Injection-molded Plates Research Articles

Influence of injection molding parameters on the electrical resistivity of polycarbonate filled with multi-walled carbon nanotubes

Two polycarbonate (PC) composites with 2 and 5 wt% multi-walled carbon nanotube (MWNT) content were injection molded using a two-level, four-factor factorial design to evaluate the influences of holding pressure, injection velocity, mold temperature, and melt temperature on the electrical surface and volume resistivities. For both composites variations in resistivity of the injection-molded plates up to six orders of magnitude were found. The highest impact was determined for the injection velocity followed by the melt temperature and the interaction of both. The resistivity varied also locally within the plates showing differences up to five orders of magnitude for 2 wt% and up to two orders for 5 wt% MWNT. Thereby, areas of equal resistivity are formed in a semicircular shape with values increasing with the flow path. Transmission electron microscopy (TEM) investigations indicated a skin layer with highly oriented nanotubes in case of high injection velocity and low melt temperature, but a network-like structure even in the skin area at low injection velocity and high melt temperature.

Relationship between fine structure of polyoxymethylene copolymer plates and methanol transport properties

An experimental study was made of diffusion behavior of methanol through three kinds of injection-molded plates of a polyoxymethylene (POM) copolymer with different molecular weights M at 60 °C. Fine structure of the three sample plates was also examined by wide-angle X-ray diffraction and small-angle X-ray scattering, and moreover, their dynamic properties were investigated by the dynamic mechanical analysis (DMA). It is shown that the diffusion behavior may be well explained by the one-dimensional Fick diffusion equation with a constant diffusion coefficient, and that the steady-state transport rate increases with increasing M. As for fine structure, the crystallinity decreases slightly, and the preferential orientation and the long period increase, with increasing M. The long period of the lamellar stacking structure increases with increasing M, and it also increases with methanol transport. In DMA, the loss tangent tan δ becomes higher after the methanol transport in the wide range of temperature around the glass transition one. These results indicate that amorphous regions serve as channels for methanol molecules in the lamellar stacking structure, leading to the conclusion that the dependence of the steady-state transport rate on M arises from the factors of crystallinity and long period. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 1234–1242, 2007

Effect of fibre orientation on the fatigue behaviour of a short glass fibre reinforced polyamide-6

The effect of fibre orientation on the fatigue strength of a short glass fibre reinforced polyamide-6 has been investigated. Tension–tension axial fatigue tests were conducted with specimens extracted from injection moulded plates. Specimens were cut out of plates with different orientations with respect to the longitudinal axis of plates, and therefore displayed different orientations of the reinforcing fibres. Results are presented in the form of S– N curves, showing the variation of the fatigue strength as a function of the specimen orientation. The experimental data, both tensile and fatigue tests, have been compared with values predicted by a failure criterion derived from the Tsai–Hill formula. The influence on the fatigue lives of the fibre orientation distribution in the thickness of the specimen is discussed.

Determination of orientational states in impact-polystyrene specimens by near-infrared polarization spectroscopy

To measure the orientational state of opaque high-impact polystyrene (HIPS) samples, the dichroic ratios of different absorption bands have been determined in the near-infrared (NIR) transmission spectra measured with polarized radiation. For injection molded plates, dichroic ratios close to one were observed, indicating very low orientation effects in these specimens. However, a larger anisotropy was detected in the thermoformed cups and additionally different processing conditions could be discriminated by the dichroic effects in the NIR polarization spectra. Thus, NIR transmission spectroscopy with polarized radiation proved to be a convenient and rapid tool to determine orientation phenomena in HIPS samples and may also be implemented as a light-fiber-coupled on-line process control technique. POLYM. ENG. SCI. 46:381–383, 2006. © 2006 Society of Plastics Engineers

Processing-induced Properties in Glassy Polymers

Abstract A new method is employed to directly predict the development of yield stress distributions in injection molded products of glassy polymers. The approach is based on the results of a study on the temperature dependence of the evolution of yield stress during annealing of polycarbonate below Tg. In combination with the process-related thermal history, derived from numerical simulations of the injection molding process, an estimate of the yield stress distribution throughout a product can be obtained. Computed yield stresses of injection molded plates prove to be in excellent agreement with the experimental values, including their dependence on mold temperature and plate thickness.

Fibre Orientation and Mechanical Properties of Short Glass Fibre Reinforced PP Composites

The fibre orientation and distribution in short glass fibre (SGF) reinforced polypropylene (PP) composites were measured and an orientation function ( f) was introduced to describe the fibre orientation distribution. The fibre orientation distribution in the same level plane depended on the fibre position in an injection-moulded plate. The fibres close to the boundary wall of the injection cavity were parallel to the injection direction and those close to the back wall were perpendicular to the injection direction. A commercial maleic anhydride grafted PP (MAPP) was used as a compatibilizer. With the addition of MAPP, the fibre orientation distribution in PP/SGF/MAPP composites was slightly different from that of PP/SGF, but MAPP improved the interfacial adhesion between SGF and PP and consequently the mechanical properties. The use of SGF increased the tensile strength of the PP composites from 26.5 to 45.9 MPa (73% improvement), and for the PP/MAPP/SGF composite the tensile strength increased further to 55.4 MPa. The effect of SGF on the tensile strength can be expressed by a fibre efficiency factor (λσ). The SGF efficiency factor of PP/MAPP/SGF (0.198) was 45% higher than that of PP/SGF (0.137). The impact strength of PP/MAPP/SGF (64.7 MPa) was not only higher than that of PP/SGF (40.8 MPa) by 59% but also higher than that of PP (48.3 MPa) by 34%.

Electrically conductive thermoplastic blends for injection and compression molding of bipolar plates in the fuel cell application

AbstractThis study aimed at developing highly conductive, lightweight, and low‐cost bipolar plates for use in proton exchange membrane fuel cells. Injection and compression molding of carbon‐filled polypropylene, PP, and polyphenylene sulfide, PPS, were used to fabricate the bipolar plates. Loadings up to 60 wt% in the form of graphite, conductive carbon black, and carbon fibers were investigated. The developed compositions have a combination of properties and processability suitable for fuel cell bipolar plate manufacturing, such as good chemical resistance, sufficient fluidity, and good electrical and thermal conductivity. Two bipolar plate designs were successfully fabricated by molding the gas flow channels over aluminum plates to form a metallic/polymer composite plate or simply by direct injection molding of the conductive polymer composite. For the first design, overall plate volume resistivities of 0.2 and 0.1 Ohm‐cm were respectively attained using PP and PPS based blends as the conductive overmolded layer. A lower volume resistivity of around 0.06 Ohm‐cm was attained for the second design with injection molded plates made of the PPS‐based blend. Polym. Eng. Sci. 44:1755–1765, 2004. © 2004 Society of Plastics Engineers.

Fiber orientation structures and mechanical properties of injection molded short glass fiber reinforced ribbed plates

AbstractIn this paper we describe a study of the fiber orientation structures present within a model ribbed injection molded plate. The details of the fiber orientation at each chosen location on the injection molded parts were measured using an in‐house developed image analysis system, which enabled large areas to be scanned (up to 200 mm2) up to a limit of 1 million fiber images. Two materials were used for these experiments, short glass fiber filled PBT and short glass fiber filled nylon 66. First, a comparison was made between the fiber orientation at an identical position, 28 mm from the injection gate on a transverse rib, on two plates made from glass fiber filled PBT. It was found that the fiber orientation in these two separately manufactured components was virtually identical when comparing the whole scanned area, but the differences became more significant when comparing areas on the length scale of an individual fiber (∼ 200 μm). Second, the fiber orientation at the same position was compared for two plates made using the glass/PBT and glass/nylon 66 materials. The differences for the complete scanned areas were small, confirming that mold geometry plays a crucial role in determining fiber orientation structures, and that matrix properties are secondary. Third, the fiber orientation structures at various positions across one of the glass/PBT plates were examined in greater detail, in particular across a number of the transverse ribs: the chosen ribs were of various widths and heights. Differences in structure were found depending on the local rib geometry. Finally, the effect of the measured fiber orientation structures in determining the mechanical properties of the ribbed plate was investigated using simple modeling schemes. While the stiffness of the rib/web assembly was found to depend on the average fiber orientation of the two parts, the different thermal expansions of the web and the rib, caused by the different fiber orientation in the two regions, led to significant warpage of the rib/web assembly. Polym. Compos. 25:237–254, 2004. © 2004 Society of Plastics Engineers.

The Effect of Holding Pressure on the Shrinkage and Birefringence of Injection Moulded Polypropylene Plates

This paper shows the results of shrinkage and birefringence of injection moulded plates. The study was done with two polypropylenes with different MFI. Focus is made on the effect of the holding pressure on the experimental as-moulded shrinkage and birefringence in the flow direction. The mouldings are edge gated flat rectangular plate. The moulding process was continuously monitored with pressure transducers. The results show that the rise of holding pressure determines the decrease of the shrinkage. Also, it was observed that the increase of the pressure applied during the holding phase and the associated flow induce re-orientation of the melt, especially in the case of higher molecular weight PP. Consequently, an increase in the birefringence results.

Structure and Mechanical Properties of Injection Moulded CF/LCP Thin Plates

The effect of injection speed on the structure and bending properties of carbon fibre (CF) reinforced liquid crystal polymer (LCP) injection moulded plates were investigated. The distribution of the modulus in injection moulded LCP and CF/LCP thin plates made using two different injection speeds was investigated by taking slices at 0.1 mm intervals. Moreover the modulus of the CF/LCP thin plate was estimated by the Cox formula and by laminate theory from values of the fibre orientation and modulus of each LCP layer. It was found that the differences in the bending properties and structures due to changes in injection speed were not remarkable. The differences in the fracture behaviours with changes in the cutting position were caused by the differences in the modulus of each layer as in the laminated composite. It was confirmed that injection moulded CF/LCP can be treated in general as a laminated composite, and that the distribution of the modulus in injection moulded CF/LCP could be evaluated by the slicing technique. Comparing the calculated modulus with the experimental one in the machine direction and the transverse direction revealed that the degree of local orientation of the LCP domains near the fibres was different from that of LCP without any fibre. Moreover, it was shown by evaluating the fibre orientation and modulus of the sliced specimens that fibre orientation contributed to the difference in the modulus near the surface when the injection speed changed. It was also verified that the prediction method suggested in this paper is valid to estimate the modulus of thin injection moulded CF/LCP plates.

Fibre orientation structures and their effect on crack resistance of injection moulded transverse ribbed plate

An extensive study of the fibre orientation structures developed in a transverse ribbed plate during injection moulding, and the use of these structures to investigate the effect of local fibre orientation state on crack initiation resistance, is reported. The fibre orientation results for the ribbed plate, measured using large area image analysis system developed at Leeds University, showed that after an initial settling down period, the central core region, where the fibres are aligned perpendicular to the flow direction, decreased in size monotonically, with an associated monotonic increase in the outer shell regions, where the fibres are aligned preferentially along the injection direction. Interestingly, the level of orientation in the two regions remained almost constant: only the proportions of the two regions were found to change with flow length. Across the plate, close to the gate, the central core region was found to have a lens-like shape, while at the other end of the plate the core was thinner and also consistent in thickness across the sample width. The transverse rib was found to cause little disturbance to the fibre orientation of the base plate. The different proportions of the shell and core regions at different locations over the ribbed plate provided an ideal case to test the proposition of Friedrich that the crack resistance of a short fibre reinforced material depends on the number of fibres that are perpendicular to the crack tip. The impact test results gathered in this way confirmed this hypothesis of Friedrich.

Glass Fibre Contents of PP Plates and their Properties: Part I: Shrinkage and Changes in Time

Glass Fibre Contents of PP Plates and their Properties: Part I: Shrinkage and Changes in Time

Investigations into moulding window of gas assisted injection moulded polystyrene plates with various gas channel designs

The design of the gas channel plays an important role in the successful application of gas assisted injection moulding. Although empirical guidelines for gas channel design have been proposed by the various equipment suppliers, quantitative rules based on well designed experiments have not been reported previously. To investigate the effects of geometry on gas penetration for two plate thicknesses, transparent polystyrene (PS) plates designed with semicircular gas channels of differing radii and with rectangular gas channels of differing width to height ratios have been produced using gas assisted injection moulding. Moulding windows and criteria for gas penetration were also chosen so that design rules could be defined quantitatively. The mouldability index was also classified into five levels (excellent, good, fair, poor, and bad) based on the relative areas of the moulding windows. From a plot of mouldability index against R eq , the ratio of equivalent gas channel radius to plate thickness, it was found that to obtain an appropriate moulding window (i.e at least a fair mouldability index) R eq should be greater than 2. Gas channels with a semicircular cross-section provide better mouldability than those with rectangular cross-sections of the same cross-sectional area. For the same equivalent radius, the ratio of width to height in rectangular gas channels also affects the mouldability index. The present investigation provides part designers with preliminary quantitative design/moulding guidelines for choosing the effective gas channel design that allows the parts to be moulded within an appropriate moulding window, so that the uncertainty in both simulation and process control can be overcome. A methodology for the establishment of guidelines for quantitative design of gas channels is also proposed.

Glass Fibre Content of PP Plates and their Properties: Part II: Tensile Mechanical Properties

Glass Fibre Content of PP Plates and their Properties: Part II: Tensile Mechanical Properties

Lamellae Formation and Deformation of Isotactic Polypropylene due to Push-Pull Processing

Like all thermoplastic materials, the mechanical performance of isotactic polypropylene (i-PP) depends strongly on morphology. Using techniques such as push-pull processing or SCORIM (Shear Controlled Orientation in Injection Moulding), it is possible to obtain i-PP with an oriented lamellae structure via injection moulding. Due to an overall shear induced crystallisation and molecular alignment both the amorphous and the crystalline phases are oriented. The orientation can be visualised using X-ray-diffraction methods (WAXD, SAXS) and Raman-Spectroscopy techniques as well as light and electron microscopy (SEM, TEM). Although the molecular chains are oriented parallel to the flow direction (and so the c-axis in the crystallites) lamellae are oriented perpendicular. Results for DSC measurements and micro-hardness over the cross-section of injection-moulded plates accompany the microscopical investigations. This lamellar structure has been deformed under tensile and shear load. The results are discussed with reference to mechanical tests and microscopic investigations. The deformation on a structural level shows a fibrillar, crazed structure that is examined using SEM and plasma etching preparation techniques.

Zero-birefringence polymer by the anisotropic molecule dope method

We propose using the anisotropic molecule dope method for synthesizing a zero-birefringence polymer that showed no orientational birefringence at any orientation degree of polymer chains. In this method a rodlike molecule with polarizability anisotropy was chosen to compensate orientational birefringence. The zero-birefringence polymer was synthesized by doping of 3-wt.% trans-stilbene as an anisotropic molecule into poly(methyl methacrylate). The zero-birefringence at the 590-nm wavelength in the drawn film and the injection-molded plate made from the zero-birefringence polymer was confirmed by the rotating parallel nicols method. Furthermore, high transparency (37.2 dB/km) of the zero-birefringence polymer at the 633-nm wavelength was confirmed by the light-scattering measurement.

Plastic deformation in polypropylene/(ethylene–propylene) copolymer blend during paint debonding

Polypropylene (PP) and polypropylene/(ethylene–propylene) copolymer (EP) blend (PP/EP) injection-moulded plates were painted with a chlorinated polypropylene (CPO) based adhesion promoter. Three-point bending and crosscut adhesion tests performed on these plates show that adherence is better on PP/EP than on PP. X-ray photoelectron spectroscopy (XPS) and force modulation microscopy (FMM) analyses were performed on the failure surface obtained after the three-point bending test. XPS shows that the Cl atomic fraction is less than 0.5% on the polymer side and close to 4% on the paint side indicating that the fracture is located at the CPO–polymer interface. The analysis of the PPEP surface by FMM reveals that paint debonding occurs with plastic deformations located into the EP elastomer phase. The magnitude of the plastic deformation is dependent on the depth location of the nodules. The adherence improvement induced by the presence of EP nodules can thus be explained by energy dissipation occurring during EP deformation.

Notched tensile strength for long- and short-fiber reinforced polyamide

The point stress criterion and the average stress criterion are applied for evaluating the notched tensile strength of injection-moulded plates with machined and moulded-in notches. Use is made of a characteristic length which is found to be geometry and material dependent.

Injection molding of poly(ethylene terephthalate): Differential scanning calorimetry and confocal micro-raman spectroscopy investigations of the skin-core morphology

The skin-core structure of injection-molded plates of poly(ethylene terephthalate) (PET) has been investigated using Confocal Micro-Raman Spectroscopy (CMRS) and Differential Scanning Calorimetry (DSC). CMRS has proved to be a useful tool for finding out about quantitative conformational changes in polymeric materials. In order to obtain a complete picture of isomer distributions in the injection molded industrial PET samples, spectroscopic measurements were correlated with DSC measurements. The spectroscopic results reveal two different structural zones related to the fabrication process: a skin layer and the core. The correlation between the apparent degree of crystallinity obtained by DSC and the percentage of the trans isomer obtained by CMRS has allowed the latter parameter to be separated into crystalline and amorphous trans isomer and to follow its change along the injection molded plate.

Investigations on the Impact Performance of Gas-Assisted Injection Molded Polystyrene Parts

The effects of geometrical factors, introduced by part thickness and various gas channel designs, including semicircular gas channel and rectangular gas channels with different geometry, on the impact properties of gas-assisted injection molded (GAIM) polystyrene parts were investigated via impact energy measurement. Test results were also compared with those of conventional injection molded (CIM) parts and plate parts without gas channel. It was found that gas channel design results in part impact reinforcement and gas-assisted injection molded parts show better impact performance than conventional injection molded parts. Generally speaking, impact strength of GAIM parts increases when the size of the gas channel is increased. Meanwhile, for five gas channel designs, both gas channel designs attached with top rib (shapes D and E) show the higher impact energy and impact value. These two gas channel designs provide the best enhancement effect in impact performance. The present investigation provides a design guideline for choosing the most effective gas channel design to achieve the part impact performance.