US3688384A

US3688384A - Method of producing a synthetic resin box with double wall structure

Info

Publication number: US3688384A
Application number: US63677A
Authority: US
Inventors: Kyohei Mizushima; Yukio Yada
Original assignee: Mitsubishi Rayon Co Ltd
Current assignee: Mitsubishi Rayon Co Ltd
Priority date: 1969-09-09
Filing date: 1970-08-14
Publication date: 1972-09-05
Anticipated expiration: 1989-09-05
Also published as: DE2044713C3; FR2060670A5; DE2044713A1; DE2044713B2; GB1307276A

Abstract

A method of producing a synthetic resin-made box with double wall structure, adapted for use as the box frame of a refrigerator or the like, comprising the steps of molding a double-walled box body of a synthetic resin in a metal mold by injection molding in such a manner that wall sections constituting the inner wall of the box body are formed integrally, while wall sections constituting the outer wall of the box body are formed separately extending obliquely outwardly with one edge each thereof connected to said respective inner wall sections, after molding bringing said outer wall sections to the normal positions extending horizontally or vertically in adjacent relation to said inner wall sections respectively, connecting the adjacent edges of said outer wall sections with each other by means of coupling members, filling the space defined between said inner wall and said outer wall with a heat insulating material and closing the open rear end of the box body with a closure plate.

Description

Unite States Patet Mizushima et a1.

[451 Sept. 5, 1972 Inventors: Kyohei Mizushima; Yukio Yada,

both of Tokyo, Japan [73] Assignee: Mitsubishi Rayon Company Ltd.,

Tokyo, Japan [22] Filed: Aug. 14, 1970 U.S. Cl. "129/455, 29/475, 29/526, 220/9 F, 264/DIG. 66, 264/295, 312/214 Int. Cl. ..B2ld 39/00, B23p 19/04 Field of Search ..29/455, 526, 475; 220/9 F; 264/DIG. 66, 295; 312/214 References Cited UNITED STATES PATENTS Rill et a1. ..220/9 F Morrison ..29/455 X Dusel et a1 ..220/9 F 3,601,463 5/1970 Watt ..220/9 F X 3,616,012 10/1971 Salvarani ..264/295 X 3,622,418 ll/197l Black ..264/295 X FOREIGN PATENTS OR APPLICATIONS 237,227 12/1964 Austria ..,'..264/DIG. 66

Primary ExaminerCharlie T. Moon Att0meyWenderoth, Lind & Ponack [5 7] ABSTRACT A method of producing a synthetic resin-made box with double wall structure, adapted for use as the box frame of a refrigerator or the like, comprising the steps of molding a double-walled box body of a synthetic resin in a metal mold by injection molding in such a manner that wall sections constituting the inner wall of the box body are formed integrally, while wall sections constituting the outer wall of the box body are formed separately extending obliquely outwardly with one edge each thereof connected to said respective inner wall sections, after molding bringing said outer wall sections to the normal positions extending horizontally or vertically in adjacent relation to said inner wall sections respectively, connecting the adjacent edges of said outer wall sections with each other by means of coupling members, filling the space defined between said inner wall and said outer wall with a heat insulating material and closing the open rear end of the box body with a closure plate.

1 1 Claims, 16 Drawing Figures PATENTE D SE? 5 I972 3.688.384

sum 1 or 5 a H I I I 1; 1 I} 31C INVENTOR.

PATENTEB SEP 5 I972 SHEET 2 BF 6 I NVENTOR.

KYOHE I MIZUSHIMA YUKIO YADA PATENTEUSEP 5 m2 3,688,384

sum

3 or 6 IN\-ENTOR. KYOHEI MIZUSHIMA YUKIO YADA PAIENTEMEP' 5 I912 SHEEI l; BF 6 INVENTOR. KYOHEI MIZUSHIMA YUKIO YADA PATENTEDsEP' 51912 SHEEI 6 BF 6 I NVEN'TOR.

m Q m m mm WWW METHOD OF PRODUCING A SYNTIC RESIN BGX WHTH DOUBLE WALL STRUCE The present invention relates to a method of producing synthetic resin-made boxes with double wall structure which are primarily used as the box frames of refrigerators, hot boxes, dish-washing machines, etc.

With reference to a refrigerator, the inner and outer walls of the box frame thereof are usually made from a steel plate having a special rustproof coating thereon. Therefore, it is possible that the coating is separated from the steel plate due to collision against other articles or during use of the refrigerator for an extended period, degrading the appearance of said refrigerator. Once such condition has occurred, the box frame rusts immediately as a result of water attaching to the steel plate exposed to the outside, not only further degrading the appearance of the box frame but also causing the box frame to be progressively damaged from the rusted portion. However, when the box frame is made of a synthetic resin, particularly of ABS resin material excelling in mechanical properties, such as shock resistant property, hardness and tensile strength, the entire box frame can be maintained in a beautiful condition over an extended period, without being damaged by collision against other articles and being rusted with water or other liquids attaching thereto, and thus the aforesaid disadvantage can be eliminated. In the production of such a box frame, it is most advantageous to mold the box frame integrally by means of an injection molding machine, from the standpoint of the cost of metal mold and the working cycle. However, by the conventional methods, it is practically impossible to mold the box frame integrally since the box frame is double-walled. Further, when the injection molding machine is used, the metal molds must have at least 1 of gradient to provide for separation of the molds and, in molding a large sized box, such as a box frame of refrigerator, such a slight gradient will result in a large dimensional difference between the top and bottom of the box, which causes a wedge-shaped gap to be formed between the outer wall surface of the refrigerator and the adjacent wall when the refrigerator is placed, for instance, at the corner of a kitchen, spoiling the appearance of the kitchen.

The present invention has been made with these points in mind, and the principal object of the invention is to provide a novel method of producing a synthetic resin-made box with double wall structure by means of an injection molding machine, in which the inner and outer wall sections can be molded integrally and which enables the influence of a slip gradient of the metal molds used to be eliminated from the outer wall sections.

Another object of the invention is to provide a novel method of producing a synthetic resin-made box with double wall structure, of the character described above, in which a section of the outer wall of the box is provided separately after molding, so that a desired shape and function may be imparted to one section of the box.

Still another object of the invention is to provide a novel method of producing a synthetic resin-made box with double wall structure, of the character described above, in which an optional pattern is formed on the outer wall sections.

Still another object of the invention is to provide a novel method of producing a synthetic resin-made box with double wall structure, of the character described above, in which shelf-supporting ribs are formed on the inner wall sections.

Still another object of the invention is to provide a novel method of producing a synthetic resin-made box with double wall structure, of the character described above, in which metal molds used for the formation of the inner and outer wall sections are stabilized and are so designed that the projection area can be decreased, whereby the injection molding machine is rendered compact and small insize.

Still another object of the invention is to provide a novel method of producing a synthetic resin-made box with double wall structure, of the character described above, in which the outer wall sections formed have a uniform strength, without being fragile locally.

Still another object of the invention is to provide a novel method of producing a synthetic resin-made box with double wall structure, of the character described above, in which the inner and outer wall sections molded can be separated from the metal molds easily and a back panel of the box can be connected smoothly.

Still another object of the invention is to provide a novel method of producing a synthetic resin-made box with double wall structure, of the character described above, in which, in the formation of a band-shaped joint between the inner and outer walls, attachment fitting holes are formed in said joint by means of interlocking projections and holes formed on and in the cooperating metal molds respectively and said metal molds are stabilized in an engaged position by said interlocking projections and holes.

Still another object of the invention is to provide a novel method of producing a synthetic resin-made box with double wall structure, of the character described above, in which the box is molded with means which facilitate mounting of fittings for a door panel.

A further object of the invention is to provide a novel method of producing a synthetic resin-made box with double wall structure, of the character described above, in which a molten resin can be injected into the clearance between the metal molds smoothly and the inner and outer wall section can be molded quickly.

In order to attain the objects of the invention set forth above, according to the invention there is provided a method of producing a synthetic resin-made box with double wall structure, which comprises molding a double-walled box body of a synthetic resin in a metal mold by injection molding in such a manner that the inner wall sections of the box body are formed integrally and the outer wall sections thereof are formed having one end each connected to said inner wall sections respectively with the other portions being separated therefrom and obliquely extending outwardly from said one ends, and after removing the box body from the mold bringing said outer wall sections into the normal positions extending vertically or horizontally in adjacent relation to said respective inner wall sections and connecting the adjacent edges thereof with each other.

According to the invention, as stated above, in producing a synthetic resin-made box with double wall structure by injection molding the inner wall sections of the box body are formed integrally by a metal mold and the outer wall sections are formed, with one end each connected to said inner wall sections respectively and with the other portions being separated therefrom and extending obliquely outwardly from said one ends. Namely, while the inner wall of the box body is formed integrally in a progressively converged shape as usual, the sections of the outer wall of the box body are formed separately and distributed in such a manner as to a slip gradient to the metal mold. Therefore, in the formation of the outer wall sections both the use of a metal mold having a slip gradient and the integral formation of the outer wall sections become possible. Furthermore, since the outer wall sections are formed separately, they can be formed in a rectangular or square shape and, since the outer wall sections thus formed are subsequently brought into the normal positions extending vertically or horizontally and the adjacent edges thereof are connected with each other, the outer wall of the box can be assembled into the shape of cube or rectangllar parallelepiped, regardless of the slip gradient of the metal mold. Thus, according to the invention the outer wall of the box can be formed integrally, even by the use of an injection molding machine and a metal mold having a slip gradient and moreover the outer wall thus formed is in the shape of cube or rectangular parallelepiped. Therefore, the method of this invention has such an excellent advantage that a large sized box, such as the box frame of a refrigerator, can be molded of a synthetic resin in a desired shape highly efficiently and at a low cost.

Other objects, features and advantages of the present invention will become apparent from the following description on the embodiments of the invention when taken in conjunction with the accompanying drawings, in which:

FIG. 1 is a vertical cross-sectional view exemplifying a metal mold used in the formation of a synthetic resin box according to the method of the present invention;

FIG. 2 is a perspective view of an injection molding machine incorporating the metal mold of FIG. 1, which is used for practicing the method of this invention;

FIG. 3 is a fragmentary perspective view showing the essential portion of the injection molding machine;

FIG. 4 is a perspective view a box body molded by the metal mold of FIG. 1;

FIG. 5 is a plan view of the box body shown in FIG. 4;

FIG. 6 is a fragmentary perspective view showing, in transverse cross-section, the adjacent outer wall sections coupled together by means of a coupling member;

FIG. 7 is a fragmentary perspective view exemplifying means for closing the back side of the box body;

FIG. 8 is a vertical cross-sectional view a box with double wall structure obtained from the box body molded by the metal mold of FIG. 1;

FIG. 9 is a perspective view of the box, with shelfsupporting ribs, notches and a motor housing concurrently formed therewith and with a separate panel attached to the top end thereof;

FIG. 10 is a perspective view of the box body, showing the back side thereof;

FIG. 11 is a vertical cross-sectional view showing another form of the metal mold;

FIG. 12 is a plan view of a box body molded by the metal mold of FIG. 1 1;

FIG. 13 is a vertical cross-sectional view of a doublewalled box obtained from the box body molded by the metal mold of FIG. 11;

FIG. 14 is a vertical cross-sectional view of still another form of the metal mold;

FIG. 15 is a vertical cross-sectional view of a box body molded by by a metal mold similar to that shown in FIG. 14; and

FIG. 16 is a vertical cross-sectional view of still another form of the metal mold.

The present invention will be described in detail hereinafter with reference to an embodiment in which the method of the invention is applied to the production of a box frame of a refrigerator. In the production of the box frame, use is made of a metal mold unit consisting of a pair of cooperating metal molds, such as metal molds A and B shown in FIG. 1, which are capable of molding a doubled-walled box body in such a manner that the inner wall sections or panels thereof are formed integrally into the shape of a progressively inwardly converged box and the outer wall sections or panels are formed separately from each other, with one edge each thereof connected to the free edges of said inner wall sections, and extending obliquely outwardly from said one edges; and the box body is molded by said metal molds A, B and an injection molding machine C, shown in FIGS. 2 and 3, which is of the type known in the art, in the usual manner.

Referring to FIGS. 2 and 3, the injection molding machine C includes a hopper D for feeding a pelletized resin therethrough, a screw conveyor E for melting the pelletized resin and conveying the molten resin towards a nozzle F to extrude it therethrough, a mold mount G being movable in the direction of the arrows with one of the metal molds A mounted thereon, and another mold mount H with the other metal mold B mounted thereon in opposed relation to said metal mold A. In operation, the mold mount G is advanced with the metal mold A thereon, to bring the metal mold A into engagement with the metal mold B in the state shown in FIG. 1, and then the molten resin is injected into the clearance defined between the metal molds A and B through the nozzle F. After the resin has been solidified, the mold mount G is retracted with the metal mold A thereon and then the resultant molding is removed from said metal mold A. In case of the present invention, since the metal molds A, B are so constructed as shown in FIG. 1, the resultant molding will take the form as shown in FIGS. 4 and 5. Namely, in the embodiment illustrated the molding, that is a doublewalled box body, has an outer wall consisting of

outer wall sections

1, 2, 3, 4 which are separated from each other and extend obliquely outwardly from one

edges

1a, 2a, 3a, 4a thereof respectively, and an inner wall 5 which is formed integrally in the shape of a progressively inwardly converged box. The

outer wall sections

1 and 2, and 3 and 4, Le, the opposed outer wall sections are formed in the same shape and size, and these

outer wall sections

1, 2, 3, 4 are connected to the inner wall 5 through connecting portions 6 which respectively extend from said one

edges

10, 2a, 3a, 4a of said outer wall sections towards the inner wall 5. Each of the

outer wall sections

1, 2, 3, 4 is concurrently formed at each side edge thereof with an enlarged

engaging portion

1b, 2b, 3b, or 4b as best shown in FIG. 6. The

outer wall sections

1, 2, 3, 4 thus formed are brought into the normal vertical positions in the directions of the arrows respectively and the engaging

portions

1b, 2b, 3b, 4b of the adjacent outer wall sections are coupled together by means of a coupling means 7 as shown in FIG. 6, which coupling means 7 serves simultaneously as a decorative element. Thus, the

outer wall sections

1, 2, 3, 4 are assembled into an integral outer wall.

Now, the method described above will be further illustrated by way of practical example.

A box body of the type described above was molded of Diapet ABS (the tradenarne of the ABS resin produced by Mitsubishi Rayon Company, Ltd.), which box body had an inner wall of 700 X 370 X 400 mm in size, an outer wall of which one opposed wall sections are 850 X 453 mm in size and the other opposed wall sections are 450 X 453 mm in size, and a wall thickness of 3 mm, the width of the connecting portions 6 being 40 mm. In this case, metal molds defining an angle of inclination (the angle a in FIG. 1) of 930 and having 12 gates open at the connecting portions 6 were used. The resin was injected at a temperature of 270 C., using an injection molding machine having a moldtightening capacity of 2,200 tons, while maintaining the metal molds at a temperature of 55 65 C. After the box body thus molded was removed from the metal molds, the outer wall sections were brought into the normal positions and coupled together at their side edges by means of the metallic coupling means 7 as shown in FIG. 6.

In the manner described, a synthetic resin box with double wall structure is obtained, but in this state, a heat insulating material has not been filled in the space between the inner and outer walls of the box. Further, the back side of the box has not been closed. However, the heat insulating material may be filled in the space between the inner and outer walls after or at the same time when the outer wall sections are coupled together. In the latter case, the heat insulating material is inserted between the outer wall and the inner wall, immediately before the

outer wall sections

1, 2, 3, 4 are coupled together, whereby the charging of the heat insulating material and the coupling of the

outer wall sections

1, 2, 3, 4 can be accomplished concurrently and the subsequent heat insulating material-charging operation can be eliminated. For closing the back side of the box, a pair of the opposed outer wall sections 1, 2 may be made longer than those shown in FIGS. 4 and 5, and the extended portions of said outer wall sections may be flexed in the directions of the arrows shown in FIG. 7 and coupled together at their confronting edges by means of suitable means, such as the coupling means 7 shown in FIG. 6, or alternatively a back wall or panel, formed separately, may be attached to the open back side of the box and secured thereto by suitable means such as the coupling means 7 of FIG. 6. Thus, a doublewalled box is formed, with the space between the inner and outer walls thereof being filled with the heat insulating material, which can be used as the box frame of a refrigerator of the type having an opening and closing door. The state of the box is shown in FIG. 8, in which reference numeral 8 designates the back panel and 9 designates the heat insulating material, such as foamed polyurethane or foamed styrol. Besides the ABS resin, the box may be molded of such resins as polypropylene, polyethylene, etc.

Other advantageous features of the present invention will be described hereunder: As is well known, the surface of the top panel 4 of a refrigerator is used as dresser, etc. and, therefore, it must be resistive to heat and wear. In case of the top panel being made of a synthetic resin material, however, such properties cannot be imparted to the top panel, so that the box is molded with no top plate and a metallic top panel 10, prepared separately, is attached to the top end of the box by means of the coupling members 7 of FIG. 6, as shown in FIGS. 9 and 10. It may be necessary to use a top panel of complicated configuration or to provide a condensing unit (not shown) on the bottom plate 3, depending upon the type of refrigerator, and in such a case, the

outer section

3 or 4 of the box body is not needed. In these cases, the desired shape may be imparted or the desired provision may be made, in the same manner as described above.

In the method illustrated in FIGS. 1, 4 and 5, the shaping surfaces of the metal molds A and B are flat and hence the surfaces of the

outer wall sections

1, 2, 3, 4 of the molded box body become flat and smooth. Where a pattern is desired to be formed on these surfaces, an aventurine or engraved pattern is previously formed on the shaping surfaces of the metal molds A, B, whereby a pattern as shown in FIGS. 3, 9 and 10 can be obtained on the outer surfaces, for example, of the wall sections 1 and 2, concurrently with the molding of the box, and the subsequent pattern-forming operation can be eliminated.

Further, in the method illustrated in FIGS. 1, 4 and 5, since the shaping surfaces of the metal molds A, B are flat as stated above, the inner surface of the inner wall also becomes flat and smooth. However, by forming recesses in the shaping surfaces prior to the molding, the complementary ribs 15 (or projections) are formed on the inner surface of the inner wall as exemplified in FIGS. 9 and 10, which may be used as shelf-supporting ribs when the box is used as the box frame of a refrigerator. Such practice is advantageous in eliminating the operation of forming such ribs or projections, otherwise required subsequently of the molding operation.

Still further, in the method illustrated in FIGS. 1, 4 and 5, it is possible to mold a double-walled box integrally and in a desired shape, such as the shape of a rectangular parallelepiped, owing to the fact that the outer wall of the box is not subjected to the influence of the slip gradient of the metal molds, as stated previously. On the other hand, however, the following problem arises: Namely, in the method described above the

outer wall sections

1, 2, 3, 4 are molded extending obliquely outwardly from edges la, 2a, 3a, 40 thereof. Therefore, when the method is applied to the molding of a large-sized box, such as that used as the box frame of a refrigerator, and the gates of the metal molds are located at the connecting portions 6 between the inner and outer walls, the area of projection will become so large, i.e., the length l in FIG. 1 becomes so large, that I an extremely large molding machine will be required to be used. For instance, in case of the box frame of a refrigerator on the order of X 530 mm in size, the area of projection is about 900 cm and thus it becomes necessary to use a molding machine which has a moldtightening capacity of 3,500 tons. However, the maxirnum mold-tightening capacity of the presently available molding machines is about 2,500 tons and the production of a large box as mentioned above requires a molding machine of such a large capacity as mentioned above to be newly provided which is expensive. This is disadvantageous economically. However, if the metal molds A, B are constructed such that a land B formed on the metal mold B is received in a depression A formed in the metal mold A, at the time of molding, as shown in FIG. 11, the area of projection of the metal molds is decreased accordingly and it becomes unnecessary to use a large capacity molding machine. The state of the box body immediately after molding by such metal molds is shown in FIG. 12. Namely, the molded double-walled box body has an outer wall consisting of sections 1 to 4 formed separately and extending obliquely outwardly, and an inwardly converged inner wall 5, the outer wall sections 1 and 2 being rectangular in shape and in the same size, the

outer wall sections

3 and 4 being rectangular in shape and in the same size, and said

outer wall sections

1, 2, 3i, 4 being connected to said inner wall 5 through connecting portions 6. An opening 11 is formed in the bottom wall of the inner wall 5, leaving a rim of a width 50, around the periphery thereof. After molding the box body in the manner described, the outer wall sections are brought into the normal positions extending vertically as viewed in FIG. 4 and the side edges lb to 4b of the adjacent ones thereof are coupled together by means of the coupling members 7 to form an integral outer wall. The opening 11 is closed by a plate member 12 as shown in FIG. 13, which is formed as by vacuum forming. For the sake of decreasing the area of projection, the opening 11 is preferably as large as possible, but in case of the box frame of a refrigerator, the size and shape of the opening 11 should be suitably selected in consideration of various equipments, such as motor, to be provided at this portion. An embodiment thereof is shown in FIGS. 9 and 10. In the embodiment of FIGS. 9 and 10, the opening 11 is seen at the center of the box frame, and at the right lower portion of the opening is formed a box-shaped motor compartment 13 which is open to the back side of the box frame. In case of a box, such as the box frame of a refrigerator, it is unnecessary to form all of the four outer wall sections as stated previously and, in this embodiment, the outer wall section corresponding to the bottom plate of the box frame is omitted. As shown in FIG. 11, the metal molds A, B are respectively provided with projections A" and bores B" for receiving said projections A". The interlocking engagement between these projections A" and bores B", plus the interlocking engagement between the aforesaid land A and depression B, stabilizes the relative position of the metal molds A and B, and enables a molding of high precision to be obtained. However, the interlocking engagement between the projections A" and the bores B" results in the formation of holes 13 in the molding, as shown in FIGS. 12 and 13, and these holes 13 must be closed by suitable means. Where the holes 13 are formed at suitable locations and rectangular in shape, these holes may be utilized for mounting an instrument, such as a control panel. Likewise, the opening 11 may also be utilized for mounting equipments although it is finally closed with the closure plate 12 as stated above. Namely, in a refrigerator or the like, a condenser is mounted on the outside surface of the back panel of the box frame and an evaporator in the upper portion of the box frame. When the opening 1 l is formed in the back panel of the box frame, mounting of these equipments can be achieved only by previously assembling the major portion of the refrigeration system and inserting the evaporator into the box frame from the back side. Thus, the mounting operation of the equipment can be extremely simplified. The difference between the metal molds A, B of FIG. 1 and the metal molds A, B of FIG. 1 1 brings about the following difference in the injection molding machine used: Namely, in molding a doublewalled box body having an inner wall of 700 X 370 X 400 mm in size, an outer wall consisting of a front and back wall sections of 850 X 450 mm in size and side wall sections of 450 X 450 mm in size, a connecting portion 6 of 40 mm in width and a wall thickness of 3 mm, using Diapet ABS (the tradename of the ABS resin produced by Mitsubishi Rayon Company, Ltd.), at the angle of inclination of the metal molds (the angle a in FIG. 1) of 930, by injecting said resin at the connecting portion 6, when the opening 11 was not intended to be formed in the inner wall, the area of projection was about 5,000 cm and a molding machine of 2,200 tons in mold tightening capacity was required. Whereas, when the opening 11 of 650 X 320 mm in size was intended to be formed in the inner wall, the area of projection was decreased to about 2,700 cm and a molding machine of only 1,250 tons in mold tightening capacity was required for molding the box body.

In practicing the method of this invention in each of the embodiments described above, the following problem will be encountered: Namely, when the box body to be molded is large in size and used as the box frame of a refrigerator or the like, the edges la, 2a, 3a, 4a of the box body must be equal to or even larger in thickness than the other portions, since a door is connected thereto, and in this case, difficulty is encountered in bringing the outer wall sections back to the normal positions after molding. For this reason, in case of a large box body, it would be considered that this portion is formed in a thickness smaller than that of the other portions to facilitate the movement of the outer wall sections, but such practice is unsatisfactory because, in case of the box frame of a refrigerator or the like, the subject portion undergoes an external force every time the door, connected thereto, is opened or closed, and will be destructed in a short period of time. However, if the

outer wall sections

1, 2, 3, 4 of the box body are molded in an arcuate shape by the use of metal molds A, B of the type shown in FIG. 14, it will be possible to mold the

portions

1a, 2a, 3a, 4a of the respective outer wall sections in a thickness equal to or larger than that at the other portions and at right angles to the connecting portions 6. Namely, since the

outer wall sections

1, 2, 3, 4 are curved gently as a whole, they can be brought back to the normal positions by deforming them little by little and it is unnecessary to reduce the thickness at any portion thereof. In addition, since a heat insulating material, such as foamed polyurethane or foamed styrol, is placed in the space between the inner wall and the outer wall sections when the outer wall sections are brought back to the normal positions, as stated previously, the heat insulating material aids in shaping each outer wall section into a flat panel and consequently the appearance of the box body is not impaired. In practice, however, it is desirable to form the subject portions in the manner as indicated by numeral 14 in FIG. 15. In the metal molds of FIG. 14, the radius of curvature r is selected within the range of m r 3m wherein m is the length of the curved wall section.

In practicing the embodiments described above, there also arises the following problem: Namely, in removing the molded box body from the metal molds upon separating said metal molds from each other, the box body tends to remain on the mold B, since the contacting area of the box body is larger with the mold B than with the mold A, and in this case the

outer wall sections

1, 2, 3, 4 and the inner wall are held so deep in the mold B that the removal of the box body is not easy and takes a considerably long time, making quick molding impossible. If the box body is removed with an unreasonable force, the outer surfaces of the

outer wall sections

1, 2, 3, 4 will possibly be damaged. However, such a problem may be solved by the following method: Namely, recesses A are formed in the metal mold A on which the molded box body is desired to be retained, as shown in FIG. 16, so as to form

stoppers

1d, 2d, 3d, 4d (1d and 2d being not apparent) on the inside surface of the

other edge portions

10, 2c, 30, 4c and 2c being not apparent) of the outer wall sections. In this case, it is necessary that the inward surfaces of the

stoppers

1d, 2d, 3d, 4d are inclined by an angle [3 (about i 1 relative to a horizontal plane) inwardly relative to the inside surfaces of the respective

outer wall sections

1, 2, 3, 4 (see FIG. 16). By so doing, the molded box body is retained on the metal mold A, when the metal molds A and B are separated from each other, due to the engagement of the stoppers 1d, 2d, 3d, M with the respective recesses A. After the metal mold B has been relatively moved away from the metal mold A, the molded box body is removed from the metal mold A, by pulling the inner wall 5 in the direction of the arrow Y upon releasing the

outer wall sections

1, 2, 3, 4 by pulling them in the directions of the arrows X shown in FIG. 16. The

stoppers

1d, 2d, 3d, 4d formed on the inside surface of the other edge portions 1c, 2c, 30, 4c of the outer wall sections can advantageously be used for mounting the back panel 8.

Where the box body thus molded is used as the box frame of a refrigerator, hinges for opening and closing the door and a magnet for holding the door in its closed position under magnetic attraction must be provided on the connecting portions 6. The operation of attaching such elements to the box frame may be eliminated if the hinges and the magnet are placed, prior to the injection of a molten resin, in recesses formed in the metal mold A at the portions corresponding to the connecting portions 6 of the molded box body and then injecting the molten resin, whereby said parts are embedded in the connecting portions concurrently with molding. It is also advantageous, for achieving smooth injection of the molten resin and thereby molding even a large sized box body quickly, to progressively reducing the width of the gap, formed between the metal molds A and B, at a location remoter from the resin injecting side (the connecting portions 6) and thereby progressively reducing the thicknesses of the

outer wall sections

1, 2, 3, 4 and the inner wall section 5 accordingly.

Although the present invention has been described and illustrated herein in terms of specific embodiments thereof, it should be understood that the invention is not restricted only to the embodiments but many changes and modifications are possible without deviating from the spirit of the invention.

We claim:

1. A method of producing a synthetic resin-made box with double wall structure, which comprises molding a double-walled box body of a synthetic resin in a metal mold by injection molding in such a manner that the inner wall sections of the box body are formed integrally and the outer wall sections thereof are formed having one end each connected to said inner wall sections respectively with the other portions being separated therefrom and obliquely extending outwardly from said one ends, and after removing the box body from the mold bringing said outer wall sections into the normal positions extending vertically or horizontally in adjacent relation to said respective inner wall sections and connecting the adjacent edges thereof with each other.

2. A method of producing a synthetic resin-made box with double wall structure, as defined in claim 1, in which any one of the two sets of opposed outer wall sections are extended and said extended portions are folded inwardly of each other and coupled together at the adjoining edges thereof, so as to close the back side of the box body.

3. A method of producing a synthetic resin-made box with double wall structure, as defined in claim 1, in which one or two of said outer wall sections are not formed during molding of the box body and the portion or portions of the box body where the outer wall sections are not formed is or are closed with a suitable panel or panels produced separately. 4

4. A method of producing a synthetic resin-made box with double wall structure, as defined in claim 1, in which an aventurine or engraved pattern is formed on the shaping surfaces of said metal mold, so that the corresponding pattern may be formed on the outer surfaces of said outer wall sections concurrently with molding of the box body.

5. A method of producing a synthetic resin-made box with double wall structure, as defined in claim 1, in which recesses such as elongate grooves are formed in the shaping surfaces of said metal mold, whereby shelf supporting projections or ribs are formed on the inside surfaces of said inner wall sections concurrently with molding of the box body.

6. A method of producing a synthetic resin-made box with double wall structure, as defined in claim 1, in which said mold consists of a mold member having a depression formed therein and a cooperating mold member having a projecting land formed thereon for interlocking engagement with said depression, said depression and said projecting land being located on the side remote from the side on which molten resin in injected, whereby the area of projection can be decreased, providing for the use of a small-sized injection molding machine, stabilization of the metal mold and hence production of a precise box body.

7. A method of producing a synthetic resin-made box with double wall structure, as defined in claim 1, in which the outer wall section molding surfaces of said mold are outwardly gently curved from the portions corresponding to said one ends of said outer wall sections and, in bringing the molded outer wall sections back into the normal positions thereof, said outer wall sections are deformed little by little over the range of the lengths thereof, whereby the necessity of forming a thin portion at said one end of each outer wall section can be eliminated.

8. A method of producing a synthetic resin-made box with double wall structure, as defined in claim 1, in which recesses are formed in those of the shaping surfaces of the mold which shape the inside surfaces of the outer wall sections, so as to form complementary projections on the inside surfaces of said outer wall sections, whereby the molded box body can be retained on that one of the mold members from which it can be separated relatively easily, by the engagement between said recesses and said projections, whereby said projections can be used for mounting a back panel on the box body after said box body is removed from the mold.

9. A method of producing a synthetic resin-made box with double wall structure, as defined in claim 1, in which said metal mold consists of a mold member having bores formed therein and a cooperating mold member having projections formed thereon for interlocking engagement with said bores, said bores and said projections being located at those shaping surfaces of the mold which form connecting portions between the outer wali sections and the inner wall sections, being held in interlocking engagement with each other, whereby equipment mounting holes are formed in the molded box body and the mold members are held stable in the molding position, providing for the production of a precise box body.

10. A method of producing a synthetic resin-made box with double wall structure, as defined in claim 1, in which recesses are formed in those of the shaping surfaces of the mold which form the connecting portions between the outer wall sections and the inner wall sections, and elements, such fittings for a door are placed in said recesses respectively prior to molding, whereby said elements are cast in said connecting portions concurrently with molding of the box body.

11. A method of producing a synthetic resin-made box with double wall structure, as defined in claim 1, in which the gap formed between the molding members is progressively reduced in width at a location remoter from the side where the molten resin is injected so as to facilitate injection of the molten resin into said gap.

Claims

3. A method of producing a synthetic resin-made box with double wall structure, as defined in claim 1, in which one or two of said outer wall sections are not formed during molding of the box body and the portion or portions of the box body where the outer wall sections are not formed is or are closed with a suitable panel or panels produced separately.

9. A method of producing a synthetic resin-made box with double wall structure, as defined in claim 1, in which said metal mold consists of a mold member having bores formed therein and a cooperating mold member having projections formed thereon for interlocking engagement with said bores, said bores and said projections being located at those shaping surfaces of the mold which form connecting portions between the outer wall sections and the inner wall sections, being held in interlocking engagement with each other, whereby equipment mounting holes are formed in the molded box body and the mold members are held stable in the molding position, providing for the production of a precise box body.