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 is a platform for publication of results from current research in the field of Plastics Technology. The Journal`s aim is to broaden public access to scientifically qualified, German and English contributions from the fields of material, construction and processing of plastics. Downloads are free of charge.

In this research project an algorithm is developed and tested, which enables the automated optimization of dynamic mixing elements. For this purpose, selected geometry parameters are optimized by a genetic algorithm on a parameterized spiral shear mixer in such a way that flow conditions that favor mixing are established. In order to avoid manual evaluation of the flow results, quality criteria are introduced which are automatically calculated. A total of nine different quality criteria are used, which not only assess the potential of the flow field to implement distributive and dispersive mixing mechanisms, but also include processing aspects such as the fluid change effectiveness of the system. The results of the test show that an automated optimization can be carried out with the selected quality criteria and further improvement measures for the algorithm can be derived.

Automated optimization of dynamic mixing elements

In the present study, the pressure-dependent orientation relaxation was investigated by varying the mold temperature and cavity pressure in the context of a new injection-compression molding process with dynamic mold temperature control. For this purpose, isothermal pvT measurements were used to define a process window around the pressure-dependent glass transition region within which the injection-compression molding experiments were carried out. Initially, orientations were systematically introduced into the component via the filling phase, and their pressure- and temperature-dependent relaxation was observed during an isothermal and isobaric holding phase by means of photoelasticity images and path difference measurements. A clear pressure and temperature dependence of the orientation relaxation could be found, whereby with increasing mold temperature and decreasing cavity pressure, smaller path differences could be measured as a result of lower orientations.

Spritzprägen

Dynamische / variotherme Temperierung

Pressure-dependent orientation relaxation in injection compression molding with dynamic mold temperature control

Leakage in vacuum bags for fibre composite production is a frequently occurring defect, especially in the manufacture of components using the open mould process. The previous limit values for assessing the leak tightness of a vacuum set up are mostly historical and based on empirical values. In this paper, the effects of leaks that have been introduced in a defined manner into a vacuum bag are investigated. The physical processes that occur during curing within the leaking vacuum structure are analysed optically as well as by means of ultrasonic testing and thickness measurement and a better understanding of the effects on laminate quality is generated. It is found that there is a difference between leakages in the vacuum bag only and leakages that affect both the vacuum and release film.

Verbundwerkstoffe / Werkstoffverbunde

Influence of leaky vacuum bags on the quality of composite parts made from prepreg material

The process stability of injection moulding is negatively influenced by disturbing influences. An adaptive, close-phase process control strategy, which defines the reference curve for a subordinate cavity pressure control, should enable constant part quality. The influences of the cyclical changes are analysed for different control strategies by evaluating the process variable curves and the part weight during the start-up process. Up on that, it is evaluated whether the examined process variables are relevant for process control and therefore cyclical adjustments of the cavity pressure reference are necessary.

Werkzeuginnendrucksensoren

Spritzgießen

Prozesssimulation

Effects of cyclical process influences on the cavity pressure curve and part quality in the injection moulding process

The article presents a process simulation environment for the additive fused layer modeling process. It allows the simulation of an extruded melt strand as a function of process parameters which are specified by the machine in additive manufacturing and whose influence cannot currently be predicted and computer-aided determined. The challenge here is the implementation of pressure and temperature dependent properties, since these are not provided for in common CFD environments in terms of fluid mechanics. The validation of the process simulation developed in OpenFOAM is based on experimental thermographic analysis.

Verfahren für die additive Fertigung

Development of a Computer-Aided Approach for Modelling the Strand Deposition in Extrusion-Based Additive Manufacturing

For simulations of the mechanical behavior of short-fiber reinforced plastics, a variety of commercial software tools are available. The planning of modern product development processes makes a decision regarding the selection of a suitable tool necessary. Therefore, a suitable methodology is necessary for this selection. Using the methodology presented in this study, the mechanical behavior of a PP-GF30 is simulated using four commercial tools. It turns out that there are significant difference between the tools used, particularly with regard to material failure under multiaxial load. The presented methodology creates a reliable basis for the application-dependent selection of the most suitable software.

Faserverbundwerkstoffe

Evaluation methodology of commercially available homogenization software for the optimized simulation of the mechanical behavior of short fiber reinforced plastics

In addition to the previous state of the art in science and technology, in this publication spruce fibres from a thermo-mechanical pulping process were directly compounded with polypropylene and coupling agent using an injection moulding compounder. An increasing wood fibre content of up to 30 wt.% was investigated in direct single-stage processing. The characterisation of the material combinations is carried out by tensile tests, melt-flow-rate measurements, density determination, microscopy and impact tests. An increase in tensile properties, viscosity and density is observed with increasing wood fibre content. The impact strength is lower compared to the unchanged base polymer, but remains constant within the standard deviation with increasing wood fibre content.

Reinforcement of polypropylene with wood fibres by direct compounding

Conventional additive manufacturing processes such as Fused Layer Manufacturing (FLM) technology make it possible to produce any structures from a wide variety of materials without tools in a very short time. In addition, layer-by-layer processes hardly make use of the technical manufacturing degrees of freedom with regard to the filling strategy and existing anisotropy. Therefore, in the present publication an optimality strategy for 2.5D-FLM structures shall be presented which is based on a principal normal stress analysis. The goal is to generate a structure design that is suitable for the requirements and production process by means of simulation-based methods. In addition to the methodical imple-mentation, experiments on generic open hole plates with different filling strategies serve to validate and first confirm the method.

Application of a principle normal stress oriented filling strategy for Fused Layer Manufacturing structures

Currently, a nozzle consisting of a series of round tubes (round nozzle) is used as standard for hot gas welding. A newly developed nozzle (top nozzle), which surrounds the weld joint during heating, enables a reduction of the heating time of the resin by up to 60 % during the hot gas welding process. For PA66-GF35, for example, the heating time can be reduced up to 15 s compared to a round nozzle system. In addition, the melting of the resin is much more uniform and does not have a crater-like structure on the surface. The reduction of the heating time leads to lower process gas consumption and ultimately to a reduction of the total energy consumption and thus lower process costs.

Reduction of the heating time for hot gas welding

Air inclusions in fiber reinforced plastics (FRP) are among the most common imperfections that can affect both the resulting stiffness and strength. For the calculation of the stiffness of a perfect UD layer, semi-empirical mixing rules can be used which show good agreement with experimental data. Since the content of air inclusions is generally low compared to the constituents of an FRP, the Mori-Tanaka scheme can be exploited in a following step to homogenise the perfect phase and air inclusions. By introducing a geometry function into Mori-Tanaka scheme, influence of air inclusions’ appearance can be investigated.

Calculation of stiffness for FRP with air inclusions with respect to their geometry

Investigations and prediction of the melting processes in single-screw extrusion have been of scientific interest for many decades. In this study, the melting zone of a high-performance extruder with a continuous helically grooved plasticizing zone is experimentally investigated for the first time. For this purpose, an axially divisible extruder was developed to expose the barrier screw together with the plastic melt. An optical examination of the frozen melt validates assumptions of the existing melting model. In addition, the results of the experimental investigation are compared with those of the analytical model. These investigations confirm the essential mathematical model assumptions and are in good agreement with previous investigations on an axially grooved single-screw extruder.

Investigation of the melting zone of high-performance extruders using a helically grooved plasticizing zone

Contrary to the plastification in injection molding processes, thermoset molding compounds could be more heated in a twin screw extruder due to its stronger shearing effect which leads to the rise of the melting temperature above the initial curing temperature. Besides the screw speed, the screw configuration has an extensive impact on the shear-induced energy input and on the melt temperature profile along the extruder. The aim of this research is the development of an analytical method to investigate the influence of different processing conditions on the flow-curing behavior. Furthermore, possible implications to the resulting properties of the foamed semi-finished parts are discussed.

Analysis of the flow-curing behavior of chemically foamable phenolic molding compounds in the extrusion process

Effects of post-processing thermal treatments of vibration-welded polypropylene and its silica nanocomposite were systematically studied by using a heating cabinet and in-line infrared radiation. Experimental results demonstrate that thermal treatments, independent on the heating sources, exert obviously positive effect on improving the weld quality of polypropylene materials, i.e. stiffness and strength of the weld. Despite the short cycles used for infrared heating, the results are better and the effort is less.

Response of mechanical and thermal properties of a vibration-welded polypropylene/SiO<sub>2</sub> nanocomposite on different thermal treatments

This research work presents a recycling methodology for carbon fibre composites processed with a novel room temperature infusible and curable thermoplastic matrix ELIUM 150 from Arkema. Herein we demonstrate that by dissolving the composites in acetone at room temperature it is possible to separate the matrix from the fibres and to reuse both the matrix and the fibres in form of scrim (preform). A comparison of the thermomechanical, thermal and mechanical properties of the composites shows only marginal performance changes in bulk properties due to the proposed recycling strategy.

Closed-loop room temperature recycling of Elium CFRPs and its influence on the 2nd generation composite properties

Changes in the specific volume during the processing of thermosetting resin systems result in reduced dimensional accuracy of the components due to residual stresses and, in the case of fibre reinforced plastics (FRP), in surface waviness. A specially developed dilatometer is used to characterise the chemical shrinkage close to the process. The tests show a significant influence of the mixing ratio of resin and curing agent, which leads to a reduced chemical shrinkage. An increased moisture content of the resin component tends to lead to a lower chemical shrinkage.

Investigation of the Shrinkage Behaviour of a Fast Curing Epoxy Resin as a Function of Moisture, Gas Content and the Mixing ratio

A flexible air guiding system in blown film extrusion enables a mass throughput increase of more than 60 %. The development encloses the bubble expansion zone and creates a flow gap between the film bubble and a flexible membrane. In this gap, the flow velocity of the cooling air can be increased, resulting in higher heat extraction. As a side effect, the increase in flow velocity creates a local vacuum in the gap, causing the film bubble to open earlier than in the conventional process. Because of this surface enlargement the heat extraction can be improved again. During the extrusion process, the flow gap can be adjusted to the current bubble geometry independent of the process parameters.

Increased cooling capacity when using a flexible air guiding system due to the Venturi effect in blown film extrusion

In this investigation, a possibility for shortening the cycle time in the manufacture of thick-walled optical components by compression induced solidification (CIS) was studied in a numerical manner and experimentally tested for validity. The test specimens produced with a reduced cycle time were compared with the manufacturing processes commonly used for such components, i.e. injection molding, injection-compression molding and conventional CIS, with regard to dimensional accuracy, internal stresses and microstructural impression. It could be shown that by delaying the heating of the mold, the adapting time and thus the cycle time of the process can be reduced by up to 55 %, while maintaining the same high quality of the components.

Application optimized compression induced solidification

Automated lay-up of carbon fiber reinforced polymers is an established method for producing large parts in the aerospace industry. Currently, the quality control for this automated manufacturing process is performed by humans. In the last years, research has focused on increasing productivity of automated lay-up, thus the automation of the quality control has been taken into consideration. The objectives of the paper are to review the requirements for quality control based on an overview of the possible defects in automated lay-up and to evaluate their fulfilment by both visual quality control by humans and automated quality control systems.

A literature review of quality control for automated lay-up processes of CFRP

The Journal of Plastics Technology is a peer reviewed internet periodical published under the auspices of the Scientific Alliance of Polymer Technology (WAK)