AbstractThe major disadvantage of rotational molding is the cycle time, which is very long compared to other plastic processing methods. A major percentage of the cycle time besides heating and cooling results from the time necessary to remove gas inclusions from the polymer melt, which are trapped while sintering the polymer powder. In this work the formation of gas inclusions is investigated by conducting a cycle time variation on a uniaxial rotational molding machine. The influence of low pressure during melting on the formation of inclusions is investigated by examining sintering experiments with a pressure variation during the melting of the polymer. Sintering experiments are conducted with different melt residence times to investigate the mechanisms of gas inclusion removal. By comparing the time to reach a pore-free polymeric melt, the cycle time reduction potential under low-pressure application while melting the polymeric powder is estimated.
Rotational molding is a plastic processing method that allows for the production of seamless, hollow parts. Defined shaping of the polymeric material only takes place on the outer surface where contact to the tooling is given. The inner surface forms by surface tension effects. By sequential adding of materials, complex multilayer build-up is possible. Besides pure, single materials, filled, or multiphase systems can be processed as well. In this work, possibilities to generate bonding between supposedly incompatible materials by adding a mix-material interlayer are investigated. Interlock mechanisms on a microscale dimension occur and result in mechanical bonding between the used materials, polyethylene (PE) and thermoplastic polyurethane (TPE-U). The bonding strength between the materials was investigated to reveal the correlations between processing parameters, resulting layer build-up, and bonding strength. The failure behavior was analyzed and inferences to the influence of the varied parameters were drawn.
AbstractIn rotational molding, shaping is achieved by elevating the temperature of polymer powder particles above their melting temperature, causing them to adhere to the mold wall. Multi-layer parts can be processed by sequential adding of different polymeric materials to the mold, while the thickness of each layer is defined by the amount of each material. In this work, the influence of adding time of a second component on the shape and development of the interface region between different materials for multi-layer rotational molding is investigated. Therefore, rotational molding experiments were conducted using an uniaxial rotating molding machine and a cylindrical mold. Varying the time of second material addition yielded different specific interface regions. The cross sections of the resulting multi-layer parts were analyzed using transmitted light microscopy and characteristic values were derived to describe the interfaces. Single-layer parts were produced to verify the built-up of the polymer layer and the development of the inner melt surface in rotational molding.
Reaction injection molding is a plastic processing method to produce net shape parts using reactive systems. By integrating semi-finished products as inserts, complex multi-layer parts can be generated in highly integrative and energy efficient processes. The material by far mostly used is polyurethane, a polymer which results from the reaction of isocyanate and polyol. By adding blowing agents, like for example water, to the polyol component, foamed parts can be realized. In contrast to thermoplastic injection molding a chemical reaction takes part during molding within the cavity. Therefore the processing parameters have a significant effect on this chemical reaction and on the properties of the finished part.In this work the influences of different processing parameters like for example mold temperature and injection volume on the resulting foam structure are investigated for reaction injection foam molding. Therefore multi-layer parts based on polyurethane materials (thermoplastic and reactive) were molded varying relevant processing parameters. The foaming took place within an open cavity. The resulting foam structures were characterized using scanning electron microscopy (SEM). Additional the multi-layer parts were characterized mechanically to reveal the resulting effects on the mechanical properties of parts containing a foamed reactive polyurethane component.
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