Closure cap made of plastic material
By designing the connecting ring and sealing cone structure of the PET sealing cap, combined with low-temperature injection molding and short cycle time, the problem of high hardness of PET material was solved, achieving reliable sealing and efficient recycling of the PET sealing cap.
Patent Information
- Application Number
- CN202480020054.6
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- Priority Date
- 2023-03-28
- Filing Date
- 2024-03-26
- Publication Date
- 2025-10-21
AI Technical Summary
PET material is unsuitable for use as a sealing cap due to its high hardness, and current technology requires expensive injection molds to manufacture PET sealing parts, and its recyclability is limited.
The closure is made of PET and includes a connecting ring and a sealing cone. The connecting ring has a wall thickness of 0.2 mm to 0.4 mm and the surface roughness of the internal and external threads is between 0.1 µm and 0.2 µm. Combined with low-temperature injection molding and short cycle time, it ensures both sealing and flexibility.
It achieves a reliable seal for PET caps, avoids friction welding, reduces manufacturing costs, and improves the recyclability of PET, making it suitable for food safety containers.
Smart Images
Figure CN120826355A_ABST
Abstract
Description
Technical Field
[0001] The invention relates to a closure cap made of plastic material according to the preamble of claim 1 , a combination of a closure cap according to the preamble of claim 9 with a container made of PET, and a method for producing such a closure cap according to the preamble of claim 12 . Existing technology
[0002] Polyethylene terephthalate (PET) is polar, meaning strong intermolecular forces exist. The molecule is also linear in structure, lacking crosslinks. Therefore, it is suitable for manufacturing rigid objects with high breaking strength, such as stable PET bottles capable of withstanding high internal pressure. Hardness and rigidity can be further increased by stretching PET preforms in the radial and axial directions. Due to its hardness, PET is unsuitable for closures, which typically require softer, and therefore more effective, plastic materials. Therefore, polyolefins are primarily used for closures. However, a disadvantage of polyolefins compared to PET is their limited recyclability, even when they are of food-safe quality.
[0003] Therefore, experiments were conducted using an injection mold with improved thermal conductivity, which allows the temperature in the injection mold to be increased and decreased rapidly. This was intended to make it possible to inject PET into the mold and release it from the mold.
[0004] Purpose of the present invention The described disadvantages of the prior art give rise to the problem of producing a plastic closure which can be made from the same PET material as the container it closes and which reliably seals the container without requiring expensive injection moulds to produce the closure.
[0005] describe The solution to the proposed problem is achieved in a closure cap made of PET for closing a container made of PET by the features specified in the characterizing part of claim 1. The dependent claims relate to developments and / or advantageous alternative embodiments.
[0006] The present invention is characterized in that a connecting ring is disposed between the sealing cone and the top plate, the connecting ring having a wall thickness between 0.2 mm and 0.4 mm and connecting the sealing cone to the top plate, and in that the closure cap, the sealing cone, and the connecting ring are made of PET. The very thin-walled connecting ring imparts flexibility to the sealing cone comparable to that of HDPE and PP. This means that the sealing cone can smoothly abut the inside of the container neck, ensuring a reliable seal between the sealing cone and the container neck. This is despite the fact that the cap is made entirely of relatively hard and brittle PET.
[0007] In a preferred embodiment, the internal thread has a surface roughness of between 0.1 μm and 0.2 μm, and preferably between 0.14 μm and 0.16 μm. This prevents the internal thread from being too smooth and thus from risking welding to the external thread of the container neck when the internal thread is first screwed onto the container neck. The increased roughness of the internal thread prevents friction welding caused by fast-moving capping machines in the packaging industry.
[0008] It has proven useful if the outer side of the sealing cone is polished and has a surface roughness of less than 0.15 μm and preferably less than 0.02 μm. The smooth surface improves the sealing effect of the sealing cone when it rests on the inside of the container neck.
[0009] The closure cap conveniently includes a retaining element onto which the retaining element engages when the closure cap is ejected from the injection mold. Conventional demolding of hard PET closures is difficult because the threaded portion is not easily deformed due to its wall thickness, and the internal threads intersect by more than 0.4 mm. The retaining element allows the closure cap to be removed by rotating the injection core. When the core is unthreaded from the closure cap, the retaining element prevents the cap from rotating. The retaining element can be a hole, a rib, or a notch. Notches are preferred because they can be formed during injection molding using a simple mold.
[0010] Conveniently, the closure cap is colorless. This means the cap is transparent or white. This means that during recycling, the cap does not introduce any disruptive color pigments.
[0011] In another particularly preferred embodiment of the present invention, the closure cap is made of up to 100% food-safe rPET. Compared to PP (polypropylene) or HDPE (high-density polyethylene), the plastic materials used primarily for closure cap production, PET is very recyclable, even into high-quality food-safe rPET. The closure cap according to the present invention, including the thin-walled connecting ring, can be made from this rPET.
[0012] The closure cap conveniently comprises a protective ring which is held on the opening edge of the threaded portion by a plurality of predetermined breaking connections, which means that if the predetermined breaking connections break, the cap can be immediately identified as having been opened for the first time.
[0013] Another aspect of the invention relates to a combination of a closure cap and a container as described above, the cap being screwed onto the container, wherein the outer thread of the container neck has a surface roughness of between 0.1 µm and 0.2 µm, and preferably between 0.14 µm and 0.16 µm. As explained above, this prevents friction welding of the outer and inner threads during the first rapid screwing together using a fully automatic capping machine.
[0014] In another preferred embodiment, the inside of the container neck has a surface roughness of between 0.1 μm and 0.2 μm and preferably between 0.14 μm and 0.16 μm. This also prevents friction welding of the sealing cone to the container neck, even if the sealing cone is polished to improve the sealing function.
[0015] In a particularly preferred embodiment of the invention, the outer diameter of the sealing cone, when threaded onto the container neck, is between 0.4 mm and 0.6 mm larger than the inner diameter of the container neck after the closure has shrunk by between 0.4% and 0.6%. PET always exhibits shrinkage, which must be taken into account when sizing the sealing cone so that the outer diameter of the sealing cone does not become too small upon shrinkage and the closure does not become leaky.
[0016] Another aspect of the present invention relates to a method for producing the described closure cap from PET. This method is characterized in that the mold temperature of the injection mold during injection molding is between 12°C and 52°C, preferably between 20°C and 40°C, and particularly preferably between 25°C and 35°C. Surprisingly, at these low temperatures, residual stresses in the closure can be kept low and the closure does not become brittle. The selected mold temperature prevents stress cracks that can grow over time and lead to deformation of the sealing cone and leakage.
[0017] In another particularly preferred embodiment of the present invention, the cycle time between injecting the molten PET and demolding the closure cap is less than 15 seconds, preferably between 6 and 12 seconds, and particularly preferably between 8 and 12 seconds, resulting in the closure cap not having time to retract onto the injection core. These short cycle times prevent the retracting material from developing tensile stresses in height and circumference, which could lead to microscopic cracks (crazes) that damage the material through embrittlement. These stress cracks become larger over time, causing increased embrittlement of the material. Frozen stresses, which should be avoided, can also lead to deformation and corresponding shrinkage of the sealing cone over time, and subsequently to leakage.
[0018] Preferably, the residence time of the PET in the injection extruder is less than 350 s, and preferably between 50 and 250 s, and the temperature of the PET in the injection extruder is less than 300° C., and preferably between 270° C. and 285° C. By combining a short residence time with the selected temperature, degradation of the PET material, which would lead to its embrittlement, is prevented.
[0019] In another preferred embodiment of the present invention, molten PET is injected from an injection extruder into the mold at an injection speed of between 15 g / s and 35 g / s and an injection pressure of greater than 1000 bar, preferably between 1500 bar and 3500 bar. The high injection speed enables a thin-walled PET closure cap (with a minimum wall thickness of 0.2 mm at the connecting ring) to fill the entire cavity of the injection mold without the PET material freezing and the cavity in the area of the connecting ring remaining unfilled. The high injection pressure enables high injection speeds.
[0020] The invention is also preferably characterized in that after reaching the filling point, the pressure is reduced as quickly as possible to below 1000 bar, and preferably to a pressure below 500 bar. The rapid pressure reduction prevents crystallization of the PET material, which occurs at high injection pressures and leads to undesirable embrittlement.
[0021] In another particularly preferred embodiment of the present invention, the injection core is rotated out of the closure cap to demold it. A retaining element prevents the closure cap and the injection core from rotating together. For forced demolding of relatively hard PET closure caps, the threaded section is too thick and the undercut of the internal thread is too deep. Therefore, rotating the core out provides an alternative demolding method to forced demolding, which is possible and common for closures made of PP or HDPE.
[0022] Further advantages and features will become apparent from the following description of embodiments of the invention with reference to the accompanying schematic drawings, in which, in a schematic diagram not drawn to scale: Figure 1 : a closure cap and a container (preform) to which the closure cap can be screwed in an axonometric view; Figure 2 : A closure cap threaded onto a container shown in side view; Figure 3 :Along Figure 2 A sectional view taken along section line III-III of FIG. Figure 4 : Cross-sectional view through the closing cover.
[0023] Figures 1 to 4 A closure cap made of PET is shown, which is designated as a whole by the reference numeral 11. The closure cap 11 is intended to be screwed onto a container 13. The container is also made of PET and is Figures 1 to 3Shown in FIG. 1 is a preform 13 which is stretch blow molded to form a container or bottle.
[0024] The lid 11 includes a circular top plate 15 and a cylindrical threaded portion 17 surrounding the top plate 15. The threaded portion 17 has an opening edge 19 and an internal thread 21. A sealing cone 23 protrudes on the inner side of the top plate 15. The container 13 includes a container neck 25 having an external thread 27 that mates with the internal thread 21. The container neck 25 defines a container opening 29 through which filling material is poured into and out of the container 13.
[0025] When screwed together, the sealing cone 23 penetrates into the container opening 29 and thereby seals the opening 29 against the lid 11. This allows the lid 11 to reliably seal the opening 29.
[0026] Connecting ring 31 is positioned between the sealing cone and the top plate. Connecting ring 31 has a wall thickness between 0.2 mm and 0.4 mm and connects sealing cone 23 to top plate 15. Because the wall thickness of connecting ring 31 is very small, the sealing cone remains flexible and can seal the opening, even if it is made of hard PET, which is less flexible than HDPE or PP. Thanks to connecting ring 31, sealing cone 23 is as flexible as a closure made of PP or HDPE.
[0027] The internal and external threads 21, 27 have a surface roughness between 0.14 and 0.16 µm. This high surface roughness ensures that friction welding of the threads 21, 27 occurs when the closure is screwed together, and that the torque required to reliably seal the opening 29 can be applied. The surface roughness of the inside of the container neck 25 also lies between 0.14 and 0.16 µm, preventing friction welding of the sealing cone to the container neck.
[0028] The closure cap 11 has a notch 33 on which the retaining element can engage when the closure cap 11 is ejected from the injection mold. Thus, the closure cap 11 can be ejected from the injection mold by rotating the injection core and can be ejected despite the presence of undercuts and low PET deformability.
[0029] The outer diameter 35 of the sealing cone must be slightly larger than the inner diameter 37 of the container neck in order to achieve a reliable sealing function. This excess is also called prestressing force. Because PET shrinks after injection molding, this must be taken into account to obtain the correct prestressing force. Otherwise, due to the unaccounted shrinkage, the outer diameter 35 of the sealing cone becomes too small and the container neck 25 can no longer be sealed. The shrinkage of the closure cap is approximately 0.5%, which means that the outer diameter of the sealing cone shrinks by approximately 0.1 mm to 0.4 mm. When the shrinkage is complete, the outer diameter 35 when screwed onto the container neck exceeds the inner diameter 37 of the container neck by 0.4 mm to 0.6 mm. Because the shrinkage or retraction process is complete, this excess is sufficient to reliably seal the container neck using the sealing cone.
[0030] The closure cap 11 is preferably colorless or transparent. This makes it particularly easy to process the closure cap together with the container 13 to form a container made of rPET.
[0031] The method for producing the described closure caps in an injection mold has the following distinguishing features which reduce the brittleness of the PET material and thus contribute to making PET a suitable material for closure caps: The mold temperature of the injection mold during injection molding is between 12°C and 52°C, preferably between 20°C and 40°C, and particularly preferably between 25°C and 35°C. This allows the residual stresses in the closure 11 to be kept low and prevent the closure from becoming brittle. The selected mold temperature prevents stress cracks that grow over time and lead to deformation and leakage of the sealing cone 23.
[0032] The cycle time between injecting the molten PET and demoulding the closure cap is preferably between 6 and 12 seconds, and particularly preferably between 8 and 12 seconds. The closure cap thus does not have time to retract onto the injection core. This retraction onto the injection core would lead to undesirable tensile stresses in the cap, which are prevented by the selected cycle time.
[0033] By ideally reducing the residence time in the injection extruder to 50 to 250 seconds, combined with a temperature preferably between 270°C and 285°C, degradation of the PET material of the cap 11 and the associated brittleness can be achieved. However, such a low melting temperature can cause the PET material to freeze. Therefore, the injection speed into the injection mold is selected to be between 15 g / s and 35 g / s. In order to be able to inject the connecting ring 31 so that it has a very small wall thickness of preferably 0.25 mm and fill the area of the connecting ring 31 in the injection mold, a high injection pressure is selected. The injection pressure is preferably between 1500 bar and 3500 bar.
[0034] A disadvantage of high injection pressure is that the injected PET material orients itself and crystallizes at the molecular level, particularly around the injection point under the holding pressure. This crystallization leads to undesirable hardening and increased brittleness of the PET material. To avoid crystallization, the holding pressure is reduced to below 1000 bar immediately after reaching the filling point.
[0035] Although it is made of PET, the closure cap 11 has sufficient sealing function to seal the container opening 29. Advantageously, the closure cap 11 can also be made of food-safe rPET, because the flexible sealing cone can also be made of rPET. Therefore, the closure cap 11 is suitable for closing the container 13 or bottle into which the beverage is poured.
[0036] Reference Signs List : 11 Closing cap 13 Containers, preforms 15 Top Plate 17 Threaded part 19 More open edges 21 internal thread 23 Sealing cone 25 Container neck 27 external thread 29 Container opening 31 connecting ring 33 Recess, retaining element 35 Outer diameter of sealing cone 37 Inner diameter of container neck
Claims
1. A closure cap (11) made of plastic material for closing a container (13) made of PET, the closure cap comprising: - circular top plate (15), - a cylindrical threaded portion (17) surrounding the top plate (15), the threaded portion comprising an opening edge (19) and an internal thread (21), and - a sealing cone (23) protruding on the inner side of the top plate, It is characterized by: A connecting ring (31) is provided between the sealing cone (23) and the top plate (15), the connecting ring having a wall thickness between 0.2 mm and 0.4 mm and connecting the sealing cone (23) to the top plate (15), and The invention is characterized in that the closing cap (11) together with the sealing cone (23) and the connecting ring (31) are made of PET.
2. The closure cap according to claim 1, wherein: The internal thread 21 has a surface roughness between 0.1 μm and 0.2 μm and preferably between 0.14 μm and 0.16 μm.
3. The closure cap according to claim 1 or claim 2, wherein: The outer side of the sealing cone (23) is polished and has a surface roughness of less than 0.15 μm and preferably less than 0.02 μm.
4. Closure cap according to any one of the preceding claims, characterized in that The closure cap (11) comprises a retaining element (33) on which a retaining element can engage when the closure cap is ejected from the injection mold.
5. The closure cap according to claim 5, wherein: The retaining element is a notch (33) provided on the opening edge (19) of the threaded portion (17).
6. Closure cap according to any one of the preceding claims, characterized in that The closure cap (11) is colorless.
7. Closure cap according to any one of the preceding claims, characterized in that The closure cap (11) is made of up to 100% food-safe rPET.
8. Closure cap according to any one of the preceding claims, characterized in that The closure cap (11) comprises a protective ring which is held on the opening edge (19) of the threaded portion (17) by a plurality of predetermined breaking connections.
9. Combination of a closure cap (11) according to any one of the preceding claims and a container (13) made of PET, the container comprising a container neck (25) having an external thread (27), the closure cap (11) being screwable onto and unscrewable from the external thread, It is characterized by: The external thread (27) has a surface roughness between 0.1 µm and 0.2 µm and preferably between 0.14 µm and 0.16 µm.
10. The combination according to claim 9, characterized in that The inside of the container neck (25) has a surface roughness between 0.1 µm and 0.2 µm and preferably between 0.14 µm and 0.16 µm.
11. The combination according to claim 9 or claim 10, characterized in that After the closure cap (11) has shrunk by between 0.4% and 0.6%, the outer diameter (25) of the sealing cone (23) is between 0.4 mm and 0.6 mm larger than the inner diameter (37) of the container neck (25).
12. A method for producing a closure cap (11) according to any one of claims 1 to 8 in an injection mould comprising a mould and an injection core, It is characterized by: The mold temperature of the injection mold during injection molding is between 12°C and 52°C, preferably between 20°C and 40°C, and particularly preferably between 25°C and 35°C.
13. The method according to claim 12, characterized in that The cycle time between injecting the molten PET and demoulding the closure cap (11) is less than 15 s, preferably between 6 and 12 s, and particularly preferably between 8 and 12 s, resulting in the closure cap not having time to retract onto the injection core.
14. The method according to claim 12 or claim 13, characterized in that The residence time of the PET in the injection extruder is less than 350 s, and preferably 50 to 250 s, and the temperature of the PET in the injection extruder is less than 300°C, and preferably between 270°C and 285°C.
15. The method according to any one of claims 12 to 14, characterized in that The molten PET is injected from the injection extruder into the mold at an injection speed between 15 g / s and 35 g / s and an injection pressure greater than 1000 bar and preferably between 1500 bar and 3500 bar.
16. The method according to claim 15, characterized in that After reaching the filling point, the holding pressure is reduced as quickly as possible to a pressure below 1000 bar and preferably to a pressure below 500 bar.
17. The method according to any one of claims 12 to 16, characterized in that In order to demould the closure cap, the injection core is rotated out of the closure cap (11), and a retaining element prevents the closure cap (11) from rotating together with the injection core.