Tim A. Osswald

Compression Mold Filling Simulation for Non-Planar Parts

Abstract We describe a simulation of compression mold filling that combines a Galerkin finite element solution of the governing equations with a control volume scheme for tracking the moving flow front. The method can model thin shell-like parts in three dimensions, single or multiple charges, and multiply-connected domains. Either the generalized Hele-Shaw formulation or the thin-charge limit of Barone and Caulks model can be used for the governing equations. The numerical techniques are explained, with special attention to the recovery of approximate flow front shapes. Predictions compare favorably to experiments, including laboratory experiments on model fluids as well as two large commercial parts molded from sheet molding compound. The simulation is most accurate when the initial charge thickness in uniform, but can also model multi-thickness charges.

Design of Dispersive Mixing Devices

Abstract Mixing is one of the main functions of screw extruders. In the analysis of mixing, there are two mixing mechanisms that need to be considered: distributive and dispersive mixing. It is well known that single screw extruders generally have poor dispersive mixing capability, even when dispersive mixing elements are incorporated into the screw design. This paper will discuss the requirements for dispersive mixing, current dispersive mixing elements used in single screw extruders will be analyzed, and the lack of efficient dispersive mixing will be explained. A new generation of dispersive mixing elements will be introduced for use in both single and twin screw extruders and internal mixers. A two and three dimensional flow simulation will be used to analyze the mixing performance of these new mixers. Experimental work on the new mixing elements will be presented in a follow-up paper; the results demonstrate that it is possible to achieve dispersive mixing on single screw extruders that is as good as what can be achieved on twin screw extruders. The implications of these results on the compounding industry will be briefly discussed.

Prediction of Shrinkage and Warpage of Fiber Reinforced Thermoset Composite Parts

One of the most challenging tasks in designing plastic parts, especially those that are fiber reinforced, is to predict shrinkage and warpage of the molded parts. Shrinkage and warpage result from material inhomogeneities caused by flow induced fiber orientation, curing, poor thermal mold lay-out, and processing conditions. Shrinkage and warpage are directly related to residual stresses which result from locally varying strain fields that occur during the curing or solidification stage of a manufacturing process. This paper presents research conducted in modeling, analysis and process simulation of the thermomechanical behavior of compression molded fiber reinforced composite parts. The theory behind shrinkage and warpage of fiber reinforced composite parts is described first, followed by a description of finite element/finite difference simulation of the thermo mechanical behavior of fiber reinforced composites during a parts manufacturing process. A coupled temperature and stress simulation program with a three-noded shell element formulation was developed to calculate the residual stress build-up during curing and solidification stages of a compression molding process. The effects of fiber content, part thickness, unsymmetric curing and flow-induced fiber orientation on the shrinkage and warpage of the molded parts are also investigated.

Incorporation of Mg particles into PDLLA regulates mesenchymal stem cell and macrophage responses

In this work, we investigated a new approach to incorporate Mg particles within a PDLLA matrix using a solvent-free commercially available process. PDLLA/Mg composites were manufactured by injection moulding and the effects of Mg incorporated into PDLLA on MSC and macrophage responses were evaluated. Small amounts of Mg particles (≤ 1 wt %) do not cause thermal degradation of PDLLA, which retains its mechanical properties. PDLLA/Mg composites release hydrogen, alkaline products and Mg(2+) ions without changing pH of culture media. Mg-containing materials provide a noncytotoxic environment that enhances MSC viability. Concentration of Mg(2+) ions in extracts of MSCs increases with the increment of Mg content in the composites. Incorporation of Mg particles into PDLLA stimulates FN production, ALP activity, and VEGF secretion in MSCs, an effect mediated by degradation products dissolved from the composites. Degradation products of PDLLA induce an increase in MCP-1, RANTES, and MIP-1α secretion in macrophages while products of composites have minimal effect on these chemokines. Regulation of MSC behavior at the biomaterials interface and macrophage-mediated inflammatory response to the degradation products is related to the incorporation of Mg in the composites. These findings suggest that including small amounts of Mg particles into polymeric devices can be a valuable strategy to promote osseointegration and reduce host inflammatory response.

A Novel Cure Reaction Model Fitting Technique Based on DSC Scans

A numerical methodology has been developed to fit cure kinetic reaction models based on DSC scans only, and not isothermally as traditionally done. A non-linear least squares Levenberg-Marquardt algorithm was used to fit the reaction rates with the autocatalytic Kamal-Sourour kinetic model. The technique avoids the use of isothermal DSC data which causes additional problems in the fitting procedure, such as oscillations in low temperature scans or lost information for fast reaction materials at high process temperature scans. The new methodology is based on a series expansion of the kinetic parameters as a function of temperature. The technique was first tested with the vulcanization process of silicone rubber which is dominated by a cross-linking single-event curing reaction. The method was also tested with phenolic formulations, which according to the hardener, present one or two reaction events. Additionally, the glass transition temperature, of these materials, during the curing process is mostly below the vitrification line, thus, diffusion effects can be neglected. The results show good agreement between the experimental DSC scans and the predicted reaction rates. The technique presented in this paper is intended for use in process simulation, and not to infer kinetic reaction mechanisms.

Flow Analysis in Screw Extruders-Effect of Kinematic Conditions

Abstract Recently, several investigators have challenged the simplifying assumption of using a stationary screw with a rotating barrel when analyzing the flow in single screw extruders. The investigators claim that there are significant differences between the realistic and simplified cases, such as a 12 percent difference in throughput [1]. This paper presents an analytical derivation for the flow in a laid out system and one which includes the curvature of the screw. An experimental set-up was built which allows the rotation of the barrel with a stationary screw and the rotation of the screw with a stationary barrel, and a three dimensional flow simulation program was used to model the flow in the extruder using the two conditions. The analytical, experimental and numerical analyses show that it is justified to use the stationary screw assumption when modeling the flow in single screw extruders.

Simulating Polymer Mixing Processes Using the Boundary Element Method

Abstract The mixing of plastic into filled and unfilled polymer blends has been an important issue in the polymer industry. Processing difficulties with these polymers have been encountered in the mixing quality as well as in the thermal degradation due to viscous heating. Mixing often occurs as an element of the processing step, e.g., inside single and twin screw extruders used in the fabrication of final parts or sheets, and inside internal mixers such as the Banbury type mixer. Quantifying the mixing inside an extruder or an internal mixer and predicting the thermal degradation due to viscous heating is an extremely difficult task. A better understanding of the mixing process and control of viscous heating will lead to optimal parts and may eventually allow us to increase the relative amount of fillers within the material. This paper presents a boundary element simulation of the flow of filled and unfilled polymer blends inside extruders and internal mixers. First, the general equations that govern such flows are shown. The boundary integral equation and the fundamental solutions are then formulated followed by the numerical implementation and logistics of the simulation. After the theoretical background is presented, a numerical example is given. The paper then shows how the simulation was used to analyze the flow inside a single screw extruder and internal batch mixers. Such a simulation can be used to eliminate some of the tedious trial-and-error tasks that are typically performed in the early stages of material synthesis and can also be used by manufacturers of mixing equipment when optimizing the geometries of cavities and mixing heads to achieve optimum mixing with reduced viscous heating. This research will significantly increase our knowledge of the behavior of filled and unfilled polymer blends and expand our understanding of the complex phenomena that take place during their mixing process.

Influence of Pressure on Volume, Temperature and Crystallization of Thermoplastics during Polymer Processing

Abstract The main shrinkage and dimensional stability impact factors are processing temperature and pressure. For that reason, the pvT-behaviour is often used to estimate plastic part shrinkage. As there are still differences between predicted and actual part dimensions, a closer look was taken at these influencing factors. During processing, temperature changes occur due to heat conduction to the mold walls, and the pressure varies in the filling stage and holding time along the flow path. However, these pressure gradients are typically not taken into account in plastics processing. Hence, the influence of pressure was examined in detail and further investigations on pvT-behavior of polymers were performed. For that purpose, fundamental examinations of the behavior of amorphous thermoplastics during cooling and compression were made. These include analysis in the different phases and at different compression speeds as well as variation of the pressure-temperature-cycles and their succession. It was found that the specific volume is not defined by one value of p and T, but is dependent on the pathway of the process. That applies for both thermal expansion and compressibility. The adiabatic compression heating was also examined and a numeric solution was found to easily adapt the pvT-results on the base of compressibility measurements. These findings are discussed by means of the free volume theory. Additionally, a choice of these investigations was performed with semi-crystalline thermoplastics. It was found, that compression heating is even more important for these polymers because it can superpose crystallization heating at appropriate parameters. Furthermore, it can be seen that the density, morphology and lamellae thickness is influenced by pressure. In the outlook of this paper, the impact of these results on the injection molding process is discussed in detail.

The dual-reciprocity method for heat transfer in polymer processing

Abstract This paper presents a boundary-element simulation of the heat transfer during polymer processing. The governing equations, boundary integrals and fundamental solutions, with their numerical implementation for heat transfer and creeping flows, are presented. Viscous dissipation and internal-heat generation for curing polymers is included in the equations. In order to calculate the transient-heat transfer and internal-heat generation, the dual-reciprocity boundary-element method is applied. The heat-transfer-simulation results are in good agreement with analytical and other numerical results.

Flow and Heat Transfer Simulation of the Mixing of Polymer Blends Using the Boundary Element Method

Abstract In most polymer processes the quality of the final part stems back in great part to the mixing of the polymer blends and the viscous heating that occurs during the process. To date, these processes have been simulated using the finite difference or finite element methods which rely on cumbersome remeshing techniques, restricting the analysis to simple geometries. However, mathematical manipulation of the governing equations results in boundary integrals, allowing the solution of complicated moving boundary problems without domain meshing. The resulting boundary element model and simulation results of flow with heat transfer and the blending of two or more fluids of different viscosity effects during mixing of polymer blends is presented in this paper. The results give insight to the complex phenomena that take place during mixing processes.