Christian Hopmann

Einführung in die Kunststoffverarbeitung

ISBN 978-3-446-44627-4 Das nun schon in der 7. Auflage vorliegende Lehrbuch vermittelt in verstandlicher, auch zum Eigen studium geeigneter Form Studierenden sowie Praktikern aus Industrie und Handwerk einen um fassenden Uberblick uber die wesentlichen Kunststoffverarbeitungsprozesse, ihre Funktionsweise und verfahrenstechnischen Hintergrunde. Zahlreiche Beispiele und Bilder sollen ein grundlegendes Verstandnis erzeugen und eine Faszination fur die Moglichkeiten der Kunststofftechnik wecken.

Development of an integrative simulation method to predict the microstructural influence on the mechanical behaviour of semi-crystalline thermoplastic parts

Abstract The mechanical properties of injection moulded plastic parts depend on the morphology, the degree of crystallinity and the molecular orientation of the formed microstructure. In order to take the variation of the microstructure into account in a structural analysis, a novel multi-scale, integrated simulation approach is presented here. At first, a coupled mould filling and heat transfer analysis is achieved at the macroscale and its temperature field is transferred to the micromodel. Based on the concept of cellular automata, a 3-D microstructure evolution model is developed. It specifies the nucleation of the spherulite germs and describes their expansion rate. To evaluate the effective mechanical properties of the simulated microstructures, the homogenisation method is applied directly to the spherulites, assembled in few classes according to their crystallinity degree. These local properties are then introduced into a new multilinear material model for structural analysis of thermoplastics. Finally, the influence of the microstructure on macroscopic behaviour is outlined for a polypropylene tensile bar, extracted from an injection moulded plate.

Injection Molding of Superhydrophobic Liquid Silicone Rubber Surfaces

Superhydrophobic functional surfaces have numerous applications. Their self-cleaning ability and the associated savings in energy, water and cleaning agents enhance the sustainability of products and often make active cleaning of these surfaces unnecessary. Silicone surfaces, which aim to imitate the surface of the lotus plant, were prepared using a microstructured injection mold. The conical micro structures were varied in diameter and height ranging from 5 to 20 μ

Preliminary results in anterior cervical discectomy and fusion with an experimental bioabsorbable cage - clinical and radiological findings in an ovine animal model

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Modeling of the heating process during the laser transmission welding of thermoplastics and calculation of the resulting stress distribution

m as were the process parameters within the framework of a statistical experimental plan. The molded structures were evaluated by scanning electron microscopy and confocal laser microscopy, and the resulting contact angle was measured. In contrast to the structural dimensions, the process parameters had only a minor impact on the contact angle. Smaller base diameters of the individual cones and the resulting smaller distances between the cone tips produced larger contact angles. Larger aspect ratios and increasing heights at equal intervals of the individual structures led to smaller standard deviations from the mean measured contact angles. Subsequent mechanical load tests showed the resistance of the functionalization. Our results reveal that it is possible to produce robust superhydrophobic surfaces in a single-step liquid silicone injection molding process.

A self-optimising injection moulding process with model-based control system parameterisation

BackgroundBioabsorbable implants are not widely used in spine surgery. This study investigated the clinical and radiological findings after anterior cervical discectomy and fusion (ACDF) in an ovine animal model with an experimental bioabsorbable cage consisting of magnesium and polymer (poly-ϵ-caprolactone, PCL) in comparison to a tricortical bone graft as the gold standard procedure.Materials and Methods24 full-grown sheep had ACDF of C3/4 and C5/6 with an experimental bioabsorbable implant (magnesium and PCL) in one level and an autologous tricortical bone graft in the second level. The sheep were divided into 4 groups (6 sheep each). After 3, 6, 12, or 24 weeks postoperatively, the cervical spines were harvested and conventional x-rays of each operated segment were conducted. The progress of interbody fusion was classified according to a three-point scoring system.ResultsThere were no operation related complications except for one intraoperative fracture of the anterior superior iliac spine and two cases of screw loosening and sinking, respectively. In particular, no vascular, neurologic, wound healing or infectious problems were observed. According to the time of follow-up, both interbody fusion devices showed similar behaviour with increasing intervertebral osseointegration and complete arthrodesis in 10 of 12 (83.3%) motion segments after 24 weeks.ConclusionsThe bioabsorbable magnesium-PCL cage used in this experimental animal study showed clinically no signs of incompatibility such as infectious or wound healing problems. The radiographic results regarding the osseointegration are comparable between the cage and the bone graft group.

Mesoscale simulation of the solidification process in injection moulded parts

At present, various welding techniques are used in many different process chains and steps. Moreover, when joining metallic materials, welding processes are used to create integrated material bonds of organic as well as inorganic materials. One of the methods used to join thermoplastics is laser transmission welding. In addition to the low heat introduced into the welded parts, high welding speed is one of the reasons why this process is used in industry. Highly accurate simulation of the heating process would improve understanding of the process, facilitate and reduce the time required for process installation in the long run, and provide a significant contribution for computer-aided component design. The quality of the weld line in laser transmission welded plastic parts is affected by material properties, machine parameters, and their interactions with each other. For these reasons, a thermal simulation model was developed as part of a research project due to support the operator of the laser transmission welding process in setting up the process faster and more reliably. First, the presented model considers the various interferences of the welding process by the material, the thermal, and optical properties which are often not known exactly enough. Second, interaction between the laser beam and the material was determined and implemented. The developed calculation model allows for highly reliable determination of the size of the heat affection zone. The exact process window can be identified and predicted. Furthermore, the heating and cooling process can be reproduced using the spatial temperature distribution, which allows cycle time to be optimized on the basis of these results. In addition to the impact on weld quality, residual stresses can be found in welded thermoplastic parts as a result of local thermal expansion and shrinkage. Especially, tensile stresses are undesired effects of the welding process because they can have an unfavorable impact on the local mechanical properties of the welded components. Mainly, in laser transmission welded parts, these tensile residual stresses occur in the weld seam. These tensile stresses can overlap with permitted operating stresses, and this may result in early failure of the component. This can take place, in particular, when the structural part is exposed to oscillating and/or highly corrosive stresses. These are the reasons why it is important to know the residual stresses introduced into the component, in order to minimize the distribution of these stresses by means of suitable process management. To calculate the mechanical behavior in the structural simulation of a welded part, precisely mapping the temperature field is of fundamental importance. Residual stresses and warping are directly based on the temperature field. Based on thermal calculations, the paper presents thermo-mechanical simulations for the determination of residual stress distribution in the weld seam, depending on the welding parameters.

Mechanical testing of an absorbable hybrid fusion cage for the cervical spine.

The consideration of the pvT-behaviour (pressure, specific volume and temperature) of the plastic material in combination with closed-loop cavity pressure control allows for compensation of variable boundary conditions in the injection moulding process. By suitably implementing cavity pressure control, repeatability and product quality in injection moulding processes can be improved. However, there are still obstacles for industrial application. As the process behaviour is greatly dependent on the mould – which is interchangeable and typically designed and manufactured independently of the machine’s control system – challenges arise in designing a cavity pressure controller that yields high performance and at the same time is robust enough to be suitable for universal use. The use of a model predictive controller (MPC) for cavity pressure control is being researched and found to be helpful to overcome these issues. Unlike controllers such as proportional-integral-derivative controllers, the control output is not determined using a well-tuned, but mathematically relatively simple algorithm. Instead, it performs an online optimisation based on a process model in order to obtain the control outputs. In order to operate as intended, the model used by the MPC has to be adjusted with every significant change of the system, in particular the machine, material and mould. Therefore, a process model as well as a suitable strategy for in-process identification of the necessary parameters is developed and presented. For automated parameterisation, a strategy based on two experiments is suggested and first experimental results are presented.

Meta-modeling for manufacturing processes

Abstract Due to their wide range of applications and their complex material properties, it is desirable to be able to predict the behaviour of injection moulded parts with the help of simulation tools. For semi-crystalline materials, this can only take place with considerable accuracy if the inhomogeneous material properties are taken into account. Because of this, it is necessary to calculate the microstructure of the solidified melt and to incorporate these findings in the simulation. We present an integrative, multiscale simulation approach in which the manufacturing process is calculated on a macroscale and the solidification process on a mesoscale. A multiphase filling and cooling simulation is done to calculate temperature and velocity fields, which are used as boundary conditions for the calculation of the spherulite distribution in the part. We present the used nucleation and growth model and shortly describe the parallelisation approach of the mesoscale simulation.

Towards an accurate simulation of the crystallisation process in injection moulded plastic components by hybrid parallelisation

Abstract Conventional fusion devices (“cages”) are often used to join two vertebrae of the human spine and generally remain in the body for a lifetime and can theoretically lead to any complications. Therefore, an absorbable hybrid fusion cage consisting of a magnesium skeleton infiltrated with an absorbable polymer [poly-ε-caprolactone (PCL)] has been developed. The primary objective of the cage is to ensure an adequate stiffness of the disc space directly after the operation and to encourage the ingrowth of the new bone tissue to secure long life stability. Once a sufficiently rigid bone connection is formed, the implant should be absorbed. The purpose of this first study on the new absorbable fusion cage was to investigate the mechanical properties in vitro. Tensile tests were performed with tensile specimens type 1BA according to DIN EN ISO 527 made of PCL foamed using controlled expansion of saturated polymers (CESP). Furthermore, cyclic compression tests and compression tests with steady movement were performed with different designs of the new cage. Compression tests were also performed with vertebral endplates of ovine cadaveric spines. Foaming of PCL resulted in a modulus of elasticity of 135 MPa, which is approximately one third of unfoamed PCL. The results indicate that the initial compression strength of the implants should be adequate for the implantation in the cervical spine.