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Residual Wall Thickness of Water-Powered Projectile-Assisted Injection Molding Pipes.

INTRODUCTION

The production of hollow parts is well known and very common in the plastics industry. Various hollow molding methods using injection molding have been proposed in recent years, such as the fusible core method, two-shell molding/welding method, and die-slide injection molding method [1]. However, it is difficult or costly to produce a long, three-dimensional pipe with a bent portion using any of these molding methods.

Fluid-assisted injection molding technique (FAIM), such as the gas-assisted injection molding (GAIM) process and the water-assisted injection molding (WAIM) process, enables the efficient production of complex, highly integrated parts in one process step with the help of pressurized fluid [2-8]. In the case of WAIM, the polymer melt is displaced by water. Due to the considerably more efficient cooling effect of water, cycle times can be reduced significantly [4,9]. Furthermore, larger part diameters are possible with WAIM [9]. In FAIM, however, the residual wall thickness (RWT) is determined largely by the rheological properties of the polymer and fluid used [2-8]. FAIM is known to have some problems in the uniformity of the inner diameter, the uniformity of the RWT, the smoothness of the inner surface, and the production of pipes with diameters greater than 40 mm [4,8,10-13]. For FAIM parts with large diameters, the achievable wall thicknesses are relatively thick, which leads to an unfavorably heavy part weight and relatively long cycle times to cool down the final product [14]. Moreover, for WAIM, a lot of plastic grades are not suitable or require expensive material modification [9,14].

One process variant of FAIM, namely fluid-powered projectile-assisted injection molding (F-PAIM, PIT in German) process, can be used to solve the above problems. The basic idea of F-PAIM was first described in a Japanese patent [15]. In the F-PAIM process, the hollow channel in the part is formed by a projectile (it is also named a floating core) which is pushed through the polymer melt by a pressurized fluid. Compared with GAIM or WAIM, thinner and more uniform wall thicknesses with smoother surfaces can be achieved [16]. Moreover, more standard materials, which need to be specially modified for WAIM, can be used in the FPAIM process for that the molten core is displaced by a rigid projectile and the fluid comes only after the RWT is formed. Figure 1 shows how the F-PAIM process works [17,18]. A projectile is initially placed on the fluid injector which stretches into the cavity. After the mold is closed, the melt is injected into the mold cavity and the projectile is fully covered by the melt. Then the pressurized fluid is injected directly onto the projectile and the projectile is driven through the molten plastic core to form a hollow channel. After that, the part cools down while the fluid holding pressure compensates for any shrinkage. Finally, the fluid is drained before mold opening and the part can be ejected. Two main process variants of F-PAIM are the short shot method and full shot method [18-21]. In the short shot method, the cavity is first partially filled with a polymer melt, followed by the injection of pressurized fluid to drive the projectile through the core of the polymer melt. In contrast, in the full shot method, the cavity is completely filled with a polymer melt, followed by a high pressure fluid that drives the projectile through the molten polymer core into an overflow cavity. Similar to the FAIM process, the F-PAIM process can be categorized into two types: water-powered projectile-assisted injection molding (W-PAIM) and gas-powered projectile-assisted injection molding (G-PAIM). Due to the more efficient cooling effect of water, cycle times can be reduced significantly in W-PAIM.

Many researchers have investigated the penetration behaviors of water, the distribution of the RWT [11,22-24], and the optimization of processing parameters in WAIM via experimentation [25-27]. To the best knowledge of the authors, only some researchers from the Institute of Plastic Processing (1KV) at RWTH Aachen University have studied the F-PAIM process. They showed the potential and limitations of the new process [17,19,20]. Unfortunately, the RWT of the W-PAIM pipes has not been systematically studied.

The present research aims to study the effects of projectile, the processing parameters and cavity geometry on the RWT of the W-PAIM pipes. The injection mold cavity with a changeable inserts was developed and the corresponding laboratory experimental platform was set up at first, and then the experimental and numerical investigations were conducted. These investigations mainly include the following: (1) Studying the effect of the projectile on the RWT of W-PAIM pipe by comparing with that of WAIM pipe and investigating the mechanism of the penetration behaviors of the projectile; (2) Studying the effect of the processing parameters, including the melt temperature, melt injection pressure, mold temperature, water delay time, and water pressure, on the RWT and (3) Studying the effect of the cavity geometry, including bending radius and deflection angle, on the RWT. It was found that the RWT of W-PAIM pipe was much thinner than that of WAIM pipe. The introduction of the projectile has a great impact on the RWT and makes the flow field in W-PAIM very different from that in WAIM. It was also found that the RWT was mainly influenced by some of the melt temperature, melt injection pressure, mold temperature, and water delay time. A thin RWT at the inner concave side and a thick RWT at the outer convex side were found at the curved sections. The RWTs at the two sides of the curved sections decreased with increasing deflection angle while the increase of bending radius decreased the RWT at outer convex side and increased that at the inner concave side.

EXPERIMENTATION

Equipment Setup

A lab-developed experimental platform for W-PAIM process was set up. It consisted of an injection molding machine, a mold with changeable inserts, a water injection unit, a mold temperature controller, and projectiles. The molding machine was a full-automatic injection molding machine (TTI-250FT; Donghua Machinery Co., Ltd., China). The cavities in the changeable inserts have the same diameters of 16 mm and different curved sections. There was a rapid cooling area at the runner system to prevent the water from penetrating into the runner and flowing into the barrel. An overflow cavity was set at the end of the pipe cavity. The water-injection unit developed in our lab was comprised of a high pressure pump with a maximum pressure of 20 MPa. a water tank and a water injector. The mold temperature controller (BTM-09 W; Borack Machinery Co., Ltd., China) uses water as the heat transfer medium and the mold temperature could be controlled from room temperature to 393 K. The projectile used in the experiments was made of polyoxymethylene (POM), which had very good mechanical properties. It had an outer diameter of 12 mm.

Geometry of the Cavities and RWT Measurement

As shown in Figure 2a, a pipe with two deflection angles of 30[degrees] and 60[degrees] had a bending radius of 20 mm, namely pipe I, was chosen as the basic test case. It was used to investigate the influences of the introduction of the projectile and the processing parameters on the RWT. The RWTs were calculated at five positions along the straight sections of pipe I, that is, P1-P5, as shown in Figure 2a. The RWT at each position was a mean value, which was obtained by averaging the measured values of the four quartering points around the cross section of the position. The RWT of each pipe was calculated by averaging the RWTs of five specimens.

Another pipe with two larger deflection angles of 90[degrees] and 120[degrees] and a bending radius of 20 mm, namely pipe II, as shown in Figure 2b, was used as a counterpart to study the effect of the deflection angle on the RWT at the bending portions. For the bending portion with 90[degrees] deflection angle, two changeable inserts with bending radii of 30 mm and 40 mm were added to investigate the effect of bending radius on the RWT at the curved sections. The RWTs at the inner concave and outer convex side of the curved section were measured at the bisector of the bending angle, that is, D30, D60, D90, and D120, as shown in Figure 2. The RWT was obtained by averaging the measured values of five specimens.

The basic RWT could be calculated by subtracting the projectile outer radius from the cavity inner radius. For the two cases used in this study, the basic RWT was 2 mm.

Materials

For the sake of comparison between the W-PAIM and WAIM, polypropylene was used in the experiments for its applicability in the WAIM process. The polypropylene (Grade: PPH-T03) with a melt flow rate (MFR) of 24.2 g/10 min (463 K/2.16 kg, ASTM D-1238) was provided by the Maoming Petrochemical Inc., China. Its properties are listed in Table 1.

Processing Parameters

The full-shot W-PAIM process was focused on in this study for its more uniform RWT compared to that of short-shot process. The basic processing parameters for the experiments are indicated in Table 2. In order to understand the effects of the processing parameters on the RWT of the W-PAIM pipes, single factor experiments, in which one factor changed while the other factors remained constant, were carried out. Here five factors--the melt temperature, the melt injection pressure, the mold temperature, the water pressure, and the water delay time--were investigated.

NUMERICAL SIMULATION METHOD

In order to understand the experimental results well, numerical simulations were performed. CFD models of a straight pipe and a curved pipe were modeled, as shown in Figure 3. The entrance size is 6 mm, and the exit size is 10 mm. The straight pipe model was used to study the characteristics of the flow field when the projectile penetrates through the molten plastic. The pipe with one curved section was utilized to explore the influence mechanism of the cavity geometry on the RWT. The variables R and A were the bending radius and deflection angle, as shown in Figure 3b. The curved pipes with a constant R of 20 mm and variable A of 30[degrees], 60[degrees], 90[degrees], and 120[degrees] were used to study the effect of the deflection angle on the RWT around the curved section. The curved pipes with a constant A of 90[degrees] and variable R of 20 mm, 30 mm, and 40 mm were adopted to explore the influence of the bending radius on the RWT at the bending segment. The RWT was calculated at the middle of the curved section. Due to the symmetry of the pipes, two-dimensional (2D) simulation models were built to simplify the computational complexity and save the computer time consumption. Two-dimensional analysis can qualitatively reflect the influence of bending portions on the RWT at the symmetry surface.

The boundary conditions of these models were specified as pressure-inlet, pressure-outlet, no-slip, and constant wall temperature of 300 K. The inlet pressure was 5 MPa. The outlet was set as atmospheric pressure because the section of the overflow chamber is very large. The projectile was considered as a solid body (Rigid Body) and the heat exchange between the projectile and the melt was not taken into account.

In the initial state, the front and outside of the projectile are filled with melt, and the inside of the projectile is filled with water. The initial temperature of the melt was 483 K, and the water injection temperature was 298 K, and the water delay time was not considered.

Polypropylene with grade of NB71 provided by Hanwha Total Petrochemical Co. Ltd was selected as the molding material. It has a MFR of 25 g/10 min, which is similar to the resin used in the experiments. Cross-WLF viscosity model [28,29] was adopted and the parameters were found from the Moldflow material library, as shown in Table 3.

Gambit software was applied for 2D modeling and meshing. The numerical simulations were computed with FLUENT [30,31]. A user-defined function was coded to calculate the melt viscosity. The motion of projectile was simulated via the application of dynamic mesh in six degrees of freedom model, spring smoothing method, and local-remeshing method. The multi-fluid Volume of Fluid (VOF) model was adopted to simulate the penetrations of the projectile and high pressure water in the melt. The Pressure-Implicit with Splitting of Operators (PISO) algorithm was adopted to solve the pressure-velocity coupling equations for its high accuracy and capability to deal with unsteady flow.

RESULTS AND DISCUSSION

Mechanism of the Penetration Behaviors of the Projectile

The experimental samples of the basic test case, produced by WAIM and W-PAIM under the basic processing parameters listed in Table 2, were cut along the longitudinal symmetrical plane, as shown in Figure 4. The RWTs at the five specified positions are shown in Figure 5. Figure 5 shows that the RWT of the WAIM specimen increased gradually from 2.8 mm to 3.0 mm along the flow direction, while the RWT of the W-PAIM specimen was almost stable at around 1.6 mm. The RWT of the W-PAIM specimen was much thinner than that of the WAIM specimen. Obviously the projectile has a crucial influence on the RWT. Moreover, the actual RWT of the W-PAIM specimen was thinner than 2 mm, the basic RWT.

In the WAIM process, the high pressure water is injected directly into the molten plastic core. The water always tends to penetrate through the core melt for its low flow resistance. The size of the water penetration section depends on the water pressure and the penetration resistance which is mainly affected by the melt temperature, mold temperature, water delay time, and cavity section [25,32-35]. In the W-PAIM process, high pressure water provides the driving force for the penetration of the solid projectile in the melt. The projectile moves just like a flexible piston driven by high pressure water. The penetration of a projectile during the W-PAIM process of a straight pipe with a diameter of 16 mm was simulated. The simulated RWT was about 1.6 mm and agree well with the experimental result. The velocity of the simulation at 0.0228 s, as shown in Figure 6, can be used to illuminate the thinner RWT than the difference between the cavity inner radius and the projectile outer radius. It can be seen from the Figure 6a that the velocity contour result at that time indicates that the velocity field in front of the projectile was very different from that behind of it. The velocity profiles of three positions of 0.09, 0.10, and 0.11 m from the entrance, marked by y-0.09, y-0.10, and y-0.11, were shown in Figure 6b. The velocity profile at y-0.11, the position in front of the projectile, appeared a typical plunger flow and the equal velocity zone was just close to the projectile section size, which determined the basic penetration section size. And the penetration resistance of the projectile from the front was the shear of the melt near the wall. The velocity profile at the current position of the projectile, that is, y-0.10, showed the shape of an isosceles trapezoid. The horizontal segment at the middle represented the velocity of the water trapped by the projectile. The linear segments on both sides were the velocity of the melt at the outside of the projectile, which implied that the melt flow at the outside of the projectile was drag flow. The velocity profile at y-0.09, the position behind the projectile, was approximately parabolic, which was a characteristic of pressure flow. Because of the great viscosity difference between the water and the melt, the velocity gradient at the melt zone was much smaller than that of the water. Moreover, it was seen that the velocity of the drag flow at the both sides of the projectile was faster than that of the subsequent pressure flow near the wall. Consequently, the water followed the projectile was partially supplemented to the drag flow area, as shown in the circle marked area of Figure 6c. Therefore, the simulated RWT was thinner than the basic RWT.

Moreover, there are two other factors that should be noted. One is the solidified layer of the melt adhered on the surface of the projectile, the other is the shrinkage of the formed RWT after the penetration of the projectile. All these factors previously mentioned resulted in a larger penetration section size than the projectile cross-section and a thinner actual RWT than the basic RWT defined by the projectile cross-section.

Effect of Processing Parameters on RWT

The effects of processing parameters, including the melt temperature, melt injection pressure, mold temperature, water injection delay time, and water injection pressure, on the RWT of W-PAIM pipes were shown in Figure 7.

Melt Temperature. The molding temperature range of the polypropylene used in the experiment is 453 K to 543 K. Therefore, the experiments were carried out at 10 levels from 453 K to 543 K with an interval of 10 K, and the average RWT for the pipes is shown in Figure 7a.

As illustrated in Figure 7a, the RWT of the pipe decreased from 1.76 mm to 1.52 mm as the melt temperature increased from 453 K to 513 K, and then increased to 1.58 mm with the melt temperature risen up to 543 K. This was perhaps due to the fact that when the melt temperature was lower than 513 K, the melt viscosity decreased as the melt temperature risen, so the penetration resistance of the projectile reduced and the penetration velocity increased. Consequently, the melt at the side of the projectile flowed faster and more water was compensated into the rear region of the drag flow area, thus resulting in a thinner RWT. In addition, the higher the melt temperature, is the greater the cooling shrinkage of resin. Both of these two factors led to a thinner RWT. When the melt temperature was higher than 513 K, partial melting occurred on the surface of the projectile made of POM, a material with a relatively low melt point of 438 K, which led to a smaller actual size of the projectile. The partial melting of the projectile resulted in a thicker RWT. Thus the RWT of the pipe first decreased and then increased with increasing melt temperature.

Melt Injection Pressure. Based on several trials, the melt injection pressure range of 4 MPa-14 MPa was preferred. And the experimental investigations were performed at an interval of 2 MPa. The mean RWT of the W-PAIM pipe is shown in Figure 7b. The RWT of the pipe decreased from 1.68 mm to 1.50 mm as the melt injection pressure increased from 4 MPa to 8 MPa, and then further increased to 1.64 mm with increasing the melt injection pressure to 14 MPa. On the one hand, the melt injection velocity increased with increasing the melt injection pressure. As a consequence the higher wall shear led to a lower viscosity of the melt near the wall. Thereby, the penetration resistance of the projectile reduced and the penetration velocity increased. Thus the melt at the outer side of the projectile flowed faster and more water was compensated to the drag flow area, resulting in a thinner RWT. However, on the other hand, the melt became denser with an excessive injection pressure, leading to a lower shrinkage, resulting in a thicker RWT.

Mold Temperature. According to the operating temperature of the mold temperature machine used in the experiments and the molding process window of polypropylene, the experiments were conducted at the mold temperature of 298 K-388 K with an interval of 10 K. The results were shown in Figure 7c. With the increase of the mold temperature, the residual wall pipe thickness gradually decreased at first and then approached a constant after the mold temperature was over 358 K. It was due to the fact that increasing mold temperature reduced the temperature difference between the mold and the melt, as well as the cooling rate near the wall. The melt near the wall had a lower viscosity and the high pressure water can push the projectile more quickly, resulting in a thinner RWT. When the mold temperature was higher than 353 K, the flow viscosity at the drag flow area was lower, but the RWT tended to be stable for the constant thickness of the solidification layer at the wall.

Water Delay Time. The water delay time of 0 s to 15 s were selected to conduct the experimental investigations with an interval of 3 s. As shown in Figure 7d, the RWT of the pipe gradually increased from 1.56 mm to 1.73 mm with increase of water injection delay time. This was because increasing water delay time increased the cooling time and the melt viscosity near the wall due to the heat transfer across the cavity wall. Then the penetration resistance of the projectile from the near wall increased, leading to a slower penetration speed, as well as the flow velocity of the melt at the drag flow area. Moreover, the solidifying layer near the wall was thicker. Therefore, the RWT became thicker.

Water Pressure. The water pressure range, ranged from 3 MPa to 9 MPa. An interval of 1 MPa was chosen. The mean RWT of the W-PAIM pipe is shown in Figure 7e. Surprisingly, the RWT almost kept at around 1.7 mm over the investigated range. It came to the conclusion that the water pressure had little impact on the RWT.

Effect of the Cavity Geometry on RWT at the Curved Section

Deflection Angles. In order to investigate the influence of the deflection angle of the pipe on the RWT at the curved section, the RWTs at the inner concave side and the outer convex side of the curved section of the pipe were measured. The statistical results of the specimens and the simulated RWTs were shown in Figure 8. Both the experiment and the simulation results shared the common feature that the W-PAIM curved pipe had a thin RWT at the inner concave side and a thick one at the outer convex side. With the increasing of deflection angle, the inner concave RWT reduced. The RWTs at the two sides of the simulation results were thinner than those of the experiment specimens. This was perhaps attributed to the two facts: one was the difference between the CFD model and the experimental model, the other was neglect of delay time of water injection in the numerical simulation. With the increase of deflection angle, the RWTs of the specimens at the outer convex side slightly increased, while those of the simulation results decreased. This may have been due to the partial melting at the front of the projectile for its low melt point while it keeps rigid in the simulation.

To some extent, the reliability of the simulation is verified. So the simulation results such as the pressure field, velocity field near the bend while the projectile passing through the bend are examined to analyze the influence of the deflection angle on the projectile penetration. Figure 9 show the simulated pressure field and velocity field at the bends. It can be seen from Figure 9a that the area of the high-pressure zone at the outer convex side was obviously larger than that at the inner concave side, which caused a pressure difference between the outer convex side and the inner concave side. The pressure difference forced the projectile penetration close to the inner concave side. Increasing the deflection angle increased the pressure difference leading to a thinner RWT at the inner concave side. It should be noted that the cross-section of the envelope surface formed by the projectile moving at the curved section was larger than the projectile cross-section size, which resulted in a larger penetration section than the cross section of the projectile. The larger the deflection angle, the closer the head of the projectile to the outer convex side. Thus the RWT at the outer convex side decreased slightly with the increasing deflection angle. Figure 9b illuminates that the velocity near the outer convex side was greater than that near the inner concave side. The greater the deflection angle was, the greater the velocity difference between the inner concave side and the outer convex one was. Consequently, a more sharp turn of the projectile occurred at the curved section with a larger deflection angle.

Bending Radius. Three curved sections with a deflection angle of 90[degrees] and the different bending radii of 20 mm, 30 mm, and 40 mm were investigated, respectively. Figure 10 showed the contrast of the experimental and the numerical RWTs at the inner concave side and the outer convex side. The simulation results agree well with the experimental results that increase of bending radius decreased the RWT at outer convex side of the bend and increased that at the inner concave side.

It can be seen from Figure 11a that the increase of the bending radius decreased the area deference of the high pressure acting on both sides of the projectile, and decreased the deviation of the projectile to the inner concave side. Consequently, the RWT at outer convex side of the bend decreased and that at the inner concave side increased. The velocity field shown in Figure 11b indicated that for the case of bending radius 20 mm, the velocity at the outer convex side near the projectile was much faster than that at the inner concave side, and the projectile turned around the bend sharply close to the inner side. With the increase of bending radius, the velocity difference between the outer convex side and the inner concave side near the projectile decreased, and the projectile tended to penetrate along the center of the cavity leading to a more uniform RWT at the bend.

CONCLUSION

RWT is an important quality indicator of W-PAIM pipes. Experimental investigations were carried out to examine the effects of the projectile, the processing parameters, and the geometry of the cavity bending portion on the RWT of W-PAIM pipe. The VOF model provided by software FLUENT was used to perform the numerical simulations and the mechanisms of the penetration behavior of the projectile in the melt at straight and curved sections was explored. The main conclusions drawn from this work are as follows:

1. Compared with the WAIM process, the RWT of the WPAIM pipe was much thinner than that of the WAIM pipe. In W-PAIM process, the melt flow in front of the projectile was a plug flow and that at the side of the projectile was a drag flow, while the water flow behind the projectile was a pressure flow. The section size of the projectile determined the basic penetration section size. The velocity of the drag flow at the side of the projectile was faster than that of the subsequent pressure flow near the wall leading to the supplement of the water to the drag flow area. Therefore, the actual RWT was thinner than the basic RWT which is defined by the projectile cross section.

2. With the increase of the melt temperature, the RWT of the pipes first decreased and then increased for the partial melting of the projectile. Increasing the melt injection pressure decreased the RWT, while an excessive melt injection pressure increased the RWT. With the increase of the mold temperature, the RWT decreased at first and then tended to keep in a constant. Increasing the water injection delay time increased the RWT of the pipe. And the water pressure was found to have little effect on the RWT.

3. The W-PAIM curved pipes have a thin RWT at the inner concave side and a thick RWT at the outer convex side. With the increase of deflection angle, the pressure difference between the outer convex side and the inner concave side increased leading to a thinner RWT at the inner concave side. With the increasing deflection angle, the RWT at the outer convex side decreased slightly. Increasing of bending radius decreased the RWT at outer convex side of the bend and increased that at the inner concave side.

Finally, the uniformity of the RWT of W-PAIM pipes with bending portions might be improved by selecting more suitable projectile material, adjustment of local mold temperature, as well as an optimized projectile geometry. It will be investigated in our further works.

ACKNOWLEDGMENTS

This study was supported financially by the National Natural Science Foundations of China (51563010, 21664002, and 11272093) for which the authors are very grateful. And the financial support of the Science and Technology Research Project of Education Department of Jiangxi Province (grant number 150528) for this research work is also gratefully acknowledged.

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Tang-Qing Kuang [iD], (1) Jun-Yu Pan, (1) Qiang Feng, (1) He-Sheng Liu, (2) Bai-Ping Xu [iD],3 Wen-Wen Liu, (1) Lih-Sheng Turng [iD] (4)

(1) School of Mechatronics & Vehicle Engineering, East China Jiaotong University, Nanchang, 330013, China

(2) School of Mechatronics & Vehicle Engineering, East China University of Science and Technology, Nanchang, 330013, China

(3) Technology Development Center for Polymer Processing Engineering of Guangdong Province, Guangdong Industry Technical Polytechnic, Guangzhou, 510300, China

(4) Polymer Engineering Center and Wisconsin Institute for Discovery, University of Wisconsin-Madison, Madison, Wisconsin, 53706

Correspondence to: T.-Q. Kuang; e-mail: tkuang@ecjtu.edu.cn or B.-P. Xu; e-mail: xubaiping2003@163.com

Contract grant sponsor: National Natural Science Foundation of China; contract grant numbers: 51563010; 21664002; 11272093. contract grant sponsor: Education Department of Jiangxi Province; contract grant number: 150528.

DOI 10.1002/pen.24904

Caption: FIG. 1. Schematic representation of F_PAIM process: (a) Projectile placement (b) Melt injection (c), Fluid injection and holding stage, (d) Fluid being drained. [Color figure can be viewed at wileyonlinelibrary.com!

Caption: FIG. 2. Schematic of molded curved pipe samples and positions for measuring the RWTs: (a) Pipe I. (b) Pipe II. All dimensions are in millimeters.

Caption: FIG. 3. CFD models for the numerical simulations: (a) Straight pipe, (b) Pipe with one curved section. The variables A and R are deflection angle and bending radius, respectively. All dimensions are in millimeters. [Color figure can be viewed at wileyonlinelibrary.com]

Caption: FIG. 4. Longitudinal sections of the specimens: (a) WAIM. (b) W-PAIM. [Color figure can be viewed at wileyonlinelibrary.com]

Caption: FIG. 5. Comparison of RWTs between WAIM and W-PAIM specimens of pipe I. [Color figure can be viewed at wileyonlinelibrary.com!

Caption: FIG. 6. Velocity distribution at time 0.0228 s for the W-PAIM process: (a) Velocity magnitude contour, (b) Velocity profiles at three positions near the projectile, (c) Velocity vector near the projectile. [Color figure can be viewed at wileyonlinelibrary.com]

Caption: FIG. 7. Effects of the processing parameters on the RWT of W-PAIM pipes: (a) Melt temperature, (b) Melt injection pressure, (c) Mold temperature, (d) Delay time, (e) Water injection pressure. [Color figure can be viewed at wileyonlinelibrary.com]

Caption: FIG. 8. RWTs at the outer and inner sides of the bend with various deflection angles. E denotes the experimental result, and S denotes the simulation result. [Color figure can be viewed at wileyonlinelibrary.com]

Caption: FIG. 9. Numerical results at the bend with various deflection angles: (a) Pressure field, (b) Velocity field. Left to right: 30[degrees], 60[degrees], 90[degrees], and 120[degrees]. [Color figure can be viewed at wileyonlinelibrary.com!

Caption: FIG. 10. RWTs at the outer and inner sides of the bend with various bending radii. E denotes the experimental result, and S denotes the simulation result. [Color figure can be viewed at wileyonlinelibrary.com]

Caption: FIG. 11. Numerical results at the bend with various bending radii: (a) Pressure field, (b) Velocity field. Left to Right: 20 mm, 30 mm, and 40 mm. [Color figure can be viewed at wileyonlinelibrary.com]
TABLE 1. Properties of polypropylene
used in the experiments

Property                 Measure       Value

Melt index (g/10 min)    ASTM D-1238   24.2
Impact strength (J/m)    ASTM D-256    78.6
Tensile strength (MPa)   ASTM D-638    29.1
Flexural modulus (MPa)   ASTM D-790    1090
Heat distortion          ASTM D-648    349.6
temperature (K)

TABLE 2. Basic processing parameters used in the experiments

Processing parameters            Value

Melt temp. (K)                    483
Melt injection pressure (MPa)       6
Water delay time (s)                3
Water pressure (MPa)                5
Packing time (s)                    8
Mold temp. (K)                    298

TABLE 3. Cross-WLF model constants for the
polypropylene used in the simulations

Model constant                Value

n (Pa)                        0.3568
[[tau].sup.*] (Pa)            31870.2
[D.sub.1] (Pa x s)     6.7266 x [10.sup.13]
[D.sub.2] (K)                 263.15
[D.sub.3] (K/Pa)                0
[A.sub.1]                     29.925
[A.sub.2] (K)                  51.6
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Article Details
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Author:Kuang, Tang-Qing; Pan, Jun-Yu; Feng, Qiang; Liu, He-Sheng; Xu, Bai-Ping; Liu, Wen-Wen; Turng, Lih-Sh
Publication:Polymer Engineering and Science
Article Type:Technical report
Date:Feb 1, 2019
Words:6067
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