Effect of molecular weight and molding conditions on the replication of injection moldings with micro‐scale v‐groove features

Injection molded optical plastic parts require accurate replication of micro‐scale features. The effects of melt viscosity and molding conditions on replication of microscopic v‐groove features in injection molded parts were examined for PC with different molecular weight. The micro‐scale feature size was a continuous v‐groove with 20 μm in depth and 50 μm in width. For injection molding conditions, melt temperature, mold temperature, injection velocity and holding pressure were varied in three levels. As the result, the mold temperature had significantly affected replication for all polymers with different molecular weight. Additionally, the molding conditions that lower melt viscosity led to improved replication. In the case of polymer with high molecular weight, the viscosity decreased with increasing melt temperature. It has been found that high replication of micro‐scale features could be achieved by higher mold temperature and higher melt temperature even with high viscosity PC. POLYM. ENG. SCI., 2008. © 2008 Society of Plastics Engineers     

Properties of glass‐filled thermoplastic polyesters

The thermal, mechanical, and rheological properties of glass‐filled poly(propylene terephthalate) (GF PPT) were compared to glass‐filled poly(butylene terephthalate) (GF PBT). The impetus for this study was the recent commercial interest in PPT as a new glass‐reinforced thermoplastic for injection‐molding applications. This article represents the first systematic comparison of the properties of GF PPT and GF PBT in which differences in properties can be attributed solely to differences in the polyester matrices, that is, glass‐fiber size and composition, polymer melt viscosity, nucleant content and composition, polymerization catalyst composition and content, and processing conditions were kept constant. Under these controlled conditions, GF PPT showed marginally higher tensile and flexural properties and significantly lower impact strength compared to GF PBT. The crystallization behavior observed by cooling from the melt at a constant rate showed that GF PBT crystallized significantly faster than did GF PPT. Nucleation of GF PPT with either talc or sodium stearate increased the rate of crystallization, but not to the level of GF PBT. The slower crystallization rate of GF PPT was found to strongly affect thermomechanical properties of injection‐molded specimens. For example, increasing the polymer molecular weight and decreasing the mold temperature significantly increased the modulus drop associated with the glass transition. In contrast, the modulus–temperature response of GF PBT was just marginally influenced by the polymer molecular weight and was essentially independent of the mold temperature. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 74: 889–899, 1999     

注射成型工艺对PBT结晶和降解行为的影响

通过采用不同的注射条件,分别考察了熔体温度、模具温度、螺杆转速、注射速率、背压、保压压力、保压时间、滞留时间等多个重要的注射参数对聚对苯二甲酸丁二醇酯(PBT)小型制件降解程度、结晶形态和力学性能的影响。利用偏光显微镜、DSC、黏度计等,着重研究了各注射工艺参数对制件皮层厚度、结晶度、相对分子质量以及断裂伸长率的影响,从而为优化注射成型工艺过程提供依据。结果表明:熔体温度对制件各项性能的影响最为显著,而螺杆转速、背压、保压压力等的影响相对较小;随着熔体温度的提高和滞留时间的延长,PBT的降解程度迅速加剧,相对分子质量急剧下降,从而使制件断裂伸长率严重降低,力学性能变差。     

Evaluation of molecular orientation in injection molded parts with microscale features by Raman spectroscopy

Molecular orientation of polycarbonate (PC) in injection‐molded parts with microscale features was characterized by means of polarized Raman spectroscopy, and the relationship between microstructure and replication was discussed. The microscale feature size of continuous v‐groove was 20 μm in depth and 50 μm in width. PC injection‐molded parts were molded with various molding conditions. The molecular orientation distribution along flow direction on the cross‐section of molding parts were evaluated by the intensity ratio of the bands at 635 to 703 cm^?1 (I₆₃₅/I₇₀₃) in the Raman spectra. Molecular orientation along the flow direction inside the v‐groove was higher than that of the core and the opposite surface region. In particular, the highest molecular orientation was at the surface of the v‐groove. Among the injection molding conditions, the mold temperature showed significant effect on the molecular orientation and replication. Higher mold temperature caused high replication and low molecular orientation. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Fatigue performance of injection‐molded short e‐glass fiber reinforced polyamide‐6,6. II. Effects of melt temperature and hold pressure

Tensile and fatigue properties of an injection molded short E‐glass fiber reinforced polyamide‐6,6 have been studied as a function of two key injection molding parameters, namely melt temperature and hold pressure. It was observed that tensile and fatigue strengths of specimens normal to the flow direction were lower than that in the flow direction, indicating inherent anisotropy caused by injection molding. Tensile and fatigue strengths of specimens with weld line were significantly lower than that without weld lines. For specimens in the flow direction, normal to the flow direction and with weld line, tensile strength and fatigue strength increased with increasing melt temperature as well as increasing hold pressure. The effect of specimen orientation on the tensile and fatigue strengths is explained in terms of the difference in fiber orientation and skin‐core morphology of the specimens. POLYM. COMPOS., 2011. © 2010 Society of Plastics Engineers.     

Effects of filler size and content on shrinkage and gloss of injection molded PBT/PET/talc composites

Effects of filler size and content on the shrinkage of injection molded poly(butylene terephthalate)/poly(ethylene terephthalate)/talc (PBT/PET/talc) composites were investigated. Circular plate specimens were examined by a field emission scanning electron microscope and a numerical analysis code was used for modeling of the injection molding. Orientation effects of polymer molecules and fillers on the shrinkage behavior of PBT/PET/talc composites were illustrated by using schematic diagrams. It was found that the planes of talc particles were aligned parallel to the mold wall due to applied shear stresses and the largest axes of the disk‐like talc particles were oriented along the flow direction. Shrinkage of the PBT/PET/talc composites was decreased more rapidly along the flow direction than along the transverse direction as the talc content was increased. Small‐sized talc was more effective for reduction of the surface shrinkage of PBT/PET composites than titanium dioxide (TiO₂) or large‐sized talc. Although the PBT/PET polymer matrix almost covered the surface of talc particles, surface gloss of the particle filled composites was decreased as the size of mineral filler was increased. POLYM. COMPOS., 2010. © 2009 Society of Plastics Engineers     

聚碳酸酯注射成型制品力学性能的Taguchi分析

采用Taguchi分析方法考察了注射成型工艺参数对聚碳酸酯(PC)力学性能的影响。结果表明,注射成型工艺参数对PC的拉伸强度影响较大,对弯曲强度和冲击强度影响较小;方差分析结果表明,模具温度对拉伸强度的影响最大,注射压力和冷却时间次之,熔体温度的影响最小;模具温度降低,熔体冷却固化层厚度增加,取向程度增大,拉伸强度增加;同时结果表明Taguchi分析方法可以准确地对优选出的成型工艺制备的PC试样的拉伸强度进行预测,其预测值与测量值的偏差仅为0.75%。     

The morphology and property of HDPE in the presence of oscillation pressure and poly(ethylene terephthalate)

The injection‐molded specimens of neat HDPE and the PET/HDPE blends were prepared by conventional injection molding (CIM) and by pressure vibration injection molding (PVIM), respectively. The effect of oscillation pressure and PET phase with different shapes on superstructure and its crystal orientation distribution of injection molded samples were characterized by differential scanning calorimetry (DSC), scanning electron microscopy (SEM), and two‐dimension wide‐angle X‐ray diffraction techniques (2D‐WAXD). Hermans' orientation functions were determined from the wide‐angle X‐ray diffraction patterns. With the PET particles added, the shear viscosity of blend increase and crystallization rate of HDPE phase is enhanced. For the neat HDPE samples, with the promotion from oscillation shear, the orientation parameter experienced a large increase, moreover, the PVIM can induce transverse lamellae (kebabs) twisting in growth direction. Because of the redefined flow field and nucleation effect of PET particles, the crystal orientation of blend is also increased. So the tensile strength of vibration samples enhanced and elongation at break declined. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Fracture Toughness of Injection-Molded Poly(Butylene Terephthalate) Composites

Abstract:

The influences of various fiber amounts and injection molding process conditions on the fracture toughness of injection-molded short fiber-reinforced poly(butylene terephthalate) (PBT) composites were investigated. Three materials of various fiber amounts, neat PBT, 15 wt.%, and 30 wt.% short fiber-reinforced PBT composites, were used in this study. The compact tension (CT) specimens were prepared by various injection molding process conditions, wherein filling time, melt temperature, mold temperature, and packing pressure were design parameters used to measure fracture toughness. The morphology of the specimens, which consisted of frozen, intermediate, and core layers, was evaluated by scanning electron microscopy (SEM) and related to the fracture toughness. The fracture surfaces were also observed by SEM to understand the difference between the fracture mechanisms of neat PBT and fiber-reinforced PBT. It was found that the fracture toughness of neat PBT was significantly increased by the addition of short glass fibers. However, the variation of the injection molding design parameter had little effect on the fracture toughness. The fracture toughness depended on the thickness of the layers where fibers oriented perpendicular to the crack direction. The layer thickness was strongly affected by the fiber amounts.     

Frozen-in birefringence and anisotropic shrinkage in optical moldings: II. Comparison of simulations with experiments on light-guide plates

The thermally-, flow-induced and total birefringence components and anisotropic shrinkages in LGP moldings were simulated by using a combination of a CV/FEM/FDM technique nonlinear viscoelastic and photoviscoelastic constitutive equations and orientation functions, as described in Part I of this study. The simulated results were compared with measurements on LGP moldings of a polystyrene (PS) and two optical grade polycarbonates (PCs) OQ1030 and OQ3820 having low and high molecular weights. The thermally-induced birefringence was simulated by a combination of constrained and free cooling during molding. In LGP moldings of PS, the simulated thermally-induced birefringence indicated a minor variation with location in the mold plane, a parabolic shape in the core region and an increase towards the wall. Compared to the flow-induced birefringence, the thermal birefringence provided a minor contribution to the total transverse birefringence Δn₁₂. In LGP moldings of PCs, the simulated thermally-induced birefringence showed a significant variation with location in the mold plane, nearly constant value in the core region and high value in the wall region. In LGP moldings of both PCs, the contributions of the thermally- and flow-induced birefringence to the total transverse birefringence Δn₁₂ were significant. The effect of processing conditions on the development of the normal birefringence in LGP moldings of PCs was ranked from most to least: the packing pressure, mold temperature, melt temperature, injection speed and packing time. However, in LGP moldings of PS the packing time effect was significant due to a longer gate freezing time. Simulated and measured normal birefringence along the flow direction was in fair agreement, but simulations were unable to describe the observed birefringence maximum arising near the gate. The averaged luminance of LGP moldings exhibited some correlation with the averaged normal birefringence. LGP moldings of PC OQ1030 indicated a pronounced maximum in the simulated transverse flow birefringence in the core but a low value near the wall. In contrast, the LGP molding of PC OQ3820 showed a high simulated birefringence near the wall and a low value of maximum in the core. The simulated and measured total transverse birefringence in LGP moldings was in fair agreement. LGP molding of both PCs showed similar tendency in shrinkage variation with processing conditions. However, the thickness shrinkage was higher in LGP moldings of PC OQ3820. The effect of processing conditions on the development of shrinkage in LGP moldings of both PCs was ranked from most to least: the packing pressure, melt temperature, mold temperature, injection speed and packing time. In LGP moldings of PS, the thickness shrinkage slightly increased with increasing melt temperature and significantly increased with reducing packing time. A good agreement between the simulated and measured anisotropic shrinkages in LGP moldings at various processing conditions was observed.     

Nonisothermal crystallization of poly(ethylene terephthalate) and its blends in the injection-molding process

We investigated the nonisothermal crystallization during the cooling process of injection molding of poly(ethylene terephthalate) (PET), PET/talc, and PET/Surlyn blends. We applied the isothermal crystallization parameters obtained by the Hoffman–Lauritzen theory to the kinetics of nonisothermal crystallization and then calculated the relative crystallinity χ/χ_c as a function of the mold temperature. χ/χ_c were nicely interpreted by calculation without effect of the pressure history on crystallization in PET and PET/talc (1 wt %) blends. In contrast, in the PET/Surlyn (3 wt %) blend, crystallization occurred at a lower mold temperature than predicted by our calculation. The transmission electron micrograph near the surface of the injection-molded PET/Surlyn blend showed deformation and stretching of dispersed Surlyn particles, suggesting that orientation of the PET matrix proceeds with the flow in processing. The orientation of the PET matrix resulted in acceleration of the crystallization in the injection molding. © 1995 John Wiley & Sons, Inc.     

Formation mechanism of high-pressure water penetration induced fiber orientation in overflow water-assisted injection molded short glass fiber-reinforced polypropylene

Recently, there has been growing interest in water-assisted injection molding (WAIM) not only for its advantages over gas-assisted molding (GAIM) and conventional injection molding (CIM), but also for its great potential advantages in industrial applications. To understand the formation mechanism of water penetration induced fiber orientation in overflow water-assisted injection molding (OWAIM) parts of short glass fiber-reinforced polypropylene (SGF/PP), in this work, the external fields and water penetration process within the mold cavity were investigated by experiments and numerical simulations. The results showed that the difference of fiber orientation distribution in thickness direction between WAIM moldings and CIM moldings was mainly ascribed to the great external fields generated by water penetration. Besides, fiber orientation depended on the position both across the part thickness and along the flow direction. Especially in the radial direction, fiber orientation varied considerably. The results also showed that the melt temperature is the principal parameter affecting the fiber orientation along the flow direction, and a higher melt temperature significantly facilitated more fibers to be oriented along the flow direction, which is quite different from the results as previously reported in short-shot water-assisted injection molding (SSWAIM). A higher water pressure, shorter water injection delay time, and higher melt temperature significantly induced more fibers to be orderly oriented in OWAIM moldings, which may improve their mechanical performances and broaden their application scope.     

Visualization analysis of reciprocating‐screw plastication process of glass fiber‐reinforced resins in a glass‐inserted heating cylinder

One challenge in injection molding of long fiber‐reinforced resins is minimizing the fiber breakage during resin plastication and resin flow into a mold. Such fiber breakage reduces the strength of the molded product. Reciprocating plastication is subjected to periodic positional fluctuations of the screw and cylinder in the molding cycle. In this study, visualization experiments were conducted on the reciprocating plastication of long glass fiber (GF)‐reinforced polypropylene with 50 wt% GF. It was found that: (1) temporary voids form in the resin during the metering process, and the pellets rotate from an orthogonal to parallel orientation in the flight direction; (2) length of the weight‐averaged fiber temporarily decreased in the middle stage of the injection process; (3) length of the weight‐averaged fiber was most strongly influenced by the waiting time; and (4) for constant waiting time, the fiber breakage could be minimized by lowering the rotation speed, lowering the back pressure, and shortening the charge stroke. POLYM. ENG. SCI., 59:846–853, 2019. © 2018 Society of Plastics Engineers     

Numerical prediction of residual stresses and birefringence in injection/compression molded center‐gated disk. Part II: Effects of processing conditions

The accompanying paper, Part I, has presented the physical modeling and basic numerical analysis results of the entire injection molding process, in particular with regard to both flow‐induced and thermally‐induced residual stress and birefringence in an injection molded center‐gated disk. The present paper, Part II, investigates the effects of various processing conditions of injection/compression molding process on the residual stress and birefringence. The birefringence is significantly affected by injection melt temperature, packing pressure and packing time. However, the thermally‐induced birefringence in the core region is insignificantly affected by most of the processing conditions. On the other hand, packing pressure, packing time and mold wall temperature affect the thermally‐induced residual stress rather significantly in the shell layer, but insignificantly in the core region. The residual stress in the shell layer is usually compressive, but could be tensile if the packing time is long, packing pressure is large, and the mold temperature is low. The lateral constraint type turns out to play an important role in determining the residual stress in the shell layer. Injection/compression molding has been found to reduce flow‐induced birefringence in comparison with the conventional injection molding process. In particular, mold closing velocity and initial opening thickness for the compression stage of injection/compression molding have significant effects on the flow‐induced birefringence, but not on the thermal residual stress and the thermally‐induced birefringence.     

Effect of polymer properties on the structure of injection-molded parts

In this study, the distributions of both molecular orientation and crystallinity along the flow direction as well as across the thickness direction of injection-molded specimens of poly(ethylene terephthalate) (PET) molded at different mold temperatures were investigated. The degree of molecular orientation at the surface of the specimens was compared with that of other injected materials (polystyrene, high density polyethylene, liquid crystal polymer) showing different thermal, rheological, and crystallization characteristics. It was found that the molecular orientation at the skin layer of the molding increases with the polymer relaxation time, the rigidity of the polymer molecules, and the crystallization rate of the polymer. Moreover, in the case of PET, it was found that the crystallinity at the skin layer and in the core of the molding depends on the mold temperature. For low mold temperatures, near the gate, the maximum of crystallinity was observed at the subskin layer because of the “shear-induced crystallization” generated during the filling stage. On increasing the mold temperature, the maximum of crystallinity was found to shift to the skin layer as a result of the decrease of the thickness of this layer. For low mold temperatures, the variation of the molecular orientation in the thickness direction was found to be much the same as for the crystallinity of the polymer. This result indicates that the shear-induced crystallization process improves the degree of molecular orientation in the flow direction since it inhibits the relaxation process of the polymer molecules.     

Thin‐wall injection molding of polypropylene using molds with different laser‐induced periodic surface structures

In injection molding, high pressure is required to completely replicate the mold geometry, due to the viscosity of thermoplastic polymers, the reduced thickness of the cavity, and the low mold temperature. The reduction of the drag required to fill a thin‐wall injection molding cavity can be promoted by inducing the strong slip of the polymer melt over the mold surface, which occurs within the first monolayer of macromolecules adsorbed at the wall. In this work, the effects of different laser‐induced periodic surface structures (LIPSS) topographies on the reduction of the melt flow resistance of polypropylene were characterized. Ultrafast laser processing of the mold surface was used to manufacture nano‐scale ripples with different orientation and morphology. Moreover, the effects of those injection molding parameters that mostly affect the interaction between the mold surface and the molten polymer were evaluated. The effect of LIPSS on the slip of the polymer melt was modeled to understand the effect of the different treatments on the pressure required to fill the thin‐wall cavity. The results show that LIPPS can be used to treat injection mold surfaces to promote the onset of wall slip, thus reducing the injection pressure up to 13%. POLYM. ENG. SCI., 59:1889–1896, 2019. © 2019 Society of Plastics Engineers     

The hydrolytic stability of glass fiber reinforced poly(butylene terephthalate), poly(ethylene terephthalate) and polycarbonate

The hydrolytic stability of glass fiber reinforced poly(butylene terephthalate) (PBT), poly(ethylene terephthalate) (PET) and polycarbonate (PC) was studied. The activation energies in kcal/mole for hydrolysis are 26 for PBT and 23 for PET. Both PBT and PET contain 30 percent glass fiber reinforcement. The hydrolysis rates for a series of experimental PC's containing 10, 30 and 40 percent glass were obtained from GPC data. These increase with glass concentration but are lower than that of the unreinforced PC. Melt flow rate changes are a good measure of the hydrolytic degradation of PET. However, in the time scale of these experiments, the tensile properties of glass reinforced PBT and PC do not correlate well with M?_w changes, unlike unreinforced PBT and PC polymers. Consequently, to compare these three glass fiber reinforced polymers, estimates of failure time must be based on changes in tensile strength rather than melt flow rate.     

Improved through‐plane electrical conductivity in a carbon‐filled thermoplastic via foaming

Flow‐induced orientation of the conductive fillers in injection molding creates parts with anisotropic electrical conductivity where through‐plane conductivity is several orders of magnitude lower than in‐plane conductivity. This article provides insight into a novel processing method using a chemical blowing agent to manipulate carbon fiber (CF) orientation within a polymer matrix during injection molding. The study used a fractional factorial experimental design to identify the important processing factors for improving the through‐plane electrical conductivity of plates molded from a carbon‐filled cyclic olefin copolymer (COC) containing 10 vol% CF and 2 vol% carbon black. The molded COC plates were analyzed for fiber orientation, morphology, and electrical conductivity. With increasing porosity in the molded foam part, it was found that greater out‐of‐plane fiber orientation and higher electrical conductivity could be achieved. Maximum conductivity and fiber reorientation in the through‐plane direction occurred at lower injection flow rate and higher melt temperature. These process conditions correspond with foam flow during filling of the mold cavity, indicating the importance of shear stress on the effectiveness of a fiber being rotated out‐of‐plane during injection molding. POLYM. ENG. SCI., 2008. © 2008 Society of Plastics Engineers