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Visualization Analysis of Reciprocating-Screw Plastication Process of Glass Fiber-Reinforced Resins in a Glass-Inserted Heating Cylinder.

INTRODUCTION

During injection molding, resins are mixed with fillers such as glass or carbon fibers to improve their mechanical strength in heavy-industry applications such as manufacturing injection-molded automobile and mechanical parts. As the mechanical strength of the resin depends on the fiber length, long glass-fiber pellets are often preferred to shorter ones. However, previous studies have found that long glass fibers (GFs) tend to break at the following points of the injection process: (1) when the resin is plasticized inside the heating cylinder [1-7]. (2) when the resin bypasses the check ring [8-12], and (3) when the resin flows into the mold [13,14]. These findings suggest that the original fiber length cannot be maintained throughout the manufacturing process of the molded product. Therefore, understanding the fiber breakage process during injection molding is an important research topic to establish a more robust molding process that yields reliable and strong shaped products.

Based on the visualization results of continuous plastication experiments, the authors previously proposed a melting model of long glass fiber-reinforced polypropylene (L-GFPP) [15]. In contrast to the steady-state process of continuous plastication, reciprocating plastication is an unsteady process in which the relative positions of the screw and cylinder fluctuate intermittently and periodically during the molding cycle. The reciprocating motion is also generated by the screw of the Buss Kneader, which moves forward and backward. White and Lyu published many papers on the Buss Kneader, including experimental research, theoretical models, and non-isothermal and viscoelastic simulations [16-18]. Ville et al. [19] further investigated the fiber-breakage situation in the Buss Kneader. However, when performing injection molding, the positions of the screw and cylinder always fluctuate considerably during the metering process. This reciprocating plastication process is affected by various molding conditions such as the screw design, cylinder temperature, rotation speed, back pressure, and cycle time [20]. The cylinder internal pressure, torque, resin-melting situation, and other factors affecting non-reinforced resins have been predicted in simulation models of the resin-melting process [21-24]. Tatsuno and co-workers [25,26] also conducted visualization experiments of the reciprocating plastication processes of various unreinforced resins, but without considering the relationship between the reciprocating plastication of L-GFPP and the fiber-length distribution in the resin. To investigate this relationship, this study visualizes the reciprocating plastication process of L-GFPP and observes the changes in the fiber-length distribution during the injection process under different molding conditions (rotation speed, back pressure, charge stroke, and waiting time).

EXPERIMENTAL METHODS AND APPARATUSES

Visualization and Image Analysis Methods

Figure 1 shows a cross-section of the visualization-heating cylinder. This heating cylinder, developed by Yokoi [27]. is divided into three parts in the axial direction. It includes a highly heat-and pressure-resistant long quartz-glass block with a camera-viewing window. When inserted into the cylinder, this apparatus enlarges the visualization range in the cylinder's axial direction [27,28]. As the observation window is only 6 mm wide, the high-speed video camera cannot simultaneously capture the entire plastication process. To overcome this limitation, we joined a time series of snapshots captured through the observation window, forming extended images [27]. In this process, we extracted thin video-frame slices from the center of the observation window (Fig. 2), and arranged them in sequential order to capture the dynamic state of the resin. The time axis was represented by the laminating direction. The image was extended from the bottom upwards and was then rotated by 90[degrees] to the right.

In previous studies [25,26]. the extended image was binarized by a luminance threshold that distinguished the solid bed (SB) from the melt pool (MP). The SB ratio was calculated as the quotient of the SB width at each time point and the screw channel width. The high luminance band at the base of the flight in the MP region, which would cause error in the binarized image, was removed by a masking treatment. The maximum SB ratio in these studies was 80%. In this study, we propose a new analytical method that extracts the non-fibrillated fiber bundles floating in the melting zone (see Fig. 3). Information contained in the 10-46 mm region of the observation window (where 36 mm corresponds to 1 pitch) was extracted from the extended image. The straight line in the image was visually identified as the screw flight. The uniform and non-uniform parts in the screw channel between the flights were identified as the melted and un-melted MP regions, respectively. The non-uniform part was classified into the un-melted pellet group SB and the non-fibrillated glass fiber bundle GF (the white striped region drawn out from the un-melted SB). The SB and GF boundaries of the image were traced and painted green and yellow. Finally, each area was digitized using analysis software, and the SB and GF ratios were obtained by dividing the SB and GF widths at each time interval by the screw channel width.

Experimental Method and Plastication Conditions

The experimental equipment was a Si-80 V injection-molding machine (Toyo Machinery & Metal Co. Ltd., Hyogo Prefecture, Japan) equipped with a prototype observation cylinder with a 36-mm screw diameter and a high-speed video camera (MEMRECAM n-Box C-cam, NAC Image Technology Inc.). Figures 4 and 5 show the experimental setup and the screw and cylinder arrangement at the most advanced position of the screw, respectively. The high-speed video camera was located in front of the cylinder and fixed to a movable table that moved along the axial direction of the screw. In turn, the movable table was fixed to the bracket of the screw shaft, enabling the camera to follow a fixed position on the moving screw surface. The heating cylinder was installed with three camera-viewing windows (one each at the feed zone B, the compression zone D, and the metering zone F). Additionally, two NP400 resin pressure sensors with a sensor diameter of 8 mm (Dynisco Co., Ltd.) were placed at 90[degrees] from the center line along the width of the compression zone at 393.5 and 483.5 mm from the hopper center (see Fig. 5). The extended images from the video capture were created by the Time-im image processing system (Library Co., Ltd.). The screw was a full-flight screw with a compression ratio of 2.3. The screw dimension and molding conditions are shown in Tables 1 and 2, respectively (the underlined conditions in Table 2 are the standard molding conditions). The metering process was started and the screw retracted from the most screw advance limit to the set stroke. After the set waiting time, the injection process was started and returned to the forward screw limit. After 6-10 repeats of this cycle, two shots were videotaped. The resin material was long glass fiber-reinforced polypropylene (L-5050P from Prime Polymer Co. Ltd.) with a fiber content of 50 wt%, GF diameter of 16 [micro]m, and a GF length of 8 mm. The original fiber pellets were cold cut with an average diameter and length of 3 and 8 mm, respectively.

Measurement of Fiber Length

The lengths of the glass fibers dispersed in water were measured under a LEXT OLS 4000 laser microscope (Olympus Corporation). To extract the fibers, the resin sample was incinerated by heating for 3 h at 600[degrees]C in an F-120-P electric furnace (Tokyo Glass Co. Ltd.). As the accuracy of the fiber-length distribution depends on the number of fibers measured [29], we measured 800-900 fibers from each sample to obtain a statistically viable result. The fiber-length distribution was computed as follows:

[L.sub.w] = [SIGMA][n.sub.i][L.sup.2]/[SIGMA][n.sub.i]L, (1)

where L is the measured fiber length, [n.sub.i] is the fiber number, and [L.sub.w] is the length of the weight-averaged fibers.

RESULTS AND DISCUSSION

Extended Images

The extended images of the feed zone, compression zone, and metering zone under standard conditions are shown in Fig. 6. We now discuss the time-dependent changes of the resin during plastication at the fixed camera-viewing positions in the axial direction of the screw.

In the feed zone, at the start of the metering process, the MP (dark area) spread to the side of the pushing flight, whereas the long fiber pellets were un-melted on the trailing flight. The width of the SB increased from 0.5 to 3.5 s and the pellets were stretched along the rotational direction as shown by the fibrous streaks. It was assumed that the fibers had separated from the fiber bundles in the pellets without significant breakage. After 3.5 s, the screw channels were filled with pellets. During continuous plastication [15]. the pellets advance orthogonally to the flight, but in reciprocating plastication, they tend to rotate from the orthogonal to the parallel direction, suggesting that many of the pellets in the SB agglomerate parallel to the flight. As the metering progressed, the pellets successively filled the void areas at B0, B1, B2, and B3 in the SB.

One second after the metering process, discrete pellet shapes appeared in the SB of the compression zone. The SB was located at the hopper side of the observation area. Between 1 and 3 s, the pellet shapes became less defined and numerous white streaks extended from the SB. These steaks are the fibers at the outer region of the melted pellets, which drew out in an unfibrilated situation during the waiting time at the trailing edge of the SB breakup (BUP). Thus, the shape of the pellet is visibly unclear, suggesting that melting has advanced. We hence deduced that at the beginning of metering, the pellets were softened and melted by the heat applied during the waiting time, and were then transported to the nozzle side. Presumably, the fibers remained in the softened pellets and were protected from the shearing force of the cylinder wall, so breakage was suppressed. The pellet shapes also appeared in the SB piece, which gradually increased, then disintegrated after 3 s. At approximately 6.5 s, BUP was clearly seen in one region of the SB, as the fibers were drawn out in streaks that flowed in the rotational direction. The SB was separated by the interaction of the screw and cylinder inner wall, suggesting that some of the fiber bundles rapidly deflect in the SB. These fibers stretch and flow into the MP. The fiber breakage was largely attributed to such BUP phenomenon. In reciprocating plastication, the cohesive force of the SB is weakened by the many unfilled areas between the pellets from the feed zone. Although many pellets in the SB are oriented parallel to the flight and the SB is not easily divisible along the channel (flight) direction, the lack of cohesive force likely explains the high frequency of BUP in reciprocating plastication.

At 0-0.5 s, obscured elongated fiber bundles were observed in the metering zone. Some of the fibers in the bundle might have passed through the check ring to be injected without breaking. Large quantities of streaky fibers appeared at 1.5 s, which were temporarily decreased between 2.5 and 10.0 s. During this period, the channel interior exhibited a uniform state. The disappearance of the streaky fibers indicates a homogeneous melting material. The streaky fibers reappeared at 16-16.5 and 18.5-19 s, but SB fragments were absent until the end of the metering process. The same trend was observed in the compression zone.

The SB and GF ratios in the compression zone provide quantitative measures of the plastication process. They also correlate the molding conditions with the fiber-length distribution of the injection resin.

Influence of Molding Conditions on Fiber-Length Distribution

As the rotating screw moved backward during the metering process, the metering resin sequentially accumulated in the reservoir area. During low-speed injection from the reservoir, resin flowed from the nozzle and weighted average fiber length formed in this resin was measured with each 10-mm advance of the screw. Figure 7 plots the weight-averaged fiber length at 10-mm length of the screw position in different stages of the injection process, and Fig. 8 shows the changing SB and GF ratios during the metering process. The average SB and GF ratios as functions of screw rotation speed and waiting time are plotted in Fig. 9.

Charge Stroke. The influence of charge stroke on the fiber-length distribution is shown in Fig. 7a. For all three charge strokes (80, 100, and 120 mm), the fiber length decreased during the first 50 mm of the charge stroke (from the start of the injection process). The shortening effect enhanced with lengthening charge stroke, presumably because of the longer metering time, residence time, and shear time at longer charge strokes. When the charge stroke was increased in 100 and 120 mm, the shortening effect of the fiber after first 50 mm of the charge stroke was followed by fiber lengthening. This rather peculiar phenomenon (a temporary decrease in the weight-averaged fiber length in the middle injection stage) is further described in Model of Fiber-Length Distribution Section. The weighted average fiber length of the early and the middle injection stage are focused and analyzed below.

Rotation Speed. The influence of rotation speed on the fiber-length distribution is shown in Fig. 7b. At 60 rpm, the fiber length in the early and middle stages was little changed. Increasing the rotation speed to 90, 120, and 150 rpm shortened the fiber length in both stages. At rotation speeds of 90 rpm and higher, the fiber length was shorter in the middle stage than in the early stage. When the screw began rotating (at any speed), the fibers at the outer part of the melted pellets were simultaneously dispersed and started flowing as shown in Fig. 8a. The SB ratio in the unmelted region increased when the screw position crossed 30 mm (50 mm at 60 rpm). As the rotation speed increased, the average SB ratio and GF ratio became higher and lower, respectively (see Fig. 9a).

At high rotation speeds, the pellets transported to the nozzle side were insufficiently heated by the cylinder during rotation. As the temperature of the melt film (MF) decreased, the MF became thinner and its shear rate (amount of deformation per unit time) increased. Here, the MF refers to the molten resin interposed in the gap between the inner wall of the cylinder and the SB. The increased rotation speed in turn increased the transportation speed of the resins and the viscosity of the molten resin. Therefore, the low temperature/high viscosity resin, which included the unmelted resin, was plasticized and transported, enhancing the torque. As the rotation speed increases, the viscosity of the molten resin also increases. This increase the amount of entangled fibers that cannot follow the increased shear rate. Hence, we assumed that the fibers were subjected to high stress originating from their intertwined portions. Consequently, we expect that the frequency of fiber shearing also increased, causing fiber breakage.

Back Pressure. The influence of back pressure on the fiber-length distribution is shown in Fig. 7c. At higher back pressure, the weight-averaged fiber length was shorter in both the early and middle injection stages, and the pressure in the screw channel also increased at the upstream side of the check ring. Increasing the back pressure and the metering time increased the residence and shearing times. In this state, the resin was subjected to high-pressure shearing stress during the plastication process. The enlarged shear stress was thought to increase the fiber breakage.

Waiting Time. The influence of waiting time on the fiber-length distribution is shown in Fig. 7d. In both the early and middle stages of the injection process, the fibers were longer at longer waiting times, but the effect was gradual from the early to middle stages. Comparing the SB-ratio trends at waiting times of I and 6 s (Fig. 8b), we observe that the SB ratios almost coincided at screw positions beyond 50 mm. At 30 s waiting time and screw positions up to 70 mm, the resin was nearly melted and was transported to the nozzle side. Increasing the waiting time from 1 to 30 s decreased the average SB ratio by nearly 50%, and increased the average GF ratio (Fig. 9b). As the waiting time increased, more of the pellets underwent softening and melting, reducing the screw torque and shear stress and thereby minimizing the fiber breakage.

Model of Fiber-Length Distribution

As confirmed in the experimental results, the weight-averaged fiber length decreased in the middle stage of the injection process at high charge strokes (L [greater than or equal to] 100 mm). Figure 10 shows the fiber-length distributions in the early, middle, and final stages of the injection process under standard conditions. The number of fibers with lengths between 5.4 and 8.0 mm was large in the early injection stage, reduced in the middle stage, and then recovered in the final stage. This trend is presumed to greatly affect the weight-averaged fiber length. To understand this phenomenon, we analyzed the relationship between each of the experimental results and the middle-stage fiber-length shrinkage. Figure 11a and b shows the fluctuations in weight-averaged fiber length and torque during the metering process, respectively, and Fig. 11c shows the variations of the GF and SB ratios during the metering process. For correlation comparisons with Fig. 11a-c, the melting behavior in the screw channel at each point in the metering process is schematized in Fig. 11d. Based on these correlations, the fluctuations in the fiber-length distribution during the metering process were attributed to the following behaviors:

Early Stage of the Metering Process. When the previous shot preserves the interior channel conditions after completion, the screw injects and advances, transporting the resins to the nozzle side. During the pre-injection waiting period, heat is transferred to the un-melted resin in the channel, softening and melting it. When the screw begins to rotate during the early metering stage, the softened resin (which embeds the long fibers and protects them from breakage) is transported and advanced through the check ring into the reservoir. This behavior characterizes the resin at the beginning of the injection process. The screw torque is lowered by the softening of the resin, and the GF ratio is high in the region containing the long fibers streaking out from the un-melted pellets.

Middle Stage of the Metering Process. At the start of the metering process, the MF is formed at the interface between the inner-wall cylinder surface and the SB. The MF contains long fibers that are drawn out without breaking, which align in the rotational direction to form white streaks. When scratched by the screw flight, the MP was formed with these fibrous streaks. As described in Early Stage of the Metering Process Section, the resin is transported through the check ring in this state. The melting state of the resin is promoted in the pre-injection waiting time.

New pellets fed into the channel from the hopper are transferred to the nozzle side and pushed through, gradually refilling the inside of the screw channel with new pellets. In this way, the GF region disappears and the ratio of un-melted SB increases. The increased shearing action between the low temperature/high viscosity SB surface and the inner-wall cylinder surface/screw surface then enhances the torque. The increased screw torque shortens the fibers by breaking them. This behavior reflects an increased shear action on the SB surface.

Latter Stage of the Metering Process. As the screw advances during the injection process, the pellets fall from the hopper into the empty screw channel. The pellet density in the channel is low, with large voids between the pellets, and in the subsequent waiting time, the surface of the pellet is heated via radiant heat transfer. Consequently, BUP is certain because a cohesive force between the pellets in the pressurized and agglomerated SB induced by the subsequent screw rotation decreases (BUP generation is evidenced by the appearance of break points in the SB-ratio curves during the latter stage of the metering process). An SB with low cohesive force is easily broken by brittle fracturing, as the inner-wall surface of the cylinder exerts tension in the channel direction. Meanwhile, the SB fragment (un-melted pellet group) retains the long pellet form, transporting the unbroken fibers to the nozzle side. Furthermore, the relative position between the hopper (pellet falling position) and the screw shifts to the screw head side, thereby shortening the shear history of each fallen pellet. The fiber length recovers under the low cohesive force of the pellets and short shear history.

Final Stage of the Metering Process. The shear history of the resin in the screw channel is further shortened as shown in Latter Stage of the Metering Process Section. The screw torque continues to increase. However, the SB, which is characterized as a low-temperature pellet mass, significantly breaks up. As the SB breakup intensifies, the effects of long-fiber retention in the BUP become more obvious. Under these effects, the fibers become longer than at any previous stage of the metering process.

Waiting Time after the Metering Process. The waiting time after the metering process is characterized by heat softening and melting of the SB and BUP (the aggregated resin pellets transferred from the hopper and the-just formed BUP fragments). Heating is enhanced by heat transfer from the cylinder.

Considering the correspondences among the various factors analyzed in Influence of Molding Conditions on Fiber-Length Distribution Section, the charge stroke, rotation speed, and back pressure are the most sensitive during Stages 2-4, whereas the waiting time is the most sensitive during Stages 1 and 5. That is, shortening the charge stroke increases the fiber length and stabilizes the fiber-length distribution, because the residence time and shear time decrease in the cylinder. Increasing the rotation speed increases the shear stress exerted by the cylinder wall, causing fiber breakage. When the back pressure increases, the increased shear time and residence time in the cylinder encourage fiber breakage. When the waiting time is sufficiently long, the pellets are sufficiently heated to a soft and plasticized state, which decreases the shear stress and minimizes the fiber breakage. For a constant waiting time, fiber breakage can be minimized by lowering the rotation speed and back pressure, and shortening the charge stroke.

CONCLUSIONS

We visualized the reciprocating plastication process of long glass fiber-reinforced resin containing 50 wt% GF in an injection molding machine with a full-flight screw. We focused on the melting state conditions inside the channels of the molding machine. The following observations and conclusions were derived from the visualization results and the fiber-length distributions during the injection process:

1. During the metering process, there were temporary unfilled areas in the feed zone, and the pellets changed their rotational orientation from orthogonal to parallel to the flight direction. The breakup-phase BUP was easily generated in the compression zone.

2. The weight-averaged length of the resin fibers temporarily reduced in the middle stage of the injection process. During the early stage, the long fibers were preserved by melting and softening of the un-melted pellets during the waiting time; during the later stage, they were recovered in the mixed BUP fragments resulting from the BUP generation of SB in the metering process.

3. For a constant waiting time, fiber breakage can be minimized by lowering the rotation speed, lowering the back pressure, and shortening the charge stroke.

4. The waiting time most significantly influenced the weight-averaged fiber length. During plastication, fiber breakage can be suppressed by sufficiently heating the glass fiber-reinforced resins prior to metering.

ACKNOWLEDGMENTS

The authors would like to express their gratitude to Prime Polymer Co., Ltd. for providing the resins, and members of this study, which was conducted as part of the 2015 and 2016 "Ultimate Injection Molding Project" funded by the Foundation for the Promotion of Industrial Science.

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Sai Ma, [iD] (1) Kazuyuki Shibata, (2) Hidetoshi Yokoi (1)

(1) Institute of Industrial Science, The University of Tokyo, Tokyo, 153-8505, Japan

(2) TOYO MACHINERY AND METAL CO. Ltd., Akashi, Hyogo, 674-0091, Japan

Correspondence to: S. Ma; e-mail: masai@iis.u-tokyo.ac.jp Contract grant sponsor: The Foundation for the Promotion of Industrial Science.

DOI 10.1002/pen.25023

Published online in Wiley Online Library (wileyonlinelibrary.com).

Caption: FIG. 1. Cross-sectional structure of the inserted glass cylinder in the longitudinal direction.

Caption: FIG. 2. Process of generating the extended image (t denotes time).

Caption: FIG. 3. Measurement of solid bed (SB) and glass fiber (GF) ratios in long GF-reinforced polypropylene.

Caption: FIG. 4. Setup of the visualization experiment. [Color figure can be viewed at wileyonlinelibrary.com]

Caption: FIG. 5. Relationship between the axial coordinates of the screw and cylinder.

Caption: FIG. 6. Extended images of the screw channels (H, hopper side; N, nozzle side).

Caption: FIG. 7. Weight-averaged fiber lengths as functions of screw position for different values of charge stroke (a), screw rotation speed (b), back pressure (c), and waiting time (d).

Caption: FIG. 8. SB and GF ratios under different molding conditions.

Caption: FIG. 9. Average SB and GF ratios as functions of screw rotation speed (a) and waiting time (b).

Caption: FIG. 10. Distributions of glass-fiber length at different screw positions (indicated by the square, circle, and triangle symbols).

Caption: FIG. 11. Model of the plastication process (d) deduced from the relationships among (a-c).
TABLE 1. Screw dimension (Unit: mm).

                              Length allocation

Effective   Metering   Compression   Feed    Outer
length        zone        zone       zone   diameter

720           144          216       360       36

                  Channel depth

Effective   Metering   Feed   Compression
length        zone     zone      ratio

720           2.7      6.3        2.3

TABLE 2. Experimental conditions.

Cylinder temperature ([degrees]C)   230-230-230-220-210-40
Screw revolution (rpm)              60/[bar.90]/120/150
Back pressure (MPa)                 0/[bar.6]/12
Charge stroke (mm)                  80/100/[bar.120]
Waiting time (s)                    1/[bar.6]/15/30
Pressure holding time (s)           5
Injection rate ([cm.sup.3]/s)       13
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Author:Ma, Sai; Shibata, Kazuyuki; Yokoi, Hidetoshi
Publication:Polymer Engineering and Science
Article Type:Case study
Date:Apr 1, 2019
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