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An experimental study of the water-assisted injection molding ofglass fiber filled poly-butylene-terephthalate (PBT) compositesShih-Jung Liu*, Ming-Jen Lin, Yi-Chuan WuPolymer Rheology and Processing Lab, Department of Mechanical Engineering, Chang Gung University, Tao-Yuan 333, TaiwanReceived 12 September 2005; received in revised form 29 June 2006; accepted 11 September 2006Available online 7 November 2006AbstractThe 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 acrosscenter was used. Various processing variables were examined in terms of their influence on the length of water penetration in moldedparts, and mechanical property tests were performed on these parts. X-ray diffraction (XRD) was also used to identify the materialand structural parameters. Finally, a comparison was made between water-assisted and gas-assisted injection molded parts. It was foundthat the melt fill pressure, melt temperature, and short shot size were the dominant parameters affecting 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 higherdegree 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 theskin surface.? 2006 Elsevier Ltd. All rights reserved.Keywords: Water assisted injection molding; Glass fiber reinforced poly-butylene-terephthalate (PBT) composites; Processing parameters; B. Mechanicalproperties; Crystallinity; A. Polymer matrix composites; Processing1. IntroductionWater-assisted injection molding technology 1 hasproved itself a breakthrough in the manufacture of plasticparts due to its light weight, faster cycle time, and relativelylower resin cost per part. In the water-assisted injectionmolding process, the mold cavity is partially filled withthe polymer melt followed by the injection of water intothe core of the polymer melt. A schematic diagram of thewater-assisted injection molding process is illustrated inFig. 1. Water-assisted injection molding can produce partsincorporating both thick and thin sections with less shrink-age and warpage and with a better surface finish, but with ashorter cycle time. The water-assisted injection moldingprocess can also enable greater freedom of design, materialsavings, 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 structuralparts with a built-in water channel network. On the otherhand, despite the advantages associated with the process,the molding window and process control are more criticaland difficult since additional processing parameters areinvolved. Water may also corrode the steel mold, and somematerials including thermoplastic composites are difficultto mold successfully. The removal of water after moldingis also a challenge for this novel technology. Table 1 liststhe advantages and limitations of water-assisted injectionmolding technology.0266-3538/$ - see front matter ? 2006 Elsevier Ltd. All rights reserved.doi:10.1016/pscitech.2006.09.016*Corresponding author. Address: 259, Wen-Hwa 1st Road, Kwei-San,Tao-Yuan 333, Taiwan.E-mail address: shihjung.tw (S.-J. Liu)./locate/compscitechComposites Science and Technology 67 (2007) 14151424COMPOSITESSCIENCE ANDTECHNOLOGYWater assisted injection molding has advantages over itsbetter known competitor process, gas assisted injectionmolding 5, because it incorporates a shorter cycle timeto successfully mold a part due to the higher cooling capac-ity of water during the molding process. The incompress-ibility, low cost, and ease of recycling the water makes itan ideal medium for the process. Since water does not dis-solve and diffuse into the polymer melts during the moldingprocess, the internal foaming phenomenon 6 that usuallyoccurs 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 asmall residual wall thickness. Table 2 lists a comparisonof water and gas assisted injection molding.With increasing demands for materials with improvedperformance, which may be characterized by the criteriaof lower weight, higher strength, and a faster and cheaperproduction cycle time, the engineering of plastics is a pro-cess that cannot be ignored. These plastics include thermo-plastic and thermoset polymers. In general, thermoplasticpolymers have an advantage over thermoset polymers interms of higher impact strength, fracture resistance andstrains-to-failure. This makes thermoplastic polymers verypopular materials in structural applications.Poly-butylene-terephthalate (PBT) is one of the mostfrequently used engineering thermoplastic materials, whichis formed by polymerizing 1.4 butylene glycol and DMTtogether. Fiber-reinforced composite materials have beenadapted to improve the mechanical properties of neat plas-tic materials. Today, short glass fiber reinforced PBT iswidely used in electronic, communication and automobileapplications. Therefore, the investigation of the processingof fiber-reinforced PBT is becoming increasingly important710.This report was made to experimentally study the water-assisted injection molding process of poly-butylene-tere-phthalate (PBT) materials. Experiments were carried outon an 80-ton injection-molding machine equipped with alab scale water injection system, which included a waterpump, a pressure accumulator, a water injection pin, awater tank equipped with a temperature regulator, and acontrol circuit. The materials included a virgin PBT anda 15% glass fiber filled PBT composite, and a plate cavitywith a rib across center was used. Various processing vari-ables were examined in terms of their influence on thelength of water penetration in molded parts, whichincluded melt temperature, mold temperature, melt fillingspeed, short-shot size, water pressure, water temperature,water hold and water injection delay time. Mechanicalproperty tests were also performed on these molded parts,and XRD was used to identify the material and structuralTable 1Advantages and disadvantages of water-assisted injection moldingAdvantagesDisadvantages1. Short cycle time2. Low assisting medium cost (water is much cheaper and can be easilyrecycled)3. No internal foaming phenomenon in molded parts1. Corrosion of the steel mold due to water2. Larger orifices for the injection pin required (easier to get stuck by thepolymer melt)3. Some materials are more difficult to mold (especially amorphousthermoplastics)4. Removal of water after molding is requiredTable 2A comparison of water and gas-assisted injection moldingWaterGas1. Cycle timeShortLong2. Medium costLowHigh3. Internal foamingNoYes4. Residual wall thicknessSmallLarge5. Outside surface roughnessLowHigh6. Outside surface glossHighLow7. FingeringGreaterLess8. Asymmetrical penetrationMorestableUnstable9. Material crystallinityLowHigh10. Part transparencyHighLow11. Internal surface (semi-crystallinematerials)SmoothLesssmooth12. Internal surface (amorphous materials)RoughSmoothFig. 1. Schematic diagram of water-assisted injection molding process.1416S.-J. Liu et al. / Composites Science and Technology 67 (2007) 14151424parameters. Finally, a comparison was made betweenwater-assisted and gas-assisted injection molded parts.2. Experimental procedure2.1. MaterialsThe materials used included a virgin PBT (Grade1111FB, Nan-Ya Plastic, Taiwan) and a 15% glass fiberfilled PBT composite (Grade 1210G3, Nan-Ya Plastic, Tai-wan). Table 3 lists the characteristics of the compositematerials.2.2. Water injection unitA lab scale water injection unit, which included a waterpump, a pressure accumulator, a water injection pin, awater tank equipped with a temperature regulator, and acontrol circuit, was used for all experiments 3. An ori-fice-type water injection pin with two orifices (0.3 mm indiameter) on the sides was used to mold the parts. Duringthe experiments, the control circuit of the water injectionunit received a signal from the molding machine and con-trolled the time and pressure of the injected water. Beforeinjection into the mold cavity, the water was stored in atank with a temperature regulator for 30 min to sustainan isothermal water temperature.2.3. Molding machine and moldsWater-assisted injection molding experiments were con-ductedonan80-tonconventionalinjection-moldingmachine with a highest injection rate of 109 cm3/s. A platecavity with a trapezoidal water channel across the centerwas used in this study. Fig. 2 shows the dimensions ofthe cavity. The temperature of the mold was regulated bya water-circulating mold temperature control unit. Variousprocessing variables were examined in terms of their influ-ence on the length of water penetration in water channelsof molded parts: melt temperature, mold temperature, meltfill pressure, water temperature and pressure, water injec-tion delay time and hold time, and short shot size of thepolymer melt. Table 4 lists these processing variables aswell as the values used in the experiments.2.4. Gas injection unitIn order to make a comparison of water and gas-assistedinjection 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 gasinjection unit setup can be found in the Refs. 1115.The processing conditions used for gas-assisted injectionmolding were the same as that of water-assisted injectionmolding (terms in bold in Table 4), with the exception ofgas temperature which was set at 25 ?C.2.5. XRDIn order to analyze the crystal structure within thewater-assisted injection-molded parts, wide-angle X-raydiffraction (XRD) with 2D detector analyses in transmis-sion mode were performed with Cu Ka radiation at40 kV and 40 mA. More specifically, the measurementswere performed on the mold-side and water-side layers ofthe water-assisted injection-molded parts, with the 2h angleranging from 7? to 40?. The samples required for theseanalyses were taken from the center portion of thesemolded parts. To obtain the desired thickness for theXRD samples, the excess was removed by polishing theTable 3Characteristics of the glassfiber reinforced PBT compositePropertyASTMPBT15% G.F.PBTYield strength (kg/cm2)D-6386001000Bending stress (kg/cm2)D-5709001500Hardness (R-scale)D-785119120Heat distortion temperature (?C)(18.6 kg/cm2)D-64860200Melt flow index (MFI)D-12384025Impact strength (Kg-cm/cm)D-25655Melting temperature (?C)DSC224224Fig. 2. Layout and dimensions of mold cavity (unit: mm).S.-J. Liu et al. / Composites Science and Technology 67 (2007) 141514241417samples on a rotating wheel on a rotating wheel, first withwet silicon carbide papers, then with 300-grade silicon car-bide paper, followed by 600- and 1200-grade paper for abetter surface smoothness.2.6. Mechanical propertiesTensile strength and bending strength were measured ona tensile tester. Tensile tests were performed on specimensobtained from the water-assisted injection molded parts(see Fig. 3) to evaluate the effect of water temperature onthe tensile properties. The dimensions of specimens forthe experiments were 30 mm 10 mm 1 mm. Tensile testswere performed in a LLOYD tensiometer according to theASTM D638M test. A 2.5 kN load cell was used and thecrosshead 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 fromparts (Fig. 3) subjected to various water temperatures.Thedimensionsofthespecimenswere20 mm 10 mm 1 mm. Bending tests were performedin a micro tensile tester according to the ASTM D256test. A 200 N load cell was used and the crosshead speedwas 50 mm/min.2.7. Microscopic observationThe fiber orientation in molded specimens was observedunder a scanning electron microscope (Jeol Model 5410).Specimens for observation were cut from parts moldedby water-assisted injection molding across the thickness(Fig. 3). They were observed on the cross-section perpen-dicular to the flow direction. All specimen surfaces weregold sputtered before observation.3. Results and discussionAll experiments were conducted on an 80-ton conven-tional injection-molding machine, with a highest injectionrate of 109 cm3/s. A plate cavity with a trapezoidal waterchannel across the center was used for all experiments.Table 4The processing variables as well as the values used in the experimentsABCDEFMelt pressure (Mpa)Melt temperature (?C)Short shot size (%)Water pressure (Mpa)Water temperature (?C)Mold temperature (?C)140280 (270)7688080126275 (265)7797575114270 (260)7810707098265 (255)8011656584260 (250)81126060*The values in the parentheses are the melt temperatures used for virgin PBT materials.Fig. 3. Schematically, the positioning of the samples cut from the molded parts for tensile and bending tests and microscopic observations.1418S.-J. Liu et al. / Composites Science and Technology 67 (2007) 141514243.1. Fingerings in molded partsAll molded parts exhibited the water fingering phenom-enon at the channel to plate transition areas. In addition,molded glass fiber filled composites showed more severewater fingerings than those of non-filled materials, asshown photographically in Fig. 4. Fingerings usually formwhen a less dense, less viscous fluid penetrates a denser,more viscous fluid immiscible with it. Consider a sharptwo phase interface or zone where density and viscositychange rapidly. The pressure force (P2?P1) on the dis-placed fluid as a result of a virtual displacement dx of theinterface can be described by 16,dP P2? P1 l1? l2U=K?dx1where U is the characteristic velocity and K is the perme-ability. If the net pressure force is positive, then any smalldisplacement will be amplified and lead to an instabilityand part fingerings. For the displacement of a dense, vis-cous fluid (the polymer melt) by a lighter, less viscousone (water), we can have Dl = l1?l2 0, and U 0 16.In this case, instability and the relevant fingering resultwhen a more viscous fluid is displaced by a less viscousone, 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. Thismight be due to the fact that the viscosity difference 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.3.2. Effects of processing parameters on water penetrationVarious processing variables were studied in terms oftheir influence on the water penetration behavior. Table 4lists these processing variables as well as the values usedin the experiments. To mold the parts, one central process-ing condition was chosen as a reference (bold term in Table4). By changing one of the parameters in each test, we wereable to better understand the effect of each parameter onthe water penetration behavior of water assisted injectionmolded composites. After molding, the length of waterpenetration was measured. Figs. 510 show the effects ofthese processing parameters on the length of water penetra-tion in molded parts, including melt fill pressure, melt tem-perature,moldtemperature,shortshotsize,watertemperature, and water pressure.The experimental results in this study suggest that waterpenetrates further in virgin PBT than in glass fiber filledPBT 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.Fig. 4. Photograph of water-assisted injection molded PBT compositepart.8498112126140101214161820 PBT PBT+15% G.F.Melt fill pressure (MPa)Length of penetration (cm)Fig. 5. Effects of melt fill pressure on the length of water penetration inmolded parts.S.-J. Liu et al. / Composites Science and Technology 67 (2007) 141514241419The length of water penetration decreases with the meltfill pressure (Fig. 5). This can be explained by the fact thatincreasing the melt fill pressure increases the flow resistanceinside the mold cavity. It is then more difficult for the waterto penetrate into the core of the materials. The length ofwater penetration decreases accordingly 3.The melt temperature was also found to reduce thewaterpenetrationinmoldedPBTcompositeparts(Fig. 6). This might be due to the fact that increasing themelt temperature decreases viscosity of the polymer melt.A lower viscosity of the materials helps the water to packthe water channel and increase its void area, instead ofpenetrating further into the parts 4. The hollow core ratioat the beginning of the water channel increases and thelength of water penetration may thus decrease.Increasing the mold temperature decreases somewhatthe length of water penetration in molded parts (Fig. 7).This is due to the fact that increasing the mold temperaturedecreases the cooling rate as well as the viscosity of thematerials. The water then packs the channel and increasesits void area near the beginning of the water channel,instead of penetrating further into the parts 3. Moldedparts thus have a shorter water penetration length.Increasing the short shot size decreases the length ofwater penetration (Fig. 8). In water-assisted injectionmolding, the mold cavity is partially filled with the polymer89101112101214161820 PBT PBT+15% G.F.Water pressure (MPa)Length of penetration (cm)Fig. 10. Effects of water pressure on the length of water penetration inmolded parts.2502552602652702752808101214161820 PBT PBT+15% G.F.Length of penetration (cm)Melt temperature (C)Fig. 6. Effects of melt temperature on the length of water penetration inmolded parts.6065707580101214161820 PBT PBT+15% G.F.Mold temperature (C)Length of penetration (cm)Fig. 7. Effects of mold temperature on the length of water penetration inmolded parts.767778798081101214161820 PBT PBT+15% G.F.Short shot size (%)Length of penetration (cm)Fig. 8. Effects of short shot size on the length of water penetration inmolded parts.6065707580101214161820 PBT PBT+15% G.F.Water temperature (C) Length of penetration (cm)Fig. 9. Effects of water temperature on the length of water penetration inmolded parts.1420S.-J. Liu et al. / Composites Science and Technology 67 (2007) 14151424melt followed by the injection of water into the core of thepolymer 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 length of water penetration inmolded parts. Increasing the water temperature decreasesthe cooling rate of the materials and keeps the polymermelt hot for a longer time; the viscosity of the materialsdecreases accordingly. This will help the water penetratefurther into the core of the parts 3. Increasing the waterpressure also helps the water penetrate into the materials.The length of water penetration thus increases.Finally, the deflection of molded parts, subjected to var-ious processing parameters, was also measured by a profile-meter. The maximum measured deflection is considered asthe part warpage. The result in Fig. 11 suggests that thepart warpage decreases with the length of water penetra-tion. This is due to the fact that the longer the water pen-etration, the more the water pressure can pack thepolymeric materials against the mold wall. The shrinkageas well as the relevant part warpage decreases accordingly.3.3. Crystallinity of molded partsPBT is a semi-crystalline thermoplastic polyester with ahigh crystallization rate. In the water-assisted injectionmolding process, crystallization occurs under non-isother-mal conditions in which the cooling rate varies with coolingtime. Here the effects of various processing parameters(including melt temperature, mold temperature, and watertemperature) on the level of crystallinity in molded partswere studied. Measurements were conducted on a wide-angle X-ray diffraction (XRD) with 2D detector analyses(as described in Section 2). The measured results inFig. 12 showed that all materials at the mold-side layerexhibited a higher degree of crystallinity than those at thewater-side layer. The result indicates that the water has abetter cooling capacity than the mold during the cooling0204060801001200510152025Length of water penetration (mm)Part warpage (mm)Fig. 11. Measured warpage of molded parts decreases with the length ofwater penetration.260265270275280101520253035404550 Water Side Mold Side6065707580101520253035404550 Water Side Mold Side6065707580101520253035404550 Water Side Mold SideMelt temperature (C)Crystallinity (%)Crystallinity (%)Crystallinity (%)abcMold temperature (C) Water temperature (C)Fig. 12. Effect of melt temperature, mold temperature, and watertemperature on the level of crystallinity of water-assisted injection moldedparts.S.-J. Liu et al. / Composites Science and Technology 67 (2007) 141514241421process. This matches our earlier finding 17 by measuringthe in-mold temperature distribution. In addition, theexperimental result in Fig. 12c also suggests that the crys-tallinity of the molded materials generally increases withthe water temperature. This is due to the fact that increas-ing the water temperature decreases the cooling rate of thematerials during the cooling process. Molded parts thusexhibited a higher level of crystallinity.On the other hand, to make a comparison of the crysal-linity of parts molded by gas and water, gas-assisted injec-tion molding experiments were carried out on the sameinjection molding machine as that used with water, butequipped with a high-pressure nitrogen gas injection unit1115. The measured results in Fig. 13 suggests thatgas-assisted injection molded parts have a higher degreeof crystallinity than water-assisted injection mold parts.This is due to the fact that water has a higher coolingcapacity and cools down the parts faster than gas. Partsmolded by water thus exhibited a lower level of crystallinitythan those molded by gas.3.4. Mechanical propertiesTensile tests were performed on specimens obtainedfrom the water-assisted injection molded parts to examinethe effect of water temperature on the tensile properties.Fig. 14 showed the measured decrease subjected to variouswater temperatures. As can be observed, both yieldstrength and the elongational strain at break of waterassisted molded PBT materials decrease with the watertemperature. On the other hand, bending tests were alsoperformed at room temperature on water-assisted injectionmolded parts. The measured result in Fig. 15 suggests thatthe bending strength of molded parts decreases with thewater temperature.Increasing the water temperature generally decreases thecooling rate and molds parts with higher level of crystallin-0510152025303540 Water Water (gas) sideCrystallinity ( %)Mold side GasFig. 13. Level of crystallinity of parts molded by gas and water injectionmolding.6065707580303540455055606570Water temperature (oC)Yield strength (MPa)60657075802468101214161820Elongational strain at break (%)Water temperature (oC)abFig. 14. Effect of water temperature on the tensile properties of moldedPBT parts.6065707580102030405060Bending strength (MPa)Water temperature ( C)Fig. 15. Effect of water temperature on the bending strength of moldedPBT parts.1422S.-J. Liu et al. / Composites Science and Technology 67 (2007) 14151424ity. As is usually encountered in semi-crystalline thermo-plastics, a higher degree of crystallization means a lowercontent of free volume and therefore an increasing levelof stiffness. However, the experimental results here suggestthat the quantitative contribution of crystallinity to PBTsmechanical properties is negligible, while there is a moreimportant quantitative increase of tensile and bendingstrength for the PBT materials. The mechanical propertiesof molded materials are dependent on both the amount andthe type of crystalline regions developed during processing.The fact that the ductility of PBT decreases with the degreeof crystallinity may indicate that a more crystalline andstiffer PBT developed at a lower cooling rate during pro-cessing and did not exhibit higher stress values in tensiletests because of a lack of ductility, and therefore did notbehave as strong as expected from their stiffness 18. Nev-ertheless, more detailed experiments will be needed for thefuture works to investigate the morphological parametersof water-assisted injection molded parts and their correla-tion with the parts mechanical properties.3.5. Fiber orientation in molded partsSmall specimens were cut out from the middle of moldedparts in order to observe their fiber orientation. The posi-tion of the specimen for the fiber orientation observationis as shown in Fig. 3. All specimen surfaces were polishedand gold sputtered before observation. Fig. 16 shows themicrostructure of the water-assisted injection molded com-posite parts. The measured result suggests that the fiber ori-entation distribution in water-assisted injection moldedparts is quite different from that of conventional injectionmolded parts.In conventional injection molded parts, two regions areusually observed: the thin skin and the core. In the skinregion near the wall, all fibers are oriented parallel to theflow direction, while at the core, the fibers are oriented ran-domly in the flow plane 19,20. Compared to conventionalinjection molding, water-assisted injection molding tech-nology is different in the way the mold is filled. With a con-ventionalinjectionmoldingmachine,onecycleischaracterized by the phases of filling, packing and cooling.In the water-assisted injection molding process, the moldcavity is partially filled with the polymer melt followedby the injection of water into the core of the polymer melt.The novel filling process influences the orientation of fibersand matrix in a part significantly.From Fig. 16, the fiber orientation in water-assistedinjection molded parts can be approximately divided intothree zones. In the zone near the mold-side surface wherethe shear is more severe during the mold filling, fibers areprincipally parallel. For the zone near the water-side sur-face, the shear is smaller and the velocity vector greater.In this case, the fiber tends to be positioned more trans-versely in the direction of injection. At the core, the fiberstend to be oriented more randomly. Generally speaking,the glass fibers near the mold-side surface of molded partswere found to be oriented mostly in the flow direction, andoriented substantially perpendicular to the flow directionwith increasing distance from the mold-side surface.Finally, it shou
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