Barrel extrusion

Specimen Location Coolant piping connections in extrusion barrel cooling jacket... 

Figure 9.16 depicts the arrangement of connections in the extrusion barrel cooling jacket. Embrittlement and cracking of these stainless steel connections (Figs. 9.17 and 9.18) were occurring as frequently as every 4 to 6 weeks. 

Polycarbonate melts adhere strongly to metals and if allowed to cool in an injection cylinder or extrusion barrel may, on shrinkage, pull pieces of metal away from the wall. It is therefore necessary to purge all equipment free of the resin, with a polymer such as polyethylene, after processing. 

Microencapsulation using extrusion is mainly described for glassy carbohydrate matrices [14-16, 28-29]. The glassy carbohydrates, such as starch and maltodextrins, are melted at elevated temperature and low water contents and are intensively mixed with the active in the extrusion barrel. Extrusion has been used for volatile and unstable flavours. The shelf life of flavour oils could be extended from several months to 5 years, compared with 1 year for spray-dried materials. The main drawbacks of the technology are the high investments costs and the formation of rather large particles (500-1,000 pm). 

Figure 3.6 Nomenclature of an injection barrel (top) and extrusion barrel (courtesy of Sp ...

Hinged-type extruders. This type of special de-airing extruder developed for use in the advanced ceramic industry is fitted with an extrusion barrel which can be adjusted from a vertical to horizontal position (9CM) ), and is employed for the extrusion of non-plastic special bodies and oxide ceramics (Fig. 2). 

Instead of the extrusion barrel diameter, the same dimensions are frequently quoted when referring to the auger diameters, which are almost identical but are, in fact, approx. 1-10 mm smaller than the extruder barrel diameter. Table 3 gives a rough guideline as to how the extrusion barrel diameters can be allocated to the application range and type of products. 

Table 3 Range of application of extruders determined by the extrusion barrel diameter...

Cylindrical extrusion barrel. The extruders commonly in use nowadays are usually provided with a cylindrical extrusion barrel and accordingly also a cylindrical extrusion auger. The pressure build-up is achieved by extrusion augers with a degressive pitch, i.e. the pitch decreases in direction of material feed. 

Conical extrusion barrel. When using a conical extrusion barrel, the diameter of the auger is gradually reduced in the direction of feed, whereby the augers have a linear pitch. The pressure build-up is in this case generated by the conicity of the extruder barrel. An extrusion auger of pure conical design is however rarely employed nowadays. 

Enlarged extrusion barrel. With this design the auger diameter increases towards the front end this version is pre-eminent in the extrusion of slugs. The purpose of using a conical extrusion barrel is to facilitate backward release of air still entrapped in the material. 

Expanded extrusion barrel. This version represents an option to the enlarged extrusion barrel, for coimection to a cylindrical auger. The aim is to obtain a perfect uniformly-condensed column. If this is a necessity, especially when extruding different column sections, the expanded barrel proves to be of benefit. 

Fig. 2 Piston extruder with hinged extrusion barrel, circa 1870...

Gebr. Sachsenberg in Rosslau/Elbe (Germany) build the first expression rolls machine in Germany Henry Clayton introduces a double-acting piston extruder at the London World Exhibition De Wolfe builds an extruder in USA for cable sheathing Lipowitz carries out first trials for heating the clay with a double-walled extrusion barrel... 

L. Schmelzer jr. develops an extrusion barrel with spiral-shaped inside contour to prevent the material from twisting and proposes replaceable barrel inserts made of hard casting Schmelzer develops the so-called replaceable compensation augers in connection with a tapered extruder barrel for a better utilisation of the wear volume of the augers... 

Injection molding produces solid parts in large volumes at high production rates. It permits close tolerances and the manufacture of very small pieces which are difficult to fabricate in quantity by other methods. Scrap losses are minimal compared to other forming methods because it is one of the only commercial processes where scrap can be reground and remolded at the machine. Melt material is injected into the mold from an extrusion barrel where it is maintained underpressure (Figure 1.6). When the part has sufficiently solidified, the mold opens and the part is ejected. 

The continuous microcellular process is also based on the concept of thermodynamic instability and a much shorter time is needed for saturation of polymer with gas [67]. As described by Park, when the polymer is melted in the extrusion barrel, a metered amount of gas is delivered to the polymer melt [67]. The injected gas diffuses into the polymer matrix at a much higher rate because of convective diffusion induced in the extmsion barrel at an elevated temperature [65]. As for a batch process, there are three specific steps in a continuous microcellular extrusion process formation of polymer/gas solution, cell nucleation, and shaping and cell growth [23,24,65-71]. A typical schematic of the overall continuous microcellular experimental equipment is illustrated in Figure 17.6. 

The transfer of fine powders causes air entrapment and fluffing to a low bulk density. All of this air must be removed during plastication or bubbles will develop in the plastic as the pressure is released after extrusion or on demolding. This is done by venting the injection or extrusion barrel and... 

It will not damage or scratch injection cylinders and extrusion barrels. 

The standard two-piece configuration consisted of a separate water-cool feedblock with a flanged extrusion barrel bolted on the downstream end of the feed block. This barrel also has (11) melt transdncers to record the internal pressures at every (2) L/D along the axial length of the barrel. Figure 2 shows the barrel feedblock configuration. 

The integral configuration consisted of an extrusion barrel that fits through a water-cooled feed block with a feed port machine directly into the cylinder. This barrel also has (11) melt pressure transducers located at every (2)... 

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