Patent Publication Number: US-7588809-B2

Title: Hollow body of synthetic resin, method and mold for injection molding of the hollow body

Description:
This is a divisional of application Ser. No. 10/764,601 filed Jan. 27, 2004 now U.S. Pat. No. 7,147,816, the entire disclosure of the prior application, application Ser. No. 10/764,601 is hereby incorporated by reference. 
   The invention is based on Japanese Patent Application No. 2003-16954, the entire contents of which are incorporated herein by reference. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   This invention relates to a hollow body of a synthetic resin, a method for its injection molding and a mold for its molding, and more specifically, to a hollow body of a synthetic resin made by joining a pair of semi-hollow bodies along a joining space, a method for molding such a hollow body of a synthetic resin by primary and second injection molding and a mold used for carrying out the molding method. More particularly, it relates to a hollow body of a synthetic resin which is suitable for a gasoline tank to be mounted in a motor vehicle, a method for its injection molding and a mold for its molding. 
   2. Related Art 
   A molding method using an injection molding machine is known as one of the methods for molding a hollow body, or a hollow molded product of a synthetic resin. A mold used for carrying out such a method for injection molding is generally composed of a stationary mold and a slidable mold as set forth in, for example, Japanese Patent Publications Nos. JP-B-2-38377, JP-A-6-23789 and JP-A-6-246781. One primary semi-hollow body is formed in the stationary mold and the other primary semi-hollow body in the slidable mold. In primary molding, therefore, the slidable mold is set in the first position and a pair of primary semi-hollow bodies are so molded that each may have a butting or joining portion about its open end, and after they have solidified, the slidable mold is slid to the second position so that the primary semi-hollow bodies in pair may but each other at their butting portions. In secondary molding, a molten resin is injected into the joining space and the primary semi-hollow bodies in pair are joined to each other at their butting portions to form a hollow body of a synthetic resin. 
   The injection molding method as described above has the advantage of making it possible to automate the steps and produce a large quantity of hollow bodies, since a hollow body can be obtained by butting a pair of primary semi-hollow bodies formed by primary molding, and injecting a molten resin to fill the butted portion by secondary molding. The injection molding of a pair of primary semi-hollow bodies has features including the possibility of manufacturing even a hollow body having a complicated shape. It is, however, likely that the joined portion may be low in strength.  FIGS. 6A  though  6 C are sectional views showing a part of an example of a hollow body obtained by the injection molding method as described above, and reference is made to  FIG. 6A  for the explanation of the reason for low joining strength. In  FIG. 6A , A′ denotes a first primary semi-hollow body and B′ a second primary semi-hollow body, and the first and second primary semi-hollow bodies A′ and B′ are united together by the molten resin injected for secondary molding to fill the joining space S′. The joining space S′ is defined about the butt ends T′ of the open ends of the first and second primary semi-hollow bodies A′ and B′ and when the molten resin for secondary molding is injected to fill the joining space S′, therefore, the first and second primary semi-hollow bodies A′ and B′ are joined by the joining space S′ alone and the notch portions, or the butt ends T′ are not welded together. Accordingly, the welding force is low. It may be thought of to reduce the thickness of the butt ends T′, but as a low injection pressure for the injection of the molten resin for secondary molding into the joining space S′ gives a low welding or bonding force, it is necessary to inject it with a reasonably high injection pressure. Its injection with an injection pressure required for welding causes the deformation of the thin butt ends T′ and the leakage of the molten resin into the inside. For these reasons, the butt ends T′ have a specific thickness. 
   While there is no problem if only a compressive force acts between the butt ends T′ not joined together, the action of, for example, an external tensile force causes the separation of the butt ends T′ from inside. Then, cracking starts at the separated portion and extends to the joining space S′, as is generally the case with the destruction of a material. For these reasons, the structure as described above is unsuitable for any use calling for joining strength, for example, a gasoline tank mounted in a motor vehicle for which even an accident has to be anticipated. 
   Therefore, various attempts have so far been made to realize an elevation of joining strength. For example, JP-A-10-16064 proposes a joint shaped as shown in  FIG. 6B . In this joint, the butt ends T′ of first and second primary semi-hollow bodies A′ and B′ are formed in an inclined way from a corner of a joining space S′. The joint proposed by JP-A-6-246781 is made by flanges formed integrally at the open ends of first and second primary semi-hollow bodies A 7  and B′ during primary molding and connected with a resin R′ during secondary molding, as shown in  FIG. 6C . 
   The butt ends T′ of the joint shown in  FIG. 6B  are more resistant to deformation by the pressure of secondary molding, since they are formed in an inclined way from the corner of the joining space S′. The degree to which the pressure of secondary molding acts upon the butt ends T′ is lower, since they are formed at the corner. As the butt ends T′ are not joined to each other, however, a tensile force acting upon that region causes the separation of the butt ends T′ and allows cracking to extend easily to the joining space S′, as explained with reference to  FIG. 6A . In other words, the shape of the joint still fails to solve the problems explained with reference to  FIG. 6A . On the other hand, the first and second primary semi-hollow bodies A′ and B′ shown in  FIG. 6C  can be said to have a high joining strength, since their flanges are joined to each other by the secondary molding resin R′ like a rivet. A third mold is, however, specifically required for secondary molding and is likely to add to the cost. Moreover, the outward appearance is not always a desirable shape. 
   SUMMARY OF THE INVENTION 
   This invention is aimed at providing a hollow body of a synthetic resin, a method for its injection molding and a mold for its molding which overcome the problems as stated above, and more specifically providing a hollow body of a synthetic resin obtained by joining a pair of primary semi-hollow bodies together and having their joint which is unlikely to crack easily by the action of not only a compressive force, but also a tensile force, and an injection molding method and a mold which enable such a hollow body of a synthetic resin to be formed at a low cost. 
   (1) In order to attain the object stated above, the invention provides a hollow body of a synthetic resin made by butting the open ends of a pair of primary semi-hollow bodies formed by primary injection molding, whereby a joining space is defined about the butt ends, and injecting a molten resin by secondary injection molding to unite the pair of primary semi-hollow bodies into a single body, the molten resin for the secondary molding filling the space between the butt ends. 
   (2) The invention may provides a hollow body of a synthetic resin made by butting the butt ends formed along inner peripheral sides of the open ends of a pair of primary semi-hollow bodies formed by primary injection molding, whereby a joining space is defined about them, and injecting a molten resin by secondary injection molding to unite the pair of primary semi-hollow bodies into a single body, the molten resin for the secondary molding filling the space between the butt ends. 
   (3) The invention may be so arranged that when a hollow body is obtained by primary injection molding for injecting a molten resin into a cavity defined by a slidable mold and a movable mold to form a pair of primary semi-hollow bodies so that the butting of their open ends may define a joining space about the butt ends, and secondary injection molding for uniting the pair of primary semi-hollow bodies into a single body by injecting a molten resin into the joining space defined about the butt ends of the open ends of the pair of primary semi-hollow bodies butted by sliding the slidable mold relative to the movable mold after the primary injection molding, the movable mold is opened to a specific degree during the secondary injection molding to fill the space between the butt ends with the resin for the secondary injection molding, while filling the joining space with it. 
   (4) The invention may be so arranged that when a hollow body is obtained by primary injection molding for injecting a molten resin into a cavity defined by a slidable mold and a movable mold to form a pair of primary semi-hollow bodies so that the butting of the butt ends defined along the inner peripheral sides of their open ends may define a joining space about the butt ends, and secondary injection molding for uniting the pair of primary semi-hollow bodies into a single body by injecting a molten resin into the joining space defined about the butt ends of the open ends of the pair of primary semi-hollow bodies butted by sliding the slidable mold relative to the movable mold after the primary injection molding, the movable mold is opened to a specific degree during the secondary injection molding to fill the space between the butt ends with the resin for the secondary injection molding, while filling the joining space with it. 
   (5) In the invention, the movable mold may be opened to a specific degree by the filling pressure of the resin for the secondary injection molding to fill the space between the butt ends. 
   (6) In the invention, the molten resin filling the joining space and the space between the butt ends may be compressed again after the space between the butt ends is filled. 
   (7) The invention may provides a mold comprising a slidable mold and a movable mold, each having on the side of their parting line a cavity for molding the main portion of one of at least a pair of primary semi-hollow bodies and a cavity for molding a joining portion at the open end of the main portion, the movement of the slidable mold to a first position for closing the molds and for primary molding allowing the cavities to form a pair of primary semi-hollow bodies each having a butt end and a part of a joining space at its open end, the movement of the slidable mold to a second position for closing the molds with the pair of primary semi-hollow bodies held therein allowing the butting of the butt ends of the open ends of the pair of primary semi-hollow bodies to define the joining space about them, the pair of primary semi-hollow bodies being united into a single body by filling the joining space by injection with a molten resin for secondary molding, wherein the movable mold can be opened to a specific degree during the secondary molding in the second position. 
   (8) The invention may provides a mold comprising a slidable mold and a movable mold, each having on the side of their parting line a cavity for molding the main portion of one of at least a pair of primary semi-hollow bodies and a cavity for molding a joining portion at the open end of the main portion, the movement of the slidable mold to a first position for closing the molds and for primary molding allowing the cavities to form a pair of primary semi-hollow bodies each having a butt end and a part of a joining space at its open end, the movement of the slidable mold to a second position for closing the molds with the pair of primary semi-hollow bodies held therein allowing the butting of the butt ends of the open ends of the pair of primary semi-hollow bodies to define the joining space about them, the pair of primary semi-hollow bodies being united into a single body by filling the joining space by injection with a molten resin for secondary molding, wherein the movable mold can be opened to a specific degree in the second position by the filling pressure of the molten resin for the secondary molding. 
   (9) In the invention, the mold opened to a specific degree in the second position can be closed again. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIGS. 1A through 1B  are views showing various stages of molding of a hollow body of a synthetic resin according to this invention, or cross sectional views showing a pair of primary semi-hollow bodies;  FIG. 1A  is in the position in which a pair of primary semi-hollow bodies are butted against each other,  FIG. 1B  is in the position in which the joining space between the primary semi-hollow bodies is filled with a secondary molding resin,  FIG. 1C  is in the position in which the space between the butt ends of the primary semi-hollow bodies is filled with the resin, and  FIG. 1D  is in the position in which the resin filling the joining space between the primary semi-hollow bodies and the space between their butt ends is compressed. 
       FIGS. 2A  though  2 C are views showing a mode of embodying a mold according to this invention, or  FIG. 2A  is a cross sectional view of the whole,  FIG. 2B  is an enlarged cross sectional view of the portion marked X in  FIG. 2A , and  FIG. 2C  is an enlarged cross sectional view of the portion marked Y in  FIG. 2A . 
       FIGS. 3A through 3B  are views showing stages of molding of a hollow body of a synthetic resin according to this invention, or  FIG. 3A  is a cross sectional view showing the stage in which primary molding has been finished, and  FIG. 3B  is a cross sectional view showing the stage in which the joining space has been filled with a molten resin for secondary molding. 
       FIGS. 4A and 4B  are views showing stages of molding of a hollow body of a synthetic resin according to this invention, or  FIG. 4A  is a cross sectional view showing the stage in which the movable mold has been opened and the joining space and the space between the butt ends have been filled with a molten resin for secondary molding, and  FIG. 4B  is a cross sectional view showing the stage in which the movable mold has been tightly closed again and the molten resin in the joining space and between the butt ends has been compressed. 
       FIGS. 5A  though  5 D are views showing a part of other modes of embodying a joint on a hollow body of a synthetic resin according to this invention, or  FIG. 5A  is a cross sectional view showing a first other mode,  FIG. 5B  is a cross sectional view showing the state in which its joining space is filled with a resin,  FIG. 5C  is a cross sectional view showing a second other mode, and  FIG. 5D  is a cross sectional view showing the state in which its joining space is filled with a resin. 
       FIGS. 6A  though  6 C are views showing known examples of joints on a hollow body of a synthetic resin, or cross sectional views showing different known examples. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The molding principle of a hollow body of a synthetic resin according a mode of carrying out the invention will first be described with reference to  FIGS. 1A  though  1 D. A first and a second primary semi-hollow body A and B are formed by primary injection molding. At the same time, outwardly projecting flanges Fa and Fb are integrally formed at the open ends of the first and second primary semi-hollow bodies A and B. While they stay in the mold, the mold is so closed that the notch portion, or butt ends T at their open ends may butt each other, as shown in  FIG. 1A . As a result, a joining space S is defined about the butt ends T. A molten resin for secondary molding is injected at a specific pressure to fill the joining space S. The resin as injected is shown in  FIG. 1B . Then, the mold is opened to a specific degree, for example, by 0.5 to 5.0 mm. The pressure of the molten resin being injected causes a part of the molten resin to fill the space between the butt ends T, as shown in  FIG. 1C . After cooling and solidification, the mold is opened to yield a hollow body of a synthetic resin joined along the joining space S and the butt ends T. 
   Alternatively, the mold is closed before the solidification of the molten resin after the molten resin for secondary molding has filled the space between the butt ends T, as shown in  FIG. 1C . As a result, the resin in the joining space S and between the butt ends T is compressed and a hollow body of a synthetic resin having a still higher joining strength can be obtained, though a part of the resin may leak to the inside, as shown in  FIG. 1D . 
   A hollow body of a synthetic resin can also be obtained as will now be described. The mold is so closed for the secondary molding as to be opened to a specific degree by a specific resin pressure. For example, the mold is closed for the secondary molding by means of a fluid pressure, or a spring, or other elastic material. Then, the secondary molding is started as described before. The molten resin for the secondary molding is injected to fill the joining space and when its pressure has risen to a specific level, the mold is opened to a specific degree by resisting the elastic material. As a result, the space between the butt ends is also opened to the specific degree and filled. After cooling and solidification, the mold is opened to yield a hollow body of a synthetic resin joined between the butt ends T, too. The resin in the joining space S and between the butt ends T is compressed after filling, as stated before, if required. 
   In order to obtain a hollow body of a synthetic resin as described above, it is desirable to carry out secondary molding while holding a pair of first and second primary semi-hollow bodies in the molds in which they have respectively been formed, it is also possible to form primary semi-hollow bodies in separate molds, insert them in a mold for secondary molding and fill the space between the butt ends, too, with the secondary molding resin as described above. While the molten resin for secondary molding is injected into the joining space through a runner and a gate, the runner may be a ring runner, or ring passage for secondary molding surrounding the joining space, and the gate may be composed of a plurality of gates situated at specific intervals. It may alternatively be a disk gate keeping continuous communication between the ring runner and the joining space. 
   More specific modes of carrying out the invention will now be described by way of a mold for forming a hollow body and a molding method employing it. The mold embodying this invention is shown in  FIG. 2A  and is composed of a stationary plate  1  situated on the right-hand side of the figure, a stationary mold  5  secured to the stationary plate  1 , a slidable mold  10  which is driven for sliding vertically in the figure, a movable mold  30  attached to a movable plate  28  by a movable mold plate  29 , etc. 
   The stationary plate  1  has an opening  3  made across it substantially in its central axial portion for having an injector nozzle  2  inserted therein. The stationary mold  5  has a main sprue  6  aligned with the opening  3 . An apparatus for driving the slidable mold  10  vertically is composed of a piston-cylinder unit  11  which includes a piston rod  12  having an end secured to the top of the slidable mold  10  and a cylinder having a bottom connected to the top of the stationary mold  5  by a horizontally extending arm  13  and a supporting post  14 . Thus, when a working fluid is supplied to and discharged from the piston-cylinder unit  11 , the slidable mold  10  is driven for sliding vertically of the stationary mold  5  between a first and a second position. 
   On its side facing the stationary mold  5 , the slidable mold  10  driven for sliding as described above has a primary runner  15  for primary molding connected with the main sprue  6 . A first and a second primary sprue  16  and  17  for primary molding extend from the primary runner  15  across the slidable mold  10  and vertically symmetrically of the main sprue  6  as viewed in the figure. It also has a secondary runner  18  for secondary molding formed below the primary runner  15  on its side facing the stationary mold  5 . A secondary sprue  19  for secondary molding formed across the slidable mold  10  is connected with the secondary runner  18 . 
   The slidable mold  10  has a specifically sized slidable core  20  formed in its upper portion, protruding outwardly from a parting line P and having a square shape in side elevation, as shown in  FIGS. 2A and 2B . The first primary sprue  16  stated before is open in the top of the slidable core  20  via a gate. The slidable core  20  is surrounded by a small annular core  21  of relatively small height spaced from the core  20  by a specific distance equal to the wall thickness of a first primary semi-hollow body. The slidable core  20  cooperates with the movable mold  30  in defining a cavity for molding a first primary semi-hollow body as will later be described in detail, while the small annular core  21  cooperates with the movable mold  30  in defining a cavity for molding the butt end of the first primary semi-hollow body and a half of a joining space. 
   The slidable mold  10  has a specifically sized slidable concavity  22  formed in its lower portion as viewed in  FIG. 2A , open on the side of the parting line P and having a square shape in side elevation. The second primary sprue  17  is open in the bottom of the slidable concavity  22  via a gate. The slidable concavity  22  is surrounded by a small annular concavity  23  of relatively small depth spaced from the slidable concavity  22  by a specific distance equal to the wall thickness of a second primary semi-hollow body, as shown enlarged in  FIG. 2C . The small concavity  23  is surrounded by a secondary ring passage  24  connected with the secondary sprue  19 . The secondary ring passage  24  is connected with a secondary gate  25  composed of a plurality of gates or film gates. The secondary gate  25  is open in a side of a small core  34  on the movable mold  30  as will be described. The slidable concavity  22  cooperates with the movable mold  30  in defining a second cavity for molding the main portion of a second primary semi-hollow body. The small concavity  23  and the small core on the movable mold define a cavity for molding the butt end of the second primary semi-hollow body and a half of the joining space. 
   The movable mold  30  has a specifically sized movable concavity  31  formed in its upper portion as viewed in  FIG. 2A , open on the side of the parting line P and having a square shape in side elevation. The movable concavity  31  and the slidable core  20  described above define the first cavity for molding the first primary semi-hollow body. The movable concavity  31  is surrounded by a small concavity  32  of relatively small depth having a specific distance from the movable concavity  31 , as shown in  FIG. 2B . The small concavity  32  and the small core  21  on the slidable core  10  described above define the cavity for molding the butt end of the first primary semi-hollow body and a half of the joining space. 
   The movable mold  30  has a specifically sized movable core  33  formed in its lower portion, protruding outwardly from the parting line P and having a square shape in side elevation. The movable core  33  is surrounded by a small core  34  of relatively small height having a specific distance from the core  33 , as shown enlarged in  FIG. 2C . The movable core  33  and the slidable concavity  22  cooperate in defining the cavity for molding the main portion of the second primary semi-hollow body and the small core  34  and the small concavity  23  of the slidable mold  10  define the cavity for molding the butt end of the second primary semi-hollow body and a half of the joining space. 
   Description will now be made of a molding method employing the slidable mold  10 , movable mold  30 , etc. as described above, for forming a first and a second primary semi-hollow body by primary injection molding and for forming a hollow body by injecting a molten resin into a joining portion at their open ends for secondary molding. The movable mold  30  can be opened to a specific degree, for example, by 0.5 to 5.0 mm during secondary injection molding, and closed again, though no mold clamping device is shown in the drawings. 
   The slidable mold  10  is slid to its first position as shown in  FIG. 2A  to close the mold. A first cavity C 1  in which the main portion of a first primary semi-hollow body is molded is defined by the slidable core  20  of the slidable mold  10  and the movable concavity  31  of the movable mold  30 , and a small cavity c 1  in which a joining portion is formed is defined about it. Likewise, a second cavity C 2  in which the main portion of a second primary semi-hollow body is molded is defined by the slidable concavity  22  of the slidable mold  10  and the movable core  33  of the movable mold  30 , and a small cavity c 2  in which the joining portion is formed is defined about it. 
   A molten resin for primary molding is injected through the injector nozzle  2  in a known way. The molten resin flows through the main sprue  6  in the stationary plate  1 , is distributed by the primary runner  15  and flows through the first and second primary sprues  16  and  17  and the gates to be injected into and fill the first and second cavities C 1  and C 2 . The first and second primary semi-hollow bodies A and B as molded upon injection and filling are shown in  FIG. 3A . The primary molding forms a flange Fa or Fb, a butt end T and a half of a joining space S at the open end of each of the first and second primary semi-hollow bodies A and B, as shown in  FIGS. 1A through 1D . 
   After a certain degree of cooling and solidification, the movable mold  30  is opened by a specific distance. Then, the slidable mold  10  is slid upward with the second primary semi-hollow body B left therein to its second position in which the open end of the second primary semi-hollow body B is aligned with that of the first primary semi-hollow body A. Then, the mold is closed. As a result, the butt ends T of the first and second primary semi-hollow bodies A and B abut on each other as shown in  FIGS. 3B and 1A , and the joining space S is defined by the outer surfaces of the butt ends, the inner surfaces of the flanges Fa and Fb, the wall surface of the small concavity  23  of the slidable mold  10  and the wall surface of the small concavity  32  of the movable mold  30 . The mold as closed in its second position is shown in  FIG. 3B . When the mold is closed in its second position, the main sprue  6  is connected with the secondary runner  18 . Then, a molten resin for secondary molding is injected. The molten resin flows through the main sprue  6 , secondary runner  18  and secondary sprue  19  into the secondary ring passage  24 . Then, it flows through the secondary gate  25  and fills the joining space S. During its filling, the movable mold  30  is opened to a specific degree. As a result, the butt ends T are opened to a specific degree, as the movable mold  30  is opened while holding the first primary semi-hollow body A therein. The pressure of the molten resin for secondary molding causes it to reach between the butt ends T. Thus, it fills the space between the butt ends T, too. The space between the butt ends T as filled with the resin is shown in  FIG. 4A . After its cooling and solidification, the movable mold is opened, whereupon a hollow body is ejected by an ejector pin not shown. 
   Alternatively, the movable mold  30  is tightly closed again as shown in  FIG. 4B  before the solidification of the resin filling the space between the butt ends T, too. As a result, the resin in the joining space S and between the butt ends T is compressed to yield a hollow body having a still higher joining strength. 
   This invention is not limited to the modes of carrying it out as described above, but can be carried out in various other forms, too. For example, it is obvious that the shapes of the joining space and the butt ends of the first and second primary semi-hollow bodies, etc. are not limited to the modes as described above, but can be realized in other forms, too. Other forms of butt ends are shown in  FIGS. 5A  though  5 D.  FIG. 5A  shows butt ends T having a zigzag cross section and the zigzag form gives a higher joining strength owing to a longer resin filling, or joining distance than a straight line, as shown in  FIG. 5B . If a ring-shaped cut K is formed along the inner edges of the butt ends T as shown in  FIG. 5C , the cut K traps any excessive resin occurring from the tight closure of the mold, as shown in  FIG. 5D , when the mold is tightly closed again after the space between the butt ends T is filled with the molten resin for secondary molding. 
   Although the modes as described before have been such that the movable mold is opened to a specific degree during secondary molding, or is opened to a specific degree by the pressure of the secondary molding resin to allow the molten resin to fill the space between the butt ends T, too, it is also possible to carry out the invention in such a way that the movable mold is opened to a specific degree before the injection of the secondary molding resin to allow the molten resin to fill the space between the butt ends T. In this connection, it is also possible to close the movable mold tightly before the solidification of the molten resin filling the space to compress the secondary molding resin. 
   The hollow body of a synthetic resin according to this invention is a hollow body made by butting the open ends of a pair of primary semi-hollow bodies formed by primary injection molding, or the butt ends formed along the inner peripheral sides of the open ends thereof, whereby a joining space is defined about the butt ends, and injecting a molten resin by secondary injection molding to unite the pair of primary semi-hollow bodies into a single body, and as the pair of primary semi-hollow bodies are joined together along the butt ends and the joining space surrounding them, no cracking occurs to the joint even if not only a compressive force, but also an external tensile force may act upon the joint, as explained before. In other words, the hollow body of a synthetic resin according to this invention can withstand both internal and external pressure. 
   According to another aspect of the invention, it is possible to obtain a hollow body of a synthetic resin which can withstand both internal and external pressure, since when a hollow body is obtained by primary injection molding for injecting a molten resin into a cavity defined by a slidable mold and a movable mold to form a pair of primary semi-hollow bodies so that the butting of their open ends, or the butting of the butt ends defined along the inner peripheral sides of their open ends may define a joining space about the butt ends, and secondary injection molding for uniting the pair of primary semi-hollow bodies into a single body by injecting a molten resin into the joining space defined about the butt ends of the open ends of the pair of primary semi-hollow bodies butted by sliding the slidable mold relative to the movable mold after the primary injection molding, the movable mold is opened to a specific degree, or is opened to a specific degree by the filling pressure of the resin for the secondary injection molding to fill the space between the butt ends with the resin for the secondary injection molding, while filling the joining space with it, during the secondary injection molding. It is possible to mold such a hollow body of a synthetic resin having a high joining strength accurately, at a low cost and automatically by an injection molding method without employing any special technology. 
   According to still another aspect of the invention, it is possible to obtain a hollow body of a synthetic resin having a still higher joining strength, as the molten resin filling the joining space and the space between the butt ends is compressed again.