C. Hopmann

Surface pre-treatment for barrier coatings on polyethylene terephthalate

Polymers have favourable properties such as light weight, flexibility and transparency. Consequently, this makes them suitable for food packaging, organic light-emitting diodes and flexible solar cells. Nonetheless, raw plastics do not possess sufficient barrier functionality against oxygen and water vapour, which is of paramount importance for most applications. A widespread solution is to deposit thin silicon oxide layers using plasma processes. However, silicon oxide layers do not always fulfil the requirements concerning adhesion and barrier performance when deposited on films. Thus, plasma pre-treatment is often necessary. To analyse the influence of a plasma-based pre-treatment on barrier performance, different plasma pre-treatments on three reactor setups were applied to a very smooth polyethylene terephthalate film before depositing a silicon oxide barrier layer. In this paper, the influence of oxygen and argon plasma pre-treatments towards the barrier performance is discussed examining the chemical and topological change of the film.It was observed that a short one-to-ten-second plasma treatment can reduce the oxygen transmission rate by a factor of five. The surface chemistry and the surface topography change significantly for these short treatment times, leading to an increased surface energy. The surface roughness rises slowly due to the development of small spots in the nanometre range. For very long treatment times, surface roughness of the order of the barrier layers thickness results in a complete loss of barrier properties. During plasma pre-treatment, the trade-off between surface activation and roughening of the surface has to be carefully considered.

Influence of layer type and order on barrier properties of multilayer PECVD barrier coatings

Due to their macromolecular structure, plastics are limited in their scope of application whenever high barrier functionality against oxygen and water vapour permeation is required. One solution is the deposition of thin silicon oxide coatings in plasma-enhanced chemical vapour deposition (PECVD) processes. A way to improve performance of barrier coatings is the use of multilayer structures built from dyad layers, which combine an inorganic barrier layer and an organic intermediate layer. In order to investigate the influence of type and number of dyads on the barrier performance of coated 23 µm PET films, different dyad setups are chosen. The setups include SiOCH interlayers and SiOx-barrier layers deposited using the precursor hexamethyldisiloxane (HMDSO). A single reactor setup driven in pulsed microwave plasma (MW) mode as well as capacitively coupled plasma (CCP) mode is chosen. In this paper the effects of a variation in intermediate layer recipe and stacking order using dyad setups on the oxygen barrier properties of multilayer coatings are discussed with regard to the chemical structure, morphology and activation energy of the permeation process.Changes in surface nano-morphology of intermediate layers have a strong impact on the barrier properties of subsequent glass-like coatings. Even a complete failure of the barrier is observed. Therefore, when depositing multilayer barrier coatings, stacking order has to be considered.

Model Predictive Control of Cavity Pressure in an Injection Moulding Process

Abstract Cavity pressure control is a means of improving repeatability and product quality in injection moulding processes. As the system behaviour is greatly dependent on the mould - which is interchangeable and typically designed and manufactured independently of the machines control system - challenges arise in designing a controller that yields high performance and robustness to be suitable for universal use. A cavity pressure controller intended to be used for a wide variety of moulds will likely need some form of reparametrisation. In order to gain user acceptance, the process of manual or automatic parametrisation of the controller to a new mould needs to be simple enough to be performed and understood by staff that are not necessarily control experts. Addressing this issue, the authors suggest an approach using a Model Predictive Controller that is based on a physically motivated grey-box model. The model is simple enough to be intuitively checked for plausibility but sophisticated enough to reproduce the dominant behaviour of the system. For automated parametrisation, a strategy based on two experiments is suggested. The experiments are tailored to be suitable for incorporation into the regular production process. The concept is presented and first experimental results are shown.

Design Criteria in Numerical Design of Profile Extrusion Dies

Abstract. The rather unintuitive and non-linear behavior of plastics melts is a well-known obstacle in the design and manufacturing cycle of profile extrusion dies. This is reflected, for example, in the so-called running-in experiments, in which the already manufactured die is modified up to 15 times until the final product, shaped by the die, matches the quality requirements. Besides a homogeneous outflow velocity and thus homogeneous material distribution, an appropriate die swell is a second design objective which complicates the reworking of the manufactured die. We are conducting work to shorten the manual running-in process by the means of numerical shape optimization, making this process significantly less costly and more automatic. From a numerical point of view, the extrusion process is not as challenging as high-speed flows, since it can be described by steady Stokes equations without major loss of accuracy. The drawback, however, is the need for ac- curate modeling of the plastics behavior, which generally calls for shear-thinning or even viscoelastic models, as well as for 3D computations, leading to large computational grids. The intention of this paper is to investigate the application of specific geometry features in extrusion dies and their influence on objective functions in an optimization framework. However, representative objective functions concerning die swell and the incorporation of known geometry features, as used by experienced die designers, into the optimization framework still remain a challenge. Hence, the topics discussed are the influence of the mentioned geometry features on existing objective functions as well as an outlook on an algorithmic implementation into the optimization process with regard to representative objective functions.

The effect of UV radiation from oxygen and argon plasma on the adhesion of organosilicon coatings on polypropylene

The influence of ultraviolet (UV) radiation from oxygen and argon pretreatment plasmas on a plastic substrate has not been fully understood yet. In particular, its influence on the adhesion properties has not been sufficiently researched so far. This paper addresses this issue by comparing the bond strength of a plasmapolymerized silicon organic coating (SiO x C y H z ) on polypropylene (PP) after oxygen and argon plasma pretreatment and pretreatment by UV radiation emitted by the same plasmas. The UV radiation is isolated from the other species from the plasma by means of a magnesium fluoride (MgF2) optical filter. It could be shown that UV radiation originating from an oxygen plasma has a significant impact on both substrate surface chemistry and coating adhesion. The same maximum bond strength enhancement can be reached by pretreating the polypropylene surface either with pulsed oxygen plasma, or with only the UV radiation from this oxygen plasma. Also, similar surface chemistry and topography modifications are induced. For argon plasma no significant influence of its UV radiation on the substrate could be observed in this study.

Analysis of the injection moulding process of soft magnetic plastics compounds

Electric/electronic devices become increasingly more complex and are used in highly integrated assembly groups. Plastics bonded magnets can fulfil some of upcoming requirements with respect to design freedom and advantageous processability. However, the magnetic properties of plastics-bonded magnets are significantly reduced since the maximum magnetic filler content incorporated into plastics is limited. Due to a comparably high relative permeability soft magnetic plastics compounds allow for a further increase in magnetic flux density of conventionally used plastics-bonded hard magnets. In cooperation with industrial partners IKV further develops soft magnetic plastics compounds within a joint project called “Mikromag”. The paper deals with the injection moulding of soft magnetic plastics compounds. Achievable soft magnetic properties of different fillers are shown. The influence of process parameters, inductive heating and magnetic coils within an injection mould on the processing and magnetic properties is discussed.

FE‐Analysis of Stretch‐Blow Moulded Bottles Using an Integrative Process Simulation

The two‐stage stretch‐blow moulding process has been established for the large scale production of high quality PET containers with excellent mechanical and optical properties. The total production costs of a bottle are significantly caused by the material costs. Due to this dominant share of the bottle material, the PET industry is interested in reducing the total production costs by an optimised material efficiency. However, a reduced material inventory means decreasing wall thicknesses and therewith a reduction of the bottle properties (e.g. mechanical properties, barrier properties). Therefore, there is often a trade‐off between a minimal bottle weight and adequate properties of the bottle.In order to achieve the objectives Computer Aided Engineering (CAE) techniques can assist the designer of new stretch‐blow moulded containers. Hence, tools such as the process simulation and the structural analysis have become important in the blow moulding sector. The Institute of Plastics Processing (IKV) at RWTH ...

Modelling IR-Heating in Stretch-Blow Moulding and Thermoforming

The two-stage stretch-blow moulding process has been established for the large scale production of high quality PET containers with excellent mechanical and optical properties. Thermoforming is the process of choice for manufacturing thin-gauge or large-area parts for packaging or technical applications. Both processes allow lightweight thermoplastic parts to be produced rapidly and economically.In both processes thermoplastic semi-finished products are formed by pressurised air under the influence of heat. To enable forming of the thermoplastic materials, the semi-finished products need to be transferred into a thermoelastic state. IR-heating is widely used due to short heating times.From a cost perspective, about 7 % of the total production costs of a stretch-blow moulded bottle are spent for energy in order to heat and form the preform to the later bottle. Depending on machine, semi-finished product type and cycle time, energy costs in thermoforming account for around 1–5 % of the total production costs. Modern roll-fed automatic thermoforming machines use about 22 % of the energy consumption for the heating step and around 70 % for the production of pressurised air. Due to this significant share and due to increasing energy costs during recent years, the packaging industry is interested in increasing the energy efficiency of these processes.The most important quality criterion for both processes is a uniform wall thickness distribution. The production of high-quality parts requires optimised temperature profiles of the semi-finished product depending on the particular product geometry. Simulation is an approved tool for the prediction of the influence of the heater setting on the temperature profile.Over the last decade IKV has developed an integrative three-dimensional process simulation which models the complete path of a preform through a stretch-blow moulding machine. An essential first step is the heating simulation where the temperature profile of the preform is computed. Based on this data the temperature-dependent material behaviour of PET can be considered during the inflation simulation.This work shows the influence of a thoughtful temperature profile on the wall thickness distribution in stretch-blow moulding. The focus is on modelling the reheat phase of the stretch-blow moulding process in FEA. Beyond that, a purposeful heating offers the possibility to cut down energy waste.Copyright

Transfer of Wire Arc Sprayed Metal Coatings onto Plastic Parts

By means of In-Mold-Metal-Spraying (IMMS), metal coatings deposited by means of arc spraying process (ASP) can be transferred onto plastic parts during injection molding, thus realizing an efficient production of metallized plastic parts. Parts produced by means of IMMS can be used in electrical applications. In the current study, the electrical resistivity of coatings applied with different feedstock materials was determined. As a starting point, pressurized air is used as atomizing gas for ASP. In contrast to Zn coatings, Cu coatings applied with pressurized air exhibit a significantly higher electrical resistivity in comparison with massive material. One possible reason is the more pronounced oxidation of Cu particles during ASP. Therefore, N2 and a mixture of N2 and H2 were used as atomizing gas. As a result, the electrical resistivity of coatings applied by means of IMMS could be significantly reduced. Furthermore, standoff distance, current and pressure of the atomizing gas were varied to investigate the influence of these process parameters on the electrical resistivity of Zn coatings using a full factorial experiment design with center point. It can be observed that the electrical resistivity of the Zn coatings increases with decreasing current and increasing standoff distance and pressure.

Tailoring the heat transfer on the injection moulding cavity by plasma sprayed ceramic coatings

Inhomogeneous material shrinkage in injection moulding can cause warpage in thermoplastic components. To minimise the deformations of the injection moulding parts, the heat transfer during the cooling phase can be adjusted according to the local cooling demand on the surface of the mould cavity by means of plasma sprayed coatings with locally variable thermal resistance over the surface of the mould. Thermal resistance is a function of thermal conductivity and thickness of the coatings, where thermal conductivity of thermal barrier coatings can be adjusted by altering the chemical composition and the microstructure, which is depending on the thickness. This work evaluates the application of plasma sprayed coatings with variable thickness as thermal barrier coatings in the mould cavity. The thermal resistance of the coating and thereby the heat transfer from the melt into the mould will be influenced locally by varying the coating thickness over the cavity area according to the local cooling demand. Using the laser flash method, the thermal conduction of coatings with different thicknesses will be determined. On the basis of the experimentally determined thermal conduction, the effect of the coatings on the temperature field of the mould cavity will be numerically calculated and the required thickness distribution of the coating for an optimal temperature gradient will be determined.