The melting performance of single screw extruders

A number of recent screw designs is analyzed for melting performance, using a simple analytical approach based on Tadmor's original work. The melting length for a screw with constant depth channel is used as reference. An ideal compression screw will have a melting length of one-half the melting length of the reference screw. The Maillefer melt separation principle is discussed. The Maillefer screw melts in 2/3 of the length of the reference screw. Screws by Barr, by Dray and Lawrence and by Kim are shown to approach the ideal compression screw. A new design screw, using ideal compression and multiple channels and having a very large screw pitch, is shown to be a considerably more efficient melting device than any of the other, screws discussed

Meerlagen spuitgieten, een voorstudie : de stroming tussen twee plunjers

Als onderdeel van de ontwikkeling van een meerlaagsspuitgiettechnologie, die ook in complexe produkten centrische of excentrische dunne lagen gecontroleerd moet kunnen realiseren, is de stroming van een polymeersmelt in een cilinder tussen twee plunjers experimenteel en numeriek onderzocht. Schijfjes van een verschillende kleur van, voorlopig, hetzelfde materiaal werden tussen de plunjers geplaatst, opgesmolten, gedeformeerd door de plunjers over verschillende afstanden en later ook bij verschillende temperaturen en snelheden te verplaatsen, vervolgens weer afgekoeld en doormidden gezaagd. De aldus gevisualiseerde deformatie geschiedenissen werden ook numeriek berekend, gebruik makend van een standaard eindige elementen programma. Uit een vergelijking van de berekende en gemeten deformatie blijkt dat kleine verschillen in reologisch gedrag, grote invloed hebben op het geintegreerde snelheidsveld en dat aan de discontinuiteit tussen plunjer en cilinderwand relatief veel aandacht moet worden besteed

Static mixing for a uniform layer distribution and easy fabrication

No abstrac

On the modeling of continuous mixers. Part II: The cokneader

The Buss cokneader is a single-screw extruder with interrupted flights. Pins from the barrel are inserted into the screw channel. The screw is both rotating and oscillating. Due to this action, screw flights are continuously wiped by the pins. During one passage of the pin, the material is not only subjected to high shear stress, but it is reoriented as well, thus promoting the distributive mixing process by the local weaving action of the pins and screw flights. Attempts to model the cokneader tend to focus on a single pin passing through the hole in a screw flight (1, 2). However, a more comprehensive model can start with the same equations that apply to the corotating twin-screw extruder (3). Because the effect of leakage flows on the local pressure gradient has to be considered along with the effect of the local dragging action of the pins (neglecting the oscillatory action), experiments with model liquids have been performed to evaluate the comprehensive model. Additional experiments with a Plexiglas-wailed cokneader support the calculations concerning filled lengths in various screw geometries. These results, and those of model calculations, which are extended to the nonisothermal, non-Newtonian situation, are presented

Droplet breakup mechanisms : stepwise equilibrium versus transient dispersion

In dispersive mixing of immiscible liquids the minimum attainable dropsize is often deduced from the critical value of the Capillary number (the ratio of the shear stress to the interfacial stress) necessary for drop breakup under quasiequilibrium conditions. The critical Capillary number shows a minimum if the viscosity ratio between dispersed and continuous phase is about one. Hence, it is commonly accepted that the finest morphology is obtained if both viscosities match. In practical mixing devices, however, small drops are formed by a transient mechanism of thread breakup during extension rather than by stepwise breakup under equilibrium conditions. For Newtonian liquids, a comparison is made between the dropsizes resulting from a stepwise equilibrium and a transient breakup mechanism. Generally, the transient mechanism yields smaller drops and, more interestingly, a higher viscosity ratio between the dispersed and continuous phases results in a finer morphology, as already indicated by Tjahjadi and Ottino (1991). In the present paper the comparison is elaborated over a broad range of the relevant parameters while a compact illustrative presentation of the results is given to stress the possible consequences for practical blend morphologies

Optimization of a compact air pressure actuated membrane for inducing flow inside a bioreactor

No abstract

Influence of electron beam irradiation on the microrheology of incompatible polymer blends : thread break-up and coalescence

The microrheology of polymer blends as influenced by crosslinks induced in the dispersed phase via electron beam irradiation, is systematically investigated for the model system polystyrene/low density polyethylene (PS/LDPE). Both break-up of threads and coalescence of particles are delayed to a large extent, but are not inhibited completely and occur faster than would be expected for a nonirradiated material with a comparable viscosity. Small amplitude, dynamic rheological measurements indicated that in the irradiated materials a yield stress could exist. In contrast, direct microrheological measurements showed that this yield stress, which would prevent both break-up and coalescence, could not be realized by EB irradiation. Apparently, the direct study of the microrheology of a blend system is important for the prediction of the development of its morphology and it is not possible to rely only on rheological data obtained via other methods