JP7767979B2 - Preform - Google Patents

Description

The present invention relates to a preform for a synthetic resin container.

Containers such as bottles made from thermoplastic resins such as polyethylene terephthalate are commonly known. One known method for manufacturing such containers involves producing a bottomed, cylindrical preform by injection molding or other methods, and then molding this preform into a bottle by biaxially stretching blow molding. Bottles manufactured in this manner are widely used as containers for various beverages, seasonings, and other products. Technology related to preforms for synthetic resin containers is disclosed, for example, in Patent Document 1.

Japanese Patent Application Laid-Open No. 2000-334816

The object of the present invention is to provide a preform for a synthetic resin container that can impart the desired properties to the container.

According to one aspect of the present invention, a preform for a synthetic resin container comprises a mouth portion extending from the open end to the neck ring; a lower neck portion connected to the mouth portion and including a portion whose wall thickness gradually increases with increasing distance from the neck ring along the axial direction; a bottom portion including a closed end opposite the open end and including a portion whose wall thickness gradually increases toward the neck ring; and a body portion extending from the upper end to the lower end of the body, with the upper end being the point connected to the lower neck portion where the wall thickness begins to become constant with increasing distance from the neck ring along the axial direction, and the lower end being the point connected to the bottom portion where the wall thickness begins to become constant toward the neck ring along the axial direction, and the body portion including a thick-walled portion whose wall thickness gradually increases and then decreases with increasing distance from the neck ring along the axial direction.

The present invention provides a preform for a synthetic resin container that can impart the desired properties to the container.

FIG. 1 is a schematic diagram showing an outline of a configuration example of a preform according to an embodiment. FIG. 2 is a schematic diagram showing an outline of a configuration example of a bottle formed using a preform according to the embodiment. FIG. 3 is a schematic diagram showing an outline of a configuration example of a preform according to a first modified example. FIG. 4 is a schematic diagram showing an outline of a configuration example of a mold used in manufacturing a preform according to the embodiment. FIG. 5 is a schematic diagram showing an outline of a configuration example of a mold used to manufacture a preform according to the first modified example. FIG. 6 is a schematic diagram showing an outline of a configuration example of a preform according to a second modified example.

One embodiment will be described with reference to the drawings. This embodiment relates to a synthetic resin container. In particular, this embodiment relates to a preform used in the manufacture of a bottle molded by biaxial stretch blow molding or the like.

Figure 1 is a diagram showing an outline of an example configuration of a preform 1 according to this embodiment. The left side of Figure 1 shows a schematic longitudinal cross-sectional shape of the preform 1. For clarity, this figure is expanded horizontally to exaggerate the thickness. The actual preform 1 is thinner than that shown in Figure 1. The right side of Figure 1 shows the wall thickness at each position on the preform 1 corresponding to the left side of the figure. The preform 1 is basically symmetrical about the central axis, and has a roughly cylindrical shape with one end open and the other closed. These ends will be referred to as the open end 11 and the closed end 51.

The preform 1 is made of a thermoplastic resin. For example, the preform 1 is made using an ethylene terephthalate-based thermoplastic polyester resin such as polyethylene terephthalate. The thermoplastic polyester resin is not limited to polyethylene terephthalate; polybutylene terephthalate, polyethylene naphthalate, amorphous polyarylate, polylactic acid, polyethylene furanoate, or copolymers thereof may also be used. Mixtures of these resins, or mixtures of these resins with other resins, may also be used. Polycarbonate, acrylonitrile resin, polypropylene, propylene-ethylene copolymer, polyethylene, etc. may also be used.

The preform 1 of this embodiment is suitable for producing a bottle 100 as shown in Figure 2. The preform 1 is stretched by blow molding, such as biaxial stretch blow molding, to form the bottle 100 from the preform 1. The bottle 100 shown in Figure 2 is, for example, a beverage bottle, with a capacity of approximately 2 L. The bottle 100 comprises a mouth 110, a shoulder 120, a body 130, a heel 151, and a bottom 152. The body 130 of the bottle 100 has an approximately rectangular cylindrical shape, and is generally referred to as a rectangular bottle. The following explanation will use up and down defined based on the orientation of the bottle 100 when it is placed upright on a horizontal surface with the mouth 110 facing up, as shown in Figure 2.

The mouth 110 is a cylindrical portion that serves as a spout for pouring the contents. A ring-shaped neck ring 112 protrudes from the lower end of the mouth 110. The bottle 100 widens radially at a shoulder 120 that extends from the bottom of the neck ring 112 toward the body 130. The body 130 makes up the majority of the bottle 100. The lower end of the body 130 is connected to a closed bottom 152 via a heel 151.

The body 130 can be divided into an upper body portion 131, a waist portion 132, and a lower body portion 133, from top to bottom. The waist portion 132 is located in the center of the body 130 and is radially narrower than the other portions. The waist portion 132 has recesses 135 on the front and back sides as viewed in Figure 2. The bottle 100 has a shape that makes it easy to hold, as a person's fingers can fit into the recesses 135 when gripping it. The upper body portion 131 and lower body portion 133 have multiple grooves in the circumferential direction to improve the rigidity of the body 130.

Lighter bottles are generally required. In a bottle 100 such as that shown in Figure 2, if the portion of the preform 1 corresponding to the body 130 is made uniformly thin throughout, the container wall that makes up the waist 132 will also be thin. As a result, when a person grasps the waist 132, the waist 132 may not be strong enough to make the bottle 100 difficult to handle, or the depression 135 in the waist 132 may invert due to an impact such as being dropped. The preform 1 according to this embodiment is designed so that the body 130 is thick, particularly in the area corresponding to the center, to provide sufficient strength to the body 130 of such a bottle 100.

Returning to Figure 1, the preform 1 will now be described in detail. The preform 1 has a mouth portion 10 that will not be stretched by blow molding and will become the mouth portion 110 of the bottle 100 as is. The mouth portion 10 includes the area from the open end 11 to the neck ring 12 that will become the neck ring 112 of the bottle 100.

The preform 1 is stretched below the neck ring 12 by blow molding. When blown, the preform 1 has a lower neck portion 20 that roughly corresponds to the shoulder portion 120 of the bottle 100, a body portion 30 that roughly corresponds to the body portion 130, and a bottom portion 50 that roughly corresponds to the heel portion 151 and bottom portion 152. The lower neck portion 20 and body portion 30 have a roughly cylindrical shape, and the bottom portion 50 has a roughly hemispherical shape.

In this example, the inner diameter of the lower neck portion 20 of the preform 1 is constant. Because the outer diameter of the body portion 130 of the bottle 100 is relatively large, the outer diameter of the lower neck portion 20 gradually increases from a certain point toward the lower body portion 30, i.e., as it moves away from the neck ring 12 in the axial direction, and the wall thickness of the preform 1 gradually increases. The portion where the wall thickness gradually increases is referred to as the increasing wall thickness portion 21. In the example shown in Figure 1, the outer diameter is constant from position P1 to position P2 directly below the neck ring 12, and gradually increases from position P2 to position P3, and the region from position P2 to position P3 is the increasing wall thickness portion 21.

The point (position P3) where the wall thickness begins to become constant as it continues from the lower neck portion 20 and moves away from the neck ring 12 in the axial direction is defined as the upper end 31 of the body portion, and the area below this point is defined as the body portion 30. Therefore, the maximum outer diameter of the body portion 30 is larger than the diameter directly below the neck ring 12.

The bottom portion 50 also includes a portion where the thickness gradually increases from the closed end 51 (position P8) toward the upper body portion 30, i.e., toward the neck ring 12. The point (position P7) where the thickness begins to become constant along the axial direction toward the neck ring 12, continuing from the bottom portion 50, is defined as the body lower end 32, and the area above this point is defined as the body portion 30.

The body portion 30 includes a thick-walled portion 35 whose thickness gradually increases and then decreases with increasing distance from the neck ring 12 in the axial direction. The end (position P4) on the neck ring 12 side where the thickness begins to increase is referred to as the thick-walled portion upper end 36, and the end (position P6) on the bottom 50 side where the thickness ends is referred to as the thick-walled portion lower end 37. The region from the thick-walled portion upper end 36 to the thick-walled portion lower end 37 is referred to as the thick-walled portion 35. The thickest part of the thick-walled portion 35 (position P5) is referred to as the maximum thickness portion 38. The portion of the body portion 30 closer to the neck ring 12 than the thick-walled portion upper end 36 is referred to as the upper uniform thickness portion 34a, and the portion closer to the bottom 50 than the thick-walled portion lower end 37 is referred to as the lower uniform thickness portion 34b. The upper uniform thickness portion 34a and the lower uniform thickness portion 34b are collectively referred to as the uniform thickness portion 34.

By providing the thick-walled portion 35 in the axial center of the body 30, when the bottle 100 is formed by blow molding, a sufficient amount of material is used to form the waist portion 132 near the center of the body 130 of the bottle 100, ensuring sufficient strength of the waist portion 132. As a result, the waist portion 132 is easier to grip and defects such as the inversion of the depression formed in the waist portion 132 are prevented.

The location of the thick-walled portion 35 depends on the shape and function of the bottle 100. The thick-walled portion 35 is located in a position on the bottle that requires strength or a position that requires material due to a thicker or more complex shape. For example, the thickest portion 38 can be positioned so that the length from just below the neck ring 12 to the thickest portion 38 (the length from position P1 to position P5) is in the range of 40% to 60% of the length from just below the neck ring 12 to the closed end 51 (the length from position P1 to position P8). For example, in the case of bottle 100, when considering balance when gripping it, the area near the center of bottle 100 is considered to be the easiest to grip, and it is expected that sufficient strength and a shape that makes it easy to grip are required in this area.

Furthermore, for example, the thick-walled portion upper end 36 can be positioned so that the length from just below the neck ring 12 to the thick-walled portion upper end 36 (the length from position P1 to position P4) is in the range of 22% to 32% of the length from just below the neck ring 12 to the closed end 51 (the length from position P1 to position P8). Furthermore, for example, the thick-walled portion lower end 37 can be positioned so that the length from just below the neck ring 12 to the thick-walled portion lower end 37 (the length from position P1 to position P6) is in the range of 67% to 77% of the length from just below the neck ring 12 to the closed end 51 (the length from position P1 to position P8).

The thick-walled portion 35 is configured as follows: the inner diameter of the preform 1 gradually decreases from the thick-walled portion upper end 36 to the maximum thickness portion 38. The point where the inner diameter begins to gradually decrease as the distance from the neck ring 12 increases along this axial direction is referred to as the inner diameter reduction start portion 42, the point where the decrease in the inner diameter ends is referred to as the inner diameter reduction end portion 43, and the region from the inner diameter reduction start portion 42 to the inner diameter reduction end portion 43 is referred to as the inner diameter reduction portion 41. In the body portion 30, the region where the inner diameter does not change other than the inner diameter reduction portion 41 is referred to as the uniform inner diameter portion 44. The uniform inner diameter portion 44 closer to the neck ring 12 than the inner diameter reduction portion 41 is referred to as the upper uniform inner diameter portion 44a, and the uniform inner diameter portion 44 closer to the bottom portion 50 than the inner diameter reduction portion 41 is referred to as the lower uniform inner diameter portion 44b.

The outer diameter of the preform 1 gradually decreases from the maximum thickness portion 38 to the thick-walled portion lower end 37. The point where the outer diameter begins to gradually decrease with increasing distance from the neck ring 12 along this axial direction is referred to as the outer diameter reduction start portion 47, the point where the outer diameter reduction ends is referred to as the outer diameter reduction end portion 48, and the region from the outer diameter reduction start portion 47 to the outer diameter reduction end portion 48 is referred to as the outer diameter reduction portion 46. In the body portion 30, the region where the outer diameter does not change other than the outer diameter reduction portion 46 is referred to as the uniform outer diameter portion 49. The uniform outer diameter portion 49 closer to the neck ring 12 than the outer diameter reduction portion 46 is referred to as the upper uniform outer diameter portion 49a, and the uniform outer diameter portion 49 closer to the bottom portion 50 than the outer diameter reduction portion 46 is referred to as the lower uniform outer diameter portion 49b.

In this embodiment, the position of the inner diameter reduction start portion 42 corresponds to the position of the thick-walled portion upper end 36, and the position of the inner diameter reduction end portion 43 corresponds to the position of the maximum thickness portion 38. In the inner diameter reduction portion 41, the outer diameter of the preform 1 is constant, forming an upper uniform outer diameter portion 49a. Therefore, in the inner diameter reduction portion 41, the wall thickness increases as the inner diameter decreases. Furthermore, the position of the outer diameter reduction start portion 47 corresponds to the position of the maximum thickness portion 38, and the position of the outer diameter reduction end portion 48 corresponds to the position of the thick-walled portion lower end 37. In the outer diameter reduction portion 46, the inner diameter of the preform 1 is constant, forming a lower uniform inner diameter portion 44b. Therefore, in the outer diameter reduction portion 46, the wall thickness decreases as the outer diameter decreases.

Not limited to this embodiment, as long as the inner diameter reduction end portion 43 is located at the same position as the outer diameter reduction start portion 47 in the axial direction, or the inner diameter reduction end portion 43 is located closer to the neck ring 12 than the outer diameter reduction start portion 47, the thick-walled portion 35 will be formed regardless of the shapes of the inner diameter reduction portion 41 and the outer diameter reduction portion 46. If the outer diameter reduction start portion 47 is located closer to the neck ring 12 than the inner diameter reduction end portion 43 in the axial direction, the thick-walled portion 35 can be formed by appropriately setting the inner surface shape of the inner diameter reduction portion 41 and the outer surface shape of the outer diameter reduction portion 46.

When the inner diameter reduction end portion 43 and the outer diameter reduction start portion 47 are located in the same position in the axial direction, as in this embodiment, the maximum thickness portion 38 does not have a width. On the other hand, if the inner diameter reduction end portion 43 and the outer diameter reduction start portion 47 are located in different positions and there is a continuous axial section where the outer surface and the inner surface are equidistant in the direction perpendicular to the axis, the maximum thickness portion 38 may have a width. For example, if the inner diameter reduction end portion 43 is located closer to the neck ring 12 in the axial direction than the outer diameter reduction start portion 47, the distance between the inner surface and the outer surface in the direction perpendicular to the axis will be equidistant in the section where the lower inner diameter uniform portion 44b and the upper outer diameter uniform portion 49a face each other, and therefore the maximum thickness portion 38 will have a width.

In this embodiment, the thickness of the upper uniform thickness portion 34a and the lower uniform thickness portion 34b are equal. However, this is not limited to this, and the thickness of the upper uniform thickness portion 34a and the thickness of the lower uniform thickness portion 34b may be different. The upper uniform thickness portion 34a may be thicker than the lower uniform thickness portion 34b, or the lower uniform thickness portion 34b may be thicker than the upper uniform thickness portion 34a, but the thick portion 35 is thicker than both the upper uniform thickness portion 34a and the lower uniform thickness portion 34b.

In this embodiment, the overall mass of the preform 1 is, for example, 27 g or less, and may be 26.5 g, for example. For example, an example of the wall thickness of a preform 1 having a neck 10 diameter of 30 mm may be 2.6 mm for the constant thickness portion 34 and 2.7 mm for the maximum thickness portion 38. In this example, the mass of the neck 110 of the bottle 100 may be 5.0 g, the mass of the shoulder portion 120 may be 4.9 g, the mass of the upper body portion 131 may be 3.3 g, the mass of the waist portion 132 may be 4.3 g, the mass of the lower body portion 133 may be 5.0 g, and the mass of the heel portion 151 and the bottom portion 152 may be 4.0 g. In this example, the mass of the waist portion 132 of the bottle 100 increases by approximately 10% compared to when the entire body portion 30 of the preform 1 has a thickness of 2.6 mm. In this way, the thickness of the maximum thickness portion 38 may be 3% to 8% thicker than the thickness of the constant thickness portion 34. By making it thicker to this extent, the portion of the bottle corresponding to the thick-walled portion 35 can be given the desired functionality.

[First Modification]
A first modified example will be described. Differences from the above-described embodiment will be described below, and the same parts will be denoted by the same reference numerals and will not be described again. FIG. 3 shows an outline of a configuration example of a preform 2 according to the first modified example. As shown in this figure, the preform 2 according to this modified example has a constant inner diameter from the neck portion 10 to the body portion 30. Meanwhile, the wall thickness of the preform 2 is designed in the same manner as the preform 1 according to the above-described embodiment, as described with reference to FIG. 1. Therefore, in the preform 2 according to the first modified example shown in FIG. 3, the outer diameter gradually increases in the increasing wall thickness portion 21 as it moves away from the neck ring 12 in the axial direction, is constant in the upper uniform thickness portion 34a, gradually increases from the upper end 36 of the thick wall portion to the maximum thickness portion 38, gradually decreases from the maximum thickness portion 38 to the lower end 37 of the thick wall portion, and is constant in the lower uniform thickness portion 34b. Even with this shape, the same effects as those of the preform 1 according to the above-described embodiment can be obtained.

[Injection molding molds]
The preforms according to the above-described embodiment and the first modified example are formed by, for example, injection molding, compression molding, etc. Here, a mold used for manufacturing the preform by injection molding will be described.

Figure 4 is a schematic diagram showing an example configuration of a mold 210 used to manufacture the preform 1 according to the embodiment described above with reference to Figure 1. While the gate is not shown in this figure, the gate may be located, for example, at the closed end 51. As shown in Figure 4, the mold 210 has a core mold 211, a mouth mold 212, which is a cavity mold, and a body mold 215. To prevent undercuts from occurring in the body mold 215, the mold split position P11 between the mouth mold 212 and the body mold 215 corresponds to the position of the upper end 31 of the body. Therefore, the body mold 215 forms the outer shape of the body portion 30 and bottom portion 50 of the preform 1 from the upper end 31 of the body toward the closed end 51. Meanwhile, the mouth mold 212 forms the outer shape of the mouth portion 10 and lower neck portion 20 of the preform 1 from the upper end 31 of the body toward the open end 11. The mouth mold 212 is a split mold and has a first mouth mold 213 and a second mouth mold 214.

Figure 5 is a diagram showing an outline of an example configuration of a mold 220 used to manufacture the preform 2 according to the first modified example described above with reference to Figure 3. As shown in Figure 5, the mold 220 has a core mold 221, a mouth mold 222, which is a cavity mold, and a body mold 225. To prevent undercuts in the body mold 225, the mold split position P12 between the mouth mold 222 and the body mold 225 corresponds to the position of the maximum thickness portion 38. Therefore, the body mold 225 forms the outer shape of the lower side of the body portion 30 and the bottom portion 50 from the maximum thickness portion 38 toward the closed end 51 of the preform 1. On the other hand, the mouth mold 222 forms the outer shape of the mouth portion 10, the lower neck portion 20, and the upper side of the body portion 30 from the maximum thickness portion 38 toward the open end 11 of the preform 1. The mouth mold 222 is a split mold and has a first mouth mold 223 and a second mouth mold 224.

Compare the production of preform 1 shown in FIG. 1 using mold 210 shown in FIG. 4 with the production of preform 2 shown in FIG. 3 using mold 220 shown in FIG. 5. Compared to mold 220 shown in FIG. 5, mold 210 shown in FIG. 4 has a larger body mold 215 and a smaller split mouth mold 212. Generally, compared to cylindrical molds such as body molds 215 and 225, split molds such as mouth molds 212 and 222 have more restrictions on the design of cooling water channels, and therefore tend to have lower cooling capacity. Compared to mold 210 shown in FIG. 4, mold 220 shown in FIG. 5 has a relatively large split mouth mold 222, and its lower cooling capacity results in a relatively slow molding cycle. For this reason, the production of preform 1 shown in FIG. 1 using mold 210 shown in FIG. 4 is more efficient and more preferable.

[Second Modification]
A second modified example will be described. Here, differences from the above-described embodiment will be described, and the same parts will be assigned the same reference numerals and their description will be omitted. FIG. 6 shows an outline of a configuration example of a preform 3 according to the second modified example. This preform 3 is configured to be suitable for bottles with a smaller capacity than the preform 1 of the above-described embodiment described with reference to FIG. 1. This preform 3 is for a bottle with a capacity of, for example, 500 mL. Therefore, in this preform 3, the lower neck portion 20, the body portion 30, and the bottom portion 50 are smaller than the mouth portion 10. On the other hand, a thick portion 35 is provided in the center of the body portion 30, making the central portion thick.

As shown in Figure 6, the inner and outer diameters of the preform 3 change as follows, moving away from the neck ring 12 in the axial direction. That is, both the inner and outer diameters decrease in the lower neck portion 20 up to the upper end 31 of the body. Here, because the reduction in the inner diameter is greater than the reduction in the outer diameter, the thickness of the lower neck portion 20 gradually increases with increasing distance from the neck ring 12.

The body portion 30 has the same configuration as the preform 1 of the embodiment described above with reference to Figure 1. That is, the inner diameter is constant from the body portion upper end 31 to the inner diameter reduction start portion 42, gradually reduces in the inner diameter reduction portion 41 from the inner diameter reduction start portion 42 to the inner diameter reduction end portion 43, and is constant from the inner diameter reduction end portion 43 to the body portion lower end 32. The outer diameter is constant from the body portion upper end 31 to the outer diameter reduction start portion 47, gradually reduces in the outer diameter reduction portion 46 from the outer diameter reduction start portion 47 to the outer diameter reduction end portion 48, and is constant from the outer diameter reduction end portion 48 to the body portion lower end 32. The inner diameter reduction start portion 42 is closer to the neck ring 12 than the outer diameter reduction start portion 47, the inner diameter reduction end portion 43 is closer to the neck ring 12 than the outer diameter reduction end portion 48, and the inner diameter reduction end portion 43 and the outer diameter reduction start portion 47 are in corresponding positions.

As a result, an upper uniform thickness portion 34a with a uniform thickness is formed closer to the neck ring 12 than the inner diameter reduction start portion 42, a thick portion 35 is formed in which the thickness gradually increases from the inner diameter reduction start portion 42 to the inner diameter reduction end portion 43 (outer diameter reduction start portion 47) and gradually decreases from the outer diameter reduction start portion 47 (inner diameter reduction end portion 43) to the outer diameter reduction end portion 48, and a lower uniform thickness portion 34b is formed from the outer diameter reduction end portion 48 toward the bottom 50.

As such, the preform 3 according to the second modification also has a thick-walled portion 35 in the center of the body 30, making this preform 3 suitable for producing bottles with designs that require strength or material in the center of the body, even if the bottle has a small capacity.

Furthermore, in the mold used to form the preform 3 according to the second modified example by injection molding, the mold split position between the mouth mold of the cavity mold and the body mold can be set at a position corresponding to the neck lower part 20, as in the case described with reference to Figure 4. For example, the mold split position can be set at the upper end 22 of the thickened portion 21 of the neck lower part 20, or directly below 23 the neck ring 12. As a result, the body mold can be made relatively large, resulting in high production efficiency.

The present invention has been described above by showing preferred embodiments, but it goes without saying that the present invention is not limited to the above-described embodiments, and various modifications are possible within the scope of the present invention.

1, 2, 3 Preform 10 Mouth portion 11 Open end 12 Neck ring 20 Lower neck portion 21 Increasing wall thickness portion 22 Upper end of increasing wall thickness portion 23 Directly below neck ring 30 Body portion 31 Upper end of body portion 32 Lower end of body portion 34 Uniform thickness portion 34a Upper uniform thickness portion 34b Lower uniform thickness portion 35 Thick portion 36 Upper end of thick portion 37 Lower end of thick portion 38 Maximum wall thickness portion 41 Reduced inner diameter portion 42 Start of inner diameter reduction portion 43 End of inner diameter reduction portion 44 Uniform inner diameter portion 44a Upper uniform inner diameter portion 44b Lower uniform inner diameter portion 46 Reduced outer diameter portion 47 Start of outer diameter reduction portion 48 End of outer diameter reduction portion 49 Uniform outer diameter portion 49a Upper uniform outer diameter portion 49b Lower uniform outer diameter portion 50 Bottom portion 51 Closed end 100 Bottle 110 Mouth portion 112 Neck ring 120 Shoulder portion 130 Body portion 131 Upper body portion 132 Waist portion 133 Lower body portion 135 Depression 151 Heel portion 152 Bottom portion 210 Mold 211 Core mold 212 Mouth portion mold 213 First mouth portion mold 214 Second mouth portion mold 215 Body portion mold 220 Mold 221 Core mold 222 Mouth portion mold 223 First mouth portion mold 224 Second mouth portion mold 225 Body portion mold

Claims (8)

a mouth portion including an opening end to a neck ring;
a neck lower portion including a portion connected to the mouth portion and having a wall thickness that gradually increases with increasing distance from the neck ring along an axial direction;
a bottom portion including a closed end opposite the open end, the bottom portion including a portion whose thickness gradually increases toward the neck ring;
a body portion that is a region from the upper end of the body portion to the lower end of the body portion, wherein a point that is connected to the lower neck portion and where the wall thickness starts to become constant as the distance from the neck ring in the axial direction increases is defined as an upper end of the body portion, and a point that is connected to the bottom portion and where the wall thickness starts to become constant toward the neck ring in the axial direction is defined as a lower end of the body portion,
The body portion includes a thick-walled portion whose thickness gradually increases and decreases with increasing distance from the neck ring along the axial direction.
Preform. The body portion is
a reduced inner diameter portion whose inner diameter gradually decreases with increasing distance from the neck ring in the axial direction;
a reduced outer diameter portion whose outer diameter gradually decreases with increasing distance from the neck ring in the axial direction,
The preform according to claim 1. A preform as described in claim 2, wherein, in the axial direction, the inner diameter reduction end portion, which is the end of the inner diameter reduction portion closer to the bottom, is located closer to the neck ring than or at the same position as the outer diameter reduction start portion, which is the end of the outer diameter reduction portion closer to the neck ring. A preform according to any one of claims 1 to 3, wherein the maximum thickness portion of the thick-walled portion, where the thickness is greatest, is positioned so that the length from the neck ring to the maximum thickness portion is in the range of 40% to 60% of the length from the neck ring to the closed end. A preform according to any one of claims 1 to 4, wherein the thickness of the thickest part of the thick-walled portion is 3% to 8% thicker than the thickness of the body portion excluding the thick-walled portion. A preform according to any one of claims 1 to 5, wherein the thickness of the body portion on the neck ring side of the thick portion is equal to the thickness of the body portion on the bottom side of the thick portion. A preform according to any one of claims 1 to 6, wherein the lower neck portion includes a portion whose outer diameter gradually increases with increasing distance from the neck ring in the axial direction. 8. The preform according to claim 1, having an overall mass of 27 g or less.