# Effects of Operating Conditions during Three-Layered Polymeric Balloon Catheter Extrusion

Authors: G. B. Jin; K. S. Wang; W. J. Hua; Y. F. Jin; P. Qin; J. Y. Bian; G. H. Dai; Q. Q. Zhang
Journal: Advances in Materials Science and Engineering
 (2022)

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## Abstract

Multilayered balloon catheters are attracting more attention in recent years. Multilayered balloon catheters have some unique properties such as meso-/microscale dimensions, small wall thickness, and strict requirements of dimensional accuracy. In this study, three-layered polymeric balloon catheters composed of polyamide, ethylene vinyl acetate, and polypropylene (PA/EVA/PP) are successfully fabricated using a three-layered coextrusion die and the polymer melt flowing behavior through the extrusion die are analyzed. Both the diameter and wall thicknesses have been selected as the targeted structural parameters to investigate the effects of pulling speed, air injection volume rate, and screw speed on the quality of the extrudates. The optimal combination of these three operating conditions has been achieved. It is found that pulling speed can significantly affect the diameters of the extruded balloon catheters. The variation interval values of outer profile diameter and inner cavity diameter with the extrusion experiments are 20.95% and 13.04%, respectively. The screw speed of each layer has a great influence on the wall thickness of each layer. Through the influence of outer screw speed, middle screw speed and inner screw speed, the variation interval values for outer layer wall thickness, middle layer wall thickness, and inner layer wall thickness are 31.06%, 35.69%, and 31.35%, respectively. The air injection volume rate has negligible effect.


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## Body

## 1. Introduction

With the development of precision forming technology, various polymeric products with different functions have been investigated and widely used in the interventional medical field [1–3]. One of the most important components of interventional medical devices are balloon catheters, which are gaining attention in recent years [4–6]. Multilayered balloon catheters possess some unique properties such as excellent pressure resistance, good balloon compliance, and reduced interfacial friction coefficient, to name a few [7–9]. However, the effects of operating conditions during multilayered balloon catheter fabrication still need further investigation.For the processing of multilayered films, some researchers have already made some achievements. The coextrusion blown films are investigated and found that the polymer material properties, flow rate ratio, extrusion die dimensions, and operating conditions are the four main factors to affect the interfacial stability during extrusion [10]. The flowing process during polymer coextrusion is analyzed and investigated the effects of viscosity on the interfacial location using experiments [11]. The effects of surface roughness on the optical properties of multilayered polymeric films are discussed. It is discovered that spectral technology is an effective method to study the surface properties [12]. The interfacial instability between polyethylene (PE) and polystyrene (PS) during the coextrusion process is investigated by both experiments and theoretical studies. It is revealed that the default of extrudates at the exit of the die can be predicted using convection linear stability analysis of polymer melts during coextrusion [13]. The multilayered micro-coextrusion process has been found that the loss of die temperature increases with the increasing number of extruded layers due to the shear friction between adjacent layers [14]. Polypropylene and ethylene vinyl alcohol (PP + EVOH)/PP are coextruded to form microlayers. The results show that the dispersed form of EVOH in PP matrix leads to a faster decrease of the nitrogen permeability coefficient, but there is a 27-fold increase in the elongation at the break [15]. The multilayered nanostructure and thermal stability of polymethyl methacrylate and polystyrene (PMMA/PS) amorphous films are studied. At the temperature of 140°C or above, the interface driving layer fracture and the multilayered structures eventually disappear [16]. The effects of the wall thickness of multilayered polymeric films on the glass transition temperature (Tg) and dimension of collaborative rearrangement area (CRR) are researched by temperature modulated differential scanning calorimetry (TMDSC). When the wall thickness decreases to 125 nm or lower, the molecular mobility in each polymer changes in a completely different way. During glass transition, the synergistic volume of polycarbonate (PC) decreases significantly, while that of PMMA changes slightly [17]. The relationship between uniaxial stretching of polyamide nylon 6/polyethylene (PA6/PE) multilayered films and uniaxial stretching of PA6 single-layered films has been considered [18]. The effects of interfacial polarization and layer thickness on the electrical insulation properties of multilayered polysulfone (PSF)/polyvinylidene dioxide (PVDF) films have been reported. It is discovered that the films with thinner PVDF and PSF layers have lower breakdown strength, shorter lifetime, and higher conductivity [19]. The structure and properties of multilayered films with the layer thickness greater than 150 nm have been studied. The crack propagation resistance increases with the increase of layer number by measuring the notched specimen during the tensile test [20]. The coextrusion operating conditions and the membrane layout of PA/PE multilayered nanocomposite films for food packaging are optimized [21]. The interface of low density polyethylene and high density polyethylene (LDPE/HDPE/LDPE) three-layered coextrusion blown films is simulated by Polyflow. In the coextrusion channel, the pressure difference between two adjacent layers can cause interface deformation. When each layer encounters the coextrusion channel, the position of the interface changes due to the rearrangement of the flow path [22]. In order to optimize the wall thickness distribution of medical balloon, kyphoplasty balloon was chosen as the research object, the uniformity of wall thickness distribution was taken as the evaluation index, and the inﬂuence of stretch blow molding process on the uniformity of kyphoplasty balloon was investigated [23]. Under the same molding process conditions, different outer materials with large strength difference were selected to study the change law of balloon catheter size. Therefore, the influence of molding process on balloon catheters (PA-EVA-PP and TPE-EVA-PP) with different outer materials was studied [24]. The aforementioned studies are all related researches on multilayered films, but there is little research on the processing of multilayered balloon catheters. Multilayered balloon catheters are widely used in the interventional medicine field because of the better functionality than single-layered balloon catheters. Thus, it is of great significance to investigate the operating conditions during the fabrication of multilayered balloon catheters to achieve the desired dimensional accuracy.Multilayered balloon catheters usually have small diameters, thin wall thickness for each layer, and high dimensional accuracy requirements [25–28]. In this study, the polymer flowing behavior is analyzed and a three-layered coextrusion die is created to fabricate polyamide, ethylene vinyl acetate, and polypropylene (PA/EVA/PP) three-layered balloon catheters. Using the change rates of the diameter and wall thicknesses as the main structural parameters, the effects of pulling speed, air injection volume rate, and screw speed on the quality of the produced balloon catheters are investigated by the extrusion experiments. The results can guide the design of multilayered extrusion dies and the selection of operating conditions during the multilayered balloon catheter coextrusion process.


## 2. Experiment

### 2.1. Cross Section Design of Three-Layered Balloon Catheter and Coextrusion Die

The cross section of three-layered balloon catheter consists of a tube cavity and three layers of different material wall thickness as shown in Figure1. The three layers including outer layer, middle layer, and inner layer are made from polyamide 12 (PA12), EVA, and PP, respectively. The corresponding coextrusion die is created. The exit of the die is illustrated in Figure 2. The diameter of die is D = 3.6 mm. The outside diameter of mandrel is d = 2.0 mm.Figure 1
Cross section of three-layered balloon catheter.Figure 2
Graphics of micro-coextrusion die exit.


### 2.2. Materials, Equipment, and Methods

PA 12 (L25, EMS Grilamid, Switzerland), EVA (QW2023, Sumitomo Chemical Co., Ltd, Japan), and PP (MR-12-01-25, Hyosung Group, Korea) were selected as the build materials to fabricate the multilayered balloon catheters. The extrusion equipment was the high precision microextrusion system (Hightrichja Precision Extrusion Machinery Co., Ltd). The high precision microextrusion system (Figure3) was composed of three single-screw extruders (HRJSJ-Φ25, diameter of 25 mm, and length-to-diameter ratio of 28 : 1), a microextrusion die, an air flow rate control system, a vacuum water tank, caliper and puller.Figure 3
Microextrusion system.Based on the material properties of PA, EVA, and PP and the plasticizing performance of the extruders, the five heating zones were divided to include three heating zones from the hopper to the exit of the extruder (Zones I, II, and III), flange zone, and extrusion die zone. The temperatures range from 190°C to 220°C as shown in Table1.Table 1
Temperature control during extrusion experiments (°C).
TemperatureZone IZone IIZone IIIFlangeExtrusion dieOuter layer extruder and die220220220220220Middle layer extruder and die195195195195195Inner layer extruder and die190190190190190During the experiments, PA, EVA, and PP were fed simultaneously into the corresponding extruders. Since the temperatures inside the extruders were higher than the glass transition temperatures of these polymers, the polymer melts were formed. The polymers were coextruded through the designed three-layered extrusion die to generate three-layered tubes. The semimelt status tubes were pulled by the puller through the vacuum water tank for cooling down at the exit of the extrusion die. The three-layered balloon catheters were formed, cut into segments by the cutter, and then blown dry. Five segments under different operating conditions were collected. The diameters and wall thicknesses were measured and the averages were used as the experimental results.


### 2.3. Measurement and Characterization of Catheter Cross Section

The cross-sectional dimensions of the cut balloon catheters were measured using the digital imaging tool microscope (VTM-3020F, Shuzhou Ouka Precision Optical Instrument Co., Ltd.) with an accuracy of ±0.001 mm.The dimensions of the final three-layered balloon catheters were highly dependent on the geometry of the tubes at the exit of the die (before entering the vacuum water tank). The velocity arrangement, die swelling, atmosphere pressure, injected air pressure, drawing force, and gravity can affect the profile and wall thickness of each layer of the tube. The dimensions of the extruded products were characterized based on five structural parameters including the outer diameter of the catheter (Dw), inner diameter of the catheter (dn), wall thicknesses of the outer layer (Bw), middle layer (Bz), and inner layer (Bn) (Figure 1).The change rate of catheter section size relative to die size was characterized by the catheter diameter change ratey and the catheter wall thickness change rate X respectively, which were defined as follows:The change rate of the outer contour diameter of the catheter:(1)Y1=DwD+d.The change rate of the cavity diameter of the catheter:(2)Y2=dnD+d.The change rate of outer wall thickness of the catheter:(3)X1=BwD/2−d/2.The change rate of middle wall thickness of the catheter:(4)X2=BzD/2−d/2.The change rate of inner wall thickness of the catheter:(5)X3=BnD/2−d/2.


### 2.4. Analysis of Three-Layered Polymer Melt Flowing Behavior during Coextrusion

As shown in Figure1, the coextrusion of polymer melts through the extrusion die is complicated. The liquid phase is composed of three different polymer melts coaxially flowing through the die as shown in Figure 4. To simplify the flowing analysis, two main assumptions are created as follows:(1)
The polymer melts are non-Newtonian fluids with the constitutive equation of power law;(2)
The flowing is an isothermal process, and both the gravity and inertial force have no effect on the flowing process.Figure 4
Schematic of the geometry of the three-layered polymer melt coextrusion flow.When extruding along thez-direction, the density differences between adjacent polymer melts have no effect on the flowing process and the interfaces between adjacent melts are pronounced.Figure4 illustrates the geometry of the three-layered polymer melts flowing in the coextrusion die. From the outer layer to the inner layer, the polymer melts are PA (Phase A), EVA (Phase B), and PP (Phase C), respectively. In addition, the radius of the outer layer is defined as R0, the radius of the interface between Phases A and B is defined as βR0, the radius of the interface between Phases B and C is defined as κR0, and the mandrel of the coextrusion die has the radius of δR0. Herein, the outlet direction of the coextrusion die is set as z-axis and λ is the dimensionless radius ratio with the maximum flowing velocity. When the radius at any location r=λR0, the polymer melts have the maximum flowing velocity and the shear stress τrz equals to 0. Since the maximum velocity must occur in one of the three layers, the corresponding λ can be estimated as: β ≤ λ ≤ 1 for Phase A, κ ≤ λ ≤ β for Phase B, and δ ≤ λ ≤ κ for Phase C.The momentum conservation equation alongz-direction can be simplified as(6)−∂p∂z+1r∂∂rrτrz=0R0≤r≤R1,where p is the pressure. Thus, the momentum conservation equations for different phases can be expressed as follows:(7)−∂pA∂z+1r∂∂rrτrzA=0βR0≤r≤βR0,(8)−∂pB∂z+1r∂∂rrτrzB=0kR0≤r≤βR0,(9)−∂pC∂z+1r∂∂rrτrzC=0δR0≤r≤kR0.Since the pressure drop for each layer is consistent, the following relation can be obtained:(10)−∂pA∂z=−∂pB∂z=−∂pC∂z=G.By substituting (10) into, equations (7) and (8), (9), the corresponding shear stress for each phase can be calculated as(11)τrzA=−Gr2+crβR0≤r≤R0,(12)τrzB=−Gr2+crkR0≤r≤βR0,(13)τrzC=−Gr2+crδR0≤r≤kR0,where c is the constant. When r=λR0, the maximum flowing velocity happens and the shear stress equals to 0 (τrz=0). If the ratio between the radius at any location and the radius of the outer layer is defined as ρ=r/R0, equations (11) and (12), (13) can be rewritten as(14)τrzA=GR02λ2−ρ2ρβ≤ρ≤1,(15)τrzB=GR02λ2−ρ2ρk≤ρ≤β,(16)τrzC=GR02λ2−ρ2ρδ≤ρ≤k.The boundary conditions are listed as follows:(17)whenr=R0, i.e.,ρ=1,vzA=0,(18)whenr=βR0,i.e.,ρ=β,vzA=vzB,(19)whenr=κR0,i.e.,ρ=κ,vzB=vzC,(20)whenr=δR0, i.e.,ρ=δ,vzC=0.Since the power law is used as the constitutive equation for PA, EVA, and PP, the shear stresses can be obtained as(21)τrzA=KAdvzAdrnA=KAR0nAdvzAdρnA,(22)τrzB=KBdvzBdrnB=KBR0nBdvzBdρnB,(23)τrzC=KCdvzCdrnC=KCR0nCdvzCdρnC,where Ki, ni, and vzi are the consistency index, power law index, and velocity of the PA (i = A), EVA (i = B), and PP (i = C) polymer melts, respectively.(1)
When the maximum velocity of the polymer melt occurs in the outer layer (Phase A), the corresponding shear stress is 0 (τrzA=0). Based on the (16) and (23) and boundary conditions (20), the velocity distribution of polymer melt PP can be obtained as(24)vzC1ρ=R0GR02KC1/nC∫δκλ2−ρ2ρ1/nCdρ.Similarly, the velocity distributions of polymer melts EVA and PA can be calculated as (25), (26), and (27), respectively:(25)vzB1ρ=R0GR02KB1/nB∫κβλ2−ρ2ρ1/nBdρ+vzC1ρ.(26)vzA1ρ=R0GR02KA1/nA∫λ1ρ2−λ2ρ1/nAdρλ≤ρ≤1,(27)vzA1ρ=R0GR02KA1/nA∫βλλ2−ρ2ρ1/nAdρ+vzB1ρβ≤ρ≤λ.(2)
When the maximum velocity of the polymer melt occurs in the middle layer (Phase B), the corresponding shear stress is 0 (τrzB=0). Based on the (14) and (21) and boundary conditions (17), the velocity distribution of polymer melt PA can be obtained as(28)vzA2ρ=R0GR02KA1/nA∫β1ρ2−λ2ρ1/nAdρ.Using the similar method, the velocity distributions of polymer melts PP and EVA can be calculated as (29), (30), and (31), respectively:(29)vzC2ρ=R0GR02KC1/nC∫δκλ2−ρ2ρ1/nCdρ,(30)vzB2ρ=R0GR02KB1/nB∫λβρ2−λ2ρ1/nBdρ+vzA2ρλ≤ρ≤β,(31)vzB2ρ=R0GR02KB1/nB∫κλλ2−ρ2ρ1/nBdρ+vzC2ρk≤ρ≤λ.(3)
When the maximum velocity of the polymer melt occurs in the inner layer (Phase C), the corresponding shear stress is 0 (τrzC=0). Based on the (14) and (21) and boundary conditions (17), the velocity distribution of polymer melt PA can be obtained as(32)vzA3ρ=R0GR02KA1/nA∫β1ρ2−λ2ρ1/nAdρ.The velocity distributions of polymer melts EVA and PP can be calculated as (33) and (34) and (35), respectively:(33)vzB3ρ=R0GR02KB1/nB∫κβρ2−λ2ρ1/nBdρ+vzA3ρ,(34)vzC3ρ=R0GR02KC1/nC∫λκρ2−λ2ρ1/nCdρ+vzB3ρλ≤ρ≤k,(35)vzC3ρ=R0GR02KC1/nC∫δλλ2−ρ2ρ1/nCdρδ≤ρ≤λ.Based on the aforementioned equations, the velocity distribution for each phase depends on some parameters including the pressure gradient (G), R0, δR0, K, n, and location of the interfaces (βR0 and κR0). All these parameters can be controlled by the rotation speed (N) of the corresponding extruder. Thus, it is possible to control the wall thickness of one layer by adjusting the rotation speed of the extruder, which makes the rotation speed be one of the most important operating conditions during catheter fabrication.When extruded out of the coaxial extrusion die, other operating conditions such as pulling speed and air injection volume rate can influence the cross-sectional dimensions of the extrudates. For example, with the increase of the pulling speed, the overall diameter of the catheter decreases. In addition, during extrusion, high pressure air is injected into the lumen of the catheter to balance the inner pressure with the ambient pressure, resulting in the stable shape of the catheter. Thus, the change of the air injection volume rate also affects the dimensions of the three-layered balloon catheters.


## 2.1. Cross Section Design of Three-Layered Balloon Catheter and Coextrusion Die

The cross section of three-layered balloon catheter consists of a tube cavity and three layers of different material wall thickness as shown in Figure1. The three layers including outer layer, middle layer, and inner layer are made from polyamide 12 (PA12), EVA, and PP, respectively. The corresponding coextrusion die is created. The exit of the die is illustrated in Figure 2. The diameter of die is D = 3.6 mm. The outside diameter of mandrel is d = 2.0 mm.Figure 1
Cross section of three-layered balloon catheter.Figure 2
Graphics of micro-coextrusion die exit.


## 2.2. Materials, Equipment, and Methods

PA 12 (L25, EMS Grilamid, Switzerland), EVA (QW2023, Sumitomo Chemical Co., Ltd, Japan), and PP (MR-12-01-25, Hyosung Group, Korea) were selected as the build materials to fabricate the multilayered balloon catheters. The extrusion equipment was the high precision microextrusion system (Hightrichja Precision Extrusion Machinery Co., Ltd). The high precision microextrusion system (Figure3) was composed of three single-screw extruders (HRJSJ-Φ25, diameter of 25 mm, and length-to-diameter ratio of 28 : 1), a microextrusion die, an air flow rate control system, a vacuum water tank, caliper and puller.Figure 3
Microextrusion system.Based on the material properties of PA, EVA, and PP and the plasticizing performance of the extruders, the five heating zones were divided to include three heating zones from the hopper to the exit of the extruder (Zones I, II, and III), flange zone, and extrusion die zone. The temperatures range from 190°C to 220°C as shown in Table1.Table 1
Temperature control during extrusion experiments (°C).
TemperatureZone IZone IIZone IIIFlangeExtrusion dieOuter layer extruder and die220220220220220Middle layer extruder and die195195195195195Inner layer extruder and die190190190190190During the experiments, PA, EVA, and PP were fed simultaneously into the corresponding extruders. Since the temperatures inside the extruders were higher than the glass transition temperatures of these polymers, the polymer melts were formed. The polymers were coextruded through the designed three-layered extrusion die to generate three-layered tubes. The semimelt status tubes were pulled by the puller through the vacuum water tank for cooling down at the exit of the extrusion die. The three-layered balloon catheters were formed, cut into segments by the cutter, and then blown dry. Five segments under different operating conditions were collected. The diameters and wall thicknesses were measured and the averages were used as the experimental results.


## 2.3. Measurement and Characterization of Catheter Cross Section

The cross-sectional dimensions of the cut balloon catheters were measured using the digital imaging tool microscope (VTM-3020F, Shuzhou Ouka Precision Optical Instrument Co., Ltd.) with an accuracy of ±0.001 mm.The dimensions of the final three-layered balloon catheters were highly dependent on the geometry of the tubes at the exit of the die (before entering the vacuum water tank). The velocity arrangement, die swelling, atmosphere pressure, injected air pressure, drawing force, and gravity can affect the profile and wall thickness of each layer of the tube. The dimensions of the extruded products were characterized based on five structural parameters including the outer diameter of the catheter (Dw), inner diameter of the catheter (dn), wall thicknesses of the outer layer (Bw), middle layer (Bz), and inner layer (Bn) (Figure 1).The change rate of catheter section size relative to die size was characterized by the catheter diameter change ratey and the catheter wall thickness change rate X respectively, which were defined as follows:The change rate of the outer contour diameter of the catheter:(1)Y1=DwD+d.The change rate of the cavity diameter of the catheter:(2)Y2=dnD+d.The change rate of outer wall thickness of the catheter:(3)X1=BwD/2−d/2.The change rate of middle wall thickness of the catheter:(4)X2=BzD/2−d/2.The change rate of inner wall thickness of the catheter:(5)X3=BnD/2−d/2.


## 2.4. Analysis of Three-Layered Polymer Melt Flowing Behavior during Coextrusion

As shown in Figure1, the coextrusion of polymer melts through the extrusion die is complicated. The liquid phase is composed of three different polymer melts coaxially flowing through the die as shown in Figure 4. To simplify the flowing analysis, two main assumptions are created as follows:(1)
The polymer melts are non-Newtonian fluids with the constitutive equation of power law;(2)
The flowing is an isothermal process, and both the gravity and inertial force have no effect on the flowing process.Figure 4
Schematic of the geometry of the three-layered polymer melt coextrusion flow.When extruding along thez-direction, the density differences between adjacent polymer melts have no effect on the flowing process and the interfaces between adjacent melts are pronounced.Figure4 illustrates the geometry of the three-layered polymer melts flowing in the coextrusion die. From the outer layer to the inner layer, the polymer melts are PA (Phase A), EVA (Phase B), and PP (Phase C), respectively. In addition, the radius of the outer layer is defined as R0, the radius of the interface between Phases A and B is defined as βR0, the radius of the interface between Phases B and C is defined as κR0, and the mandrel of the coextrusion die has the radius of δR0. Herein, the outlet direction of the coextrusion die is set as z-axis and λ is the dimensionless radius ratio with the maximum flowing velocity. When the radius at any location r=λR0, the polymer melts have the maximum flowing velocity and the shear stress τrz equals to 0. Since the maximum velocity must occur in one of the three layers, the corresponding λ can be estimated as: β ≤ λ ≤ 1 for Phase A, κ ≤ λ ≤ β for Phase B, and δ ≤ λ ≤ κ for Phase C.The momentum conservation equation alongz-direction can be simplified as(6)−∂p∂z+1r∂∂rrτrz=0R0≤r≤R1,where p is the pressure. Thus, the momentum conservation equations for different phases can be expressed as follows:(7)−∂pA∂z+1r∂∂rrτrzA=0βR0≤r≤βR0,(8)−∂pB∂z+1r∂∂rrτrzB=0kR0≤r≤βR0,(9)−∂pC∂z+1r∂∂rrτrzC=0δR0≤r≤kR0.Since the pressure drop for each layer is consistent, the following relation can be obtained:(10)−∂pA∂z=−∂pB∂z=−∂pC∂z=G.By substituting (10) into, equations (7) and (8), (9), the corresponding shear stress for each phase can be calculated as(11)τrzA=−Gr2+crβR0≤r≤R0,(12)τrzB=−Gr2+crkR0≤r≤βR0,(13)τrzC=−Gr2+crδR0≤r≤kR0,where c is the constant. When r=λR0, the maximum flowing velocity happens and the shear stress equals to 0 (τrz=0). If the ratio between the radius at any location and the radius of the outer layer is defined as ρ=r/R0, equations (11) and (12), (13) can be rewritten as(14)τrzA=GR02λ2−ρ2ρβ≤ρ≤1,(15)τrzB=GR02λ2−ρ2ρk≤ρ≤β,(16)τrzC=GR02λ2−ρ2ρδ≤ρ≤k.The boundary conditions are listed as follows:(17)whenr=R0, i.e.,ρ=1,vzA=0,(18)whenr=βR0,i.e.,ρ=β,vzA=vzB,(19)whenr=κR0,i.e.,ρ=κ,vzB=vzC,(20)whenr=δR0, i.e.,ρ=δ,vzC=0.Since the power law is used as the constitutive equation for PA, EVA, and PP, the shear stresses can be obtained as(21)τrzA=KAdvzAdrnA=KAR0nAdvzAdρnA,(22)τrzB=KBdvzBdrnB=KBR0nBdvzBdρnB,(23)τrzC=KCdvzCdrnC=KCR0nCdvzCdρnC,where Ki, ni, and vzi are the consistency index, power law index, and velocity of the PA (i = A), EVA (i = B), and PP (i = C) polymer melts, respectively.(1)
When the maximum velocity of the polymer melt occurs in the outer layer (Phase A), the corresponding shear stress is 0 (τrzA=0). Based on the (16) and (23) and boundary conditions (20), the velocity distribution of polymer melt PP can be obtained as(24)vzC1ρ=R0GR02KC1/nC∫δκλ2−ρ2ρ1/nCdρ.Similarly, the velocity distributions of polymer melts EVA and PA can be calculated as (25), (26), and (27), respectively:(25)vzB1ρ=R0GR02KB1/nB∫κβλ2−ρ2ρ1/nBdρ+vzC1ρ.(26)vzA1ρ=R0GR02KA1/nA∫λ1ρ2−λ2ρ1/nAdρλ≤ρ≤1,(27)vzA1ρ=R0GR02KA1/nA∫βλλ2−ρ2ρ1/nAdρ+vzB1ρβ≤ρ≤λ.(2)
When the maximum velocity of the polymer melt occurs in the middle layer (Phase B), the corresponding shear stress is 0 (τrzB=0). Based on the (14) and (21) and boundary conditions (17), the velocity distribution of polymer melt PA can be obtained as(28)vzA2ρ=R0GR02KA1/nA∫β1ρ2−λ2ρ1/nAdρ.Using the similar method, the velocity distributions of polymer melts PP and EVA can be calculated as (29), (30), and (31), respectively:(29)vzC2ρ=R0GR02KC1/nC∫δκλ2−ρ2ρ1/nCdρ,(30)vzB2ρ=R0GR02KB1/nB∫λβρ2−λ2ρ1/nBdρ+vzA2ρλ≤ρ≤β,(31)vzB2ρ=R0GR02KB1/nB∫κλλ2−ρ2ρ1/nBdρ+vzC2ρk≤ρ≤λ.(3)
When the maximum velocity of the polymer melt occurs in the inner layer (Phase C), the corresponding shear stress is 0 (τrzC=0). Based on the (14) and (21) and boundary conditions (17), the velocity distribution of polymer melt PA can be obtained as(32)vzA3ρ=R0GR02KA1/nA∫β1ρ2−λ2ρ1/nAdρ.The velocity distributions of polymer melts EVA and PP can be calculated as (33) and (34) and (35), respectively:(33)vzB3ρ=R0GR02KB1/nB∫κβρ2−λ2ρ1/nBdρ+vzA3ρ,(34)vzC3ρ=R0GR02KC1/nC∫λκρ2−λ2ρ1/nCdρ+vzB3ρλ≤ρ≤k,(35)vzC3ρ=R0GR02KC1/nC∫δλλ2−ρ2ρ1/nCdρδ≤ρ≤λ.Based on the aforementioned equations, the velocity distribution for each phase depends on some parameters including the pressure gradient (G), R0, δR0, K, n, and location of the interfaces (βR0 and κR0). All these parameters can be controlled by the rotation speed (N) of the corresponding extruder. Thus, it is possible to control the wall thickness of one layer by adjusting the rotation speed of the extruder, which makes the rotation speed be one of the most important operating conditions during catheter fabrication.When extruded out of the coaxial extrusion die, other operating conditions such as pulling speed and air injection volume rate can influence the cross-sectional dimensions of the extrudates. For example, with the increase of the pulling speed, the overall diameter of the catheter decreases. In addition, during extrusion, high pressure air is injected into the lumen of the catheter to balance the inner pressure with the ambient pressure, resulting in the stable shape of the catheter. Thus, the change of the air injection volume rate also affects the dimensions of the three-layered balloon catheters.


## 3. Results and Discussion

As aforementioned, the diameter and wall thicknesses of the three-layered balloon catheters were affected by some operating conditions, including the pulling speed of the puller (V), air injection volume rate (Q), and screw speeds of the outer layer extruder (Nw), middle layer extruder (Nz), and inner layer extruder (Nn). The range of each operating condition was determined based on some preliminary experiments as shown in Table 2.Table 2
Range of operating conditions.
Operating conditions during extrusionRangeV (m/min)4∼12Q (mL/min)4∼28Nw (r/min)6∼34Nz (r/min)6∼34Nn (r/min)6∼34


### 3.1. Effect of Pulling Speed on Diameter and Wall Thicknesses

The air injection volume rate was set as 5 mL/min, the screw speeds of three extruders were set as 10 r/min, and the pulling speed increased from 4 m/min to 12 m/min with an interval of 2 m/min to investigate the effects of pulling speed on the diameters and wall thicknesses. The profiles of the achieved three-layered balloon catheters at different pulling speeds were imaged (Figure5). When the pulling speed was increased, both the outer and inner diameters of the catheter were decreased rapidly. The ovality of the profiles was increased due to the insufficient air injection volume.Figure 5
Graphics of cross sections of catheters at different pulling speeds. (a)V1 = 4 m/min, (b) V2 = 6 m/min, (c) V3 = 8 m/min, (d) V4 = 10 m/min, and (5) V5 = 12 m/min.The relationships between pulling speed and diameter/wall thicknesses were illustrated in Figure6. When the pulling speed was increased, both the outer diameter (Dw) and inner diameter (dn) of the catheter were decreased nonlinearly. Their change rates (Y1 and Y2) are 23.27%–44.22% and 11.95%–24.99% respectively. The wall thicknesses of the three layers (Bw, Bz, and Bn) were decreased with the increase of pulling speed. Their change rates (X1, X2, and X3) are 9.75%–14.5%, 13.5%–21.79%, and 17.88%–26.25% respectively. The variance of the wall thicknesses presented a smaller amplitude when compared with the change of the diameters as shown in Figures 6(c)–6(e).Figure 6
Diameter change rates ((a) outer and (b) inner diameter) and wall thickness change rates ((c) outer layer, (d) middle layer, and (e) inner layer wall thicknesses) of three-layered balloon catheters as a function of pulling speed.
(a)(b)(c)(d)(e)


### 3.2. Effect of Air Injection Volume Rate on the Diameters and Wall Thicknesses

The pulling speed was set at 5 m/min, the screw speeds of three extruders were set at 10 r/min, and the air injection volume rate increased from 4 mL/min to 28 mL/min with an interval of 2 mL/min to investigate the effects of air injection volume rate on the diameters and wall thicknesses. The profiles of the achieved three-layered balloon catheters at different air injection volume rates were imaged and measured in Figures7 and 8, respectively.Figure 7
Graphics of cross sections of catheters at different air injection volume rates. (a)Q1 = 4 mL/min, (b) Q2 = 6 mL/min, (c) Q3 = 8 mL/min, (d) Q4 = 10 mL/min, (e) Q5 = 12 mL/min, (f) Q6 = 14 mL/min, (g) Q7 = 16 mL/min, (h) Q8 = 18 mL/min, (i) Q9 = 20 mL/min, (j) Q10 = 22 mL/min, (k) Q11 = 24 mL/min, (l) Q12 = 26 mL/min, and (m) Q13 = 28 mL/min.Figure 8
Diameter change rates ((a) outer and (b) inner diameters) and wall thickness change rates ((c) outer layer, (d) middle layer, and (e) inner layer wall thicknesses) of three-layered balloon catheters as a function of air injection volume rate.
(a)(b)(c)(d)(e)When the air injection volume rate was increased, the profiles of the catheters present good roundness and negligible dimensional changes. The inner and outer diameters were increased slightly and linearly, while the wall thickness of each layer was decreased with the increase of air injection volume rate as shown in Figures8(a)–8(e).


### 3.3. Effect of Screw Speed of Outer Layer Extruder on Diameters and Wall Thicknesses

Four variables were modified to investigate the effects of screw speed of the outer layer extruder on the geometry of the catheters. The pulling speed was set as 5 m/min. The air injection volume rate was set as 5 mL/min. The screw speeds of the middle and inner layer extruders were set as 10 r/min. The screw speed of out-layer extruder was increased from 6 r/min to 34 mL/min with an interval of 2 mL/min. The profiles of the achieved three-layered balloon catheters at different screw speeds (Nw) were imaged and measured as shown in Figures 9 and 10, respectively.Figure 9
Graphics of cross sections of catheters at different screw speeds (Nw). (a) Nw1 = 6 r/min, (b) Nw2 = 8 r/min, (c) Nw3 = 10 r/min, (d) Nw4 = 12 r/min, (e) Nw5 = 14 r/min, (f) Nw6 = 16 r/min, (g) Nw7 = 18 r/min, (h) Nw8 = 20 r/min, (i) Nw9 = 22 r/min, (j) Nw10 = 24 r/min, (k) Nw11 = 26 r/min, (l) Nw12 = 28 r/min, (m) Nw13 = 30 r/min, (n) Nw14 = 32 r/min, and (o) Nw15 = 34 r/min.Figure 10
Diameter change rates ((a) outer and (b) inner diameter) and wall thickness change rates ((c) outer layer, (d) middle layer, and (e) inner layer wall thicknesses) of three-layered balloon catheters as a function of screw speed of the outer layer extruder.
(a)(b)(c)(d)(e)When the screw speed of the outer layer extruder was increased, the wall thickness of the outer layer was increased linearly (Figures9 and 10(c)). The change rates (X1) are10.94%–42%. The velocity distribution could be calculated based on equations (24)–(27). The wall thicknesses of both the middle layer and inner layer were decreased linearly due to the radial stress provided by the increasing wall thickness of the outer layer. The extruded polymer melts had constant volumes in the middle and inner layers. The wall thickness change rates for the two layers were not pronounced as shown in Figures 9 and 10(d) and 10(e). The outer diameter of the catheter was increased rapidly due to the increase of the outer layer wall thickness. The inner diameter was increased slightly and linearly with the increasing screw speed of the outer layer extruder as illustrated in Figure 10(a) and 10(b). Their change rates (Y1 and Y2) are 31%–44.8% and 16.2%–24.91% respectively.


### 3.4. Effect of Screw Speed of Middle Layer Extruder on Diameters and Wall Thicknesses

Four variables were modified to investigate the effects of screw speed of the middle layer extruder on the geometry of the catheters. The pulling speed was set at 5 m/min. The air injection volume rate was set at 5 mL/min. The screw speeds of the outer and inner layer extruders were set at 10 r/min. The screw speed of the middle layer extruder was increased from 6 r/min to 34 mL/min with an interval of 2 mL/min. The profiles of the achieved three-layered balloon catheters at different screw speeds (Nz) were imaged in Figure 11. The dimensions of these catheters were further measured as shown in Figure 12.Figure 11
Graphics of cross sections of catheters at different screw speeds (Nz). (a) Nz1 = 6 r/min, (b) Nz2 = 8 r/min, (c) Nz3 = 10 r/min, (d) Nz4 = 12 r/min, (e) Nz5 = 14 r/min, (f) Nz6 = 16 r/min, (g) Nz7 = 18 r/min, (h) Nz8 = 20 r/min, (i) Nz9 = 22 r/min, (j) Nz10 = 24 r/min, (k) Nz11 = 26 r/min, (l) Nz12 = 28 r/min, (m) Nz13 = 30 r/min, (n) Nz14 = 32 r/min, and (o) Nz15 = 34 r/min.Figure 12
Diameter change rates ((a) outer and (b) inner diameter) and wall thickness change rates ((c) outer layer, (d) middle layer, and (e) inner layer wall thicknesses) of three-layered balloon catheters as a function of screw speed of the middle layer extruder.
(a)(b)(c)(d)(e)When the screw speed of the middle layer extruder was increased, this is a significant increase of the wall thickness of the middle layer. The change rates of the wall thickness were increased linearly from 10.56% to 46.25% when the screw speed was increased from 6 r/min to 34 r/min. The velocity distribution could be calculated based on equations (28)-(31). The increase of screw speed of the middle layer extruder led to an increase of both the inner and outer diameters as shown in Figure 12(a)-12(b). Due to the resistance of the inner layer, the variance of the outer diameter was larger than that of the inner diameter. Their change rates (Y1 and Y2) are 32.48%–45.45% and 18.05%–25%, respectively. The wall thicknesses of both the outer and inner layers were decreased slightly due to the radial stress provided by the increase of the middle layer wall thickness as illustrated in Figures 11 and 12(c)–12(e).


### 3.5. Effect of Screw Speed of Inner Layer Extruder on Diameters and Wall Thicknesses

Four variables were modified to investigate the effects of screw speed of the middle layer extruder on the geometry of the catheters. The pulling speed was set as 5 m/min. The air injection volume rate was set as 5 mL/min. The screw speeds of the outer and middle layer extruders were set as 10 r/min. The screw speed of the middle layer extruder was increased from 6 r/min to 34 mL/min with an interval of 2 mL/min. The profiles of the achieved three-layered balloon catheters at different screw speeds (Nz) were imaged in Figure 13. The relationships between the cross-sectional dimensions of these catheters and the screw speed were illustrated in Figure 14.Figure 13
Graphics of cross sections of catheters at different screw speeds (Nn). (a) Nn1 = 6 r/min, (b) Nn2 = 8 r/min, (c) Nn3 = 10 r/min, (d) Nn4 = 12 r/min, (e) Nn5 = 14 r/min, (f) Nn6 = 16 r/min, (g) Nn7 = 18 r/min, (h) Nn8 = 20 r/min, (i) Nn9 = 22 r/min, (j) Nn10 = 24 r/min, (k) Nn11 = 26 r/min, (l) Nn12 = 28 r/min, (m) Nn13 = 30 r/min, (n) Nn14 = 32 r/min, and (o) Nn15 = 34 r/min.Figure 14
Diameter change rates ((a) outer and (b) inner diameter) and wall thickness change rates ((c) outer layer, (d) middle layer, and (e) inner layer wall thicknesses) of three-layered balloon catheters as a function of screw speed of the inner layer extruder.
(a)(b)(c)(d)(e)The increase of the screw speed of the inner layer extruder resulted in a significant increase of the wall thickness of the inner layer (Bn). The change rates (X3) are from 15.46% to 46.81%. The velocity distribution could be calculated based on equations (32)–(35). The increase of the screw speed caused a linear increase of the inner and outer diameters of the catheters. Their change rates (Y1 and Y2) are 31.73%–45.7% and 16.49%–28.71% respectively. The increase of Bn in the outward radial stress caused a decrease of wall thicknesses in both the middle and outer layers as shown in Figures 14(c) and 14(d).The optimal combination of these operating conditions could achieve three-layered balloon catheters with high accuracy and fabrication efficiency. The optimal combination was a pulling speed of 5 m/min, an air injection volume rate of 12 mL/min, and a screw speed of at each extruder of 10 r/min. Using these operating conditions, balloon catheters could be fabricated with well-defined geometries (Figure15). The dimensions of the catheter were close to the designed values and the product had no visible defaults such as grooves and glitches on the surface as shown in Figures 1 and 15.Figure 15
Cross section of three-layered balloon catheter with the optimal operating conditions.


## 3.1. Effect of Pulling Speed on Diameter and Wall Thicknesses

The air injection volume rate was set as 5 mL/min, the screw speeds of three extruders were set as 10 r/min, and the pulling speed increased from 4 m/min to 12 m/min with an interval of 2 m/min to investigate the effects of pulling speed on the diameters and wall thicknesses. The profiles of the achieved three-layered balloon catheters at different pulling speeds were imaged (Figure5). When the pulling speed was increased, both the outer and inner diameters of the catheter were decreased rapidly. The ovality of the profiles was increased due to the insufficient air injection volume.Figure 5
Graphics of cross sections of catheters at different pulling speeds. (a)V1 = 4 m/min, (b) V2 = 6 m/min, (c) V3 = 8 m/min, (d) V4 = 10 m/min, and (5) V5 = 12 m/min.The relationships between pulling speed and diameter/wall thicknesses were illustrated in Figure6. When the pulling speed was increased, both the outer diameter (Dw) and inner diameter (dn) of the catheter were decreased nonlinearly. Their change rates (Y1 and Y2) are 23.27%–44.22% and 11.95%–24.99% respectively. The wall thicknesses of the three layers (Bw, Bz, and Bn) were decreased with the increase of pulling speed. Their change rates (X1, X2, and X3) are 9.75%–14.5%, 13.5%–21.79%, and 17.88%–26.25% respectively. The variance of the wall thicknesses presented a smaller amplitude when compared with the change of the diameters as shown in Figures 6(c)–6(e).Figure 6
Diameter change rates ((a) outer and (b) inner diameter) and wall thickness change rates ((c) outer layer, (d) middle layer, and (e) inner layer wall thicknesses) of three-layered balloon catheters as a function of pulling speed.
(a)(b)(c)(d)(e)


## 3.2. Effect of Air Injection Volume Rate on the Diameters and Wall Thicknesses

The pulling speed was set at 5 m/min, the screw speeds of three extruders were set at 10 r/min, and the air injection volume rate increased from 4 mL/min to 28 mL/min with an interval of 2 mL/min to investigate the effects of air injection volume rate on the diameters and wall thicknesses. The profiles of the achieved three-layered balloon catheters at different air injection volume rates were imaged and measured in Figures7 and 8, respectively.Figure 7
Graphics of cross sections of catheters at different air injection volume rates. (a)Q1 = 4 mL/min, (b) Q2 = 6 mL/min, (c) Q3 = 8 mL/min, (d) Q4 = 10 mL/min, (e) Q5 = 12 mL/min, (f) Q6 = 14 mL/min, (g) Q7 = 16 mL/min, (h) Q8 = 18 mL/min, (i) Q9 = 20 mL/min, (j) Q10 = 22 mL/min, (k) Q11 = 24 mL/min, (l) Q12 = 26 mL/min, and (m) Q13 = 28 mL/min.Figure 8
Diameter change rates ((a) outer and (b) inner diameters) and wall thickness change rates ((c) outer layer, (d) middle layer, and (e) inner layer wall thicknesses) of three-layered balloon catheters as a function of air injection volume rate.
(a)(b)(c)(d)(e)When the air injection volume rate was increased, the profiles of the catheters present good roundness and negligible dimensional changes. The inner and outer diameters were increased slightly and linearly, while the wall thickness of each layer was decreased with the increase of air injection volume rate as shown in Figures8(a)–8(e).


## 3.3. Effect of Screw Speed of Outer Layer Extruder on Diameters and Wall Thicknesses

Four variables were modified to investigate the effects of screw speed of the outer layer extruder on the geometry of the catheters. The pulling speed was set as 5 m/min. The air injection volume rate was set as 5 mL/min. The screw speeds of the middle and inner layer extruders were set as 10 r/min. The screw speed of out-layer extruder was increased from 6 r/min to 34 mL/min with an interval of 2 mL/min. The profiles of the achieved three-layered balloon catheters at different screw speeds (Nw) were imaged and measured as shown in Figures 9 and 10, respectively.Figure 9
Graphics of cross sections of catheters at different screw speeds (Nw). (a) Nw1 = 6 r/min, (b) Nw2 = 8 r/min, (c) Nw3 = 10 r/min, (d) Nw4 = 12 r/min, (e) Nw5 = 14 r/min, (f) Nw6 = 16 r/min, (g) Nw7 = 18 r/min, (h) Nw8 = 20 r/min, (i) Nw9 = 22 r/min, (j) Nw10 = 24 r/min, (k) Nw11 = 26 r/min, (l) Nw12 = 28 r/min, (m) Nw13 = 30 r/min, (n) Nw14 = 32 r/min, and (o) Nw15 = 34 r/min.Figure 10
Diameter change rates ((a) outer and (b) inner diameter) and wall thickness change rates ((c) outer layer, (d) middle layer, and (e) inner layer wall thicknesses) of three-layered balloon catheters as a function of screw speed of the outer layer extruder.
(a)(b)(c)(d)(e)When the screw speed of the outer layer extruder was increased, the wall thickness of the outer layer was increased linearly (Figures9 and 10(c)). The change rates (X1) are10.94%–42%. The velocity distribution could be calculated based on equations (24)–(27). The wall thicknesses of both the middle layer and inner layer were decreased linearly due to the radial stress provided by the increasing wall thickness of the outer layer. The extruded polymer melts had constant volumes in the middle and inner layers. The wall thickness change rates for the two layers were not pronounced as shown in Figures 9 and 10(d) and 10(e). The outer diameter of the catheter was increased rapidly due to the increase of the outer layer wall thickness. The inner diameter was increased slightly and linearly with the increasing screw speed of the outer layer extruder as illustrated in Figure 10(a) and 10(b). Their change rates (Y1 and Y2) are 31%–44.8% and 16.2%–24.91% respectively.


## 3.4. Effect of Screw Speed of Middle Layer Extruder on Diameters and Wall Thicknesses

Four variables were modified to investigate the effects of screw speed of the middle layer extruder on the geometry of the catheters. The pulling speed was set at 5 m/min. The air injection volume rate was set at 5 mL/min. The screw speeds of the outer and inner layer extruders were set at 10 r/min. The screw speed of the middle layer extruder was increased from 6 r/min to 34 mL/min with an interval of 2 mL/min. The profiles of the achieved three-layered balloon catheters at different screw speeds (Nz) were imaged in Figure 11. The dimensions of these catheters were further measured as shown in Figure 12.Figure 11
Graphics of cross sections of catheters at different screw speeds (Nz). (a) Nz1 = 6 r/min, (b) Nz2 = 8 r/min, (c) Nz3 = 10 r/min, (d) Nz4 = 12 r/min, (e) Nz5 = 14 r/min, (f) Nz6 = 16 r/min, (g) Nz7 = 18 r/min, (h) Nz8 = 20 r/min, (i) Nz9 = 22 r/min, (j) Nz10 = 24 r/min, (k) Nz11 = 26 r/min, (l) Nz12 = 28 r/min, (m) Nz13 = 30 r/min, (n) Nz14 = 32 r/min, and (o) Nz15 = 34 r/min.Figure 12
Diameter change rates ((a) outer and (b) inner diameter) and wall thickness change rates ((c) outer layer, (d) middle layer, and (e) inner layer wall thicknesses) of three-layered balloon catheters as a function of screw speed of the middle layer extruder.
(a)(b)(c)(d)(e)When the screw speed of the middle layer extruder was increased, this is a significant increase of the wall thickness of the middle layer. The change rates of the wall thickness were increased linearly from 10.56% to 46.25% when the screw speed was increased from 6 r/min to 34 r/min. The velocity distribution could be calculated based on equations (28)-(31). The increase of screw speed of the middle layer extruder led to an increase of both the inner and outer diameters as shown in Figure 12(a)-12(b). Due to the resistance of the inner layer, the variance of the outer diameter was larger than that of the inner diameter. Their change rates (Y1 and Y2) are 32.48%–45.45% and 18.05%–25%, respectively. The wall thicknesses of both the outer and inner layers were decreased slightly due to the radial stress provided by the increase of the middle layer wall thickness as illustrated in Figures 11 and 12(c)–12(e).


## 3.5. Effect of Screw Speed of Inner Layer Extruder on Diameters and Wall Thicknesses

Four variables were modified to investigate the effects of screw speed of the middle layer extruder on the geometry of the catheters. The pulling speed was set as 5 m/min. The air injection volume rate was set as 5 mL/min. The screw speeds of the outer and middle layer extruders were set as 10 r/min. The screw speed of the middle layer extruder was increased from 6 r/min to 34 mL/min with an interval of 2 mL/min. The profiles of the achieved three-layered balloon catheters at different screw speeds (Nz) were imaged in Figure 13. The relationships between the cross-sectional dimensions of these catheters and the screw speed were illustrated in Figure 14.Figure 13
Graphics of cross sections of catheters at different screw speeds (Nn). (a) Nn1 = 6 r/min, (b) Nn2 = 8 r/min, (c) Nn3 = 10 r/min, (d) Nn4 = 12 r/min, (e) Nn5 = 14 r/min, (f) Nn6 = 16 r/min, (g) Nn7 = 18 r/min, (h) Nn8 = 20 r/min, (i) Nn9 = 22 r/min, (j) Nn10 = 24 r/min, (k) Nn11 = 26 r/min, (l) Nn12 = 28 r/min, (m) Nn13 = 30 r/min, (n) Nn14 = 32 r/min, and (o) Nn15 = 34 r/min.Figure 14
Diameter change rates ((a) outer and (b) inner diameter) and wall thickness change rates ((c) outer layer, (d) middle layer, and (e) inner layer wall thicknesses) of three-layered balloon catheters as a function of screw speed of the inner layer extruder.
(a)(b)(c)(d)(e)The increase of the screw speed of the inner layer extruder resulted in a significant increase of the wall thickness of the inner layer (Bn). The change rates (X3) are from 15.46% to 46.81%. The velocity distribution could be calculated based on equations (32)–(35). The increase of the screw speed caused a linear increase of the inner and outer diameters of the catheters. Their change rates (Y1 and Y2) are 31.73%–45.7% and 16.49%–28.71% respectively. The increase of Bn in the outward radial stress caused a decrease of wall thicknesses in both the middle and outer layers as shown in Figures 14(c) and 14(d).The optimal combination of these operating conditions could achieve three-layered balloon catheters with high accuracy and fabrication efficiency. The optimal combination was a pulling speed of 5 m/min, an air injection volume rate of 12 mL/min, and a screw speed of at each extruder of 10 r/min. Using these operating conditions, balloon catheters could be fabricated with well-defined geometries (Figure15). The dimensions of the catheter were close to the designed values and the product had no visible defaults such as grooves and glitches on the surface as shown in Figures 1 and 15.Figure 15
Cross section of three-layered balloon catheter with the optimal operating conditions.


## 4. Conclusions

In this paper, the flowing behavior during PA/EVA/PP coextrusion was analyzed and the effects of the main operating conditions on the structural parameters were investigated using extrusion experiments. Three-layered balloon catheters with high accuracy and well-defined geometries were successfully fabricated using the optimal combination of these operating conditions. The main conclusions from this paper are as follows:(1)
The pulling speed can significantly affect the diameter and wall thicknesses of the catheters. The diameters and wall thicknesses of different layers are decreased with the increase of pulling speed. The variation interval values of outer profile diameter and inner cavity diameter with the extrusion experiments are 20.95% and 13.04%, respectively.(2)
The screw speed of each layer extruder may affect the geometry of the extruded products. The increase of screw speed of the outer layer extruder causes both the outer diameter and the wall thickness of the outer layer to increase significantly. The inner diameter and wall thicknesses of the middle and inner layers decrease slightly. When the screw speed is increased with the middle layer extruder, the inner diameter, the outer diameter, and the wall thickness of the middle layer increase rapidly. Both the outer layer wall thickness and the inner layer wall thickness decrease gradually. The inner diameter, outer diameter, and the wall thickness of the inner layer of the catheter are increased with the increase of screw speed of the inner layer extruder. The wall thicknesses of the middle and outer layers are decreased with the increasing screw speed. The screw speed of each layer has a great influence on the wall thickness of each layer. Through the influence of outer screw speed, middle screw speed and inner screw speed, the variation interval values for outer layer wall thickness, middle layer wall thickness, and inner layer wall thickness are 31.06%, 35.69%, and 31.35%, respectively.(3)
Air injection volume rate can affect the roundness of the cross section of the fabricated three-layered balloon catheters, but has no significant effect on the structural parameters such as the diameter and wall thicknesses.The conclusions show that the extrusion process parameters have a great influence on the cross-sectional size of the extruded three-layer balloon catheter. However, Some quantitative studies still need further experiments.



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*Source: 9409055-2022-04-07.xml*