Injection molding and injection compression molding of thin‐walled light‐guided plates with V‐grooved microfeatures

In this study, we investigated the feasibility of injection molding (IM) and injection compression molding (ICM) for fabricating 3.5‐in. light‐guided plates (LGPs). The LGP was 0.4 mm thick with v‐grooved microfeatures (10 μm wide and 5 μm deep). A mold was designed to fabricate LGPs by IM and ICM. Micromachining was used to make the mold insert. The Taguchi method and parametric analysis were applied to examine the effects of the process parameters on the molding quality. The following parameters were considered: barrel temperature, mold temperature, packing pressure, and packing time. Mold temperature in this investigation was in the conventional range. Increasing the barrel temperature and mold temperature generally improved the polymer melt fill in the cavities with microdimensions. The experimental results for the replication of microfeatures by IM and ICM are presented and compared. The height of the v‐grooved microfeatures replicated by ICM was more accurate than those replicated by IM. Additionally, the flatness of the fabricated LGPs showed that ICM was better than IM for thin‐walled molding. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Wall Slip of Linear Polymer Melts During Ultrasonic‐assisted Micro‐injection Molding

The wall slip of linear polymer melts under ultrasonic vibration is investigated by correcting the slip mechanism, and melt flow behaviors in ultrasonic‐assisted micro‐injection molding (UμIM) method are discussed. Based on the effect mechanism of ultrasonic vibration on the melt, theoretical models of the critical shear stresses for the onset of weak and strong wall slip during UμIM are established, and the change in rheological properties due to the onset of wall slip under ultrasonic vibration is experimental investigated by a built measurement system. The results show that the onset of weak and strong wall slip of the melt in micro cavity are promoted by ultrasonic vibration, which agree with the built theoretical models, and the melt filling capability in micro cavity is enhanced by reducing apparent viscosity and releasing shear stress of the polymer melt, which improves the molding quality of micro polymer parts via UμIM method. POLYM. ENG. SCI., 59:E7–E13, 2019. © 2018 Society of Plastics Engineers     

Bi‐axial self‐reinforcement of high‐density polyethylene induced by high‐molecular weight polyethylene through dynamic packing injection molding

Previously, bi‐axial self‐reinforcement of high‐density polyethylene (HDPE) was achieved through a uni‐axial shear stress field introduced by dynamic packing injection molding technology. Here, further improvement of tensile strength along the flow direction (MD) was achieved by blending a small amount of high‐molecular‐weight polyethylene (HMWPE) with HDPE, while the tensile strength along the transverse direction (TD) still substantially exceeded that of conventional moldings. Tensile strengths in both flow and transverse directions were considerably enhanced, with improvements from 23 MPa to 76 MPa in MD and from 23 MPa to 31 MPa in TD. The effect of HMWPE content and molding parameters on tensile properties was also investigated. The tensile strength along MD was highly dependent on HMWPE content, oscillating cycle, mold temperature, melt temperature and packing pressure, while that along TD was insensitive to composition and processing parameters within the selected design space. According to the stress–strain curves, samples with HMWPE produced by dynamic packing injection molding had a special tensile failure mode in MD, different from both typical plastic and brittle failure modes. There were no yielding and necking phenomena, which are characteristic during tensile testing of plastic materials, but there was still a considerably higher elongation compared to those of brittle materials. However, in TD, all dynamic injection molding samples exhibited plastic failure as did typical conventional injection molding samples. Copyright © 2006 Society of Chemical Industry     

Investigation of melt manipulation phenomena during injection molding via in situ birefringence observation

This study investigates the effects of melt manipulation on the development of molecular orientation during injection molding processing. Vibration‐assisted injection molding (VAIM), a particular method of melt manipulation, is a variation of conventional injection molding in which oscillatory energy is imparted to the polymer melt by vibrating the injection screw axially during the injection and packing stages of the molding cycle. Previous studies have shown that this process positively affects the tensile strength of polystyrene parts, but that the magnitude of the increase is dependent upon the processing parameters. Observation of birefringence patterns in VAIM processed samples show a significant impact on molecular orientation. A specially designed mold and associated image capture system has been developed and is used in this study to record the birefringence patterns of the polymer melt within the cavity during processing. Observation of birefringence shows that orientation develops primarily during post‐vibration packing of the part and not during the vibration phase as previously thought. The observed effects of process parameters such as melt temperature, packing pressure, and vibration duration are discussed. POLYM. ENG. SCI. 46:1691–1697, 2006. © 2006 Society of Plastics Engineers     

Primary and secondary gas penetration during gas‐assisted injection molding. Part II: Simulation and experiment

Simulation and experimental studies have been carried out on the transient gas‐liquid interface development and gas penetration behavior during the cavity filling and gas packing stage in the gas‐assisted injection molding of a spiral tube cavity. The evolution of the gas/melt interface and the distribution of the residual wall thickness of skin melt along with the advancement of gas/melt front were investigated. Numerical simulations were implemented on a fixed mesh covering the entire cavity. The residual thickness of a polymer layer and the length of gas penetration in the moldings were calculated using both the simulation and model developed in Part I of this study and commercial software (C‐Mold). Extensive molding experiments were performed on polystyrene at different processing conditions. The obtained results on the gas bubble dynamics and penetration behaviors were compared with those predicted by the present simulation and C‐Mold, indicating the good predictive capability of the proposed model. Polym. Eng. Sci. 44:992–1002, 2004. © 2004 Society of Plastics Engineers.     

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     

Simulation of injection–compression‐molding process. II. Influence of process characteristics on part shrinkage

A numerical algorithm is developed to simulate the injection–compression molding (ICM) process. A Hele–Shaw fluid‐flow model combined with a modified control‐volume/finite‐element method is implemented to predict the melt‐front advancement and the distributions of pressure, temperature, and flow velocity dynamically during the injection melt filling, compression melt filling, and postfilling stages of the entire process. Part volumetric shrinkage was then investigated by tracing the thermal–mechanical history of the polymer melt via a path display in the pressure–volume–temperature (PVT) diagram during the entire process. Influence of the process parameters including compression speed, switch time from injection to compression, compression stroke, and part thickness on part shrinkage were understood through simulations of a disk part. The simulated results were also compared with those required by conventional injection molding (CIM). It was found that ICM not only shows a significant effect on reducing part shrinkage but also provides much more uniform shrinkage within the whole part as compared with CIM. Although using a higher switch time, lower compression speed, and higher compression stroke may result in a lower molding pressure, however, they do not show an apparent effect on part shrinkage once the compression pressure is the same in the compression‐holding stage. However, using a lower switch time, higher compression speed, and lower compression stroke under the same compression pressure in the postfilling stage will result in an improvement in shrinkage reduction due to the melt‐temperature effect introduced in the end of the filling stage. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 75: 1640–1654, 2000     

Assessing the effect of processing variables on the mechanical response of polysytrene molded using vibration‐assisted injection molding process

The present work is focused on the study of vibration‐assisted injection molding (VAIM) process, using polystyrene as a model polymeric system. This recently developed polymer processing operation is based on the concept of using motion of the injection screw to apply mechanical vibration to polymer melt during the injection and packing stages of injection molding process, to control the polymer behavior at a molecular level, which would result in improvements/alterations to the mechanical behavior of molded products. In this study, the afore‐mentioned concept was verified experimentally from monotonic tensile experiments and birefringence measurements of VAIM molded polystyrene in comparison with those of conventional injection molding process. The results of our study indicate that the actual degree of strength improvement depends on at least four parameters, namely, vibration frequency, vibration amplitude, vibration duration, and the delay time between the injection start and the vibration start. Furthermore, when these parameters were optimized, as much as a 28% strength improvement was observed, accompanied by an increase in toughness. Furthermore, birefringence measurements revealed that VAIM processing significantly altered the residual stress distribution throughout final products, but it did not, however, change the material density in the products. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci, 2006     

Processing–Nanostructure–Property Relationships of All‐Polyethylene Composites Reinforced by Flow‐Induced Oriented Crystallization of UHMWPE

All‐polyethylene composites exhibiting substantially improved toughness/stiffness balance are readily produced during conventional injection molding of high density polyethylene (HDPE) in the presence of bimodal polyethylene reactor blends (RB40) containing 40 wt% ultrahigh molar mass polyethylene (UHMWPE) dispersed in HDPE wax. Scanning electron microscopy (SEM) and differential scanning calorimetry (DSC) analyses shows that flow‐induced crystallization affords extended‐chain UHMWPE nanofibers forming shish which nucleates HDPE crystallization producing shish‐kebab structures as reinforcing phases. This is unparalleled by melt compounding micron‐sized UHMWPE. Injection molding of HDPE with 30 wt% RB40 at 165 °C affords thermoplastic all‐PE composites (12 wt% UHMWPE), improved Young's modulus of 3400 MPa, tensile strength of 140 MPa, and impact resistance of 22.0 kJ/m². According to fracture surface analysis, the formation of skin‐intermediate‐core structures accounts for significantly improved impact resistance. At constant RB40 content both morphology and mechanical properties strongly depend upon processing temperature. Upon increasing processing temperature from 165 °C to 250 °C the average shish‐kebab diameter increases from the nanometer to micron range, paralleled by massive loss of self‐reinforcement above 200 °C. The absence of shish‐kebab structure at 250 °C is attributed to relaxation of polymer chains and stretch‐coil transition impairing shish formation.     

Luminance of injection‐molded V‐groove light guide plates

Injection‐molded V‐groove light‐guide plates (LGPs) were made of two optical grade polycarbonates of different viscosities at various molding conditions. Their luminance as a function of viewing angle was measured. The depth of melt filling of the grooves in LGPs was measured at different locations selected based on the melt front propagation during the cavity filling. The depth of melt filling of grooves was mostly completed during the cavity‐filling stage. It was only slightly affected by the packing pressure and packing time and strongly affected by other molding conditions and layout of the V‐grooves. Luminance of LGPs showed a strong correlation with the depth of melt filling of the V‐grooves. POLYM. ENG. SCI., 2008. © 2008 Society of Plastics Engineers     

Influence of different sterilization procedures on the morphological parameters and mechanical properties of ultra‐high‐molecular‐weight polyethylene

The objectives of this study were to examine the effects of the processing conditions, sterilization, and thermal treatment on the morphological and mechanical properties of ultra‐high‐molecular‐weight polyethylene (UHMWPE) in medical applications by means of thermal analysis, Fourier transform infrared spectroscopy, and nanoindentation. It is well known that manufacturing, irradiation, and thermal treatments significantly alter the microstructure of materials, which results in changes in their mechanical properties. UHMWPE was found to be barely sensitive to processing conditions but strongly influenced by sterilization treatments. Great emphasis was given to the characterization of the so‐called first generation of highly crosslinked UHMWPE because the thermal history of this material differed from that of γ‐irradiated materials. The physical and mechanical properties of UHMWPE were influenced as a result of γ and electron‐beam irradiation and the remelting procedure. Lower crystallinity, different lamellar thickness distributions, and lower hardness and modulus values were estimated. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Exploring the entangled state and molecular weight of UHMWPE on the microstructure and mechanical properties of HDPE/UHMWPE blends

Three types of ultra-high molecular weight polyethylene (UHMWPE) with different entangled state and molecular weight were blended with high-density polyethylene (HDPE) matrix by melt blending. Rheology, 2D-SAXS, 2D-WAXD, DSC, and mechanical tests were used to study the evolution and difference of microstructure and mechanical properties of the blends. The addition of weakly entangled UHMWPE enhanced the chain diffusion and chain orientation ability under a specific flow field. Thus, the rheological properties and mechanical properties of the blends were improved with the mix of weakly entangled UHMWPE. The mechanical properties enhancement effect of HDPE/UHMWPE blends with weakly entangled UHMWPE was owing to the shish-kebab structure formed in the injection molding process. The molecular chains of UHMWPE with a low degree of entanglement and high molecular weight increased the lamella size and crystallinity of the blends during processing. This leads to the formation of more oriented shish structures and more kebab lamella. Besides, the molecular chains of weakly entangled UHMWPE were better interlocked and intertwined with other polyethylene chains in the amorphous region, acting as the tie molecules, significantly improving the impact resistance.     

Structure and physical property relationships in processed polybutene‐1

The effect of SCORIM was investigated on three grades of polybutene‐1 and one grade of ethylene–butene‐1 copolymer. The methods and processing conditions used for injection molding and the properties of the moldings are reported. Phase transformations and their relationship with mechanical properties are discussed in detail. Both, conventional and shear‐controlled orientation injection molding (SCORIM) were employed to produce moldings. SCORIM is based on the application of specific macroscopic shears to a solidifying melt. The multiple shear action enhances molecular alignment. The moldings were investigated by performing mechanical tests, fractographic analysis, differential scanning calorimetry studies, wide‐angle X‐ray diffraction, polarized light microscopy, and atomic force microscopy. The application of SCORIM improves the mechanical performance. Molecular orientation results in the formation of shish‐kebab morphology. One grade of polybutene‐1 exhibited a greater than fivefold increase in Young's modulus. The application of high cavity pressures favored the formation of the stable Form I' in polybutene‐1. The formation of Form I' led to a decrease in crystallinity and mechanical properties. However, this loss was by far smaller than the gain obtained via the formation of shish‐kebab morphology. The relationship between mechanical properties and micromorphologies of the investigated materials is explained. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 88: 814–824, 2003     

注射压缩成型聚碳酸酯制品的低温拉伸力学性能

以聚碳酸酯为材料,利用自行设计带有压缩功能的模具,采用常规注塑成型(IM)和注射压缩成型方法(ICM)对比研究制品在常温和低温环境下的拉伸力学性能;基于单因素实验方法,研究熔体温度、模具温度、模板压缩距离、延迟时间和压缩力对ICM制品残余应力和低温拉伸性能的影响规律。结果表明:在相同的环境温度下,ICM制品较IM制品有较大的屈服应力和弹性模量;低温环境下样品的拉伸性能有所提升,并在-40℃附近出现了聚碳酸酯分子的次级玻璃化转变;残余应力是影响ICM制品低温拉伸性能的主要因素,较高的熔体温度、模具温度、模板压缩距离,以及较短的延迟时间,较小的压缩力会减小ICM制品的残余应力,提高制品的低温拉伸性能。     

High‐density polyethylene/ultrahigh‐molecular‐weight polyethylene blend. I. The processing,thermal, and mechanical properties

Various blend ratios of high‐density polyethylene (HDPE) and ultrahigh‐molecular‐weight polyethylene (UHMWPE) were prepared with the objective of determining their suitability as biomaterials. Although the presence of HDPE in the blends enabled melt processing, the presence of UHMWPE helped to improve the toughness of the resulting blends. The processability of the blends was investigated with the Brabender torque, which was used as an indication of the optimum blend conditions. The blends were characterized with differential scanning calorimetry. The mechanical tests performed on the blends included tensile, flexural, and impact tests. A 50:50 (w/w) blend yielded optimum properties in terms of the processability and mechanical properties. The tensile property of the 50:50 blend was intermediate between those of HDPE and UHMWPE, but the strain at break increased 200% in comparison with that of both neat resins. The energy at break of the 50:50 blend revealed an improvement in the toughness. The fracture mechanism was also investigated with scanning electron microscopy. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 97: 413–425, 2005     

Characterization of the microinjection molding process

The commonly used plunger injection system in the microinjection molding (μIM) process has separate screw melting, metering, and injection units. As a result, extra operating parameters and complexity are introduced, in comparison with conventional injection molding. In this study, a μIM machine was used to obtain micromoldings of polyoxymethylene, high‐density polyethylene, and polycarbonate. A data acquisition system was developed to record traces of data regarding the evolution of process variables with time. Cavity filling was followed, at the millisecond time scale, using short‐shot experiments and traces of injection pressure, runner pressure, and plunger position. Six characteristic process parameters (CPPs) were defined to characterize both the cavity filling and packing stages. The method of design of experiments was used to investigate the effects of machine settings on the CPPs. Metering size, which was optimized for each set of machine variables, was also used as a CPP. Injection speed was the most significant factor affecting plunger velocity and injection pressure during cavity filling, while the effects of mold and melt temperature varied with the material and machine settings. POLYM. ENG. SCI., 2010. © 2010 Society of Plastics Engineers     

Effect of packing pressure on fiber orientation in injection molding of fiber‐reinforced thermoplastics

Injection molding of fiber‐reinforced polymeric composites is increasing with demands of geometrically complex products possessing superior mechanical properties of high specific strength, high specific stiffness, and high impact resistance. Complex state of fiber orientation exists in injection molding of short fiber reinforced polymers. The orientation of fibers vary significantly across the thickness of injection‐molded part and can become a key feature of the finished product. Improving the mechanical properties of molded parts by managing the orientation of fibers during the process of injection molding is the basic motivation of this study. As a first step in this direction, the present results reveal the importance of packing pressure in orienting the fibers. In this study, the effects of pressure distribution and viscosity of a compressible polymeric composite melt on the state of fiber orientation after complete filling of a cavity is considered experimentally and compared with the simulation results of Moldflow analysis. POLYM. COMPOS. 28:214–223, 2007. © 2007 Society of Plastics Engineers     

Investigation of fiber orientation states in injection‐compression molded short‐fiber‐reinforced thermoplastics

Injection‐compression molding (ICM) has received increased attention because of its advantages over conventional injection molding (CIM). This article aims to investigate the effects of five dominating ICM processing parameters on fiber orientation in short‐fiber‐reinforced polypropylene (SFR‐PP) parts. A five‐layer structure of fiber orientation is found across the thickness under most conditions in ICM parts. This is quite different from the fiber orientation patterns in CIM parts. The fibers orient orderly along the flow direction in the shell region, whereas most fibers arrange randomly in the skin and the core regions. Additionally, the fiber orientation changes in the width direction, with most fibers arranging orderly along the flow direction at positions near the mold cavity wall. The results also show that the compression force, compression distance, and compression speed play important roles in determining the fiber states. Thicker shell regions, in which most fibers orient remarkably along the flow direction, can be obtained under larger compression force or compression speed. Moreover, the delay time has an obvious effect on the fiber orientation at positions far from the gate. However, the effect of compression time is found to be negligible. POLYM. COMPOS., 31:1899–1908, 2010. © 2010 Society of Plastics Engineers.     

Properties of compression molded ultra‐high molecular weight polyethylene products pretreated by eccentric rotor extrusion

Low processing efficiency and fusion defects limit the application of ultra‐high molecular weight polyethylene (UHMWPE) in artificial joint implants. These problems result from the high melt viscosity of UHMWPE. Here, we use an eccentric rotor extruder (ERE) based on elongational flow to pretreat UHMWPE. Compression molded UHMWPE is obtained without and with ERE pretreatment (EP‐UHMWPE). The processing efficiency of EP‐UHMWPE is improved compared with direct compression molded UHMWPE. This is because the preheating time can be omitted during the molding process, and the residence time of UHMWPE in the extruder is less than 90 s. The mechanical properties and friction resistance of EP‐UHMWPE are significantly improved compared with those of direct compression molded UHMWPE. The yield strength increases from 21 MPa to 23 MPa, the tensile strength increases from 36 MPa to 46 MPa, the elongation at break increases from 610% to 700%, and the abrasion loss decreases from 1.73 mg/1000 r to 0.93 mg/1000 r when UHMWPE is subjected to ERE pretreatment. We attribute these improvements to the elongational flow enhancing the orientation and disentanglement of UHMWPE molecular chains, which in turn improves particle fusion. The molecular weight is well maintained when subjected to ERE pretreatment. UHMWPE components pretreated by ERE have good prospects in artificial joint implants. © 2019 Society of Chemical Industry     

Structure and properties of microfibrillar‐reinforced composites based on thermoplastic PET/LDPE blends after manufacturing by means of pultrusion

In situ microfibrillar‐reinforced composites (MFC) based on blends from poly(ethyleneterephthalate) (PET) and low‐density polyethylene (LDPE) were prepared under industrial relevant conditions by melt extrusion, followed by continuous cold drawing in weight ratios of PET/LDPE equal to 50/50. Test specimens were prepared by pultrusion (Pult) of the drawn blend at a processing temperature below the melting temperature of PET. This was the first attempt to pultrude such a material. By varying the Pult parameters, rectangular cross‐sectional profiles have been successfully produced using a self‐designed Pult line. For comparison, plates were also prepared by compression (CM) and injection molding (IM). Samples of each stage of MFC manufacturing and processing were characterized by means of scanning electron microscopy (SEM), wide‐angle X‐ray scattering (WAXS), and mechanical testing. SEM and WAXS showed that the highly oriented blends are converted into MFC‐structured polymer–polymer composites during the Pult, CM, and IM process. POLYM. ENG. SCI., 2010. © 2009 Society of Plastics Engineers