Limiting thickness estimation in polycarbonate lenses injection using CAE tools

Some commercial software packages that simulate injection processes are available. It is possible to design moulds and to optimise the processes by using computer-aided engineering (CAE) tools. In addition, we employ C-MOLD, one of the best commercial packages, to determine the limiting thickness in polycarbonate lenses manufactured by injection. In this work we use a simple geometry to study the variation of different magnitudes as a function of the lowest thickness in biconcave and biconvex lenses. The parameters studied are the variation of temperature, pressure and piece mass during the filling time, air traps, melt lines and weld lines. The flow problem of the plastic in the mould is very different for biconcave and biconvex lenses. While the limiting thickness is in the centre for the biconcave lens, the perimeter is the limiting zone for the flow in the biconvex lens. For both geometries, however, the minimum thickness is limited by a mechanical and an aesthetic factor. If we take into account the variation of temperature, pressure and mass with minimum thickness, the mechanical limiting thickness can be estimated as 500 μm. But, if we also consider the evolution of the air traps and the melt and weld lines, which greatly influence the aesthetic factor, the resulting limiting thickness is 650 μm in the centre of biconcave lenses and 500 μm in the peripheric area of biconvex lenses

Critical thickness estimation in ISO-MC cards injection using CAE toolsls

Injection moulding is a widely used polymer processing operation because of its ability to produce net-shaped components of good dimensional accuracy with a relatively short cycle time. Nevertheless, only few industries use CAE tools to optimise the injection process or to design the mould. Today, many industries have more confidence in their know-how than in the prediction on a commercial simulation software package. In this work we use C-MOLD to estimate the critical thickness for injection by studying the flow parameters in the thinnest section point of the ISO-MC card. We simulated the process with the geometry of a microchip credit card, which has a minimum thickness of 250 μm (memory microchip area thickness), and studied the variation of different process parameters as this thickness increased. The maximum thickness was fixed at 800 μm (global thickness) and the minimum thickness was modified from 250 to 500 μm. Different injection points and filling times were used to optimise the process. The pressure and temperature variations during the filling process are the most sensitive parameter. The plot of these parameters versus the minimum thickness allowed us to estimate the critical thickness of 360–390 μm for this geometry using different ABS plastics formulations. The plots show how the magnitudes are stabilised in a plateau when the thickness is higher than 390 μm but they increase as the thickness decreases below 360 μm

Fluorescence labels to monitor water absorption in epoxy resins

Water uptake phenomena was studied in a group of commercial epoxy based thermosets using gravimetric and fluorimetric analysis. The different epoxy formulations were labeled with two dansyl derivatives differing in the spacer length between the chain and the fluorophore moiety. The fluorimetric method consisted of monitoring the changes in the first moment of the dansyl emission band as a function of water immersion time. Using the fluorescence, it was possible to obtain the parameters that govern the water diffusion process and there was a good concordance with gravimetric results. Furthermore, the fluorescence response of the dansyl moieties was used to study the effect of the molecular structure of the polymers in the water absorption process.Authors wish to thank CICYT and UE for support under grants MAT1997-2072-CE and BE97-4472, respectively

Effect of the morphology of two phase polymer blends on glass transition temperature

Paper derivado del Póster presentado al Congreso: International Conference on Advanced Materials Processing Technologies (AMPT'01)Blends of poly(cyclohexylmethacrylate), PChMA, with poly(vinylacetate), PVAc, were prepared by casting THF and chloroform solutions. A calorimetric and morphologic study was performed. Differential scanning calorimetry investigations of the blends show two glass transition temperature that give evidence of inmiscibility. For polymer less flexible (PChMA) was observed an enhancement of Tᵍ by 20 °C approximately in blends of PVAc-rich composition. Epifluorescence microscopy, using PChMA labelled by copolymerization pirenyl-methylmethacrylate, Py, shows also phase separation providing imaging of the distribution of PChMA in the different domains and in the matrix. Blends containing 80% w/w PChMA show a bicontinuous primary morphology suggesting a spinodal phase separation mechanism. The 50 and 20% PChMA samples show morphologies composed of PChMA-rich domains in a matrix composed by PVAc mainly. Blends with domain–matrix morphology present a higher Tᵍ than pure homopolymer more rigid (PChMA) due to packing in microphases in matrix of more flexible polymer (PVAc)

Host/Guest Simulation of Fluorescent Probes Adsorbed into Low-Density Polyethylene, 1. Excimer Formation of 1,3-Di(1-pyrenyl)propane

Molecular dynamics and Rotational Isomer State/Monte Carlo techniques with a Dreiding 1.01 Force Field are employed to study the excimer formation of isolated 1,3-di(1-pyrenyl)propane and the probe adsorbed into a low-density polyethylene (LDPE) matrix model. The probability of formation of each molecular conformer at several temperatures was calculated using these theoretical techniques. Conformational statistical analysis of the four torsion angles (ϕ₁, ϕ₂, θ₁, θ₂) of Py3MPy showed that the angles —C—Cᵃʳ— (ϕ₁, ϕ₂) present two states c ± = ±90°; and the angles —C—C— (θ₁, θ₂), the three trans states = 180°, g ± = ±60°. The correlation of θ₁–θ₂ torsion angles showed that the most probable pairs were g⁺g⁻ and g⁻g⁺ for the excimer-like specimens, although these angles are distorted because of interactions with the polymer matrix. The temperature dependence of the excimer-formation probability revealed that this process was thermodynamically controlled in the isolated case. When the probe was adsorbed into the LDPE matrix, the excimer formation process was reversed at T = 375 K. At T > 375 K, the behavior was similar to the isolated case but, at T < 375 K, excimer formation probability increased with temperature as found experimentally by steady-state fluorescence spectroscopy. This temperature was coincident with the onset of the LDPE melting process, determined experimentally by thermal analysis.The authors wish to thank Brite EuRam programme (BE97-4472) and CAM (07N/0002/1998) for support

Studies of Polymerization of Acrylic Monomers Using Luminescence Probes and Differential Scanning Calorimetry

A fiber optic system has been designed to couple calorimeter and fluorimeter equipment for in situ monitoring of polymerization reactions by both techniques simultaneously. Two acrylic monomers (cyclohexyl methacrylate, CHM, and 2-ethylhexyl acrylate, EHA) were studied at different temperatures. Pyrene (Py) was employed at a low concentration (≤10⁻⁴ mol/L) as a fluorescence probe. The emission spectrum of pyrene shows a broad band peaking at 390 nm, whose intensity grows with polymerization progress. A correlation with conversion degree could be established by collecting fluorescence intensity through the optical fiber at different polymerization times. For the more flexible polymer formed, poly(ethylhexyl acrylate) (PEHA), Py emission sharply increases only when high conversions are reached and continues increasing for a long time after the limiting conversion is attained. For CHM polymerizations, S-shaped curves are found. Isochronal plots of intensity vs. scaled conversion allow elaboration of master curves for the peak emission. Data at 50°C for CHM cannot be fitted to the master curve, and this is explained in terms of vitrification.The authors acknowledge the financial support of this work by the Consejeria de Educación of Comunidad de Madrid (CAM 247 /92) and the Comisión Interministerial de Ciencia y Tecnología (MAT93-0823)

Use of CAE Tools to Determine Critical Dimensions of Lenses Processed by Plastic Injection

Se ha empleado C-MOLD (herramienta CAE, Asistencia Computacional a Ingeniería) para simular el proceso de inyección de lentes bicóncavas y se ha encontrado que para ciertas geometrías los valores de las variables del proceso de inyección comienzan a ser excesivamente altos. Se han diseñado lentes de 50 mm de diámetro y muy diferentes curvaturas y se ha establecido una relación entre los parámetros geométricos (espesor y curvatura) y las variables de proceso (presión máxima de inyección y diferencias de temperaturas al final de la inyección). De las representaciones de las variables de proceso frente a los parámetros geométricos se han podido determinar los valores críticos de los mismos, siendo de 2,2 mm para el espesor en el borde de la lente, 0,75 mm para la ságita, 420 mm para el radio de curvatura y 0,4 mm para el espesor en el centro de la lente.C-MOLD software (CAE tools, Computational Assistant Engineering) has been used to simulate the injection process of biconcave lenses. It has been found that for a given geometry the variables that control de process attain values that are excessively high. Lenses of 50 mm in diameter and very different thickness have been designed and a relationship between the geometrical parameters (thickness and curvature) and the process variables (maximum injection pressure and temperature difference at the end of injection) has been established. From the plots of the process variables against the geometrical parameters it has been possible to obtain the critical values of the geometrical parameters: 2,2 mm for the edge thickness, 0.75 mm for sagitta, 420 mm for the curvature radius and 0,4 mm for the thickness at the center of the lens.Los autores quieren agradecer al Ministerio de Ciencia y Tecnología (MAT2001-0677 y MAT2004-01347) por la financiación concedida

Comparison of Gauge Deformation Determined by Simulation of the Injection Process With Real Process Values

Se ha realizado un estudio comparativo entre las deformaciones reales de una pieza industrial obtenida por inyección y los desplazamientos obtenidos por simulación empleando C-MOLD, con el objeto de verificar la utilidad predictiva de herramientas de simulación de procesos. La pieza seleccionada es un aforador, fabricado en poliamida 6/10 con un 50% de fibra corta de vidrio, que presenta suaves rechupes en las zonas laterales como principal defecto. La simulación del proceso se realizó con las condiciones empleadas por el fabricante y, posteriormente, se optimizó para máquinas de inyección Engel 125 y 175. Con los parámetros óptimos de proceso se fabricaron piezas y se midieron las deformaciones en puntos situados en dos ejes paralelos a la dirección longitudinal de la pieza. Estas deformaciones se compararon con los valores de desplazamiento obtenidos por simulación encontrándose una coincidencia excelente entre ambos, excepto en las zonas próximas a los extremos de la pieza.A comparative study between the actual deformation of a real industrial part obtained by injection moulding and the displacement in the same simulated part has been performed using C-MOLD, with the objective of verifying the predictive capabilities of process simulation tools. The selected part is a gauge, made of polyamide 6/10 reinforced with 50% of fibber glass, that presents soft sink marks in the lateral areas as main defects. Process simulation was initially performed using the process conditions used by the manufacturer and, in a later stage, those conditions were optimised for Engel 125 and 175 injection machines. Using the optimum process parameters a set of parts was manufactured and the deformations were measured in several points located along two parallel longitudinal axis. These deformations were compared with displacement values obtained from simulation. Excellent agreement between the two sets of values was found except in the most external regions of the part.Los autores quieren agradecer al Ministerio de Ciencia y Tecnología (MAT2001-0677 y MAT2004-01347) por la financiación concedida

Micromorphology and Relaxation Processes of Low Density Polyethylene Probed by Fluorescence Spectroscopys

Fluorescence spectra of molecular guests at several temperatures are useful technique to study several types of polymer properties. In particular, it has been often employed to study polymer relaxation processes either in static or in dynamic conditions. In this work some applications of the steady-state fluorescence spectroscopy of different guests in polyethylene and in particular reports the photophysical behavior of 1,3-di(1-pyrenyl)propane (1Py(3)1Py) sorbed in low density polyethylene are shown. This molecular probe differs of pyrene by its ability to form intramolecular excimer species and we discuss the temperature dependence of this formation. In this study we recorded simultaneously the fluorescence spectra and the differential scanning calorimetry (dsc) traces. The relative dependence of fluorescence intensities on temperature of the higher energy vibronic band at 367 nm, of the isolated choromophore emission and of the excimer emission were discussed and associated with the polyethylene polymer relaxation processes. The influence of the macroscopic stretching upon the polymer relaxation processes and the fluorescence emission was also investigated and compared with previous results.J. B. thanks Brite EuRam/EU (BE97-4472) and to CICYT (MAT2000-0391-P4-02) for financial support. T. D. Z. A. thanks FAPESP and CNPq (Brazil) for financial support and a fellowship. S. B. Y. thanks FAPESP (Brazil) for a fellowship. The authors thank Prof. Carol Collins for useful discussions

Photochemical sensing of semicrystalline morphology in polymers: Pyrene in Polyethylene

Pyrene was inserted into a low-density polyethylene matrix. Fluorescence spectra as a function of temperature and the differential scanning calorimetry (DSC) trace were recorded simultaneously. Along with the usual vibronic bands, a low-intensity band at 365 nm appears at higher energies with respect to the 0−0 transition in the pyrene fluorescence spectra. The fluorescence intensity of this small band increased with temperature, and an isoemissive point was observed to occur at 368 nm. This emission was interpreted as arising from pyrene molecules located in the outer rigid interfacial region of polymer crystallites. Its temperature-dependent fluorescence was interpreted in terms of electron−phonon coupling; two phonons which coincide with fundamental vibrations of polyethylene were necessary to fit experimental data. Coupling with a high-energy phonon was possible at low temperature, whereas, above the β relaxation temperature, phonon coupling occurs with a lower-energy phonon. The α relaxation was detected as a maximum in fluorescence intensity since above its characteristic temperature, nonradiative processes begin to operate.J. Baselga thanks CAM and CICYT (Projects No. 247/92 and MAT 93-0823} for financial support. T.D.Z.A. thanks FAPESP, FINEP, and CNPq (Brazil} for financia} support