Extrusion cooling length

The stretching force is a direct consequence of the distance of polymer solidification (DPS) in both processes (i.e., FLH in film blowing processes or cooling length in ribbon extrusion). In this sense, controlling DPS offers the possibility to control the final properties of films and ribbons. Table 24.3 lists typical values of stretching force as function of FLH and DR for blown films of LDPE. 

Resin temperature affects viscosity and part quality by causing variations in the parison extrusion time, length, thickness, and cooling time. This temperature is a critical parameter that must be monitored carefully and the source of any variations must be accurately identified. Extruder factors that can cause resin temperature variations include ... 

The convention extrusion blow moulding process may be continuous or intermittent. In the former method the extruder continuously supplies molten polymer through the annular die. In most cases the mould assembly moves relative to the die. When the mould has closed around the parison, a hot knife separates the latter from the extruder and the mould moves away for inflation, cooling and ejection of the moulding. Meanwhile the next parison will have been produced and this mould may move back to collect it or, in multi-mould systems, this would have been picked up by another mould. Alternatively in some machines the mould assembly is fixed and the required length of parison is cut off and transported to the mould by a robot arm. 

Example 3. Hydrolysis in an Extruder.69 PET reactive extrusion experiments were carried out on a 25-mm Berstorff ZE25 corotating twin-screw extruder with a barrel length-to-screw ratio of 28 1. The extruder consisted of six barrel sections equipped for heating, cooling, and controlling the temperature of each section of the extruder. Initially reaction extrusion of PET and water was performed with cold water at room temperature injected into the extruder. Typical operating conditions were reaction temperatures of 230-265° C, extruder speeds... 

Extrusion (Fig. 16.9) is a method of producing lengths of plastic material of uniform cross section. The extruder is similar to a domestic mincing machine with the added facility that it can be heated and cooled. The pellets enter the screw section via the hopper, are melted, and then pass through the... 

Extrusion accounts for about 30% of nylon produced and is used in various processes (24). Nylons can be extruded on conventional equipment having the following characteristics. The extruder drive should be capable of continuous variation over a range of screw speeds. Nylon often requires a high torque at low screw speeds typical power requirements would be a 7.5-kW motor for a 30-mm machine or 25-kW for 60-mm. A nylon screw is necessary and should not be cooled. Recommended compression ratios are between 3.5 1 and 4 1 for nylon-6,6 and nylon-6 between 3 1 and 3.5 1 for nylon-11 and nylon-12. The length-to-diameter ratio, L D should be greater than 15 1 at least 20 1 is recommended for nylon-6,6, and 25 1 for nylon-12. 

Fig. 9.20 Cross sections obtained from cooling experiments of a 2.5-in-diameter 26.5 length-to-diameter ratio screw extruder. Material rigid PVC. Operating conditions are listed in the figure Tb is the barrel temperature, N the screw speed, P the pressure at the die, and G the mass flow rate. Numbers denote turns from the beginning (hopper side) of the screw. The screw was of a metering type with a 12.5 turn feed section 0.37 in deep, a 9.5 turn transition section, and a 4.5 turn metering section 0.127 in deep. [Reprinted by permission from Z. Tadmor and I. Klein, Engineering Principles of Plasticaling Extrusion, Van Nostrand Reinhold, New York, 1970. The experiments were carried out at the Western Electric Engineering Research Center, Princeton, NJ.]...

Tubes and blown Aims can be produced as multilayer structures by employing multiple extruders and coextrusion manifolds and dies. Figure 12.44 is a schematic representative of a conventional and new spiral coextrusion die. The designs can be used for both blown-film and blown-molding parison dies. In the extrusion of tubes, such as rigid PVC or PE pipe, the extrudate passes over a water-cooled mandrel and enters a cold-water bath whose length depends on the tube thickness the tube leaves the bath well below its Tm (if it is crystalline) or Tg (if it is amorphous) and is sectioned to the desired lengths. 

Two-stage EBM treats the parison as a true preform by separating the functions of parison preparation and BM. Parisons are produced by conventional tube extrusion methods (Chapter 5). The tube is cooled and cut into parison lengths that are stored before being reheated and BM in the normal way. 

In the continuous extrusion design process, the parison is continuously extruded between the open mold halves from an accumulator head. When the required length of parison has been produced, the mold is closed, trapping the parison that is severed usually by a hot knife from the die. Figure 6.7 provides a simplified schematic of a continuous BM process. Land or pinch-off areas on the mold compress and seal the upper and lower ends of the parison to make an elastic airtight part. Compressed air is introduced through the blow pin into the interior of the sealed parison that expands to take up the shape of the mold cavities. The cooled mold chills the blown object that can then be ejected when the mold opens. 

Almost exclusively used for extrusion of PVC-U into pipe profile and sheet, extrusion equipment consists of a cylindrical barrel containing two close fitting, intermeshed, Archimedean screws. PVC dry blend is fed in via a hopper at the start of the screws and is conveyed forward, softened, sheared and finally gelled by rotation of the screws and the heat of the barrel. The resulting homogeneous viscous melt is forced through a heated die fitted to the exit end of the barrel. Thereafter the downstream equipment sizes or calibrates, cools and pulls off the product which is also cut to desired lengths. 

Figure 21-22 Fibers of synthetic polymers are made by extrusion of the molten material through tiny holes, called spinnerets. After cooling, nylon fibers are stretched to about four times their original length to orient the polymer molecules.

Figure 7.7 shows a typical rubber profile extrusion line. Several days prior to the extrusion itself, the raw rubber is prepared in an internal mixer, where certain materials such as sulfur or carbon black are added to achieve the required properties. One or more extruders form the head of the production line. There, the rubber is kneaded, heated, intermixed, and then extruded through specially formed dies (final delivery elements) which are responsible for attaining the required profile geometry. A major part of the line is taken by the vulcanization process, usually achieved by near infra red and microwave heating. After optical quality control, the profile is cooled down and cut into pieces of a certain length. 

Extrusion is a method of producing a length of material with a uniform cross section [5], Extruders can be heated and cooled as required. A screw provides pressure to the material after it is fed into the barrel through the feed hopper. The pellets are melted and pass through a breaker plate into the die. The material is then forced out of the die its cross section is determined by the shape of the die [5],... 

It was discovered that a number of PVDF film properties, including electrical, could be altered by the extrusion and orientation conditions.The relationship between the extrusion and orientation process and the electrical properties of the film are important because of the application of PVDF films in miniaturized capacitors which are used in apparatus such as defibrillators. To compare the properties, extruded film, while in molten state, was brought in contact with chilled rollers at 80°C and cooled rapidly. The oriented film was stretched at 150°C in the longitudinal direction by a stretch ratio of 3.5, that is, its original length was increased by 3.5 times [Eq. (6.6)]. The dielectric constant and dissipation factor were measured as seen in Table 6.8. Orientation increased the dielectric constant and reduced the dielectric loss... 

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