JPH0688313B2 - Preform for molding resin containers and molding die therefor - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a preform for a resin container having a mouth to which a screw cap can be attached, and a molding die for molding the preform.

BACKGROUND ART Containers made of resin, such as polyethylene terephthalate (hereinafter referred to as "PET"), and having a mouth to which a screw cap can be attached are widely used as containers for various soft drinks and the like.

The contents of such containers, particularly soft drinks containing fruit juice, are generally sterilized at high temperatures before being filled into the containers. Therefore, such resin containers are required to have sufficient heat resistance. In particular, the mouth of the container is not stretched and therefore has poor heat resistance. Furthermore, since a cap must be attached, the mouth must have high heat resistance to prevent thermal deformation and ensure that the threads do not become misaligned.

A proposal for a container having the characteristics required for the mouth of such a container is disclosed, for example, in Japanese Patent Publication No. 61-35056.

In this technique, as shown in FIG.
The container is composed of a biaxially stretched container body 2 and a non-biaxially stretched mouth portion 5. The mouth portion 5 has a screw portion 3 on its outer periphery.
and a support ring 4.
The resin container 1 has the advantage that the opening 5 is less likely to deform even at high temperatures, and high dimensional accuracy can be maintained.

However, according to the above-mentioned conventional method, the opening 5 and the container body 2
Therefore, if the degree of crystallization of the mouth portion 5 is to be increased sufficiently, there is a problem that crystallization occurs in a part of the container body 2 that is continuous with the mouth portion 5. In other words, if the mouth portion 5 is crystallized at the preform stage, the mouth portion 5
The resin container 1 is also crystallized in a part of the container body 2 adjacent to the resin container 1, and the crystallized part makes it difficult to perform biaxial stretching in the next step.
If the mouth portion 5 is crystallized at the stage of forming the container body 2, not only the mouth portion 5 but also a part of the shoulder portion or the body portion of the container body 2 may be crystallized and whitened.

Also, as disclosed in Japanese Patent Publication No. 2-5,564 and U.S. Patent No. 4,715,504, for example, a preform is known in which an insert piece having a threaded portion on the outer periphery is made of a resin with good heat resistance, and the mouth portion other than the insert piece and the preform body are made of PET or the like. A method is disclosed for molding such a preform in which the insert piece is first injection molded from a thermoplastic resin with good heat resistance, and then the PET is injection molded to embed the insert piece on the outer periphery of the mouth portion.

A preform using such an insert piece can eliminate the drawback of the conventional technology in that when the entire mouth portion is crystallized, part of the container body is also crystallized, and the heat resistance of the mouth portion can be increased to a level sufficient for practical use.

However, even in preforms using these insert pieces, the bonding strength between the insert piece and the resin that makes up the preform body is not always sufficient, and in the worst case, the insert piece can partially peel off due to the load applied when attaching the cap.

The present invention solves the problems of the prior art, and its purpose is to provide a preform having sufficient mechanical strength, in which an insert piece is made of a resin having high heat resistance and mechanical strength, and in which this insert piece is firmly bonded to the resin that constitutes the preform body and the inner layer of the mouth portion, and a molding die for molding this preform.

[Disclosure of the Invention] The preform for molding resin containers of the present invention comprises a cylindrical preform body, and a mouth portion in which an outer layer is formed by an insert piece having properties different from those of the preform body, and at least a part of the inner layer is formed by the same resin as the preform body, wherein the insert piece includes an upper outer layer portion having a threaded portion on its outer circumferential surface, and a lower outer layer portion located below the upper outer layer portion and having at least a support ring, and wherein the insert piece is formed by a crystallized resin.

With this configuration, the insert piece, which is required to have dimensional stability even when heated, is made of a crystallized resin, making it possible to form a mouth with extremely excellent heat resistance. Therefore, even when filling with high-temperature liquid or during a heating process for sterilizing the container or its contents, shrinkage and deformation of the mouth due to heat can be prevented, resulting in a perfect seal between the mouth and the cap.

Another preform for molding resin containers of the present invention has a cylindrical preform body, and a mouth portion whose outer layer is formed by an insert piece having properties different from those of the preform body, and at least a part of whose inner layer is formed by the same resin as the preform body, wherein the insert piece includes an upper outer layer portion having a threaded portion on its outer peripheral surface, and a lower outer layer portion located below the upper outer layer portion and having a support ring, and the support ring of the insert piece is characterized in that a joint reinforcement portion formed by the resin used in injection molding of the preform body is integrally formed in a direction intersecting the axial direction of the preform body.

According to this configuration, by forming a joint reinforcement portion integrally with the support ring during injection molding of the preform body and the mouth inner layer, the contact area between the insert piece and the other resin layers is increased, thereby making the bond between the two stronger.

Furthermore, by forming the joint reinforcement portion in a direction intersecting the axial direction of the preform body, the connection of the insert piece can be ensured not only in the axial direction of the preform body but also in the radial direction. That is, the insert piece is firmly integrated into the preform's mouth portion by the joint between the inner wall of the insert piece and the resin layer constituting the inner layer of the mouth portion in the axial direction, and by the joint reinforcement portion in the radial direction. Specifically, the buckling strength of the mouth portion and the peel strength of the insert piece can be dramatically improved.

Furthermore, the molding die for the preform for molding a resin container of the present invention comprises a mouth molding die, a cavity die and a core die, and injection molding of the preform is carried out with an insert piece held in the mouth molding die, the insert piece including an upper outer layer portion having a threaded portion on its outer peripheral surface and a lower outer layer portion having at least a support ring and an engaging recess, and the mouth molding die has on its inner wall a thread relief groove portion having a depth greater than the height of the threaded portion of the insert piece, and an engaging portion to be inserted into the engaging recess of the lower outer layer portion, and the insert piece can be inserted and fixed in the mouth molding die in any circumferential direction.

According to this molding die, a thread relief groove that is the same height as or deeper than the thread is formed in the neck molding die to avoid interference with the threaded portion. Therefore, when setting the insert piece, there is no need to position the insert piece in the circumferential direction. Therefore, the device and steps required for positioning can be omitted, and the device and process can be simplified.

Furthermore, the mouth forming die can securely hold the insert piece at its lower outer layer portion by inserting the engaging portion into the engaging recess of the insert piece.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a partial cross-sectional view showing the essential parts of a first embodiment of the present invention.

FIG. 2 is an explanatory diagram showing the manufacturing process of a synthetic resin container.

3A, 3B and 3C are explanatory diagrams showing the operation in the eject/insert process section.

FIG. 4 is a cross-sectional view that schematically shows the preform injection process.

FIG. 5 is a partial cross-sectional view showing the main part of a second embodiment of the present invention.

FIG. 6 is a partial cross-sectional view showing the main part of a third embodiment of the present invention.

FIG. 7 is a bottom view of the insert piece shown in FIG.

FIG. 8 is an explanatory view showing an apparatus for testing the strength of the mouth portion of a preform using an insert piece in which a joint reinforcement portion is not formed on the support ring.

FIG. 9 is an explanatory view showing an apparatus for testing the strength of the mouth portion of a preform having a joint reinforcement portion formed on a support ring.

FIG. 10 is a partial cross-sectional view showing the main part of a fourth embodiment of the present invention.

FIG. 11 is a partial cross-sectional view showing the main part of a fifth embodiment of the present invention.

FIG. 12 is an external view showing an example of a conventional resin container.

BEST MODE FOR CARRYING OUT THE INVENTION First Embodiment FIG. 1 is a cross-sectional view showing a mouth forming die 10 constituting the preform forming die of the present invention and an insert piece 20 held therein.

In this first embodiment, the insert piece 20 is a cylindrical body with a hollow interior, and its upper half portion forms an upper outer layer portion 22 having a threaded portion 22a on the outer periphery, and its lower half portion forms a lower outer layer portion 24 having a locking ring 26 and a support ring 28.

The thickness of the upper outer layer portion 22 is the same as the thickness of the mouth portion of the preform 30 to be formed, and the preform 30 has a single layer structure in this portion.
The inner diameter of the upper outer layer portion 22 is set to be larger than that of the upper outer layer portion 22, and the upper outer layer portion 22 is formed to be thin.

The insert piece 20 must be made of a resin material that has the heat resistance and other properties required for the opening of a resin container. Here, heat resistance refers to the property of not deforming at the temperature of the contents of the resin container to be formed, i.e., the temperature during high-temperature sterilization (e.g., 70 to 95°C). Examples of synthetic resins with such excellent heat resistance include:
For example, various resins such as polyethylene terephthalate, polybutylene terephthalate, polyethylene naphthalate, polypropylene, polyamide, polyacrylonitrile, polyvinylidene chloride, nylon, polyethylene, polycarbonate, etc. Among these resins, crystalline resins can be used after being crystallized to improve their heat resistance.

Furthermore, when a resin different from the resin material constituting the preform body 34 is used as the material for the insert piece 20, the resin may be a blend of the different resin and the resin constituting the preform body. Alternatively, the insert piece may be formed by crystallizing only the resin constituting the preform body. Using the same type of resin material in this way has the advantage of, for example, increasing the bonding strength between the insert piece 20 and the preform body 34 or facilitating container recycling.

The mouth forming mold 10 holds the insert piece 20 and forms the mouth 32 of the preform 30, and is made up of a separable cylindrical body that can be moved back and forth in the radial direction by a moving means.

In this mouth forming die 10, a thread relief groove 12 is formed so that when the insert piece 20 is loaded, the threaded portion 22a formed on the outer periphery of the upper outer layer portion 22 does not interfere with the inner wall of the mouth forming die 10. That is, the upper outer layer portion 22
By forming a ring-shaped thread relief groove portion 12 that is deeper than the height of the thread portion 22a along the inner periphery of the mouth forming die 10 that faces the region where the thread portion 22a is formed, when the mouth forming die 10 is closed to clamp the insert piece 20,
Therefore, when loading the insert piece 20 into the mouth forming mold 10, all that is required is a means for holding and transferring the insert piece 20, which simplifies the structure and process of the mouth forming mold.

The insert piece 20 is held in the mouth forming mold 10 as follows:
The upper surface 22b of the insert piece 20 abuts against the lower end of the first engagement portion 14 of the mouth forming mold 10, thereby restricting the upward movement.
The position of the insert piece 20 is restricted in the vertical and horizontal directions by the engagement between the second engagement portion 16 of the mouth forming die 10 inserted between the support ring 28 and the third engagement portion 18 inserted under the support ring 28.
20 is set in the mouth forming mold 10 stably and with good positional accuracy.

Next, such an insert piece 20 and a mouth forming mold
A method for manufacturing a preform for a synthetic resin container using the mold 10 will be outlined with reference to Figs. 2 to 4.

FIG. 2 shows the entire process cycle for injection stretch blow molding of a resin container in one stage, in which 40 is the eject/insert process section, 42 is the preform injection molding process section, 44 is the temperature control process section, and 46 is the stretch blow molding process section.

3A, 3B, and 3C show the eject/insert process section 40.
FIG. 3A shows a completed synthetic resin container 10.
10 indicates the stage where the molded product is ejected from the mouth forming mold 10 by the opening operation of the mouth forming mold 10, which also serves to transport and hold the molded product.

FIG. 3B shows the insert piece 20 being inserted into the mouth mold 10.
This shows the stage where the insert piece is being loaded.
The insert piece 20 is previously injection molded from the heat-resistant thermoplastic resin described above, and is carried to a clamping position in the mouth forming mold 10 by the piece conveying means 52. Then, as shown in Figure 3C, when the mouth forming mold 10 is driven to close, the insert piece 20 is securely held in place by the engagement relationship described above.

FIG. 4 shows the preform injection molding process section 42.
The insert piece 20 held in the mouth forming mold 10 is inserted into a predetermined position in the cavity mold 60, and then inserted into the core mold 62.
In this state, a resin material for preform injection molding, such as PET, is injected from the gate 60a. By this injection molding, the insert piece 20
The other areas are filled with PET to form the preform 30.

The preform 30 has a cylindrical preform body 34 made of PET and a mouth portion in which the insert piece 20 is embedded.
1, the upper half of the mouth portion 32 is formed of the upper outer layer portion 22 of the insert piece 20, forming a single layer. The lower half of the mouth portion 32 has an outer layer formed of the lower outer layer portion 24 of the insert piece 20, and an inner layer 32a formed of PET that forms the main body 34 of the preform 30, forming a two-layer structure.

In this embodiment, the thickness of the upper outer layer 22 of the insert piece 20 is set to be the same as the thickness of the mouth 32 of the preform 30, and the upper half of the mouth 32 is formed only by the upper outer layer 22. This allows the inflow of molten resin during injection molding of the preform to be directed downward from the lower outer layer 22, i.e., the lower outer layer 24.
Therefore, the molten resin does not flow into the inside of the upper outer layer portion 22 during injection molding, so the pressure of the molten resin does not press the upper outer layer portion 22 toward the thread relief groove portion 12, causing deformation, and the dimensional accuracy of the upper outer layer portion 22 is not impaired.

Next, after the injection molding of the preform 30 is completed, as shown in Fig. 2, the preform 30 is carried into a temperature control process section 44 while still held in the neck mold 10, and is adjusted to a temperature suitable for stretching. Thereafter, as the final process, stretch blow molding is carried out by a known method in a stretch blow process section 46. That is, the preform is biaxially stretched and blow-molded in the blow cavity mold by the axial movement of a stretch rod and the blowing of pressurized fluid, which are commonly performed, to form a container.
100 moldings are completed.

Second Embodiment Fig. 5 is a partial cross-sectional view of a second embodiment of the present invention. Members having substantially the same functions as those in the first embodiment shown in Fig. 1 are designated by the same reference numerals, and detailed descriptions thereof will be omitted.

In the second embodiment, the structure for holding the insert piece 20 relative to the mouth forming die 10 is the same as in the first embodiment.
The difference from the first embodiment is that the insert piece
The upper outer layer 22 of the molded article 20 is thin except for a portion of its upper end, and the thickness of the upper outer layer 22 is set to be approximately the same as the thickness of the lower outer layer 24. Therefore, during preform injection molding, the lower outer layer
The molten resin penetrates not only into the upper outer layer portion 24 but also into the inside of the upper outer layer portion 22 except for a portion of the upper end.
The insert piece 20 has a two-layer structure consisting of a resin layer 32a that forms a preform body 34 formed inside the insert piece 20.

In the second embodiment, as described above, the molten resin flows into the inside of the insert piece 20 except for a portion of the upper end, and in order to prevent deformation of the upper outer layer portion 22, which requires high dimensional accuracy, due to the pressure of the molten resin, the depth L of the thread relief groove portion 12 of the neck forming die 10 is made the same as the height of the thread portion 22a of the upper outer layer portion 22. By doing so, the upper outer layer portion 22 is positioned with the tip of its thread portion 22a in contact with the bottom surface of the thread relief groove portion 12, and therefore the upper outer layer portion 22
Even if an outward pressure is applied by the resin flow,
No problematic deformation occurs.

Third Embodiment FIG. 6 is a partial cross-sectional view of this embodiment, and FIG. 7 is an end view of the insert piece of this embodiment as seen from the bottom side.

In this embodiment, the inner wall of the insert piece 20 is level, and the upper outer layer 22 and the lower outer layer 24 have the same thickness. Therefore, during preform injection molding, the molten resin that constitutes the preform body 34 fills the entire inner wall of the insert piece 20 from the top to the bottom, and the mouth 32 has a two-layer structure consisting of the insert piece 20 and the inner layer 32a.

A feature of this embodiment is that a recess 28 is formed on the bottom surface of the support ring 28, which is in communication with the internal space of the insert piece 20.
The recess 28 is formed by recessing the inner surface of the support ring 28 except for the peripheral edge 28a in a ring shape.
As shown in FIG. 7, multiple locations (four locations in this embodiment)
A recessed engaging portion 42 is formed in the recessed portion 42.
The molten resin used in molding the preform flows into the recessed engaging portion 40 and the recessed engaging portion 42 and hardens, thereby forming the joint reinforcement portion 36 that is integral with the preform body 34 and the support ring 28.

By forming this joint reinforcement portion 36, the contact area between the insert piece 20 and the preform body 34 increases, and the joint therebetween becomes stronger accordingly.
Since the recess 40 is formed in a state in which it is expanded in a direction perpendicular to the axial direction of the preform body 34, the joint reinforcement portion 36 is formed in the radial direction of the preform body 34. Therefore, the joint between the insert piece 20 and the preform body 34 and the inner layer 32a of the mouth portion 34, which is made of the same resin as the preform body 34, is
The joint is sufficiently rigid in both the axial and radial directions.

The peripheral portion 28a of the support ring 28 is not essential, but if the insert piece 20 and the preform body 34 have different colors, providing the peripheral portion 28a makes it possible to unify the overall color of the appearance of the mouth portion 32, which is preferable from an aesthetic standpoint.

In addition, in this embodiment, the insert piece 20 is formed to be thinner overall compared to the first and second embodiments. However, if the thickness of the insert piece 20 is too thin, there is a risk that the insert piece 20 itself will be deformed by the resin pressure during injection molding.
The thickness of the insert piece 20 at its thinnest point must be at least 1.
A thickness of 2 mm, preferably 1.3 to 1.5 mm is required.

(Experimental Example) Next, the results of measurements of the buckling load and peel strength performed on a preform using an insert piece having the configuration of the third embodiment will be described.

FIG. 8 is an explanatory diagram of an experimental device using a preform in which no joint reinforcement portion is formed on the support ring 28 as a measurement object, and FIG.
10 is an explanatory diagram of an experimental device using a preform 28 having a joint reinforcement portion 36 formed therein.

Purpose of the experiment: To compare the bond strength of a preform having a support ring 28 with that of a preform having no support ring 28.
The extent of the difference in peel strength of the insert piece 20 and the buckling load of the opening 32 is confirmed.

Here, the peel strength of the insert piece refers to the magnitude of the load at the time when peeling begins to occur between the insert piece 20 and the inner layer 32a.

The test equipment used was a compression testing machine "TC" manufactured by Shinko Tsushin Kogyo Co., Ltd.
This compression tester is comprised of a base 60 having a cavity in the center into which the preform body 34 is inserted, a pressing part
The preform to be measured is set in place with its support ring 28 placed on the base 60 and the pressing part 70 placed on the top surface of the mouth part 32.

The measurement was carried out at room temperature or 90°C at a compression speed of 10 mm/min.

The sizes a to f of the mouth portion 32 of the preform to be measured are as follows:

a; 24.94 mm b; 22.36 mm c; 22.52 mm d; 33.0 mm e; 5.03 mm f; 0.5 mm The preform body 34 of the preform is made of PET, and the insert piece 20 is made of polycarbonate.

Experimental results a. Measurement results at room temperature for the preform shown in Figure 8 Peel strength: 263 kg/cm ² Buckling load: 468 kg/cm ² b. Measurement results at room temperature for the preform shown in Figure 9 Even when a load of 500 kg/cm ² , which is the upper limit of the compression force of the compression tester, was applied, the insert piece did not peel or buckle. Therefore, although the exact values are unknown,
The peel strength and buckling load were confirmed to be over 500 kg/ ^cm2 .

c. The buckling load of the preform shown in FIG. 9 measured at 90° C. was 280 kg/cm ² , and no peeling of the insert piece was observed when buckling occurred.

The above experimental results show that the buckling strength of the mouth and the peel strength of the insert piece are significantly improved at room temperature by providing the joint reinforcement 36 on the support ring 28. Furthermore, it was confirmed that providing the joint reinforcement 36 on the support ring 28 provides sufficiently high buckling strength even under high temperature conditions, and also provides very high joint strength for the insert piece.

Fourth Embodiment FIG. 10 is a partial cross-sectional view showing a fourth embodiment of the present invention.

This embodiment has a configuration basically similar to that of the third embodiment. The difference between this embodiment and the third embodiment is that a taper 50 is provided at the upper end of the insert piece 20. This taper 50 is formed by a surface whose inner diameter gradually expands linearly as it extends upward. This taper 50 is formed above the upper end of the threaded portion 22a so as not to reduce the mechanical strength of the threaded portion, which is subjected to a load when the cap is attached.

By forming such a taper 50, the molten resin can easily flow completely up to the upper end of the mouth portion 32 during injection molding of the preform body 34, thereby preventing molding defects from occurring.

Fifth Embodiment FIG. 11 is a partial cross-sectional view showing a fifth embodiment of the present invention.

This embodiment has basically the same configuration as the fourth embodiment. The difference between this embodiment and the fourth embodiment is that the shape of the taper 52 formed at the upper end of the insert piece 20 is an arc-shaped curved surface in cross section, and the support ring 28 does not have a peripheral portion on the bottom surface, but the joint reinforcement portion 36
is formed along the entire bottom surface of the support ring 28.

By configuring the taper 52 as an arched curved surface, the insert piece 20 is less likely to peel off when subjected to top pressure from the capper, compared to the linear taper 50 of the fourth embodiment, as it can distribute the force.

Furthermore, in the fourth embodiment in which a peripheral portion 28a is provided on the outer periphery of the support ring 28, peeling of the insert piece 20 may occur near the bend of the peripheral portion 28a, whereas in this embodiment, the bond between the support ring 28 and the joint reinforcement portion 36 is advantageously made stronger against compressive forces in the vertical direction.

Although the embodiments of the present invention have been described above, the present invention is not limited to the above-described embodiments, and various modifications are possible within the scope of the present invention. For example, the specific structure and shape of the insert piece 20 can take various forms as long as it has at least an upper outer layer 22 having a threaded portion 22a and a lower outer layer 24 that can be loaded into the mouth forming mold 10.
The shape of the thread relief groove 12 of 10 is not particularly limited as long as it can accommodate the threaded portion 22a. Furthermore, the properties required for the mouth of the preform are not limited to heat resistance, but also include safety and mechanical strength, and the insert piece can be formed from an appropriate resin selected from a wide variety of resins depending on these required properties.

As described above, according to the present invention, it is possible to provide a preform for molding a resin container, in which an insert piece is made of a resin with good heat resistance and this insert piece is attached to the mouth of the preform with a sufficiently large bonding strength, and a molding die for this preform.

───────────────────────────────────────────────────────── Continued from the front page (51) Int.Cl. ⁵ Identification symbol Internal reference number FI Technical display location B29C 49/48 7619−4F // B29L 22:00 4F

Claims (13)

[Claims] [Claim 1] A preform for molding a resin container, comprising: a cylindrical preform body; and a mouth portion having an outer layer formed by an insert piece having properties different from those of the preform body, and at least a portion of an inner layer formed by the same resin as the preform body, wherein the insert piece includes an upper outer layer portion having a threaded portion on its outer peripheral surface, and a lower outer layer portion located below the upper outer layer portion and having at least a support ring, and wherein the insert piece is formed by crystallized resin. [Claim 2] A preform for molding a resin container, comprising: a cylindrical preform body; and a mouth portion having an outer layer formed by an insert piece having properties different from those of the preform body, and at least a portion of an inner layer formed by the same resin as that of the preform body, wherein the insert piece includes an upper outer layer portion having a threaded portion on its outer peripheral surface, and a lower outer layer portion located below the upper outer layer portion and having at least a support ring, and wherein the support ring of the insert piece has a joint reinforcement portion formed by the resin used in injection molding of the preform body, integrally formed in a direction intersecting the axial direction of the preform body. [Claim 3] A preform for molding a resin container according to claim (2), characterized in that the joint reinforcement portion of the insert piece is constituted by a resin layer formed on the bottom surface of the support ring. [Claim 4] A preform for molding a resin container, as set forth in claim (3), characterized in that the joint reinforcement portion of the insert piece is constituted by a resin layer filled in a recess formed by recessing the inside of the bottom surface of the support ring in a ring shape, excluding the peripheral portion. [Claim 5] A preform for molding a resin container according to claim (3), characterized in that a recess and/or a protrusion is formed on the surface of the recess of the support ring to strengthen the bonding of the insert piece to the mouth portion. [Claim 6] A preform for molding a resin container, as set forth in claim (2), characterized in that the mouth portion has a two-layer structure along the axial direction of the mouth portion, with an inner layer made of the same resin as the preform body and the insert piece, and the upper end of the insert piece has a structure in which its inner diameter expands upward. 7. A preform for molding a resin container according to claim 6, wherein the upper end of the insert piece is tapered so that the inner diameter changes linearly. 8. A preform for molding a resin container according to claim 6, wherein the upper end of the insert piece is configured with a curved surface having an arcuate cross section. [Claim 9] In claim (2), the resin constituting the insert piece is polyethylene terephthalate, polybutylene terephthalate, polyethylene naphthalate, polypropylene, polyamide, polyacrylonitrile, polyvinylidene chloride, nylon,
A preform for molding a resin container, which is made of a resin selected from polyethylene and polycarbonate. 10. The preform for molding a resin container according to claim 2, wherein the insert piece is crystallized. 11. A preform for molding a resin container according to claim (2), wherein the insert piece is made of a resin obtained by crystallizing the same resin as that constituting the preform body. 12. The method according to claim (2), wherein the insert piece has a minimum wall thickness of at least 1.2 mm.
A preform for molding a resin container, characterized in that: [Claim 13] A molding die for a preform for molding a resin container, comprising a mouth molding die, a cavity die and a core die, wherein a preform is injection molded while an insert piece including an upper outer layer portion having a threaded portion on its outer peripheral surface and a lower outer layer portion having at least a support ring and an engaging recess is held within the mouth molding die, wherein the mouth molding die has on its inner wall a thread relief groove portion having a depth greater than the height of the threaded portion of the insert piece, and an engaging portion to be inserted into the engaging recess of the lower outer layer portion, wherein the insert piece can be inserted and fixed within the mouth molding die in any circumferential direction.