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Visualization analysis of side-edge flow phenomena in different thickness/width rectangular cavities using a rotary runner exchange system.

INTRODUCTION

Visualization is a very important technique for analyzing various phenomena during plastic injection molding. In particular, the understanding of resin behavior inside the cavity is an important theme, as it will also help clarify the causes of molding defects. Some static methods for visualizing resin flow are as follows: the colored material embedding method in which colored materials are embedded in resins and pushed inside the cavity using a plunger; the colored material laminate method in which colored sheets are laminated and pushed into the mold using a plunger [1-4]; the cylinder method that links resins of two different colors using two injection cylinders; and the gate magnetization method that marks resins mixed with magnetic powder by magnetization during flow [5]. The progress of resin flow is frozen inside the sample by replacing some resins with colored resins; however, because this information is obtained by cutting the sample after molding, time factors cannot be taken into account. With the gate magnetization method, time factors can be taken into account by high-precision marking and instantaneous pulse magnetization, but because the marked magnetized area can only be extracted using magnetic detection fluid, it is difficult to determine complicated flow patterns. In contrast, the dynamic visualization method, which uses a visualization mold and sandwich molding technology, is able to switch colored resins halfway through the injection molding process. However, the use of the conventional sandwich nozzle causes the following problems [1]. Injection stops during resin switching and [2] the flow of colored resins is interrupted at the spool, runner, and gate located at the upstream of the cavity. These problems result in the core layer shape changing from its original shape inside the cavity. To resolve these problems, the authors proposed a multistep resin switching method based on color marking as a new technique for visualizing two types of resins inside the mold [6-8] using a glass-insert mold [9]. This method is capable of not only dynamic visualization of resins but also static visualization by observation of the sample cross section in which colored resins switched at multilayers at the gate spread into different layers inside the sample and freeze. As this method enables complicated three-dimensional (3D) flow behavior to be visualized easily inside the mold, it is expected to help clarify unknown flow behavior.

In this study, the authors carried out visualization of the flow behavior on both sides of a rectangular cavity using the rotary runner exchange system. The sides of the cavity are restrained by the mold surface in three directions, forming a 3D flow behavior area. This area is also known as the "ear flow phenomenon," in which the flow front flows faster at the sides of the cavity than in the center. In relation to this phenomenon, there have been reports of experimental analyses by internal resin temperature measurement using observation of flow behavior and an integrated thermocouple sensor [10], static visualization analysis of internal resin How behavior using the gate magnetization method, and simulation analysis, indicating that areas of high-resin temperature are formed at both sides of the cavity instead of the center [11-13]. As all of these studies were only aimed at clarifying the ear flow phenomenon, universal 3D resin flow behavior along the sides of the cavity has not been clarified despite its importance. For this reason, in this study, the authors focused on clarifying this universal 3D resin flow behavior observed on both sides of a simple rectangular cavity. Specifically, dynamic visualization using the multistep resin switching method based on color marking, static visualization by cross-sectional observation of the sample, and dynamic visualization using a glass-insert mold were carried out to accurately determine the 3D flow behavior on both sides of the cavity (hereafter called "side-edge flow"). The aspect ratio [t/W], between the cavity width [W] and thickness [t], was also changed to review the generation of side-edge flow under different conditions. In addition, we also observed the generation of side-edge flow of not only rectangular cavities but also circular and semicircular cavities, and on the basis of these visualization results, we proposed a side-edge flow model.

EXPERIMENTAL METHODS

Rotary Runner Exchange System

Figure 1 shows the basic structure of the rotary runner exchange system. It is composed of a rack and pinion gear, rotary runner exchanger, and hydraulic cylinder. The rotary runner exchanger rotates 90[degrees] via the pinion gear as a result of the reciprocal movements of the rack. As the force acting on the runner of the rotary runner exchanger is constantly symmetrical around the axis, rapid switching is realized even during the injection process. The switching time is about 100 ms, and the time taken for the rotary runner exchanger to completely obstruct resin flow is about 15 ms. Figure 2 shows the visualization method using the multistep switching mode of the rotary runner exchange system. By temporarily using colored resins for the resins flowing through the gate, the flow patterns of resins inside the cavity can be visualized. Specifically, the resins are switched twice: transparent to colored resins and again to transparent resins. The flow history of colored resins flowing into the cavity in pulse pattern can be obtained, allowing static visualization by observing the sample cross sections. In addition, by combining the use of the 2D glass insert mold, the flow behavior of colored resins inside the cavity can also be dynamically visualized.

[FIGURE 1 OMITTED]

[FIGURE 2 OMITTED]

Molding Conditions

We conducted molding experiments using the double switching mode of the rotary runner exchange system described earlier to investigate the flow patterns of colored resins through observation of sample cross sections and dynamic visualization. The molding machine used was a two-color molding machine (FE80S5ASED, Nisei Plastic Industrial) and the resins used were GPPS679 (PS Japan). Red pigment is used to color the resin. The content ratio of this red pigment was set at 1.5 wt%. No difference is seen in the viscosity curves of the colored and transparent resins, indicating that the two resins have more or less the same properties. Figure 3 shows the cavity shapes used. Experiments were conducted using a rectangular cavity with a length of 95 mm at various conditions where the aspect ratio [thickness t/width W (mm)] of the cross section was varied [t2/W40], [t5/W40], [t5/W20], [t5/W10], and [t5/W5]. We also investigated a circular cavity ([phi]5) and a semicircular cavity (R2.5) to compare with a rectangular cavity. Table 1 shows the molding conditions. In the dynamic visualization experiments, resin flow behavior was recorded on video using a glass-insert mold and high-speed video camera HSV-500 (NAC IMAGE TECHNOLOGY). The injection time of the resin under these molding conditions was about 1 s. Given that the time taken for the rotary runner exchanger to obstruct resin flow during switching operations is about 15 ms, the effects of switching on How behavior are considered small.

[FIGURE 3 OMITTED]
TABLE 1. Molding conditions for multistep switching mode: Rectangular
cavity [t2:W40].

Molding condition No.                     RC-1

Inj. rate ([cm.sup.3]/s)
Inj. pattern
Inj. volume (cm)          7.4 [right arrow]0.5[right arrow]6.4
Resin temp. ([degrees]C)
Mold temp. ([degrees]C)   50 (Stationary and movable side mold)

Molding condition No.                     RC-2

Inj. rate ([cm.sup.3]/s)                  15.3
Inj. pattern                 A[right arrow]B[right arrow]A
Inj. volume (cm)          7.4[right arrow]0.5[right arrow]6.9
                          7.4[right arrow]0.5[right arrow]6.4
                          7.4[right arrow]0.5[right arrow]5.8
                          7.4[right arrow]0.5[right arrow]4.7
                          7.4[right arrow]0.5[right arrow]3.7
Resin temp. ([degrees]C)                  200
Mold temp. ([degrees]C)  50 (Stationary and movable side mold)

Molding condition No.                     RC-3

Inj. rate ([cm.sup.3]/s)
Inj. pattern
Inj. volume (cm)          7.2[right arrow]0.5[right arrow]6.6
                          7.7[right arrow]0.5[right arrow]6.1
                          8.0[right arrow]0.5[right arrow]5.8
                          8.5[right arrow]0.5[right arrow]5.3
Resin temp. ([degrees]C)
Mold temp. ([degrees]C)   50 (Stationary and movable side mold)

A Transparent material; B Colored material.


RESULTS AND DISCUSSION

Flow Behavior of Colored Resins in Rectangular Cavity

Figure 4 shows the short-shot sample undergoing fountain flow after the resin flowing into the cavity from the gate reaches the flow front at the molding conditions given in Table 1-RC-l. In the figure, colored resins near the center of the sample (III) are found to be trapped inside the skin layer. This is thought that the colored resins reached the front and became fountain flow at this position. On the other hand, at both sides of the cavity, the colored resins are seen to be stretched far out in a horn shape(s), most reaching the flow front. These colored resins are the resins injected inside the cavity at the first switching, as indicated by the dotted line in Fig. 4. The horn shape(s) indicates that resins at the beginning of injection always reach near the flow front.

[FIGURE 4 OMITTED]

Figure 5 shows the cross sections at positions (I) to (IV) of Fig. 4, and Fig. 6 shows the enlargement of the dotted line areas of Fig. 5. Figure 5 shows that near the flow front, the colored resins flow along both sides of the cavity as shown at (I) to form a circular cross section. At (II) and (III), the colored resins surround the outer circumference of the sample in the shape of a closed curve along the skin layer. In Fig. 6, colored resins can be seen in one layer at (II) and in two layers (inner and outer) at (III). At (IV), colored resins remain only in the inner layer and form the shape of a closed curve. As in (g) and (h), during the integration process (II) of the double-layer structure at (III), the sides of (II) were also found to close in the shape of a bag. Next, Fig. 7 shows the cross section of the p-p' section obtained by cutting the sample in Fig. 4 along the flow direction. A Z-shaped flow pattern [1] can be clearly seen near the skin layer as the trail of the colored resins forming fountain flow at (III). This is because of the dragging of the colored resins toward the flow direction by shear flow. Similar to position u, the colored resins are stretched into a horn shape. The "u" position corresponded to the thin band-like colored area extending from (III) to (II) in the direction of the cavity width. Figure 8 shows the cross section of the sample cut along the center of the cavity thickness (top half of the cavity). This figure shows the cross section of the horn-shaped portion protruding out until near the flow front. At (III), a Z-shaped flow can again be observed. However, the area stretched out by shear flow that follows it forms the s portion, which is the pattern corresponding to the u portion of Fig. 7. Consequently, the shear flow area, which distributes at a low speed to the skin layer after fountain flow, is stretched out in the flow direction in a horn shape in the inner layer. However, the stretched out distance is considerably long at the cavity side, always reaching up to the flow front.

[FIGURE 5 OMITTED]

[FIGURE 6 OMITTED]

[FIGURE 7 OMITTED]

[FIGURE 8 OMITTED]

Analysis of Resin Flow Behavior at Both Sides of Cavity

In the previous section, we used only one switching time of the rotary runner exchange system and one injection volume. We also reviewed the changes in the shape of the colored resins according to the differences in molding conditions. As shown in Table 1-RC-2, we observed in detail the flow behavior of colored resins near the sides by changing the injection volume while maintaining the switching timing from transparent resins to colored resins at a constant rate. Figure 9 shows the results, which confirmed that the colored resins near the sides of the cavity are stretched out as flow front progresses. Next, we observed changes in the distribution of colored resins according to the differences in switching timing at the conditions shown in Table 1-RC-3. The sample was cut in the direction vertical to the flow direction 63.5 mm from the gate, and the colored resin areas were obtained by image processing of the cross section photographs. Figure 10 shows the areas displayed overlapping with each other. From this figure, it could be confirmed that flux is slow near the cavity sides, there exists an area with prolonged time flow, and resins flowing into the cavity during the initial stage of filling are stretched out and distributed. At the cavity sides, old resins that were filled first start to accumulate concentrically at the boundaries between the core layer in the center and skin layer, and these resins are thought to be stretched out in the flow direction by shear flow.

[FIGURE 9 OMITTED]

[FIGURE 10 OMITTED]

The reason that colored resins in the side-edge flow area are capable of flowing over a long time compared with the center of the cavity may be attributed to the difference in cooling effects. These colored resins flow through the shear flow area near the cavity wall, and the cooling effect of the heat conducted to the cavity wall lowers the resin temperature, causing the flow to stop eventually. Along the cavity thickness, shear flow distributes from the cavity wall to shallow areas, facilitating cooling. Thus, resins stop flowing in a short time. On the other hand, along the width at the center of the cavity thickness, the actual cavity thickness is equivalent to the cavity width. In this area, because resin velocity gradient near the wall is moderate, as shown by the s portion in Fig. 8, the colored resins in the shear flow area extend to the area at a distance from the cavity wall. The cooling effects from the cavity side were relatively small compared with the thickness at the cavity center, and the areas on both sides of the center are thought to remain as melt flow areas until the end. Consequently, the colored resins distributing to the sides of the cavity are able to continue shear flow for a comparatively long period time compared with the center area, forming a horn pattern along the sides. This may be due to the influence of shear heating effects near the cavity side walls along the cavity width direction. The ear flow phenomenon already reported in previous papers [10-12] is thought to be due to such shear heating effects. However, shear heat has strong influence under high-injection rate conditions, but its effects are thought to be small under low-injection rate conditions, like the ones applied in this experiment.

Effects of Cavity Cross Section

In the previous section, the causes of side-edge flow at [t2/W40] were discussed. This section reviews the effects of the aspect ratio [t/W] of cavity cross sections on the generation of side-edge flow. The side-edge flow was observed at different widths under the conditions shown in Table 2 at a constant thickness (t = 5 mm). Figure 11 shows cross section photos of sample side areas corresponding to Fig. 4(I-III). Areas where the same flow was seen were found to transform gradually into corner areas stretched out in the direction of the thickness with decreasing width. When the aspect ratio was 1, these areas were found to divide completely into four corners. Figure 12 shows the results of analyzing the dynamic behavior of side-edge flow. The figure shows the changes with time in the distance between the flow front and the side-edge flow ([DELTA]X) and the distance between the side-edge flow and center of the cavity ([DELTA]Y). [DELTA]X indicates an increase of about 2 mm at each condition. Compared with this, [DELTA]Y shows a large change, increasing over 6 mm with the progress of flow. This confirms that the side-edge flow advances while maintaining a constant distance with the flow front.

[FIGURE 11 OMITTED]

[FIGURE 12 OMITTED]
TABLE 2. Molding conditions for multistep switching mode: Rectangular
cavity [t5:W5, 10, 20, 40], circular shape[[PHI]5], semicircular
shape [R 2.5].

Cavity thickness[t]:width[W]                 t5:W40
(mm)

Inj. rate ([cm.sup.3]/s)                       38.2
Inj. Pattern
Inj. volume ([cm.sup.3])        10.6[right arrow]0.8[right arrow]14.5
Resin temp. ([degrees]C)
Mold temp. ([degrees]C)

Cavity thickness[t]:width[W]                 t5:W20
(mm)

Inj. rate ([cm.sup.3]/s)                       19.1
Inj. Pattern
Inj. volume ([cm.sup.3])        8.8[right arrow]0.5[right arrow]10
Resin temp. ([degrees]C)
Mold temp. ([degrees]C)

Cavity thickness[t]:width[W]                 t5:W10
(mm)

Inj. rate ([cm.sup.3]/s)                        9.6
Inj. Pattern                        A[right arrow]B[right arrow]A
Inj. volume ([cm.sup.3])        7.6[right arrow]0.3[right arrow]6.5
Resin temp. ([degrees]C)                        200
Mold temp. ([degrees]C)        50 (Stationary and movable side mold)

Cavity thickness[t]:width[W]                   t:W5
(mm)

Inj. rate ([cm.sup.3]/s)                        4.8
Inj. Pattern                        A[right arrow]B[right arrow]A
Inj. volume ([cm.sup.3])        6.9[right arrow]0.2[right arrow]2.5
Resin temp. ([degrees]C)                        200
Mold temp. ([degrees]C)         50 (Stationary and movable side mold)

Cavity thickness[t]:width[W]                 [PHI] 5
(mm)

Inj. rate ([cm.sup.3]/s)                        3.8
Inj. Pattern
Inj. volume ([cm.sup.3])        6.6[right arrow]0.2[right arrow]2.9
Resin temp. ([degrees]C)
Mold temp. ([degrees]C)

Cavity thickness[t]:width[W]                   R2.5
(mm)

Inj. rate ([cm.sup.3]/s)                        1.9
Inj. Pattern
Inj. volume ([cm.sup.3])        6.3[right arrow]0.2[right arrow]2.7
Resin temp. ([degrees]C)
Mold temp. ([degrees]C)

A Transparent material; B Colored material.


Next, the side-edge flow was found to generate in the same way at the circular cross section of [phi]5 and semicircular cross section of R2.5, both of which have a constant distance from the center to the cavity wall. Figure 13 shows photos of the cross sections of the respective molded samples. From Fig. 13a, no side-edge flow can be seen at the circular cross section, and the shear flow area was found to be stretched uniformly near the skin layer. In contrast to this, side-edge flow can be seen at the corners of both edges of the semicircular cross section.

[FIGURE 13 OMITTED]

It should be noted that a side gate is used in rectangular cavities and such side-edge flow is considered to be a phenomenon occurring at the side gate. Experiments were therefore conducted under the same conditions at the fan gate, and the same side-edge flow was found to occur as that seen with the side gate. This indicated that the phenomenon of side-edge flow was not caused by the shape of the gate. These results thus clarify that the area of side-edge flow generated also varied according to the changes in the cavity cross section, and that side-edge flow was not generated with circular cross sections.

Side-Edge Flow Model

Based on the results described in the earlier sections, the generation of side-edge flow is thought to be caused by the growth of the solidified layer and nonflow layer by cooling from the cavity wall for various types of cross sections, as well as the change in the shape of the residual melt flow region around the center of the cross section that accompanies this growth. Since the cooling speed from the cavity wall is uniform, the solidified layer and nonflow layer are thought to grow uniformly from the cavity wall. For this reason, in rectangular cross sections, where the width is longer than the thickness, the shape of the residual melt flow region around the center of the cross section changes from rectangular to ellipse. Such shape changes of the residual melt flow areas during the flow process causes the formation of a region which stops flowing and region which continues flowing. In rectangular cross sections, as a result of flow history of regions, where shear flow selectively continues over a long time, a horn-shaped side-edge flow is formed near both sides of the cavity. In square cross sections, where thickness and width are the same, the shape of the residual melt flow region during the flow process is thought to change from square to a circle-like shape. As the resins distributing at the four corners of the cavity section continue shear flow for a long time, side-edge flow is thought to generate near the four comers. On the other hand, in circular cavities, as growth of the layer solidified by cooling and nonflow layer from the cavity wall is uniform, the residual flow region in the cross section is constantly round. For this reason, side-edge flow is not seen in circular cavities. In addition, at the side gate, a large area of resins accumulating is also formed at the left and right comers of the gate. Colored resins distributing to these areas are considered to flow into the cavity in early stages of filling, then are gradually pulled out to the downstream side by shear flow as flow progresses, and are continuously supplied to the side-edge flow area. Consequently, side-edge flow can be observed more clearly at side gates with large areas of resin accumulation at the left and right corners of the gate.

Figure 14 shows the flow model of the general edge flow taking the above into consideration. The G area in the figure indicates the skin layer formed by colored resins, and the H area indicates the core layer flow. The side-edge model when the cavity width is greater than the thickness is represented in (a), and (b) is for a square cross section; (i) shows the shape of the colored resins immediately after the fountain flow, and (ii) shows these resins after filling progressed from the (i) state. For the rectangular cross section (a), the colored resins at the cavity side do not form a horn shape at (i) where the G portion of the colored resins protrudes out uniformly. At (ii), where flow has advanced, the colored resins in the shear flow area near the skin layer gradually advances forward, but due to the uniform growth of the solidified layer and nonflow layer by cooling from the cavity wall, the colored resins S2 stop flowing from an early stage near the cavity wall. Compared with this, colored resins S1' flowing in the inner layer away from the cavity wall are able to continue shear flow over a longer period, showing flow behavior that protrudes into a horn shape. At the side gate, because colored resins continue to stretch out constantly from the large area of resins accumulated at the cavity corner at the upstream E, the horn-shape phenomenon can be observed clearly and continuously for a long time. For the square cross section (b), in the (i) state, colored resin region protruding out in a horn shape is not formed. At (ii), where flow has advanced, the area continuing shear flow for a long time divides into four corners as the residual melt flow region changes its shape during flow. Consequently, the colored resins S3' distributing to the four corners protrude to form a horn shape.

[FIGURE 14 OMITTED]

CONCLUSIONS

Using the multistep switching mode of the rotary runner exchange system, the resin flow behavior of the side-edge flow of rectangular, circular, and semicircular cavities was visualized, and the flow process, generation mechanism, and the effects of changing the aspect ratio of the cavity width and thickness were reviewed.

As a result of carrying out visualization of resin flow behavior at both sides of the rectangular cavity, resins in the initial stage of filling were found to stretch out in a horn shape along both sides of the cavity and most reach the flow front, confirming the unique side-edge flow. It was found that the side-edge flow is formed by resins flowing into the cavity at the initial stage of filling the closer it is to the center of the cavity. It was clarified that resins filled at the beginning along the cavity side flow over a long distance, taking a long time. Side-edge flow is stretched in the thickness direction with decreasing width and divides into corners. For square cross sections, where the aspect ratio is 1, the side-edge flow was seen to divide completely into four corners. In the case of a circular cross section with uniform distance from the center of the cross section to the cavity wall, uneven flow protruding out into a horn shape was not seen.

For all the rectangular cavity cross section shapes discussed, a solidified layer and nonflow layer grow uniformly from the cavity wail during the flow process, and this causes the shape of the residual melt flow region around the center of the cross section to change during the How process. As a result of flow history of regions, where shear flow selectively continues over a long time, a horn-shaped side-edge flow is formed near both sides of the cavity or its four corners. At the side gate and other gate shapes, because colored resins from the initial stage of filling are continuously supplied from the area of accumulated resins generated at the corners of the cavity at the gate, side-edge flow could be observed even more clearly than in other areas.

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Yoshinori Kanetoh, Hidetoshi Yokoi

Institute of Industrial Science, The University of Tokyo, Meguro-ku, Tokyo 153-8505, Japan

Correspondence to: Y. Kanetoh; e-mail: ykaneto@iis.u-tokyo.ac.jp

DOI 10.1002/pen.21880

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

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Author:Kanetoh, Yoshinori; Yokoi, Hidetoshi
Publication:Polymer Engineering and Science
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Geographic Code:9JAPA
Date:Apr 1, 2011
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