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Material Challenges in Medical Micromolding Applications
Processors in the medical field are being challenged to mold parts with smaller and smaller features. Many new lab-on-a-chip devices are designed to move biological fluids with micro liter volumes. This requires flow channels and other features that are on the order of microns. Molding parts with features of this size and with the tolerances required for medical applications presents several unique challenges in the selection and processing of the appropriate material. This paper discusses some general principles for material selection in the medical micro-molding field. In addition, several case studies are provided to illustrate solutions on real parts.
Mathematical Modeling of the In-Mold Coating Process for Injection Molded Thermoplastic Parts
In-mold coating (IMC) for thermoplastic parts is employed either to protect products from outdoor exposure or to improve the surface appearance as in automotive applications, or both. The coating material is injected into the closed mold and advances by compressing the thermoplastic substrate. This paper presents a mathematical model for the process. The compressibility of the thermoplastic substrate is taken into account. The corresponding computer code for the coating material filling and packing in a simple geometry has been developed and the results have been verified by experiments.
Measurement of Peroxide Content of Crosslinkable Polyethylene by Differential Scanning Calorimetry
The first rotational molding grades of polyethylene appeared in the 1950's. In subsequent years considerable effort was expended to improve the performance of these polyethylenes through peroxide-initiated crosslinking. Success was achieved due to the availability of peroxides with sufficient stability to be processed at elevated temperatures without significant scorching or pre-curing of the resin. The performance of crosslinkable polyethylene is strongly dependent upon the level of peroxide in the resin. This paper presents data showing that peroxide levels can be quantitatively and reproducibly characterized using differential scanning calorimetry. This method can be used for quality control in determining peroxide loss when grinding the resin into powder for rotational molding.
Measuring Dart Impact Strength of Polyethylene Films
Dart impact strength is widely used by resin manufacturers and film converters for quality control and for selecting the appropriate resin for a desired application. ASTM D 1709, Method A and B are generally used in the industry for dart impact strength measurement. This standard allows a surprising amount of latitude in the details of the test. This paper will describe a systematic study of the dart impact strength measurement technique. Some of the variables that have been explored are the material of construction of the dart head, age of the dart head and the diameter of the incremental weights. Variations of this type, all within the limits set in ASTM procedure D 1709, revealed significant influence on the measured dart impact strength values.
Mechanical Hole Burning Spectroscopy: A Nonlinear Viscoelastic Analysis
A mechanical hole-burning scheme was constructed to compare the analogous observations to those from dielectric non-resonant spectral hole burning (NSHB) for glass forming liquids near their glass transitions. Within the framework of the BKZ nonlinear viscoelastic constitutive equation and without invoking an explicit heterogeneous or homogeneous nature of the relaxation of supercooled materials, the calculated modified shear moduli shift non-uniformly with pump frequency. Mechanical holes were burned in the loss response and exhibited most of the apparent features of those reported from dielectric NSHB.
The Mechanical Testing of Micro Injection Mouldings
Micro-injection moulding is a relatively new technology that is gaining increasing interest in the processing industry. Injection moulded parts weighing less than one milligram are now in production. One of the main issues that has to be addressed with this technology is related to the measurement of the physical properties of the parts. The work in progress at the Wolfson Centre is concerned with the mechanical testing and characterisation of micro-mouldings. We believe that the techniques most suited to this are nanoindentation supported by surface etching, atomic force microscopy and light microscopy, with scope for scanning electron microscopy in the future. The method by which the micro-mouldings are prepared for the nanoindentation testing will be described. Results from proving trials on cut sections of injection mouldings are presented and demonstrate the potential of this technique for the testing of micro injection mouldings.
Mechanical, Thermal and Permeability Properties of PP / Oligopinene Blends
Our interest of study is blending of polyolefins with oligomers from natural and synthetic sources. In this work we show the results on mechanical, thermal and permeability properties of polypropylene (PP) / Oligopinene systems from compression films about 10 microns which were prepared using quenching (liquid nitrogen) from the molten state and permeability was measured using CO2. The addition of oligopinene on PP changes stress-strain curve of the polyolefin. For all oligomer content no more yielding was observed and the elongation at break had an abrupt decrease at a concentration of 10% of oligomer. The thermal analysis revealed that the blend system has two glass transition temperatures. The permeability values changed slightly with the oligomer content in the blends.
Mechanical-Morphology Relationship of PS Foams
The relationship between the morphology and the mechanical behavior of commercial PS foams has been investigated. The foams studied had a closed cell morphology with densities between 25 and 60 kg/m3 and number-average cell sizes between 75 and 230 ?m, and a normal cell size distribution (dv/dn ? 1.20). Mechanical results showed that the compressive strength and modulus could be expressed as a function of the foam morphology, using a unique morphological parameter taking into account the cell size and foam density. Flat sheet impact tests showed that three stages, i.e. initiation, propagation and collapse, could be identified in the impact behavior of the foams, which could be related to the morphological parameter proposed. A transition from a brittle to a ductile behavior could be rationalized using the proposed parameter.
Melt Flow Instability Studies of Metallocene Catalyzed LLDPE in Pelletizing Dies
The capillary flow behavior of metallocene catalyzed LLDPE was studied in the melt flow instability region of the resin. Processing variables, as well as die materials of construction and geometric/design variables of the dies were explored. The extrudate performance variable that this study focused on was the detailed microscopic structures (topographies) of the extrudate surfaces, in order to obtain insights on the die-related origins of the melt instabilities involved. It was speculated that the surface melt fracture of extrudate was similar in nature to other mechanical fracture that is initiated from the formation of small cracks on the extrudate surface, caused by high wall shear stress with increasing flow rate and/or high elongation stress due to exit singularity (or velocity discontinuity). Cohesive failure creating peeling/tearing cracks observed can propagate and evolve into a well-organized structure, depending on the excess stress energy levels on the extrudate surface at the exit and cohesive strength of the polymer melt. Possible methods to mitigate the surface melt fracture in capillary dies were also suggested from the proposed fracture mechanism.
Melt Mixing Improves Hot Runner Balance and Improves Part Quality
One method of balancing multicavity molds is to restore the cross-sectional symmetry of the shear induced hot/cold lamination of the melt before a runner branch. While, this method has been used successfully with H pattern runners it's less compatible with molds including intersections with more than two branches. In this work it is demonstrated that melt homogenizing before critical runner intersections improves the balance of molds independently of their hot runner configuration. In addition, when a mixing nozzle is used to homogenize the melt before delivering it to the cavity, preferential cavity filling can be eliminated and more dimensionally stable parts can be produced with uniform microstructure and mechanical properties.
Melt Processability of Polyethylene with Long Chain Branches
Linear viscoelastic results are presented for several polyethylenes which each exhibit, to varying degree, an increased zero shear viscosity (?0) relative to that observed for a linear polyethylene with the same weight average molecular weight (Mw). This is a well-recognized rheological signature of the presence of long chain branching (LCB). Examination of the small amplitude oscillatory shear data for the branched polyethylenes clearly reveals the presence of two separate relaxations. We examine the utility of considering these polyethylenes as blends of branched and linear species. A unique power-law behavior is observed for the dynamic viscosity in the intermediate frequency region bounded by the distinct relaxations of the linear and long chain branched components. Characterizing these rheological features appears to be a key element in formulating an understanding of the processability of polyethylenes which possess entangled branches.
Melt Processing of Polymers Using Supercritical Fluids
In-line rheometry has been used to study the plasticising effect of sub and supercritical carbon dioxide during polymer melt processing. The extent of the plasticising effect has been investigated using model systems so that theoretical and actual viscosity reductions achieved could be compared. Results will be presented which demonstrate these effects.As a result of this research injection moulding and extrusion processes have been optimised for a selection of polymers and highly filled polymer compositions. This has led to enhanced manufacturing process efficiency and the manufacture of components with preferential foaming.
The Melting Behavior of Amorphous Polyester in a Co-Rotating Twin Screw Extruder
The melting behavior of an amorphous polyester in an intermeshing co-rotating twin screw extruder was examined. It was found that the melting of this low Tg amorphous polyester could happen even in the partially-filled conveying screw section without any kneading or reverse screw elements, if a preformed melt" was created. Barrel heating is more than providing the energy required for melting but to create a layer of "preformed melt". Once this "preformed melt" was created the melting could be sustained with the heaters of the barrel being shut off. The "preformed melt" seems catalytic to this VED (Viscous Energy Dissipation) dominant melting process might also be important to many other melting processes."
The Melting Behavior of Polyethylenes Synthesized with Ziegler-Natta and Constrained Geometry Catalysts
The melting flux and shear stress values during melting of five polyethylenes were measured using a Screw Simulator. Four materials were produced using constrained geometry catalyst Technology (CGCT), while the fifth was produced with a traditional Ziegler-Natta (ZN) catalyst. All of the materials have nearly the same melt index value, but the solid density of each material varied. The melting flux was measured over a range of sliding velocities, temperatures and pressures typical in single screw extrusion. Melting flux values for all materials increased with velocity, temperature and applied pressure. The materials produced with CGCT had a higher melting flux than the ZN material under the same experimental conditions, in all cases. Experimental data were compared to existing melting flux and shear stress models.
The Melting Characteristics of Polycarbonate Resins
Melting of a polymer inside a single-screw plasticating extruder is described by a dissipative melting mechanism of the solid bed rubbing on the barrel surface. Both the melting rate and the shear stress of a solid bed are expected to increase with decreasing melt index (i.e., with increasing viscosity). However, the melting rates of polystyrene (PS) in controlled Screw Simulator tests were found to decrease with decreasing melt index even though the shear stresses increased. Such unexpected results were reported previously (1).This paper will report the results of controlled Screw Simulator tests for polycarbonate (PC) resins with varying melt indices and also on the melting phenomena in single-screw plasticating extrusion experiments.
Melting Model for Co-Rotating Twin-Screw Extruders
A comprehensive model describing the last stage of the melting process in co-rotating twin-screw extruders is proposed. At this stage, the melting is governed by viscous dissipation and heat transfer from the melt to the unmelted particles. The residence time in the melting section mainly determines the final melt temperature and the remaining fraction of unmelts.The effects of screw speed, polymer viscosity and particle size on the melting process are shown, leading to implications for the new high-speed, high-torque machines. The model is supported by experiments using a screw configuration in which the melting section is placed at the downstream end of the screw.
Melting of High-Heat Polyamide in a Co-Rotating Twin Screw Extruder
Experimental work is presented on the melting behavior of high-heat polyamide in co-rotating twin screw extruders. Screw designs were tested with different melting sections located near the exit of the extruder, to isolate the melting performance. A slit-die provided a melt ribbon that was drawn down for visualization of unmolten particles that together with melt temperature quantify melting performance. This method proved to be efficient in scanning an entire process window. Evident are effects of screw speed, degree of fill, screw configuration, and low melting additives. These experimental directions, along with model simulations are the basis to optimize compounding screws covering modern hi-speed, hi-torque process windows.
Melting Regimes in Modular Intermeshing Co-Rotating Twin Screw Extrusion: Effect of Barrel Temperature Screw Speed and Feed Rate
The melting of polyolefins in a modular co-rotating twin screw extruder was investigated as a function of (i) barrel temperature (ii) screw speed and (iii) feed rate. Carcasses of the melting region were removed from the twin screw extruder and sliced into sections.Our observations suggest three different mechanisms of melting; (i) Hot barrel induced melting, (ii) Screw surface induced melting, and (iii) Homogeneous bulk melting. The mechanism of (i) is favored by high barrel temperatures, the mechanism of (ii) by lower barrel temperatures and slower screw speeds, and the mechanism of (iii) by lower barrel temperatures and higher screw speeds.
Micro Molding - A New Way""
Small parts less than one quarter of a pellet require a complete rethink by molding engineers and mold designers. Total part cost justification becomes more challenging based upon the amount of material wasted and tooling costs. Micro machinery is helping to meet the requirements of accuracy through simplicity. There is no need for complicated multiple profile closed-loop servo controlled injection and metering functions.The two-stage plunger over plunger injection unit is capable of processing basic polyolefins to high temperature LCP materials. Shot pot injection provides accuracy of volumetric control by eliminating variation caused by slider or check ring shutoff inconsistency. The minimum number of components used in the design achieves simplicity.
Microcellular Foaming of Polypropylene Containing Wood Fibre in an Injection Moulding Process
Microcellular foams of polypropylene containing wood fibres, cell sizes on the order of 10 to 50?m were produced in an injection moulding process. The relationships of processing/structure/property were investigated for wood fibre-thermoplastics composites foaming with a chemical blowing agent. Wood polypropylene composites (WPC) of different wood content (30%,40%,50% and 60% by weight) have been prepared using maleic anhydride-polypropylene copolymer (5% relative to the wood fibre content) as a coupling agent. Measurement of density, cell size, void content, tensile and flexural test of the prepared WPC were carried out. The shape and distribution of the voids were investigated by optical photo examination of longitudinal sections of specimens, using polishing technique and reflected light microscopy. Density of foamed composites decreased about 24%. The cell morphology and foam properties showed improvement when the coupling agent was added. Water absorption and scanning electron microscope of the composites also investigated.
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