Method and device for evaluating and optimizing continuous extrusion processing characteristics of high-voltage cable insulation materials

Abstract

The present invention discloses a method and device for evaluating and optimizing the continuous extrusion processing characteristics of high-voltage cable insulation materials, comprising: continuously extruding the material to be tested as a melt, measuring and recording the inlet pressure P, mass growth rate w and diameter D' of the melt spline, and calculating the apparent shear viscosity η. a ; Calculate the export expansion rate δ; record the display curve η a (t) and δ(t), the time corresponding to the increase of the set percentage on the curve is taken as the starting time T of the cross-linking reaction X ; Select reference sample for testing, according to the η of the reference sample a (t) and δ(t) curves to determine the starting time T of the cross-linking reaction S According to T X and T S The index α is defined. The beneficial effects of the present invention are: uninterrupted melt extrusion is performed using an extruder in conjunction with a throttling device and a capillary die, the continuous testing time is unlimited, the cable insulation extrusion process is more realistically simulated, performance parameters that are more in line with reality are measured, experimental material consumption is saved, and the evaluation of material extrusion processing performance is more accurate and reliable.

Description

Technical Field

[0001] The present invention belongs to the technical field of cross-linked cables, and in particular relates to a method and device for evaluating and optimizing the continuous extrusion processing characteristics of high-voltage cable insulation materials. Background Art

[0002] Submarine cables (hereinafter referred to as submarine cables) are core components of cross-sea power transmission systems. Unlike land cables, submarine cable joints are more difficult and costly to manufacture. Therefore, in engineering, it is always desirable to make single-segment submarine cable products as long as possible to reduce the total number of intermediate joints or factory joints in the submarine cable. Prior art document CN 113284673 A provides a device and method for preparing long-length, jointless ultra-high voltage submarine cables. The method increases the length of long-length, jointless ultra-high voltage submarine cables by controlling the out-of-roundness of the cables, but the improvement effect is limited.

[0003] Cross-linked polyethylene (XLPE) is currently the most important insulation material for high-voltage submarine cables. It is primarily composed of polyethylene resin mixed with a certain proportion of antioxidants and cross-linking agents. The manufacturing process for cable insulation involves heating the cross-linkable polyethylene insulation material into a melt using a screw extruder at a temperature below the cross-linking agent's rapid decomposition temperature but above the polyethylene's melting point. This melt is then continuously extruded onto the cable conductor through a die head. The melt then enters a cross-linking pipe, where it undergoes continuous cross-linking at a high temperature that allows the cross-linking agent to decompose rapidly. During this process, the extruder's uninterrupted operating time is a significant limiting factor in the length of a single cable segment.

[0004] Based on the technical principles of cross-linked cable production, during the long-term extrusion process of cross-linked polyethylene cable insulation materials, due to the uneven melt fluidity and the existence of local dead corners with poor fluidity in the extruder, the material will inevitably be retained in a local position in the extruder. At the extrusion temperature, although the decomposition rate of the cross-linking agent is slow, the material that is retained for a long time will also produce pre-cross-linking due to the decomposition of the cross-linking agent. Some pre-cross-linked materials will form gel point defects in the cable insulation. The gel point defects can easily cause blockage of the extruder filter, thereby limiting the continuous extrusion time. Some gel points that penetrate through the filter will also cause a significant decrease in the electrical properties of the insulation layer. Therefore, after unacceptable pre-cross-linking occurs, the extruder equipment must be stopped and the interior of the equipment must be cleaned as a whole, which also limits the continuous processing time of the extruder.

[0005] The key to manufacturing long-length submarine cables is maximizing the continuous production time. This requires testing to select insulation materials with excellent long-term extrusion performance and optimizing extruder parameters to find optimal long-term processing conditions. However, the cable manufacturing industry currently lacks methods for accurately evaluating the continuous extrusion processing characteristics of cross-linked polyethylene cable insulation materials, nor does it have quantitative evaluation indicators to characterize these characteristics. This lacks clear criteria for selecting the optimal insulation material in the cable manufacturing industry. Furthermore, in actual submarine cable production, extruder processing parameters and the duration of continuous production must be determined empirically. If the continuous production time is set too long, pre-crosslinking of the material may occur during production, resulting in a decrease in cable insulation performance. If the continuous production time is set too short, the material's properties cannot be fully utilized, and the cable product's maximum length cannot be achieved. Due to the high price of high-voltage cable insulation materials, repeated trials in large-scale production equipment inevitably result in significant material waste and cost. Consequently, optimizing extruder processing parameters is costly and time-consuming.

[0006] How to reasonably select insulating materials that can be extruded for a long time, and how to reasonably limit the continuous extrusion time to avoid pre-crosslinking of the material during the extrusion process are key factors in determining the quality of the cable insulation layer. However, there is currently a lack of effective evaluation methods for the long-term extrusion processing performance of materials. There is a lack of reliable data support for the optimization of insulating materials, and there is also a lack of objective basis for setting the continuous extrusion time for the manufacture of long-length submarine cables. The present invention aims to provide a method and device for evaluating and optimizing the continuous extrusion processing characteristics of high-voltage cable insulation materials. This method and device can be used to evaluate and optimize cross-linkable polyethylene insulation materials suitable for long-term extrusion processing, and can also be used to optimize the process parameters during the long-term extrusion process to achieve the optimal extrusion processing process parameter setting for a given material.

[0007] Prior art document CN 114791440 A discloses a method for evaluating cross-linked inner shielding materials. By measuring the thermal expansion properties of test specimens made from the cross-linked inner shielding material, as well as the temperature rise of the extruder body during extrusion, this method can select, from a variety of inner shielding materials, a cross-linked inner shielding material that is less susceptible to pre-crosslinking. However, while the reference document qualitatively determines whether a material will pre-crosslink, the present method quantitatively evaluates the time it takes for a material to undergo continuous extrusion until pre-crosslinking occurs, rationally setting the duration of production. This approach aims to maximize the material's properties. Furthermore, the present invention can utilize more accurate quantitative indicators to screen materials, but its functionality is not limited to this. Summary of the Invention

[0008] Currently, there is a lack of effective methods for evaluating the long-term extrusion processing performance of cross-linked polyethylene insulation materials. This lack of reliable data supports the optimization of insulation materials, and there is also a lack of objective basis for setting the continuous extrusion time in the manufacture of long-length submarine cables. The present invention aims to provide a method and apparatus for evaluating and optimizing the continuous extrusion processing characteristics of high-voltage cable insulation materials. This method and apparatus can be used to evaluate and optimize cross-linked polyethylene insulation materials suitable for long-term extrusion processing, and can also be used to optimize process parameters during long-term extrusion processing to achieve the optimal extrusion processing parameter settings for a given material.

[0009] Based on the principle that pre-crosslinking of polyethylene materials causes changes in shear viscosity and melt expansion, the present invention designs a device for continuously monitoring the changes in shear viscosity and melt expansion of polyethylene cable insulation material during continuous extrusion. The continuous changes in shear viscosity and melt expansion over extrusion time quantitatively characterize the material's continuous extrusion processing characteristics. This invention achieves two main objectives: (1) evaluating the continuous extrusion processing characteristics of cross-linked polyethylene insulation materials for high-voltage cables; and (2) optimizing and determining the optimal process conditions for continuous extrusion of long-length submarine cables.

[0010] The technical solution of the present invention is:

[0011] In a first aspect, the present invention discloses a method for evaluating and optimizing the continuous extrusion processing characteristics of a high-voltage cable insulation material, which is characterized by comprising:

[0012] Step A1: The material to be tested is heated to a melt at a set temperature, and the melt is continuously extruded through a capillary die. During the extrusion process, the capillary inlet pressure P, the mass growth rate w of the melt extrusion, and the diameter D' of the melt spline are continuously measured and recorded to calculate the apparent shear viscosity η of the melt flow. a ;

[0013] Step A2, calculating the outlet expansion rate δ of the melt using the spline diameter D';

[0014] Step A3: Real-time recording and display of the curve η of the apparent shear viscosity and outlet expansion rate of the melt over time. a (t) and δ(t), in η a (t) or δ(t) curves. a The time corresponding to the increase of (t) or δ(t) by a set percentage is taken as the starting time of the cross-linking reaction and recorded as T X ;

[0015] Step A4: Select a reference sample and test it according to steps A1-A3. a (t) and δ(t) curves to determine the cross-linking reaction starting time T of the reference sample S;

[0016] Step A5, according to T X and T S The index α is defined, and the index α is a digital characteristic quantity for characterizing and evaluating the continuous extrusion processing characteristics of the material being tested.

[0017] Preferably, step A1 includes:

[0018] Step A1.1, using the mass growth rate w of the extruded melt, calculate the shear rate of the melt flow according to the following formula (1):

[0019]

[0020] Where ρ is the melt density, C = 1.02 × 10 4 / cm 3 is the device constant;

[0021] Step A1.2: Using the capillary inlet pressure data P, calculate the shear stress τ of the melt flow according to the following formula (2):

[0022]

[0023] Where P0 is the atmospheric pressure of the experimental environment, which is taken as 0.1 MPa;

[0024] Step A1.3, using the calculated τ and The apparent shear viscosity η of the melt flow is calculated according to the following formula (3): a

[0025]

[0026] Where τ is the shear stress of melt flow, is the shear rate of melt flow.

[0027] Preferably, in step A1, η is measured with 60 seconds as a data cycle. a Perform calculations.

[0028] Preferably, in step A2, the outlet expansion rate δ of the melt is calculated using the spline diameter D' according to the following formula (4).

[0029]

[0030] Preferably, in step A3, the percentage is set to 10%.

[0031] Preferably, in step A4, the reference sample is made of low-density polyethylene resin, a cross-linking agent and an antioxidant, wherein the low-density polyethylene resin is LDPE, the cross-linking agent is dicumyl peroxide-DCP, and the antioxidant is antioxidant 1010, and the sample is prepared in a ratio of 2phrDCP and 0.3phr antioxidant 1010 to 100phrLDPE.

[0032] Preferably, in step A4, the speed of the screw extruder is adjusted during the test so that the initial shear rate of the extruded melt is between 1000 and 1200 s -1 The experimental test is carried out continuously at the rotation speed to finally determine the starting time of the cross-linking reaction of the reference sample.

[0033] Preferably, in step A5, the α index is defined as follows:

[0034]

[0035] If α>0, it means that the continuous extrusion processing characteristics of the tested material are higher than those of the reference sample, and the larger the value, the longer the continuous processing time of the material in the equipment;

[0036] If α<0, it means that the continuous extrusion processing characteristics of the tested material are lower than those of the reference sample, and the larger the absolute value, the shorter the continuous processing time of the material in the equipment.

[0037] In a second aspect, the present invention discloses a device for evaluating and optimizing the continuous extrusion processing characteristics of high-voltage cable insulation materials, characterized in that:

[0038] The test device includes: a single-screw extruder of set specifications and a capillary extrusion die. Under set extrusion processing conditions, the melt of the material to be tested is continuously extruded from the single-screw extruder through the capillary;

[0039] A porous throttling device is provided at the inlet of the capillary die, and the polymer melt enters the extrusion capillary through the porous throttling device;

[0040] A melt pressure sensor is provided at the capillary inlet to continuously measure the melt pressure at the capillary inlet;

[0041] For the melt after being extruded through the capillary, an automatic weighing device is used to continuously measure the mass of the extruded melt per unit time;

[0042] A non-contact optical diameter measuring instrument is set at the outlet of the extruded melt to continuously measure the diameter of the extruded melt specimen.

[0043] Preferably, in the test device, the specifications of the single-screw extruder are: screw diameter Φ20mm, aspect ratio 20:1, compression ratio 1:1.18; weighing device range 100g, accuracy 1mg; diameter gauge range 5mm, accuracy 5μm.

[0044] Preferably, the throttling device includes two 100-mesh stainless steel filter screens and one 500-mesh stainless steel filter screen. The three stainless steel filter screens form a sandwich combination structure. The throttling device is installed at the inlet of the capillary mold. The capillary mold with the throttling device is connected to the single-screw extruder as a whole.

[0045] Preferably, the parameters of the capillary core are: capillary diameter D = 1.0 ± 0.013 mm, length L = 30.0 ± 0.13 mm, inlet angle A = 40° ± 0° 30'.

[0046] In a third aspect, the present invention discloses a method for optimizing a continuous extrusion process of a cross-linked polyethylene insulation material for a high-voltage cable, characterized by comprising:

[0047] Step B1: Using a predetermined cross-linkable polyethylene insulation material, and based on the number of heating sections of a large extruder used in actual production, select a small single-screw extruder with a similar structure and the same number of heating sections;

[0048] Step B2: heating the cross-linkable polyethylene insulation material into a melt under set processing conditions, and continuously extruding the melt through a capillary die by a single-screw extruder. During the extrusion process, the capillary inlet pressure P, the mass growth rate w of the melt extrusion, and the diameter D' of the melt spline are continuously measured and recorded to calculate the apparent shear viscosity η of the melt flow. a ;

[0049] Step B3, calculating the outlet expansion rate δ of the melt using the spline diameter D';

[0050] Step B4, real-time recording and display of the curve η of the apparent shear viscosity and outlet expansion rate of the melt changing with time a (t) and δ(t), in η a (t) or δ(t) curves. a The time corresponding to the increase of (t) or δ(t) by a set percentage is taken as the starting time of the cross-linking reaction and recorded as T X ;

[0051] Step B5: Set multiple groups of different process conditions. Under each group of process conditions, continuously extrude the melt of the material to be tested through the capillary die, and retest the cross-linking reaction starting time T of the material. X , obtain each group of T related to the processing conditions X value;

[0052] Step B6, T X The shear viscosity η corresponding to the maximum value a As the numerical characteristic of the melt in the best flow state, the process conditions corresponding to the shear viscosity are taken as the optimal conditions for the extrusion of long-length submarine cables;

[0053] Step B7: according to the process parameter rules obtained from the small and medium-sized extruder test in step B6, the production process parameters of the actual extruder are set, and the optimal process parameters can be obtained through fine-tuning.

[0054] Preferably, in step B2, the processing conditions include: the screw speed of the screw extruder and the operating temperature of each heating section.

[0055] Preferably, step B2 includes:

[0056] Step B2.1, using the mass growth rate w of the extruded melt, calculate the shear rate of the melt flow according to the following formula (1):

[0057]

[0058] Where ρ is the melt density, C = 1.02 × 10 4 / cm 3 is the device constant;

[0059] Step B2.2, using the capillary inlet pressure data P, calculate the shear stress τ of the melt flow according to the following formula (2):

[0060]

[0061] Where P0 is the atmospheric pressure of the experimental environment, which is taken as 0.1 MPa;

[0062] Step B2.3, using the calculated τ and The apparent shear viscosity η of the melt flow is calculated according to the following formula (3): a

[0063]

[0064] Where τ is the shear stress of melt flow, is the shear rate of melt flow.

[0065] Preferably, in step B2, η is calculated with 60 seconds as a data cycle. a Perform calculations.

[0066] Preferably, in step B3, the outlet expansion rate δ of the melt is calculated using the spline diameter D' according to the following formula (4):

[0067]

[0068] Real-time recording and display of the curve η of the apparent shear viscosity and outlet expansion rate of the melt changing with time a (t) and δ(t).

[0069] Preferably, in step B4, the percentage is set to 10%.

[0070] Preferably, in step B5, in each set of process conditions, the temperature of each heating section and extruder head and the screw speed are different.

[0071] In a fourth aspect, the present invention discloses an optimization device for continuous extrusion processing of cross-linked polyethylene insulation materials for high-voltage cables, characterized in that:

[0072] The optimization device includes: a single screw extruder and a capillary extrusion die with set specifications,

[0073] Under the set extrusion processing conditions, the melt of the material to be tested is continuously extruded through the capillary by a single-screw extruder;

[0074] A porous throttling device is provided at the inlet of the capillary die, and the polymer melt enters the extrusion capillary through the porous throttling device;

[0075] A melt pressure sensor is provided at the capillary inlet to continuously measure the melt pressure at the capillary inlet;

[0076] For the melt after being extruded through the capillary, an automatic weighing device is used to continuously measure the mass of the extruded melt per unit time;

[0077] A non-contact optical diameter measuring instrument is set at the outlet of the extruded melt to continuously measure the diameter of the extruded melt specimen.

[0078] Preferably, the single-screw extruder has a structure similar to that of a large-scale extruder used in actual production, and has the same number of heating sections.

[0079] Preferably, in the optimized device, the specifications of the single-screw extruder are: screw diameter Φ20mm, aspect ratio 20:1, compression ratio 1:1.18; weighing device range 100g, accuracy 1mg; diameter gauge range 5mm, accuracy 5μm.

[0080] Preferably, the throttling device includes two 100-mesh stainless steel filter screens and one 500-mesh stainless steel filter screen. The three stainless steel filter screens form a sandwich combination structure. The throttling device is installed at the inlet of the capillary mold. The capillary mold with the throttling device is connected to the single-screw extruder as a whole.

[0081] Preferably, the parameters of the capillary core are: capillary diameter D = 1.0 ± 0.013 mm, length L = 30.0 ± 0.13 mm, inlet angle A = 40° ± 0° 30'.

[0082] The beneficial effects of the present invention are:

[0083] (1) The inventors have found in their long-term research that the long-term extrusion processing characteristics of cross-linkable polyethylene insulation materials are jointly determined by the rheological properties of the materials and the chemical properties of the cross-linking reaction. Traditional methods for characterizing the extrusion processing performance of cross-linkable polyethylene insulation materials mostly use various types of rheometers (such as torque rheometers, rotational rheometers, high-pressure capillary rheometers, etc.) to test the rheological properties of the materials. Due to the small total mass of the test material, the cavity space where the melt is located is small, and the continuous test time is short, under the material extrusion processing temperature conditions, the material rheological characteristic parameters are poorly sensitive to pre-crosslinking. Therefore, the test parameters related to the pre-crosslinking characteristics of the material must be obtained at a higher temperature (higher than the decomposition temperature of the cross-linking agent DCP), and the relevant tests can only make qualitative and inaccurate inferences about the long-term extrusion processing performance. Since the test temperature deviates from (is higher than) the long-term extrusion processing conditions of the actual production process of the insulation material, the reference value of the test results is poor. The present invention adopts an extruder in conjunction with a throttling device and a capillary die to perform uninterrupted melt extrusion, and the continuous test time is not limited. Under the premise of more realistically simulating the cable insulation extrusion process, not only can more realistic performance parameters be measured, but also a large amount of experimental material consumption can be saved.

[0084] (2) The extrusion processing and testing device of the present invention can provide multiple parameters for quantitatively evaluating the long-term extrusion processing performance of cross-linkable polyethylene insulation materials. This can be used to optimize materials based on these quantitative parameters and can also provide clear optimization targets for setting extruder processing parameters. By using a low-power extruder to simulate large-scale production equipment, the optimal process parameters can be explored with minimal material consumption, which not only saves a large amount of production equipment capacity, but also reduces a large amount of experimental cycles and material consumption.

[0085] (3) Compared with the traditional testing method of periodically and manually measuring the expansion rate and the quality of the extruded melt, the multiple quantitative parameters described in the present invention are obtained and continuously recorded by the automated testing of the equipment, which can continuously and uninterruptedly obtain the changes in important parameters during the long-term extrusion processing of the material, and can obtain more information on the changes in the material during the extrusion processing. Therefore, the exact time point when the material's performance changes due to pre-crosslinking can be accurately discovered, and the evaluation of the material's long-term extrusion processing performance is more accurate and reliable.

[0086] (4) The present invention provides a sandwich structure throttling device composed of multiple layers of filter screens. The throttling device is composed of high-precision filter screens. The filter screens have a certain obstructive effect on the flow of the melt, and together with the capillary die connected to it, they constitute a part of the area with poor melt fluidity (which can be figuratively called a flow dead corner). This combination method simulates the situation where there are processing dead corners in the extruder during the actual extrusion production process. The existence of dead corners will cause the retained melt material to be pre-cross-linked, thereby deteriorating the quality of the insulation layer, and the cable will therefore not be able to be continuously extruded for a long time. The device provided by the present invention can fully test the problems of pre-cross-linking and long-term extrusion performance deterioration caused by the melt flow dead corners, and fully simulate the specific problems in the actual cable production process. Therefore, the results obtained according to the method of the present invention have good practical application reference value. BRIEF DESCRIPTION OF THE DRAWINGS

[0087] Figure 1 This is a flow chart of the method for evaluating the continuous extrusion processing characteristics of insulating materials in the present invention;

[0088] Figure 2 This is a flow chart of the process optimization method for the continuous extrusion process of the insulating material in the present invention;

[0089] Figure 3 It is a schematic diagram of the melt extrusion testing device in the present invention;

[0090] Figure 4a It is a schematic diagram of the throttling device in the present invention;

[0091] Figure 4b This is a schematic diagram of the matching method of the capillary mold, throttling device, caliper, and pressure sensor in the present invention;

[0092] Figure 5 Schematic diagram of the capillary mold core in the present invention;

[0093] Figure 6 is η in the present invention a (t), schematic diagram of the curve of δ(t) changing with time.

[0094] In the figure, 1 is a 100-mesh stainless steel filter; 2 is a 500-mesh stainless steel filter; 3 is the inlet of the capillary mold; 4 is a pressure sensor; 5 is a heating jacket; 6 is the outlet of the capillary mold; 7 is a non-contact optical diameter gauge DETAILED DESCRIPTION

[0095] To make the objectives, technical solutions, and advantages of the present invention more clear, the technical solutions of the present invention will be clearly and completely described below in conjunction with the accompanying drawings in the embodiments of the present invention. The embodiments described in this application are only part of the embodiments of the present invention, not all of them. Based on the spirit of the present invention, other embodiments obtained by ordinary technicians in this field without making creative efforts are all within the scope of protection of the present invention.

[0096] Based on the principle that pre-crosslinking of polyethylene materials causes changes in shear viscosity and melt expansion, the present invention designs a device for continuously monitoring the changes in shear viscosity and melt expansion of polyethylene cable insulation material during continuous extrusion. The continuous changes in shear viscosity and melt expansion over extrusion time quantitatively characterize the material's continuous extrusion processing characteristics. This invention achieves two main objectives: (1) evaluating the continuous extrusion processing characteristics of cross-linked polyethylene insulation materials for high-voltage cables; and (2) optimizing and determining the optimal process conditions for continuous extrusion of long-length submarine cables.

[0097] Example (1): Evaluation of continuous extrusion processing characteristics of cross-linked polyethylene insulation material for high-voltage cables

[0098] In order to characterize and evaluate the continuous extrusion processing characteristics of cross-linked polyethylene cable insulation materials and objectively reflect the differences between different materials, it is actually necessary to find the processing time length from the start of processing to the occurrence of noticeable pre-crosslinking under the same extrusion processing conditions. This time length is used as a quantitative indicator to characterize the continuous processing characteristics of the material.

[0099] The technical principles and test methods for characterizing the continuous extrusion processing characteristics of high-voltage submarine cable insulation materials are as follows:

[0100] In step A1, the material to be tested is heated to a melt at 115±2°C and continuously extruded through a capillary die using a single-screw extruder at an appropriate screw speed (the appropriate screw speed should be set according to the conditions given when testing the reference specimen, and the shear rate of the test specimen and the reference specimen should be set to the same). During the extrusion process, the capillary inlet pressure P (MPa), the mass growth rate of the melt extrusion w (g / s), and the diameter D' (μm) of the melt spline are continuously measured and recorded. The following calculations are performed with a data period of 60 seconds:

[0101] Step A1.1: Calculate the shear rate of the melt flow using the mass rate of the extruded melt according to formula (1):

[0102]

[0103] Where ρ(g / cm3 ) is the melt density, C = 1.02 × 10 4 / cm 3 is the device constant.

[0104] Step A1.2: Using the capillary inlet pressure data, calculate the shear stress τ (MPa) of the melt flow according to equation (2):

[0105]

[0106] Where P0 is the atmospheric pressure of the experimental environment, which can usually be taken as 0.1 MPa.

[0107] Step A1.3, using the calculated τ and The apparent shear viscosity η of the melt flow is calculated numerically according to formula (3): a (Pa·s)

[0108]

[0109] Step A2, while completing step A1, calculate the outlet expansion rate δ (%) of the melt using the spline diameter D' according to formula (4)

[0110]

[0111] Step A3: Real-time recording and display of the curve η of the apparent shear viscosity and outlet expansion rate of the melt over time. a (t) and δ(t).

[0112] If the molecular chain of the material does not cross-link during the extrusion process, the shear viscosity and outlet expansion rate of the melt are stable values, that is, η a (t) and δ(t) are almost horizontal curves. If the material undergoes a certain degree of pre-crosslinking, a gel point will form in the melt. At this time, the flux of the melt will decrease when passing through the throttling device, causing a significant increase in the measured apparent shear viscosity and an increase in the melt outlet expansion rate.

[0113] With η a (t) or δ(t) increases by more than a certain percentage (preferably 10%) as the criterion, when η a When either η(t) or δ(t) reaches 1.1 times or more of the initial value, it indicates that the material has begun to pre-crosslink. a Find η on the (t) or δ(t) curve a The time corresponding to a 10% increase in (t) or δ(t) is taken as the starting time of the cross-linking reaction and recorded as T X The curve is as follows. Figure 6 shown.

[0114] Step A4: Select a reference sample and test it according to steps A1-3. a (t) and δ(t) curves to determine the cross-linking reaction starting time T of the reference sample S .

[0115] To minimize the impact of differences in experimental equipment parameters and structure on the test results, reference samples were used to further refine the test results. The reference sample is recommended to be made from low-density polyethylene (LDPE), a crosslinker (dicumyl peroxide - DCP), and an antioxidant (Antioxidant 1010) commonly used in the industrial manufacture of high-voltage cable insulation. The ratio of DCP to antioxidant 1010 is 2 phr per 100 phr of LDPE.

[0116] According to the above principle, the η of the reference sample is tested a (t) and δ(t) curves. When starting the test, the screw extruder speed should be adjusted so that the initial shear rate of the extruded melt is between 1000 and 1200 s -1 The experimental test was carried out continuously at the rotation speed, and the starting time of the cross-linking reaction of the reference sample was finally determined, which was recorded as T S .

[0117] Step A5, according to T X and T S The α index is defined as a numerical characteristic quantity that characterizes the continuous extrusion processing characteristics of the material being tested. The α index is defined according to the following formula (5):

[0118]

[0119] If α>0, it means that the continuous extrusion processing characteristics of the tested material are higher than those of the reference sample, and the larger the value, the longer the material can be continuously processed in the equipment.

[0120] If α<0, it means that the continuous extrusion processing characteristics of the tested material are lower than those of the reference sample, and the larger the absolute value, the shorter the time the material can be continuously processed in the equipment.

[0121] The principle block diagram of the test device is as follows: Figure 3 As shown, the device can continuously obtain the time-varying curves of three parameters: the pressure P of the melt of the tested material flowing through the capillary inlet, the mass m of the extruded melt per unit time, and the diameter D' of the extruded melt spline, as basic data.

[0122] The test device specifically includes a single-screw extruder of specific specifications and a capillary extrusion die, which are used to continuously extrude the melt of the tested material through the capillary under specific extrusion processing conditions (the extruder temperature is set between 105 and 120°C); the recommended specifications of the single-screw extruder in the test device are: screw diameter Φ20mm, aspect ratio 20:1, compression ratio 1:1.18; weighing device range 100g, accuracy 1mg; diameter gauge range 5mm, accuracy 5μm.

[0123] A multi-hole throttling device is installed at the inlet of the capillary die, through which the polymer melt enters the extrusion capillary. The throttling device consists of a sandwich structure consisting of two 100-mesh and one 500-mesh stainless steel screens, as shown in Figure 4(a). The throttling device is installed at the inlet of the capillary die, as shown in Figure 4(b). The capillary die with the throttling device is connected to the single-screw extruder.

[0124] A melt pressure sensor is set at the capillary inlet to continuously measure the melt pressure at the capillary inlet; the capillary mold core is as follows: Figure 5 The recommended parameters are: capillary diameter D = 1.0 ± 0.013 mm, length L = 30.0 ± 0.13 mm, and inlet angle A = 40° ± 0° 30'.

[0125] For the melt after being extruded through the capillary, an automatic weighing device is used to continuously measure the mass of the extruded melt per unit time;

[0126] A non-contact optical diameter measuring instrument is set at the outlet of the extruded melt to continuously measure the diameter of the extruded melt specimen.

[0127] Example (II): Optimizing and Determining the Optimal Process Conditions for Continuous Extrusion of Long-Length Submarine Cables

[0128] In order to optimize and determine the optimal process conditions for continuous extrusion of long-length submarine cables, it is necessary to evaluate the processing time of the material from the beginning of processing to the occurrence of noticeable pre-crosslinking under different extrusion process conditions, while determining the material category. This time length is used as a quantitative indicator for evaluating and adjusting the processing parameters. Through experiments, a set of process parameters that maximizes this time length can be optimally obtained. This set of process parameters can provide data reference for determining and optimizing the process parameters for long-length continuous extrusion.

[0129] The technical principles and optimization methods for optimizing and determining the optimal process conditions for continuous extrusion of long-length submarine cables are as follows:

[0130] Step B1: Using a predetermined cross-linkable polyethylene insulation material, and based on the number of heating sections of a large extruder used in actual production, select a small single-screw extruder with a similar structure and the same number of heating sections;

[0131] In step B2, the material to be tested is heated to a melt under a set of predetermined process conditions (including the temperature of each heating section and the screw speed). The material is then continuously extruded through a capillary die using a single-screw extruder. During the extrusion process, the capillary inlet pressure P (MPa), the melt extrusion mass rate w (g / s), and the melt spline diameter D' (μm) are continuously measured and recorded. The following calculations are performed using a 60-second data cycle:

[0132] Step B2.1, calculate the shear rate of melt flow using the mass rate of the extruded melt according to formula (1):

[0133]

[0134] Where ρ(g / cm 3 ) is the melt density, C = 1.02 × 10 4 / cm 3 is the device constant.

[0135] Step B2.2, calculate the shear stress τ (MPa) of the melt flow using the capillary inlet pressure according to formula (2)

[0136]

[0137] Where P0 is the atmospheric pressure of the experimental environment, which can usually be taken as 0.1 MPa.

[0138] Step B2.3, using the calculated τ and The apparent shear viscosity η of the melt flow is calculated numerically according to formula (3): a (Pa·s)

[0139]

[0140] Step B3, while completing step B2, calculate the outlet expansion rate δ (%) of the melt using the spline diameter D' according to formula (4)

[0141]

[0142] Step B4, real-time recording and display of the curve η of the apparent shear viscosity and outlet expansion rate of the melt changing with time a (t) and δ(t). If the molecular chain of the material does not cross-link during the extrusion process, the shear viscosity and outlet expansion rate of the melt are stable values, η a(t) and δ(t) are almost horizontal curves. If the material undergoes a certain degree of cross-linking, a gel point will form in the melt. At this time, the flux of the melt will decrease when passing through the throttling device, causing a significant increase in the measured apparent shear viscosity and an increase in the melt outlet expansion rate.

[0143] With η a (t) or δ(t) increases by more than a certain percentage (preferably 10%) as the criterion, when η a When either η(t) or δ(t) reaches 1.1 times or more of the initial value, it indicates that the material has begun to pre-crosslink. a Find η on the (t) or δ(t) curve a The time corresponding to a 10% increase in (t) or δ(t) is taken as the starting time of the cross-linking reaction and recorded as T X The curve is as follows. Figure 6 shown.

[0144] Step B5: Using the screw speed and operating temperature of the screw extruder as processing conditions, adjust these parameters to set multiple sets of different process conditions (including the temperature and screw speed of each heating section and extruder head). Under each set of process conditions, the melt of the tested material is continuously extruded through the capillary die, and the cross-linking reaction start time T of the material is retested. X , obtain each group of T related to the processing conditions X value.

[0145] Step B6, T X The shear viscosity corresponding to the maximum value is taken as the numerical characteristic of the melt being in the optimal flow state, and the process conditions corresponding to the shear viscosity are taken as the optimal conditions for the extrusion processing of long-length submarine cables.

[0146] Step B7, according to the process parameter rules obtained from the small extruder test, set the production process parameters of the actual extruder and make fine adjustments to obtain the optimal process parameters, thereby shortening the test time on the large extruder and reducing the waste of materials and extruder production capacity.

[0147] The principle block diagram of the optimization device is as follows: Figure 3 As shown, the device can continuously obtain the time-varying curves of three parameters: the pressure P of the melt of the tested material flowing through the capillary inlet, the mass m of the extruded melt per unit time, and the diameter D' of the extruded melt spline, as basic data.

[0148] The optimized equipment specifically includes a small single-screw extruder with a similar structure to the large extruder used in actual production, including the same number of heating sections; a weighing device with a range of 100g and an accuracy of 1mg; and a diameter gauge with a range of 5mm and an accuracy of 5μm. The number of heating sections can be set based on the number of heating sections in the extruder to be used in actual cable production. If necessary, the extruder specifications can be adjusted to accommodate the desired number of heating sections.

[0149] A multi-hole throttling device is installed at the inlet of the capillary die, through which the polymer melt enters the extrusion capillary. The throttling device consists of two 100-mesh and one 500-mesh stainless steel screens in a sandwich structure, as shown in Figure 4(a). The throttling device is installed at the inlet of the capillary die, as shown in Figure 4(b). The capillary die with the throttling device is connected to the single-screw extruder.

[0150] A melt pressure sensor is set at the capillary inlet to continuously measure the melt pressure at the capillary inlet; the capillary mold core is as follows: Figure 5 The recommended parameters are: capillary diameter D = 1.00 ± 0.01 mm, length L = 30.0 ± 0.1 mm, and inlet angle A = 40° ± 0° 30'.

[0151] The melt after being extruded through the capillary is continuously measured by an automatic weighing device in terms of the mass of the extruded melt per unit time;

[0152] A non-contact optical diameter measuring instrument is set at the outlet of the extruded melt to continuously measure the diameter of the extruded melt specimen.

[0153] The beneficial effects of the present invention are as follows:

[0154] (1) The inventors have found in their long-term research that the long-term extrusion processing characteristics of cross-linkable polyethylene insulation materials are jointly determined by the rheological properties of the materials and the chemical properties of the cross-linking reaction. Traditional methods for characterizing the extrusion processing performance of cross-linkable polyethylene insulation materials mostly use various types of rheometers (such as torque rheometers, rotational rheometers, high-pressure capillary rheometers, etc.) to test the rheological properties of the materials. Due to the small total mass of the test material, the cavity space where the melt is located is small, and the continuous test time is short, under the material extrusion processing temperature conditions, the material rheological characteristic parameters are poorly sensitive to pre-crosslinking. Therefore, the test parameters related to the pre-crosslinking characteristics of the material must be obtained at a higher temperature (higher than the decomposition temperature of the cross-linking agent DCP), and the relevant tests can only make qualitative and inaccurate inferences about the long-term extrusion processing performance. Since the test temperature deviates from (is higher than) the long-term extrusion processing conditions of the actual production process of the insulation material, the reference value of the test results is poor. The present invention adopts an extruder in conjunction with a throttling device and a capillary die to perform uninterrupted melt extrusion, and the continuous test time is not limited. Under the premise of more realistically simulating the cable insulation extrusion process, not only can more realistic performance parameters be measured, but also a large amount of experimental material consumption can be saved.

[0155] (2) The extrusion processing and testing device of the present invention can provide multiple parameters for quantitatively evaluating the long-term extrusion processing performance of cross-linkable polyethylene insulation materials. This can be used to optimize materials based on these quantitative parameters and can also provide clear optimization targets for setting extruder processing parameters. By using a low-power extruder to simulate large-scale production equipment, the optimal process parameters can be explored with minimal material consumption, which not only saves a large amount of production equipment capacity, but also reduces a large amount of experimental cycles and material consumption.

[0156] (3) Compared with the traditional testing method of periodically and manually measuring the expansion rate and the quality of the extruded melt, the multiple quantitative parameters described in the present invention are obtained and continuously recorded by the automated testing of the equipment, which can continuously and uninterruptedly obtain the changes in important parameters during the long-term extrusion processing of the material, and can obtain more information on the changes in the material during the extrusion processing. Therefore, the exact time point when the material's performance changes due to pre-crosslinking can be accurately discovered, and the evaluation of the material's long-term extrusion processing performance is more accurate and reliable.

[0157] (4) The present invention provides a sandwich structure throttling device composed of multiple layers of filter screens. The throttling device is composed of high-precision filter screens. The filter screens have a certain obstructive effect on the flow of the melt, and together with the capillary die connected to it, they constitute a part of the area with poor melt fluidity (which can be figuratively called a flow dead corner). This combination method simulates the situation where there are processing dead corners in the extruder during the actual extrusion production process. The existence of dead corners will cause the retained melt material to be pre-cross-linked, thereby deteriorating the quality of the insulation layer, and the cable will therefore not be able to be continuously extruded for a long time. The device provided by the present invention can fully test the problems of pre-cross-linking and long-term extrusion performance deterioration caused by the melt flow dead corners, and fully simulate the specific problems in the actual cable production process. Therefore, the results obtained according to the method of the present invention have good practical application reference value.

[0158] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention rather than to limit it. Although the present invention has been described in detail with reference to the above embodiments, ordinary technicians in the field should understand that the specific implementation methods of the present invention can still be modified or replaced by equivalents. Any modification or equivalent replacement that does not depart from the spirit and scope of the present invention should be covered by the scope of protection of the claims of the present invention.

Claims

1. A method for evaluating and optimizing the continuous extrusion processing characteristics of high-voltage cable insulation materials, characterized in that: include: Step A1: The material to be tested is heated to a melt at a set temperature, and the melt is continuously extruded through a capillary die. During the extrusion process, the capillary inlet pressure P, the mass growth rate w of the melt extrusion, and the diameter D' of the melt spline are continuously measured and recorded to calculate the apparent shear viscosity η of the melt flow. a ; Step A2, calculating the outlet expansion rate δ of the melt using the spline diameter D'; Step A3: Real-time recording and display of the curve η of the apparent shear viscosity and outlet expansion rate of the melt over time. a (t) and δ(t), in η a (t) or δ(t) curves. a The time corresponding to the increase of (t) or δ(t) by a set percentage is taken as the starting time of the cross-linking reaction and recorded as T X ; Step A4: Select a reference sample and test it according to steps A1-A3. a (t) and δ(t) curves to determine the cross-linking reaction starting time T of the reference sample S ; Step A5, according to T X and T S The index α is defined, and the index α is a digital characteristic quantity used to characterize and evaluate the continuous extrusion processing characteristics of the material being tested; In step A1, it includes: Step A1.1, using the mass growth rate w of the extruded melt, calculate the shear rate of the melt flow according to the following formula (1): Where ρ is the melt density, C = 1.02 × 10 4 / cm 3 is the device constant; Step A1.2: Using the capillary inlet pressure data P, calculate the shear stress τ of the melt flow according to the following formula (2): Where P0 is the atmospheric pressure of the experimental environment, which is taken as 0.1 MPa; Step A1.3, using the calculated τ and The apparent shear viscosity η of the melt flow is calculated according to the following formula (3): a Where τ is the shear stress of melt flow, is the shear rate of melt flow; In step A2, the outlet expansion rate δ of the melt is calculated using the spline diameter D' according to the following formula (4): In step A5, the α index is defined as follows: If α>0, it means that the continuous extrusion processing characteristics of the tested material are higher than those of the reference sample, and the larger the value, the longer the continuous processing time of the material in the equipment; If α<0, it means that the continuous extrusion processing characteristics of the tested material are lower than those of the reference sample, and the larger the absolute value, the shorter the continuous processing time of the material in the equipment.

2. The method for evaluating and optimizing the continuous extrusion processing characteristics of high-voltage cable insulation materials according to claim 1, characterized in that: In step A1, η is measured with 60 seconds as a data cycle. a Perform calculations.

3. The method for evaluating and optimizing the continuous extrusion processing characteristics of high-voltage cable insulation materials according to claim 1, characterized in that: In step A3, the percentage is set to 10%.

4. The method for evaluating and optimizing the continuous extrusion processing characteristics of high-voltage cable insulation materials according to claim 1, characterized in that: In step A4, a reference sample is prepared using low-density polyethylene resin, a crosslinking agent, and an antioxidant, wherein the crosslinking agent is dicumyl peroxide (DCP), and the antioxidant is antioxidant 1010, and the sample is prepared in a ratio of 2 phr DCP and 0.3 phr antioxidant 1010 to 100 phr low-density polyethylene.

5. The method for evaluating and optimizing the continuous extrusion processing characteristics of high-voltage cable insulation materials according to claim 1, characterized in that: In step A4, the screw extruder speed is adjusted during the test so that the initial shear rate of the extruded melt is between 1000 and 1200 s -1 The experimental test is carried out continuously at the rotation speed to finally determine the starting time of the cross-linking reaction of the reference sample.

6. A method for optimizing the continuous extrusion process of cross-linked polyethylene insulation material for high-voltage cables, characterized in that: include: Step B1: Using a predetermined cross-linkable polyethylene insulation material, and based on the number of heating sections of a large extruder used in actual production, select a small single-screw extruder with a similar structure and the same number of heating sections; Step B2: heating the cross-linkable polyethylene insulation material into a melt under set processing conditions, and continuously extruding the melt through a capillary die by a single-screw extruder. During the extrusion process, the capillary inlet pressure P, the mass growth rate w of the melt extrusion, and the diameter D' of the melt spline are continuously measured and recorded to calculate the apparent shear viscosity η of the melt flow. a ; Step B3, calculating the outlet expansion rate δ of the melt using the spline diameter D'; Step B4, real-time recording and display of the curve η of the apparent shear viscosity and outlet expansion rate of the melt changing with time a (t) and δ(t), in η a (t) or δ(t) curves. a The time corresponding to the increase of (t) or δ(t) by a set percentage is taken as the starting time of the cross-linking reaction and recorded as T X ; Step B5: Set multiple groups of different process conditions. Under each group of process conditions, continuously extrude the melt of the material to be tested through a capillary die, and retest the cross-linking reaction starting time T of the material. X , obtain each group of T related to the processing conditions X value; Step B6, T X The shear viscosity η corresponding to the maximum value a As the numerical characteristic of the melt in the best flow state, the process conditions corresponding to the shear viscosity are taken as the optimal conditions for the extrusion of long-length submarine cables; Step B7: setting the production process parameters of the actual extruder according to the process parameter rules obtained from the small-scale extruder test in step B6, and obtaining the optimal process parameters through fine-tuning; In step B2, it includes: Step B2.1, using the mass growth rate w of the extruded melt, calculate the shear rate of the melt flow according to the following formula (1): Where ρ is the melt density, C = 1.02 × 10 4 / cm 3 is the device constant; Step B2.2, using the capillary inlet pressure data P, calculate the shear stress τ of the melt flow according to the following formula (2): Where P0 is the atmospheric pressure of the experimental environment, which is taken as 0.1 MPa; Step B2.3, using the calculated τ and The apparent shear viscosity η of the melt flow is calculated according to the following formula (3): a Where τ is the shear stress of melt flow, is the shear rate of melt flow; In step B3, the outlet expansion rate δ of the melt is calculated using the spline diameter D' according to the following formula (4): Real-time recording and display of the curve η of the apparent shear viscosity and outlet expansion rate of the melt changing with time a (t) and δ(t).

7. The method for optimizing the continuous extrusion process of cross-linked polyethylene insulation material for high-voltage cables according to claim 6, characterized in that: In step B2, the processing conditions include: the screw speed of the screw extruder and the operating temperature of each heating section.

8. The method for optimizing the continuous extrusion process of cross-linked polyethylene insulation material for high-voltage cables according to claim 6, characterized in that: In step B2, η is calculated with 60 seconds as a data cycle. a Perform calculations.

9. The method for optimizing the continuous extrusion process of cross-linked polyethylene insulation material for high-voltage cables according to claim 6, characterized in that: In step B4, the percentage is set to 10%.

10. The method for optimizing the continuous extrusion process of cross-linked polyethylene insulation material for high-voltage cables according to claim 6, characterized in that: In step B5, in each set of process conditions, the temperature of each heating section and extruder head and the screw speed are not exactly the same.

Images