Quality of extrudate

Peri, C., Barbieri, R., and Casiraghi, E. M. (1983). Physical, chemical and nutritional quality of extruded com germ flour and milk protein blends. Int. J. Food Sci. Technol. 18,43-52. 

A study was made of relationships between compound rheological properties, microwave vulcanisation parameters and accelerators on the quality of extruded EPDM foam seals for the automotive industry. The influence of these factors on cell size and structure, density and mechanical properties was investigated. Correlations were found between the chemical composition of the compound, variations in processing parameters and the quality of the finished product. 12 refs. 

Malleshi, N. G., Hadimani, N. A., Chinnaswamy, R., and Klopfenstein, C. (1996). Physical and nutritional qualities of extruded weaning foods containing sorghum, pearl millet, or finger millet blended with mung beans and nonfat dried milk. Plant Foods Hum. Nutr. 49,181-189. 

Overall, it seems that neither the power-law index, nor the consistency index, nor temperature dependencies of the both, nor polymer processability ratios (in terms of MFl) is a good or appropriate indicator of the expected quality of extruded composite profiles. 

Rehydration The quality of extruded cereal products, such as pasta, depends on extrusion temperature and the variety of wheat used in the extrusion process. The effect of varying the ratio of hard and soft wheat on the rehydration kinetics of extruded spaghetti has been investigated by MRI. By... 

This method covers the determination of the quality of extruded rigid poly(vinyl chloride) (PVC) plastic pipe as indicated by reaction to immersion in anhydrous acetone. 

This method is not intended for use in purchasing specifications because the conditions of processing plastic compounds vary widely, and the degree of correlation of data obtained by this method with chemical and physical properties has not been completely determined. However, despite this limitation, this method does yield data of value in establishing such correlations and in judging the quality of extruded PVC pipe. 

In terms of processing there is no need for pre-drying PCHE granules, a standard extruder screw as used for polycarbonate may be used and discs are said to release well from the mould. Question marks remain on the oxidative stability of the polymer and on the quality of adhesion of the reflective layer but Dow claim that metallising is possible. 

In commercial extruders, additional zones may be included to improve the quality of the output. For example there may be a mixing zone consisting of screw flights of reduced or reversed pitch. The purpose of this zone is to ensure uniformity of the melt and it is sited in the metering section. Fig. 4.4 shows some designs of mixing sections in extruder screws. 

When a molten plastic is forced through a die it is found that under certain conditions there will be defects in the extrudate. In the worst case this will take the form of gross distortion of the extrudate but it can be as slight as a dullness of the surface. In most cases flow defects are to be avoided since they affect the quality of the output and the efficiency of the processing operation. However, in some cases if the flow anomaly can be controlled and reproduced, it can be used to advantage - for example, in the production of sheets with matt surface finish. Flow defects result from a combination of melt flow properties, die design and processing conditions but the exact causes and mechanisms are not completely understood. The two most common defects are... 

An extruder is a complicated device to control. Often the barrel is divided into three sections, and the temperature at the exit of each section determines the additional amount of electrical energy to be supplied. Most of the energy for heating is provided by the screw. The throughput is usually set by the rate at which the screw rotates, and is maintained constant. Work is currently being done on the effect of extruder operating conditions on product quality. Preliminary conclusions indicate that conditions should be kept as constant as possible if reproducible results are desired. 

Extrusion with the aid of a vacuum applied to the screw. The process is applied particularly in the preparation of extrudates to be vulcanised by either the liquid curing medium or fluid bed techniques. It assists in removing the volatiles from the compound and thus improves the quality of the extrudate. 

The gear pump can be fitted to a normal cold-feed, pin or vacuum extruder and enhances the accuracy and quality of the product. 

The extrudate samples and the solidification experiment clearly show the importance of the melting process on the mixing quality of the extruder discharge. The cases shown here are for color masterbatches in natural or white resins, but the concept applies to any solid mixture or blend added to the hopper of an extruder. Color masterbatches were used here because it is much easier to visualize than other compositional variations or thermal gradients. 

At a maximum rate of 440 kg/h, the extruder was rate limited by motor torque and not by the quality of the extrudate. This extruder was fabricated with a gearbox and sheave ratio such that the maximum screw speed was 105 rpm. Since the process was operating at a screw speed of 45 rpm, the sheave ratio could be changed such that the maximum screw speed would be about 75 rpm. This might allow the process to operate at rates up to 700 kg/h. Changing the sheave ratio will increase the torque to the inlet side of the gearbox. Care must be taken to make sure that the additional torque does not harm the gearbox. 

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