Manufacturing method for synthetic resin containers

Abstract

To suppress the occurrence of burst at the thinnest part of a bottom part which is stretched thinner when blow-molding a blow molded synthetic resin container.SOLUTION: The following relation is established between a thickness T of the thinnest part of a bottom 5 of a container 1 formed by blow-molding a bottomed cylindrical preform P and a thickness T0 of a portion of the preform P corresponding to the thinnest part of the bottom 5: T0 / T=K×T-0.94 (1), 3.26≤K≤4.40 (2), and 3.83×T-0.94≤30 (3).SELECTED DRAWING: Figure 6

Description

[Technical field]

[0001] The present invention relates to a synthetic resin container. [Background technology]

[0002] Conventionally, synthetic resin containers are produced by preparing a preform from a polyester resin such as polyethylene terephthalate, and then molding the preform into a bottle shape by biaxial stretch blow molding or the like, and are used in a wide range of fields as containers for various beverages, etc. This type of container is generally known as a PET bottle.

[0003] In recent years, a recycling technology called "bottle to bottle" has been considered, in which polyester resin molded products such as used PET bottles are collected and reused as recycled materials to manufacture PET bottles. For example, Patent Document 1 proposes a preform manufacturing device that includes a decontamination machine that removes contaminants from thermoplastic resin flakes, an injection molding machine that injection molds the thermoplastic resin supplied from the decontamination machine, a filter disposed between the decontamination machine and the injection molding machine, and a gear pump disposed between the filter and the injection molding machine. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Patent No. 6895990 Summary of the Invention [Problem to be solved by the invention]

[0005] According to the study of the present inventors, when using recycled materials to blow mold synthetic resin containers such as PET bottles, the frequency of bursting increases at the thinnest part of the bottom, which is stretched to be thinner. Based on this finding, the present inventors have investigated the cause of the high frequency of bursting and further studied to suppress the occurrence of bursting, which resulted in the completion of the present invention.

[0006] That is, an object of the present invention is to suppress the occurrence of bursting at the thinnest part of the bottom, which is stretched to be thinner, during blow molding of a synthetic resin container. [Means for solving the problem]

[0007] The synthetic resin container according to the present invention is a synthetic resin container obtained by blow molding a bottomed cylindrical preform into a predetermined container shape having a mouth portion, a shoulder portion, a body portion, and a bottom portion, and the thickness T of the thinnest part of the bottom portion and the thickness T0 of a portion of the preform corresponding to the thinnest part are such that T0 / T=K×T -0.94 (1) 3.26≦K≦4.40 (2) 3.83×T -0.94 ≦30 (3) This is a structure in which the following relationship holds true. Effect of the Invention

[0008] According to the synthetic resin container of the present invention, the occurrence of bursting at the thinnest part of the bottom, which is stretched to be thinner during blow molding, can be suppressed. [Brief description of the drawings]

[0009] [Figure 1] 1 is a perspective view showing an outline of a synthetic resin container according to an embodiment of the present invention. [Diagram 2] 1 is a front view showing an outline of a synthetic resin container according to an embodiment of the present invention. [Diagram 3]1 is a bottom view showing an outline of a synthetic resin container according to an embodiment of the present invention. [Figure 4] 1 is an explanatory diagram showing a part of a process for manufacturing a synthetic resin container according to an embodiment of the present invention by blow molding. [Diagram 5] 1 is a graph showing the correlation between the dimensionless stretch ratio T0 / T and the burst occurrence rate. [Figure 6] 1 is a graph showing the correlation between the thickness T of the thinnest part and the dimensionless draw ratio T0 / T. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0010] Hereinafter, preferred embodiments of the present invention will be described with reference to the drawings.

[0011] Fig. 1 is a perspective view showing a schematic of a synthetic resin container according to this embodiment, Fig. 2 is a front view of the same, and Fig. 3 is a bottom view of the same. The container 1 shown in these figures has a mouth portion 2, a shoulder portion 3, a body portion 4, and a bottom portion 5.

[0012] The mouth 2 is a cylindrical portion that serves as an outlet for pouring the liquid content. A screw thread 21 is provided on the side of the open end of the mouth 2 for attaching a lid (not shown) when sealing the container 1 filled with the liquid content.

[0013] The mouth 2 is provided with a neck ring 22 that protrudes annularly in the circumferential direction. The mouth 2 includes a portion that hangs down cylindrically with approximately the same diameter from directly below the neck ring 22. The lower end of the mouth 2 is connected to a shoulder 3 that connects the mouth 2 and the body 4. In the example shown in the figure, the shoulder 3 is formed in a rounded truncated cone shape, but the shape of the shoulder 3 is not limited to this. For example, it may be formed in a so-called hanging neck shape.

[0014] The body 4 occupies most of the height of the container 1 , and is formed roughly cylindrically. The upper end of the body 4 is connected to the shoulder 3 , and the lower end is connected to the bottom 5 . Here, the height direction refers to the direction perpendicular to the horizontal plane when container 1 is placed upright on the horizontal plane with mouth 2 facing up, and defines the up-down, left-right and length-width directions of container 1 in this state (as shown in Figure 2).

[0015] In this embodiment, the container 1 is configured as a pressure-resistant bottle for carbonated beverages having a petaloid-shaped bottom 5 with multiple (five in the illustrated example) legs 51 arranged rotationally symmetrically and radially at equal intervals around the central axis C of the container 1 so that the container's stability is not compromised even when positive pressure is created inside the container.

[0016] In the bottom 5 having such a shape, between adjacent leg parts 51, bottom groove parts 56 that are fitted into the inside of the container are formed so as to extend radially from the bottom center part 50. Meanwhile, the leg parts 51 have an outer peripheral surface part 52 that hangs down (in the illustrated example, hangs down while inclining slightly toward the inside of the container) from the upper end of the bottom 5 that is connected to the body part 4 and has a circumferential length (width) that is narrowed at the lower end side, side parts 53 that extend from both circumferential ends of the outer peripheral surface part 52 toward the bottom groove part 56, and a leg bottom surface part 54 that extends from the bottom center part 50 toward the lower end of the outer peripheral surface part 52, inclining downward. The connecting part between the outer peripheral surface part 52 and the leg bottom surface part 54 and the vicinity thereof are formed to become a grounding part 55 that comes into contact with the ground surface when the container 1 is uprighted.

[0017] In addition, the container 1 can be manufactured by using recycled materials prepared by applying recycling technologies known as mechanical recycling, chemical recycling, etc. from polyester resin molded products such as collected used PET bottles, producing a preform P molded into a bottomed cylindrical shape by injection molding, compression molding, etc., and molding this preform P into a predetermined container shape by biaxial stretch blow molding, etc.

[0018] When blow molding the container 1, for example, as shown in Fig. 4, it can be molded using a blow molding die M equipped with a body die M1 consisting of an openable and closable split die designed in consideration of die opening, and a bottom die M2 to be incorporated into the bottom side of the body die M1. More specifically, after being softened by heating to a state in which blow molding is possible, the preform P set in the blow molding die M is stretched in the axial direction (vertical direction) by a stretching rod (not shown) with the point directly below the neck ring 22 supported by the blow molding die M as the starting point of stretching, and is stretched in the axial direction and radial direction (horizontal direction) by high-pressure fluid blowing. Then, the cavity shape of the blow molding die M is transferred to the stretched portion, so that the shoulder part 3, the body part 4, and the bottom part 5 are formed, and the container 1 having a predetermined container shape is molded.

[0019] In the example shown in Figure 4, the blow molding mold M is shown in cross section cut along a plane parallel to the paper surface including the central axes of the preform P and the container 1, with Figure 4(a) showing the state in which the preform P is set in the blow molding mold M, and Figure 4(b) showing the state in which the blow molding mold M has been opened to remove the blow-molded container 1.

[0020] The inventors investigated the cause of the high frequency of bursting at the thinnest part of the bottom 5, which is stretched to be thinner, when blow molding the container 1 using recycled material in this manner, and discovered that if fine particles less than about 50 μm in size, which are difficult to capture by filters when preparing the recycled material, are present in the part that will become the thinnest part of the bottom 5, then bursting will occur from these fine particles during blow molding.

[0021] It is believed that such bursts originating from fine particles occur during the process in which the preform P is stretched and the corresponding portion is molded into the thinnest portion of the bottom 5. Therefore, focusing on the amount of stretching in the thinnest portion of the bottom 5, in other words, the partial stretching ratio in the thinnest portion of the bottom 5, the following study was attempted regarding the relationship with the burst occurrence rate when the container 1 is blow molded using recycled material.

[0022] That is, in the container 1 configured as a pressure-resistant bottle with a capacity of 500 mL and a mass of 17 g excluding the mouth part 2, the ground diameter and ground width of the bottom part 5 formed in a petaloid shape were changed to adjust the stretch amount at the thinnest part of the bottom part 5, and the relationship with the burst occurrence rate when blow molding was performed using recycled materials was examined. In this case, the stretch amount at the thinnest part of the bottom part 5 was calculated as the ratio of the thickness T0 of the part of the preform P corresponding to the thinnest part of the bottom part 5 before stretching to the thickness T of the thinnest part of the bottom part 5 (hereinafter referred to as the "dimensionless stretch ratio") T0 / T. An example of a sample used in the study is shown in Table 1 and FIG. 5.

[0023] 3, a circle including ground contact portion 55 of each leg portion 51 is shown by a chain line, the radius of the circle is the ground contact diameter, and the length along the circumferential direction of ground contact portion 55 is the ground contact width. The vicinity of both ends in the circumferential direction of ground contact portion 55 of each leg portion 51 becomes the thinnest portion that is stretched to be thinner during blow molding. 4(a), the lower end of the cylindrical body of the preform P corresponds to the thinnest part of the bottom 5. The thickness T0 can be measured, for example, by making a plurality of annular notches parallel to the axial direction along the circumferential direction on the lower end, and identifying in advance the part corresponding to the thinnest part of the bottom 5 from the traces of the notches remaining on the blow-molded container 1.

[0024] [Table 1]

[0025] As shown in the graph of Fig. 5, which is a regression line obtained as a result of the regression analysis, a strong positive linear relationship is observed between the dimensionless stretch ratio T0 / T and the burst occurrence rate. Therefore, from these results, it can be confirmed that the occurrence of bursts during blow molding using recycled materials can be suppressed by reducing the dimensionless stretch ratio T0 / T of the thinnest part of the bottom part 5 so that the amount of stretching at the thinnest part of the bottom part 5 is reduced.

[0026] In order to adjust the amount of stretch at the thinnest part of the bottom 5, in addition to changing the ground contact diameter and ground contact width of the bottom 5, the depth of the bottom groove 56 formed between adjacent leg parts 51 may be changed. Furthermore, the amount of stretch at the thinnest part of the bottom 5 can also be adjusted by the blow molding conditions (including the use of a device for reducing the amount of local stretch) and the preform shape (including the setting of the stretch ratio, the setting of the local thickness of the preform, etc.).

[0027] Furthermore, as shown in Table 2 and Fig. 6, even if the volume of the container 1 and the mass of the preform P used in the blow molding (particularly the mass of the portion to be stretched excluding the mouth portion) are different, it can be confirmed that there is a strong correlation between the thickness T of the thinnest part of the bottom 5 and the dimensionless stretch ratio T0 / T. That is, Table 2 and Fig. 6 show an example of a sample used in the study, which differs in volume, mass, and shape of the bottom 5. In the graph shown in Fig. 6, the regression curve obtained as a result of the regression analysis is shown by a dotted line, and the regression curve is expressed as T0 / T=K×T -0.94 It can be seen that the distribution is within the range of ±15% (3.26≦K≦4.40) with K=3.83 as the center value.

[0028] Furthermore, the results of evaluating the occurrence of bursting based on the following criteria are also shown in Table 2. It can be seen that even if the capacity of the container 1 is different, the occurrence of bursting during blow molding using recycled materials can be suppressed by reducing the dimensionless stretch ratio T0 / T of the thinnest part of the bottom 5.

[0029] [Burst Rating] ⊚: No burst was observed. ○: Sporadic bursts were observed from time to time. ×: Many bursts occurred.

[0030] [Table 2]

[0031] From the above, between the thickness T of the thinnest part of the bottom part 5 and the thickness T0 of the part of the preform P corresponding to the thinnest part, T0 / T=K×T -0.94 (1) 3.26≦K≦4.40 (2) 3.83×T -0.94 ≦30 (3) It has been found that the establishment of the above relationship can be an index for designing the container 1 so as to suppress the occurrence of bursting during blow molding using recycled materials. Therefore, the container 1 designed based on such an index suppresses the occurrence of bursting at the thinnest part of the bottom 5, which is stretched to be thinner.

[0032] In this way, when designing the container 1 so as to suppress the occurrence of bursting when blow molding is performed using recycled materials, the following formula is used instead of the above formula (3): 3.83×T -0.94 ≦26 (4) It is preferable that the following relationship is satisfied.

[0033] It is preferable to appropriately set the lower limit of the dimensionless stretch ratio T0 / T of the thinnest part of the bottom 5 so that the amount of stretching at the thinnest part of the bottom 5 is reduced, while the amount of stretching at other parts is not insufficient, which goes against the demand for a thinner and lighter container 1 (for example, a range preferably not less than 6, more preferably not less than 7).

[0034] As described above, taking into consideration that the lower end portion of the cylindrical body of the preform P corresponds to the thinnest portion of the bottom portion 5, the wall thickness of the lower end portion of the cylindrical body of the preform P is preferably 2.3 to 4.4 mm. Furthermore, it is preferable that the wall thickness of the body portion 4 is as thin as possible within the range of 0.09 to 0.60 mm so as to ensure the thickness of the bottom portion 5. These aspects are advantageous in reducing the dimensionless draw ratio of the thinnest portion of the bottom portion 5 so that the amount of drawing at the thinnest portion of the bottom portion 5 is alleviated.

[0035] Although the present invention has been described above by showing preferred embodiments, it goes without saying that the present invention is not limited to the above-described embodiments, and various modifications can be made within the scope of the present invention. [Explanation of symbols]

[0036] 1 container 2 Mouth 3 Shoulder 4. Torso 5 Bottom 51 Legs P Preform

Claims

1. A method for manufacturing a synthetic resin container, comprising blow-molding a bottomed cylindrical preform into a predetermined container shape having a mouth, shoulders, body, and bottom, The thickness T of the thinnest part of the bottom and the thickness T of the part of the preform corresponding to the thinnest part. ０ Between, T ０ / T=K×T －０．９４ ・・・(1) 3.26 ≤ K ≤ 4.40 ... (2) 3.83×T －０．９４ ≦30 ・・・(3) A method for manufacturing a synthetic resin container characterized by the existence of the following relationship.

2. Instead of the above formula (3), 3.83×T －０．９４ ≦26 ・・・(4) A method for manufacturing a synthetic resin container according to claim 1, wherein the following relationship holds true.

3. A method for manufacturing a synthetic resin container according to claim 1 or 2, comprising a bottom having a plurality of legs arranged radially at equal intervals and rotationally symmetrically with respect to a central axis.

4. A method for manufacturing a synthetic resin container according to Claim 1, wherein the relationships 6 ≤ T 0 / T and / or 2.3 ≤ T 0 ≤ 4.4 hold.

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