A protective film substrate for circuit board drilling and its preparation method

Abstract

The present invention relates to the technical field of protective films for drilling circuit boards, and provides a protective film substrate for circuit board drilling and a preparation method thereof. The present invention utilizes high-haze low-density polyethylene (LDPE) in the middle and inner layers of the protective film substrate. The polyethylene has the characteristics of low crystallinity, high haze, easy degassing, and is not prone to producing raised bubbles during winding, and is easy to process. This results in a protective film substrate having a certain haze, which virtually eliminates bubbles when attached to a circuit board, and does not affect the deep rotation of the drill bit during drilling, thereby preventing damage to the drill bit. Furthermore, the present invention strictly controls the parameters of the raw materials in each layer and the mass ratio of the three layers during extrusion, resulting in a film with low transverse and longitudinal tensile strength. When the film is used for drilling circuit board holes, damage to the drill bit due to external forces causing the protective film to rotate with the drill bit is avoided during drill bit rotation.

Description

Technical Field

[0001] The present invention relates to the technical field of protective films, and in particular to a protective film substrate for drilling holes in circuit boards and a preparation method thereof. Background Art

[0002] Circuit boards primarily consist of copper-clad laminates (CCLs) and printed circuit boards (PCBs). PCBs, also known as printed circuit boards (PCBs), are important electronic components, typically used to achieve electrical connections between multiple electronic components. PCBs provide mechanical support for component assembly, enabling wiring, electrical connections, and insulation between electronic components. Copper-clad laminates (CCLs), also known as copper-clad laminates (CCLs), are sheet-like materials made by impregnating electronic fiberglass cloth or other reinforcing materials with resin, then coating one or both sides with copper foil and heat-pressing.

[0003] During the PCB manufacturing process, after lamination, a drill is used to cut through the board at high speed, creating through-holes at predetermined locations. These through-holes are then metallized, meaning the inner walls of the through-holes are copper plated and plated, allowing electrical signals to be transmitted between different conductive layers. These holes are typically categorized as vias, blind vias, and buried vias. Drilling is crucial in the printed circuit board (PCB) production process and cannot be neglected. This is because drilling creates the necessary vias on the board to provide electrical connections and secure component performance. Improper operation can result in components not being secured to the PCB, impacting performance at best and rendering the entire board useless at worst. Therefore, the drilling process is crucial during the PCB manufacturing process.

[0004] During the drilling process, the traditional process generally uses the glue-coating drilling method. However, the glue coating is expensive, the curing time is long, the efficiency is low, and there are technical defects such as inaccurate positioning during the drilling process. In addition, the cured glue is prone to powdering during the drilling process and is not easy to clean. Therefore, a method of protecting the circuit board during the drilling process has emerged by applying a film. The advantage of the film is that it does not require a long curing time, which improves efficiency. The film has a certain elasticity, and the drill bit is accurately positioned without deviation or powdering. However, there are two main technical defects in the film. First, the film is not tightly attached to the circuit board, resulting in bubbles, which affects drilling. Second, the tensile strength of the film is too high, causing the drill bit to cause wires. The above two points will cause damage to the drill bit. Summary of the Invention

[0005] In light of this, the present invention provides a protective film substrate for circuit board drilling and a method for preparing the same. The protective film substrate provided by the present invention adheres tightly to the circuit board and has low transverse and longitudinal tensile strength, preventing damage to the drill bit when used in circuit board drilling.

[0006] In order to achieve the above-mentioned object of the invention, the present invention provides the following technical solutions:

[0007] A protective film substrate for drilling holes in circuit boards, made of a three-layer co-extruded blown film comprising an outer layer, a middle layer, and an inner layer;

[0008] The outer layer is made of low-density polyethylene, the middle layer is made of high-mist low-density polyethylene, and the inner layer is made of high-mist low-density polyethylene.

[0009] The low-density polyethylene has a melt index of 1.5 to 2.5 g / 10 min and a density of 0.918 to 0.930 g / cm 3 ;

[0010] The high-fog low-density polyethylene used in the middle layer and the inner layer has a melt index of 1.5 to 2.5 g / 10 min and a density of 0.915 to 0.925 g / cm 3 ;

[0011] The haze of the high haze low density polyethylene used in the middle layer and the inner layer is independently 20 to 26%;

[0012] Taking the total mass of the outer layer, the middle layer and the inner layer as 100%, the mass fraction of the outer layer is 20-30%, the mass fraction of the middle layer is 40-55%, and the mass fraction of the inner layer is 20-30%.

[0013] Preferably, the model of the low-density polyethylene is 2420H, and the model of the high-fog low-density polyethylene used in the middle layer and the inner layer is LF449.

[0014] Preferably, the protective film substrate for circuit board drilling has a haze of 18 to 22%, a longitudinal tensile strength of 16 to 20 MPa, and a transverse tensile strength of 12 to 17 MPa.

[0015] The present invention also provides a method for preparing the protective film substrate for circuit board drilling described in the above scheme, comprising the following steps:

[0016] The raw materials for the outer layer, the middle layer and the inner layer are co-extruded in three layers to obtain the protective film substrate for drilling holes in a circuit board.

[0017] Preferably, the three-layer co-extrusion process includes: feeding the raw materials for the outer layer, middle layer and inner layer into the outer layer, middle layer and inner layer extruders respectively for melting and plasticizing, conveying the obtained glue to the die head, extruding and blowing the film, and fusing the molten raw materials for the outer layer, middle layer and inner layer into one layer, and then blowing it out through the die mouth. After the blown film bubble is cooled, it is stabilized, thickness measured and pulled in turn to obtain a protective film substrate for drilling holes in the circuit board.

[0018] Preferably, the film bubble is cooled by a dynamic air ring, and the temperature of the cooling air blown out by the automatic air ring is 15-25°C.

[0019] Preferably, the outer layer extruder is provided with 5 heating zones, which are sequentially recorded as zones 1 to 5 according to the order in which the raw materials pass through; the temperature of zone 1 of the outer layer extruder is 150-160°C, the temperature of zone 2 is 155-165°C, the temperature of zone 3 is 160-170°C, the temperature of zone 4 is 155-165°C, and the temperature of zone 5 is 150-160°C;

[0020] The extrusion pressure of the outer layer extruder is 232-260 bar.

[0021] Preferably, the middle layer extruder is provided with 5 heating zones, which are sequentially recorded as zones 1 to 5 according to the order in which the raw materials pass through; the temperature of zone 1 of the middle layer extruder is 145-155°C, the temperature of zone 2 is 150-160°C, the temperature of zone 3 is 150-160°C, the temperature of zone 4 is 140-150°C, and the temperature of zone 5 is 140-150°C;

[0022] The extrusion pressure of the middle layer extruder is 240-250 bar.

[0023] Preferably, the inner layer extruder is provided with 5 heating zones, which are sequentially recorded as zones 1 to 5 according to the order in which the raw materials pass through; the temperature of zone 1 of the inner layer extruder is 145-155°C, the temperature of zone 2 is 150-160°C, the temperature of zone 3 is 150-160°C, the temperature of zone 4 is 145-155°C, and the temperature of zone 5 is 140-150°C;

[0024] The extrusion pressure of the inner layer extruder is 220-280 bar.

[0025] Preferably, the die head is provided with 4 heating zones, which are recorded as zones 1 to 4 in the order in which the raw materials pass through. The temperature of zone 1 of the die head is 155-165°C, the temperature of zone 2 is 160-170°C, the temperature of zone 3 is 160-170°C, and the temperature of zone 4 is 155-165°C.

[0026] The present invention provides a protective film substrate for drilling holes in circuit boards, which is made of a three-layer co-extruded blown film of an outer layer, a middle layer and an inner layer; the outer layer is made of low-density polyethylene, the middle layer is made of high-mist low-density polyethylene, and the inner layer is made of high-mist low-density polyethylene; the low-density polyethylene has a melt index of 1.5 to 2.5 g / 10 min and a density of 0.918 to 0.930 g / cm 3 The melt index of the high-mist low-density polyethylene used in the middle and inner layers is independently 1.5 to 2.5 g / 10 min, and the density is independently 0.915 to 0.925 g / cm 3The haze of the high-haze low-density polyethylene used in the middle layer and the inner layer is independently 20-26%. Taking the total mass of the outer layer, the middle layer and the inner layer as 100%, the mass fraction of the outer layer is 20-30%, the mass fraction of the middle layer is 40-55%, and the mass fraction of the inner layer is 20-30%. The present invention uses high-haze low-density polyethylene in the middle and inner layers of the protective film substrate. The polyethylene has the characteristics of low crystal point, high haze, easy degassing, and is not prone to producing raised bubbles when the film is rolled up and easy to process. The resulting protective film substrate has a certain haze, and almost no bubbles are generated when it is attached to the circuit board. The drilling process does not affect the deep rotation of the drill bit and does not damage the drill bit. In addition, the present invention strictly controls the parameters of the raw materials in each layer and the mass ratio of the three layers during extrusion to obtain a film with low transverse and longitudinal tensile strength. When the film is used for drilling holes in the circuit board, the drill bit will not be damaged due to external force causing the protective film to rotate with the drill bit.

[0027] The present invention also provides a method for preparing the protective film substrate for circuit board drilling described in the above solution. The preparation method provided by the present invention has simple steps, is easy to operate, and is suitable for industrial production. DETAILED DESCRIPTION

[0028] The present invention provides a protective film substrate for drilling holes in circuit boards, which is made of a three-layer co-extruded blown film comprising an outer layer, a middle layer and an inner layer;

[0029] The outer layer is made of low-density polyethylene, the middle layer is made of high-mist low-density polyethylene, and the inner layer is made of high-mist low-density polyethylene.

[0030] The low-density polyethylene has a melt index of 1.5 to 2.5 g / 10 min and a density of 0.918 to 0.930 g / cm 3 ;

[0031] The high-fog low-density polyethylene used in the middle layer and the inner layer has a melt index of 1.5 to 2.5 g / 10 min and a density of 0.915 to 0.925 g / cm 3 ;

[0032] The haze of the high haze low density polyethylene used in the middle layer and the inner layer is independently 20 to 26%;

[0033] Taking the total mass of the outer layer, the middle layer and the inner layer as 100%, the mass fraction of the outer layer is 20-30%, the mass fraction of the middle layer is 40-55%, and the mass fraction of the inner layer is 20-30%.

[0034] In the present invention, the outer layer is made of low-density polyethylene, the melt index of the low-density polyethylene is preferably 1.8 to 2.2 g / 10 min, more preferably 2.0 g / 10 min, and the density of the low-density polyethylene is preferably 0.918 to 0.925 g / cm 3 , more preferably 0.924 g / cm 3 In a specific embodiment of the present invention, the low-density polyethylene is preferably 2420H, a product produced by Basel (LyondellBasell Industries, Netherlands), with a melt index of 2.0 g / min and a density of 0.924 g / cm 3 The present invention adopts low-density polyethylene that meets the above requirements, which has the characteristics of high cleanliness, few crystal points, and easy processing.

[0035] In the present invention, the raw material for preparing the middle layer is high-fog low-density polyethylene, and the melt index of the high-fog low-density polyethylene is preferably 2.0g / 10min, and the density is independently 0.924g / cm 3 The high-mist low-density polyethylene is a low-density polyethylene produced by a kettle process, and has a very high mist; in a specific embodiment of the present invention, the model of the high-mist low-density polyethylene is preferably LF449, a product produced by JPE (Japan Polyethylene Co., Ltd. (JPE)), with a melt index of 2.0 g / min and a density of 0.924 g / cm 3 The present invention adopts the high-fog low-density polyethylene required above, which has the characteristics of low crystal point, high fog, easy exhaust, no protruding bubbles in the film, and easy processing.

[0036] In the present invention, the raw material for preparing the inner layer is high-fog low-density polyethylene, and the melt index of the high-fog low-density polyethylene is preferably 2.0g / 10min, and the density is independently 0.924g / cm 3 In a specific embodiment of the present invention, the type of high-mist low-density polyethylene used in the inner layer is preferably consistent with that of the middle layer, which will not be repeated here.

[0037] In the present invention, the haze of the high-haze low-density polyethylene used in the middle layer and the inner layer is preferably independently 20-26%. The haze of the high-haze low-density polyethylene is specifically the haze obtained by testing a film with the same thickness as the target film (protective film substrate for drilling holes in circuit boards) prepared using the raw material as a single raw material. The specific testing method can be found in the national standard GB / T2410-2008. In a specific embodiment of the present invention, the haze is obtained by testing a film with a thickness of 50 μm prepared from the high-haze low-density polyethylene.

[0038] In the present invention, taking the total mass of the outer layer, the middle layer and the inner layer as 100%, the mass fraction of the outer layer is 22-28%, the mass fraction of the middle layer is 45-50%, and the mass fraction of the inner layer is 22-25%.

[0039] In the present invention, the haze of the protective film substrate for circuit board drilling is preferably 18-22%, the longitudinal tensile strength is preferably 16-20 MPa, more preferably 18-19.5 MPa, and the transverse tensile strength is preferably 12-17 MPa, more preferably 13-15 MPa; the longitudinal elongation at break is preferably 160-170%, more preferably 165-166%, and the transverse elongation at break is preferably 420-435%, more preferably 425-430%.

[0040] In the present invention, the total thickness of the protective film substrate for drilling holes in a circuit board is preferably 50 μm.

[0041] The present invention also provides a method for preparing the protective film substrate for circuit board drilling described in the above scheme, comprising the following steps:

[0042] The raw materials for the outer layer, the middle layer and the inner layer are co-extruded in three layers to obtain the protective film substrate for drilling holes in a circuit board.

[0043] In the present invention, the three-layer co-extrusion process includes: feeding the raw materials for the outer layer, middle layer and inner layer into the outer layer, middle layer and inner layer extruders respectively for melting and plasticizing, conveying the obtained glue to the die head, extruding and blowing the film, and fusing the molten raw materials for the outer layer, middle layer and inner layer into one layer, and then blowing it out through the die mouth. After the blown film bubble is cooled, it is stabilized, thickness measured and pulled in sequence to obtain a protective film substrate for drilling holes in a circuit board.

[0044] In the present invention, the outer layer extruder is preferably provided with 5 heating zones, which are sequentially recorded as zones 1 to 5 according to the order in which the raw materials pass through; the temperature of zone 1 of the outer layer extruder is preferably 150-160°C, more preferably 155°C, the temperature of zone 2 is preferably 155-165°C, more preferably 160°C, the temperature of zone 3 is preferably 160-170°C, more preferably 165°C, the temperature of zone 4 is 155-165°C, more preferably 160°C, and the temperature of zone 5 is 150-160°C, more preferably 155°C.

[0045] In the present invention, the extrusion pressure of the outer layer extruder is preferably 232 to 260 bar.

[0046] In the present invention, the middle layer extruder is preferably provided with 5 heating zones, which are sequentially recorded as zones 1 to 5 according to the order in which the raw materials pass through; the temperature of zone 1 of the middle layer extruder is preferably 145-155°C, more preferably 150°C, the temperature of zone 2 is preferably 150-160°C, more preferably 155°C, the temperature of zone 3 is preferably 150-160°C, more preferably 155°C, the temperature of zone 4 is preferably 140-150°C, more preferably 145°C, and the temperature of zone 5 is preferably 140-150°C, more preferably 145°C;

[0047] In the present invention, the extrusion pressure of the middle layer extruder is preferably 240 to 250 bar.

[0048] In the present invention, the inner layer extruder is provided with 5 heating zones, which are sequentially recorded as zones 1 to 5 according to the order in which the raw materials pass through; the temperature of zone 1 of the inner layer extruder is preferably 145-155°C, more preferably 150°C, the temperature of zone 2 is preferably 150-160°C, more preferably 155°C, the temperature of zone 3 is preferably 150-160°C, more preferably 155°C, the temperature of zone 4 is preferably 145-155°C, more preferably 150°C, and the temperature of zone 5 is preferably 140-150°C, more preferably 145°C;

[0049] In the present invention, the extrusion pressure of the inner layer extruder is preferably 220 to 280 bar.

[0050] In the present invention, the die head is preferably provided with 4 heating zones, which are sequentially recorded as zones 1 to 4 according to the order in which the raw materials pass through. The temperature of zone 1 of the die head is preferably 155-165°C, more preferably 160°C, the temperature of zone 2 is preferably 160-170°C, more preferably 165°C, the temperature of zone 3 is preferably 160-170°C, more preferably 165°C, and the temperature of zone 4 is preferably 155-165°C, more preferably 160°C.

[0051] In a specific embodiment of the present invention, the raw materials of each layer are preferably firstly dedusted by a dust removal device and then added to an automatic batching device, and then added to the outer layer, middle layer and inner layer extruders according to a set ratio.

[0052] In the present invention, the cooling of the film bubble is preferably performed by a dynamic air ring, and the temperature of the cooling air blown out by the automatic air ring is preferably 15 to 25°C.

[0053] In a specific embodiment of the present invention, the thickness is preferably measured using an online thickness gauge rotating at 360°. The online thickness gauge measures the thickness distribution of the entire film bubble and feeds this information back to an automatic air ring. The film bubble is then clamped into a flattened cylindrical film by a herringbone arrangement and then enters an upper traction rotating device. After further cooling, the protective film substrate for circuit board drilling of the present invention is obtained. The present invention has no special requirements for the equipment used in each of the above processes; equipment familiar to those skilled in the art can be used.

[0054] The technical solutions of the present invention will be described clearly and completely below in conjunction with the embodiments of the present invention.

[0055] Example 1

[0056] The protective film substrate for circuit board drilling prepared in this embodiment has a three-layer structure, namely an outer layer, a middle layer, and an inner layer. Taking the total weight of the three layers as 100%, the extrusion weight fractions of the outer layer, the middle layer, and the inner layer are 30%, 40%, and 30%, respectively. The raw material components of each layer are as follows:

[0057] Outer layer: low density polyethylene 2420H;

[0058] Middle layer: high fog low density polyethylene LF449;

[0059] Inner layer: high fog low density polyethylene LF449;

[0060] The preparation method is as follows:

[0061] (1) The raw materials of each layer are dedusted by a dust removal device and added to the automatic batching device according to the set ratio. After mixing, they enter the outer layer, middle layer, and inner layer extruders for melting and plasticization. The molten glue is transported to the die head, extruded and blown to form a film blank with three layers fused into one layer, and then blown out through the die mouth;

[0062] (2) Cooling into the required film bubble through the 15-25℃ cooling air of the automatic air ring;

[0063] (3) After cooling, the film bubble is stabilized by a bubble stabilizing rack, and the thickness distribution of the entire film bubble is measured by an online thickness gauge rotating 360°. The thickness adjustment system feeds the information back to the automatic air ring adjustment;

[0064] (4) The film bubbles entering the herringbone row are clamped into flat tubes and enter the upper traction rotating device;

[0065] (5) After cooling, a protective film substrate for drilling holes in a circuit board is obtained.

[0066] The temperature and pressure of the extruder and die are shown in Table 1:

[0067] Table 1 Extruder temperature and pressure

[0068] condition Zone 1 / ℃ Zone 2 / ℃ Zone 3 / ℃ Zone 4 / ℃ Zone 5 / ℃ Pressure / bar Outer layer 155 160 165 160 155 245 Middle level 150 155 155 145 145 298 Inner layer 150 155 155 150 145 265 die head 160 165 165 160

[0069] Example 2

[0070] Taking the total weight of the three layers as 100%, the extrusion weight fractions of the outer layer, the middle layer and the inner layer are 25%, 55% and 20% respectively;

[0071] Other conditions and preparation methods are consistent with those in Example 1.

[0072] Example 3

[0073] Taking the total weight of the three layers as 100%, the extrusion weight fractions of the outer layer, the middle layer and the inner layer are 20%, 50% and 30% respectively;

[0074] Other conditions and preparation methods are consistent with those in Example 1.

[0075] Comparative Example 1

[0076] The outer layer, middle layer and inner layer are all made of low-density polyethylene 2420H;

[0077] Other conditions and preparation methods are consistent with those in Example 1.

[0078] Performance parameter test:

[0079] The mechanical properties, haze and rolling state of the protective films prepared in Examples 1 to 3 and Comparative Example 1 were tested, and the results are shown in Table 2;

[0080] Table 2 Performance parameter test results of the protective films obtained in Examples 1 to 3 and Comparative Example 1

[0081]

[0082] The data in Table 2 show that the protective films prepared in Examples 1-3 of the present invention exhibit low transverse and longitudinal tensile strength and elongation at break. When used for drilling circuit boards, the drill bit does not suffer damage due to external forces causing the protective films to rotate with the drill bit. Furthermore, the protective films prepared in Examples 1-3 exhibit high haze and are smooth and free of bubbles and protrusions when rolled up. This characteristic ensures that they adhere to the circuit board with virtually no bubbles, preventing the drill bit from rotating deeper and damaging the drill bit during drilling. The protective film prepared in Comparative Example 1 exhibits high tensile strength and elongation at break, as well as low haze, with more bubbles and protrusions when rolled up.

[0083] Application Example 1

[0084] The protective film prepared in Example 1 was applied to PCB board drilling. Specifically, the protective film prepared in Example 1 was coated with glue and then applied to upper and lower aluminum backing plates. For the production of multi-layer PCB boards, the backing plates were required to be uniform in thickness, flat, free of warping, scratches, and bubbles. The drilling room was maintained in a clean room with a temperature of 1-22°C and a relative humidity of ≤50%. The upper and lower backing plates and the PCB board were secured to a drill table. The drilling parameters were: a linear speed of 150-160 m / min; a drill bit rotation speed of 40,000-90,000 rpm; and the drilling process was performed without breakage, smooth hole walls, brushing burrs, or plugged holes.

[0085] The above is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principles of the present invention. These improvements and modifications should also be regarded as within the scope of protection of the present invention.

Claims

1. A protective film substrate for drilling holes in circuit boards, characterized in that: Made of three layers of co-extruded blown film: outer layer, middle layer and inner layer; The outer layer is made of low-density polyethylene, the middle layer is made of high-mist low-density polyethylene, and the inner layer is made of high-mist low-density polyethylene. The low-density polyethylene has a melt index of 1.5-2.5 g / 10 min and a density of 0.918-0.930 g / cm 3 ; The high-fog low-density polyethylene used in the middle layer and the inner layer has a melt index of 1.5-2.5 g / 10 min and a density of 0.915-0.925 g / cm 3 ; The haze of the high haze low density polyethylene used in the middle layer and the inner layer is independently 20-26%; Based on the total mass of the outer layer, the middle layer and the inner layer being 100%, the mass fraction of the outer layer is 20-30%, the mass fraction of the middle layer is 40-55%, and the mass fraction of the inner layer is 20-30%; The model of the low-density polyethylene is 2420H, and the model of the high-haze low-density polyethylene used in the middle layer and the inner layer is LF449; the haze of the protective film substrate used for circuit board drilling is 18~22%, the longitudinal tensile strength is 16~20MPa, and the transverse tensile strength is 12~17MPa.

2. The method for preparing a protective film substrate for circuit board drilling according to claim 1, characterized in that: The following steps are involved: The raw materials for the outer layer, the middle layer and the inner layer are co-extruded in three layers to obtain the protective film substrate for drilling holes in a circuit board.

3. The preparation method according to claim 2, characterized in that The three-layer co-extrusion process includes: feeding the raw materials for the outer layer, middle layer and inner layer into the outer layer, middle layer and inner layer extruders respectively for melting and plasticizing, conveying the obtained glue liquid to the die head, extruding and blowing the film, and fusing the molten raw materials for the outer layer, middle layer and inner layer into one layer, and then blowing it out through the die mouth. After the blown film bubble is cooled, it is stabilized, thickness measured and pulled in sequence to obtain a protective film substrate for drilling holes in circuit boards.

4. The preparation method according to claim 3, characterized in that The film bubble is cooled by an automatic air ring, and the temperature of the cooling air blown out by the automatic air ring is 15~25℃.

5. The preparation method according to claim 3, characterized in that The outer layer extruder is provided with 5 heating zones, which are recorded as zones 1 to 5 in the order in which the raw materials pass through; the temperature of zone 1 of the outer layer extruder is 150-160°C, the temperature of zone 2 is 155-165°C, the temperature of zone 3 is 160-170°C, the temperature of zone 4 is 155-165°C, and the temperature of zone 5 is 150-160°C; the extrusion pressure of the outer layer extruder is 232-260 bar.

6. The preparation method according to claim 3, characterized in that The middle layer extruder is provided with 5 heating zones, which are recorded as zones 1 to 5 in the order in which the raw materials pass through; the temperature of zone 1 of the middle layer extruder is 145-155°C, the temperature of zone 2 is 150-160°C, the temperature of zone 3 is 150-160°C, the temperature of zone 4 is 140-150°C, and the temperature of zone 5 is 140-150°C; the extrusion pressure of the middle layer extruder is 240-250 bar.

7. The preparation method according to claim 3, characterized in that The inner layer extruder is provided with 5 heating zones, which are recorded as zones 1 to 5 in the order in which the raw materials pass through; the temperature of zone 1 of the inner layer extruder is 145-155°C, the temperature of zone 2 is 150-160°C, the temperature of zone 3 is 150-160°C, the temperature of zone 4 is 145-155°C, and the temperature of zone 5 is 140-150°C; the extrusion pressure of the inner layer extruder is 220-280 bar.

8. The preparation method according to claim 3, characterized in that The die head is provided with 4 heating zones, which are recorded as zones 1 to 4 in the order in which the raw materials pass through. The temperature of zone 1 of the die head is 155-165°C, the temperature of zone 2 is 160-170°C, the temperature of zone 3 is 160-170°C, and the temperature of zone 4 is 155-165°C.

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