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Effects of SCF Content, Injection Speed, and CF Content on the Morphology and Tensile Properties of Microcellular Injection-Molded CF/PP Composites.

INTRODUCTION

In recent years, reduction of C[O.sub.2] emission via improved efficiency has become imperative in the automotive industry. One of the effective approaches for enhancing efficiency is to lighten the weight of automotive components, such as metallic parts, high-strength steel plates for car bodies and aluminum alloy sheets for automotive body panels [1, 2]. Furthermore, metallic parts have been constructed using polymers and their composites to reduce weight [3]. This substitution reduces the use of fastener components due to integral molding of complex parts and simplifies the component design due to weight reduction [3]. The use of polymeric materials in automotive parts has increased, and the average modem car weighing 1,500 kg contains between 12% and 15% polymeric materials [4]. However, further weight reduction of automotive plastic parts is required to improve fuel efficiency. As one technique to reduce the weight of thermoplastic parts, Suh et al. developed the microcellular injection molding method in the 1980s (commercially known as the MuCell[R] process) [5]. In this process, nitrogen or carbon dioxide in a supercritical state (supercritical fluid, SCF) is used as a physical blowing agent. This method offers such advantages as low environmental impact, decreased cell diameter lower than 100 [micro]m [6], improved dimensional stability [7-9], and reduced injection pressure.

It is known that fillers such as silica, nanoclay, carbon nanotubes, and so forth are effective for obtaining uniform and fine foam structures in microcellular foaming processes. Xiang el al. investigated the effects of silica on the cellular structure and nucleation of silicone rubber composite foams prepared by a batch foaming process. The silica acted as the nucleation agent, and a silicone rubber nanofoam was obtained [10]. Hwang et al. reported that the addition of nanoclay (montmorillonite, MMT) to low-density polyethylene (LDPE) [11], polystyrene (PS) [12], polybutylene terephthalate (PBT) [13], polyamide 6 (PA6) [14], and thermoplastic olefin elastomers (TPO) [15] significantly improved the tensile strength and thermal stability, reduced the cell diameter, and increased the cell density of the products obtained using the MuCell[R] process. Wang et al. studied the microcellular structure and mechanical properties of thermoplastic polyurethane (TPU)/organoclay composite foams prepared by the MuCell[R] process. According to the results, the organoclay acted as an effective nucleation agent and increased the cell nucleation rate and cell density in TPU/organoclay nanocomposite foams. In addition, the tensile strength also increased with the increase in organoclay content [16]. Li et al. reported on multiwalled carbon nanotubes (MWCNTs), MMT and talc blended with polyetherimide (PEI) as a nucleation agent, and samples were subsequently prepared by microcellular injection molding. The results indicated that MWCNTs mostly functioned as a nucleation agent to improve the cellular structure and the mechanical properties of the composites [17]. Thus, the fillers affect not only the mechanical properties but also the cellular structure. Wang et al. prepared carbon fiber (CF) and polypropylene (PP) composite foams by batch foaming and revealed that the cell size decreased and the cell density increased with the addition of CFs [18]. In addition, carbon fillers may possibly confer conductivity. Electrically conductive thermoplastics have been used in such applications as electrostatic dissipation and electromagnetic interference (EMI) shielding [19-24]. Ameli et al. used PP as a matrix and CF as a conductive additives, and the electrical and EMI shielding properties of CF/PP composite foams were studied. The through-plane conductivity was enhanced due to foaming because the foaming changed the fiber orientation and decreased the skin layer thickness [25]. Furthermore, the specific EMI shielding effectiveness (SE) increased with foaming as a consequence of reducing the volume fraction of the percolation threshold [26]. Additionally, most analysis of the cellular structure of foams has been performed by scanning electron microscopy (SEM) for observation of fracture surfaces [27]. Recently, X-ray CT scanning, a nondestructive test method, was used in the observation of the cellular structure of foams [28], Although X-ray CT scanning allows researchers to examine the three-dimensional cellular structure of foams, SEM micrographs are used to measure the cell diameter and cell density because morphological observation of foams using X-ray scanning has not been established.

In addition, it is known that the morphology of injection-molded parts is generally a three-layer structure with skin, intermediate, and core layers. The three-layer structure is strongly dependent on the molding conditions and influences the mechanical properties of injection-molded parts [29, 30]. In particular, the cellular structures of foamed samples have been observed, such as a solid skin layer, an intermediate layer with highly stretched cells, and a core layer with spherical cells. Dong et al. investigated the formation mechanism and structural characteristics of skin layers in a low-pressure injection molding process [31]. However, few works have addressed the formation mechanism of the three-layer structure and the relationship between the three-layer structure and injection molding conditions or mechanical properties. This knowledge is necessary for practical use of microcellular injection molding and for better understanding of the formation mechanism of the three-layer structure and the effect of injection molding conditions on fiber orientation, morphology, and mechanical properties.

In this study, to advance the practical use of microcellular injected parts in many fields, CF/PP composite foams were prepared by microcellular injection molding. Morphological observations were performed using X-ray CT scanning, and the layer thickness and cell diameter of the CF/PP composite foams were measured by using X-ray CT imaging to establish a morphological analysis method. We investigated the effects of SCF content, injection speed, and CF content on the morphology of CF/PP composite foams. Furthermore, the effects of CF contents and morphology on the tensile properties were evaluated, and the reinforcing effects of CF and the cellular structure of CF/PP composite foams were studied separately.

MATERIALS AND METHODS

Materials

Three commercial grades of carbon-fiber reinforced polypropylene (PYROFIL pellet, MITSUBISHI RAYON CO., LTD., Japan) were used. The material properties are listed in Table 1. PAN-based short carbon fiber with an average diameter 7 [micro]m was used as reinforcement, and N2 with a purity of 0.999 (TAIHEI YOZAI Co., Ltd., Japan) was used as the physical blowing agent.

Sample Preparation by Microcellular Injection Molding

An injection molding machine (NEX-180III-25E-MuCell, NISSEI PLASTIC INDUSTRIAL CO., LTD, Japan) equipped with a SCF delivery system (SII-TRJ-10A, Trexel, Inc) was used in the preparation of CF/PP foams. The mold contained a single side gate, as shown in Fig. 1. Microcellular injection molding with a low pressure (short shot) process was used. The weight reduction of foams was approximately 10% compare with the solids weight. Dumbbell and rectangle specimens were injection-molded from CF/PP pellets. The nozzle temperature was 230[degrees]C, the mold temperature was maintained at 60 [degrees]C, and the back pressures for solids and foams were 5 and 13 MPa, respectively. The SCF delivery pressure was 13.5 MPa, and the SCF content was 0, 0.5, 0.7, or 1 wt%. The injection speed was 50, 100, or 150 mm/s. The samples with different conditions and compositions prepared by microcellular injection molding are listed in Table 2.

Morphological Analysis by X-Ray CT

The radiographic sample was prepared from the central portion of a dumbbell specimen (5 mm) (gray area of Fig. 2). The X-ray CT image was obtained using a Micro CT scanner (TOSCANNER32300[mu]FD-Z, TOSHIBA IT & CONTROL SYSTEMS CO., Japan). The sample was placed on the stage of an X-ray CT system, as shown in Fig. 3, and the central portion of the sample was scanned.

The spatial resolution was 5 [micro]m, the number of projection views was 1,200, and the number of averages was 4. A binarized image of the cross-section of the flow direction (machine direction, MD) and thickness direction (TD) was examined using image analysis measurement (Win ROOF 2013, MITANI CO., Japan). The cell diameters, cell numbers, and cross-sectional areas were measured. At a spatial resolution of 5 [micro]m, the cells with diameters less than 10 [micro]m were accurately eliminated to measure the cell diameter. Three X-ray CT images of each sample were used to obtain the cell size distribution, average cell diameter, and cell density. At least 700 cells were measured from the X-ray CT image of each cross-section. The cell density was calculated as the cell number per measured cross-sectional area of each cross-section. The cell size distribution, average cell diameter, and cell density were obtained from three images of the cross-section and subsequently averaged. The standard deviation was obtained from three measurements for each sample. Furthermore, the thicknesses of the skin, intermediate, and core layers were also measured using image analysis measurement. The number of measurements was more than 5, and the thickness of each layer was evaluated as the percentage share of the skin, intermediate, and core layers.

Tensile Test

The tensile test was conducted with a universal testing machine (5,967, INSTRON) based on IS0527-1. The tensile speed was 10 mm/min, and the room temperature was set at 23 [degrees]C [+ or -] 2 [degrees]C.

The Young's modulus was determined from the initial slope of the stress-strain curves with a strain range from 0.05% to 0.25%. Five specimens were tested to obtain the average values. Figure 2 shows the geometry of the tensile specimen.

RESULTS AND DISCUSSION

Effect of SCF Content on Cellular Structure

Figure 4 shows the X-ray CT images of the MD and TD at different SCF contents (CF content: 10 wt%, injection speed: 50 mm/s). The left side presents the mold surface, and the right side shows the central portion of the molded samples in each figure. As in conventional injection molding, a three-layer structure is observed with the increase in distance from the mold surface [29, 30]. The skin layer in contact with the mold is solid, the intermediate layer consists of stretched cells oriented parallel to the flow direction, and the core layer contains comparatively spherical cells. Figure 5 schematically illustrates the formation mechanism of the three-layer structure in the foams during microcellular injection molding in this study, according to reference 31. The SCF dissolves in the melting polymer and becomes oversaturated at high pressure. When the melted polymer is injected into the mold, cell nucleation occurs due to a rapid pressure drop, and cell growth begins. Dong et al. reported that the skin layer consisted of two regions during the filling process, namely, a thin frozen layer and thick solid-like layer [31], In this study, the formation mechanism of the skin layer was similar to this process. The stretched cells are pushed out to the mold wall and broken by contact with the mold wall, and the thin frozen skin layer is subsequently formed (Fig. 5a). The melt pressure increases gradually from the flow front to the gate, and the stretched cells redissolved into the polymer melt under critical pressure. Thus, the thick solid-like skin layer is formed (Fig. 5b). Subsequently, the intermediate layer is formed during the filling process. This layer has stretched cells that are oriented in the direction of flow. The stretched cells, which are not redissolved into the polymer melt because of low melt pressure, are foamed into the intermediate layer due to solidification from the outer layer (Fig. 5b). Finally, the core layer is solidified after the filling stage and contains relatively spherical cells. Because the core layer is located far from the mold wall, due to high temperature and a low effect of flow, the bubbles grow spherically and solidify (Fig. 5c). This three-layer structure was especially observed at the MD due to the high effect of melt flow (Fig. 4a-c). The stretched cells at the intermediate layer of the MD tended to decrease with the increase in the SCF content. Because the viscosity of the melt polymer decreased and the share force decreased with the increase in the SCF content, the deformation of cells was suppressed. On the other hand, as observed from X-ray CT images of the TD (Fig. 4d-f), the stretched cells at the intermediate layer were not observed due to the low impact of flow. The cell size at the intermediate layer appeared smaller than that at the core layer. Because the cell growth time of the intermediate layer was shorter than that of the core layer, the cells at the intermediate layer were spherical and small.

The cell size distribution of the MD and TD of the foams is shown in Fig. 6. The cell diameter of 20 pm was most often observed, and its frequency increased with the increase in the SCF content at the MD. Therefore, the cell size distribution became sharper with the increase in the SCF content. With the increase in the SCF content from 0.5 to 1 wt%, the average cell diameter of the MD decreased from 41 [+ or -] 4 (mean [+ or -] standard deviation, hereinafter the same) to 34 [+ or -] 3 [micro]m, and the cell density of the MD increased from 1.1 [+ or -] 0.03 x [10.sup.4] to 1.5 [+ or -] 0.09 x [10.sup.4] cell/[cm.sup.2]. In the case of the TD, while the cell diameter decreased from 33 [+ or -] 3 to 26 [+ or -] 4 pm with the increase in the SCF content from 0.5 to 1 wt%, the maximum cell density was obtained at 0.7 wt%. However, the TD had similar tendency because the cellular structure improved with the increase in the SCF content. In the case of the foamed composites, cell nucleation may occur at the interface between CF and the matrix, and the cell growth continues until solidification by cooling. The cell nucleation increased with an increase in the SCF content, and thus, the cell density increased. Therefore, the SCF content plays an important role in improving the cellular structure. In addition, the smallest cell diameter in this molding condition (20 [micro]m) was most frequently observed. Cell coalescence subsequently occurred because a cell diameter larger than 40 [micro]m was two times greater than that of the smallest cell diameter. Since previous studies have shown that the mechanical properties of foamed composites are improved by fine bubbles [12-17], this improvement of the cellular structure might affect the tensile properties.

In addition, the morphology of the short shot process indicates a three-layer structure similar to that of conventional injection molding. However, the core-back process tends to produce a more uniform morphology. Therefore, the morphology of the short shot process differs from that of the core-back process. Furthermore, it is known that the thicknesses of three layers have an effect on the mechanical properties of injection molded samples, such as the fatigue failure properties [29, 30]. In this study, we focused on the three-layer structure of the short shot process and measured the thickness of each layer. Figure 7 shows the effect of the SCF content on the thickness of each layer. From this figure, the intermediate layer thickness, which formed during the filling process, was approximately 50% of whole thickness and is the thickest of the three layers. The skin layer and core layer thickness were approximately half that of the intermediate layer. When the SCF content increased from 0.5 to 1 wt%, the intermediate layer decreased by approximately 5%, and the core and skin layer thickness subsequently increased by 2% and 3%, respectively. The thickness change ratio of the intermediate layer was larger than that of the others. The melt viscosity of polymer decreased with the increase in the SCF content, and the share force subsequently decreased due to the improvement of fluidity. Since the intermediate layer foams during filling and is strongly affected by share force, its thickness decreased due to a decrease in share force.

Effect of Injection Speed on Morphology

Figure 8 shows the X-ray CT images of the MD and TD at different injection speeds (CF content: 10 wt%, SCF content: 1 wt%). The CT images of the MD and TD at an injection speed of 50 mm/s are presented in Fig. 4c,f, respectively. A three-layer structure was observed for the foams in which the skin layer was solid, the intermediate layer consisted of stretched cells, and the core layer contained spherical cells. In addition, the cellular structure of the intermediate and core layers tended to be more uniform with an increase in the injection speed.

Figure 9 shows the results of the cell size distribution of the MD and TD of the foams. In the case of the MD, the most frequently observed cell diameters at injection speeds of 50, 100 and 150 mm/s were 20, 30, and 50 pm, respectively. Therefore, the most frequently observed cell diameter was shifted to larger sizes, and the cell size distribution became broader with increased injection speed. The average cell diameter of the MD increased from 34 [+ or -] 3 to 43 [+ or -] 4 [micro]m, and the cell density of the MD decreased from 1.4 [+ or -] 0.09 x [10.sup.4] to 1.1 [+ or -] 0.07 x [10.sup.4] cell/[cm.sup.2] with the increase in the injection speed from 50 to 150 mm/s. A similar tendency was observed at the TD. Thus, the increase in the injection speed tends to reduce the frequency of microcellular structures and increase the cell diameter due to the coalescence of cells, depending on the cell growth time. When the injection speed is high, the time from filling to solidification increases, and the cell growth time increases. Consequently, the cell size increases, and cell coalescence occurs. Furthermore, it is inferred that the shared force increased with the increase in the injection speed. Thus, cell coalescence easily occurred due to the agitation action when the melting polymer at a high temperature was injected into the mold cavity. Therefore, under the molding conditions of this study, a cellular structure with numerous fine cells was easily formed at the slower injection speed (Fig. 4f). Additionally, according to Fig. 10, when the injection speed increased from 50 to 150 mm/s, the skin layer thickness decreased by 3%. With the increase in the injection speed, the filling time of the polymer became shorter, leading to a decrease in the skin layer thickness and a promotion of heat release. In addition, the intermediate layer thickness decreased by 2%, and the core layer thickness increased by 6% with the increase in the injection speed. Since the intermediate layer was formed during the filling process, the intermediate layer thickness decreased due to a shorter filling time with increasing injection speed. Therefore, the solidification time of the core layer after the filling process became longer, and the core layer thickness subsequently increased with the increase in the injection speed.

Effect of CF Content on Morphology

Figure 11 shows the X-ray CT images of the MD and TD at different CF contents (SCF content: 1 wt%, injection speed: 50 mm/s). The CT images of the MD and TD with 10% CF are displayed in Fig. 4c,f, respectively. As previously stated, the skin layer is solid, the intermediate layer contains stretched cells, and the core layer consists of spherical cells. When the CF content increased from 10 to 30 wt%, the average cell diameter decreased from 34 [+ or -] 3 to 24 [+ or -] 3 [micro]m at the MD and from 26 [+ or -] 4 to 18 [+ or -] 5 [micro]m at the TD. Concomitantly, the cell density doubled from 1.4 [+ or -] 0.09 x [10.sup.4] to 2.8 [+ or -] 0.08 x [10.sup.4] cell/[cm.sup.2] at the MD and from 1.9 [+ or -] 0.13 x [10.sup.4] to 3.8 [+ or -] 0.15 x [10.sup.4] cell/[cm.sup.2] at the TD. Additionally, the ratio of the long diameter to the short diameter of cells, which were stretched parallel to the flow direction at the intermediate layer, decreased with the increase in the CF content. Thus, the cellular structure with numerous fine cells was easily formed with the increase in the CF content. Moreover, the results of the cell size distribution of the MD and TD in Fig. 12 indicate that the smallest cell diameter (20 [micro]m) was most frequently observed at all conditions, and the frequency of the cell diameter drastically improved from 32% to 43% at the MD and from 31% to 47% at the TD with the increase in the CF content from 10 to 30 wt%. The cell size distribution became sharper with the increase in the CF content. The formation mechanism of the microcellular structure in response to changes in the CF content is explained as follows. When the CF content increased, the interface between the matrix and CF is increased, and cell nucleation is promoted. The cell growth and cell coalescence are suppressed at the micro region due to the network formation of CF. Furthermore, the increase in the CF content decreases the volume percent of the matrix, and the SCF content relatively increases. As a result, the microcellular structure is improved.

Relationship between Morphology and Mechanical Properties

Figure 13 shows the effect of the CF content on the tensile strength and Young's modulus of solids and foams (SCF content: 1 wt%, injection speed: 50 mm/s). The tensile strength and Young's modulus greatly increased with the increase in the CF content for all samples. This improvement was mainly caused by the reinforcing effect of the CF with high mechanical strength. When the CF content increased from 10 to 30 wt%, the tensile strength improved from 101 to 140 MPa, by 39%, and the Young's modulus increased from 5.1 to 9.1 GPa, by 78% for solids. On the other hand, in the case of foams, the tensile strength improved from 78 to 111 MPa, by 42%, and the Young's modulus drastically increased from 4 to 8.5 GPa, by 113%. In this study, the short shot process was used, and the weight of the foams decreased by 10% compared with the solids. The void fractions of foams at the CF content of 10, 20, and 30 wt% were 15%, 13%, and 15%, respectively, and no significant differences were noted. Although the CF volume of foams was the same as that of solids, the ratio of strength increase for the foams was larger than that of the solids. Therefore, the improvement in tensile properties for the foams was also affected by the morphology. Particularly, the Young's modulus of the foams was improved by approximately 35% due to the improvement in the microcellular structure, as measured by subtracting the improvement ratio of the solids (78%) from that of the foams (113%). As described previously, this improvement was caused by the change in the microcellular structure of the foams with the increase in the CF content.

Previous research has suggested that the three-layer structure has a significant effect on mechanical properties [29, 30]. Figure 14 shows the effect of layer thickness on the tensile strength and Young's modulus: (a) skin layer, (b) intermediate layer and (c) core layer (CF content: 10 wt%, SCF content: 0.5, 0.7, and 1 wt%, injection speed: 50, 100 and 150 mm/s). From Fig. 14a, the changes in the tensile strength and Young's modulus were minimally affected by the skin layer thickness. Conversely, although the intermediate layer thickness had a low impact on the Young's modulus, the tensile strength tended to decrease with increases in the intermediate layer thickness (Fig. 14b). Although the Young's modulus was minimally affected by the core layer thickness, the tensile strength was improved with the increase in the core layer thickness (Fig. 14c). The thick core layer with many microcells has advantages in maintaining the tensile strength. The tensile strength is affected by the microcellular structure, rather than the three-layer structure of the cross-section of the foams. The tensile properties are only determined by cross-sectional area and that of the foams depends on the void fraction. Conversely, the solids are affected by the three-layer structure [29, 29, 30], Therefore, the material characteristics of the foams are far different from those of solids.

CONCLUSIONS

To widen the practical application of CF/PP composite foams, this study investigated the effects of molding conditions on the morphology and tensile properties of the CF/PP foams prepared by microcellular injection molding. The following conclusions were reached:

1. Similar to conventional injection molding, the CF/PP composite foams prepared by microcellular injection molding achieved a three-layer structure. The skin layer was solid, the intermediate layer contained stretched cells parallel to the flow direction, and the core layer consisted of spherical cells. The formation mechanism of the three-layer structure was affected by the melt pressure and solidification.

2. When the SCF content increased from 0.5 to 1 wt%, the average cell diameter of the MD decreased from 41 to 34 [micro]m, the cell density of the MD increased from 1.1 x [10.sup.4] to 1.4 x [10.sup.4] cell/[cm.sup.2], and the intermediate layer thickness decreased by 5% because the cell nucleation increased and the viscosity decreased with the increase in the SCF content.

3. The average cell diameter of the MD increased from 34 to 43 [micro]m when the injection speed increased from 50 to 150 mm/s because cell coalescence easily occurred due to high agitation action and longer cell growth time. Furthermore, the skin and intermediate layer thicknesses decreased by 3% and 2%, respectively, and the core layer thickness increased by 6%, with an increase in the injection speed due to a decrease in the filling time and an increase in the cell growth time.

4. As a result of the increase in cell nucleation, the network formation of CF, and the relative ratio of the SCF content/molten polymer, the microcellular structure was easily formed with the increase in the CF content. Notably, when the CF content increased from 10 to 30 wt%, the average cell diameter of the MD decreased from 34 to 25 [micro]m, and the cell density increased from 1.4 x [10.sup.4] to 2.9 x [10.sup.4] cell/[cm.sup.2].

5. When the CF content increased from 10 to 30 wt%, the Young's modulus of the solids and foams increased by 78% and 113%, respectively. Thus, the Young's modulus of the foams was improved by 35% due to the improvement in the cellular structure.

6. The difference in the three-layer structures of the foams had a low impact on the tensile properties of the CF/PP foams. The tensile properties were affected by the microcellular structure, rather than the three-layer structure of the cross-sections of the foams.

The results obtained in this study reveal potential applications for CF/PP composite foams in the manufacturing of lightweight automotive products with enhanced tensile properties.

ACKNOWLEDGMENTS

The authors thank Dr. Eiichi Sakai and Dr. Wendi Liu for helpful suggestions and comments in the early stages of this work.

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Rie Nobe [iD], (1,3) Jianhui Qiu, (2) Makoto Kudo, (3) Kazushi Ito, (2) Masaki Kaneko (1)

(1) Graduate School of Systems Science and Technology, Akita Prefectural University, Yurihonjo 015-0055, Japan

(2) Faculty of Systems Science and Technology, Akita Prefectural University, Yurihonjo 015-0055, Japan

(3) Ecological Material Development Section, Akita Industrial Technology Center, Akita 010-1623, Japan

Correspondence to: J. Qiu; e-mail: qiu@akita-pu.ac.jp

DOI 10.1002/pen.25120

Caption: FIG. 1. Cavity shape.

Caption: FIG. 2. The geometry of the tensile specimen.

Caption: FIG. 3. Schematic of X-ray CT.

Caption: FIG. 4. X-ray CT images at different SCF contents: (a) 0.5%, MD, (b) 0.7%, MD, (c) 1%, MD, (d) 0.5%, TD, (e) 0.7%, TD, and (f) 1%, TD (CF content: 10 wt%, injection speed: 50 mm/s).

Caption: FIG. 5. Schematic illustration of the formation of three-layer structure. [Color figure can be viewed at wileyonlinelibrary.com!

Caption: FIG. 6. Cell size distribution of (a) MD and (b) TD at different SCF contents (CF content: 10 wt%, injection speed: 50 mm/s).

Caption: FIG. 7. Effect of SCF contents on the thickness of each layer (CF content: 10 wt%, injection speed: 50 mm/s).

Caption: FIG. 8. X-ray CT images at different injection speeds: (a) 100 mm/s, MD, (b) 150 mm/s, MD, (c) 100 mm/s, TD, and (d) 150 mm/s, TD (CF content: 10 wt%, SCF content: 1 wt%).

Caption: FIG. 9. Cell size distribution of (a) MD and (b) TD at different injection speeds (CF content: 10 wt%, SCF content: 1 wt%).

Caption: FIG. 10. Effect of injection speeds on the thickness of each layer (CF content: 10 wt%, SCF content: 1 wt%).

Caption: FIG. 11. X-ray CT images at different CF contents: (a) CF20, MD, (b) CF30, MD, (c) CF20, TD, and (d) CF30, TD (SCF content: 1 wt%, injection speed: 50 mm/s).

Caption: FIG. 12. Cell size distribution of (a) MD and (b) TD at different CF contents (SCF content: 1 wt%, injection speed: 50 mm/s).

Caption: FIG. 13. Effect of CF contents on the tensile strength and Young's modulus (SCF content: 1 wt%, injection speed: 50 mm/s).

Caption: FIG. 14. Effect of layer thickness on tensile strength and Young's modulus: (a) skin layer, (b) intermediate layer, and (c) core layer (CF content: 10 wt%, SCF content: 0.5, 0.7, 1 wt%, injection speed: 50, 100, 150 mm/s).
TABLE 1. Material properties of carbon-fiber reinforced
polypropylene.

                                PP-C-10A   PP-C-20A   PP-C-30A

CF content (wt%)                   10         20         30
Density (g/cm)                    0.95       1.01       1.06
Melt volume rate [cm/10 min]       36         20         11
(230[degrees]C X21 N)

TABLE 2. Sample list.

Sample                   1     2     3     4     D    6      7

CF content (wt%)         10   10    10    10    10    10    10
SCF content (wt%)        0     0     0    0.5   0.7   1      1
Injection speed (mm/s)   50   100   150   50    50    50    100

Sample                     8       9      10     11     12

CF content (wt%)           10      20     20     30     30
SCF content (wt%)          1       0      1      0      1
Injection speed (mm/s)    150      50     50     50     50
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Author:Nobe, Rie; Qiu, Jianhui; Kudo, Makoto; Ito, Kazushi; Kaneko, Masaki
Publication:Polymer Engineering and Science
Article Type:Report
Geographic Code:9JAPA
Date:Jul 1, 2019
Words:5556
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