SPE Library

In this paper, we present a new continuum framework to formulate models to study flow induced crystallization in polymers. The models are developed in a general thermo-mechanical setting and are able to incorporate the main features of the crystallization process. A consistent framework is developed to model the transition from a fluid like behavior to a solid like behavior. The anisotropy of the crystalline phase is included in the model and depends on the deformation in the melt. Particular models are generated by choosing specific forms for the internal energy, entropy and the rate of dissipation. Equations governing the evolution of the natural configurations and the rate of crystallization are obtained by maximizing the rate of dissipation. The initiation criterion, marking the onset of crystallization, arises naturally in this setting in terms of the thermodynamic functions. The model is used to simulate bi-axial extension in a polymer film that is undergoing crystallization.

Dynamic feed is the injection molding process whereby the machine's polymeric flow is attenuated by a series of independent valves that are placed in a hot runner manifold just prior to each cavities entry gate. Each valve is controlled dynamically, in real time, to follow a pre-programmed pressure profile using feedback from a pressure transducer located downstream. The advantages of having control over each cavity (or open/close sequencing along with pressure profiling) are many. Parts of widely varying fill needs can each have a tailor made pressure profile specific to the needs of that particular part geometry. Strikingly dissimilar parts can be made in a single shot that would not be possible on a conventionally equipped machine. Watkins and Hume have discussed the primary advantages of this technology previously, they focused on the particular advantages of using the technique with modular tooling1. Kazmer discussed the process in detail5. Of concern in this work is the inherent stability of the dynamic feed process and the apparent potential for increased process variation due to anticipated material variations2.

Processing flows are known to accelerate polymer crystallization kinetics, strongly altering the orientation distribution of the crystallites and producing dramatic changes in material properties. Our research probes the molecular level processes that give rise to these effects. To clarify the role of macromolecular relaxation, we investigate the effects of shear history on the crystallization of isotactic polypropylenes. A unique apparatus enables us to subject a subcooled melt to precisely controlled intervals of shear at stress levels similar to those encountered in industrial processes.(1) Brief intervals of shear enhance the rate of subsequent crystallization by orders of magnitude. Previous rheo-optical experiments have indicated that the creation of long-lived, oriented structures during flow is controlled by the dynamics of the melt.(2) We present polarimetry and synchrotron wide-angle x-ray diffraction (WAXD) data obtained during and after shear of an iPP believed to contain chains with long branches. Results suggest that shearing near the nominal melting temperature induces the formation of a slow relaxing species that templates subsequent oriented crystal growth, emphasizing the importance of rheology to shear-enhanced crystallization.

This research involves the area of rapid prototyping (RP) and a new concept called functional prototyping. The overall goal of this project was to determine if current rapid prototyping methods allow for the prediction of the mechanical performance of a molded snap-fit. The Rapid Prototyping methods that were evaluated are Fused Deposition Modeling (FDM), Selective Laser Sintering (SLS), and machining plastic from stock shape. The results show that machining is the best method to use for functional testing, followed by SLS, and FDM. An attempt to ratio the results using the modulus of elasticity and yield strength are not quite satisfactory. But it can still be used for the rough estimation. Each of the prototypes types has its own tendency to deviate from the actual value.

Prepregs of an amine-rich mixture of the tetrafunctional epoxy tetraglycidyl 4,4-diaminodiphenyl methane (TGDDM) and the tetrafunctional amine 4,4'-diaminodiphenylsulfone (DDS) were characterized with temperature-modulated DSC (TMDSC) as well as dynamic mechanical analysis (DMA). The baseline shift of the glass transition was separated from the curing exotherm by using temperature-modulated and step scan DSC temperature scans. Likewise, the baseline shift in heat capacity due to vitrification was isolated using TMDSC isotherms. Using the TMDSC glass transition temperature, degree of conversion, and vitrification results, combined with the gelation data generated from DMA, a time-temperature-transformation (TTT) diagram was constructed, providing information necessary for optimization of industrial processing of the epoxy prepreg. Thus, effects of storage, preprocessing, and postprocessing on the overall curing process are taken into account.

One of the ways of increasing fuel efficiency of a typical automobile is to reduce its overall weight. To this extent, plastics, especially fiber reinforced plastics are finding an increasing role as automotive structural components. The automotive structural systems made up of these structural fiber reinforced plastic components, should satisfy the needs in terms of safety, strength, NVH and durability, in addition to being affordable, manufacturable with desired fit and finish and recyclable. In general, structural components made up of fiber reinforced plastics are adhesively bonded together to form structural systems, capable of carrying automotive structural loads under static and dynamic conditions. Fiber reinforced plastics and the adhesive used to bond them to form a structure are inherently viscoelastic in behavior. It is imperative therefore, to understand the behavior of these adhesively bonded fiber reinforced plastic components, in terms of their load carrying capacity at different temperatures and different load or strain rates. One of the key factors in this understanding is to characterize the adhesive failure itself at different temperatures and different strain rates of loading. The present paper is an attempt to present some results from an ongoing research work on fiber reinforced adhesively bonded large injection molded thermoplastic automotive structural systems. In particular the paper presents the results from the test methodology and the mathematical models used to characterize the failure mechanics of adhesively bonded automotive body sections, at different temperatures and different load or strain rates.

A competitive intelligence methodology has been developed which ensures an actionable customer focus is reflected in an assessment of the competitive arena. This is accomplished by combining the classic top down" or Industry analysis of a competitor (or group of competitors) with a "bottoms up" view from the customer perspective. The Industry approach assesses factors such as external environment market position technology supply chain issues and corporate culture among others identifying strengths weaknesses and apparent strategy. The Customer view obtained by in depth field interviews provides insights into unmet needs emerging trends and supplier performance against the strategy. The result is an externally focused actionable plan. This paper will detail the combined methodologies employed and the value added obtained by a review of case studies."

Because of the increasing use of thermoplastics and thermoplastic composites in load-bearing applications, welding methods are becoming important for part cost reduction. Welding requires the melting of the surfaces to be joined, followed by a solidification of the interfacial molten layers under pressure. Many welding techniques using different means for heating the joint interface are available [1]. One such versatile technique is induction welding, in which the surfaces to be joined are brought to the melting temperature" by induction heating a specially made gasket placed in the joint interface. Applications of this technique include welding of hot-water kettle housings and the welding of high-pressure water tanks. This process is not well understood. In this paper tensile tests on induction butt-welds of amorphous and semicrystalline materials are used to characterize achievable weld strengths. Microscopy is used to correlate the strengths achieved with the morphology of the failure surface."

Synthetic lightweight aggregate has been produced by melt compounding high concentrations of high carbon fly ash into various thermoplastic binders. The composite material is being developed as a synthetic lightweight aggregate for use in applications such as lightweight concrete. In this study, a series of lightweight aggregates have been produced using several fly ash concentrations, and several different thermoplastic binders. The synthetic aggregates have been produced using flexible thermoplastic binders, rigid thermoplastic binders, and a mixed thermoplastic binder formulation. The physical properties of the melt compounded aggregate materials have been evaluated in an effort to determine the relationship between variables, such as the binder stiffness, and the aggregate stiffness. Lightweight concrete test samples have also been prepared and evaluated. The results of the study show that the lightweight aggregate properties are influenced by both the fly ash concentration and the thermoplastic binder composition. However, the effect that the thermoplastic binder has on the physical properties of the aggregate becomes less significant at high fly ash concentrations. At fly ash concentrations of 80%, the physical properties of the aggregate are fairly insensitive" to the composition of the thermoplastic binder. The aggregates produced using a mixed plastic composition had properties that were quite similar to those produced using the individual (control) thermoplastic binders indicating that low value mixed plastic waste may be a candidate binder material for the polymer bound fly ash aggregate."

Preliminary studies on the basic structure developed in nylon 6 and its blends with nanoparticles are presented. The melting transition and crystallization behavior of compression molded nylon 6 and its nanocomposites are investigated using DSC and WAXD techniques. The structural hierarchy developed along and across the flow direction is also addressed using optical microscopy. Polarized optical microscopy revealed that the presence of nanoparticles causes significant enhancement of the preferential orientation through the thickness direction of the molded parts even at mold temperatures very close to the melting temperature.

Today's competitive business environment requires companies to embark on continuous improvement projects aimed at dealing with plant inefficiencies and new product introductions. Almost every company is unsatisfied with the slow rate of implementation and unsatisfied with bottom line results. How do you ensure that your new system integration will not fall into the same trap? Once the plastic part is designed, the task of creating a cell to manufacture it is too often considered to be a mere formality. It is more than a simple matter of buying the pieces of individual equipment, positioning them on the floor, connecting them to the services and pressing GO" to start. In fact there are critical cell design choices and decisions to be made even before designing the plastic part which are crucial to the ultimate success of the project. A world class injection molding cell requires a well thought out plan detailed engineering design of the system including all the auxiliaries and finally rigorous testing of all the components and their interaction. The focus of this paper will be on the system integration which can be loosely defined as the physical placement/connection of the equipment as well as the performance testing and validation."

Thermoset recyclate fillers are considered as microcomposite reinforcements for polymers. Emphasis is given to glass fibre-reinforced phenolic and polyester waste products as functional fillers for polypropylene, where with appropriate surface modification, significant enhancement in mechanical properties can be achieved. In this respect, the role of a two component treatment package is discussed, in terms of fibre-matrix interfacial bonding, the effect on properties of the host polypropylene matrix, and the failure mechanism induced. Novel integrated compounding technology is described for the cost-effective preparation of polymer composites, containing thermoset recyclate fillers.

The birefringence and stress development of PET films has been measured by our new stretching system that couples the Spectral Birefringence Technique with the stress strain measurements. The tensile stretching machine which is specifically designed such that both clamps of the samples move away from each other. This allows for keeping the midsection of the sample stationary. The birefringence and width of the sample is measured at this location. The width of the sample is measured using laser micrometer through which true strain is obtained real time. It has been found that the birefringence development starts before strain-hardening point. The system is fast enough to follow the deformation at high rates of strains. The increased strain rates results in higher birefringence levels through all stages of deformation

A series of stretch modes: uniaxial free width (UFW) stretching, uniaxial constant width (UCW) stretching, simultaneous equall biaxial(SEB) stretching and sequential equally biaxial (SEQEB) stretching, were applied to cast amorphous films of PLA in order to investigate the effect of processing on the structure and properties. It is found that UFW stretching leads to higher crystallinity. SEQEB stretching is found to induce very high stress during this resulted in higher crystallinity development in the sequentially stretched films as compared to simultaneously stretched ones of comparable stretch ratios. This was attributed to the increased efficiency of strain induced crystallization in the sequential deformation mode stemming from the first stretching stage.

Semicrystalline polymers exhibit changes in physical properties during storage above their glass transition temperature. According to the current paradigm, physical aging above TG is explained by densification of the rigid amorphous fraction in the vicinity of the crystalline phase. Results from differential scanning calorimetry, X-ray diffraction, atomic force microscopy and creep studies lead us to propose a different model, which considers the formation of secondary crystals and the increase in conformational constraints in the residual amorphous fraction.

Development of Stress and Birefringence during uniaxial stretching of Polymers was studied using the newly developed stretching instrument that allows simultaneous recording of stress, spectral birefringence and strain during uniaxial deformation. In this new instrument the stretching mechanism is designed such that both upper and lower sample holding clamps move away from each other simultaneously in order to probe the birefringence at the stationary mid section of the sample. The spectral birefringence technique is robust enough to follow rapid changes in the retardation (and birefringence) as well as the reversals in the trend in birefringence that may be caused by the relaxation, crystallization and/or melting depending on the processing conditions. This deviced unique instrument will allow us to study the details of the structural ordering mechanisms real time at industrially meaningful temperatures and deformation rates.

The influence of composition and pre-orientation on the development of structural hierarchy during heat setting of PEEK/PEI films was investigated using on-line birefringence, X-ray scattering and thermal analysis techniques. When the PEEK/PEI blends are drawn to deformation levels below the onset of strain hardening, the subsequent heat setting at high temperature starts with a large relaxation process followed by fast crystallization stage and long-term slow structural rearrangement process. When the films are pre-drawn beyond a critical structural level (crystallinity and orientation), the initial relaxation stage completely disappears. This signifies that beyond a critical structural order a long-range physical network, where the nodes consist of crystallized domains, is formed. The addition of non-crystallizable PEI chains was found to retard the formation of this network structure"."

Polymer blends are, with few exceptions, made in extruders. However, extrusion can be costly and requires that the previously made starting materials (blend components) be isolated (and dried in the case of polar polymers) and handled, adding to expense. A novel method has been discovered wherein polyester/acrylate polymer blends are prepared in the polyester reactor itself. This is accomplished by emulsion polymerizing acrylate monomers in the presence of a diol continous phase. The diol can then be used to synthesize a polyester. This technology allows for blending and compatibilization strategies accessible and inaccessible through conventional methods at a reduced cost. In addition to polyesters and polyacrylates, we have expanded this technology to include other condensation polymers such as polyamides and other dispersed phase polymers such as silicone and butadiene rubbers.

A potential material change for a medical packaging thermoformed pack requires costly and time-consuming re-validation. One of the major issues associated with this is the puncture resistance of the pack. This study considers a non-isothermal computational fluid dynamics (CFD) analysis of the thermoforming process of a syringe pack, followed by a non-linear structural analysis of the formed pack to estimate puncture resistance. In this way the final shape and thickness distribution of the thermoformed pack are included in the structural analysis. Two reference materials will be used in validating the usefulness of the technique.

Despite continuous growth over the years, thermoplastic composites suffer from rather low mechanical properties due to low levels of glass reinforcements and random fibers. The breakthrough of commingling thermoplastic and glass fibers during the glass forming process addresses that issue. When comparing commingled wovens to long fiber reinforced PP the significant improvement in properties opens the door to new applications. Recent developments have focused on combining woven fabrics and molding compounds in both injection and compression molding tools to take advantage of the benefits of both material forms.