Effect of injection-molding conditions on the crystallinity,orientation gradients,and mechanical properties of poly(aryl ether ketone). II. Large dumbbell parts

High-temperature thermoplastics such as poly(arylene ether ketone) form multilayer structures when they are molded at low mold temperatures. The cavity geometry, particularly the cavity thickness, plays an important role in forming these structural gradients. In this paper, we present our results on relatively thick large dumbbell specimens. Unlike the thin specimens (reported in Part I of this series of papers) that show structural gradients composed of amorphous skin-crystalline intermediate zone and amorphous core, thicker dumbbell specimens exhibit structural gradients with increasing crystallinity from skin to core at low mold temperatures. Optical profiling on thin-sliced specimens indicate the presence of six layers from skin to core at low mold temperatures, and the number of layers decrease with the increase of mold temperature. As the mold temperature increases, the regions near the surface increase in crystallinity while the core crystallinities remain high and unaffected by the changes in the process variables. At very high mold temperatures where thermally activated crystallization is dominant, the crystallinities become uniform throughout the thickness of the specimens. Wide-angle X-ray pole figure investigations indicate that the skin layers possess high preferential orientation with uniplanar axial (200) [001] texture and the core regions possessing lower orientation levels exhibit simple uniaxial texture with the chain axes oriented primarily along the flow direction. Elongation to break, tensile strength, and impact strength increase with the decrease of injection speed. The implications of these structural gradients on the mechanical properties are discussed. © 1993 John Wiley & Sons, Inc.     

Properties of SiC-Si made via binder jet 3D printing of SiC powder,carbon addition,and silicon melt infiltration

We report the physical and mechanical properties of ceramic composite materials fabricated by binder jet 3D printing (BJ3DP) with silicon carbide (SiC) powders, followed by phenolic resin infiltration and pyrolysis (IP) to generate carbon, and a final reactive silicon melt infiltration step. After two phenolic resin infiltration and pyrolysis cycles; porosity was less than 2%, Young's modulus was close to 300 GPa, and the flexural strength was 517.6 ± 24.8 MPa. However, diminishing returns were obtained after more than two phenolic resin infiltration and pyrolysis cycles as surface pores in carbon were closed upon the formation of SiC, resulting in reaction choking and residual-free carbon and porosity. The instantaneous coefficient of thermal expansion of the composite was found to be independent of the number of phenolic IP cycles and had values of between 4.2 and 5.0 ppm/°C between 300 and 1000℃, whereas the thermal conductivity was found to have a weak dependence on the number of phenolic IP cycles. While the manufacturing procedures described here yielded highly dense, gas impermeable, siliconized SiC composites with properties comparable to those of bulk siliconized silicon carbide processed according to conventional techniques, BJ3DP enables the manufacture of objects with complex shape, unlike conventional techniques.     

Adhesive wafer bonding with photosensitive polymers for MEMS fabrication

Adhesive wafer bonding is a technique that uses an intermediate layer (typically a polymer) for bonding two substrates. The main advantages of using this approach are: low temperature processing (maximum temperatures lower than 400°C), surface planarization and tolerance to particles contamination (the intermediate layer can incorporate particles with the diameter in the layer thickness range). The main bonding layers properties required by a large field of applications/designs can be summarized as: isotropic dielectric constants, good thermal stability, low Young’s modulus, and good adhesion to different substrates. This paper reports on wafer-to-wafer adhesive bonding using SINR^TM polymer materials. Substrate coating process as well as wafer bonding process parameters optimization was studied. Statistical analysis methods were used to show repeatability and reliability of coating processes. Features of as low as 15 μm size were successfully resolved by photolithography and bonded. An unique megasonic-enhanced development process of the patterned film using low cost solvent was established and proven to exceed standard development method performance.     

Context-based literature digital collection search

We identify two issues with searching literature digital collections within digital libraries: (a) there are no effective paper-scoring and ranking mechanisms. Without a scoring and ranking system, users are often forced to scan a large and diverse set of publications listed as search results and potentially miss the important ones. (b) Topic diffusion is a common problem: publications returned by a keyword-based search query often fall into multiple topic areas, not all of which are of interest to users. This paper proposes a new literature digital collection search paradigm that effectively ranks search outputs, while controlling the diversity of keyword-based search query output topics. Our approach is as follows. First, during pre-querying, publications are assigned into pre-specified ontology-based contexts, and query-independent context scores are attached to papers with respect to the assigned contexts. When a query is posed, relevant contexts are selected, search is performed within the selected contexts, context scores of publications are revised into relevancy scores with respect to the query at hand and the context that they are in, and query outputs are ranked within each relevant context. This way, we (1) minimize query output topic diversity, (2) reduce query output size, (3) decrease user time spent scanning query results, and (4) increase query output ranking accuracy. Using genomics-oriented PubMed publications as the testbed and Gene Ontology terms as contexts, our experiments indicate that the proposed context-based search approach produces search results with up to 50% higher precision, and reduces the query output size by up to 70%.     

Optical properties of simultaneous biaxially stretched poly(ethylene terephthalate) films

We offer a detailed study on the anisotropic optical properties in uni and simultaneous biaxially stretched Poly(ethylene terephthalate) (PET) films. Cast amorphous sheets of PET were stretched to a series of extension ratios in two mutually perpendicular directions at 80, 90, and 100°C. Additionally, 0selected films were subsequently “heat-set” by annealing with their width and lengths constrained. The principal refractive indices at sodium D wavelength of these, asstretched and heat-set films were obtained using a modified Abbe refractometer. The changes in the principal refractive indices with the processing history were correlated with the orientation of PET chains and phenyl plane normals, which were determined independently by wide angle X-ray (WAXS) pole figure technique.     

Crystallization kinetics and nucleating agents for enhancing the crystallization of poly(p-phenylene sulfide)

An experimental study of crystallization kinetics and the influence of nucleating agents on the solidification of poly(p-phenylene sulfide) (PPS) is described. The effect of molecular weight is considered by investigating PPS samples having different viscosity levels. We studied the effect of a range of nucleating agents including aluminum oxide, calcium oxide, silicon dioxide, titanium dioxide, kaolin, and talc. All of these compounds were found to enhance the rate of crystallization; in particular, silicon dioxide, kaolin, and talc were the most effective nucleating agents. An effort to study particle size effects of the silicon dioxide showed that the nucleation was very sensitive to the source of the material. These studies did, however, show that nucleation rates tended to increase with decreasing particle size and increasing loading of silicon dioxide. Comparison of PPS crystallization rates with those of other polymers indicates that it crystallizes much more slowly than polyethylene or isotactic polypropylene and is slower than polyetherether-ketone, when comparisons are made on an equivalent basis. PPS crystallizes at similar rates to polyethylene terephthalate (PET). However, our nucleated PPS does not crystallize as rapidly as nucleated PET.     

Structural gradients developed in injection-molded syndiotactic polystyrene (sPS)

In this experimental research, we investigated the influence of processing history on the development of structural gradients in injection-molded syndiotactic polystyrene (sPS). The structures formed during injection molding of this slow-to-crystallize polymer are explained by the complex interplay between the mechanical history of the sample and the cooling conditions dictated by such variables as mold temperature, cavity geometry, and injection speed. When crystallized under shear, sPS exhibits unusually high preferential chain orientation along the flow direction; this orientation gradually decreases toward the core. The mechanical properties are related to the degree of orientation and crystallinity. © 1996 John Wiley & Sons, Inc.     

Effect of annealing on the structure of injection-molded PEN

The influence of annealing at temperatures where thermally activated crystallization rates become significant on the structural variations in the injection-molded polyethylene naphthalate (PEN) was investigated. In these studies, two distinct shear crystallized layers are observed. The shear crystallized layer near the skin forms during the filling stage, and the second shear crystallized layer forms during the packing stage but remains optically unobservable until the annealing takes place. Upon annealing, additional layers with larger crystallite sizes form around these preformed highly oriented layers. The α and β phases were observed in both of the shear crystallized layers. The β phase is generally grown either under high deformation fields or under quiescent conditions at high temperatures. © 1996 John Wiley & Sons, Inc.     

Instrumented thermoforming of advanced thermoplastic composites. III: Relative performance of various prepregs in forming double curvature parts

Thermoforming of a double curvature hemispherical cap-shaped part from different continuous carbon fiber-reinforced thermoplastic composite prepregs has been investigated. A novel three-piece matched die mold was used for shaping the prepregs directly from 16-ply unconsolidated lay-ups. For poly(ether ether ketone) (PEEK)-based prepregs, PEEK/graphite fiber unidirectional tapes and commingled knitted three-dimensional fabric yielded parts that possessed good shape conformity and were free of any major delamination. Encouraging results were also obtained for modified polysulfone (PXM-8505) based unidirectional fabric as well as PEEK/graphite knitted bidirectional fabrics. Several other prepregs performed rather poorly in forming acceptable quality parts. Fiber alignment was found to be better for fabric prepregs than unidirectional tapes. Both stretching and “draw in” of the material occur during deformation, even though the relative contribution depends strongly on the prepreg form. Our thermoforming studies suggest a cycle time of 10 min or less to form good quality parts, which is an attractive feature, considering that cycle times are typically several hours for conventional thermosetting resins.