Injection molding of semicrystalline polymers. I. Material characterization

Various material data for an isotactic polypropylene were acquired for the simulation of the injection molding of this material. Viscosity as a function of shear rate and temperature was measured using a capillary rheometer at high shear rates and a cone-and-plate rheometer at low shear rates. Heat-flow properties, characterizing kinetics and induction time of quiescent crystallization, were obtained from DSC measurements. Material data characterizing shear-induced crystallization were obtained from extrusion experiments through a slit die with subsequent quenching of the material in the die after various rest times. The thickness of the shear-induced crystallization layer was measured along with the birefringence in this layer. A model of shear-induced crystallization developed by Janeschitz-Kriegl and co-workers was used to fit the kinetic data. Thus, kinetic parameters such as the limiting shear rate below which no shear-induced crystallization can occur and the characteristic time for the relaxation of birefringence were obtained. © 1995 John Wiley & Sons, Inc.     

Modeling and experimental study of birefringence in injection molding of semicrystalline polymers

The prediction of birefringence developed in injection moldings is very important in order to satisfy required specification of molded products. A novel approach for the numerical simulation of the flow-induced crystallization and frozen-in birefringence in moldings of semicrystalline polymers was proposed. The approach was based on the calculation of elastic recovery that becomes frozen when the flow-induced crystallization occurred. The flow effect on the equilibrium melting temperature elevation due to the entropy reduction between the oriented and unoriented melts was incorporated to model crystallization. To find the entropy reduction and the frozen-in elastic recovery during crystallization, a non-linear viscoelastic constitutive equation was used. From the ultimate elastic recovery the crystalline orientation function was calculated. The crystalline and amorphous contributions to the overall birefringence were obtained from the crystalline orientation function and the flow birefringence, respectively. The birefringence profiles were measured and predicted in moldings of polypropylenes of different molecular weights obtained at various melt temperatures, injection speeds, holding times and mold temperatures. The resulting predictions were in fair agreement with corresponding experimental data.     

Experimental and numerical investigation of warpage of semicrystalline polymers in rotational molding

Warpage of various semicrystalline polyethylenes (linear low density polyethylene [LLDPE]) has been investigated under typical rotational molding conditions, which means slow cooling from only one side. We have developed an experimental technique that is able to quickly rate different materials with respect to warpage under typical process conditions. We have also developed a numerical model simulating the experiments assuming a thermoelastic material including crystallization. As has been observed in practical rotational molding, it has been found in both experiments and simulations that materials with high crystallinity have in general higher warpage. The simulations also showed that the crystallization kinetics has implications on the warpage because the crystallinity gradient during solidification depends on the rate of crystallization. POLYM. ENG. SCI., 45:945–952, 2005. © 2005 Society of Plastics Engineers     

Injection molding of thermotropic liquid crystal polymers

Thermotropic liquid crystal polymers consist of rod-like molecules and are often called “self reinforcing thermoplastics.” Their rheological behaviors as well as orientation development during processing are often very similar to those of short fiber-filled composites. Without reinforcement, the polymer shows superior mechanical properties to conventional glass fiber-reinforced engineering resins. The orientation distribution in the crosssection as well as flow patterns in the molded thermotropic polymers are clearly visible to the naked eye due to color differences. This makes it particularly convenient to study the orientation distribution as well as the flow patterns of packing, back flow, jetting, flow instabilities, and weld line formation in injection molding. This paper discusses physical properties of a typical ther motropic polymer and their relationship to mold filling process in the injection molding.     

PVT measurement methodology for semicrystalline polymers to simulate injection‐molding process

This article discusses the specific volume‐measurement methods for semicrystalline polymers needed in order to obtain reliable data. Particularly, the effect of the cooling rate is analyzed, taking into account the thermal gradient in a cylindrical sample. Experimental results for a polypropylene form the basis for the study. In a first step our thermal model was validated by comparing the calculated results with the experimental ones for a temperature range higher than the crystallization zone with different cooling rates and by analyzing the stabilization time of the measured specific volume after cessation of the cooling. Secondly, specific‐volume evolutions from 220°C to 50°C for different cooling rates and different pressures were analyzed, revealing that when the data are corrected to eliminate the thermal gradient effect, the transition zone is much narrower than the experimental one. Moreover, the effect of the pressure and the cooling rate on the relative crystallinity function—that is, on the crystallization kinetics—can be more accurately evaluated. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 79: 302–311, 2001     

Electroplating on crystalline polypropylene. II. Injection molding and adhesion

This paper describes the effect of injection molding conditions (melt temperature, mold temperature, and fill time) and etch conditions on metal adhesion in electroplated isotactic polypropylene (PP). It is found that injection molding PP homopolymer produces a lamellar surface morphology which can consistently develop after-plated peel strengths of 30 lb/in or better as measured by the Jacquet peel test. Surface etching of PP homopolymer prior to plating develops crack patterns characteristic of injection molding; a directional crack pattern is always evident in specimen surfaces crystallized under shear. The surface pattern is developed in the oxidative process by swelling of amorphous material, followed by oxidative dissolution and oxidative stress cracking. Additionally, the depth and number of the surface cracks is a function of the solvent swell and acid etch times. Crack depth increases in lamellar surfaces as the sample immersion times are increased; however, as crack depth increases, crack density decreases. Metal-to-polymer adhesion, as measured by the peel test, represents a balance between crack depth and diminished surface strength incurred in the oxidative cracking process. Although peel adhesion usually increases with crack depth, overetching may actually reduce adhesion even though the crack depth has been increased. Any advantage from deeper cracks may, therefore, be offset by a loss in the surface strength of the polymer. Comparison of the surface and cross-sectional crack patterns in TiO₂-filled PP indicates that the surface morphology is similar to that of unfilled polymer. Molding conditions that produce the desired morphology is similar to that of unfilled polymer. Molding conditions that produce the desired morphology are important for high peel adhesion values but appear to be less critical than in unfilled PP. A propylene–ethylene copolymer (90/10) developed 12–15 lb/in. peel adhesion—50% lower than for the filled and unfilled homopolymer when molded under similar conditions; peel adhesion in this composite system is, however, relatively insensitive to changes in molding conditions. Aging of 2–3 weeks after plating is required for maximum peel adhesion in all samples studied.     

Compression molding of glass reinforced thermoplastics: Modeling and experiments

The shearing and extensional behavior of glass mat‐thermoplastic (GMT) material under compression molding was investigated with a special model being developed for the case of non‐lubricated mold‐plate surfaces. Mathematical expressions for the radial and through‐thickness flow velocities were derived that enabled the derivation of extensional and shear strain rates. The GMT non‐lubricated (no‐slip wall conditions) compression molding was modeled as a combination of extensional and shearing flow and the two extensional and shear viscosities were determined. Scott's approach was used in this work to determine the radial velocit in the r‐direction, which depends on the shear power‐law expression. The velocity component in the z‐direction was then calculated using the continuity equation. The velocity profiles were used to calculate the shear and extensional strain rates. Scott's shear viscosity did not satisfy the constitutive equation for the extensional part, but a power‐law expression with new parameters depending on the deformation tensors was successfully used to calculate an independent extensional viscosity using the same non‐lubricated squeezing experiment. Lubricated squeezing flow was carried out for the same material to achieve a pure extensional flow, and the extensional viscosity calculated using this approach agreed with the extensional viscosity determined using the non‐lubricated experiment. GMT material used in this study is confirmed to have two layers of continuous long fibers orientated randomly inplane, separated by short chopped fibers in the middle, which suggests that the material can be treated as an isotropic material, and the fiber‐matrix separation is seen to be high at the extremities of the flow.     

Modeling the uniaxial rate and temperature dependent behavior of amorphous and semicrystalline polymers

Strain rate and temperature dependent constitutive equations are proposed for polymer materials based on existing isotropic formulations of viscoplasticity. The proposed formulations are capable of simulating some of the important features of deformation behavior of amorphous and semicrystalline polymers. The materials model is based on the assumption that the evolution of flow stress is dependent on the rate of deformation, temperature, and an appropriate set of internal variables. The proposed theory is capable of modeling yielding, strain softening, and the orientation hardening exhibited by amorphous polymers. It is also possible to model the initial viscoplastic and subsequent nonlinear hardening behavior shown by semicrystalline polymers at large strains. Uniaxial tensile tests with uniform and hourglass specimens are made at temperatures ranging from 23 to 100°C and under various crosshead speeds. Both amorphous polycarbonate and semicrystalline polypropylene sheet materials are tested to characterize the stress and strain behavior of these materials and to determine their appropriate material constants. Load relaxation experiments are also conducted to obtain the necessary material constants describing the rate and temperature dependent flow stress behavior of polypropylene. Simulation results compare favorably against experimental data for these polymer materials.     

Deformation and entanglement in semicrystalline polymers

Transmission electron microscopy and optical studies of thin films of isotactic polystyrene (iPS) and polyoxymethylene (POM) provide evidence for a distinction between crazing mechanisms in semicrystalline polymers above and below T_g. In the latter temperature regime, deformation in iPS and POM crystallized at high supercooling has been discussed in terms of existing models for crazing in amorphous glassy polymers, based on entanglement ideas. Above T_g, where the difference in mechanical behavior of the amorphous and crystalline regions becomes marked, the fibrillar nature of local deformation appears to be a consequence of the inhomogeneity of the undeformed polymer.     

Cavitation during deformation of semicrystalline polymers

Cavitation phenomenon is observed during deformation in many semicrystalline polymers above their glass transition temperature. Numerous voids (cavities) both nanometer and micrometer size are formed inside amorphous phase between lamellae during deformation of a polymer. The cavitation is observed only in tension, never during compression or shearing. Most often used methods of voids detection are: microscopies (SEM, TEM, AFM and light microscopy), small angle X-ray scattering and measurements of density. Usually the voids are detected close to yielding or at yielding, strongly suggesting that yielding is often caused by cavitation. However, there is a competition between two processes: breaking of amorphous phase leading to cavitation and plastic deformation of lamellar crystals. Which process occurs first depends on the relation between compliances of those two phases. If the crystals are weak and defected their deformation occurs (mostly by chain slips mechanism) without cavitation. If the crystals in a polymer are thick and more perfect then the barrier for their deformation, represented by shear yielding stress, is increased and the cavitation sets in first and yielding is determined by the stress needed for cavitation. Further deformation involves deformation of crystals due to rapid local change of stress around voids. The influence of different morphological factors: crystal thickness, crystallinity degree, arrangement of crystalline elements (e.g. in spherulites), morphology of amorphous phase (free volume, entanglements, tie molecules) were analyzed. Experimental factors, such as temperature of deformation and rate of deformation influence remarkably the formation of cavities. Cavitation is generated at points where a high local triaxial state of stress is developed. Triaxiality of stress can be amplified by a notch, even very mild notch with large radius of curvature stimulates generation of cavities. Evolution of nano-cavities into micro-cavities and change of their shapes with increasing deformation were evidenced by SAXS. Initially voids are oriented perpendicularly to deformation direction, however, with increasing elongation they become oriented along deformation direction. Stress whitening is visual sign of cavitation and is caused be light scattering either by microvoids or by assemblies of nanovoids.     

Tear strength theory of semicrystalline polymers

A theroretical model describing how the tear strenght of semicrystalline polymers depends upon the interaction of the crystalline and amorphous phases is presented. It is based upon the “freevolume“ theory and a “Hooke's law -lattice energy” treatment. The strength T_M is hypothesized to be partitioned between the crystalline Q_c and amorphous Q_a phases. Therefore T_M∝? Q_c. Q_a, T_M = T_o.Q_c.Q_a, where Q_c = NKTE/bY² and e^?v/vf =G/G_oτ. Proof of this theory is obtained by measuring the oxygen transmission rates of 11 polyethylene film samples varying in tear strenght. Good agreement exists between T_M (experimental) and T_M (theroetical) for 9 of the polyethylene samples. The other 2 Samples differ from the theory by factor of 2. One explantion of this departure is the τ varies from unity.     

Solid state extrusion of semicrystalline polymers

The dynamics of solid state extrusion of polypropylene, high density polyethylene, and poly(tetrafluorethylene) have been investigated at various extrusion temperatures and piston velocities. A model of solid state extrusion of semicrystalline polymers is proposed. The formulae are obtained relating the value of the extrusion pressure with the parameters of the process and polymer properties. It has been found that for certain values of the parameters, defects of two main types emerge. A mechanism of defect formation is suggested.     

The microwave processability of semicrystalline polymers

The overall objective of these studies was to investigate the relationship between polymer structure and microwave absorptivity. In this paper, the microwave processing of semicrystalline polymers such as poly(ether ether ketone) (PEEK), nylons, and poly(ethylene terephthalate) (PET), via a cylindrical resonance wave cavity and a rectangular standing wave applicator is described. These polymeric materials were irradiated in low power (< 50W) electric fields at 2.45 GHz. Silicone flexible molds were necessary for improved processing of nylons and PEEK at temperatures below their T_c Rapid heating rates were observed between the glass transition temperature, T_g, and the melting temperature, T_m, for all these polymers provided that T_c was exceeded. Both dynamic mechanical thermal analysis (DMTA) and dielectric thermal analysis (DETA) spectra were utilized to predict the heating phenomena between amorphous and semicrystalline materials and to explain the rapid crystallizing rate of PEEK. above its glass transition temperature. Correlations were drawn between (a) the apparent activation energy and the critical temperature (T_c) and (b) the shape of the dielectric spectra at 2.45 GHz and its shape in kHz region.     

Fiber mat deformation in liquid composite molding. II: Modeling

Mathematical models are proposed to simulate the flow induced fiber mat deformation during liquid composite molding. The fiber bed is treated as an elastic beam and the load acting on the bed causes its deformation. The lubrication approximation is used to simplify the resin flow equation in the fiber free region, while Darcy's law is used to calculate the pressure and velocity fields in the deformed fiber bed. The governing equations are solved using the control volume/finite element method. The numerical results show reasonable agreement with the experimental results from Part I.     

Continuous extrusion of high-modulus semicrystalline polymers

A process has been developed by which very high-modulus semicrystalline polymer films can be extruded continuously from a melt. This is accomplished by controlled cooling of the melt in a two-stage flow channel. A temperature gradient along the flow channel quenches the melt prior to an area reduction in which the polymer undergoes solid-state orientation. Analysis of high-density polyethylene tapes extruded by this process shows that they have properties similar to samples hydrostatically extruded at 120°C. Infrared analysis was used to determine both the degree of crystallinity and degree of orientation in these tapes as well as previously prepared hydrostatically extruded samples.     

Mechanical relaxations and moduli of oriented semicrystalline polymers

A systematic investigation was carried out on the mechanical relaxations and moduli of four drawn semicrystalline polymers: polyoxymethylene, polypropylene, polyvinylidene fluoride and polychlorotrifluoroethylene. Low-frequency tensile and torsional measuremnts were made between-140 and 140°C, and ultrasonic measurements of all five moduli were made by the water-tank method between 0 and 60°C. The patterns of relaxations remain essentially unchanged upon orientation, but there is a marked reduction of the height of relaxation peaks associated with the amorphous phase and, correspondingly, a smaller drop of moduli in the relaxation region. This reflects a lowering of molecular mobility in the amorphous phase due to the constraining effect of taut tie-molecules. The modulus C₃₃ increases sharply with draw ratio λ while the other moduli show little variation, which result from the alignment of molecular chain axes and the production of taut tie-molecules. The λ-dependence of the moduli is consistent with the aggregate model only when the polymer is glassy, that is, when its amorphous phase is comparable in stiffness to the crystalline phase and the polymer can reasonably be regarded as a one-phase material for which the aggregate model is valid.     

A non-Gaussian statistical treatment of semicrystalline polymers

Summary The purpose of this paper is to characterize the molecular parameters of a Semicrystalline polymer by means of stress-strain-measurements. At low or moderate draw-ratios, the Gaussian theory of rubberlike elasticity of the amorphous regions allows to calculate the intrinsic birefringence of the amorphous region. At higher draw-ratios the non-Gaussian model makes it possible to define the number of statistical segments for any value of homogeneous draw-ratio, and then to estimate the maximum extensibility of the network chains.