Molecular orientation induced by cooling stresses. Birefringence in polycarbonate: III. Constrained quench and injection molding

Residual stress and birefringence distributions are determined in polycarbonate samples obtained by quenching in a specially designed apparatus and by injection molding. The molecular orientation is distinguished from the thermally and pressure-induced residual stresses. The birefringence in the quenched samples is found to be positive and almost constant, independent of the quench temperature, but varying strongly with initial quench temperature between 150 and 180°C. The residual stress level, as determined by layer removal and sectioning, is very low. The birefringence distribution is mainly due to a tensile equibiaxial orientation induced by transient cooling stresses built up above T_g. The samples which are injection-molded with a high injection speed and without packing pressure display the same birefringence distribution as the quenched samples, apart from a local maximum beneath the surface due to the shear flow during filling. Apart from the flow during filling and packing, the frozen-in molecular orientation in injection-molded samples is also induced by transient thermal stresses present during vitrification. The birefringence from thermally induced orientation was found to be of comparable magnitude to that from flow-induced orientation. For a correct prediction of molecular orientation the thermal strains above T_g must therefore be included in simulation programs. Because of the low level of thermal stresses, the application of a packing pressure will lead to tensile stresses at the surface in general. © 1994 John Wiley & Sons, Inc.     

Sol-Gel; Potential Creative Applications for the Artist,Focusing on the Colloidal Alkali Silicate Route

The development of the colloidal alkali silicate gelation process (the Shoup Method) has established that intricately shaped large size monolithic glass samples are achievable without the need for long drying procedures or the use of a critical drying process. The ability to fabricate gels with large pores (diameter range of 100–300 nm) and the ability to dry them simply and quickly (with the help of a microwave oven) has made Sol-Gel technology accessible to the artist. The ability to cast from flexible molding compounds, to cast without major finishing, to dope and color directly with metal oxides, establishes new possibilities for the artistic forming of glass.     

Flow analysis of the weld line formation during injection mold filling of thermoplastics

To study the weld line formation of colliding flow fronts the filling of a mold cavity was simulated. The thermo-rheological findings were used to investigate the sources of weld line weakness. In this way critical areas of the interface in regard to the lack of interdiffusion and the inappropriate molecular orientation were found to be placed near the surface of the finished parts. The main source for the weld line weakness seems to be the V-notch that arises due to the poorly bonded region near the surface in combination with the large shrinkage as a result of extremely high molecular orientations induced at the end of the filling. Furthermore, the empirical knowledge was confirmed that weld lines are rather more sensitive to the local flow situation than the global processing conditions. Melt and mold temperatures can be considered to be the most important factors which influence the weld line strength.In part presented at the 6th European Conference on Rheology, Erlangen, 2002     

Non-isothermal transient flow and molecular orientation during injection mold filling

The properties of injection molded products are directly related to the microstructure which in turn strongly depends on the flow kinematics and thermal history of the polymer melt during the filling process. In this study the mold filling process has been analyzed by using an FEM-code (FIDAP) to solve the equation of continuity, momentum, and energy under transient and non-isothermal conditions. As constitutive relation for a purely viscous fluid, the Bird-Carreau and Arrhenius model was chosen. The phenomenon at the flow front, its flow kinematics, and its significant implication on the microstructure of the part have been investigated in detail. Computed particle tracking showed good agreement with experiments under real processing conditions. Furthermore, a rather simple but effective and useful method for predicting the orientation distribution in an injection molded part was proposed. It was found that the local deformation near the solid wall may be considered as the main source for a typical layer of high orientation on the surface of the part. Received: 1 December 1999 Accepted: 9 August 2000     

Construction of Pre-Deformed Shapes for Rapid Tooling in Injection Molding

The major issues in the development of injection molding technology include the progress in CAE and the developments in tool design methodology such as rapid tooling. The applicability of rapid tooling in injection molding was examined using unbalanced cooling to analyze the warpage and shrinkage. Moldflow Plastics Insight simulation software was used for the deformation calculations with different mold thermal conductivities. It can be concluded that the decreasing mold thermal conductivity will dramatically increase the volumetric shrinkage and the warpage as well. Because of these effects, it is of fundamental importance to compensate for the shrinkage and warpage, so in the paper a new design methodology is suggested for rapid tooling, which is based on the pre-deformed model.     

Rheological Properties of Nanosized AlN Powder Suspensions for Advanced Ceramics

The rheological properties (flow curves and viscoelastic behavior) of injection molding suspensions of a plasma-processed AlN nanosized powder (nanopowder) in paraffin are investigated over a broad range of shear rates (0.07–1350 s^–1). Two viscosity plateaux are observed on the flow curves and two values of the yield stress are obtained. The lower value of the strain amplitude (0.66%), exceeding the linearity limit of periodic shear, is restricted by the rheometer resolution. The ultrasound treatment and shear deformation of suspensions affect the structure of particle packing, which is responsible for the dependence of their rheological properties on the prehistory of mechanical actions.     

Elastomeric sizings for glass fibers and their role in fiber wetting and adhesion in resin transfer molded composites

Mold fill velocities of 0.067 cm³/s and 2.66 cm³/s were used to impregnate glass fiber preforms with different architectures and sizing types in two force-controlled resin transfer molding (RTM) fixtures. The fabrication of disk-shaped parts at high molding speed and high post-cure fill pressure was proven successful in reducing the amount of flow-induced defects for reinforcements with a random nonlayered structure. Investigations on the effect of fiber/matrix interface modification with controlled-thickness elastomeric films obtained by the admicellar polymerization technique were carried out to assess the structural integrity levels attained with these less expensive polymeric sizings. In particular, parts reinforced with fibers coated with a thin film of styrene-isoprene copolymer performed significantly better than the uncoated control samples in the tensile and flexural tests. For the same sizing type, the interlaminar shear strength was more than 30% higher than the desized composite and compared statistically to the adhesion level exhibited by commercially sized reinforcements. Greater data scatter and poorer adhesion performance was observed for those composites containing fibers with a thin polystyrene coat. We infer that beneficial effects of a nanometer-thick elastomer interlayer are more evident when extensive cooperative segmental motions take place, that is, when the surface glass transition temperature of the sizing is far below the room temperature. These results have implications for composite manufacture applications involving tailored interfaces with flexible sizings.     

Poiseuille/squeeze flow-induced crystallization in microinjection-compression molded isotactic polypropylene

Microinjection-compression molding was used to fabricate isotactic polypropylene part with microscale thickness. The combined effect of shear (up to the order of 10⁵ s⁻¹) and elongational deformations imposed by the Poiseuille (injection stage) and squeeze (compression stage) flow resulted in pronounced flow-induced crystallization under rapid quenching. Hierarchical crystalline morphology, characterized by two oriented layers, a transitional layer in between, and an isotropic core layer, was detected through the thickness in the upstream region initially filled in injection stage, whereas skin-core morphology appeared in the downstream region filled in subsequent compression stage. Under the molding conditions imposing sufficient strain rates, predominant shish-kebabs developed in the oriented layer. Furthermore, the oriented layer thicknesses, crystallinity, β-form content, and melting behavior of molded parts, all of which were closely correlated with the calculated strain rates, as well as the location of inner oriented layer could be manipulated via varying the compression-related parameters. © 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2013     

On the origin of the “core‐free” morphology in microinjection‐molded HDPE

This study investigates the morphology of a high‐density polyethylene processed with microinjection molding. Previous work pointed out that a “core‐free” morphology exists for a micropart (150‐μm thick), contrasting with the well‐known “skin‐core” morphology of a conventional part (1.5‐mm thick). Local analyses are now conducted in every structural layer of these samples. Transmission electron microscopy observations reveal highly oriented crystalline lamellae perpendicular to the flow direction in the micropart. Image analysis also shows that lamellae are thinner. Wide‐angle X‐ray diffraction measurements using a microfocused beam highlight that highly oriented shish–kebab morphologies are found through the micropart thickness, with corresponding orientation function close to 0.8. For the macropart, quiescent crystallized morphologies are found with few oriented structures. Finally, the morphology within the micropart is more homogeneous, but the crystalline structures created are disturbed due to the combined effects of flow‐induced crystallization and thermal crystallization during processing. © 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 49: 1470–1478, 2011     

Polycyanurate networks from anethole dimers: Synthesis and characterization

Novel biorenewable bisphenols are obtained through simple and rapid chemical transformation of the natural product trans‐anethole. The corresponding dicyanate esters are thermally cured to give polycyanurate networks. The thermal properties from differential scanning calorimetry, thermogravimetric, and dynamic thermomechanical analyses of the new dicyanate esters compare favorably with similar commercial products. © 2012 Wiley Periodicals, Inc.^? J Polym Sci Part A: Polym Chem, 2012