Closure cap made of plastic
Patent Information
- Application Number
- EP2024712543
- Authority / Receiving Office
- EP · EP
- Patent Type
- Applications
- Current Assignee / Owner
- Priority Date
- 2023-03-28
- Filing Date
- 2024-03-26
- Publication Date
- 2026-02-11
AI Technical Summary
Polyethylene terephthalate (PET) is unsuitable for closure caps due to its hardness, which prevents effective sealing, while polyolefins used for caps have limited recyclability in food-safe quality.
A PET closure cap with a thin-walled connecting ring and optimized surface roughness for the internal thread, combined with a method using controlled injection molding temperatures and pressures to prevent brittleness, allowing PET to be used for both containers and caps without expensive molds.
The solution enables reliable sealing and easy removal of PET caps from molds, maintaining material flexibility and recyclability, preventing embrittlement and leakage, while ensuring food safety and recyclability.
Smart Images
Figure EP2024058044_03102024_PF_FP_ABST
Abstract
Description
[0001] Plastic cap
[0002] Field of the invention
[0003] The invention relates to a closure cap made of plastic according to the preamble of claim 1, a combination of the closure cap with a container made of PET according to the preamble of claim 9 and a method for producing such a closure cap according to the preamble of claim 12.
[0004] State of the art
[0005] Polyethylene terephthalate (PET) is polar, which results in strong intermolecular forces. The molecule also has a linear structure without cross-links. This makes it suitable for the production of hard objects with high fracture strength, such as sturdy PET bottles that can withstand high internal pressures. The hardness and rigidity can be further increased by stretching a PET preform in the radial and axial directions. Due to its hardness, PET is unsuitable for use as a closure cap, as a closure cap generally requires a softer and therefore better-sealing plastic. Therefore, polyolefins are predominantly used to manufacture closure caps. Compared to PET, however, polyolefins have the disadvantage of limited recyclability to food-safe quality.
[0006] Therefore, experiments are being conducted with injection molds that have improved thermal conductivity, allowing the temperature in the injection mold to be raised and lowered quickly. This should allow PET to be injected into and removed from the mold.
[0007] Object of the invention
[0008] The disadvantages of the described prior art give rise to the task of creating a plastic closure that can be manufactured from the same PET material as the container it closes and reliably seals the container without the need for an expensive injection mold to produce the closure. Description
[0009] The stated problem is solved by a closure cap made of PET for closing a container made of PET by the features stated in the characterizing portion of patent claim 1. Further developments and / or advantageous embodiments are the subject of the dependent patent claims.
[0010] The invention is characterized by the fact that a connecting ring is provided between the sealing cone and the head plate. This connecting ring has a wall thickness of between 0.2 and 0.4 mm and connects the sealing cone to the head plate. The closure cap, together with the sealing cone and the connecting ring, is made of PET. The very thin-walled connecting ring gives the sealing cone a flexibility comparable to that of HDPE and PP. As a result, the sealing cone can smoothly adhere to the inside of a container neck, enabling a reliable seal between the sealing cone and the container neck. This is despite the closure cap being made entirely of comparatively hard and brittle PET.
[0011] In a preferred embodiment, the internal thread has a surface roughness between 0.1 pm and 0.2 pm, and preferably between 0.14 pm and 0.16 pm. This prevents the internal thread from being too smooth, which could lead to the risk of welding to the external thread when screwed onto the container neck for the first time. Friction welding, caused by the high-speed seamers in the packaging industry, is prevented by the increased roughness of the internal thread.
[0012] It has proven to be advantageous if the outside of the sealing cone is polished and has a surface roughness of less than 0.15 pm, and preferably less than 0.02 pm. The smooth surface improves the sealing effect of the sealing cone when it rests against the inside of the container neck.
[0013] The closure cap advantageously has retaining elements that can be engaged by retention elements during demolding of the closure cap from an injection mold. Demolding the rigid PET closure is difficult in the conventional manner because the threaded portion is difficult to deform due to its wall thickness, and the internal thread represents an intersection of more than 0.4 mm. The retaining elements allow the closure cap to be removed by rotating the injection mold core. The retaining elements can prevent the cap from rotating when the core is unscrewed from the closure cap. The retaining elements can be holes, ribs, or notches. Notches are preferred because they can be produced during injection molding using a simple mold.
[0014] For convenience, the cap is uncolored. This means the cap is transparent or white. This prevents any disruptive color pigments from entering the bottle during recycling.
[0015] In another particularly preferred embodiment of the invention, the closure cap is made of up to 100% food-safe rPET. Compared to PP (polypropylene) or HDPE (high-density polyethylene), which are plastics primarily used to manufacture closure caps, PET can be recycled very easily, even into high-quality food-safe rPET. This rPET can be used to produce closure caps according to the invention with a thin-walled connecting ring.
[0016] The closure cap advantageously features a tamper-evident ring secured to the open edge of the threaded portion by a plurality of predetermined breaking ribs. This allows the closure cap to be immediately identified as having been opened for the first time if the predetermined breaking ribs are broken.
[0017] A further aspect of the invention relates to a combination of the above-described closure cap and a container onto which the cap is screwed, wherein the external thread of the container neck has a surface roughness between 0.1 pm and 0.2 pm, and preferably between 0.14 pm and 0.16 pm. As already explained above, this prevents friction welding of the external and internal threads during rapid initial screwing with fully automatic cappers.
[0018] In a further preferred embodiment, the inside of the container neck has a surface roughness between 0.1 pm and 0.2 pm, and preferably between 0.14 pm and 0.16 pm. 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.
[0019] In a particularly preferred embodiment of the invention, the outer diameter of the sealing cone is between 0.4 and 0.6 mm larger than the inner diameter of the container neck after a shrinkage of the closure cap of between 0.4% and 0.6% when screwing it onto the container neck. PET always exhibits shrinkage, which must be taken into account when dimensioning the sealing cone to ensure that the outer diameter of the sealing cone does not become too small during shrinkage and the closure does not leak.
[0020] A further aspect of the invention relates to a method for producing the described closure cap made of PET. The method is characterized in that the mold temperature of the injection mold during injection molding is between 12 and 52°C, preferably between 20 and 40°C, and particularly preferably between 25 and 35°C. Surprisingly, at these low temperatures, the residual stress of the closure can be kept low and it does not become brittle. Stress cracks that grow larger over time, which lead to deformation of the sealing cone and leaks, are prevented by the selected mold temperature.
[0021] In a further particularly preferred embodiment of the invention, the cycle time between the injection of the molten PET and the demolding of the closure cap is less than 15 s, preferably between 6 and 12 s, and particularly preferably between 8 and 12 s, whereby the closure cap does not have time to shrink onto the injection core. These short cycle times prevent the shrinking material from developing tensile stresses in height and circumference, which lead to microscopic cracks (crazes) that damage the material through embrittlement. These stress cracks grow larger over time, increasing material embrittlement. Frozen stresses that are to be avoided also lead over time to deformation and corresponding shrinkage of the sealing cone and subsequently to leakage.
[0022] Preferably, the residence time of the PET in the injection extruder is less than 350 s and preferably between 50 and 250 s, with the temperature of the PET in the injection extruder being less than 300°C and preferably between 270 and 285°C. The short residence time in combination with the selected temperature prevents degradation of the PET material, which leads to embrittlement of the PET material.
[0023] In a further preferred embodiment of the invention, the molten PET is injected from an injection extruder into the mold at an injection speed between 15 and 35 g / s and an injection pressure of more than 1000 bar, and preferably between 1500 and 3500 bar. The high injection speed enables the thin-walled PET closure cap, with a minimum connecting ring wall thickness of 0.2 mm, to fill the entire cavity of the injection mold without the PET material freezing and leaving the cavity unfilled in the area of the connecting ring. The high injection pressure enables the high injection speed.
[0024] The invention is also preferably characterized in that the holding pressure is reduced as quickly as possible after reaching the filling point to a pressure below 1000 bar, and preferably below 500 bar. This rapid pressure reduction prevents crystallization of the PET material, which occurs at high injection pressures and leads to undesirable embrittlement.
[0025] In another particularly preferred embodiment of the invention, the injection core is turned out of the closure cap to demold the closure cap, with retaining elements preventing the closure cap from rotating along with the injection core. For forced demolding of the comparatively hard PET closure cap, the threaded portion is too thick and the undercuts of the internal thread are too deep. Therefore, turning out the core provides a substitute for forced demolding, which is possible and common with closure caps made of PP or HDPE.
[0026] Further advantages and features will become apparent from the following description of an embodiment of the invention with reference to the schematic representations. These are not to scale:
[0027] Figure 1 : a closure cap and a container (preform) on which the
[0028] Screw-on cap in an axonometric view;
[0029] Figure 2: the cap screwed onto the container in a side view;
[0030] Figure 3: a sectional view along the section line lll-lll from Figure 2 and
[0031] Figure 4: a cross-sectional view of the closure cap.
[0032] Figures 1 to 4 show a closure cap made of PET, which is designated overall 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 shown in Figures 1 to 3 as a preform 13, which is stretch-formed into a container or bottle. The cap 11 has a circular head plate 15 and a cylindrical threaded portion 17 surrounding the head plate 15. The threaded portion 17 has an open edge 19 and an internal thread 21. A sealing cone 23 protrudes from the inside of the head plate 15. The container 13 has a container neck 25 with an external thread 27, which interacts with the internal thread 21. The container neck 25 delimits a container opening 29, through which filling material is poured into the container 13 and also poured out.
[0033] When screwed on, the sealing cone 23 penetrates the container opening 29 and thereby seals the opening 29 against the cap 11. This allows the cap 11 to reliably seal the opening 29.
[0034] A connecting ring 31 is provided between the sealing cone and the head plate. The connecting ring 31 has a wall thickness of between 0.2 and 0.4 mm and connects the sealing cone 23 to the head plate 15. Due to the very thin wall thickness of the connecting ring 31, the sealing cone remains flexible and can seal the opening, even though it is made of PET, which is less flexible and harder than HDPE or PP. The provision of the connecting ring 31 makes the sealing cone 23 similarly soft to a closure made of PP or HDPE.
[0035] The internal thread 21 and the external thread 27 have a surface roughness between 0.14 and 0.16 pm. This high surface roughness ensures that the threads 21 and 27 do not friction weld when the closure cap is screwed on, and that a torque can be applied that enables reliable sealing of the opening 29. The surface roughness of the inside of the container neck 25 is also between 0.14 and 0.16 pm, so that the sealing cone does not friction weld to the container neck.
[0036] The closure cap 11 has notches 33, which retaining elements can engage during demolding of the closure cap 11 from an injection mold. This allows the closure cap 11 to be demolded from the injection mold by a rotating movement of the injection core and can be demolded despite undercuts and the low deformability of PET.
[0037] The outer diameter 35 of the sealing cone must be slightly larger than the inner diameter 37 of the container neck to achieve a reliable sealing function. This excess dimension is also referred to as pre-tension. Since PET shrinks after injection molding, this must be taken into account to achieve the correct pre-tension. Otherwise, the outer diameter 35 of the sealing cone becomes too small due to the unaccounted for shrinkage and can no longer seal the container neck 25. The shrinkage for the closure cap is approximately 0.5%, causing the outer diameter of the sealing cone to shrink by approximately 0.1 to 0.4 mm. Once the shrinkage is complete, the excess dimension of the outer diameter 35 when screwed onto the container neck is 0.4 to 0.6 mm compared to the inner diameter 37 of the container neck. Since the shrinkage or shrinkage process is complete, this excess dimension is sufficient to reliably seal the container neck with the sealing cone.
[0038] The closure cap 11 is preferably uncolored or transparent. This allows it to be particularly well processed together with the container 13 to form a container made of rPET.
[0039] The process for producing the described closure cap in an injection mold has the following special features, which reduce the brittleness of the PET material and thus contribute to making PET a suitable material for closure caps:
[0040] The temperature of the injection mold during injection molding is between 12 and 52°C, preferably between 20 and 40°C, and particularly preferably between 25 and 35°C. This keeps the residual stress of the closure 11 low and prevents it from becoming brittle. Stress cracks that grow larger over time, which lead to deformation of the sealing cone 23 and leaks, are prevented by the selected mold temperature.
[0041] The cycle time between the injection of the molten PET and demolding of the closure cap is preferably between 6 and 12 seconds, and particularly preferably between 8 and 12 seconds. This prevents the closure cap from shrinking onto the injection core. Shrinking onto the injection core would lead to undesirable tensile stresses in the cap, which are prevented by the selected cycle times.
[0042] The degradation of the PET material of the cap 11 and the associated brittleness could be achieved by reducing the residence time in the injection extruder to ideally 50 to 250 s in combination with a temperature preferably between 270 and 285°C. However, such low melt temperatures can lead to freezing of the PET material. Therefore, the injection speed into the injection mold is selected between 15 and 35 g / s. In order to be able to inject the connecting ring 31 with the very thin wall thickness of preferably 0.25 mm and to fill the injection mold in the area of the connecting ring 31, a high injection pressure is selected. The injection pressure is preferably between 1500 and 3500 bar.
[0043] The high injection pressure has the disadvantage that the injected PET material, particularly around the injection point, orients itself at the molecular level during the holding pressure and crystallizes. This crystallization leads to undesirable hardening and increased brittleness of the PET material. To prevent crystallization, the holding pressure was reduced to pressures below 1000 bar immediately after reaching the filling point. The closure cap 11 has a sufficient sealing function to seal the container opening 29, even though it is made of PET. Advantageously, the closure cap 11 can also be made of food-safe rPET, since a flexible sealing cone can also be made of rPET. This makes the closure cap 11 suitable for closing a container 13 or a bottle into which beverages are filled.
[0044] Legend:
[0045] 11 Cap
[0046] 13 containers, preforms
[0047] 15 Headstock
[0048] 17 Threaded part
[0049] 19 More open edge
[0050] 21 internal thread
[0051] 23 Sealing cone
[0052] 25 Container neck
[0053] 27 external threads
[0054] 29 Container opening
[0055] 31 Connecting ring
[0056] 33 notches, retaining elements
[0057] 35 outer diameter of the sealing cone
[0058] 37 inner diameter of container neck
Claims
1. A closure cap (11) made of plastic for closing a container (13) made of PET, comprising a circular head plate (15), a cylindrical threaded part (17) surrounding the head plate (15) with an open edge (19) and an internal thread (21), and a sealing cone (23) projecting on the inside of the head plate, characterized in that a connecting ring (31) is provided between the sealing cone (23) and the head plate (15), which connecting ring has a wall thickness of between 0.2 and 0.4 mm and connects the sealing cone (23) to the head plate (15), and in that the closure cap (11) together with the sealing cone (23) and the connecting ring (31) is made of PET.
2. Closure cap according to claim 1, characterized in that the internal thread (21) has a surface roughness between 0.1 pm and 0.2 pm and preferably between 0.14 pm and 0.16 pm.
3. Closure cap according to claim 1 or 2, characterized in that the outer side of the sealing cone (23) is polished and has a surface roughness of less than 0.15 pm and preferably less than 0.02 pm.
4. Closure cap according to one of the preceding claims, characterized in that the closure cap (11) has holding elements (33) on which retaining elements can engage when the closure cap is removed from an injection mold.
5. Closure cap according to claim 5, characterized in that the holding elements are notches (33) which are provided on the open edge (19) of the threaded part (17).
6. Closure cap according to one of the preceding claims, characterized in that the closure cap (11) is uncolored.
7. Closure cap according to 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 one of the preceding claims, characterized in that the closure cap (11) has a guarantee ring which is held by a plurality of predetermined breaking webs on the open edge (19) of the threaded part (17).
9. Combination of a closure cap (11) according to one of the preceding claims and a container (13) made of PET with a container neck (25) having an external thread (27) onto which the closure cap (11) can be screwed on and off, characterized in that the external thread (27) has a surface roughness between 0.1 pm and 0.2 pm and preferably between 0.14 pm and 0.16 pm.
10. Combination according to claim 9, characterized in that the inside of the container neck (25) has a surface roughness between 0.1 pm and 0.2 pm and preferably between 0.14 pm and 0.16 pm.
11. Combination according to claim 9 or 10, characterized in that the outer diameter (25) of the sealing cone (23) is between 0.4 and 0.6 mm larger than the inner diameter (37) of the container neck (25) after a shrinkage of the closure cap (11) of between 0.4% and 0.6% when screwing onto the container neck (25).
12. A method for producing a closure cap (11) according to one of claims 1 to 8 in an injection mold with a die and an injection core, characterized in that the tool temperature of the injection mold during injection molding is between 12 and 52 °C, preferably between 20 and 40 °C and particularly preferably between 25 and 35 °C.
13. A method according to claim 12, characterized in that the cycle time between the injection of the molten PET and the demolding of the closure cap (11) is less than 15 s, preferably between 6 and 12 s and particularly preferably between 8 and 12 s, whereby the closure cap does not have time to shrink onto the injection core.
14. The method according to claim 12 or 13, characterized in that the residence time of the PET in an injection extruder is less than 350 s and preferably 50 to 250 s, the temperature of the PET in the injection extruder being less than 300°C and preferably between 270 to 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 of between 15 and 35 g / s and an injection pressure of more than 1000 bar and preferably between 1500 and 3500 bar.
16. Method according to claim 115, characterized in that the holding pressure is reduced as quickly as possible after reaching the filling point to a pressure below 1000 bar and preferably to below 500 bar.
17. Method according to one of claims 12 to 16, characterized in that for demoulding the closure cap the injection core is turned out of the closure cap (11), wherein retaining elements prevent the closure cap (11) from rotating with the injection core.