The thermomechanical environment and the mechanical properties of injection moldings

Axisymmetric specimens were injection molded in a propylene copolymer with systematic variations of the melt and mold temperatures and the injection flow rate, in a total of 15 different processing conditions. From computer simulations of the mold filling stage using commercially available software packages, two thermomechanical indices were calculated. They aim at evaluating the level of orientation of the skin and the degree of crystallinity of the core layers. Assuming that these morphological features determine the mechanical response of the moldings, the thermomechanical indices were weighted by the relative thickness of the skin and core layers. The tensile behavior of the moldings was assessed at two velocities of 3.3 × 10^?5 (2 mm/min) and 3 m/s. The mechanical properties studied were the initial modulus, the yield stress and the strain at break. The relationships between the weighted thermomechanical indices and these mechanical properties were analyzed from 3D response surfaces obtained by polynomial fittings. Globally, a marked effect of the strain rate on the mechanical response along with a distinct sensitivity on the weighted thermomechanical indices was found. At high strain rates the microstructural differences were enhanced. The dependence of the yield stress on the thermomechanical indices was not significantly affected by the strain‐rate. However, the strain‐rate dependence of the other mechanical properties was strongly influenced by the initial microstructural state. Furthermore, the maximization of different mechanical properties could not be made simultaneously due to their distinct microstructural dependences. The concept of the thermomechanical indices is evidenced as a simple, valid and valuable tool to establish straightforward relationships between the processing and the mechanical behavior. Polym. Eng. Sci. 44:1522–1533, 2004. © 2004 Society of Plastics Engineers.     

Effect of processing conditions on morphology and mechanical properties of injection-molded poly(l-lactic acid)

This work investigates the relationship among the processing, morphology, and the mechanical properties of injection-molded poly(L -lactic acid) (PLLA). Melt processing temperature, mold temperature, injection flow rate, and holding pressure were systematically changed following a design of experiments array. The thermomechanical environment imposed during processing was estimated by computer simulations for the mold-filling phase, which allows the calculation of shear stress, shear rate, and the thickness of frozen skin layer. The morphology was characterized by differential scanning calorimetry and hot recoverable strain measurements. The analysis of variance results of influence of processing factors on the morphology are in good agreement with the analysis of thermomechanical parameters on the morphology. The primary factor for inducing the crystallinity in PLLA product was the stress-induced crystallization, whereas the thermal induced crystallization had a little effect. The morphology–mechanical property relationships were established. The crystallinity developed during processing has little effect on elastic modulus, increases the yield strength, and severely decreases the elongation at break. The level of molecular orientation developed during processing has little effect on elastic modulus, but increases both the yield strength and the elongation at break. POLYM. ENG. SCI., 47:1141–1147, 2007. © 2007 Society of Plastics Engineers     

Assessment of weld line performance of PP/Talc moldings produced in hot runner injection molds

Weld lines are weak regions in thermoplastic injection moldings caused by low molecular entanglement and unfavorable orientation. Their occurrence may lead to a significantly reduced mechanical performance of the products. Therefore, when weld lines are likely to occur in molded products, they must be taken into account during the mechanical and technological design processes. The weld lines become more critical when particulate fillers are compounded with the polymer. The performance of weld lines in talc‐filled polypropylene box moldings produced with a double‐gated hot runner mold is assessed in this work. The processing conditions were varied in order to cause morphology and tensile‐impact resistance changes. The weld performance at room temperature was assessed in terms of the energy absorbed in the impact tests. It was found that the performance depends on the injection temperature, the injection rate, and the orientation of the talc particles in the weld‐line plane. J. VINYL ADDIT. TECHNOL., 13:159–165, 2007. © 2007 Society of Plastics Engineers     

Mechanical properties of rotationally molded nyrim

Rotational molding is used primarily for the manufacture of products from powdered plastics. However, there are many advantages to be gained from the use of a liquid plastic feedstock. This paper describes the results of an investigation to study the effect of process variables, such as mold temperature, on the morphology and mechanical properties of parts manufactured by the rotational molding of a reactive liquid nylon, caprolactam plus a diol. Initial mold temperature has a significant effect on the degree of crystallinity, levels drop off sharply at mold temperatures in excess of 140°C, while spherulite size increases after this point. Flexural properties improve with increasing degree of crystallinity while impact strength decreases. A similar trend is observed in moldings containing fillers. A brief study of water uptake and dimensional stability of molded parts is also described.     

Distribution of higher-order structures in injection moldings of particulate-filled polypropylenes

Flexural test specimens were injection-molded from polypropylenes filled with flaky talc or spherical calcium carbonate at levels of 0-40 wt % under cylinder temperatures of 200-320°C. Distributions in the flow direction of higher-order structures such as thickness of skin layer, a*-axis-oriented component fraction [A*], and crystalline orientation functions and distributions in the thickness direction of higher-order structures such as crystallinity, β-crystal content, [A*], and crystalline orientation functions were studied. These higher-order structures are inhomogeneous in the flow and thickness directions also for injection moldings of particulate-filled polypropylenes, which strongly influences the product properties such as mechanical and thermal properties. Molecular orientation process in injection molding was theoretically analyzed from a viewpoint of growth of recoverable shear strain at the gate and its relaxation in the cavity, which could considerably well explain the influences of particulate filling and cylinder temperature on not only the mean values but also the changes in the flow and thickness directions, of the quantities such as thickness of the skin layer and crystalline orientation functions, which express the degree of molecular orientation.     

Structure and properties of injection‐molded polypropylenes with different molecular weight distribution and tacticity characteristics

Two series of polypropylenes with different molecular weight distribution and tacticity characteristics were injection molded into flexural test specimens by varying cylinder temperature and the effects of the molecular weight distribution and tacticity on the structure and properties of the moldings were studied. Measured propertied were flexural modulus, flexural strength, heat distortion temperature, Izod impact strength, and mold shrinkage and structures studied were crystallinity, the thickness of skin layer, a*‐axis‐oriented component fraction and crystalline orientation functions. The relations between the structures and properties were also studied. It was found that the molecular weight distribution and tacticity characteristics affect the properties mainly through the molecular orientation and crystallinity, respectively. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 84: 2142–2156, 2002     

Influence of processing conditions on the mechanical properties of SEBS/PP/oil blends

A common type of thermoplastic elastomer is a blend of styrene-ethylene-butylene-styrene copolymer (SEBS) with polypropylene and mineral oil. The knowledge about the processing parameters that influence the blend of these components is important to achieve the best material properties. In this study, were evaluated the influence of screw configuration (low and high shear) and speed (200, 300, and 400 rpm), molding form (compression and injection) and SEBS type (high and low molecular weight). The specimens were characterized according to their morphology, crystallinity, thermomechanical and mechanical properties. Samples processed in a screw with more kneading zones had a lower modulus value. Compression molded specimens had lower mechanical performance than injected samples. Samples prepared with low molecular weight SEBS presented a loss of mechanical properties. It was possible to correlate processing form with all the properties evaluated. The analyzed variables influenced the morphology, crystallinity and thermomechanical properties, but had low influence on the tensile properties.     

Thermomechanical processing environment and morphology development of a thermotropic polymer liquid crystal

We have studied a longitudinal polymer liquid crystal consisting of poly(ethylene terephthalate) (PET) and p‐hydroxybenzoic acid, namely PET/0.6PHB, where 0.6 is the mole fraction of the second component. The material was injection molded with systematic variations of the melt and mold temperatures and injection flow rate using design of experiments based on a Taguchi orthogonal array. Thermomechanical environment defined by local melt temperatures and shear rates and stresses imposed during processing was estimated by computer simulations of the mold‐filling phase. The morphology of the moldings was characterized by optical and scanning electronic microscopy, wide‐ and small‐angle X‐ray scattering, and differential scanning calorimetry. An analysis of variance approach identified the significant processing variables and their contributions to variations of morphological parameters. The processing environment affects strongly the melt viscosity, and there is a strong thermo‐mechanical coupling. The result is a complex multilaminated and hierarchical microstructure, whose morphological features are very sensitive to the processing conditions. Relationships between local thermomechanical variables (rather than global ones) and the morphological parameters are established. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Tailoring toughness of injection molded bar of polypropylene random copolymer through processing melt temperature

In this study, a facile route to realize the superior toughness of injection molded polypropylene random copolymer (PPR) is reported. The toughness of PPR is increased about twofold when the processing melt temperature increases from 180 to 250 °C. Systematic and detailed structural characterizations have been carried out to establish the structure–property relationships by using polarized light microscopy, scanning electron microscopy, infrared microscopy and dynamic mechanical analysis. It is found that increasing the melt temperature is beneficial for the coalescence of rubbery domains and enhanced molecular mobility which are mainly responsible for the improvement in toughness. Other factors, such as molecular orientation, crystallinity and so on, seem to have little effect. The vital role of enhanced molecular mobility in improving toughness is further demonstrated by the annealing of injection molded samples at elevated temperature, i.e. 110 °C. Copyright © 2011 Society of Chemical Industry     

Compounding and injection molding of polybutene‐1/polypropylene blends

The effect of shear‐controlled orientation injection molding (SCORIM) was investigated for polybutene‐1/polypropylene blends. This article reports on the methods and processing conditions used for blending and injection molding. The properties of SCORIM moldings are compared with those of conventional 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 with mechanical tests, differential scanning calorimetry studies, and polarized light microscopy. The application of SCORIM improved Young's modulus and the ultimate tensile strength. The gain in stiffness was greater for higher polybutene‐1 content blends. A drastic decrease in the strain at break and toughness was observed in SCORIM moldings. The enhanced molecular orientation of SCORIM moldings resulted in a featureless appearance of the morphology. Interfacial features due to segregation were visible in the micrographs of SCORIM moldings. Both conventional and SCORIM moldings exhibited form I′ in polybutene‐1. This article explains the relationship between the mechanical properties and micromorphologies. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 88: 806–813, 2003     

Mechanical properties of glass fiber reinforced polypropylene injection molded with a rotation mold

A special mold (Rotation, Compression, and Expansion Mold) was used to impose a controlled shear action during injection molding of short glass fiber reinforced polypropylene discs. This was achieved by superimposing an external rotation to the pressure‐driven advancing flow front during the mold filling stage. Central gated discs were molded with different cavity rotation velocities, inducing distinct levels of fiber orientation through the thickness. The mechanical behavior of the moldings was assessed, in tensile and flexural modes on specimens cut at different locations along the flow path. Complete discs were also tested in four‐point flexural and in impact tests. The respective results are analyzed and discussed in terms of relationships between the developed fiber orientation level and the mechanical properties. The experimental results confirm that mechanical properties of the moldings depend strongly on fiber orientation and can thus be tailored by the imposed rotation during molding. POLYM. ENG. SCI. 46:1598–1607, 2006. © 2006 Society of Plastics Engineers.     

The crystallinity of poly(phenylene sulfide) and its effect on polymer properties

The crystallinity and crystallizability of poly(phenylene sulfide) have been examined by a number of common techniques. Several provided qualitative information, but only one, x-ray diffraction, was considered sufficiently reliable and reproducible to allow quantitative comparisons. Based on x-ray measurements, an approximate degree of crystallinity, termed crystallinity index (C_i), could be readily assigned. According to this method, virgin polymer possesses significant crystallinity (C_i ≈ 65%). Curing (crosslinking) the resin below its melting point did not change the crystallinity but did affect the crystallizability. Lightly cured resin suitable for molding and film extrusion was easily quenched from the melt to give amorphous polymer. The amorphous samples crystallized rapidly when heated to temperatures > 121°C (250°F). At mold temperatures below 93°C (200°F), moldings with very low surface crystallinity were produced. Annealing (204°C, 400°F) caused rapid crystallization of such moldings, and changes in crystallinity were correlated with observed changes in physical properties. The resin crystallizes so rapidly that these quenched moldings possessed a crystallinity gradient, the internal crystallinity being substantially greater. At high mold temperatures (121–204°C, 250–400°F), moldings very similar to fully annealed specimens were obtained.     

The local thermomechanical conditions and the fracture behavior of an injection‐molded poly(oxymethylene)

This work explores the fracture behavior of an injection‐molded engineering thermoplastic, poly(oxymethylene), POM. Lateral gated discs of 120 mm diameter and 1.5 mm thick were molded with variations of the melt temperature and injection flow rate. The local thermomechanical environment was characterized by computer simulations of mould filling phase. This also allows the computation of two thermomechanical indices related to the morphological state of the moldings. From the molded discs, double edge notched tensile (DENT) specimens were meticulously cut at different angles with respect to the flow direction and with distinct ligament lengths. The fracture tensile tests were performed at 2 mm/min at controlled room temperature. As the test configuration (specimen geometry) imposes a plane stress state, the stress intensity factor, K_I, was calculated as function of the processing conditions and orientation angle with respect to the flow direction. The K_I values and their degree of anisotropy are dependent upon processing conditions. Finally, the dependences of K_I values upon the thermomechanical indices (and therefore on the expected morphological state of the moldings) are established. POLYM. ENG. SCI. 46:181–187, 2006. © 2005 Society of Plastics Engineers     

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     

Processing–mechanical property relationships in injection moldings of polybutylene

Two unfilled nonpigmented extrusion grades of polybutylene have been injection-molded into a tensile bar mold under a wide range of barrel and mold temperatures. The overall structure of the moldings has been determined and correlated with processing conditions. The short term tensile mechanical properties of the moldings have been ascertained and correlated with molding structure. For low mold temperatures, the Young's modulus and tensile strength of injection moldings of polybutylene are controlled by the extent of and structure within the highly oriented skin. Low barrel temperatures can give rise to highly crystalline thick skins that treble the Young's modulus and fracture stress, when compared to high barrel temperature moldings. Increasing the mold temperature introduces a brittle response in polybutylene injection moldings. Modulus is controlled, at the high mold temperatures, by the skin thickness and by the crystallinity of the material comprising the core of the molding.     

Effects of molding conditions on properties of injection-molded polycarbonates

Statistically designed experiments were carried out to study the effects of molding conditions on the properties of two types of polycarbonate, which were synthesized by the solvent process and the melt process, respectively. The properties tested in this study were classified into two groups with respect to the effect of molding conditions. One, which included birefringence, heat shrinkage at 180°C, and surface resistance to Taber abrasion, was mainly affected by stock temperature and was slightly affected by holding pressure. The other, which included resistance to solvent crack, Rockwell hardness, density, and heat shrinkage at 120°C, was affected by mold temperature and holding pressure. Mechanically isotropic moldings with a low degree of frozen orientation could be molded at a high stock temperature and at a low holding pressure, where stock temperature was more effective than holding pressure. Moldings with low residual stresses could be molded at a high mold temperature and at a low holding pressure. Essentially there was no difference in the molding conditions and properties by the method of synthesis. However, under the same molding conditions polycarbonate synthesized by the melt process gave a higher degree of frozen orientation and somewhat more rigid moldings.     

Structural development of HDPE in injection molding

This study investigated some relevant structure/properties relationships in shear‐controlled orientation in injection molding (SCORIM) of high‐density polyethylene (HDPE). SCORIM was used to deliberately induce a strong anisotropic character in the HDPE microstructure. Three grades with different molecular weight characteristics were molded into tensile test bars, which were subsequently characterized in terms of the mechanical behavior by tensile tests and microhardness measurements. The structure developed upon processing was also characterized by polarized light microscopy (PLM), scanning electron microscopy (SEM), differential scanning calorimetry (DSC), and wide‐angle X‐ray diffraction (WAXD). SCORIM allows the production of very stiff molded parts, exhibiting a very well‐defined laminated morphology. This morphology is associated with both an M‐shaped microhardness profile and a pronounced mechanical anisotropy. These characteristics are supported by an analogous variation in the crystallinity and a high level of molecular orientation, as indicated, respectively, by calorimetric measurements and X‐ray diffraction results. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 89: 2079–2087, 2003     

Relationship between processing method and microstructural and mechanical properties of poly(ethylene terephthalate)/short glass fiber composites

Composites of recycled poly(ethylene terephthalate) (PET) reinforced with short glass fiber (GF) (0, 20, 30, and 40 wt %) were compounded in a single‐screw extruder (SSE) and in a intermeshing corotating twin‐screw extruder (TSE). An SSE fitted with a barrier double‐flight screw melting section in between two single‐flight sections and a TSE with a typical screw configuration for this purpose were used. The composites were subsequently injection molded at two different mold temperatures (10 and 120°C), with all other operative molding parameters kept constant. The effects of processing conditions on composite microstructure, PET degree of crystallinity, and composite mechanical properties were evaluated. Appropriate dispersive and distributive mixing of the glass fiber throughout the PET matrix as well as fine composite mechanical and thermal‐mechanical properties were achieved regardless of whether the composites were prepared in the SSE or TSE. The performance of the SSE was attributed to the efficiency of the barrier screw melting section in composite mixing. The mold temperature influenced the mechanical properties of the composites, by controlling of the degree of crystallinity of the PET in the composites. For a good balance of mechanical and thermal‐mechanical properties, high mold temperatures are desirable, typically, 120°C for a mold cooling time of 45 s. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci 2008     

Predicting mechanical properties of acrylonitrile-butadiene-styrene terpolymer in injection molded plaque and box

A methodology to predict mechanical properties in injection molded parts has been developed. Knowledge of part properties before actual molding and testing will be of immense help to part and mold designers in modification of design. This methodology involved the application of connectionist learning systems, injection molding computer simulation, and experimental evaluation of mechanical properties, to relate the thermomechanical history of injection molded parts to the resulting part properties of injection molded parts are dependent upon their thermomechanical history which in turn is greatly influenced by the processing conditions and part geometry. As the relationships between engineering properties and thermomechanical history are complex and highly nonlinear, the methodology developed was based on a backpropagation neural network algorithm that provided the means for a nonparametric mapping between the part properties and thermomechanical history. The proposed methodology has been successfully applied to two geometries, plaque and box. This methodology provides designers with the ability to predict mechanical properties in injection molded parts when significant thermomechanical history can be obtained from injection molding simulation.