Residence time of the melt

We now discuss the residence times and temperature increases of layers close to the die wall using the preceding equations At r — Ra 0.1 mm and r — R 0.01 mm, the residence time and the temperature increase of the two layers are, respectively, 0.008 s, 77°C and 0.082 s, 704°C. On the other hand, on the core surface (r — Rf), the residence time is 0.0005 s, and the temperature increase only 38°C. It is obvious that the closer the melt layer is to the die wall, the residence time is longer and the melt temperature during transit increases in an exponential fashion. Despite the very high temperature increases, the residence time of the melt layers near the wall is short and much shorter than the degradation induction time 0(T) (see Fig. E5.1(a), which is for unplasticized PVC). Thus, degradation is not likely to occur, and the wall melt layer has such a small viscosity that it precludes melt fracture. 

The efficiency of ultrasonic degassing in a melt flow corresponds to the completeness of the processes of cavitation nucleation, growth and evolution of hydrogen bubbles. So with increasing flow of a melt through an ultrasonic setup, the increase in a number of working ultrasonic sources and in the duration of the residence time of the melt in the cavitation region are required. 

For the thermoplastic processing the differences between PLA and PHB referring to their thermal stability during the melting are essential. By using PHB, the residence times of the melt have to be kept to less than 4 min to limit thermal degradation. PLA turns out to be relatively thermally stable under similar conditions. 

In general, extruders are located as close to the feedblock or multimanifold die as possible to minimize the length of transfer lines (which connect the extruder to the feedblock or die) and so reduce the residence time of the melt. Typically, th extruder located closest to the feedblock contains the heat-sensitive polymer. Adhesive resins tend to cross-link if their residence time is too long. The barrier resin, poly(vinylidene chloride), is also heat sensitive. 

It can be shown that the change in striation thickness on mixing, and hence in the degree of laminar mixing, is a simple function of the total shear strain imposed on the system. However, at the end of the process the components of the mixture still exist as discrete components. The total shear strain exerted on the melt is a function of the residence time of the melt in the process. As a result of the complex velocity profile of the melt in the screw channel, the residence time of the melt in the channel varies as a function of the position of the melt in the screw channel as well as the down-channel velocity of the melt. 

High residence time of the melt in the runner Poor thermal uniformity and cold spots Mismatches between hot runner components Melt channel turns and splits Hot to cold transition in the gate well Row obstructions in the runner such as valve stems, valve stem guides or nozzle tips Material differences (base resins and colorants) between the new and old color... 

Any refractory material that does not decompose or vaporize can be used for melt spraying. Particles do not coalesce within the spray. The temperature of the particles and the extent to which they melt depend on the flame temperature, which can be controlled by the fueLoxidizer ratio or electrical input, gas flow rate, residence time of the particle in the heat zone, the particle-size distribution of the powders, and the melting point and thermal conductivity of the particle. Quenching rates are very high, and the time required for the molten particle to soHdify after impingement is typically to... 

Differences in residence time can lead to increased support of a daughter nuclide if the parent residence time in the melting column exceeds that of the daughter (see Bourdon et al. 2003). We can quantify this effect using the system as an... 

To reach steady state, the residence time of the fluid in a constant stretch rate needs to be sufficiently long. For some polymer melts, this has been attained however, for polymer solutions this has proved to be a real challenge. It was not until the results of a world wide round robin test using the same polymer solution, code named Ml, became available that the difficulties in attaining steady state in most extensional rheometers became clearer. The fluid Ml consisted of a 0.244% polyisobutylene in a mixed solvent consisting of 7% kerosene in polybutene. The viscosity varied over a couple of decades on a logarithmic scale depending on the instrument used. The data analysis showed the cause to be different residence times in the extensional flow field... 

Spherical particles proved to be superior in several applications owing to their favorable properties. Thus, they are used in thermal spraying for their excellent flowabil-ity, in powder metallurgy because of their excellent reproducibility in manufacturing parts with controlled porosity and as a filler material, as well. Metal microspheres can be easily produced by melt atomization. Similar method in the case of ceramics is impractical. Micron-sized ceramic particles, however, can be smelted by thermal plasmas that provide exceptional conditions for spheroidization due to its high temperature. In terms of purity and residence time of the particles in the hot temperature core, RF plasmas provide better conditions as compared to arc plasmas. 

The residence time of the polymer in the different process sections, the heat exchange surface, i. e., the inner barrel surface, or the surface renewal of the melt layers are further important parameters when scaling up reaction, heat exchange, or devolatization processes and should be kept constant in a scale-up. 

Residence times of the real drawing are extremely short some milliseconds compared with the normal half-time of crystallisation (f 1/2 of the order of 50s) it is clear that the crystallisation process during spinning with high-speed winding is many decades faster than that in the isotropic melt. 

Because the plastic is processed between the required heat and its critical heat of degradation, the time of heat becomes extremely critical and an important part of the complete process. For example the processor will minimize the amount of melt in the nip of the rolls. The residence time of the plastic flux at high heat must be controlled and limited. PVC is especially sensitivity to heat and time at heat. What is required is proper setting of the machine controls and operation within set limits. The processing variables of a PVC plastic (such as flow, heat stability and softening point) are strongly influenced by polymerization technique, MWD, and the extent of any polymerization (Chapter 1). 

Although the residence time of plastic melt in the dehydrochlorination process is very long, the amount of HCl gas evolved is less than in the liquefaction processes of Niigata and Sapporo. This may indicate that PVC and PVDC materials are removed effectively... 

Extruders Extruders can also be used to blend powders, but usually only in conjunction with a subsequent melting operation. They consist of parallel screws that are constructed of individual sections that are specifically designed to move or mix the material as it passes through the extruder. The configuration of the screws affects the residence time of the material in the extruder, which can affect the degree of blending achieved. 

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