JP2013014020A - Resin-made bottle and method of molding the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To solve the technical problem of controlling local orientation deformation in the bottom of a blow-molded bottle and thinning down due to the deformation in producing a small and thick bottle by biaxial-orientation blow molding based on the hot parison method and thus provide a small and thick bottle having a high-quality appearance like that of glass bottles.SOLUTION: In the method of molding a resin-made bottle by biaxial-orientation blow molding which is based on the hot parison method and uses a bottomed cylindrical resin-made preform formed by injection molding, the preform is injection-molded by using an injection molding mold having a core mold in which a convex portion is arranged in the center of the tip portion corresponding to the center of the inner peripheral surface of the bottom of the preform.

Description

The present invention relates to a synthetic resin casing by biaxial stretch blow molding, and more particularly to a casing having a thick bottom portion and a molding method thereof.

Conventionally, a biaxially stretched blow molded casing made of polyethylene terephthalate (hereinafter referred to as PET) resin has excellent transparency, mechanical strength, heat resistance, gas barrier properties, etc., and is widely used as containers for various beverages. However, the use as a substitute for small glass bottles for cosmetics is also progressing.
And in the case as a substitute of this kind of glass bottle, it is calculated | required that a surrounding wall, especially a bottom part should be thickly formed so that the texture of a glass bottle may appear.
For example, in Patent Document 1, a test tubular preform having a cylindrical cylindrical portion made of PET resin is biaxially stretch blow-molded, and is a small, thick-walled round intended for cosmetic use. An invention relating to a shape body is described.

  In addition, with respect to a rectangular casing having a small, thick-walled body having a rectangular plane cross-sectional shape, the peripheral wall of the test tubular preform is locally stretched and deformed (hereinafter, simply referred to as partial stretching in some cases). However, there is a problem that the wall thickness of the peripheral wall, particularly the corner wall of the trunk part and the corner part of the bottom part becomes thin, and such a problem has been considered by the inventors of the present invention. The problem is being solved by using a preform having a bottom rectangular tube shape.

  Here, the biaxial stretch blow molding method includes a hot parison method and a cold parison method. The hot parison method is a method in which the preform maintains the preheating of the injection molding and continuously moves to the blow molding process. The parison method is a method in which a preform injection molding process and a blow molding process are separated, and the injection molded preform is once cooled and then reheated for blow molding.

In Cited Document 1, a thick region is formed in a part of the bottom of the preform, and the hot parison method is biaxially stretch blow-molded to locally maintain the temperature of the thick region at a relatively high temperature. Description that the shape of the inner surface of the bottom wall is formed into a dish shape, that is, a concave shape as a whole, for example, and the pouring property of the remaining content liquid by the pouring pump can be improved. There is.

JP 2009-286457 A

The hot parison method is a method in which the preform continuously moves to the blow molding process while maintaining the preheating of the injection molding as described above. The partial stretchability of the portion that is formed and stretched and deformed at a higher temperature than the surroundings can be positively utilized.
In addition, among the studies of the inventors of the present invention, by adopting the hot parison method, as described above, a glass bottle-like small and thick rectangular casing using a bottomed rectangular cylindrical preform is used. When biaxial stretch blow molding is used, the thickness at the corners of the peripheral wall of the preform is increased to increase the stretch deformability of the corners, and the wall thickness at the peripheral wall of the trunk and the bottom wall of the casing is increased. It has been found that there is an advantage that can be more equalized.

  However, the hot parison method, on the other hand, has a thick region locally on the peripheral wall of the preform or a region where the progress of cooling after the injection molding of the preform is delayed for some reason. In the biaxial stretch blow molding, which is a so-called hot spot, the hot spot is unevenly stretched, and an unintended thin area is generated in the casing, and the overall thickness distribution is non-uniform. Have a problem.

FIG. 10 is a schematic explanatory view showing a main part of an injection molding apparatus used for injection molding of the test tubular preform 111 shown in FIG.
The mold 41 includes a cavity mold 42 and a core mold 43, and a nozzle portion 47 is fixed to the mold 41 via a heat insulating ring 49.
When the resin flow path is opened by moving the shut-off pin 48 upward (FIG. 10 shows a closed state), the molten resin injected from the tip of the nozzle portion 47 passes through the cavity 44 via the gate 45. The preform 111 is formed by filling.

  In the hot parison method, after cooling and solidifying in the mold 41 for an appropriate time, the mold is released while maintaining the preheating of the injection molding, and the gate portion 117 extended to the bottom 115 of the preform 111 is cut. Thus, the casing is formed by biaxial stretch blow molding in a continuous process.

Here, since the bottom portion 115 of the preform 111 is cooled with the gate portion 117 formed of a molten resin extending in the center of the outer peripheral surface as described above, the center portion of the bottom wall 116 is the hot spot HS. It becomes.
For this reason, when the bottom of the preform 111 is made thick so as to make the bottom of a small casing thick so that the texture of the glass bottle appears, the inside of the mold caused by the connecting gate part 117 described above is used. In combination with the delayed action of cooling, there is a problem that, in the initial stage of biaxial stretching, partial stretching occurs in the vicinity of the hot spot HS located at the center of the bottom, and the bottom is conversely thinned.

In the case of a transparent or semi-transparent container, when the bottom wall of the casing is thinned as described above, the appearance is visible from the outside, and further, the thickness is not uniform and the stretch deformation is not uniform. Since the resulting optical spots appear, clear transparency like a glass bottle is impaired, and the appearance of the product may be greatly deteriorated. This is a major problem particularly in containers for cosmetics and the like intended to differentiate products with high-quality packaging.
Furthermore, if the above-mentioned local stretching deformation is concentrated at the bottom, in the extreme case, the grounding function, which is one of the important functions of the bottom, is impaired, and the standing posture of the housing becomes unstable. There is also a fear.

The present invention has been made in view of the problems associated with a small and thick casing by biaxial stretch blow molding, which is associated with such a hot parison method, and in particular, local stretch deformation at the bottom of the blow molded casing. In addition, it is a technical object to suppress the thinning due to this deformation, and therefore, it is an object to provide a small and thick casing having a high-quality appearance like a glass bottle.

The present invention comprises an invention relating to a synthetic resin casing and an invention relating to a molding method thereof. Hereinafter, the invention relating to the molding method will be described first.
The main configuration according to the molding method of the present invention is a molding method of a synthetic resin casing by biaxial stretch blow molding by a hot parison method of a bottomed cylindrical synthetic resin preform by injection molding,
The preform is injection-molded using an injection mold having a core mold in which a convex portion is provided in the center of the tip portion corresponding to the center of the inner peripheral surface of the bottom of the preform. It is.

According to the above method, when the preform is injection-molded, by using the core mold in which the convex portion is provided in the center of the tip portion and arranged, the thickness of the central portion of the bottom portion of the preform is partially increased by the convex portion. Since the cooling of the part is promoted by the convex part of the core mold, the cooling in the mold due to the gate part extending to the bottom of the preform described above is possible. Can alleviate the delay,
In the biaxial stretch blow molding by the hot parison method, it is possible to effectively suppress the uneven stretching due to the hot spot at the bottom, and the wall thickness distribution of the bottom wall at the bottom of the housing is made evenly thick. It becomes possible.
In particular, this molding method is a small and thick casing like a glass bottle, and is effective for increasing the thickness of the bottom.

  Another configuration according to the molding method of the present invention is the above main configuration, wherein the diameter of the base end portion of the convex portion is substantially equal to the diameter of the gate of the injection mold located opposite to the tip portion of the core mold, It is said that the diameter is reduced toward the tip.

The shape of the protrusions protruding from the tip of the core mold, such as the diameter and protrusion height, is the cooling effect at the bottom of the preform, the effect of suppressing uneven stretching in biaxial stretch blow molding, and the injection of the preform Although it can be appropriately determined in consideration of formability and the like, the above configuration presents a standard shape of the convex portion.
By making the diameter of the base portion approximately equal to the diameter of the gate, the cooling delay action by the gate portion can be effectively suppressed.

In addition, when the preform is injection-molded, since the molten resin injected from the gate collides with the tip of the core mold, so-called jetting due to flow spots occurs at the bottom of the preform. By disposing the convex portion having the shape described above at the tip of the core mold, the flow behavior of the molten resin can be made smooth, and the occurrence of jetting can be effectively suppressed.
In particular, in the case of a transparent casing, since the state of the bottom portion is also seen from the outside, such jetting is poor in appearance, and its suppression is an important requirement for maintaining a high-quality appearance. .

  Another configuration relating to the molding method of the present invention is the above-described main configuration, wherein the shape of the preform is set up, and a cylindrical mouth portion is erected at the upper end of the cylindrical portion in which the flat cross-sectional shape of the outer peripheral surface of the peripheral wall is rectangular. It is said that it is set as the shape.

The above configuration is for providing a particularly small and thick rectangular casing, and the shape of the preform is cylindrical at the upper end of the cylindrical portion in which the planar cross-sectional shape of the outer peripheral surface of the peripheral wall is rectangular. By forming the mouth portion upright (hereinafter sometimes referred to as “rectangular cylindrical preform”), when viewed in a flat cross section, from the outer peripheral surface of the peripheral wall of the preform, the housing of the mold cavity The distance to the position corresponding to the square wall can be reduced, and local stretching deformation in the vicinity of the square wall can be mitigated.
Further, as described above, the local stretching deformation in the vicinity of the square wall can be alleviated, so that the remarkable local stretching in the corner portion formed from the lower end portion and the bottom wall of the trunk portion can be alleviated. Combined with the effect of suppressing the occurrence of spots, it is possible to suppress the thinning in the vicinity of the corner walls and corners, and to provide a small and thick rectangular casing that exhibits a high-quality appearance like a glass bottle. be able to.

Next, the synthetic resin casing of the present invention will be described. All of these are molded by the molding method described above.
The configuration of the invention relating to the synthetic resin casing of the present invention is such that the transverse stretch ratio from the body portion to the bottom wall is 2 times or less, the longitudinal stretch ratio is 1.2 times or less, and the bottom wall of the bottom portion is It is said to be formed with a thickness of 3.5 mm or more.

  In the molding method by the hot parison method of the present invention described above, the bottom wall of the bottom portion is formed by a small stretch ratio housing in which the transverse stretch ratio is 2 times or less and the longitudinal stretch ratio is 1.2 times or less. It is possible to provide a small casing having a thick bottom and formed to a thickness of 3.5 mm or more.

Another configuration of the invention relating to the synthetic resin casing of the present invention is due to the molding method using the rectangular tubular preform described above,
It has a rectangular cylindrical body extending from the tubular part of the preform in a similar manner, and a cylindrical mouth tube part is erected on the upper end of this body part through the shoulder, and the lower end is rectangular. It has a shape closed by a bottom wall of the shape,
The transverse stretch ratio from the body part to the bottom wall is 2 times or less, the longitudinal stretch ratio is 1.2 times or less, and the bottom wall of the bottom part is formed with a thickness of 3.5 mm or more. It is.

Still another configuration of the invention relating to the synthetic resin casing of the present invention is due to the molding method using the rectangular cylindrical preform described above,
It has a rectangular cylindrical body extending from the tubular part of the preform in a similar manner, and a cylindrical mouth tube part is erected on the upper end of this body part through the shoulder, and the lower end is rectangular. It is a shape that is closed by the bottom wall of the shape, and the wall thickness is bent from the center of the bottom wall to the lower end of the four corner walls of the body wall. The structure increases in the vicinity of the connecting corner portion, and the inner peripheral surface of the corner portion is rounded in a circular arc shape when viewed in a longitudinal section.

The above two configurations are related to a square housing made of synthetic resin, which suppresses thinning in the vicinity of a corner wall or a corner portion, and exhibits a high-quality appearance like a glass bottle, which has not been heretofore, A small and thick rectangular housing is provided.

The synthetic resin casing of the present invention and its molding method have the above-described configuration, but in the molding method of the present invention,
During injection molding of preforms, a hot spot is generated in the mold due to the gate part extending to the bottom of the preform by using a core mold that has a convex part in the center of the tip. Can be suppressed,
In biaxial stretch blow molding by the hot parison method, it is possible to effectively suppress uneven stretching caused by hot spots at the bottom, and to increase the thickness distribution of the bottom wall of the bottom of the housing evenly. It is a glass bottle-like small and thick casing that is extremely effective for increasing the thickness of the bottom.

It is a perspective view of one example of a synthetic resin casing of the present invention. It is a half vertical front view of the housing of FIG. FIG. 1A is a plan view, and FIG. 1B is a bottom view. FIG. 3 is a plan sectional view taken along line AA in FIG. 2. FIG. 4 is a half vertical section of the lower half region of the housing of FIG. 1 taken along line BB in FIG. FIG. 2 is a half longitudinal front view of a preform used for forming the casing of FIG. 1. 6A is a plan view, and FIG. 6B is a cross-sectional plan view taken along line CC in FIG. It is a schematic explanatory drawing which cuts and shows the principal part of the injection molding apparatus for injection-molding the preform of FIG. FIG. 7 is a schematic explanatory view for explaining a longitudinal stretching process of a preform, in which (a) is a preform of FIG. 6, (b) is a state immediately before longitudinal stretching of a preform of a comparative example, and (c) is a profile of a comparative example. This shows the partially stretched state of the reform. It is a schematic explanatory drawing which cuts and shows the principal part of the injection molding apparatus for injection-molding a preform. It is a front view which shows the example of a typical preform partially longitudinally.

Hereinafter, an embodiment of a synthetic resin casing and a molding method thereof according to the present invention will be described with reference to the accompanying drawings.
1 to 5 are diagrams for explaining an embodiment of the synthetic resin casing of the present invention. FIG. 1 is a perspective view, FIG. 2 is a half longitudinal front view, and FIG. 3A is a plan view. 3 (b) is a bottom view, FIG. 4 is a plan sectional view taken along the line AA in FIG. 2, and FIG. 5 is a bottom view of the housing shown along the line BB in FIG. 3 (a). It is a longitudinal section of a half region.
FIGS. 6 and 7 show a preform 11 used for forming the casing 1 of the above embodiment. FIG. 6 is a front view of a half longitudinal section, FIG. 7A is a plan view, and FIG. b) is a cross-sectional plan view taken along the line CC in FIG.
FIG. 8 is a schematic explanatory view showing a main part of an injection molding apparatus for injection molding the preform 11 shown in FIGS.

The housing 1 is formed by biaxial stretch blow molding by a hot parison method made of PET resin, and has a square cylindrical body portion 4 from the upper end of the body portion 4 through the shoulder portion 3. A cylindrical mouth tube portion 2 is erected and has a bottom portion 5 whose lower end is closed by a square bottom wall 6.
This is a transparent, small, and thick rectangular housing with an overall height of 82 mm, a lateral width of 27 mm, an average thickness of the peripheral wall of the body 4 of 3.2 mm, and a thickness of the center of the bottom wall 6 of 4 mm.
A grounding portion 7 is provided along the periphery of the bottom wall 6.

A preform 11 shown in FIGS. 6 and 7 has a cylindrical portion 14 formed on the body portion 4 of the casing 1, and a cylindrical mouth portion 12 is erected from the upper end of the cylindrical portion 14. Moreover, it has the structure which has the bottom part 15 which closed the lower end with the square-shaped bottom wall 16.
Further, as shown in FIGS. 7A and 7B, the cylindrical portion 14 has a square outer cross-sectional shape on the outer peripheral surface, a circular inner cross-sectional shape on the inner peripheral surface, and a thick wall in the square wall 14c. It is trying to become. The inner peripheral surface of the cylindrical portion 14 is formed so as to extend downward from the cylindrical inner peripheral surface of the mouth portion 12 in consideration of injection moldability.

The total height of the preform 11 is 76 mm, the lateral width is 19 mm, and the thickness of the tubular portion 14 is 4.9 mm.
The thickness of the central portion of the bottom wall 16 is 5.3 mm, but a concave portion having a depth of 1 mm is formed at the center of the inner surface of the bottom wall 16 by a convex portion 43a disposed at the tip of a core mold 43 described later. 16a is formed.
When calculated based on the outer peripheral surface of the preform 11 and the outer peripheral surface of the biaxially stretched blow-molded casing 1, the transverse stretch ratio in the body portion 4 is 1.4 times, and the body portion 4 extends to the bottom portion 6. The draw ratio in the machine direction is 1.1 times.

Next, the molding method of the casing 1 of the embodiment shown in FIGS. 1 to 5 is roughly based on the following biaxial stretch blow molding by the hot parison method.
(1) The rectangular cylindrical preform 11 shown in FIGS. 6 and 7 is injection molded using an injection molding apparatus as shown in FIG.
(2) After cooling and solidifying for an appropriate time in the mold 41, the mold is released and the gate portion 17 (see FIG. 9A) is cut.
(3) The preform 11 from which the gate portion 17 has been removed is continuously set in a blow molding die (not shown) while preheating in injection molding is maintained, and biaxial stretch blow molding is performed.

The injection molding apparatus shown in FIG. 8 is substantially the same as the apparatus shown in FIG. 10 described above. However, as shown in FIG. 8, the core mold 43 has a spherical arc-shaped protrusion 43a projecting from the center of the tip. It is characteristic to use.
In the molding of the casing 1 of the present embodiment, the inner diameter of the cylindrical portion 14 of the preform 11 is 10 mm, but the diameter of the base end portion of the convex portion 43a is 4 mm according to the diameter of the gate 45, and the protruding height The thickness is 1 mm.
By projecting the convex portion 43a in this manner, the center portion of the bottom wall 16 of the bottom portion 15 of the preform 11 due to the presence of the molten resin (gate portion 17) in the gate 45 in the injection mold. Generation of hot spots HS can be effectively suppressed.

Further, when the preform 11 is injection-molded, the molten resin injected from the gate 45 collides with the tip of the core mold 43, so that the so-called jetting is caused by flow spots on the bottom 15 of the preform 11. Will occur,
By arranging the convex portion 43a having the shape as described above at the tip of the core mold, the flow behavior Rf of the molten resin indicated by the arrow in FIG. 8 can be made smooth, and the occurrence of jetting can be prevented. It was possible to effectively suppress the deterioration of the appearance due to jetting in the transparent casing as in this example.

  Here, FIG. 9 is a schematic explanatory diagram for explaining the longitudinal stretching process of the preform, (a) is the preform of FIG. 6, (b) is a state immediately before stretching of the preform of the comparative example, (c). Indicates the partially stretched state of the preform of the comparative example.

In the hot parison method, after cooling and solidifying in the mold 41 for an appropriate time, the mold is released while maintaining the preheating of the injection molding, and the gate portion 17 extending to the bottom portion 15 of the preform 11 is cut. Thus, the casing is formed by biaxial stretch blow molding in a continuous process.
In particular, in the preform 11 in which the thickness of the bottom portion 15 is increased, when the core mold 43 without the protrusion 43a is used at the tip portion, the preform 11 extends to the bottom wall 16 of the preform 11 in the injection mold. When a hot spot HS indicated by a cross hatch in FIG. 9B is generated due to the gate portion 17 to be provided, and longitudinal extension is performed by the extension pin 51, as shown in FIG. 9C, the vicinity of the hot spot HS is obtained. As a result, the bottom wall 16 is unevenly stretched and the thickness of the bottom wall of the housing is reduced, making it impossible to obtain a desired thickness.

  On the other hand, when using the core mold 43 having the protrusion 43a projecting at the tip as in the molding method of the present invention, the hot spot HS is in a state as shown in FIG. Generation | occurrence | production is suppressed effectively and desired wall thickness can be achieved, without carrying out partial extending | stretching.

Next, the features of the rectangular casing 1 of the embodiment shown in FIGS. 1 to 5 molded by the molding method of the present invention will be further described.
In the plane cross-sectional view of the casing 1 in FIG. 4, the plane cross-sectional shape of the preform 11 shown in FIG. 7B is shown to be overlapped with a two-dot chain line, but the preform 11 as in this embodiment. By using a rectangular cylindrical preform having a rectangular cross-sectional shape of the outer peripheral surface of the peripheral wall of the cylindrical portion 14, the distance L1 between the outer peripheral surface of the cylindrical body 14 and the panel wall 4p, the square wall 4c, The difference in the distance L2 can be reduced, and local stretch deformation of the peripheral wall of the preform 11 in the vicinity of the square wall 4c can be suppressed.

As a result, the thinning of the square wall seen in the conventional rectangular housing using the preform having a cylindrical tubular portion is eliminated, and the body portion 4 is formed as shown in the plane sectional view of FIG. A substantially uniform wall thickness distribution could be achieved over the entire circumference of the peripheral wall.
Furthermore, the square wall 4c of the casing 1 has an arc shape in combination with the effect of increasing the thickness of the corner 14c of the preform 11 by making the inner peripheral surface of the cylindrical body 14 of the preform 11 circular. The wall thickness t2 at the square wall 4c is thicker than the wall thickness t1 of the panel 4p.

  Next, in FIG. 5 in which a longitudinal sectional view in the diagonal direction of the lower half of the casing 1 is shown, the longitudinal sectional shape of the preform 11 shown in FIG. However, the region from the bottom wall 16 to the corner portion 16c of the preform 11 is stretched and deformed in the vertical direction in addition to the horizontal direction in the direction of the black arrow in the figure, while the bottom wall 6 and the square wall 4c of the housing 1 are A corner portion 6c connected in a bent shape at a right angle is formed.

In the casing 1 of the present embodiment, the portion near the corner portion 16c of the preform 11 formed by the hot parison method is easily stretched at a relatively high temperature, and the bottom wall 16 has a hemispherical shell shape. In addition to the fact that it is formed in a substantially flat shape, the local stretch deformation progresses as seen in the corner of a conventional rectangular housing using a preform having a cylindrical tubular portion. It can be seen that the thickness is increased in the vicinity of the corner portion 6c and the inner peripheral surface of the corner portion 6c is rounded in an arc shape.

As mentioned above, although embodiment of this invention and its effect were demonstrated along the Example, embodiment of this invention is not limited to the said Example.
For example, in the above embodiment, a case made of PET resin has been described, but the structure of the present invention is made of polypropylene resin, made of PET-G resin containing 1,4 cyclohexanedimethanol (CHDM) component as a copolymer component, aromatic It is also effective for biaxially stretched blow-molded housings made of other resins such as polyolefin resins.
Further, in the above embodiment, the forming method has been described by taking a rectangular casing having a small and thick bottom as an example, but the forming method of the present invention is a test tubular preform having a cylindrical tubular portion. Therefore, it is very effective when forming a round casing having a small and thick bottom.

As described above, the present invention provides a small, thick-walled synthetic resin-resin casing that exhibits a glass bottle-like high-quality appearance, and is expected to be used in a wide range of applications such as cosmetics. Is done.

DESCRIPTION OF SYMBOLS 1; Housing 2; Mouth part 3; Shoulder part 4; Trunk part 4p: Panel wall 4c; Square wall 5; Bottom part 6; Bottom wall 6c; Corner part 7; Cylindrical portion 14c; Corner portions 15 and 115; Bottom portions 16 and 116; Bottom wall 16a; Recess 16c; Corner portions 17 and 117; Gate portion 41; Injection mold 42; Core mold 43a; convex portion 44; cavity 45; gate 47; nozzle portion 48; shut-off pin 49;

Claims (6)

A method for forming a synthetic resin casing, in which a bottomed cylindrical synthetic resin preform by injection molding is biaxially stretch blow molded by a hot parison method, the preform (11) being the preform (11) An injection mold (41) having a core mold (43) with a projecting portion (43a) protruding and arranged at the center of the tip corresponding to the center of the inner peripheral surface of the bottom (15) A forming method characterized by: The diameter of the base end portion of the convex portion (43a) is made substantially equal to the diameter of the gate (45) of the injection mold (41) located opposite to the tip portion of the core mold (43), and directed toward the tip portion. The forming method according to claim 1, wherein the shape is reduced in diameter. The shape of the preform (11) is a shape in which a cylindrical mouth portion (12) is erected on the upper end of a cylindrical portion (14) in which the flat cross-sectional shape of the outer peripheral surface of the peripheral wall is rectangular. 2. The molding method according to 2. A casing molded by the molding method according to claim 1, 2 or 3, wherein the transverse stretch ratio from the body (4) to the bottom wall (6) is 2 times or less, and the longitudinal stretch ratio is 1. A synthetic resin casing characterized in that the bottom (5) and the bottom wall (6) are formed with a thickness of 3.5 mm or more. A casing molded by the molding method according to claim 3, comprising a rectangular cylindrical body (4) that extends in a similar manner from the cylindrical part (14) of the preform (11), A cylindrical mouth tube part (2) is erected on the upper end of the trunk part (4) via the shoulder part (3), and the lower end is closed with a rectangular bottom wall (6),
The horizontal stretch ratio from the body (4) to the bottom wall (6) is 2 times or less, the longitudinal stretch ratio is 1.2 times or less, and the bottom (5) bottom wall (6) is 3.5 mm. A synthetic resin casing made of the above-mentioned thick wall. A casing molded by the molding method according to claim 3, comprising a rectangular cylindrical body (4) that extends in a similar manner from the cylindrical part (14) of the preform (11), A cylindrical mouth tube part (2) is erected on the upper end of the body part (4) via a shoulder part (3), and the lower end is closed with a rectangular bottom wall (6). In the region from the center of (6) to the lower end of the four corner walls (4c) of the peripheral wall of the body (4), the wall thickness is substantially the same between the bottom wall (6) and the lower ends of the corner walls (4c). Synthetic resin steel plate characterized in that it increases in the vicinity of the corner portion (6c) that is bent and connected at right angles, and the inner peripheral surface of the corner portion (6c) is chamfered in an arc shape when viewed in a longitudinal section. body.

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