Band 30, Heft 4

A polymer's thermal conditions during processing in injection molding define the polymer's structure and with this the properties of the final part. Thus prediction of the temperature in the part during processing is of great interest here. One important value for calculating the temperature is the heat transfer coefficient (HTC), or the thermal contact resistance (TCR) between polymer and mold. Because of this, significant work has been done on this topic. This article gives an impression of the importance of HTC in injection molding and an overview over work conducted in measuring and calculating the HTC and the up to now known influences on it.

In the present work the benefits of using graphics processing units (GPU) to aid the design of complex geometry profile extrusion dies, are studied. For that purpose, a 3D finite volume based code that employs unstructured meshes to solve and couple the continuity, momentum and energy conservation equations governing the fluid flow, together with a constitutive equation, was used. To evaluate the possibility of reducing the calculation time spent on the numerical calculations, the numerical code was parallelized in the GPU, using a simple programing approach without complex memory manipulations. For verification purposes, simulations were performed for three benchmark problems: Poiseuille flow, lid-driven cavity flow and flow around a cylinder. Subsequently, the code was used on the design of two real life extrusion dies for the production of a medical catheter and a wood plastic composite decking profile. To evaluate the benefits, the results obtained with the GPU parallelized code were compared, in terms of speedup, with a serial implementation of the same code, that traditionally runs on the central processing unit (CPU). The results obtained show that, even with the simple parallelization approach employed, it was possible to obtain a significant reduction of the computation times.

An intermeshing co-rotating twin-screw extruder is widely used for producing polymer composites. In order to satisfy properties of the composites, it is important to understand mixing performance of segments. In this study, we discussed mixing performance of special rotor segments commercially named VCMT (various clearance mixing technology) by using a numerical analysis method. The several tip clearances of VCMT work for achievement of both self-cleaning and effective mixing. We adopted a particle tracking method to obtain the following two evaluation indices; 1) residence time for the evaluation of its uniformity and 2) maximum 1st principal stress for the evaluation of dispersive mixing. And we evaluated mixing performance of VCMT by using probability distributions of each index. Moreover, we proposed weighted probability distributions as theoretically better evaluation. The probability distributions of VCMT were compared with those of two kinds of kneading disk segments (KDs). From the comparison, it was found that the probability of VCMT in large 1st principal stress region is higher than those of KDs. As a conclusion, VCMT has better mixing performance than KDs for the geometries and operating conditions that were investigated.

To improve part quality and consistency injection molders strive to control the temperature of the molten plastic during cavity filling. In-mold temperature sensors can effectively measure the temperature of the mold surface that contacts the melt; however, they do not provide a measure of the bulk melt temperature. An analysis was developed to use data from in-mold thermocouples to predict the plastic's bulk melt temperature. This analysis integrates the heat flux through the mold steel to calculate the bulk melt temperature. A mold instrumented with an in-mold thermocouple and an IR temperature sensor was used to validate the predictions. The effects of changing barrel temperature, coolant temperature, injection velocity, and cooling time were studied using a 21 run DOE. Validation was performed by comparing bulk temperature data from the IR temperature sensor with temperature predictions derived from the in-mold temperature sensor. Trends in the melt temperature were consistently predicted, however, the magnitudes were low due to residual heat remaining in the part at the end of the molding cycle. The analysis was more sensitive to process changes than raw data from the in-mold temperature sensor, thereby improving process observability.

To improve the impact resistance and reduce the product cost, poly(lactic acid) (PLA) blends containing varying mass fraction of waste rubber powder (WRP) were fabricated via melt compounding. The effects of WRP contents on the mechanical properties, morphologies and thermal stabilities of PLA/WRP blends were investigated. Mechanical tests showed that WRP could increase the ductilities of PLA, leading to the significant improvements in the impact toughness and elongation at break. In contrast, the tensile strength was just heightened slightly, while elastic modulus declined gradually. Scanning electron microscopy observations indicated that well bonded interfacial morphologies were formed between PLA and WRP. From the results of thermo gravimetric analysis, WRP decreased the onset and peak decomposition temperatures of PLA phase and increased the char contents of samples significantly. Average activation energies of samples were increased first and then decreased with increasing WRP. Finally, theoretical lifetimes of PLA/WRP blends were also estimated.

Polymer electrolytes prepared by the complexation of lithium salts with poly(ethylene oxide) (PEO) and poly(vinylidene chloride-co-acrylonitrile) (PVdC-co-AN) will be of great use as separators in lithium polymer batteries. The amorphous nature of the blend electrolyte shows that the conductivity increases by the addition of lithium salts. The presence of C≡N and C=N in PVdC-co-AN are confirmed from the Fourier transform infrared studies. Among the various lithium salts studied, lithium trifluoro methane sulfonoimide [LiN(CF 3 SO 2 ) 2 ] based electrolyte exhibits the highest ionic conductivity of the order of 0.265 × 10 −5 Scm −1 at room temperature. The sample having a maximum ionic conductivity PEO(80 wt%)/PVdC-co-AN(20 wt%)/LiN(CF 3 SO 2 ) 2 (8 wt%) is supported by the lower optical band gap in UV-Visible analysis and low intensity in luminescence studies. Two and three dimensional topographic images of the above sample reveal the presence of micropores. Thermal stability of the prepared electrolytes is studied by thermo gravimetric/differential thermal analysis. Using differential scanning calorimetric analysis, the minimum glass transition temperature (30°C) is observed for the sample doped with LiN(CF 3 SO 2 ) 2 . The cyclic voltammetric studies reveal the strong capacitive behavior of the prepared polymer electrolytes. The electrochemical stability windows for the prepared samples are observed using linear sweep voltammetry.

Injection stretch blow molding (ISBM) is a two-step process that was designed and optimized mainly for unfilled PET resins. The present study focuses on stretch blow molding of a PET filled with different amounts of sub-micronic mineral fillers. The influence of fillers is analyzed thanks to DSC, DMA, tensile testing and processing in different configurations on a prototype apparatus and an industrial machine. It is demonstrated that fillers enhance crystallization kinetics, which leads to a reduction of the processing range. Differences in strain hardening induced by fillers make it necessary to adjust blowing temperature. However, the main effect occurs during the heating phase of the preforms. Temperature within a filled preform is much less homogeneous than in neat PET, making the temperature gradient totally different if the heating protocol is kept unchanged. Once the heating of filled preforms is controlled to reach equivalent temperature gradients, blowing is possible and rather equivalent to that of pure PET.

Chain architecture is an influential parameter in melt processing of polymeric material. In this study effect of different chain structures on polylactide (PLA) film blowing process and on the final film properties was investigated. Blends of linear PLA with three branched poly(L-lactide)s with 4-arms star shaped, comb like and hyper branched structures were prepared by twin screw extruder. In another series of blending, poly(D-lactide) of these branched structures were used to obtain stereocomplex crystal. Operating window of blends were determined in a series of film blowing process experiments with different blow up and take up ratio and results were compared to linear PLA as a reference. Moreover, different characterization techniques were applied to determine the properties of produced films. It was found that addition of branched architecture and presence of stereocomplex in a linear PLA matrix result in a greater operating window for film blowing and improve mechanical and crystallization properties of blown films.

Biodegradable PCL/PLA/Ag-MMT nanoblends with a strong antibacterial activity and a silver ion release property were successfully prepared by melt blending process for active packaging use. The presence of silver exchanged montmorillonite and its interaction with PCL/PLA matrix was evidenced by ATR/FTIR. The morphology was investigated by both XRD and TEM analyses. It was suggested the formation of mainly exfoliated structures with a random dispersion of spherical silver nanoparticles within their composite matrices. The thermal and mechanical properties of these nanomaterials were thoroughly investigated and compared to those of the unfilled blends. The potential of the silver ion release from the PCL/PLA/Ag-MMT film was estimated after immersion in water for several periods of time. The results showed an increase of the amount of silver ions released with growing immersion time of PCL/PLA/Ag-MMT nanoblends. Due to the presence of silver nanoparticles in their matrices, PCL/PLA/Ag-MMT nanoblends showed a total bacteria growth inhibition.

The study focused on the material behavior of two commercial polylactide grades affected by structural changes due to exposure to humid environment. Additionally, the impact of temperature and environment humidity on the moisture uptake and drying process of polymer granulate was examined to evaluate the necessary pre-treatment conditions prior to the polylactide processing. Also some process relevant aspects, such as the impact of polymer pre-drying on its degradation were characterised. In the first part of the experimental work, moisture absorption mechanisms in relation to conditioning parameters were investigated using Karl Fischer titration method. It was shown that the moisture uptake rate correlates with temperature and humidity of the environment. Furthermore, the drying process of polylactide pellets was conducted and analysed by additional melt flow rate measurements to determine the effect of drying conditions on the premature polymer degradation and melt properties. Investigated polymers demonstrated rheological behavior dependent not only on the residual moisture content, but also on drying temperature and time. In the last section of the study flexural and tensile tests were performed in order to determine the influence of moisture uptake on the characteristics of conditioned polylactide. Mechanical strength of polylactide decreased in both, flexural and tensile tests with increasing moisture content in the samples. Moreover, distinctive differences in polylactide mechanical behavior were observed in both tests.