H. Potente

Design of Tightly Intermeshing Co-Rotating Twin Screw Extruders

Abstract Composite models for the calculation of the filling level profiles, the pressure profiles, the melting profiles, the residence time distributions, the temperature profiles, the shear stress profiles, and the power consumption in modular tightly intermeshing co-rotating twin screw extruders (ZSK) are developed. A complex systematic design procedure was compiled, which is explained in part in this paper. The simulation of the intermeshing co-rotating machine involves both screw and kneading disc elements, including left- and right-handed sections. Kneading blocks were approximated by a screw of “equivalent pitch” with making allowance for the leakage flow across the flights from one channel to the adjacent channel. The mathematical treatment of co-rotating twin screw extruders has been based up according to the theory of single screw extruders. There was seen to be a good correlation between calculated and experimental results.

The Vibration Welding of Polyamide 66

Working on the basis of a parameter study of standard PA 66 material, comparative investigations were conducted into material modifications using heat stabiliz ers, glass fibers and minerals. The evaluation criterion applied for the weld quality was the short-time welding factor, this being the quotient of the weld strength and the strength of the base material With standard material, a weld strength almost equivalent to the strength of the base material was attained. With heat-stabilized and mineral-filled material, the welding factor fell to fs = 0.9. Glass-fiber-reinforced PA achieved a maximum value of fs = 0.58, which, in terms of absolute value, corresponds approximately to the base material strength of standard polyamide. Microtome section investigations show how the different weld results come about.

Existing Scale-up Rules for Single-screw Plasticating Extruders

Abstract This paper gives a review of existing scale-up rules for single screw plasticating extruders. It is shown that there were and still are rather varied trends. The main three are: – scale-up rules under the boundary conditions of an invariant shear rate, – scale-up rules including the power law, – scale-up rules including the zones of operation. In Europe, at least, the scale-up rules, which take the power law-flow characteristics into account, seem to have been established in practical terms. For economical reasons conventional extruders are designed in such a way, that the output increases with the square of the diameter. This however, results in the problem that the melting rate does not follow the same scale-up rules as the pumping rate and the solid conveying rate. Thus the discussion continues, to calculate the operating section lengths according to varying scale-up rules. Finally the scale-up problem is discussed once more with respect to the dominant dimensionless numbers, and despite several similarity theoretical difficulties a standardized solution will be attempted to be found.

The Gas-assisted Injection Molding Process

Abstract This paper presents an application example for the modelling of a mold used for the gas-assisted injection molding process. The simulation software used allows the computation of the distribution of the thickness-fraction of skin material all over the part depending on the process conditions. The illustrations are shown in connection with an industrial mold and the simulation results are compared with experimental data showing a reasonable agreement. The influence of variations of different operation point parameters is shown. Some effects occurring due to the gas flow into the cavity are described.

A Throughput Model for Grooved Bush Extruders

Abstract On modern extrusion plants, polyolefins are generally processed using barrels that have a grooved feed section. Accurate calculation of the melt throughput is of decisive importance for the process engineering layout of these extruders. Up to the point at which the limit speed that gives rise to the conversion from solid friction to melt film friction is attained, the conveying capacity is determined by the feed zone that is filled with granules. This paper sets out a method of calculating the throughput for speeds below this limit, paying consideration to the melt flow components in both the screw channel and the grooves. The calculation method was verified experimentally with the aid of throughput measurements for different polyolefins. The results generally fall within a tolerance range of +/−10 %.

Comparative Investigations into the Welding of Glass-Fiber-Reinforced PES

The selection of a welding process is frequently based on the requirements of the plastic being welded. The melting temperature of the material and its structure play a decisive role here. This article presents comparative investigations into the heated tool, vibration and radiant heater welding of polyethersulphone with a glass fiber content of 30% by weight. The optimum operating point was established for each of the three pro cesses by means of parameter studies. When the welding results were assessed in the short-time tensile test, radiant heater welding was seen to be the best, with affected joining zones that appear relatively homogeneous (i.e., maintain the original fiber geometry) when observed through the microscope. Shortest cycle times are achieved by vibration welding (< 10 sec.). Heated tool and radiant heater welding give cycle times from 30 sec. up to 60 sec , depending on the process parameters.

Potential Applications for Computer-aided Extruder Design

Abstract The simulation method is able to relieve the engineer of experimental work during the planning of extrusion plant and thus clearly reduces the development time. Sufficiently precise determination of the calculable parameters is possible with the process models developed so far. The empirical knowledge of the engineer is, of course, necessary as well in order to estimate the impact of influencing parameters that are not taken into account in the calculation models. The presented simulation program REX can perform these complex calculations in a comprehensive manner, with its simple and comfortable user interface. The program, however, can be employed as a valuable tool only if it is in the hands of an expert operator who can, and indeed must, be able to interpret the results as well.

An Analytical Model of Partial and Thorough Melting in Single-screw Extruders

Abstract Based on the classic Tadmor model, a number of mathematical models have been developed for calculating the melting profile in single-screw machines. The most comprehensive of these is the five-zone model from Lindt. All these models can only be solved by numerical methods. Parallel to this, attempts have been made to find analytical solutions which describe the melting profile with sufficient accuracy. In this report a comprehensive analytical solution for the whole premelting- and melting process on barrel and screw with respect to the power law will be given.

Bulk Density of Plastic Pellets in a Screw Channel

Abstract For a correct calculation of the material throughput of single screw extruders in the by solids dominated feed area, the accurate knowledge of the effective bulk density is important. In the screw channel the bulk density measured in a quasi infinite vessel cannot be achieved during processing. The effective density is reduced according to the wall influences. If a two component system exists in the feed area, for example, when processing moulding compounds or regrind, then the bulk density changes likewise. In the following a procedure is described, by which the effect of the screw core measurement and the percentage composition of two material mixtures on the effective bulk density can be calculated.

Process-concurrent calculation of moulded part properties

A large number of physico-mathematical models are now available in the field of polymer processing. These describe flow and conveying processes, melting and mixing processes, moulded part formation processes and the moulded part properties that result from these. The models are generally based on the conservation theorems and the constitutive equations specific to the individual polymers. The model for the simulation program is compiled with the aid of the finite element method, the basic principles of which are briefly outlined in this paper. Programs of this type generally give good insight into the mechanisms of moulded part formation and the resultant consequences for the design and layout of moulded parts and moulds. They thus make it possible to avoid errors at the design and production stage. So far, however, they have not been suitable for the process-concurrent calculation of moulded part properties. This problem led to the process description being drawn up on the basis of statistical models. This paper shows that models of this type (e.g. multiple regression models) are eminently suitable for calculating moulded part properties in advance from process parameters while production is still running. This is first shown taking the example of the injection moulding process. Apart from injection moulding, however, processes such as 221 Vol. 15, Nos. 3-4, 1995/6 Process-Concurrent Calculation of Moulded Part Properties extrusion blow moulding, extrusion and welding can also be monitored by multiple regression, as is subsequently shown. This type of modelling is thus ideally suited to continuous moulded part quality inspection, particularly since the models can be aligned to changing environmental conditions.