A T-shaped die with variable resistance zone thickness and flow channel design method thereof

Abstract

The present invention discloses a T-shaped die with a variable resistance zone thickness and a flow channel design method thereof. Based on the analysis of melt flow in the flow channel, a structure with a variable resistance zone thickness is adopted to reduce the extrusion pressure, and a structure with a variable manifold radius is adopted to reduce the melt residence time. Under the condition that the melt outlet flow rate is uniform along the flow channel width direction, a differential equation for the change of the resistance zone thickness along the flow channel width direction is derived using rheological theory, and the resistance zone thickness can be obtained by numerically solving and fitting the equation. A flow channel with a variable manifold radius and a variable resistance zone thickness can significantly reduce the extrusion pressure and the melt residence time. By designing the size and shape of the manifold, the melt residence time can be significantly reduced without significantly increasing the extrusion pressure, and by designing the length and thickness of the resistance zone, the extrusion pressure can be adjusted to adapt to different molding requirements without increasing the melt residence time.

Description

Technical Field

[0001] The invention relates to the technical field of mold design, and in particular to a T-shaped mold with a variable resistance zone thickness and a flow channel design method thereof. Background Art

[0002] T-type dies are often used for extrusion molding of thermoplastic sheets and cast films. The key to the design of the flow channel is that the melt outlet flow rate along the width of the flow channel should be uniform and consistent, and the residence time of the melt in the flow channel should be as short as possible. The flow channel of the T-type die consists of an inlet area, a manifold, a choke area, a relaxation area and a molding area, in which the manifold cross-section and the size of the choke area remain unchanged along the width of the flow channel. After the melt enters the manifold, while flowing along the manifold to both sides of the flow channel, a part of the melt enters the choke area and flows along the extrusion direction. The volume flow rate of the melt in the manifold gradually decreases, and the residence time of the melt is long; the pressure of the melt gradually decreases when it flows along the manifold to both sides of the flow channel, especially when the flow channel width is large, the pressure drop of the melt in the manifold is large. When a flow channel with equal choke area thickness is used, the outlet flow rate of the melt along the width of the flow channel differs greatly. Limited by the length of the die, it is usually adopted to increase the radius of the manifold to reduce the pressure drop of the melt in the manifold and reduce the thickness of the choke area to increase the pressure drop of the melt in the choke area to improve the uniformity of the melt outlet flow rate. Increasing the manifold radius significantly increases the melt residence time, while decreasing the choke thickness significantly increases the extrusion pressure.

[0003] The measures to improve the uniformity of melt outlet flow rate from the aspects of flow channel geometry and mold structure include variable baffle length, variable baffle thickness and adding baffle rods. Variable baffle length is to keep the manifold cross section and baffle thickness unchanged, and reduce the baffle length along both sides of the flow channel, but this structure is difficult to apply in actual production; variable baffle thickness is to keep the manifold cross section and baffle length unchanged, and increase the baffle thickness along both sides of the flow channel, but the melt residence time is long; the structure with baffle rods improves the uniformity of melt outlet flow rate by adjusting the baffle rods and then changing the local gap of the baffle zone during molding, but it increases the manufacturing cost of the mold. Even if the baffle rods are adjusted for poorly designed flow channels, it is difficult to achieve the ideal uniformity of melt outlet flow rate. Summary of the invention

[0004] The purpose of the present invention is to overcome the shortcomings of the prior art and provide a T-shaped die with a variable resistance flow zone thickness capable of reducing extrusion pressure and melt residence time.

[0005] Another object of the present invention is to provide a T-type die flow channel design method with a variable resistance zone thickness. The design method adopts a manifold whose radius gradually decreases along both sides of the flow channel and a resistance zone whose thickness gradually increases along both sides of the flow channel. Under the condition that the melt outlet flow rate is uniform along the width direction of the flow channel, the flow channel structure and size are theoretically designed according to rheological theory, in order to reduce the extrusion pressure and melt residence time of the T-type die.

[0006] The technical solution of the present invention is: a T-type mold with a variable resistance zone thickness, the flow channel of the T-type mold includes an inlet area, a manifold, a resistance zone, a relaxation zone and a molding zone arranged in sequence along the flow direction of the melt, with the symmetry plane in the width direction of the flow channel as the center, the cross-sectional size of the manifold gradually decreases along both sides of the flow channel, the thickness of the resistance zone gradually increases along the width direction of the flow channel, and the cross-sectional sizes of the relaxation zone and the molding zone remain unchanged along the width direction of the flow channel.

[0007] As a preference, the length of the flow blocking area remains constant along the width direction of the flow channel.

[0008] As a preference, the cross-section of the manifold is circular.

[0009] A method for designing a T-shaped mold flow channel with a variable resistance zone thickness comprises the following steps:

[0010] Constructing the physical model: Assumptions a. The melt is an incompressible fluid; b. The melt flow is a fully developed steady-state laminar flow, and the inertial force and volume force are ignored; c. The melt flows only along the axial direction of the manifold in the manifold, and only flows along the extrusion direction in the choke zone, relaxation zone and molding zone, and the melt flow in the manifold and the flow in the choke zone do not interfere with each other; d. The effect of stretching caused by the change of manifold radius on the melt flow in the manifold is ignored; e. The effect of the end walls on both sides of the flow channel width direction on the melt flow in the choke zone, relaxation zone and molding zone is ignored;

[0011] Constructing geometric model: the mold flow channel includes the inlet area, manifold, flow blocking area, relaxation area and forming area arranged in sequence along the flow direction;

[0012] Based on the analysis of melt flow in the flow channel, a structure with variable resistance zone thickness is adopted to reduce the extrusion pressure, and a structure with variable manifold radius is adopted to reduce the melt residence time. Under the condition that the melt outlet flow rate is uniform along the width of the flow channel, the rheological theory is used to derive the differential equation for the variation of the resistance zone thickness along the width of the flow channel. The resistance zone thickness can be obtained by numerically solving and fitting the equation.

[0013] As a preferred embodiment, the cross-sectional dimensions of the manifold are designed to gradually decrease along both sides of the flow channel.

[0014] As a preferred embodiment, the cross section of the manifold is circular.

[0015] As a preferred embodiment, with the symmetry plane in the width direction of the flow channel as the center, the thickness of the flow blocking area gradually increases along the width direction of the flow channel.

[0016] As a preferred embodiment, the length of the flow blocking area remains unchanged along the width direction of the flow channel, and the cross-sectional dimensions of the relaxation area and the forming area remain unchanged along the width direction of the flow channel.

[0017] As a preferred embodiment, the deduction process of rheological theory is:

[0018] The temperature of the melt remains unchanged during the flow process, and the shear viscosity of the melt is described by the power law model, that is,

[0019]

[0020] Where η is the melt viscosity; K is the consistency coefficient; is the shear rate; n is the power law exponent;

[0021] The coordinate system is constructed with the boundary line between the manifold and the choke area as the x-axis and the symmetry plane in the flow channel width direction as the y-axis. The pressure gradient of the melt flowing along the manifold is

[0022]

[0023] Where p(x) is the pressure of the melt in the manifold at x; Q(x) is the volume flow rate of the melt in the manifold at x; r(x) is the radius of the manifold at x;

[0024] Assuming that the volume flow rate of the melt at the flow channel inlet is 2Q0, and the melt outlet flow rate is required to be uniform along the flow channel width, then

[0025]

[0026] Where W is half of the channel width;

[0027] Substituting formula (3) into formula (2), we have

[0028]

[0029] When the melt outlet flow rate per unit width along the flow channel width direction is uniform, at any position x along the flow channel width direction, the melt pressure drop along the extrusion direction in the flow resistance area is

[0030]

[0031] In the formula, Δp D (x) is the pressure drop of the melt in the choke zone at x; h(x) is the thickness of the choke zone at x; L is the length of the choke zone;

[0032] The cross-sectional dimensions of the relaxation zone and the forming zone remain unchanged along the flow channel width direction, and the melt outlet flow rate is required to be uniform along the flow channel width direction. Then the melt pressure at the outlet of the choke zone remains unchanged along the flow channel width direction; at any position x in the flow channel width direction, the pressure at the entrance of the choke zone is equal to the pressure of the melt in the manifold. Therefore, the pressure gradient of the melt in the manifold along the flow direction is equal to the pressure drop of the melt in the choke zone along the flow channel width direction.

[0033]

[0034] From equations (4) to (6), we get

[0035]

[0036] Formula (7) is the differential equation of the thickness of the flow blocking area in the coordinate system, and the boundary condition is

[0037] x=W,h(x)=h E (8)

[0038] In the formula, h E is the thickness of the choke area at the end of the flow channel.

[0039] As a preferred option, ANSYS Polyflow software is used to numerically simulate the melt flow in the runner, and the ratio of the volume flow rate per unit width at a certain position along the width direction of the runner at the melt outlet to the average volume flow rate per unit width is defined as the dimensionless flow rate of the melt outlet to reflect the uniformity of the melt outlet flow rate, thereby verifying the reliability of the differential equation of the resistance zone thickness under variable manifold radius.

[0040] The principle of the present invention is that after the melt enters the manifold, while flowing along the manifold to both sides of the flow channel, a part of the melt enters the flow blocking area and flows along the extrusion direction, and the volume flow rate of the melt in the manifold gradually decreases. Therefore, the cross-sectional size of the manifold is gradually reduced along both sides of the flow channel to increase the flow rate of the melt, thereby reducing the residence time of the melt in the manifold. On the other hand, the pressure of the melt gradually decreases when flowing along the manifold to both sides of the flow channel, that is, the pressure of the melt at the entrance of the flow blocking area gradually decreases. Therefore, the thickness of the flow blocking area is gradually increased along both sides of the flow channel, so that the pressure drop of the melt flowing through the flow blocking area is reduced along both sides of the flow channel, thereby reducing the extrusion pressure.

[0041] Compared with the prior art, the present invention has the following beneficial effects:

[0042] The T-type die with variable resistance zone thickness and the flow channel design method thereof of the present invention adopt a flow channel structure with variable manifold radius and variable resistance zone thickness. Based on the condition of uniform melt outlet flow rate, a differential equation for the variation of the resistance zone thickness along the flow channel width direction is derived, and the equation is numerically solved. The equation can be used for the flow channel design of the T-type die with variable manifold radius and variable resistance zone thickness.

[0043] The present invention adopts a numerical method to solve the differential equation of the thickness of the choke zone and establishes a flow channel geometry model. The melt outlet flow rate of the designed flow channel is verified by numerical simulation, indicating that the derived differential equation of the thickness of the choke zone is reliable.

[0044] Compared with the flow channel with constant manifold radius and constant choke thickness, the flow channel with variable manifold radius and variable choke thickness can significantly reduce the extrusion pressure and melt residence time under the condition that the melt outlet flow rate is uniform along the flow channel width.

[0045] Compared with a flow channel with equal manifold radius and equal choke thickness, the present invention can significantly reduce the melt residence time without significantly increasing the extrusion pressure by designing the manifold size and shape, and adjust the extrusion pressure to adapt to different molding requirements without increasing the melt residence time by designing the choke length and thickness. BRIEF DESCRIPTION OF THE DRAWINGS

[0046] Figure 1 Schematic diagram of T-type die flow channel with variable resistance zone thickness.

[0047] Figure 2 It is the design model of the T-type die flow channel with variable resistance flow area thickness in the embodiment.

[0048] Figure 3 It is the pressure cloud diagram of the melt in the flow channel of the embodiment.

[0049] Figure 4 Graph 1 shows the variation of dimensionless flow rate of melt outlet along the flow channel width direction in the embodiment.

[0050] Figure 5 Detailed description of the embodiment shows the influence of the variation of the radius of the manifold on the thickness of the choke zone.

[0051] Figure 6 Detailed description of the embodiment shows the influence of the thickness of the choke area at the end of the flow channel on the thickness of the choke area.

[0052] Figure 7 1 is an example of the effect of the length of the choke area on the thickness of the choke area.

[0053] As shown in the figure, 1 is the inlet area, 2 is the manifold, 3 is the flow blocking area, 4 is the relaxation area, and 5 is the forming area. DETAILED DESCRIPTION

[0054] The present invention will be further described in detail below in conjunction with examples, but the embodiments of the present invention are not limited thereto.

[0055] Example

[0056] The present embodiment provides a T-shaped mold flow channel design method with a variable resistance zone thickness, which is applicable to the flow channel design of a mold that is symmetrical along the width direction, and includes the following steps:

[0057] S1. Construct a physical model: Assumptions a. The melt is an incompressible fluid; b. The melt flow is a fully developed steady-state laminar flow, and the inertial force and volume force are ignored; c. The melt flows only along the axial direction of the manifold in the manifold, and only flows along the extrusion direction in the choke zone, relaxation zone and molding zone, and the melt flow in the manifold and the flow in the choke zone do not interfere with each other; d. The effect of stretching caused by the change of manifold radius on the melt flow in the manifold is ignored; e. The effect of the end walls on both sides of the flow channel width direction on the melt flow in the choke zone, relaxation zone and molding zone is ignored; f. The temperature of the melt remains unchanged during the flow process, and the shear viscosity of the melt is described by the power law model, that is,

[0058]

[0059] Where η is the melt viscosity; K is the consistency coefficient; is the shear rate; n is the power law exponent;

[0060] S2, build geometric model: Figure 1 As shown, the mold flow channel includes an inlet area, a manifold, a choke area, a relaxation area and a molding area which are arranged in sequence along the flow direction; the cross-sectional size of the manifold gradually decreases along both sides of the flow channel, and the cross-sectional size of the manifold is circular; the length of the choke area remains unchanged along the width direction of the flow channel, and the thickness of the choke area gradually increases along the width direction of the flow channel with the symmetry plane in the width direction of the flow channel as the center; the cross-sectional sizes of the relaxation area and the molding area remain unchanged along the width direction of the flow channel.

[0061] S3. Based on the analysis of melt flow in the flow channel, a structure with variable resistance zone thickness is adopted to reduce the extrusion pressure, and a structure with variable manifold radius is adopted to reduce the melt residence time. Under the condition that the melt outlet flow rate is uniform along the width of the flow channel, the rheological theory is used to derive the differential equation for the variation of the resistance zone thickness along the width of the flow channel. The resistance zone thickness can be obtained by numerically solving and fitting the equation.

[0062] The deduction process of rheological theory is:

[0063] Considering the symmetry of the flow channel in the width direction, half of the flow channel is taken for analysis. For the convenience of analysis, the influence of the inlet area is ignored, and the manifold cross section is selected as a circle. Other shapes can be converted through the shape factor. The coordinate system is constructed with the boundary line between the manifold and the flow resistance area as the x-axis and the symmetry plane in the width direction of the flow channel as the y-axis, as shown in Figure 2 shown.

[0064] The pressure gradient of the melt as it flows along the manifold is

[0065]

[0066] Where p(x) is the pressure of the melt in the manifold at x; Q(x) is the volume flow rate of the melt in the manifold at x; r(x) is the radius of the manifold at x;

[0067] Assuming that the volume flow rate of the melt at the flow channel inlet is 2Q0, and the melt outlet flow rate is required to be uniform along the flow channel width, then

[0068]

[0069] Where W is half of the channel width;

[0070] Substituting formula (3) into formula (2), we have

[0071]

[0072] When the melt outlet flow rate per unit width along the flow channel width direction is uniform, at any position x along the flow channel width direction, the melt pressure drop along the extrusion direction in the flow resistance area is

[0073]

[0074] In the formula, Δp D (x) is the pressure drop of the melt in the choke zone at x; h(x) is the thickness of the choke zone at x; L is the length of the choke zone;

[0075] The cross-sectional dimensions of the relaxation zone and the forming zone remain unchanged along the flow channel width direction, and the melt outlet flow rate is required to be uniform along the flow channel width direction. Then the melt pressure at the outlet of the choke zone remains unchanged along the flow channel width direction; at any position x in the flow channel width direction, the pressure at the entrance of the choke zone is equal to the pressure of the melt in the manifold. Therefore, the pressure gradient of the melt in the manifold along the flow direction is equal to the pressure drop of the melt in the choke zone along the flow channel width direction.

[0076]

[0077] From equations (4) to (6), we get

[0078]

[0079] Formula (7) is the differential equation of the thickness of the flow blocking area in the coordinate system, and the boundary condition is

[0080] x=W,h(x)=h E (8)

[0081] In the formula, h E is the thickness of the choke area at the end of the flow channel.

[0082] It can be seen from formula (7) that the thickness of the choke zone is related to the change of the manifold radius along the flow channel width, the length of the choke zone, the flow channel width and the power law index of the melt, and has nothing to do with the consistency and output of the melt. The flow channel width is determined by the product specifications, and the power law index of the melt is the material parameter of the melt. After determining the manifold radius size and its change law, the thickness of the choke zone can be designed according to formula (7).

[0083] It can be seen from formula (7) that h(x) is a monotonic function of x, and the thickness of the choke zone increases monotonically along the width of the flow channel. When the radius of the manifold decreases along the width of the flow channel and the end size is not 0, it is difficult to obtain an analytical expression for formula (7). The numerical method can be used to solve and fit the thickness of the choke zone along the width of the flow channel.

[0084] The pressure drop of the melt flowing from the manifold inlet through the manifold and the choke zone along any path is equal, which can be calculated by the pressure drop of the melt in the choke zone at the symmetry plane of the flow channel, that is,

[0085]

[0086] In the formula, h C is the thickness of the choke area at the symmetry plane of the flow channel.

[0087] The residence time t(x) of the melt flowing from the inlet to any position x in the manifold is

[0088]

[0089] When the radius of the manifold decreases along the width of the flow channel and the end size is not 0, equation (10) needs to be solved by numerical method, and its boundary conditions are:

[0090] x=0,t(x)=0 (11)

[0091] S4. Verification: ANSYS Polyflow software is used to perform numerical simulation on the melt flow in the flow channel. The ratio of the volume flow rate per unit width at a certain position along the width direction of the flow channel at the melt outlet to the average volume flow rate per unit width is defined as the dimensionless flow rate of the melt outlet to reflect the uniformity of the melt outlet flow rate, thereby verifying the reliability of the differential equation of the resistance zone thickness under variable manifold radius.

[0092] Taking ABS sheet extrusion as an example, the power law model is used to describe the rheological properties of ABS at 220°C, K = 7496.8 Pa·s -0.514, n = 0.486. The sheet width is 2500mm, the length and thickness of the relaxation zone are 40mm and 4mm respectively, the length and thickness of the forming zone are 20mm and 1.5mm respectively, the length of the blocking zone is 50mm, the thickness at the end of the flow channel is 2mm, the manifold radius at the symmetry plane and the end of the flow channel is 20mm and 8mm respectively, and it decreases exponentially along the width of the flow channel, that is,

[0093] R=R E +(R C -R E )(1-x / W) m (12)

[0094] In the formula, R C is the radius of the manifold at the symmetry plane of the flow channel; R E is the radius of the manifold at the end of the flow channel; m is an exponent.

[0095] Taking m = 0.7, the fourth-order explicit Runge-Kutta method is used to solve the differential equation (7) of the thickness of the choke zone. The spline curve is used to fit the thickness of the choke zone along the width direction of the flow channel, and the flow channel is modeled using 3D design software. Considering the symmetry of the flow channel in the width direction, half of its width is taken for calculation. In order to improve the calculation accuracy, the flow channel is divided by hexahedral units, and smaller grids are used at the boundaries of the flow channel walls and the locations where the size changes suddenly. When solving the flow field, quadratic interpolation is used for velocity and linear interpolation is used for pressure.

[0096] Figure 3 and 4The melt pressure cloud diagram and the change of the dimensionless flow rate of the melt outlet along the flow channel width when the extrusion speed is 30 mm / s obtained by numerical simulation. It can be seen that when the melt leaves the choke zone, the pressure contour of the melt is parallel to the flow channel outlet. The pressure drop of the melt in the manifold obtained by simulation calculation is consistent with the pressure drop obtained by solving the pressure gradient differential equation (4) in the manifold using the fourth-order explicit Runge-Kutta method. The pressure drop of the melt in the choke zone at the end of the flow channel is consistent with the pressure drop calculated using formula (5). Along the width of the flow channel, the dimensionless flow rate of the melt outlet gradually increases. Within a range of about 10 mm at the end of the flow channel, the dimensionless flow rate of the melt outlet is less than 1. The dimensionless flow rate of the melt outlet at the symmetric plane of the flow channel is 0.989, and the maximum dimensionless flow rate of the melt outlet is 1.018. This is due to the assumption in the theoretical derivation that the melt flows only along the extrusion direction in the choke zone. Although the melt pressure at the outlet of the choke zone is equal along the flow channel width, the pressure gradient of the melt at the entrance of the choke zone along the flow channel width is not zero, causing the melt outlet flow rate to increase along the flow channel width. However, the pressure gradient of the melt in the choke zone along the flow channel width is much smaller than the pressure gradient along the extrusion direction, so the change of the dimensionless flow rate of the melt outlet along the flow channel width is very small. On the other hand, the theoretical derivation ignores the influence of the side walls at the ends of the flow channel width on the melt flow, while the simulation calculation takes into account that there is a no-slip boundary between the melt and the side wall of the flow channel in actual production, and there is a flow boundary layer, and the dimensionless flow rate of the melt outlet in the boundary layer is less than 1. Although there are certain differences in the melt outlet flow rate along the flow channel width, the difference is very small, indicating that the assumptions and theoretical derivations of this article are reasonable, and the design method of the present invention can meet engineering requirements.

[0097] S5. Discussion on the influence of flow channel structural parameters

[0098] The design of the runner of the T-type mold requires not only that the melt outlet flow rate should be uniform along the width of the runner, but also that the pressure drop of the melt flowing through the entire runner should be moderate and the residence time of the melt in the runner should be as short as possible. The pressure drop of the melt flowing from the manifold inlet along any path through the manifold and the choke area has an important influence on the design of the runner of the T-type mold. The pressure drop is jointly determined by the manifold size and its variation along the width of the runner, the length of the choke area and its thickness at the end of the runner. When the manifold size and its variation, the length of the choke area and its thickness at the end of the runner are determined, the thickness of the choke area along the width of the runner can be obtained by solving equation (7) using the fourth-order explicit Runge-Kutta method, and then the pressure drop of the melt flowing through the manifold and the choke area can be calculated by equation (9). On the other hand, near the two sides of the runner width, the residence time of the melt in the manifold is much longer than the residence time of the melt after leaving the manifold. Therefore, the residence time of the melt mainly depends on the manifold size and its variation along the width of the runner. For the convenience of comparison, the melt residence time at a distance of 0.98W from the manifold inlet in the manifold is selected for comparison.

[0099] According to the sheet specifications, extrusion speed and material mentioned above, while keeping the sizes of the relaxation zone and the molding zone unchanged, the flow channel design is carried out with equal manifold radius and equal choke zone thickness. In addition, the flow channel design is carried out with different manifold radius change rules, different choke zone thickness at the end of the flow channel and choke zone length. The pressure drop and melt residence time when the melt flows through the manifold and choke zone are compared.

[0100] S5.1. Flow channels with equal manifold radius and equal choke thickness

[0101] For flow channels with equal manifold radius and equal resistance zone thickness, the melt outlet flow rate uniformity index UI is usually used as the verification basis during design. The uniformity index is defined as the ratio of the melt volume flow rate per unit width at the end of the flow channel width and the symmetry plane. The relationship between UI and flow channel size and material parameters is:

[0102]

[0103] Where n is the power law index; H is the thickness of the choke zone; L is the length of the choke zone; R is the radius of the manifold; and W is half of the flow channel width.

[0104] When the length of the choke zone is 50 mm and the thickness is 0.7 mm and 0.9 mm respectively, in order to make UI ≥ 0.95, the manifold radius is not less than 33 mm and 40 mm, the pressure drop of the melt in the choke zone is 26.65 MPa and 16.24 MPa respectively. When the manifold radius is 33 mm and 40 mm, the pressure drop of the melt in the manifold is 0.60 MPa and 0.37 MPa respectively, and the melt residence time is 297.4 s and 437.0 s respectively.

[0105] S5.2 Effect of manifold shape

[0106] The length of the choke zone is 50 mm, the thickness at the end of the flow channel is 2 mm, the radius of the manifold at the symmetry plane and the end of the flow channel is 20 mm and 8 mm respectively, and the radius of the manifold decreases along the width of the flow channel according to formula (12). When m is 0.4, 0.7 and 1, the fourth-order explicit Runge-Kutta method is used to solve formula (7) to obtain the variation of the thickness of the choke zone along the width of the flow channel as follows: Figure 5 As shown in Figure 1, as m increases, the thickness of the obstruction area at any position in the flow channel width direction decreases, and the pressure drop of the melt flowing through the manifold and the obstruction area increases. When m is 0.4, 0.7 and 1, the pressure drop is 6.45MPa, 7.42MPa and 8.42MPa, respectively. The melt residence time obtained by solving equation (10) using the fourth-order explicit Runge-Kutta method is 56.0s, 42.5s and 35.6s, respectively.

[0107] Compared with the flow channel with equal manifold radius and equal choke thickness, the design method in this paper significantly reduces the pressure drop of the melt flowing through the manifold and choke as well as the melt residence time. In addition, even if the dimensions of the manifold at the symmetry plane and the end of the flow channel are the same, the extrusion pressure and melt residence time can be adjusted by changing the variation law of the manifold along the flow channel width, which provides greater flexibility for flow channel design.

[0108] S5.3. Influence of the size of the choke zone

[0109] The radius of the manifold at the symmetry plane and the end of the flow channel is 20 mm and 8 mm respectively. It changes in the flow channel width direction according to formula (12) with m=0.7. The influence of the length of the choke zone and the thickness of the choke zone at the end of the flow channel on the pressure drop of the melt flowing through the manifold and the choke zone is analyzed.

[0110] The length of the choke zone is 50 mm, and the thickness of the choke zone at the end of the flow channel is h. E When the thickness is 1.6 mm, 2.0 mm and 2.4 mm, the fourth-order explicit Runge-Kutta method is used to solve equation (7) to obtain the variation of the thickness of the obstruction zone along the width direction of the flow channel as follows: Figure 6 As shown. E As h increases, the thickness of the flow resistance zone at any position in the flow channel width direction increases, and the pressure drop of the melt flowing through the manifold and the flow resistance zone decreases. E When the diameter is 1.6mm, 2.0mm and 2.4mm, the pressure drop is 9.28MPa, 7.42MPa and 6.41MPa respectively.

[0111] When the thickness of the choke zone at the end of the flow channel is 2 mm and the length of the choke zone L is 40 mm, 50 mm and 60 mm, the fourth-order explicit Runge-Kutta method is used to solve equation (7) to obtain the variation of the choke zone thickness along the flow channel width direction as follows: Figure 7 As shown in the figure, as L increases, the thickness of the choke zone at any position in the flow channel width direction increases. As the length of the choke zone increases, the pressure drop of the melt flowing through the manifold and the choke zone increases. When L is 40mm, 50mm and 60mm, the pressure drop is 6.75MPa, 7.42MPa and 8.10MPa respectively.

[0112] By adopting the design method in this paper, when the size and shape of the manifold remain unchanged, that is, the melt residence time remains unchanged, the extrusion pressure can be reduced by reducing the length of the choke zone or increasing the thickness of the choke zone at the end of the flow channel, that is, the flow channel can be designed according to the molding requirements.

[0113] A T-shaped die with a variable resistance zone thickness, wherein the flow channel of the T-shaped die comprises an inlet zone, a manifold, a resistance zone, a relaxation zone and a molding zone which are sequentially arranged along the flow direction of the melt, the cross section of the manifold is circular with the symmetry plane in the width direction of the flow channel as the center, the cross section size of the manifold gradually decreases along both sides of the flow channel, the length of the resistance zone remains unchanged along the width direction of the flow channel, the thickness gradually increases along the width direction of the flow channel, and the cross section sizes of the relaxation zone and the molding zone remain unchanged along the width direction of the flow channel.

[0114] The mold is based on the specifications, extrusion speed and material of the required product, keeps the length of the choke zone unchanged along the width of the flow channel, and the cross-sectional dimensions of the relaxation zone and the molding zone unchanged along the width of the flow channel, and uses the above mold design method to determine the thickness of the choke zone to manufacture the flow channel.

[0115] In addition to the methods mentioned in the above embodiments, for manifolds of other shapes, the melt pressure drop in the manifold can be converted by the shape factor, and then the thickness of the flow blocking area along the flow channel width direction corresponding to the mold flow channel of the manifold with different cross-sectional shapes can be obtained. These transformation methods are all within the protection scope of the present invention.

[0116] In addition to the methods mentioned in the above embodiments, the radius of the manifold can be changed in other ways along the width direction of the flow channel, thereby obtaining the thickness of the flow blocking area along the width direction of the flow channel corresponding to the mold flow channel when the manifold diameter changes along the width direction of the flow channel. These changes are all within the protection scope of the present invention.

[0117] In addition to the above-mentioned embodiments, after the thickness of the blocking area is calculated by the above-mentioned design method, chamfering or rounding transitions between different areas in the flow channel do not affect the change of the blocking area along the width direction of the flow channel. These changes are all within the scope of protection of the present invention.

[0118] In addition to the methods mentioned in the above embodiments, other numerical methods can be used to solve the differential equation of the thickness of the blocking area and other reasonable methods can be used for fitting. These transformation methods are all within the protection scope of the present invention.

[0119] The above embodiments are only preferred embodiments of the present invention and are not intended to limit the scope of implementation of the present invention; that is, all equivalent changes and modifications made according to the contents of the present invention are covered by the scope of protection required by the claims of the present invention.

Claims

1. A T-type die flow channel design method with variable resistance zone thickness, characterized in that: The flow channel of the T-shaped die includes an inlet area, a manifold, a flow blocking area, a relaxation area and a molding area which are sequentially arranged along the flow direction of the melt. With the symmetry plane in the width direction of the flow channel as the center, the cross-sectional size of the manifold gradually decreases along both sides of the flow channel, and the thickness of the flow blocking area gradually increases along the width direction of the flow channel; The flow channel design method comprises the following steps: Constructing the physical model: Assumptions a. The melt is an incompressible fluid; b. The melt flow is a fully developed steady-state laminar flow, and the inertial force and volume force are ignored; c. The melt flows only along the axial direction of the manifold in the manifold, and only flows along the extrusion direction in the choke zone, relaxation zone and molding zone, and the melt flow in the manifold and the flow in the choke zone do not interfere with each other; d. The effect of stretching caused by the change of manifold radius on the melt flow in the manifold is ignored; e. The effect of the end walls on both sides of the flow channel width direction on the melt flow in the choke zone, relaxation zone and molding zone is ignored; Constructing geometric model: the mold flow channel includes the inlet area, manifold, flow blocking area, relaxation area and forming area arranged in sequence along the flow direction; Based on the analysis of melt flow in the flow channel, a structure with variable resistance zone thickness is adopted to reduce extrusion pressure, and a structure with variable manifold radius is adopted to reduce melt residence time. Under the condition that the melt outlet flow rate is uniform along the width of the flow channel, the differential equation of the variation of resistance zone thickness along the width of the flow channel is derived using rheological theory. The resistance zone thickness can be obtained by numerically solving and fitting the equation. The deduction process of rheological theory is: The temperature of the melt remains unchanged during the flow process, and the shear viscosity of the melt is described by the power law model, that is, Where η is the melt viscosity; K is the consistency coefficient; is the shear rate; n is the power law exponent; The coordinate system is constructed with the boundary line between the manifold and the choke area as the x-axis and the symmetry plane in the flow channel width direction as the y-axis. The pressure gradient of the melt flowing along the manifold is Where p(x) is the pressure of the melt in the manifold at x; Q(x) is the volume flow rate of the melt in the manifold at x; r(x) is the radius of the manifold at x; Assuming that the volume flow rate of the melt at the flow channel inlet is 2Q0, and the melt outlet flow rate is required to be uniform along the flow channel width, then Where W is half of the channel width; Substituting formula (3) into formula (2), we have When the melt outlet flow rate per unit width along the flow channel width direction is uniform, at any position x along the flow channel width direction, the melt pressure drop along the extrusion direction in the flow resistance area is In the formula, Δp D (x) is the pressure drop of the melt in the choke zone at x; h(x) is the thickness of the choke zone at x; L is the length of the choke zone; The cross-sectional dimensions of the relaxation zone and the forming zone remain unchanged along the flow channel width direction, and the melt outlet flow rate is required to be uniform along the flow channel width direction. Then the melt pressure at the outlet of the choke zone remains unchanged along the flow channel width direction; at any position x in the flow channel width direction, the pressure at the entrance of the choke zone is equal to the pressure of the melt in the manifold. Therefore, the pressure gradient of the melt in the manifold along the flow direction is equal to the pressure drop of the melt in the choke zone along the flow channel width direction. From equations (4) to (6), we get Formula (7) is the differential equation of the thickness of the flow blocking area in the coordinate system, and the boundary condition is x=W,h(x)=h E (8) In the formula, h E is the thickness of the choke area at the end of the flow channel.

2. According to claim 1, a T-shaped die flow channel design method with variable resistance zone thickness is characterized in that: The length of the flow blocking area is constant along the width direction of the flow channel, and the cross-sectional dimensions of the relaxation area and the forming area are constant along the width direction of the flow channel.

3. According to the method for designing a T-shaped die flow channel with a variable resistance zone thickness as claimed in claim 1, it is characterized in that: The cross section of the manifold is circular.

4. According to claim 1, a T-shaped die flow channel design method with variable resistance zone thickness is characterized in that: The cross-sectional dimensions of the manifold gradually decrease along both sides of the flow channel.

5. According to claim 1, a T-shaped die flow channel design method with variable resistance zone thickness is characterized in that: The cross section of the manifold is circular.

6. According to claim 1, a T-shaped die flow channel design method with variable resistance zone thickness is characterized in that: With the symmetry plane in the flow channel width direction as the center, the thickness of the flow blocking area gradually increases along the flow channel width direction.

7. According to claim 1, a T-shaped die flow channel design method with variable resistance zone thickness is characterized in that: The length of the flow blocking area remains unchanged along the width direction of the flow channel, and the cross-sectional dimensions of the relaxation area and the forming area remain unchanged along the width direction of the flow channel.

8. According to claim 1, a T-shaped die flow channel design method with variable resistance zone thickness is characterized in that: ANSYS Polyflow software was used to numerically simulate the melt flow in the flow channel, and the reliability of the differential equation of the resistance zone thickness under variable manifold radius was verified by calculating the melt outlet flow rate.

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