Low-friction-coefficient cold cylinder surface treatment method for packaging material, cold cylinder and application thereof

Abstract

The embodiment of the present application provides a cold cylinder surface treatment method, processes a cold cylinder working surface, makes the cold cylinder working surface form regularly distributed micro-pits, removes convex peaks between the micro-pits, and makes the surface profile skewness RSK of the cold cylinder working surface after processing be-0.5 to-2.0 and the kurtosis RKu be 2.5 to 5.0. By taking the combination of the surface profile skewness RSK and the kurtosis RKu as the core targeted index for controlling the stickiness, a platform-pit functional morphology is constructed on the cold cylinder working surface, the surface friction coefficient of the laminated sheet is reduced to below 0.20, and the stickiness is basically eliminated. The present application simultaneously provides a cold cylinder and application of the cold cylinder in packaging sheet lamination.

Description

Technical Field

[0001] This invention relates to the field of cold cylinder processing technology, specifically to a cold cylinder surface treatment method, a cold cylinder, and its applications. Background Technology

[0002] Liquid foods such as dairy products, beverages, and fruit juices are typically packaged using multi-layered aseptic packaging sheets. These sheets generally have a base layer of cardboard or mineral-filled polypropylene, with thermoplastic materials such as polyethylene covering both sides. Barrier layers such as aluminum foil can be added as needed. In the production of these liquid aseptic food packaging sheets, lamination processes are commonly used: molten polyethylene is extruded through a die and cast onto the substrate surface, then immediately cooled and shaped by contact with a cooling cylinder (cooling roller). The microstructure of the working surface of the cooling cylinder is directly replicated onto the polyethylene film surface during the lamination cooling process.

[0003] The stickiness, or tackiness, of the laminated composite sheet surface when in contact with a hand or adjacent sheets is a key issue affecting the quality of laminated composite sheets and the efficiency of high-speed filling production. Sheets with excessively high stickiness are prone to sticking during conveying, palletizing, and box opening, affecting production efficiency and product appearance. The morphology of the working surface of the cooling cylinder is a key factor determining the stickiness of the laminated composite sheet surface.

[0004] To address the stickiness issue, it is generally believed in the art that the stickiness is related to the surface roughness Ra of the cold cylinder working surface; that is, reducing the surface roughness Ra is desired to improve the stickiness. Based on this understanding, mechanical grinding, sandblasting, chrome plating, etc., are performed on the cold cylinder working surface to control the surface roughness Ra. However, in practice, it has been found that even when only the surface roughness Ra is controlled, or even with the addition of chrome plating or coating treatments, the stickiness of the sheet surface obtained from cold cylinder processing is still too high, making it urgent to find a new solution to this technical problem. Summary of the Invention

[0005] The present invention aims to provide a new solution to the above-mentioned technical problems.

[0006] A method for surface treatment of a cold cylinder involves processing the working surface of the cold cylinder to form regularly distributed micro-pits, and removing the peaks between the micro-pits, so that the surface profile skewness RSK of the processed cold cylinder working surface is -0.5 to -2.0 and the kurtosis RKu is 2.5 to 5.0.

[0007] Furthermore, the micro-pits are arranged in an array on the working surface of the cold cylinder, wherein the array is a structure that is regularly arranged along both the circumferential direction and the axial direction of the cold cylinder.

[0008] Furthermore, the depth of the micro-pits is 5–50 μm, and the area coverage of the micro-pits is 10%–40%.

[0009] Furthermore, the working surface of the cold cylinder is processed using a pulsed laser.

[0010] Furthermore, the pulse width of the pulsed laser is 10–500 ns, and the spot diameter is 20–100 μm.

[0011] Furthermore, the removal of the peaks between the micro-pits is carried out by ultrasonic-assisted grinding, wherein high-frequency ultrasonic vibration is applied in ultrasonic-assisted grinding, and the frequency of the high-frequency ultrasonic vibration is 20-40kHz and the amplitude is 2-15μm.

[0012] Furthermore, the Rockwell hardness of the working surface of the cold cylinder is HRC 55 to 70, and the surface profile skewness RSK of the working surface of the cold cylinder after machining is between -0.8 and -1.5, and the kurtosis RKu is between 3.0 and 4.0.

[0013] Furthermore, the working surface of the cold cylinder is treated with carburizing, nitriding, or chrome plating.

[0014] The present invention also provides a cold cylinder, the working surface of which has regularly distributed micro-dimples and relatively flat plateau areas located between the micro-dimples, formed by smoothing or removing convex peaks; the surface profile skewness RSK of the working surface is -0.5 to -2.0, and the kurtosis RKu is 2.5 to 5.0.

[0015] The present invention also provides an application of a cold cylinder in the composite packaging sheet, wherein the aforementioned cold cylinder is used to cool and shape polyethylene to produce a multi-layer composite packaging sheet, wherein the surface friction coefficient of the packaging sheet is not greater than 0.20.

[0016] Beneficial Effects: This invention provides a surface treatment method for cold cylinders, which processes the working surface of the cold cylinder to form regularly distributed micro-pits and removes the peaks between the micro-pits. This results in a surface profile skewness RSK of -0.5 to -2.0 and a kurtosis RKu of 2.5 to 5.0 on the processed working surface. By using the combination of surface profile skewness RSK and kurtosis RKu as the core target index for controlling the sticky feel, a platform-pit functional morphology is constructed on the working surface of the cold cylinder, reducing the surface friction coefficient of the coated composite sheet to below 0.20, essentially eliminating the sticky feel. This invention also provides a cold cylinder using this treatment method and a method for applying the cold cylinder. Detailed Implementation

[0017] The present invention will be further described in detail below with reference to embodiments, so that the advantages and features of the present invention can be more easily understood by those skilled in the art, thereby providing a clearer and more explicit definition of the scope of protection of the present invention. It should be noted that the following embodiments are only for illustrating the present invention and are not intended to limit the scope of protection of the present invention. Similarly, the following embodiments are only some embodiments of the present invention, not all embodiments. All other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0018] In the description of this invention, the terms "upper," "lower," "surface," "platform," "pit," etc., indicating orientation or positional relationship are only for the convenience of describing this invention and simplifying the description, and should not be construed as limiting this invention.

[0019] In the following description, for ease of understanding, the relevant terms will be explained first: "Cold cylinder" (also known as cooling roller) refers to a roller used in a production line to cool and shape composite sheets; in this application, "cold cylinder" specifically refers to a roller used in a liquid aseptic food packaging sheet lamination and composite production line to contact high-temperature molten polyethylene and cool and shape the polyethylene.

[0020] "Surface roughness Ra" refers to the arithmetic mean deviation of the unevenness of a machined surface with small spacing and minute peaks and valleys, measured according to ISO21920 or GB / T 3505.

[0021] This invention provides a surface treatment method for a cold tank used for laminating aseptic food packaging sheets, which solves the problem of excessively sticky laminated sheets obtained by existing cold tank surface processing methods.

[0022] The applicant's research found that two statistical parameters of surface profile—skewness (RSK) and kurtosis (RKu)—have a crucial impact on the stickiness of laminated composite sheets. Specifically, skewness RSK characterizes the relative dominance of peaks and valleys in the surface profile; RSK > 0 indicates peak dominance, and RSK < 0 indicates valley dominance. Kurtosis RKu characterizes the sharpness of the height distribution of the surface profile. In particular, the stickiness is determined by the combined effect of RSK and RKu: when the surface roughness Ra is essentially the same, if only RSK is negative and RKu is unsuitable, the stickiness of the sheet surface cannot be effectively eliminated; if only RKu is moderate and RSK is positive, the stickiness is still too high; only when both RSK and RKu are negative and moderate can the stickiness of the sheet surface be essentially eliminated.

[0023] Further research revealed that when a "platform-pit functional morphology" is constructed on the working surface of the cold cylinder—that is, the working surface of the cold cylinder has both regularly distributed micro-pits and relatively gentle plateau areas formed after the peaks between the micro-pits are smoothed or removed—the trace amounts of air trapped in the micro-pits during the cooling process of the coating forms an air cushion. This reduces the actual contact area between the molten polyethylene and the working surface of the cold cylinder by approximately 40% to 60%. At the same time, the plateau areas avoid concentrated contact caused by the peak tips. The combined effect of these two factors significantly reduces the surface friction coefficient of the coated composite sheet. It can be understood that the tiny cavities occupied by the trace amounts of air trapped in the micro-pits are called air-storing microcavities.

[0024] Among them, a negative skewness RSK indicates that there are more pits than peaks on the working surface of the cold cylinder, which reduces the actual contact area between the molten polyethylene and the working surface of the cold cylinder. A moderate kurtosis RKu indicates that the depth of each pit is uniform and the peak-valley boundary between the pits and the plateau is clear, ensuring that each gas storage microcavity can stably trap air and form a uniform air cushion. Understandably, both RSK and RKu are indispensable: if only RSK is negative and RKu is too high, that is, the difference in the depth of each pit is too large, then the pit depths are uneven and the air cushion is uneven, limiting the anti-sticking effect; if only RKu is moderate and RSK is positive, that is, there are more peaks than pits, then the actual contact area is still large and the sticky feeling remains.

[0025] The pit depth is preferably controlled within the range of 5 to 50 μm. This ensures that the gas storage microcavity has sufficient gas storage depth and that the pit depth far exceeds the wear amount during the normal service life of the cold cylinder, which is usually less than 2 μm. This allows the texture structure to remain effective throughout the entire life cycle of the cold cylinder.

[0026] To facilitate understanding of this invention, the quantitative definitions and measurement methods of skewness RSK and kurtosis RKu are explained below.

[0027] Skewness RSK ( () is a measure of the asymmetry of the amplitude distribution of surface profiles, and its definition is: In the formula, This is the root mean square deviation of the profile. For sampling length, The contour height function, These are the position coordinates along the evaluation direction.

[0028] kurtosis RKu ( () is a measure of the sharpness of the amplitude distribution of surface profiles, and its definition is: RKu=3 corresponds to a normal distribution, while RKu>3 indicates a sharper profile peak-valley distribution. When RKu is too low, the peak-valley boundaries are indistinct, and the morphological differences between pits and platforms are insufficient to form an effective air cushion and reduce contact. When RKu is too high, it indicates that the depth differences between pits are too large or the peaks are not effectively removed, resulting in excessive surface morphological dispersion. Controlling RKu within the range of 2.5 to 5.0 ensures sufficient morphological differences between pits and platforms while avoiding excessive morphological dispersion that could affect surface uniformity.

[0029] The RSK and RKu measurements mentioned above can be taken using any one of a white light interferometer, a confocal microscope, or a stylus profilometer, according to the sampling length and evaluation length specified in ISO 21920.

[0030] Coefficient of friction: The surface friction coefficient of the product described in this application can be tested using a coefficient of friction meter according to relevant national standards. The surface friction coefficient can serve as an objective indicator of stickiness; the lower the value, the less likely the product surface is to stick and the weaker the stickiness.

[0031] Based on the above findings, this invention uses the combination of surface profile skewness RSK and kurtosis RKu as the core target index for controlling stickiness, rather than the traditional surface roughness Ra. This invention is not limited to a specific processing method; as long as the working surface of the cold cylinder can form a plateau-dimple functional morphology with a negative RSK and a moderate RKu, the purpose of reducing the stickiness of the coated composite sheet can be achieved. Specifically, the RSK of the processed cold cylinder working surface falls between -0.5 and -2.0, and the RKu falls between 2.5 and 5.0. The combination of RSK and RKu together constitutes the plateau-dimple functional morphology, thereby reducing the surface friction coefficient of the coated composite sheet to below 0.20.

[0032] Furthermore, the cold cylinder can be made of stainless steel or carbon steel, and its working surface can undergo surface treatments such as carburizing, nitriding, or chrome plating. Understandably, the hardness of the cold cylinder's working surface affects the processing and shaping of the micro-dimples and plateau areas and their long-term retention. When the Rockwell hardness of the cold cylinder's working surface is HRC 55–70, the amount of material removed by grinding the peak tips is easier to control, and the wear during the normal service life of the cold cylinder is smaller. This allows the RSK of the processed cold cylinder's working surface to be further controlled within a smaller range of -0.8 to -1.5, and RKu to be further controlled within a smaller range of 3.0 to 4.0, further reducing the surface friction coefficient of the coated composite sheet.

[0033] Specifically, in a specific embodiment, the following steps can be used to obtain suitable RSK and RKu.

[0034] S1: A regularly arranged array of micro-pits is fabricated on the working surface of the cold cylinder using pulsed laser.

[0035] The pulse width of the pulsed laser is 10–500 ns, the spot diameter is 20–100 μm, the pit depth is 5–50 μm, and the pit area coverage is 10%–40%. The pit area coverage refers to the proportion of the sum of the projected areas of the micro-pit openings on the working surface of the cold cylinder to the total area of ​​the working surface of the cold cylinder.

[0036] Furthermore, the pulsed laser is generated by a pulsed laser, more specifically, the pulsed laser is a nanosecond pulsed fiber laser; the micro-dimple array can be an array structure that is regularly arranged along both the circumferential and axial directions of the cooling cylinder, and the spacing between adjacent micro-dimples is determined according to the area coverage of the dimples.

[0037] S2: Grinding the working surface of a cold cylinder with a micro-dimple array to remove the sharp peaks between the micro-dimples.

[0038] To ensure the preservation of micro-pits, the amount of material removed during grinding should be less than the pit depth. This ensures that the bottom of the pit is below the abrasive contact reference surface and is not contacted by the abrasive grains, while the tip of the convex peak is above the reference surface and is preferentially removed. By controlling the amount of material removed during grinding, the convex peak can be flattened while preserving the micro-pits, forming a "plateau-pit" morphology.

[0039] Understandably, since the cold cylinder is cylindrical, the grinding can be performed using external cylindrical grinding.

[0040] Preferably, the grinding can be ultrasonic-assisted grinding, that is, high-frequency ultrasonic vibration is applied to the grinding wheel during the grinding process, causing periodic micro-separation between the abrasive grains and the workpiece surface. Utilizing the alternating action of "hammering-cutting," the grinding force is reduced, grinding burns are minimized, and surface quality is improved. Specifically, the frequency of the high-frequency ultrasonic vibration can be 20–40 kHz, and the amplitude can be 2–15 μm. The amount of peak tip removal can be controlled by adjusting the matching relationship between the ultrasonic amplitude and the grinding feed rate. It is understood that ultrasonic assistance is not mandatory; ordinary grinding or polishing can also achieve peak tip removal, as long as the combination of RSK and RKu on the processed surface falls within the target range.

[0041] S3: Collect the contour data of the working surface of the cold cylinder and calculate the RSK and RKu values.

[0042] During the machining process or between machining intervals, contour data of the cold cylinder working surface can be collected, and RSK and RKu values ​​can be calculated. Specifically, the contour data can be collected using any one of a white light interferometer, a confocal microscope, or a stylus profilometer, and the cold cylinder working surface can be cleaned or purged before measurement.

[0043] S4: Adjust the operating parameters of steps S1 and S2 according to the RSK and RKu values.

[0044] The calculated RSK and RKu values ​​are compared with the target range: when RSK does not reach the target range, the removal amount of the peak tip in step S2 is increased; when RKu exceeds the upper limit, i.e., the difference in pit depth is too large and the morphology dispersion is too high, the fluctuation of pulsed laser energy in step S1 is reduced to improve the uniformity of pit depth. Through the above closed-loop process of acquisition-calculation-adjustment, the final surface morphology parameters of the cold cylinder are stabilized within the target range.

[0045] Understandably, the amount of grinding removed in step S2 mainly affects RSK, while the uniformity of the laser preformed pit in step S1 mainly affects RKu. The two work together to ensure that the surface morphology of the cold cylinder falls within the target range.

[0046] Understandably, the present invention also provides a cold cylinder, wherein the working surface of the cold cylinder has regularly distributed micro-pits and relatively gentle plateau areas located between the micro-pits, formed by smoothing or removing peaks; the surface profile skewness RSK of the working surface is -0.5 to -2.0, and the kurtosis RKu is 2.5 to 5.0. The cold cylinder can be processed by the cold cylinder surface treatment method described in any of the above embodiments.

[0047] Understandably, this invention also provides an application of a cold cylinder in packaging sheet lamination, specifically, using the aforementioned cold cylinder to cool and shape polyethylene, thereby producing multi-layered composite packaging sheets; the surface friction coefficient of the packaging sheet is no greater than 0.20, and the sticky feel is essentially eliminated. Specifically, the cold cylinder can be directly installed in a packaging sheet lamination production line to cool and shape polyethylene, allowing the polyethylene to cool and shape to form a film layer, thereby producing multi-layered composite packaging sheets.

[0048] The following specific examples further illustrate this point.

[0049] Example 1 Cold cylinder: made of stainless steel; A regularly arranged array of micro-pits was fabricated on the working surface of a cold cylinder using pulsed laser. The pulse width was 100 ns, the spot diameter was 50 μm, the pit depth was 25 μm, and the pit area coverage was 25%. Grinding is performed on the working surface of a cold cylinder with a micro-dimple array to remove the peak tips between the micro-dimples, and the amount of material removed by grinding is controlled to be less than the depth of the pit. White light interferometer is used to collect the contour data of the working surface of the cold cylinder, calculate the RSK and RKu values, and adjust the grinding removal amount and pulse laser parameters according to the RSK and RKu values ​​until RSK and RKu stably fall into the target range.

[0050] Example 2 The difference from Example 1 is that the pulse width of the pulsed laser is 20ns, the spot diameter is 30μm, the pit depth is 15μm, and the pit area coverage is 15%. The remaining steps are the same as in Example 1.

[0051] Comparative Example 1 The surface of the cold cylinder was machined using a conventional mechanical grinding method, with only the surface roughness Ra controlled. The surface roughness Ra of the cold cylinder obtained in Comparative Example 1 was basically equivalent to that in Example 1.

[0052] Comparative Example 2 The surface of the cold cylinder is machined using traditional sandblasting and chrome plating methods.

[0053] Comparative Example 3 The difference from Example 1 is that the tips of the protrusions were not removed after the micro-dimple array was processed by pulsed laser.

[0054] Comparative Example 4 The surface of the cold cylinder is machined using traditional mechanical grinding methods.

[0055] The surface friction coefficient of the coated composite sheets corresponding to the cold cylinders obtained in Examples 1, 2, and Comparative Examples 1 to 4 was tested, and the test results are shown in the table below: As can be seen from the table above: The surface roughness Ra of the cold cylinder obtained in Comparative Example 1 is basically the same as that in Example 1, but its RSK is more positive, and its surface friction coefficient is significantly higher than that in Example 1, indicating that the sticky feeling is not determined by the surface roughness Ra. The RSK of Comparative Example 2 is more positive, and its surface friction coefficient is also higher. Although the RSK of Comparative Example 3 is negative, its RKu exceeds the target range, and its surface friction coefficient is still higher, indicating that when only RSK is satisfied but RKu is not satisfied, the anti-sticking effect is limited. Although RKu of Comparative Example 4 is within the target range, its RSK is more positive, and its surface friction coefficient is still higher, indicating that when only RKu is satisfied but RSK is not satisfied, the sticky feeling cannot be eliminated.

[0056] Only when both Examples 1 and 2 satisfy the condition that RSK is negative and RKu is within the target range, does the surface friction coefficient drop below 0.20, and the sticky feeling is essentially eliminated. This demonstrates that the sticky feeling is determined by the combination of RSK and RKu; neither can be omitted, and it is not determined by either parameter alone, nor by the surface roughness Ra, thus confirming the technical principle of this invention.

[0057] The above description is merely an embodiment of the present invention and does not limit the patent scope of the present invention. Any equivalent structural or procedural transformations made based on the content of the present invention specification, or direct or indirect applications in other related technical fields, are similarly included within the patent protection scope of the present invention.

Claims

1. A method for surface treatment of a cold cylinder, characterized in that, The working surface of the cold cylinder is machined to form regularly distributed micro-pits, and the peaks between the micro-pits are removed, so that the surface profile skewness RSK of the machined working surface is -0.5 to -2.0 and the kurtosis RKu is 2.5 to 5.

0.

2. The method according to claim 1, characterized in that, The micro-pits are arranged in an array on the working surface of the cold cylinder, wherein the array is a regular array structure arranged along both the circumferential direction and the axial direction of the cold cylinder.

3. The method according to claim 1, characterized in that, The depth of the micro-pits is 5–50 μm, and the area coverage of the micro-pits is 10%–40%.

4. The method according to claim 3, characterized in that, The working surface of the cold cylinder is machined using a pulsed laser.

5. The method according to claim 4, characterized in that, The pulse width of the pulsed laser is 10–500 ns, and the spot diameter is 20–100 μm.

6. The method according to claim 1, characterized in that, The removal of the peaks between the micro-pits is carried out by ultrasonic-assisted grinding, wherein high-frequency ultrasonic vibration is applied in ultrasonic-assisted grinding, and the frequency of the high-frequency ultrasonic vibration is 20-40kHz and the amplitude is 2-15μm.

7. The method according to claim 1, characterized in that, The Rockwell hardness of the working surface of the cold cylinder is HRC 55 to 70, and the surface profile skewness RSK of the working surface of the cold cylinder after machining is -0.8 to -1.5, and the kurtosis RKu is 3.0 to 4.

0.

8. The method according to claim 7, characterized in that, The working surface of the cold cylinder is treated with carburizing, nitriding, or chrome plating.

9. A cold cylinder, characterized in that, Its working surface has regularly distributed micro-pits and relatively flat plateau areas located between the micro-pits, formed by smoothing or removing peaks; the surface profile skewness RSK of the working surface is -0.5 to -2.0, and the kurtosis RKu is 2.5 to 5.

0.

10. The application of a cooling cylinder in packaging sheet lamination, characterized in that, The polyethylene is cooled and shaped using the cooling cylinder described in claim 9 to produce a multi-layer composite packaging sheet, wherein the surface friction coefficient of the packaging sheet is not greater than 0.20.

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