METHOD OF MANUFACTURING CONTAINER COVERS

Abstract: A method for manufacturing a cover for a can-like con­ tainer providing improved peelability of a handle. The cover is made by forming a second resin layer by injection molding or compression forming on a multi-layer sheet which oxygen, water, and the like cannot permeate. A thermally weldable first resin layer is provided on the upper surface of the multi-­ layer sheet. The handle is formed by the second resin layer. A treatment is carried out for allowing the handle to be easily peeled off the multi-layer sheet to all or part of a region of the multi-layer sheet in contact with the underside of the handle.

Description: BACKGROUND OF THE INVENTION The present invention relates to an injection molding method of the kind defined by the precharacterizing features of claim 1. In an insert molding method of this type, insertion accuracy of the insert and the molding material is essential. For insertion by injection molding, the insert is inserted into the metal mold assembly in advance. However, this method suffers from a problem in that the insert can easily be displaced during molding, and the accuracy of the resultant molding is hence often lower than required. Examples of conventional methods in which an insert is inserted into a metal mold assembly in advance and molding material is injected to the insert to form an insert molding are a method in which an insert plate is set along the configuration of the core mold of the metal mold assembly, a method in which an insert plate is set in the metal mold by inserting the positioning pin of the metal mold into a hole in the insert plate, and a method in which a film plate is held by a holding mold provided for the cavity mold so that the plate is automatically cut and pushed into the cavity (see Japanese Laid-Open Patent Application No. 5747/1981). However, in these conventional methods, the positioning accuracy of the insert is low, and the insert is not satisfactorily held. Thus, it is difficult to obtain moldings high in insertion accuracy utilizing these conventional methods. Furthermore, the conventional methods are different from an injection molding method in which, according to the invention, displacement of the insert during the molding operation is prevented to obtain a molding high in insertion accuracy and which has a fold as the bottom or cover (top) of a can-like container. A can-like container to which the invention relates and its bottom and cover (top) will be described. Fig. 1 is a perspective view showing a can-like container having a side wall 1, a bottom 2 and a rectangular cover 3. Fig. 2 is also a perspective view showing a can-like container having a side wall 4, a bottom 5 and a round cover 6. Fig. 3 is a sectional view of the bottom 2 which is to be joined to the side wall 1. In Fig. 3, reference 7 designates a base material such as metal foil or sheet which air, water, etc. cannot permeate (hereinafter referred to as "a gas barrier type base material" when applicable). Examples of the base material 7 are aluminum foil, and sheets of saponified ethylene vinyl acetate copolymer, polyvinylidene chloride and polyamide. Typically, the aluminum foil is used. A resin layer (not shown in Fig. 3) which can be made molten by heating is formed on one or both sides of the gas barrier type base material. The molding shown in Fig. 3 is manufactured by inserting the gas barrier type base material or a base material having a thermally meltable resin layer into a metal mold where molten molding material is injected into the base material (or an insert) by an injection molding machine. In Fig. 3, reference numeral 8 designates a synthetic resin layer which is formed by injection. The molding in Fig. 3 must have a fold 9 to join it to the side wall 1. Fig. 4 is a longitudinal sectional view of the rectangular cover 3 (Fig. 1) which is to be joined to the side wall 1. In Fig. 4, reference numeral 10 designates a base material. In the case of Fig. 4, the base material 10 is obtained by forming thermally meltable resin layers 12 and 13 and both sides of a metal foil 11. In Fig. 4, reference numeral 14 designates a synthetic resin layer. The molding in Fig. 4 can be obtained by injecting synthetic resin molding material into the base material (insert) 10, similar to the case of the bottom shown in Fig. 3. In Fig. 4, reference numeral 11 designates a handle (a so-called "pull top") on the cover. When the handle 11 is lifted with a finger tip inserted into a groove 16, the base material 10 is torn below the position 17 so that the cover 3 is pulled off. Further in Fig. 4, reference numeral 18 designates a treatment layer to facilitate the peeling of the handle portion from the base material 10. The molding in Fig. 4 has a fold 19 similar to the case of the molding in Fig. 3. Fig. 5 is a longitudinal sectional view of the round cover 6 shown in Fig. 2. The round cover 6 is manufactured in the same manner as the molding in Fig. 4. In Fig. 5, components similar to those in Fig. 4 are designated by the same reference numerals and their descriptions are omitted. The molding in Fig. 5 also has a fold 19. In the injection molding of a flat part which, unlike the moldings in Figs. 3, 4 and 5, has no fold (9 or 19), for instance, in the case of injecting molding material into a round insert, a slight displacement thereof causes no trouble. On the other hand, if, in the case of a molding with a fold (which the invention concerns), the insert is displaced even slightly during molding, undesirable results occur. It is impossible to obtain a molding high in insertion accuracy. Further, in a conventional method of adhering a multi-layer sheet to its outer synthetic resin layer to improve the gas barrier characteristics, after the outer synthetic resin layer is formed by injection molding or the like, an adhesive is used to cause the multi-layer sheet to adhere thereto. This conventional method suffers from the following problems: (1) the number of manufacturing steps is high and the manufacturing cost high, (2) the adhesive must be hygenic, and (3) the synthetic resin layer may peel off the multi-layer sheet if repeatedly heated and cooled. It is desirable to make the outer resin layer integral with the multi-layer sheet, for instance, by insert injection molding which prevents the permeation of moisture and oxygen and the transmission of light to improve the gas barrier characteristics, thereby to provide a can-like container's cover in which the outer resin layer is strongly joined with the multi-layer sheet. However, when the outer resin layer is joined with the multi-layer sheet, the handle ("or pull top") of the cover is also joined with the multi-layer sheet, with the result that it is impossible to peel the handle off the multilayer sheet or the body of the cover. From the GB-A-1 371 102 a mold assembly is known, in which an insert is inserted in a guide member and fixedly held therein before molten molding material injection is carried out. Said insert is an elongated strip which is slid along the guide member and fixed to a suitable position, said strip thereafter being cut upon the engagement of metal molds. Thereafter said injection is applied thereto. SUMMARY OF THE INVENTION In view of the foregoing, the object of the invention is to prevent the displacement of the insert in the metal mold in the injection molding operation, thereby to obtain a molding having a high insertion accuracy which has a fold therein, and which forms the bottom or cover (top) of a can-like container. The invention resides in an injection molding method for manufacturing a readily openable can-like container cover in which an insert of multilayers is inserted in a metal mold assembly in advance and molten molding material is injected to said insert having a flat shape and such a site corresponding to said cover, wherein the insert is inserted into a guide member provided for one of at least two metal molds which form said metal mold assembly fixedly held therein before a molten molding material injection is carried out. The advantages obtained by the invention will become apparent on the study of the description of the drawings. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a perspective view showing a can-like container with a rectangular cover to which this invention relates; Fig. 2 is a perspective view showing a can-like container with a round cover to which the invention relates; Fig. 3 is a sectional side view of the bottom of the container; Fig. 4 is a sectional view of the angular cover; Fig. 5 is a sectional view of the round cover; Fig. 6 is a sectional view for a description of an insert inserting step; Fig. 7 is a sectional view for a description of the start of a mold closing operation; Fig. 8 is a sectional view for a description of a molding material injecting step which is carried out after the mold closing operation; Fig. 9 is a sectional view of a metal mold assembly using a guide member different from the one in Figs. 6, 7 and 8; Fig. 10 is a sectional view for a description of a molding material injecting step which is carried out after the metal mold assembly (Fig. 9) has been closed; Fig. 11 is a sectional view used for a description of the insert inserting oepration which utilizes a suction device; Fig. 12 is a sectional view for a description of the start of the closing operation of the metal molds in Fig. 11; Fig. 13 is a sectional view used for a description of the molding material injecting operation which is carried out after the mold closing operation; Fig. 14 is a plan view for a description of another embodiment of the invention; Fig. 15 is a sectional view used for a description of another embodiment of the invention; Figs. 16 and 18 are top views of respective further embodiments of container tops of the invention; and Figs 17 and 19 are cross-sectional views of the covers of Figs. 16 and 18, respectively. DESCRIPTION OF THE PREFERRED EMBODIMENTS The molding method of the invention will be described with reference to the case where a stripper plate is used as a guide member as shown in the drawings. A stripper plate is generally used to eject and remove a molding from the metal mold in the injection molding machine. The applicant has developed a novel injection molding method in which an insert is inserted and fixedly held in the stopper plate to improve the insertion accuracy and to prevent the displacement of the insert. Accordingly, the method of the invention is suitable for providing an insert molding having a fold supported by the insert. The invention will now be described in more detail with reference to the accompanying drawings. Fig. 6 illustrates a method of inserting an insert into a metal mold. In this embodiment, a robot (not shown) is used to insert the insert into the metal mold. An insert 21 is sucked and retained by a suction plate 20 which is connected to the arm of the robot. Under this condition, the movable cylinder 22 of the robot is moved downwardly in the direction of the arrow 23 and then moved sideways in the direction of the arrow 24 so that the insert 21 is inserted into a stripper plate 26 provided for a metal mold (core type). Even if, in this operation, the robot moves the insert 21 obliquely with respect to the inner surface of the metal mold, the insert 21 will still be suitably inserted into the metal mold because it is guided by the inner wall 27 of the stripper plate 26. That is, the insert 21 is smoothly guided by the stripper plate 26 at all times. The insert is generally larger than the hole cut in the stripper plate 26. Therefore, the insert 21 is retained and positioned by the stripper plate 26 when inserted; that is, it is fixedly held in the stripper plate 26 when inserted. In Fig. 6 and 7 through 11, the insert is shown oriented in a vertical direction by the stripper plate and fixedly held therein. However, it should be noted that the insert is retained on four sides. After the insert has been fixedly held in the stripper plate, the movable cylinder 22 of the robot is moved in the direction of the arrow 28 and then lifted in the direction of the arrow 29. Fig. 7 shows the insert fixedly held in the stripper plate 26 with the robot's cylinder having been lifted away. Under this condition, the metal mold (core-type, movable) 25 is moved laterally to start a mold closing step with a metal mold (cavity-type, stationary) 32 having an injection nose 30 and a molding material supplying path 31 communicated with the gate 30 at the center. In the closing step, the inset 21 cannot be displaced because it is fixedly held by the stripper 26. As shown in Fig. 8, at the end of the closing step, the peripheral portion of the insert 21 is formed into a fold 33, and molten molding material is injected through the injection port 30 into a cavity 34 which is formed by the core-type metal mold 25 and the cavity-type metal mold 30. The molding has a fold similar to the moldings shown in Figs. 3 through 5. In the above-described operation, the insert is not displaced at all. Therefore, the molding has a high insertion accuracy, and the molding material is applied to the fold also with a high accuracy. After molding material injection, pressurizing, cooling, mold opening, mold ejecting and mold removing, steps may be carried out according to the ordinary injection molding cycle to obtain the molding. However, according to the invention, the insert is inserted and fixedly held by means of the stripper plate, and the molding is ejected and removed with the aid of the stripper plate, so that the injection molding is achieved more efficiently. In the above-described embodiment, the insert is inserted into the stripper plate and fixedly held therein. However, the guide member in the invention may be any one which has the above-described functions and can achieve the objects of the invention. Fig. 9 illustrates another embodiment of the invention. In this embodiment, a guide member 36 is connected through springs 35 to the upper and lower portions of a stationary metal mold 32. An insert 21 is inserted into the guide member 36 and fixedly held therein. Under this condition, a mold closing operation is carried out as shown in Fig. 10, and then an injection molding operation is performed as described with reference to Figs. 6, 7 and 8. In Fig. 10, the insert 21 (Fig. 9) is not shown. Fig. 11 depicts a third embodiment of the invention. In this embodiment, the insert inserted in the guide member is more positively fixed therein by means of suction device such as a vacuum cylinder provided in the metal mold. This method is advantageously applicable to a case where molding should be carried out at higher speed or a case where a large number of moldings should be manufactured. In these cases, the method can more positively prevent the displacement of the insert and thus more completely contribute to improving the insertion accuracy. Furthermore, according to this method, the injection molding is carried out with the insert sucked and retained by the vacuum cylinder, and therefore deformation of the insert is positively prevented. The third embodiment will be described further with reference to Figs. 11, 12 and 13. As shown in Fig. 11, an insert 21 is sucked and retained by the suction plate (or suction disc) 20 of a robot. Under this condition, the movable cylinder 22 of the robot is moved downwardly in the direction of the arrow 23 and is then moved in the direction of the arrow 24. As a result, the insert 21 is inserted into a stripper plate 26 provided for a metal mold 25 while being guided by the stripper plate 26 functioning as a guide member, and is fixedly held (positioned) therein. Thus, the displacement of the insert is prevented. In this operation, the insert 21 is more firmly held therein because it is held by a vacuum cylinder 37 provided in the metal mold 25. The vacuum cylinder 37 has a through-hole 38 which is connected through a pipe 39 to a vacuum pump (not shown) so that the insert 21 is thereby attracted towards the metal mold 25. The robot's movable cylinder 22 is moved in the direction of the arrow 28 and is then lifted in the direction of the arrow 29 before the mold closing operation is carried out. Fig. 12 shows the start of the mold closing operation, and Fig. 13 the end of the mold closing operation. In the mold closing operation, the vacuum cylinder 37 is slid in a cylinder housing 41 in the direction of the arrow 40 as shown in Fig. 12; that is, the vacuum cylinder 37 moves into the metal mold 25 as shown in Fig. 13. In Figs. 12 and 13, the through-hole 38 is not shown. Upon completion of the mold closing operation, molding material is injected through the molding material supplying path 31 and the injection gate 30 of the metal mold 32 into the cavity 34 which is formed by the metal molds. The other steps are the same as those in the above-described embodiment. In the above-described embodiments, the insert is larger than the hole of the guide member. However, in a case where the insert is smaller than the hole of the guide member, the insert can be fixedly held by employing at least one rib extending from the inner periphery of the guide member. Fig. 14 is a front view of a stripper plate 26 with ribs 42. The angle (draft) of the inner wall 27 of the stripper plate 26 functioning as a guide member, which is brought into contact with the insert 21, may be suitably controlled. This will be described with reference to Fig. 15 which is an enlarged side view of the stripper plate 26. In Fig. 15, the angle (x/y) is adjusted so that a large insert can be smoothly inserted into the stripper plate and fixedly held therein. In order to clarify the effects of the invention, examples thereof will be described. Example 1 The bottom of a can-like container was molded by using an injection molding machine IS-50A(30Z) manufactured by Toshiba Co. of Japan. The insert was fabricated by forming resin layers of ethylene propylene block copolymer (MFR 1.1 g/10 min, ethylene content 9 wt%) on both sides of an aluminum layer. The thicknesses of the (outer) resin layer, the aluminum layer and the (inner) resin layer were 30 microns, 15 microns and 70 microns, respectively. The insert was inserted into the stripper plate of the injection molding machine and fixedly held therein by means of the robot. After the mold closing operation, propylene ethylene block copolymer of MFR 10.0 g/10 min was injected into the mold cavity. The molding conditions were as follows: Cylinder temperature -- 260°C Injection pressure: Primary pressure -- 60 kg/cm² Secondary pressure -- 40 kg/cm² Injection time -- 3 seconds Cooling time -- 8 seconds Molding cycle (total) -- 15 seconds Injected resin thickness -- 0.7 mm The insertion accuracy of the base material (insert) and the injected resin layer of the can-like container was 0.3 to 0.5 mm in the longitudinal direction and 0.3 to 0.5 mm in the lateral direction. Example 2 The rectangular cover shown in Figs. 1 and 4 was made in the same manner as in Example 1 with the exception that, instead of the propylene ethylene block copolymer of MFR 10.0 g/10 min, a material prepared by adding calcium carbide 40% to that resin material was used. The insertion accuracy of the rectangular cover thus constructed was similar to that of Example 1. Example 3 The round cover shown in Figs. 2 and 5 was molded in the same manner as that of Example 1. The insertion accuracy of the round cover thus molded was similar to that of Example 1. As is apparent from the above description, even if the insert is obliquely moved into the metal mold by the robot, it is accurately set in place because it is guided by the guide member such as a stripper plate, and it is fixedly held in the metal mold by the guide member and, when desired, with the aid of the vacuum cylinder. Therefore, the displacement of the insert during the molding operation is prevented. Accordingly, the insert molding produced has a high insertion accuracy and low manufacturing cost. Furthermore, as the molding operation can be achieved at high speed, the method of the invention attains high productivity. Moreover, in the above-described embodiments, the insert is inserted into the metal mold by the robot; however, the insertion may be achieved by other means, for instance, guide rails. Furthermore, in the above-described embodiments, a two-plate metal mold assembly is used; however, the technical concept of the invention is applicable to a three-plate metal mold assembly, and other metal mold assemblies. In addition, the core mold and the cavity mold may be reversed. Fig. 16 is an enlarged plan view of the rectangular cover in Fig. 1. Fig. 17 is an enlarged sectional view taken along line X-X' in Fig. 16. In Figs. 16 and 17, reference numerals 47, 48 and 49 designate second resin layers. The second resin layer 48 is distinguished from the second resin layers 47 and 49 by a groove 40. In Fig. 16, reference numeral 40' designates a bridge. A handle (or pull-top) 11 is formed in the second resin layer 48. In Figs. 16 and 17, reference numeral 42 designates an inner groove to facilitate the lifting of the handle 11. No second resin layer is formed in the inner groove 42 and in the groove 40, i.e., a multi-layer sheet 43 is exposed therein. These grooves are formed by a metal mold so that the cover can be readily opened. In the embodiment shown in Fig. 17, the multi-layer sheet 43 is manufactured by forming resin layers on both sides of a gas barrier type base material. In Fig. 17, reference numeral 44 designates a treatment layer to facilitate the peeling of the handle. The treatment layer 44 may be formed over all the area of the handle or a part of the area; however, it is preferable that it be formed on the entire rear surface of the handle. When the handle 11 is lifted, the handle 11 is readily peeled off the multi-layer sheet 43 while going inside at the position 45 to cut the multi-layer sheet 43 so that the cover is readily opened along the groove 40. In Fig. 4, reference numeral 46 designates two opposed parts of the side wall. Fig. 18 is an enlarged plan view of the round cover shown in Fig. 2. Fig. 19 is an enlarged sectional view taken along line Y-Y' in Fig. 18. In Figs. 18 and 19, those parts having the same functions as those in Figs. 16 and 17 are therefore designated by the same reference numerals. As in the rectangular cover shown in Figs. 16 and 17, the handle 11 can be readily opened. In order to clarify the effects of these embodiments of invention, specific examples thereof will be described. Example 4 A multi-layer sheet having a structure and a thickness as indicated in Table 1, which was coated with rubber series gravure printing ink, was inserted into a stripper plate provided for a metal mold and fixedly held therein by means of a robot. Under this condition, the rectangular cover shown in Fig. 16 was molded by injection molding. An injection molding machine IS-50A(30Z) made by Toshiba Co. was used. The injection molding resin material used was ethylene propylene block copolymer ("Shoaroma" MK 411C (made by Showa Denko Co.) MFR 10.0 g/10 min.). The molding conditions were as follows: Comparison Example 1 A rectangular cover was manufactured in the same manner as that of Example 4 with the exception that a multi-layer sheet not coated with the printing ink was used. Example 5 The rectangular cover shown in Fig. 16 was manufactured by compression forming. That is, an oil pressure 50-ton press made by Amino Press Co. of Japan was used. An insert prepared by applying rubber series gravure printing ink to aluminum foil having ethylene propylene block copolymer (MFR. 1.1, ethylene content 9 wt%) layers on both sides was inserted into the mold in advance. Under this condition, the compression forming was carried out by supplying a heated resin sheet of ethylene propylene block copolymer ("Shoaroma"), MFR 0.7 g/10 min). Comparison Example 2 A rectangular cover was made by compression forming in the same manner as that in Example 5 with the exception that a multi-layer sheet not coated with the printing ink was used. Examples 6 through 15 Container covers were manufactured by injection molding or compression forming with multi-layer sheets and printing ink treatment (treatment layers) as shown in Table 1. Comparison Examples 3 through 7 Container covers were formed with multi-layer sheets as shown in Table 1. These Comparison Examples 3 through 7 correspond to Examples 6 through 10 in which no peeling treatment was employed. As is apparent from the results of the above-described Examples and Comparison Examples, the printing treatment or the equivalent treatment applied to the multi-layer sheet facilitates the opening of the cover with the handle. The container cover of the invention can be manufactured at relatively low cost, and has the following excellent characteristics: (1) As the printing treatment is applied to the multi-layer sheet which is in contact with the container cover handle or to the part of the multi-layer sheet where the latter is in contact with the handle, or the treatment layer such as a paper seal is provided in order to allow the handle to peel off the multi-layer sheet, the handle can be readily peeled off the multi-layer sheet, and accordingly the cover can be readily opened. (2) The second layer is formed on the multi-layer sheet by insertion forming in such a manner that the former is welded to the latter by heat. Therefore, the second layer has a high adhesion strength and cannot be peeled off by heat cycling, for instance, in a retort treatment. (3) The second resin layer and the thermally-weldable resin layer (or the multi-layer sheet obtained by laminating the first resin layers) are laminated. Therefore, the cover can be reaily welded by heating and has a retort characteristic. Accordingly, containers excellent from a food hygiene point of view can be made with the covers according to the invention.

Claims: Injection molding method for manufacturing a readily openable can-like container cover (3), in which an insert (21) of multilayers is inserted into a metal mold assembly in advance, and molten molding material is injected to said insert (21) having a flat shape and such a size corresponding to said cover (3), characterized in that said insert (21) is inserted into a guide member (26, 36) provided for one of at least two metal molds (25, 32) which form said metal mold assembly and is fixedly held therein before a molten molding material injection is carried out. The method as claimed in claim 1, wherein said guide member (26; 36) comprises a stripper plate (26). The method as claimed in claim 1, wherein said insert (21) is inserted into said guide member (36) and fixedly held therein, and said guide member (36) is used to remove a molding from said metal mold assembly. The method as claimed in claim 1, wherein said insert (21) is fixedly held by said guide member. The method as claimed in claim 1, wherein said insert (21) is fixedly held by said guide member (36) and by suction means (37). The method as claimed in claim 1, wherein said insert (21) is inserted and fixedly held by a stripper plate (26) having ribs (42). The method as claimed in claim 1, wherein said insert (21) is inserted and fixedly held by a guide member (36) having a draft adjusted which abuts against said insert (21).

Inventors: MIYAUCHI, OTOHIKO, SUZUKI, TAKASHI, YASUDA, TETSUO, YOTSUYANAGI, JUNJI
Applicants: SHOWA DENKO KABUSHIKI KAISHA
Application year: 1984
Main classification: B29C39
IPC codes: B29C45