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1、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 system,which included a water pump, a pres

2、sure 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 o

3、f their influence on the length of water penetration in molded parts, and mechanical property tests were performed on these parts. X-ray diffraction （XRD） was also used to identify the material and structural parameters. Finally, a comparison was made between water-assisted and gas-assisted injectio

4、n molded parts. It was found that 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 higher

5、 degree of crystallinity than those molded by water. Furthermore, the glass fibers near the surface of molded parts were found to be oriented mostly in the flow direction, but oriented substantially more perpendicular to the flow direction with increasing distance from the skin surface.Keywords: Wat

6、er assisted injection molding; Glass fiber reinforced poly-butylene-terephthalate （PBT） composites; Processing parameters; B. Mechanical properties; Crystallinity; A. Polymer matrix composites;1. IntroductionWater-assisted injection molding technology 1 has proved itself a breakthrough in the manufa

7、cture 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 di

8、agram of the water-assisted injection molding process is illustrated in Fig. 1.Water-assisted injection molding can produce parts incorporating both thick and thin sections with less shrink-age and warpage and with a better surface finish, but with a shorter cycle time. The water-assisted injection

9、molding process can also enable greater freedom of design, material savings, weight reduction, and cost savings in terms of tooling and press capacity requirements 24. Typical applications include rods and tubes, and large sheet-like structural parts with a built-in water channel network. On the oth

10、er hand, despite the advantages associated with the process,the molding window and process control are more critical and difficult since additional processing parameters are involved. Water may also corrode the steel mold, and some materials including thermoplastic composites are difficult to mold s

11、uccessfully. 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.Fig. 1. Schematic diagram of water-assisted injection molding process.Water assisted injection molding has advant

12、ages 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 capacity of water during the molding process. The incompressibility,low cost, and ease of recycling the water makes it a

13、n ideal medium for the process. Since water does not dissolve and diffuse into the polymer melts during the molding process, the internal foaming phenomenon 6 that usually occurs in gas-assisted injection molded parts can be eliminated.In addition, water assisted injection molding provides a better

14、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 a

15、nd cheaper production cycle time, the engineering of plastics is a process that cannot be ignored. These plastics include thermoplastic and thermoset polymers. In general, thermoplastic polymers have an advantage over thermoset polymers in popular materials in structural applications.Poly-butylene-t

16、erephthalate （PBT） is one of the most frequently used engineering thermoplastic materials, whichis formed by polymerizing 1.4 butylene glycol and DMT together. Fiber-reinforced composite materials have been adapted to improve the mechanical properties of neat plastic materials. Today, short glass fi

17、ber reinforced PBT is widely used in electronic, communication and automobile applications. Therefore, the investigation of the processing of fiber-reinforced PBT is becoming increasingly important710.This report was made to experimentally study the waterassisted injection molding process of poly-bu

18、tylene-terephthalate （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 ci

19、rcuit. The materials included a 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, which included melt temperature, mol

20、d 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 parameters. Finally, a comparison was made bet

21、weenwater-assisted and gas-assisted injection molded parts.Table 1Advantages and disadvantages of water-assisted injection moldingAdvantagesDisadvantages1. Short cycle time1. Corrosion of the steel mold due to water2. Low assisting medium cost （water is much cheaper and can be easily recycled）2. Lar

22、ger orifices for the injection pin required （easier to get stuck by the polymer melt）3. No internal foaming phenomenon in molded parts3. Some materials are more difficult to mold （especially amorphous thermoplastics）4. Removal of water after molding is required2. Experimental procedure2.1. Materials

23、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, Taiwan）.Table 3 lists the characteristics of the composite materials.2.2. Water injection unitA lab scale water injection unit, which included a w

24、ater 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 orifice-type water injection pin with two orifices （0.3 mm in diameter） on the sides was used to mold the parts. During the experime

25、nts, the control circuit of the water injection unit received a signal from the molding machine and controlled 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 tem

26、perature.2.3. Molding machine and moldsWater-assisted injection molding experiments were conducted on an 80-ton conventional injection-molding machine with a highest injection rate of 109 cm3/s. A plate cavity with a trapezoidal water channel across the center was used in this study. Fig. 2 shows th

27、e dimensions ofthe 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 influence on the length of water penetration in water channels of molded parts: melt temperature, mold temperature,

28、meltfill pressure, water temperature and pressure, water injection delay time and hold time, and short shot size of the polymer melt. Table 4 lists these processing variables as well as the values used in the experiments.2.4. Gas injection unitIn order to make a comparison of water and gas-assisted

29、injection molded parts, a commercially available gas injection 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

30、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 _C.2.5. XRDIn order to analyze the crystal structure within the water-assisted injection-molded parts, wide-angle X-ray diffraction （XRD） with 2D detector analyses i

31、n transmission 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 7_ to 40_. The samples required for these analyses were t

32、aken from the center portion of these molded parts. To obtain the desired thickness for the XRD samples, the excess was removed by polishing theTable 3Characteristics of the glassfiber reinforced PBT compositePropertyASTM PBT 15% G.F. PBTYield strength （kg/cm2）D-638 600 1000Bending stress （kg/cm2）D-570 900 1500Hardness （R-scale）D-785 119 120Heat distortion temperature（_C）（18.6 kg/cm2）D-648 60 200Melt f