The SPE Library contains thousands of papers, presentations, journal briefs and recorded webinars from the best minds in the Plastics Industry. Spanning almost two decades, this collection of published research and development work in polymer science and plastics technology is a wealth of knowledge and information for anyone involved in plastics.
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Minimizing Part Sink Marks Using C-Mold and Genetic-Optimization Algorithm
Injection molding is widely used for mass production of polymer products. One important issue is how to determine the process conditions to produce parts of the best quality. The objective of this paper is to show how C-Mold combined with an efficient optimization system can automatically predict the optimum process to minimize sink marks. C-Mold and Genetic optimization algorithm have been integrated to solve the problem. Sensitivity analysis on part sink marks with respect to process parameters (such as filling time, hold time, cooling time, packing pressure, mold temperature and melt temperature) are also presented in this paper. Simulation results show that holding time, hold pressure and gate size have the greatest effect on part sink marks. A number of examples have been tested and the results show that sink marks can be significantly reduced after optimization.
Vibrated Gas Assist Molding: Its Benefits in Injection Molding
This paper shows how air or nitrogen can be used to impart vibration and/or pressure pulses to a melt. Air is already used to blow preforms and parisons  inside of molds, and to core out hollow articles in the process of Gas Assist Injection Molding . The methods of gas assist molding have demonstrated their great usefulness in injection molding not only to hollow parts out but also to induce an excellent surface finish. Melt vibration techniques have also been reviewed [3,4] and show great potential to reduce viscosity during filling and impart optical and mechanical benefits, i.e. stiffness, strength and clarity, without resorting to processing aids such as thinning or nucleating agents. The present paper explores the processing of injection molded plastics under gas vibration. Vibrated gas can be used for several purposes. 1. Gas can be inserted and vibrated in the mold prior to melt injection to modify the filling process mechanism, fuse knit lines, heal sink lines and other defects due to flow imperfections. 2. Compressed Vibrated Gas can act like a pressurized vibrated gas spring, which helps induce orientation benefits in the short shot during filling completion. 3. Vibrated air pressure, localized in specifically designed air-runners distributed around the runners and inside the mold, helps fill and pack the mold, core out hollow parts and balance flow in multi-cavity molds. 4. Vibrated Gas can also be used to tag parts for recognition during recycling or later inspection. The paper reviews hardware and controls requirements to apply this novel technique to injection molding.
Temperature and Molecular Weight Dependence of Viscosity Revisted: New Formulations for Rheology
Viscosity of polymers is key to their behavior in the molten state and thus to their processing. The well known equations of rheology giving the temperature and molecular weight dependence of viscosity are reviewed and tested with two independent sets of results on Polystyrene and Polycarbonate. It is shown that the admitted view that molecular weight and temperature separate in the expression of viscosity is only an approximation. Furthermore, the classical 3.4 exponenet for the variation of Newtonian viscosity with molecular weight is shown to be temperature dependent and to represeent another curve fitting approximation of the effects of entanglements on the viscosity. Another model of melt deformation and of the influenece of entanglements is presented. Based on this new model of interactive coupling kinetics other formulations of viscosity are suggested and tested on two well characterized melts of Polycarbonate and Polystyrene. The paper gives an explanation to the reptation model dilemma. why does the theory predict a power exponent of 3 whereas viscosity behaves like 3.4?
A New Formulation and Inerpretation of Shear-Thinning of Polymeric Melts. Effect of Temperature, Strain Rate and Frequency
Shear-thinning of polymeric melts is analyzed in this paper from dynamic viscosity data obtained in the non-Newtonian regime. Shear-thinning is expressed as a function of (G'/G*), where G' is the elastic modulus and G* the amplitude of the complex modulus. It is shown that viscosity depreciation, i.e. shear-thinning, is due to the cooperative coupling characteristic of the interactions between the polymer macromolecules which form a network , the EKNET network. The Newtonian character of viscosity at low strain rate is interpreted with the notion of structural re-localization of the conformers, without deformation of the potential energy of their interaction. It is purely a diffusion phenomenon. New equations are provided which define the mechanisms of shear deformation of the melt under different conditions of temperature and strain rate, and relate non-Newtonian viscosity to the number of interactive conformers stretching cooperatively versus those conformers which re-localize structurally. It is stipulated that viscous and viscoelastic effects can be interpreted from the EKNET model of interactive coupling kinetics [2-7].
Viewing Entanglements as a Two-Phase System in Polymeric Materials
All past statistical theories concentrate on the macromolecular aspect of polymeric materials, and try to determine the macroscopic properties from the characteristics of the individual macromolecules. In those theories a polymer molecule is identical to a spaghetti chain" which is able to "tie" other molecules or slip against another one creating flow and hysteresis effects. By necessity there is also place in the macromolecular theories for such a concept as chain entanglement. Accordingly two macromolecules can touch each other mechanically hook around each other in a static and stable fashion or transmit a stress as if there were a little nail pinning them together. More recently macromolecules reptate through imaginary tubes which represent the constraints imposed by neighboring chains. All those concepts are entirely directed by an approach of polymer physics around the properties of the individual singularizable macromolecules. The accent is put on determining the shape of the individual macromolecules called their configuration. The presence of neighboring and interpenetrating macromolecules is perceived as a disturbance to the ideal configuration of the chain. Macromolecules are able to rearrange their multiple configurations with the change of the thermal or mechanical energy input. This also yields a change in the conformational statistics of the bonds along the chains. For instance in the treatment of rubber elasticity it is said that the elongation of the end-to-end distance of the individual macromolecules results in a preferential orientation of the bonds in the direction of stretch."
Control of Viscosity of Polymer Melts Prior to Molding by Disentanglement Methods
Entanglement is responsible for the high viscosity of polymer melts and the molecular weight dependence of viscosity as M3.4. Disentangled polymers, at identical molecular weight, would have a much lower viscosity, proportional to M, providing a maximum viscosity reduction of (Mw/Mc)2.4. The objective of this work is to study methods to disentangle polymeric melts in order to achieve large viscosity reduction without breaking the polymer chains. We are also interested in the reverse process of re-entangling polymer melts which have been disentangled, in order to recover fully their mechanical performance after molding is performed. We first present the new concepts of interactive coupling kinetics of conformers, which naturally lead to the definition of an interface of penetration between adjacent macro-coil molecules in the melt, which we assign to entanglements". The stability of the entanglement phase depends on the potential energy of interaction between conformers. It can also vary with macroscopic variables such as temperature strain rate frequency and amplitude of melt deformation. We present a new method to increase cooperativeness between conformers in the melt to the extent that deformation occurs no longer by "reptation of the entanglement phase" which is dominated by entropic effects but by stretching resulting in disentanglement. We present viscosity reduction results for Polycarbonate and a metallocene PE obtained with a laboratory rheometer which might be proof that controlled disentanglement is indeed possible with potentially a large variety of applications for the plastic industry."
Prediction of Permeability Using Optical Coherence Tomographic Imaging of an Epoxy and Unidirectional E-Glass Composite
Knowledge of the permeability tensor in liquid composite molding is important for process optimization. Unfortunately, experimental determination of the permeability is difficult and time consuming. A rapid, non-destructive technique called optical coherence tomography (OCT) can image the microstructure of a composite in minutes. In this work, binary images were generated from the OCT data and input into a lattice Boltzmann model for permeability prediction. Calculated permeabilities agreed well with experimental values for the same fiber volume fraction.
Automated Search of the Optimal Robust Injection Molding Design against Process Validation
Injection molding optimization is unique since the injection molding process is a multifaceted process offering a number of technically feasible designs for the same optimum part quality. These solutions will vary in their quality robustness to the uncontrollable process variation. Therefore, it is of great importance to consider both response improvement and solution robustness. This paper introduces a methodology for automated search of the optimal robust design against process variability. Part warpage is chosen as the quality of interest. The warpage characteristics obtained from the proposed model are compared with those from other conventional optimization models. Each optimized design is then simulated for small plausible process fluctuations. It is seen that the optimal robust design obtained in this study exhibits the best warpage characteristics in terms of warpage mean and deviation against this mean.
Possible Applications for the Time-Temperature-Shift Principle to Describe the Impact and Long Term Materials Behavior of Thermoplastic Polymers
At the IKV a concept has been developed to describe the material behavior by means of spring/damper combinations using the time-temperature-shift principle and short-term test data. The concept called deformation model" has been used successfully to predict the uniaxial long term behavior of filled unfilled or reinforced thermoplastic polymers. In order to stimulate the dynamic material behaviour without the necessity of high speed tensile tests the deformation model is calibrated based on short-term tensile data at very low temperatures and common strain rates. To reduce the testing expenditure a concept has been developed to calibrate substitute spring/damper combination based on creep-curves from material databases (CAMPUS)."
Modeling and Experimental Validation of the Stretch Blow Molding of PET
In the two-stage stretch blow molding process for the manufacture of PET bottles, injection molded preforms are placed in an infrared oven, with axially profiled heating lamps. The subsequent inflation of the PET preform is strongly dependent on the preform geometry and the temperature profile, as hot zones will blow out faster and thin out more than colder and stiffer zones. In this work, the reheat and blowing stages of the process are both modeled and experimentally validated. The part considered is a water bottle produced at the Husky Bottle Development Center. The simulations were performed at the Industrial Materials Institute. Four oven operating settings are studied. The heat transfer in the oven is modeled by combining radiation and air convection. The preform stretching and inflation are modeled with a non-isothermal hyperelastic constitutive equation. Simulations are performed using experimentally measured temperature profiles as input.
Effect of Compounding Conditions on the Dispersion of Highly Filled CaCO3/PP Compounds in Twin-Screw Extruder
Highly filled CaCO3/PP blends (up to 60% wt) were compounded using an intermeshing corotating twin-screw extruder. The effects of screw configuration (4 geometries) and operating conditions such as RPM, total flowrate, filler feed position, temperature profile on the dispersion were investigated. A rapid and reliable method for evaluating the state of dispersion in the composites was developed. Falling weight impact and tensile tests were carried out. Composites' toughness and tensile properties (modulus, yield stress and strain), which are very sensitive to the dispersion, were correlated to the compounding conditions and to a dispersion index calculated by image analysis.
Crystallization Kinetics in Low Density Polyethylene Composites
The effect of a high electrically and thermally conductive filler on the crystallization kinetics of low-density polyethylene (LDPE) crystallites has been quantified. An increase in crystal growth rate was found which was consistent with the increase in composite thermal conductivity. However, an anomalous jump, not present in the bulk thermal conductivity, coincided with the end of the critical region in electrical conductivity. The cause of this jump is not absolutely clear; however, we believe the shift is due to the formation of a continuous network of particles causing an increase in local heat dissipation.
A Novel Test to Predict the Effect of Additives and Processing Conditions on Polypropylene Fiber Screen Pack Life
A mechanical component that is especially important in the extrusion of polymers for fiber and film applications is the screen pack. During processing this screen pack filters out material that can reduce product quality and productivity. This paper focuses on a new test method that can indicate whether a given additive/formulation or processing temperature will tend to contribute to screen pack pluggage in polypropylene. Some of the factors to be presented will be the additive formulation, melt temperature, type of neutralizer, and resin type. Key criteria for this test initially was the ability to run in a laboratory setting using normal additive loading levels and to have excellent correlation to real world" processing; these have been met with this test. Numerous resin producers worldwide have validated the results of this test in normal everyday production."
Novel All-Thermoplastic Composite (TPC) Sandwich Systems for Rapid Molding
Reinforced all-thermoplastic sandwich systems constitute a specific class of composites consisting of stiff faces and a thick and light core either porous or non-porous. They allow for a fast one-step and free of adhesive molding of continuously manufactured intermediates into complex parts (e.g. body panels, structural parts). Different all-thermoplastic sandwich systems were analyzed with respect to efficient processing techniques, manufacturing costs and mechanical performance. Furthermore, economical and technological benefits were exhibited and potential fields of application were derived. A significant potential for low-cost tooling was identified using self-expandable knitted cores whereas the application of a flowable core provided complex and stiff structures.
Characterisation of Polymers by Bubble Inflation
In order to characterise materials using a simple and relative inexpensive method, the bubble inflation technique was modified. A polymer plate is clamped between a Teflon coated heating plate and a heated cylinder. By applying air through the heating plate the polymer membrane deforms into the cylinder. The top position of the membrane is monitored by fibreoptic sensors positioned in the cylinder. The pressure difference across the membrane is measured as well. The deformation in this inflation device is non-uniform and is only equal biaxial in the top of the deformed membrane. Due to this, the response is modelled using a finite element method in 3D Cartesian coordinates. The K-BKZ constitutive equation is used to model the non-linear properties of the material. Using linear viscoelastic properties from oscillatory shear measurements and measurements of the bubble inflation, estimation of the strain dependence in the constitutive equation is possible.
Automating Online Quality Control by the Use of New Neural Network Algorithms and Neuro-Fuzzy Systems
Online quality control based upon empirical process models became a standard quality assurance tool for specific high complex and expensive parts produced in injection molding. The use of this control method is limited to special-educated engineers due to prerequisite knowledge in statistics and data analysis methods. At IKV algorithms based upon artificial intelligence have been developed to solve this problems and provide online quality control for each injection molded part without any specific education of the user. Well-known process knowledge modeled with Neuro-Fuzzy Algorithms reduces the design of experiments to a simple description of the molding. The new IKV Neural Network Algorithm enables a fully automated process modeling.
Closed Loop Fuzzy Control of Part Weight in Injection Molding of Liquid Silicone Rubber (LSR) Based on PVT-Behavior
The injection molding of Liquid Silicone Rubber (LSR) represents a cost-efficient process for the production of high quality elastomer moldings. Due to the extreme thermal conditions in the mold and the low material viscosity a precise undervolumetric filling of the cavity is required to avoid underfilling or flash formation. Since process disturbances lead to changes in the mass of material injected into the mold a closed loop control of the injected mass based on the pvT-behavior is developed and realized on an industrial injection molding machine. Dependent on the course of the cavity pressure and the mold temperature a fuzzy-based algorithm determines the required changes of the dosing volume in case of process disturbances.
Influence of mPE Grades on the Dynamic Properties of PP/mPE-Blends
Blending polypropylene (PP) with an elastomeric phase such as EPDM is often performed in order to increase its fracture toughness. With the availability of metallocene polymerized Polyethylene (mPE), a new modifier with interesting mechanical properties can be chosen for blending with PP. This research investigates the influence of different grades of mPE in PP/mPE-blends on the blend's properties. Special emphasis is placed on the dynamic characteristic quantities. The load limits of the blend for applications in which dynamic stresses are predominant are determined by using the hysteresis measurement method.
Stress Relaxation Profiles of Molded and Extruded Thermoplastics Using Dynamic Mechanical Analysis
The stress relaxation profiles across the skin-core-skin layers of injection molded polycarbonate and extruded polyethylene sheet are presented. The profiles were obtained by progressively removing the layers followed by stress relaxation tests using a Dynamic Mechanical Analyzer (DMA). The machined layers were characterized using the elastic modulus, Eo, and the relaxation time, ?o. It was found that the DMA was able to distinguish the differences in viscoelastic response across the thickness of the polymer samples. The variation in behaviour was consistent with the expected morphology and molecular orientation developed due to the processing method.
Computer Aided Design of Preforms for Injection Stretch Blow Moulding
Simulations of the injection blow moulding process have been performed using the Abaqus finite element package. The simulations have been developed using three different material models (creep, hyperelastic and Buckley) and the thickness predictions have been compared against those obtained experimentally. The Buckley model was found to be the most efficient material model to model the blow moulding process. This model is now being used to investigate the design and development of preforms. A methodology has been developed to use the material distribution produced by the simulation to predict the shelf life of the container. A FORTRAN subroutine has been written which accesses the properties of each element of the model and from these properties the shelf life can be found.
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