2.5 GPa, the body (7) thus having a mechanical strength which is sufficiently high so that the core box as to its form resists high internal and external pressures in core production. The plastic or rubber material for the inner layer (8) gives this a tear resistance of at least 10 N/mm so that the layer (8) has a tear resistance which is sufficiently high so that the surface (9) of the layer resists extensive repeated core production. Furthermore, the body (7) has a coefficient of heat expansion of at most 6.5x10<-5> mm/mm<o>C, which is sufficiently low to ensure that the core box achieves narrow tolerances at varying curing temperatures. Due to said high mechanical strength of the body (7), the core box is self-supporting and free from external support which impedes supply of microwaves in desired directions.

Description: A core box and method of manufacturing the same The present invention relates to a core box provided with a cavity of predetermined contour for the production of sand cores using water-glass as binder for casting purposes which are subjected to thermal curing using microwave energy, said core box having a pre- fabricated body of one or more plastic materials and an inner layer of a mouldable plastic or rubber material with an exposed surface forming the contour of the cavity, said layer being by means of moulding in situ di ectly or indi ectly joined to form a unit with the body, said plastic material and rubber material having sufficient transparency to microwaves. The invention also relates to a method of manufacturing such a core box. When manufacturing castings having cavities, one or more cores are used, these being placed in the casting moulds prior to casting. The cores have a contour corresponding to the internal contour, i.e. the cavity contour, of the casting, while the casting mould forms the external contour of the casting. These cores are pro¬ duced by curing a sand mixture consisting of a special sand mate¬ rial and a suitable binder, using moulds which are generally known as core boxes. These core boxes have cavities with internal sur- faces of predetermined contour which are to form the external con¬ tour of a core manufactured therein. The cores are manufactured by filling the cavity of the core box with the sand mixture, usually with the aid of compressed air. Depending on the type of curing system of the sand mixture, the subsequent curing may be carried out by means of chemical curing and/or thermal curing. In the case of thermal curing the temperature may reach about 300°C. The most usual material for the manufacture of core boxes is met¬ al, however, plastic material has also been used for manufacturing core boxes in which cores can be produced by curing at ambient temperatures. In this case, however, only short series can be pro¬ duced. Core boxes of metal have admittedly a long service life in that they have good tear resistance to the sand. However, they have a plurality of drawbacks making them less suitable and in some cases entirely unsuitable to be used. The manufacture of metal core boxes is expensive in that a careful cutting machining must be carried out and the surfaces to form the contours must then be ground and polished. This working moments are time-consuming and requires skilled personnel. There is also the risk of the binder in the sand mixture adhering to the internal surface forming the contour. The high weight of the metal core boxes makes them diffi¬ cult to handle. Furthermore, repairs or minor amendments of the form are difficult or entirely impossible to carry out. There is also a risk of part burrs being formed on the cores due to the difficulty of effecting satisfactory sealing in the part division between the parts of which a core box is usually assembled. SE 124547 describes a method of manufacturing models or core boxes for metal castings, in which a coating is effected on a sup¬ port model, using elastic, mouldable, substantially shrinkage-free and tear resistant compounds such as crude rubber and synthetic resin. The support model evidently consists of wood or metal, at least there is no mention that it should consist of a plastic ma¬ terial having specific properties. Neither, for obvious reasons, is microwave heating mentioned. Since the compounds are elastic the cores cannot be manufactured with sufficiently narrow toler- ance. DE 2 236 537 relates to a model consisting of metal and elasto er- ic material for manufacturing annul ar sand moulds with an extended l ip. Thi s technique is not suitable for core boxes, nor can it be considered for microwave heating. EP 0 060 731 descri bes a method of manufacturing cores with the purpose of effecting a so-cal led bl acking l ayer on the core in conjunction with core moulding (see Figures 9 and 10). The method requires the use of microwaves and thus a core box suited to this special core and to the microwaves. The core box consists of two parts, each having a body of a plastic material in combination with a metal frame, the plastic material being provided with a layer of silicone rubber material. The core box is thus so de¬ signed that the use of conventional moulding techniques with ac¬ companying high internal and external pressures is impossible. In¬ stead the sand mixture is poured into the core box (Figure 11) and then shaken to compact the sand (page 8, step (3)). The known method is extremely complicated and limited to a special core and the manufacture of general core goods is out of the question. Core boxes made of plastic material are also described in US 3 692 085 and US 3814624, for instance. In order to reduce adhe- sion of the sand, the walls of the cavity must be coated with a thin film of a suitable release agent such as polyethylene, poly¬ propylene or polyurethane. It is stated that the core boxes can be used for curing the sand cores with the help of microwave energy. Core boxes transparent to microwaves are also described in US 3 429 359. These consist of gypsum, polypropylene, silicone rubber or similar material, and are intended to be placed in a special chamber giving the necessary external support. From an article "Effects of Microwave Heating on Core Processing" by G.S.Cole in AFS Transactions 78-34, pages 227-236, it is clear that the micro- wave method and the use of core boxes otherwise, such as filling them with a sand mixture and curing this, place extremely high and specific demands on the core boxes themselves. However, no sugges¬ tion for core boxes which would fulfil these demands is offered. The known core boxes of plastic according to the last-mentioned paragraph are admittedly low in weight and inexpensive to manufac¬ ture since some plastics can be cast around a master so that the core box does not require much work. However, also in this case there are a number of drawbacks which make plastic core boxes not very suitable and in certain cases entirely unsuitable for use at least in conjunction with microwave heating. Thus, the known plas¬ tic core boxes cannot be used for microwave heating since they are not sufficiently transparent for the microwaves within the curing temperature range, they have insufficient mechanical strength, preventing the use of conventional core manufacturing technique with high internal and external pressures, they have insufficient tear resistance to the sand and thus a short service life, and they do not have a body with such a low coefficient of heat expan¬ sion to give narrow tolerances at varying curing temperatures. In view of all these problems entailed in the use of plastic core boxes, it has not been possible to fulfil the long-expressed de¬ sire to utilize microwave technique for the curing of sand cores containing water-glass. The object of the present invention is to eliminate the above-men¬ tioned problems and deficiencies in both the manufacture and use of the known core boxes and thus provide core boxes which in a satisfactory manner fulfil the many different demands placed upon them in each individual use. More specifically the object of the invention is to provide a core box, (a) which can be utilized for microwave heating and is thus transparent within the curing tem¬ perature range for microwaves; (b) which has high mechanical strength so that conventional core manufacturing technique using high internal and external pressures can be utilized in the same way as for a core box of metal; (c) which has high tear resistance to the sand, giving the core box long service life and thus making it suitable for extensive repeated use for manufacturing cores; (d) which has a casing with low coefficient of heat linear expan- sion so that the core box achieves narrow tolerances at varying curing temperatures; (e) which has low weight and is thus easy to handle; (f) which is easy to recondition and modify if desired; (g) which has satisfactory sealing between the parts, thus avoid¬ ing part burrs on the cores; (h) which has contour-forming sur- faces to which the sand or binder will not adhere; (i) which, due to said high mechanical strength, is self-supporting thus elimi¬ nating the need for special external supports for the body or body part and so that all sides of the core box are free for microwaves to be directed towards the core from a desired direction or a de¬ sired number of directions; (j) which can be manufactured at low costs and (k) which can be used for manufacturing all types of cores irrespective of their geometry. This object is achieved according to the invention in that the body has a tensile strength of at least 55 MPa, preferably at least 70 MPa, and a bending module of at least 2.5 GPa, the body thus having a mechanical strength which is sufficiently high so that the core box as to its form resists high internal and exter¬ nal pressures in the subsequent core production; that the plastic or rubber material used for the inner layer gives this a tear re¬ sistance of at least 10 N/mm, preferably at least 15 N/mm, and most preferably at least 20 N/mm, so that the layer has a tear re¬ sistance which is sufficiently high so that the surface of the layer resists extensive repeated core production; that the body has a coefficient of heat expansion of at most 6.5xl0~5 mm/mm°C, preferably at most 6.0xl0 -5 mm/mm°C, said coefficient of heat expansion being sufficiently low to ensure that the core box achieves narrow tolerances at varying curing temperatures occur¬ ring during thermal curing of said cores, and that due to said high mechanical strength of the body, the core box is self-sup¬ porting and thus free from external support which would otherwise impede supply of microwaves in a desired direction or directions towards the sand core. According to a preferred embodiment of the invention, the inner layer consists of a silicone rubber material moulded in situ. The invention will be described further in the following with ref¬ erence to the accompanying drawings. Figure 1 shows schematically in perspective a core box according to the invention, composed of two parts. Figure 2 shows one of the parts of the core box according to Fig¬ ure 1 with the part surface facing upwardly. Figure 3 shows a section view along the line III-III in Figure 2. Figure 4 shows a body part for a part according to Figure 2 with the part surface facing upwardly. Figure 5 is a section view along the line V-V in Figure 4. Figures 6 and 7 show schematically a further embodiment of a part of a core box according to the invention, seen from the front and side, respectively. The core box shown in Figure 1 is composed of two externally iden¬ tical parts 1 and 2, in contact with each other along their part surfaces 3 thereby forming a part joint or part division 4. The core box contains a cavity or hollow space 5 with a predetermined contour 6 which will consequently form the external contour of a core of sand produced in the core box. The core box comprises a body 7 and an inner layer 8 with an exposed or free surface 9 forming said contour 6 of the cavity 5. Since the core box con- sists of two parts 1, 2, the body 7 is formed by two body parts 7a, 7b, each having an internal layer part 8a, 8b. The layer parts together form the internal layer 8 of the core box and have ex¬ posed surface parts 9a, 9b which together form the exposed surface 9 of the core box. (The reference numbers designated by "b" have not been shown in the second part 2 since they entirely correspond to those designated by "a" in the first part 1.) According to the present invention the core box shown is manufac¬ tured from two main materials. In several respects these materials have different properties and in some respects they have similar properties so that the complete core box will have all or most of the effects and properties (a)-(k) described above, depending on the specific materials selected with respect to the particular type of sand mixture used for the cores and the equipment used for the curing process. According to the invention the main material forming the body consists of a plastic material having high me¬ chanical strength and low coefficient of heat expansion, which is important for the thermal curing process. The high mechanical - strength is required to ensure that the core box can resist- he high internal and external clamping pressures ~ usually about 0.6 MPa, prevailing in the core-shooting machine, without the core box being deformed. The low thermal expansion is necessary to maintain narrow tolerances at varying curing temperatures. The body thus gives the core box sufficient strength so that it will be self- -supporting, i.e. it requires no particular external support such as a support of metal, and so that it resists as to its form the high external and internal pressure prevailing during manufacture of cores. Such supports otherwise impede the passage of microwaves in a desired direction or directions towards the sand core. Con¬ ventional clamping means are of course used to keep the parts to¬ gether. The expression "high mechanical strength" means that the core box has a tensile strength and a bending module within the ranges spe- cified below. By the expression "low coefficient of heat expan¬ sion" is meant that this should be below 6.5xl0" 5 mm/mm°C. The other main material, forming the inner layer 8, consists ac¬ cording to the invention of a mouldable rubber material having ex¬ tremely good tear resistance and also good release properties. The mouldability means that the rubber material accurately forms ac¬ cording to the contour of the master, i.e. provides a surface 9 in exact conformity with the master both with respect to surface smoothness and dimensions. The mouldability also implies that the manufacture is simple, with resultant low labour costs. The high tear resistance implies that the wear will be minimal and the core box will therefore have a long service life. The release proper¬ ties are extremely good, tltanks to the nature of the rubber mate- rial in its solid state. By the expression "good tear resistance properties" is meant that the tear resistance is preferably above 10 N/mm. A body part 7a can be manufactured by moulding directly to the de¬ sired shape with a recess 10 for both the cavity part 5a and the layer part 8a, and optionally an intermediate layer (see below). It can also be manufactured in block form ready for machining with a cutting tool to form said recess 10. Thus, this will be an ap¬ proximate, i.e. slightly larger, replica of the corresponding part of the master for the core. The machining is performed with a few millimetres tolerance and without any demands on surface finish. Since the surface need not be smooth and the machining tolerance is large, the machining can be performed quickly, thus incurring low labour costs. A body or body part can also be constructed from semi-finished products of plastic material in the form of a plu- rality of smaller slabs or rods which are then glued together. The blank thus obtained is then machined with a cutting tool to obtain a predetermined recess therein. Before the rubber material is moulded in the body part 7a thus pre-formed, it is advisable to treat the contact surface 11, i.e. the walls of the recess 10, with an adhesive agent suitable for the internal layer part 8a, to improve the adhesion of the inter¬ nal layer part 8a to the body part 7a. The rubber material has a larger coefficient of heat expansion than the plastic material. In order to reduce any temperature ef- feet to a minimum, therefore, the internal layer 8 should be made as thin as possible. However, too thin a layer 8 will increase the machining costs since the tolerance requirements of the pre-fabri- cated body part 7a increase. However, the desired low manufactur¬ ing cost and the desired low heat expansion can be maintained with the use of a thin internal layer 8, e.g. less than about 1 mm, if an intermediate layer 12 is arranged nearest the body 7, to fill out the presumed space between the thin layer 8 and the body 7. According to a suitable embodiment of the invention, such an in¬ termediate layer 12, consists of a second mouldable rubber materi¬ al of the same basic type as the rubber material for the inner layer 8, but having a lower value of the coefficient of heat ex- pension. Such a second rubber material is also harder and has poorer tear resistance (below 5 N/mm) than the first-mentioned rubber material and cannot therefore be used for the internal lay¬ er. In order to further reduce the heat expansion and increase the hardness of the intermediate layer 12, the second rubber material for this intermediate layer may be mixed with a material in parti¬ cle or powder form which is harder and has a considerably lower value of the coefficient of heat expansion than said second rubber material. Examples of such additive material are glass beads and sand. Mixing such material into the material for the intermediate layer reduces its coefficient of heat expansion to about 40% of that of the internal layer. Since the rubber materials used for the internal and intermediate layers are of the same basic type, they join each other at the boundary surfaces during curing and form a homogenous layer without any defining limit, although this is indicated in Figure 3 merely by way of illustration. As shown in Figures 4 and 5, a groove 13 is formed in the part surface 3 around the contour or cavity part 5a. This groove pro¬ vides space for a seal 14 as illustrated in Figures 2 and 3. The seal is formed by allowing the inner layer part 8a or intermediate layer part 12a, when an intermediate layer is to be used, to fill out the groove 13 and extend outside the part surface 3 a distance of 0.2-0.4 mm, for instance, as illustrated in Figure 3. Such a seal can also be formed on the other part if desired. When the parts are assembled the cavity 5 will be satisfactorily closed thanks to the elasticity of the seal or seals. The new construction of the core box, with two laminated main parts enables minor alterations in the inner contour of the cavity of the core box and reconditioning of the inner layer when this has become worn, to be performed in a simple manner. For recondi- tioning, thus, the internal layer which has become damaged or worn normally only after repeated use is removed, possibly with the in¬ termediate layer also, and then a new layer or new such layers can be formed against the master of body part. As mentioned previously, a core box part is manufactured either by means of plastic moulding in order to obtain a pre-formed body part with predetermined recess 10 slightly larger than a master which is to form the corresponding part of the cavity of the core box, or by machining a body blank (which may have been produced by plastic moulding) so that a recess 10 is obtained having slightly larger dimensions than said master. No demands are placed on sur¬ face finish for the body part. The contact surface 11 of the re¬ cess 10 of the body is then treated with a suitable adhesive agent. The rubber material to form the inner layer is applied on the master with a brush or by immersion, or in some other suitable manner, while the rubber material to form the intermediate layer is applied on the walls of the recess 10 of the body part. If no special intermediate layer is to be built, the rubber material for the internal layer is applied (in this case normally somewhat more thickly) direct in the recess of the body part. The master, with or alternatively without the rubber material, is then pressed into the recess of the body part and secured in the correct position, and excess of rubber material will flow over and can be wiped away. The rubber material, alternatively the rubber materials is/are then subjected to a curing to produce the desired co-cured layers or alternatively the desired layer. When curing is com¬ plete, the master is removed and the core box is provided with nipples 15 in the usual manner. For most processes in which a core box is used, a first rubber a- terial having in the first hand high tear resistance can be used for the internal layer and a second rubber material having in the first place a low value of the coefficient of heat expansion can be used for the intermediate layer when such a layer is to be used, the two rubber materials being of basically the same type so that they can be joined by co-curing. A suitable rubber material for the internal layer is silicone rubber of a quality having high tear resistance. Silicone rubber is also a suitable material for the intermediate layer, but of a different quality having lower coefficient of heat expansion than that of the silicone rubber ma¬ terial used for the inner layer. The body of the core box has a tensile strenght of at least 55 MPa, preferably at east-70-MPaτ-and-a bending module of at least 2.5 GPa. The body has a low coefficient of heat expansion which is at most 6.5xl0 -5 mm/mm°C, preferably at most 6.0xl0" 5 mm/mm°C. The rubber material for the internal layer gives this a tear re¬ sistance of at least 10 N/mm, preferably at least 15 N/mm and most preferably at least 20 N/mm. When the inner layer is directly joined to the body by means of moulding in situ, the layer should preferably have a thickness of between about 1 mm and about 2 mm. When an intermediate layer is used the inner layer should be given a thickness of less than about 1 mm, whereas the intermediate layer should suitably have a thickness of less than about 5 mm. The intermediate layer has a coefficient of heat expansion as low as possible, suitably less than 17xl0~5 rπm/mm°C, and is also as hard as possible, i.e. suitably of a hardness above 50 according to Shore A. Furthermore, the plastic material and the rubber material for the different parts are chosen to enable microwaves to be used in the core production to effect thermal curing of the sand mixture con¬ taining water-glass, and the materials do not absorb any appreci¬ able amount of microwave energy, i.e. they are nearly transparent to microwaves. Suitable plastic materials for the body for such an application, taking into account other stipulations according to the invention are polypropene (up to 100°C), pol sulphone (up to 150°C) and polyether sulphone (up to 150°C). A particularly suitable silicone rubber material for the internal layer is the one marketed by the West German company, Wacker, un¬ der the designation RTV-M 540. This product is a white, syrup-like compound, i.e. relatively viscous, with a viscosity of 40.000-65.000 Pa.s. After curing, a semi-hard silicone rubber is formed having the following physical properties: A high tear re¬ sistance of 20 N/mm, a hardness of 38 according to Shore A and a coefficient of heat expansion of 20x10"-^ π_τι/mm 0 C. A silicone rubber material is also available from the same company, which is particularly suitable for building up the intermediate layer and is designated RTV-ME 428. This product is reddish brown in colour and flows easy having a viscosity of 5.000-15.000 mPa.s. After curing, a hard silicone rubber is formed having the following physical properties: A low tear resistance of 5 N/mm, a hardness of 70 according to Shore A and a coefficient of heat expansion of 15.3x10"-*-' mm/mm 0 C. The above two products are both built up of a two-component system of hardener and dimethyl polysiloxane with various additives of filler to achieve varying effects, curing being performed by condensation at ambient temperature. One and the same core box according to the invention can be used for the production of a large number of cores, at least about one thousand for instance, in a conventional core-forming machine. Figures 6 and 7 show schematically a part 42 of a core box con¬ structed in accordance with the present invention. The part has a part surface 43 provided with four semi-spherical recesses 44 ar- ranged at the corners of the part surface and connected to a non¬ return valve 45 via an inner channel system 46. The twin part (not shown) has the same semi-spherical recesses in its part surface at the corresponding places but preferably lacks said channel system and nonreturn valve. When the parts are assembled to form a core box, four spherical spaces are formed which are placed under nega¬ tive pressure by exhausting by suction the air through the non- return valve 45 which thus prevents air from returning to the spaces. An extremely strong joint is thus obtained between the parts, which is not broken until the core box arrives at a drawing station where the cured core is removed and then the negative pressure joint can be obtained again when the parts have been reassembled. Conventional clamping means around the core box can thus be entirely or for the most part omitted, thus making the core box lighter and easier to handle. The part has a cavity with an inner layer 47 of a rubber material as described earlier. A seal 48 is formed at the part surface according to the embodiments described above. Similar seals can be effected around each recess 44, which in that case are provided with similar inner layers having over-dimensions. The core boxes according to the present invention may also be pro¬ vided with evacuation openings which are in communication with the cavity and which are connected to a source of vacuum while the core box is located in a microwave heating station, in order to quickly remove the water which is heated in the core mixture, com¬ prising sand and water-glass, by the microwave energy supplied. Such core boxes have a core cavity which is closed apart from a filling opening which is so small that it normally does not permit water to escape from the uncured core mixture even during heating. Such an opening may possibly be closed or used as the only evacua¬ tion opening, or used as one of said evacuation openings.

Claims: 1. A core box provided with a cavity of predetermined contour for the production of sand cores using water-glass as binder, for casting purposes which are subjected to thermal curing using mi¬ crowave energy, said core box having a prefabricated body of one or more plastic materials and an inner layer (8) of a mouldable plastic or rubber material with an exposed surface (9) forming the contour (6) of the cavity (5), said layer (8) being by means of moulding in situ directly or indirectly joined to form a unit with --—-" the body (7), said plastic material and rubber material having sufficient transparency to microwaves, characterized in that the body (7) has a tensile strength of at least 55 MPa, preferably at least 70 MPa, and a bending module of at least 2.5 GPa, the body (7) thus having a mechanical strength which is sufficiently high so that the core box as to its form resists high internal and ex- ternal pressures in subsequent core production; that the plastic or rubber material used for the inner layer (8) gives this a tear resistance of at least 10 N/mm, preferably at least 15 N/mm, and most preferably at least 20 N/mm, so that the layer (8) has a tear resistance which is sufficiently high so that the surface (9) of the layer resists extensive repeated core production; that the body (7) has a coefficient of heat expansion of at most 6.5xl0~5 mm/mrr°C, preferably at most 6.0x10-5 mm/mm°C, said coefficient of heat expansion being sufficiently low to ensure that the core box achieves narrow tolerances at varying curing temperatures occur- ring during thermal curing of said cores, and that due to said high mechanical strength of the body (7), the core box is self- -supporting and thus free from external support which would other¬ wise impede supply of microwaves in a desired direction or direc¬ tions towards the sand core. 2. A core box according to claim 1, characterized in that the inner layer (8) consists of a silicone rubber material of a quali¬ ty giving the inner layer said properties. 3. A core box according to any of the preceding claims, charac¬ terized in that by means of moulding in situ, the inner layer (8) is directly joined into a unit with the body (7). 4. A core box according to claim 3, characterized in that the inner layer (8) has a thickness of between about 1 mm and about 2 mm. 5. A core box according to claim 2, characterized in that an intermediate layer (12) of a second mouldable silicone rubber ma¬ terial is arranged between the body (7) and the inner layer (8), said silicone rubber material is of a quality having a lower value of coefficient of heat expansion than the silicone rubber material used for the inner layer (8). 6. A core box according to claim 5, characterized in that the mouldable silicone rubber material used for the intermediate layer (12) is mixed with a material in particle form which has a consid¬ erably lower value of coefficient of heat expansion than the mouldable silicone rubber material used for the inner layer. 7. A core box according to claim 5 or 6, characterized in that the inner layer (8) has a thickness of less than about 1 mm, and that the intermediate layer (12) has a thickness of less than about 5 mm. 8. A core box according to any of the preceding claims, wherein it is composed of at least two separately prepared parts (1, 2) having part surfaces (3) laying against each other and defining a part division (4) therebetween, characterized in that in said part division at least one of the parts (1 and/or 2) has the inner lay¬ er (8) and, when occurring, the intermediate layer (12), formed with over-dimensions at said part surface (3) to form a seal (14) against the cavity (5) in said part division (4). 9. A core box according to claim 8, characterized in that at least one of the part surfaces (43) has recesses (44) which, with the opposite part surface or with corresponding recesses in this part surface of the twin part, form closed cavities, that these cavities communicate with a channel (46) in at least one part (42), said channel having an external connection and a nonreturn valve (45), the parts being arranged to be held tightly together after evacuation of air from said cavities via said channel and nonreturn valve. 10. A core box according to any of the preceding claims, charac¬ terized in that the body is made of a plastic material selected from a group consisting of polypropene for a curing temperature of up to 100°C, polysulphone for a curing temperature of up to 150°C and polyether sulphone for a curing temperature of up to 150°C. 11. A core box according to any of the preceding claims, charac¬ terized in that it is provided with one or more evacuation open¬ ings communicating with the cavity and arranged, while the core box is located in a microwave heating station, to be connected to a means for generating a negative pressure, in order to quickly remove water heated in the core mixture, consisting of water-glass and sand material, by the microwave energy supplied, the cavity of the core box being closed apart from a filling opening which is so small that it normally does not permit water to escape freely from the uncured core mixture. 12. A method of manufacturing a core box according to any of claims 1-11 having a cavity of a predetermined contour for the production of sand cores for casting purposes which are subjected to thermal curing using microwave energy, said core box having a prefabricated body (7) of one or more plastic materials and an inner layer (8) of a mouldable plastic or rubber material with an exposed surface (9) forming the contour (6) of the cavity (5), said layer (8) being by means of moulding in situ directly or in- directly joined into a unit with the body (7), said plastic mate¬ rial and rubber material being selected to have sufficient trans¬ parency to microwaves, and in which a body or at least two body parts (7a, 7b) which shall together form the body (7) of the core box are pre-formed, and then forming said contour by using a mas¬ ter to mould a mouldable first rubber material in the recess of the body or each body part (7a, 7b) thus pre-fabricated, to pro¬ duce, when the material has been cured, an inner layer or an inner layer part (8a, 8b) which together with the other layer part (8b and 8a, respectively) forms the layer of the core box, the inner layer or layer part forming a laminated unit with the body or body part, respectively, and the layer or two layer parts having an ex¬ posed surface or exposed surfaces, respectively, forming the con¬ tour of the cavity or corresponding part thereof, characterized in that the body (7) is manufactured of a plastic material which gives it a tensile strength of at least 55 MPa, preferably at least 70 MPa, and a bending module of at least 2.5 GPa, the body (7) thus having a mechanical strength which is sufficiently high so that the core box as to its form resists high internal and ex- ternal pressures in the subsequent core production; that the inner layer (8) is manufactured of a plastic or rubber material which gives this a tear resistance of at least 10 N/mm, preferably at least 15 N/mm, and most preferably at least 20 N/mm, so that the layer (8) has a tear resistance which is sufficiently high to ena- ble the surface (9) of the layer to resist extensive repeated core production; and that said plastic material for the body (7) has a coefficient of heat expansion of at most 6.5xl0 -5 mm/ m°C, prefer¬ ably at most 6.0x10-5 mrn/mτn°C, said coefficient of heat expansion is sufficiently low to ensure that the core box has narrow toler- ances at the varying curing temperatures occurring during thermal curing of said cores. 13. A method according to claim 12, characterized in that a se¬ cond mouldable rubber material is moulded in the recess of the prefabricated body or body part to form an intermediate layer af- ter curing of the material, said intermediate layer forming a lam¬ inated unit with the body or body part and the simultaneously or subsequently moulded inner layer, said second mouldable rubber ma¬ terial is of such a quality that the intermediate layer has a lower coefficient of heat expansion than the inner layer and is of the same basic type as said first rubber material enabling it to be co-cured therewith. 14. A method according to claim 13, characterized in that the inner layer has a thickness of at most about 1 mm, and that the intermediate layer (12) has a thickness of less than about 5 mmr ~~~""~ 15. A method according to any of claims 12-14, characterized in that the prefabricated body or body part is kept at an elevated temperature corresponding substantially to the temperature at which the core box shall be used and that the mouldable rubber ma- terial or materials is/are applied at this elevated temperature in the recess of the body or body part to effect said layer or part layer. 16. A method according to any of claims 12-14, characterized in that the body is made of a plastic material selected from a group consisting of polypropene for a curing temperature of up to 100°C, polysulphone for a curing temperature of up to 150°C and polyether sulphone for a curing temperature of up to 150°C.

Inventors: IVERLUND BERTIL, UDDBORN HANS, IVERLUND, BERTIL, UDDBORN, HANS
Applicants: MIKROVAOGSAPPLIKATION AB
Application year: 1985
Main classification: B22C7
IPC codes: B22C7, B22C9