Melt instability

There are three main reasons why the capillary rheometer is widely used in the plastics industry (1) shear rate and flow geometry in capillary rheometer are very similar to conditions actually encountered in extrusion and injection modeling (2) a capillary rheometer typically covers the widest shear rate ranges (10 sec to 10 sec ) and (3) a capillary rheometer provides good practical data and information on the die swell, melt instability, and extrudate defects. 

Output variations are caused hy cyclic variation or random fluctuations. These include melt fracture, variation in screw speed, feed stock, melt temperature, take-up speed and plant voltage. A fluctuation in feed rate and temperature and melting instabilities can also cause output variations. 

Melt instability (melt-flow instability) n. A term applied to the early manifestations of melt fracture. 

In the case of aliphatic diols, and for interfacial polymerizations, typically the diol is first prereacted with phosgene to form chloroformates [126,127]. The chloroformates are then condensed with BPAin a standard interfacial process to yield the copolymer with a reduced T and increased melt flow (Fig. 14.5). Selection of the diol in the process is important. Diols containing beta hydrogens, which often readily crack at elevated processing temperatures, lead to melt instabilities. This potential for poor melt stability, coupled with complications of a two-step process, has limited commercial interest in this approach. 

There has been little or no published work on the melt rheology of PHB, presumably because of problems associated with its melt instability. However, some information on melt viscosity can be derived from melt flow index data such as that given in Fig. 9, using the empirical relationship ... 

Polymer melt instabilities Polymer rheology Thermoplastic vulcanized... 

Consider a scenario where a PCR tends towards melt instability at high speeds or at normal melt temperatures. Excessive parison sag and bubble or web instability are indicative of a lack of melt strength. Therefore, the addition of a low MI resin will improve the elongational viscosity of a PCR that has a tendency to excessive shear thinning and increase its melt strength. This should improve the cycle time for blow molding, improve bubble stability in blown film or increase the line speed before the onset of draw resonance in cast film. 

The die swell was determined using the capillary rheometer at the same temperature but with a shorter die having a L/d ratio of 10/1. A laser source was installed under the die with a close proximity to the die exit. The laser beam allows the measurement of the cross-sectional area of the strand as it is extruded. These measurements were conducted at low to moderate shear rates below the onset of sharkskin and melt instabilities. The die swell is calculated as the ratio of the cross section of the extrudate to that of the die orifice. 

The viscosity-shear rate curves of the samples are shown in Figure 1. The branched PVDF samples have a similar viscosity profile to those of the reference samples across the entire shear rate range. The change in the viscosity is mostly controlled by the molecular weight of the samples. The melt viscosity of the resin plays an important role in the first step of the extrusion blow molding process where the resin is processed under moderate shear rates well below the onset of melt instabilities. This is extremely important for the formation of a smooth parison and ultimately in a quality product. 

Graphical analysis of high-speed experimental data has shown several process features that may not have been observed if only standard single-point data were available. For example, the analysis has shown interactions between the motor drive and screw speed, an inadvertent set point change in the screw speed, melting instabilities, and pressure dampening in the metering sections. 

While the severe melt instabilities such as slip-stick and gross-melt fracture were observed for the neat Exact resin, the mild sharkskin-like instability was observed for the suspensions with Dechlorane concentration above 20%. The shear rate at which the instability started to appear was decreased as the Dechlorane concentration was increased. The thixotropic behavior of Exact 5361/60% Dechlorane suspension was observed using sequential step rate. 

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