JPH048206B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for forming a plastic cap with excellent stress crack resistance and low temperature impact resistance. Plastic caps are currently widely used for plastic and glass bottles for mayonnaise, ketchup, cooking oil, instant coffee, liquid detergents, cosmetics, etc. These plastic caps are mainly molded by injection molding. On the other hand, only metal crowns or aluminum screw caps are used for plastic and glass bottles for beer and carbonated drinks. In the United States and Europe, injection molded plastic caps such as those described in U.S. Pat. No. 4,016,996 have begun to be used for plastic bottles and glass bottles for beer and carbonated beverages. However, such injection-molded plastic caps are not widely used because their performance is still not sufficient for applications such as beer and carbonated drinks that are stored at low temperatures and have high internal pressure inside the container. Unfortunately, it is not used at all in Japan. Injection molded plastic caps have poor stress cracking resistance and low temperature impact resistance.
There are problems such as circumferential cracks appearing near the top of the skirt of the cap or near the periphery of the top surface during long-term storage, and the cap easily breaking when subjected to impact during low-temperature storage. This problem was a fatal drawback for caps for containers that are used to store carbonated drinks, beer, etc. at low temperatures and where internal pressure easily builds up. On the other hand, the technology of forming plastic caps by compression molding is disclosed in, for example, published patent publications No. 56-501315 and published patent publications No. 56-501121, but the stress crack resistance of plastic caps for gas beverages is poor. , low-temperature impact resistance, and dimensional stability were extremely inadequate. The present invention proposes a molding method that solves these problems with plastic caps. According to the present invention, in a method for compression molding a plastic cap having a top surface portion and a skirt portion extending substantially perpendicularly from the peripheral edge of the top surface portion, and having a threaded projection portion on the inner surface of the skirt portion, the material temperature T is the formula: T _D ≦T≦Tm+40, preferably the formula: Tm≦T
A compression molding method characterized by satisfying ≦Tm+40 is provided. Here, T _D refers to the yield point extinction temperature of the resin used as the material. For example, for polypropylene, which has the stress strain curve shown in Figure 1, T _D is 125°C, and the stress strain curve shown in Figure 2 is 125°C. - For polyethylene with a strain curve, T _D is 95°C. Further, Tm refers to the melting point of the resin used as the material. If the material temperature is too low (less than TD), the formability will be poor, while if it is too high (more than Tm+40), the stress crack resistance will decrease.
Moreover, productivity is also lowered, which is not preferable. In this invention, melting point refers to, for example, “CRYSTALIZATION OF POLYMERS” by LEO MANDELKERN.
(McGraw-Hill Book Company, published 1964)
As described in , it is defined as the thermodynamic first-order dislocation point where the crystals of a crystalline or semi-crystalline polymer melt, and is measured by differential thermal analysis. An example of a plastic cap to which the plastic cap molding method of the present invention is applied is shown in FIG. The plastic cap mainly consists of a top part 1 and a skirt part 2, and has a pilfer-proof band 4 as an extension of the skirt part 2, and has a threaded protrusion 3 on the skirt part 2. The pilfer-proof band 4 has a bridge 6 and an inner origin 5;
When the cap is opened, the pilfer-proof band 4 is cut from the cap by the bridge 6 and remains in the bottle. The cap also has a liner 7 for better sealing. In order to mold a plastic cap in the present invention, for example, a predetermined amount of material maintained at a predetermined material temperature is placed in a cavity 8 shown in FIG. be done. The plunger 9 has a pilfer inner wall forming plunger 1.
0. A pillfur outer wall forming sleeve 11, a stripper sleeve 12 and an ejector pin 13 are attached, and an air passage 14 is connected to the cap inner wall forming plunger 9, and an air vent 15 is connected to the cap inner wall forming plunger 9 and the pillfur inner wall forming. It is provided between the plungers 10. Details of molding using these cavities, plungers, etc. will be further explained in Examples below. When compression molding the plastic cap of the present invention, the cavity temperature T _c is 0.1T _n ≦T _c ≦
Within the range of T _n , preferably 0.2T _n ≦T _c ≦0.5T _n (T _n
is preferably within the range of (the melting point of the material resin). If the cavity temperature T _c is too low, such as less than 0.1 T _n , it is not desirable because molding is impossible or even if molding is possible, the dimensional stability of the cap decreases, and low-temperature impact resistance and stress cracking resistance decrease. On the other hand, if it is too high, exceeding T _n , the cap is likely to be deformed during removal, and productivity will also be reduced, which is undesirable. Note that the temperature of the plunger is preferably maintained at the same temperature as the cavity. In this specification, the term "cavity" refers to a part of the forming mold component that forms the outer surface of the skirt and the surface of the top surface that is continuous with the outer surface of the skirt. The material resin used in the plastic cap molding method of the present invention is a polypropylene resin. Here, the polypropylene resin refers to a propylene homopolymer, a propylene-olefin copolymer such as a propylene-ethylene copolymer, and a mixture of polypropylene and polyolefin (eg, polyethylene). In the case of copolymers and mixtures, those containing 80 mol% or more of propylene are preferred. Among the above-mentioned resins, polypropylene-based resins copolymerized with ethylene are most preferred. Resins for materials such as these are melt index MI (JIS
K7210) preferably has a flow index of 3 to 10 g/10 min when measured at 230°C and a load of 2160 g; if it is too low, less than 3 g/10 min,
On the other hand, if it exceeds 10 g/10 minutes, the stress crack resistance and low temperature impact resistance will decrease, which is not preferable. The reason why plastic caps compression-molded by the plastic cap molding method of the present invention has extremely excellent stress cracking resistance, low-temperature impact resistance, and dimensional stability is not necessarily clear; One reason is that in the plastic cap molded by the molding method of the present invention, the polymer forming the resin material tends to move from the center of the top to the periphery of the top and further to the bottom of the skirt. This is presumed to be because the orientation is significant in the vertical direction. Examples of molecular orientation in such caps are shown in FIGS. 10 and 11 (control). Comparing FIG. 10 of the X-ray diffraction photograph of the plastic cap of Example 1 (experiment number P-2) and FIG. 11 of the X-ray diffraction photograph of the plastic cap of Comparative Example 1 (experiment number PR-9), Comparative Example 1, PR
In the diffraction photograph of Example 1 and P-2, the diffraction intensity of the four Debye Schierer rings from the inner side by X-rays is shown to be equal in intensity along the ring, but in the diffraction photograph of Example 1 and P-2, the diffraction intensity of the four Debye Schierer rings from the inside is shown to be equal. The diffraction intensity is locally strong along the ring. From this, it can be seen that in Example 1, crystal orientation occurs to a large extent. (The vertical direction (long side) of these photos indicates the vertical direction of the skirt part, and the horizontal direction (short side) of the photos
indicates the circumferential direction of the cap). Plastic caps according to the present invention include a variety of shapes. An example of the molding method of the present invention will be described in detail below. As shown in Fig. 3, the cap consists of a disk-shaped top surface 1, a skirt portion 2 having a screw protrusion 3 hanging down from the outer edge of the top surface in a cylindrical shape, and a skirt portion extending from the lower end of the skirt portion 2. A pilfer-proof band 4 with protrusions as shown in Figure 5 on the inside with almost the same diameter, low-density polyethylene (LDPE), linear low-density polyethylene (LLDPE) on the top surface,
Polyvinyl chloride (PVC), ethylene propylene rubber (EPR), ethylene vinyl acetate (EVA)
It has a liner 7 formed and lined with a thermoplastic such as. The pilfer-proof band 4 is connected to the lower end of the skirt by a weak bridge 6 which can be easily separated. Due to the shape of the pilfer-proof band 4, the protrusion hangs over the protruding part at the bottom of the bottle mouth when the bottle is capped, and is cut and separated from the bridge part when the bottle is opened, leaving the pilfer-proof band at the bottom of the bottle mouth. The liner 7 is melt-bonded to the top surface of the cap and/or is engaged with a ring-shaped projection 17 formed on the top surface of the cap and having an outer diameter approximately equal to the inner diameter of the bottle mouth and having a slightly inverted truncated conical shape. Ru. In the first stage of the cap, an extruder extrudes resin forming the cap top surface, skirt portion, and pilfer-proof band (hereinafter referred to as the cap outer shell) in the form of a strand while being heated by an extruder. A certain amount of the extruded strand-shaped resin is cut off by a rotary cutter and dropped into a cavity kept at a constant temperature. In the second stage, the heated resin dropped into the cavity in the first stage is compressed by the cap outer shell forming plunger and the cavity immediately after falling, as shown in FIG. 4, to form the shape of the cap outer shell. Keep compressed to cool long enough to harden. The third stage is demoulding. For details, see Figures 6 to 9.
Shown in the figure. As shown in FIG. 6, among the upper plunger, cavity, and cap outer shell held, the stripper sleeve 12 and the sleeve 11 forming the outside of the pilfer-proof band, which constitute the upper plunger, rise. Next, as shown in FIG. 7, the sleeve 10 forming the inner side of the pilfer-proof band moves upward and separates from the pilfer-proof band while rotating clockwise when viewed from the top in order to facilitate the removal of the inner protrusion of the pilfer-proof band. . Next, the cap inner wall forming plunger 9 rises together with the cap outer shell 16 and separates from the cavity 8, and then, as shown in FIG. 9, the ejector pin 13 passes through the stripper sleeve 12 and the inside of the cap inner wall forming plunger and reaches the lower surface.
are simultaneously lowered, and air is further jetted from the air passage 14 along the outer periphery of the ejector pin 13, causing the cap outer shell 16 to separate from the cap inner wall forming plunger 9. In the fourth step, a fixed amount of the strand-shaped liner forming resin heated and melted and extruded from the extruder is cut off with a rotary cutter and dropped onto the upper part of the top surface of the cap outer shell, which is positioned by a turret or the like. In the fifth step, the liner forming molten resin dropped onto the top of the cap is compression-molded by a liner forming plunger equipped with a liner forming sleeve immediately after the fall, thereby forming a liner 7 as shown in FIG. It is formed. Furthermore, in the sixth step, bridges 6 are formed by making elongated holes interrupted in the circumferential direction as shown in FIG. 3 using a rotary cutter in a portion near the base of the pilfer-proof band. This bridge forming step may be performed simultaneously with the liner forming step. There are various ways to connect each stage of the above flow. In a preferred embodiment, a rotary compression molding machine is used in which the first to third stages consist of a rotary stage having several cavities and a rotary drum having a forming plunger coaxial with the rotary stage. The cap outer shell obtained by the rotary compression molding machine is transferred to a belt conveyor turret, chute, etc. for liner formation.
A rotary liner forming apparatus is used which comprises a rotating drum having a rotating turret and a liner forming plunger rotating coaxially with the rotating turret. The cap thus obtained is brought to the bridge forming step by means of a conveyor, turret, chute, etc. Two or more of the above three processes connected by a conveyor, turret, chute, etc. may be performed by one device. Further, it should be understood that the delivery of the resin into the cavity and/or the cap shell may be accomplished by heated discs or other shaped sheets. Example 1 The stress-strain curve shown in Fig. 1 was obtained, the yield point extinction temperature was 125°C, the melting point by differential thermal analysis was 160°C, the temperature was 230°C, and the load was 2160 g (JIS K-7210). Commercially available polypropylene (Mitsubishi Noblen BC5C) with a melt index of 3.0 g/10 min under measurement conditions 14) of -7210 was used with a screen diameter of 40
mm, extruded from an extruder with a nozzle diameter of 5 mm, cut to a certain weight (2.3 g) using a rotary cutter, and dropped into a cavity. Immediately after dropping, lower the cap outer shell inner wall forming plunger to lower the resin temperature.
Cap formation was carried out at 170°C. The cavity temperature was maintained at 65°C using an internal heater. The molding pressure was approximately 800 kg/cm ² , and the bottom dead center of the cap outer wall forming plunger was set at a position where a proper cap was formed. After maintaining the compressed state for about 7 seconds for cooling, the cap was released from the mold in the same manner as in the previous three steps. Next, ethylene-vinyl acetate (EVA) is heated and melted and extruded from an extruder into the outer shell of the cap.
The copolymer was cut by a constant weight using a rotary cutter and dropped, immediately compressed using a liner-forming plunger, and then a liner-forming sleeve was lowered to form a liner. Next, adjust the resin temperature to 160℃, 180℃, 190℃, 200℃,
Caps were formed in exactly the same manner as described above, except that the molding pressure was changed according to each temperature. Table 1 shows the molding pressures (required minimum molding pressures) employed at each resin temperature of 160 to 200°C. Table 1 shows the results of the inner diameter measurement of the threaded portion of the cap after molding, the stress-crack resistance test, and the low-temperature impact resistance test conducted on 20 caps under each of the conditions obtained above. P
Shown as -1 to 5. The value of the inner diameter of the threaded portion of the cap was obtained by measuring the distance between the top of the threaded protrusion and the threaded protrusion at a position opposite to the top with the center line of the cap as an axis using a stereomicroscope. The stress-cracking test was conducted by gently filling a glass bottle with carbonated water containing approximately 4 volumes of carbon dioxide gas at 15.6°C, sealing the molded cap, and placing it in a surfactant aqueous solution (0.5% concentration) at 50°C. To,
This was done by leaving the bottles for one week so that the caps were sunken, and visually counting the number of bottles in which circumferential cracks had occurred in the caps. In the low-temperature impact resistance test, a molded cap was attached to a glass bottle filled with water, and the bottle was left in an atmosphere of 4℃ for one day and night, and then the bottle was placed on a metal surface tilted at 10 degrees from a height of 1m in an atmosphere of 4℃. This was done by dropping bottles with their tops facing down and counting the number of bottles whose caps were damaged due to the fall. Comparative Example 1 Using the same polypropylene as in Example 1,
Resin temperature 100℃, 110℃, 120℃, 125℃, 130℃,
The cap shell was molded in exactly the same manner as in Example 1 when the resin temperature was 170°C, except that the molding pressure was changed according to each temperature at 140°C, 150°C, 210°C, and 220°C. A liner was formed in the same manner as in Example 1 for the cap on which the outer shell had been formed. The caps obtained in this way were evaluated in the same way as in Example 1, and the results are shown in Table 1.
Shown as -1 to 9. As is clear from Table 1, molding cannot be performed with PR-1 to PR-3. Cap molding is possible with PR-4 to PR-7, but the required molding pressure is extremely high at 5000 kg/cm ² or more, and the productivity due to multiple molding is significantly reduced, so they cannot be used in actual production. PR-4
It was found that in the cases of 5 to 5, the variation (standard deviation) in the inner diameter of the threaded portion becomes large, so that the product yield is further reduced. PR-8 to PR-9 have poor low temperature impact resistance and stress crack resistance. Example 2 and Comparative Example 2 The melting point according to differential thermal analysis is 160°C, and JIS K-
Load 2160g at 230℃ according to 7210 (JIS K-
Melt index and T _D under condition 14) of 7210
commercially available polypropylene (Mitsubishi Noblen BC3) with 10.0g/10min and 122°C, commercially available polypropylene (Shiowa Aroma SK-111) with 0.1g/10min and 128°C, and commercially available polypropylene (Mitsubishi) with 15g/10min and 123°C, respectively. The outer shell of the cap was formed using three types of commercially available polypropylene such as Noblen BC2) at a resin temperature of 170°C in the same manner as in Example 1, except that the molding pressure was adjusted to suit each melt index.
A liner is formed in the same manner as in Example 1. The cap thus obtained was examined in the same manner as in Example 1 for the inner diameter of the cap screw portion, low-temperature impact resistance, and stress/cracking resistance. The obtained results were P-a (Example 2), P-b
(Comparative Example 2) and P-c (Comparative Example 2), P-
2 (Example 1), as is clear from Table 2, the plastic cap P-a of Example 2 has the same excellent performance as the P-2 cap of Example 1, but It can be seen that the P-b cap of Comparative Example 2 has a large variation (standard deviation) in the internal diameter of the threaded portion and is inferior in product yield, and also requires a large molding pressure and is inferior in multi-cavity moldability, that is, productivity. P-
It can be seen that although the required molding pressure of the c cap is small, it is extremely inferior to the caps of Examples 1 and 2 according to the present invention in terms of low temperature impact resistance and stress crack resistance. Example 3 and Comparative Example 3 Experiment No. P-2 of Example 1 except that the cavity temperature was kept at 35°C, 140°C, and 70°C.
A cap was molded under the same conditions as in -6. Example 1
The results of the test in the same manner as P-2-b (Example 3), P-2-a (Comparative example 3) and P-2-c (Comparative example 3) are P-2 (Example 1) They are also shown in Table 3.

【table】

[Table] *2 Percentage of the number of caps damaged when dropped compared to the number of test caps.
*3 Percentage of the number of caps with cracks compared to the number of test caps.

【table】

【table】 [Brief explanation of drawings]

FIG. 1 shows stress strain curves for the resin material used in the present invention. FIG. 2 is a side view and a partial cross-sectional view of a plastic cap molded by the method of the present invention, and FIGS. FIG. 4 is a partially enlarged perspective view of a pilfer-proof band of a plastic cap molded according to the present invention, and FIGS. 9 and 10 are X-ray diffraction images of the skirt portion of the plastic cap. It is a drawing-substituting X-ray photograph showing the figure.

Claims (1)

[Claims] 1. A polypropylene resin cap having a top surface portion and a skirt portion extending substantially perpendicularly from the peripheral edge of the top surface portion, and having a threaded protrusion on the inner surface of the skirt portion, is compressed by force punching. In the molding method, MI as polypropylene resin: 3 to 10 g/
A compression molding method characterized by using a material for 10 minutes and performing molding under conditions of a material temperature of 160 to 200°C and a cavity temperature of 16 to 65°C.