Discontinuous extruders

During moulding a polymer melt is discontinuously extruded and immediately cooled in a mould of the desired shape. 

The dense-phase regime can be further subdivided into three distinct regions,which are shown in Fig. 14.3. In continuous dense-phase flow the material moves by saltation over a stable creeping bed, in discontinuous dense-phase flow particles move as groups, and in the solid dense-phase the solids are extruded through the pipe as a continuous slag. 

Cable sheaths may be covered with paper and hessian wrappings impregnated with bituminous compounds or with extruded or taped plastics outer sheaths. At pinholes or discontinuities in protective coatings the sheath will be particularly liable to electrolytic corrosion in stray-current areas, and it is desirable to supplement this form of protection by drainage bonds or direct cathodic protection. 

Extruders play a key role in many conversions processes we use them to melt solid polymers and pump the resulting molten material to a die or a mold. After the molten polymer has been molded to the desired shape, it is cooled to form a solid product. We can feed the output of an extruder to continuous forming processes to create films, pipes, and fibers. Alternatively, we can employ discontinuous molding processes to create discrete items, such as soda bottles, lenses for single use cameras, or bathtubs. 

Fairbanks has also studied the effect of ultrasonic energy on the flow characteristics of a poly(methyl methacrylate) melt in a simulated injection moulder. Initially the ultrasound (20 kHz, 0-105 W) was applied either simultaneously or independently to both the extruder tube and the cylinder of the moulder. However, since no discernible effect was observed when ultrasound was applied at the extruder tube, further work with the horn in this position was discontinued. 

Addition of oxidizing agent to soy flour resulted in a less-puffed or undertexturized extrudates. The extrudate showed rough, discontinuous surface and small dieimeter. This result further proves that the change of disulfide linkages is an essential reaction for texture formation. 

In the three polymers just named, two more observations are worth mentioning. Lirst, at the melt fracture onset, there is no discontinuity in the flow curve (t vs. y ). Second, as expected, because the entrance is the site of the instability, increasing L/Dq decreases the severity of extrudate distortions. 

From the time they were first developed, co-rotating, closely intermeshing twin-screw extruders have proven to be highly effective for the continuous treatment of viscous products and they have successfully replaced many discontinuous technologies. They will certainly continue to be successful in other markets in the future, facilitating new production processes. 

Fig. 1. Typical flow curve of commercial LPE. There are five characteristic flow regimes (i) Newtonian (ii) shear thinning (iii) sharkskin (iv) flow discontinuity or stick-slip transition in controlled stress, and oscillating flow in controlled rate (v) slip flow. There are three leading types of extrudate distortion (a) sharkskin like, (b) alternating bamboo like in the shaded region, and (c) spiral like on the slip branch. Industrial extrusion of polyethylenes is most concerned with flow instabilities occurring in regimes (iii) to (v) where the three kinds of extrudate distortion must be dealt with. The unit shows the approximate levels of stress where the sharkskin and flow discontinuity occur respectively. There is appreciable molecular weight and temperature dependence of the critical stress for the discontinuity. Other highly entangled melts such as 1,4 polybutadienes also exhibit most of the features illustrated herein...

Many polymers exhibit neither a measurable stick-slip transition nor flow oscillation. For example, commercial polystyrene (PS), polypropylene (PP), and low density polyethylene (LDPE) usually do not undergo a flow discontinuity transition nor oscillating flow. This does not mean that their extrudate would remain smooth. The often observed spiral-like extrudate distortion of PS, LDPE and PP, among other polymer melts, normally arises from a secondary (vortex) flow in the barrel due to a sharp die entry and is unrelated to interfacial slip. Section 11 discusses this type of extrudate distortion in some detail. Here we focus on the question of why polymers such as PS often do not exhibit interfacial flow instabilities and flow discontinuity. The answer is contained in the celebrated formula Eqs. (3) or (5). For a polymer to show an observable wall slip on a length scale of 1 mm requires a viscosity ratio q/q equal to 105 or larger. In other words, there should be a sufficient level of bulk chain entanglement at the critical stress for an interfacial breakdown (i.e., disentanglement transition between adsorbed and unbound chains). The above-mentioned commercial polymers do not meet this criterion. 

It was thought in the past that the only mechanism for wall slip would be polymer desorption, i.e., an adhesive breakdown [25, 53]. However, lack of a strong temperature dependence would be inconsistent with an activation process of chain desorption. Since the onset of the flow discontinuity (i.e., stick-slip) transition was found to occur at about the same stress over a range of experimental temperatures, it was concluded from the outset [9] that the phenomena could not possibly have an interfacial origin. Thus, the idea of regarding the flow discontinuity as interfacial did not receive sufficient and convincing theoretical and experimental support in the past, not only because the transition was often accompanied by severe extrudate distortion and hysteresis, but also because the molecular mechanism for such an interfacial transition involving wall slip was elusive. 

We have surveyed the most recent progress and presented a new molecular level understanding of melt flow instabilities and wall slip. This article can at best be regarded as a partial review because it advocates the molecular pictures emerging from our own work over the past few years [27-29,57,62,69]. Several results from many previous and current workers have been discussed to help illustrate, formulate and verify our own viewpoints. In our opinion, the emerging explicit molecular mechanisms have for the first time provided a unified and satisfactory understanding of the two major classes of interfacial melt flow instability phenomena (a) sharkskin-like extrudate distortion and (b) stick-slip (flow discontinuity) transition and oscillating flow. 

We can see in Fig. 21 that, for the LDPE (FN 1010) above a given flow rate, bands of discontinuity appear in the birefringence pattern. These bands are observed parallel to the flow direction, at an intermediate position between centreline and die wall. Such "defects" may also be detected in the converging flow near the die entry (Fig. 22). For these flow conditions, the extrudate begins to exhibit distorded volume shapes, characteristic of melt fracture. By comparison with rheological studies carried out on a capillary rheometer, we may assume that these defects belong to the family referred to as "upstream instablities" in Chapter III.4. 

The characteristic curve of extrudate flow including adherence to the walls, and hence representative of shghtly to moderately entangled polymer flow in sudden two-dimensional or axisymmetrical contractions [7, 32], is represented in Fig 2. It shows a slope discontinuity above a certain pressm-e level, which depends on the pol3uner-die pair considered. With low flow rates, the flow is stable. Indeed, for these regimes, allowing for entrance effects, the flow curve is in fact representative of the shear rheometry of the polymer imder consideration, at low shear rates [34]. The slope discontinuity of the head loss curve indicates a modification in the structure of flow. It will be seen that this corresponds to the triggering of a hydrodynamic instability upstream of the contraction. 

This section analyses the appearance of the extrudate under regimes corresponding to the stable branch of the flow curves [8, 22], i.e., before the first slope discontinuity and, in certain cases, before the jump in flow rate value. 

It is now necessary to examine the appearance of the extrudate corresponding to the second branch of the Figvae 2 type flow curves in the case of slightly to moderately entangled polymers (LO, BO, LGl, BG, etc.). It was seen in section 3 that the flow becomes unstable at the moment there is a slope discontinuity in the flow curves. The extruded rod is excited by these instabilities, which trigger the well-known phenomenon of melt fracture [7]. 

The fibers are produced continuously (endless fibers, filaments) or discontinuously (short or staple fibers) depending upon the application. Most of the fibers are obtained by extruding a flowable form (melt or solution) of appropriate chemieal composition (Section 5.2.3 - 5.2.4). Other processes are based on deposition from the gas phase (Section 5.2.6.2) or thermal transformation (pyrolysis) of organic (Section 5.2.5) or organometallic polymers (Section 5.2.7). 

Glass-mat reinforced thermoplastic (GMT) is similar to SMC and is available in sheet form, reinforced with random oriented fibers that may be continuous or discontinuous. It is manufactured on a double belt press where the glass fiber mats are sandwiched between layers of extruded molten polymer before it enters the press. The thermoplastic is usually about a few millimeters thick and thus stiff, and hence stored as fiat sheets. Another method involves deposition of a molten mixture of resin, chopped fibers, and additives over a moving belt where the water is driven off. The fiber volume fraction in this compound is usually 0.1-0.3 and fiber lengths are in the range of 10-30 mm, unless the reinforcement is continuous. 

Injection molding is a cyclic discontinuous process used to form three-dimensional plastic parts (25. 26). Both thermoplastics and thermosets are injection molded. The process is shown schematically in Figure 11 for a reciprocating screw injection molder. This type of a machine has a screw rotating in a barrel similar to an extruder. However, here the melt flow is discontinuous and controlled via a check valve at the tip of the screw. Material is melted in the extruder, and the melt is accumulated in front of the screw tip until sufficient melt is at hand to fill the mold cavity (cavities). 

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