Band 24, Heft 2

Wall-slipping materials show lower melt temperature increase than wall-adhering materials on account of the reduced dissipated power component in the system, as is set out in Potente and Kurte-Jardin (2005) and Potente et al. (2005a). Up until now, however, there have not been any models for describing the development of the melt temperature in a single-screw channel when wall-slipping polymers are being processed. For this reason, a methodology of predicting the melt temperature is described. The new model is based on an existing analytical procedure that has been extended by numerical flow simulation results of isothermal 2,5D FE calculations. A comparison of the predicted mean melt temperatures with non-isothermal 3D FE flow simulation results leads to good agreement, what justifies further investigations.

A novel dilatometer to investigate the specific volume of polymers as a function of the combined effect of pressure (100 MPa), temperature (300°C), cooling rate (100°C/s) and shear rate (200 l/s) was developed. The dilatometer consists of a pressure cell, which in design is a combination of a traditional “piston-die type” dilatometer and a Couette rheometer, embedded in a custom made frame, which allowed for scaling down to a “table-sized” machine that requires only standard laboratory supplies, like pressurized air and tap water, for operation and cooling. We implemented software for fully automated control of procedures to operate non-isothermal experiments with shear steps applied at predefined temperatures. The sample rings (m ≈ 65 mg) used in the dilatometer are made with a micro injection moulding machine. Experiments with two commercial isotactic Polypropylene (iPP) grades at low cooling rates, performed by two independent groups, were compared with measurements from a commercial confined fluids dilatometer showing small relative differences in the range of 0.03 to 0.3%. As an example, additional results of an isotactic polypropylene were chosen to show the profound influence of cooling rate and melt shearing on the evolution of specific volume.

The external vacuum calibration of extruded thermoplastic materials is a complex process, which may hinder the production and even the quality of the parts. Process control requires a thorough knowledge and a deep analysis of the heat transfer phenomena between both the part and the calibrator during the cooling phase. This paper initially presents a detailed description of the experimental setting used to qualify and quantify the heat transfer modes. The experimental specimen is a 100-mm wide PVC strip, which represents one of the three sides of the U-profile. The development of physical models to predict plastic part thermal history during the forming process is a major industrial concern because it can reduce tooling development times and costs significantly. In the extrusion processing field, the external vacuum calibration stage often hinders productivity. The present study has been conducted to examine strip cooling phase so as to propose some thermal models for both global and detailed analyses of heat transfer modes. First, a transparent wall calibrator is used to view the behaviour of the extrudate inside the calibrator for a better understanding of the thermal phenomena at work. A second calibrator equipped with temperature sensors makes it possible to assess, using an inverse calculation method, the heat flux transferred from the extruded material to the calibrator as well as the temperature pattern throughout the tool.

The bleeding (internal transport) process of additives in a polypropylene film under atmospheric pressure was investigated. The experimental results were explained more precisely by a new model assuming the two-step transport between the amorphous and crystalline regions. The diffusion coefficient of a higher fatty acid such as behenic acid (docosanoic acid) in isotactic polypropylene film and that of higher fatty acid amides such as erucamaide (13-cis-docosenamide) in ethylene copolymerized polypropylene film were determined at 40 and 50°C respectively. The difference between the diffusion coefficients of three slip agents in a polypropylene film at 50°C was explained using a molecular dynamics simulation in which self-association of the slip agent molecules by hydrogen bonding was considered.

This work focuses on the development of a general finite element code devoted to the three-dimensional direct simulation of mixing processes of complex fluids. The code, developed from the CimLib © Library, is based on a domain immersion method coupled with a “level-set” approach to represent the rigid moving boundaries, such as screws and rotors, as well as free surfaces. These techniques, combined with the use of automatized parallel computing, allow calculating the time-dependent flow of generalized Newtonian fluids in large and complex processes, involving moving free surfaces which are treated by a level-set/Hamilton-Jacobi method. Three examples of flow case studies are presented in this paper: the flow within the mixing section of a twinscrew extruder, the flow in an internal mixer and the flow in a batch mixer.

The high-temperature storage stability of styrene-butadiene-styrene triblock copolymer (SBS) modified asphalt can be improved to some extent with the addition of phosphorus trichloride. The dynamic mechanical and physical properties of SBS-modified asphalts before and after adding phosphorus trichloride were studied respectively. The addition of phosphorus trichloride to asphalt improved the high-temperature performance of asphalt and reduced its temperature susceptibility to a great extent with unfavourable effect on low-temperature ductility. The addition of SBS to phosphorus trichloride modified asphalt increased the low-temperature ductility, viscosity of asphalt and improved its high-temperature performance further. Different SBS structures influenced the rheological and physical properties of asphalt before and after adding phosphorous trichloride.

The effect of bead vacuum on the minimum wet thickness obtained on horizontal and vertical slot die coatings was experimentally examined using a flow visualization technique. The commercial software package FLOW3D was used to simulate the two-dimensional coating flow. Macroscopic material balances were applied to estimate the effect of bead vacuum. Applying vacuum on the coating bead was found to reduce the minimum wet thickness and increase the maximum coating speed, with the effect on horizontal slot die coating being more pronounced. Visual observations revealed that with increasing vacuum, the upstream meniscus against the moving web was pulled back significantly for a low-viscosity solution, and the dynamic contact angle was reduced for a high-viscosity solution. The observed upstream wetting length for a low-viscosity solution subject to bead vacuum was consistent with the prediction using macroscopic balances. The sizes of coating beads predicted by FLOW3D were generally larger than those observed from flow visualization. Coating failures caused by high bead vacuum could be predicted with the aid of numerical simulations.

A previously developed model which predicts the yield stress of a polymer glass directly from processing conditions is applied to a system of miscible polymers. The selected system consists of a blend of polycarbonate with polyester and three blend compositions of increasing weight percentages polyester are investigated with respect to their aging kinetics. Based on these kinetics, the yield stress as it results form the thermal history experienced during processing is predicted and found to be in good agreement with experimental results. The parameters governing the evolution of the yield stress are shown to follow the rule of mixtures, enabling the prediction of the yield stress of any blend composition.

Hot embossing is a compression molding technique used for high replication accuracy of small features. One of the most sensitive phases of the process is the de-embossing stage during which the patterned part has to be demolded. In this paper, the demolding stage is considered as a frictional contact problem between a rigid mold insert and a viscoelastic polymer sheet as it deforms and cools inside a mold under an applied force. The contact is modeled with a modified Coulomb's law of dry friction while a generalized Maxwell model is used to describe the polymer behavior during embossing, cooling and de-embossing stages. The heat transfer between the mold insert and the patterned polymer sheet is solved through a domain decomposition method. A finite element approximation based on a penalized technique is proposed and analyzed. The purpose of this modeling approach is to predict dimensional stability and residual shape of microcomponents in the hot embossing process. Such a prediction will allow one to assign appropriate processing conditions that minimize geometrical imperfections and increase replication accuracy.

The design and performance of a flow geometry for the multipass rheometer (MPR) is described, creating an experimental setup to study in-situ and ex-situ structure and morphology development with a proper control over the processing conditions and shear history. The slit used is equipped with diamond windows, to combine flow with different experimental techniques, such as optical microscopy (OM), birefringence and X-ray scattering. In this paper we present preliminary results, obtained on isotactic polypropylene, that demonstrate the possibilities of this device for more extended future research. The focus is on the in-situ birefringence measurements of crystallization and the relation with the final morphology.

A novel screw-nested extruder is fabricated, which consists of double extrusion systems. The inner screw couples with one inner barrel, whose outer configuration acts as the screw for the outer barrel. The nested screw extrusion systems can co-extrude when the inner barrel/outer screw rotates. Each extrusion system can also work independently as one general extruder. A new mathematical model of an isothermal Newtonian flow is developed to describe the melt flow in the extruder. The analytical solutions of flow rate are well in agreement with the Campbell's experimental data for screw drag flow, screw flight displacement flow and barrel drag flow when the boundary conditions are obtained for the special cases in which the screw core, screw flight and the barrel rotate independently or in pairs. This can not be explained by conventional parallel-plate models. Further, the model is also well used to analyze the melt flow in the inner extrusion system of the novel nested screw extruder.

Powder injection moulding (PIM) is an important and accepted industrial technique for net shaping of precision components which can have a rather complex geometry. In order to meet the imposed, often rather strict, requirements with regard to dimensional accuracy, it is important to have an adequate knowledge and control of the rheological behaviour and the related processing properties of the powder/polymer melt (feedstock). Such a knowledge is furthermore of crucial importance in numerical simulations of the PIM-process. In the present work, a model system, consisting of steel powder, poly(ethylene glycol) and wax, is used in order to illustrate how the viscometric properties as well as thermal properties, such as the conductivity and the specific heat, of the system can be related to the corresponding properties of the polymeric binder system. In a similar way, the pvT (pressure-volume-temperature)-behaviour of the model system is analysed and discussed. The pvT-behaviour, which has not been extensively reported on for PIM-feedstocks, is considered to be of significant relevance for controlling the outcome of the injection moulding process.