底座模具設(shè)計(jì) 80×100×30,底座模具設(shè)計(jì),80×,100×,30,底座,模具設(shè)計(jì),80,30 form 7 November Fig. 1. Water-assisted injection molding can produce parts incorporating both thick and thin sections with less shrink- involved. Water may also corrode the steel mold, and some materials including thermoplastic composites are dicult to mold successfully. The removal of water after molding is also a challenge for this novel technology. Table 1 lists the advantages and limitations of water-assisted injection molding technology. * Corresponding author. Address: 259, Wen-Hwa 1st Road, Kwei-San, Tao-Yuan 333, Taiwan. E-mail address: shihjungmail.cgu.edu.tw (S.-J. Liu). Composites Science and Technology COMPOSITES 1. Introduction Water-assisted injection molding technology 1 has proved itself a breakthrough in the manufacture of plastic parts due to its light weight, faster cycle time, and relatively lower resin cost per part. In the water-assisted injection molding process, the mold cavity is partially filled with the polymer melt followed by the injection of water into the core of the polymer melt. A schematic diagram of the water-assisted injection molding process is illustrated in age and warpage and with a better surface finish, but with a shorter cycle time. The water-assisted injection molding process can also enable greater freedom of design, material savings, weight reduction, and cost savings in terms of tool- ing and press capacity requirements 24. Typical applica- tions include rods and tubes, and large sheet-like structural parts with a built-in water channel network. On the other hand, despite the advantages associated with the process, the molding window and process control are more critical and dicult since additional processing parameters are Abstract The purpose of this report was to experimentally study the water-assisted injection molding process of poly-butylene-terephthalate (PBT) composites. Experiments were carried out on an 80-ton injection-molding machine equipped with a lab scale water injection sys- tem, which included a water pump, a pressure accumulator, a water injection pin, a water tank equipped with a temperature regulator, and a control circuit. The materials included virgin PBT and a 15% glass fiber filled PBT composite, and a plate cavity with a rib across center was used. Various processing variables were examined in terms of their influence on the length of water penetration in molded parts, and mechanical property tests were performed on these parts. X-ray diraction (XRD) was also used to identify the material and structural parameters. Finally, a comparison was made between water-assisted and gas-assisted injection molded parts. It was found that the melt fill pressure, melt temperature, and short shot size were the dominant parameters aecting water penetration behavior. Material at the mold-side exhibited a higher degree of crystallinity than that at the water-side. Parts molded by gas also showed a higher degree of crystallinity than those molded by water. Furthermore, the glass fibers near the surface of molded parts were found to be ori- ented mostly in the flow direction, but oriented substantially more perpendicular to the flow direction with increasing distance from the skin surface. C211 2006 Elsevier Ltd. All rights reserved. Keywords: Water assisted injection molding; Glass fiber reinforced poly-butylene-terephthalate (PBT) composites; Processing parameters; B. Mechanical properties; Crystallinity; A. Polymer matrix composites; Processing An experimental study of the water-assisted glass fiber filled poly-butylene-terephthalate Shih-Jung Liu * , Ming-Jen Polymer Rheology and Processing Lab, Department of Mechanical Received 12 September 2005; received in revised Available online 0266-3538/$ - see front matter C211 2006 Elsevier Ltd. All rights reserved. doi:10.1016/pscitech.2006.09.016 injection molding of (PBT) composites Lin, Yi-Chuan Wu Engineering, Chang Gung University, Tao-Yuan 333, Taiwan 29 June 2006; accepted 11 September 2006 2006 67 (2007) 14151424 SCIENCE AND TECHNOLOGY Table 2 A comparison of water and gas-assisted injection molding Water Gas 1. Cycle time Short Long 2. Medium cost Low High 3. Internal foaming No Yes 4. Residual wall thickness Small Large 5. Outside surface roughness Low High 6. Outside surface gloss High Low 1416 S.-J. Liu et al. / Composites Science and Technology 67 (2007) 14151424 Water assisted injection molding has advantages over its better known competitor process, gas assisted injection molding 5, because it incorporates a shorter cycle time to successfully mold a part due to the higher cooling capac- Fig. 1. Schematic diagram of water-assisted injection molding process. ity of water during the molding process. The incompress- ibility, low cost, and ease of recycling the water makes it an ideal medium for the process. Since water does not dis- solve and diuse into the polymer melts during the molding process, the internal foaming phenomenon 6 that usually occurs in gas-assisted injection molded parts can be elimi- nated. In addition, water assisted injection molding pro- vides a better capability of molding larger parts with a small residual wall thickness. Table 2 lists a comparison of water and gas assisted injection molding. With increasing demands for materials with improved performance, which may be characterized by the criteria of lower weight, higher strength, and a faster and cheaper production cycle time, the engineering of plastics is a pro- cess that cannot be ignored. These plastics include thermo- plastic and thermoset polymers. In general, thermoplastic polymers have an advantage over thermoset polymers in Table 1 Advantages and disadvantages of water-assisted injection molding Advantages Disadvanta 1. Short cycle time 2. Low assisting medium cost (water is much cheaper and can be easily recycled) 3. No internal foaming phenomenon in molded parts 1. 2. 3. 4. terms of higher impact strength, fracture resistance and strains-to-failure. This makes thermoplastic polymers very popular materials in structural applications. Poly-butylene-terephthalate (PBT) is one of the most frequently used engineering thermoplastic materials, which is formed by polymerizing 1.4 butylene glycol and DMT together. Fiber-reinforced composite materials have been adapted to improve the mechanical properties of neat plas- tic materials. Today, short glass fiber reinforced PBT is widely used in electronic, communication and automobile applications. Therefore, the investigation of the processing of fiber-reinforced PBT is becoming increasingly important 710. This report was made to experimentally study the water- assisted injection molding process of poly-butylene-tere- phthalate (PBT) materials. Experiments were carried out on an 80-ton injection-molding machine equipped with a lab scale water injection system, which included a water pump, a pressure accumulator, a water injection pin, a water tank equipped with a temperature regulator, and a control circuit. The materials included a virgin PBT and a 15% glass fiber filled PBT composite, and a plate cavity 7. Fingering Greater Less 8. Asymmetrical penetration More stable Unstable 9. Material crystallinity Low High 10. Part transparency High Low 11. Internal surface (semi-crystalline materials) Smooth Less smooth 12. Internal surface (amorphous materials) Rough Smooth with a rib across center was used. Various processing vari- ables were examined in terms of their influence on the length of water penetration in molded parts, which included melt temperature, mold temperature, melt filling speed, short-shot size, water pressure, water temperature, water hold and water injection delay time. Mechanical property tests were also performed on these molded parts, and XRD was used to identify the material and structural ges Corrosion of the steel mold due to water Larger orifices for the injection pin required (easier to get stuck by the polymer melt) Some materials are more dicult to mold (especially amorphous thermoplastics) Removal of water after molding is required parameters. Finally, a comparison was made between water-assisted and gas-assisted injection molded parts. 2. Experimental procedure 2.1. Materials The materials used included a virgin PBT (Grade 1111FB, Nan-Ya Plastic, Taiwan) and a 15% glass fiber filled PBT composite (Grade 1210G3, Nan-Ya Plastic, Tai- wan). Table 3 lists the characteristics of the composite materials. polymer melt. Table 4 lists these processing variables as well as the values used in the experiments. 2.4. Gas injection unit In order to make a comparison of water and gas-assisted injection molded parts, a commercially available gas injec- tion unit (Gas Injection PPC-1000) was used for the gas- assisted injection molding experiments. Details of the gas injection unit setup can be found in the Refs. 1115. The processing conditions used for gas-assisted injection molding were the same as that of water-assisted injection molding (terms in bold in Table 4), with the exception of gas temperature which was set at 25 C176C. 2.5. XRD In order to analyze the crystal structure within the water-assisted injection-molded parts, wide-angle X-ray diraction (XRD) with 2D detector analyses in transmis- sion mode were performed with Cu Ka radiation at 40 kV and 40 mA. More specifically, the measurements were performed on the mold-side and water-side layers of the water-assisted injection-molded parts, with the 2h angle ranging from 7C176 to 40C176. The samples required for these analyses were taken from the center portion of these Fig. 2. Layout and dimensions of mold cavity (unit: mm). S.-J. Liu et al. / Composites Science and 2.2. Water injection unit A lab scale water injection unit, which included a water pump, a pressure accumulator, a water injection pin, a water tank equipped with a temperature regulator, and a control circuit, was used for all experiments 3. An ori- fice-type water injection pin with two orifices (0.3 mm in diameter) on the sides was used to mold the parts. During the experiments, the control circuit of the water injection unit received a signal from the molding machine and con- trolled the time and pressure of the injected water. Before injection into the mold cavity, the water was stored in a tank with a temperature regulator for 30 min to sustain an isothermal water temperature. 2.3. Molding machine and molds Water-assisted injection molding experiments were con- ducted on an 80-ton conventional injection-molding machine with a highest injection rate of 109 cm 3 /s. A plate cavity with a trapezoidal water channel across the center was used in this study. Fig. 2 shows the dimensions of the cavity. The temperature of the mold was regulated by a water-circulating mold temperature control unit. Various processing variables were examined in terms of their influ- ence on the length of water penetration in water channels of molded parts: melt temperature, mold temperature, melt fill pressure, water temperature and pressure, water injec- tion delay time and hold time, and short shot size of the Table 3 Characteristics of the glassfiber reinforced PBT composite Property ASTM PBT 15% G.F. PBT Yield strength (kg/cm 2 ) D-638 600 1000 Bending stress (kg/cm 2 ) D-570 900 1500 Hardness (R-scale) D-785 119 120 Heat distortion temperature (C176C) (18.6 kg/cm 2 ) D-648 60 200 Melt flow index (MFI) D- 1238 40 25 Impact strength (Kg-cm/cm) D-256 5 5 Melting temperature (C176C) DSC 224 224 Technology 67 (2007) 14151424 1417 molded parts. To obtain the desired thickness for the XRD samples, the excess was removed by polishing the samples on a rotating wheel on a rotating wheel, first with wet silicon carbide papers, then with 300-grade silicon car- bide paper, followed by 600- and 1200-grade paper for a better surface smoothness. 2.6. Mechanical properties Tensile strength and bending strength were measured on a tensile tester. Tensile tests were performed on specimens obtained from the water-assisted injection molded parts (see Fig. 3) to evaluate the eect of water temperature on 20 mm 10 mm 1 mm. Bending tests were performed in a micro tensile tester according to the ASTM D256 test. A 200 N load cell was used and the crosshead speed was 50 mm/min. 2.7. Microscopic observation The fiber orientation in molded specimens was observed under a scanning electron microscope (Jeol Model 5410). Specimens for observation were cut from parts molded by water-assisted injection molding across the thickness Table 4 The processing variables as well as the values used in the experiments AB CD E F Melt pressure (Mpa) Melt temperature (C176C) Short shot size (%) Water pressure (Mpa) Water temperature (C176C) Mold temperature (C176C) 140 280 (270) 76 8 80 80 126 275 (265) 77 9 75 75 114 270 (260) 78 10 70 70 98 265 (255) 80 11 65 65 84 260 (250) 81 12 60 60 * The values in the parentheses are the melt temperatures used for virgin PBT materials. 1418 S.-J. Liu et al. / Composites Science and Technology 67 (2007) 14151424 the tensile properties. The dimensions of specimens for the experiments were 30 mm 10 mm 1 mm. Tensile tests were performed in a LLOYD tensiometer according to the ASTM D638M test. A 2.5 kN load cell was used and the crosshead speed was 50 mm/min. Bending tests were also performed at room tempera- ture on water-assisted injection molded parts. The bend- ing specimens were obtained with a die cutter from parts (Fig. 3) subjected to various water temperatures. The dimensions of the specimens were Fig. 3. Schematically, the positioning of the samples cut from the molded (Fig. 3). They were observed on the cross-section perpen- dicular to the flow direction. All specimen surfaces were gold sputtered before observation. 3. Results and discussion All experiments were conducted on an 80-ton conven- tional injection-molding machine, with a highest injection rate of 109 cm 3 /s. A plate cavity with a trapezoidal water channel across the center was used for all experiments. parts for tensile and bending tests and microscopic observations. 3.1. Fingerings in molded parts All molded parts exhibited the water fingering phenom- enon at the channel to plate transition areas. In addition, molded glass fiber filled composites showed more severe water fingerings than those of non-filled materials, as shown photographically in Fig. 4. Fingerings usually form when a less dense, less viscous fluid penetrates a denser, more viscous fluid immiscible with it. Consider a sharp two phase interface or zone where density and viscosity change rapidly. The pressure force (P 2 C0P 1 ) on the dis- placed fluid as a result of a virtual displacement dx of the interface can be described by 16, dP P 2 C0 P 1 l 1 C0 l 2 U=KC138dx 1 where U is the characteristic velocity and K is the perme- ability. If the net pressure force is positive, then any small displacement will be amplified and lead to an instability and part fingerings. For the displacement of a dense, vis- cous fluid (the polymer melt) by a lighter, less viscous one (water), we can have Dl = l 1 C0l 2 0, and U 016. In this case, instability and the relevant fingering result when a more viscous fluid is displaced by a less viscous one, since the less viscous fluid has the greater mobility. The results in this study suggest that glass fiber filled com- posites exhibit a higher tendency for part fingerings. This might be due to the fact that the viscosity dierence Dl be- tween water and the filled composites is larger than the dif- ference between water and the non-filled materials. Water- assisted injection molded composites thus exhibit more se- vere part fingerings. S.-J. Liu et al. / Composites Science and Technology 67 (2007) 14151424 1419 Fig. 4. Photograph of water-assisted injection molded PBT composite part. 3.2. Eects of processing parameters on water penetration Various processing variables were studied in terms of their influence on the water penetration behavior. Table 4 lists these processing variables as well as the values used in the experiments. To mold the parts, one central process- ing condition was chosen as a reference (bold term in Table 4). By changing one of the parameters in each test, we were able to better understand the eect of each parameter on the water penetration behavior of water assisted injection molded composites. After molding, the length of water penetration was measured. Figs. 510 show the eects of these processing parameters on the length of water penetra- tion in molded parts, including melt fill pressure, melt tem- perature, mold temperature, short shot size, water temperature, and water pressure. The experimental results in this study suggest that water penetrates further in virgin PBT than in glass fiber filled PBT composites. This is due to the fact that with the rein- forcing glass fibers the composite materials have less volu- metric shrinkage during the cooling process. Therefore, they mold parts with a shorter water penetration length. 84 98 112 126 140 10 12 14 16 18 20 PBT PBT+15% G.F. Melt fill pressure (MPa) Length of penetration (cm) Fig. 5. Eects of melt fill pressure on the length of water penetration in molded parts. and 20 PBT PBT+15% G.F. 1420 S.-J. Liu et al. / Composites Science The length of water penetration decreases with the melt fill pressure (Fig. 5). This can be explained by the fact that increasing the melt fill pressure increases the flow resistance inside the mold cavity. It is then more dicult for the water 250 255 260 265 270 275 280 8 10 12 14 16 18 Length of penetration (cm) Melt temperature (C) Fig. 6. Eects of melt temperature on the length of water penetration in molded parts. 60 65 70 75 80 10 12 14 16 18 20 PBT PBT+15% G.F. Mold temperature (C) Length of penetration (cm) Fig. 7. Eects of mold temperature on the length of water penetration in molded parts. 76 77 78 79 80 81 10 12 14 16 18 20 PBT PBT+15% G.F. Short shot size (%) Length of penetration (cm) Fig. 8. Eects of short shot size on the length of water penetration in molded parts. 20 PBT PBT+15% G.F. 60 65 70 75 80 10 12 14 16 18 20 PBT PBT+15% G.F. Water temperature (C) Length of penetration (cm) Fig. 9. Eects of water temperature on the length of water penetration in molded parts. Technology 67 (2007) 14151424 to penetrate into the core of the materials. The length of water penetration decreases accordingly 3. The melt temperature was also found to reduce the water penetration in molded PBT composite parts (Fig. 6). This might be due to the fact that increasing the melt temperature decreases viscosity of the polymer melt. A lower viscosity of the materials helps the water to pack the water channel and increase its void area, instead of penetrating further into the parts 4. The hollow core ratio at the beginning of the water channel increases and the length of water penetration may thus decrease. Increasing the mold temperature decreases somewhat the length of water penetration in molded parts (Fig. 7). This is due to the fact that increasing the mold temperature decreases the cooling rate as well as the viscosity of the materials. The water then packs the channel and increases its void area near the beginning of the water channel, instead of penetrating further into the parts 3. Molded parts thus have a shorter water penetration length. Increasing the short shot size decreases the length of water penetration (Fig. 8). In water-assisted injection molding, the mold cavity is partially filled with the polymer 8 9 10 11 12 10 12 14 16 18 Water pressure (MPa) Length of penetration (cm) Fig. 10. Eects of water pressure on the length of water penetration in molded parts. melt followed by the injection of water into the core of the polymer melt 4. Increasing the short shot size of the poly- mer melt will therefore decrease the length of water pene- tration in molded parts. For the processing parameters used in the experiments, increasing the water temperature (Fig. 9) or the water pres- sure (Fig. 10) increases the