Post Extrusion

After the strands leave the die, they are run through a cold water bath to solidify the polystyrene. The strands are then run up through a helical rotary cutter that cleaves the strands to a length of approximately 6-9 mm. The resultant beads or pellets are passed through an annealing unit and flash cooled to set nucleation sites and cell size. 

Work done in the late 1980s [132] showed that there are many variables to achieving a perfect cut on a strand in this process. The following are the equipment and polymer conditions that must be considered  

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The mass diffusivity coefficient of isobutane blowing agent from LDPE foam was found using a onedimensional diffusion model of two concentric cylinders with Dirichlet boundary conditions. An average mass diffusivity coefficient was used to calculate the mass of isobutane remaining in the foam for different boundary conditions. The influence of temperature and additives on diffusion was also examined. The use of the mass diffusivity coefficient in assessing the flammability of PE foam in the post-extrusion period is discussed. 2 refs. USA... 

Figure 6.18 Schematic diagram of the fiber spinning process in the post-extrusion die region.

In the processing equipment of thermoplastics, many kinds of complicated flow configurations exist. The flow in a tapered die, for example, produces three components of deformation that due to flow from the reservoir into the die that due to telescopic shear within the die that to extensional flow within the die. These may be assumed separable and the separately calculated pressure drops may be added to give the total pressure drop. But in addition, each deformation mechanism contributes to post extrusion swelling. The components due to simple shear and extension at the die exit determine the swell ratio. 

The phenomenon of post-extrusion swelling or "ballooning" has been discussed in Chap. 15. It is related to the so-called Barus effect, according to which the diameter of polymer extrudates is larger than the capillary diameter when the melt is forced through an orifice. All materials with any degree of melt elasticity display this effect. The origin of the effect is related to the elastico-viscous nature of polymer melts, thus to Ni=Tu — T22. 

Calibration is a technique for maintaining the shape and dimensions of the product from the time the melt exits from the die until it can be cooled enough to solidify and stabilize it. Depending on the size and shape of the product, calibrators can use water-cooled plates, internal mandrel, internal air pressure, external vacuum, or a post-extrusion die to change the shape of the molten extrudate before it solidifies. 

Apack meat trays have a base similar to that of the produce tray. It is mated to a clear, heat sealable PLA lidstock. PLA is an inherently poor oxygen barrier, but use of a proprietary post-extrusion step reportedly extends shelf life by 50% to 6-9 days. 

The effect of the type of impact modifier on the melt flow of a PVC window profile formulation as a function of shear rates encountered during extrusion was investigated and the relationship between the melt flow and mechanical properties of the profiles evaluated. A Rheoplast Capillary Rheometer with a pre-shearing device was employed to investigate the melt viscoelastic properties of the formulations and the performance of the formulations in terms of post-extrusion shrinkage, surface gloss and enthalpy relaxation discussed. 2 refs. 

Figure 9.19 Schematic diagram of extrusion process 3.2 POST EXTRUSION...

Figure 9.20 Schematic diagram of post-extrusion process...

S17. SiEGLAFF, C. L. Rheological properties of polyvinyl chloride. Part I General flow properties. Part II Unstable flow. Part III Post extrusion swelling. SPE Trans. 4, 1 —138 (1964). 

Attempts to increase the useful life of cables and to combat premature failure has resulted in a variety of approaches. They cover improvements in design, processing and in materials and include post extrusion treatments. 

Figure 10. Growth of Listeria monocytogenes in BHI broth at 24 °C. Control (diamond), nisaplin prior to extrusion (square), nisaplin post extrusion (circle), nisaplin in compress film (up triangle), and nisaplin in blown film (down triangle).

DSC measurements were performed on a TA Instruments 2190 DSC with temperature and enthalpy calibrations performed using an indium reference. Experiments were performed under a nitrogen atmosphere with a flow rate of 50ml/min. Extruded granules of nanocomposites were heated at 2°C/min to 250°C, held isothermally for 5mins, then cooled to room temperature at 2°C/min. All samples were heated twice, first to examine the properties post extrusion and secondly to examine the preferred crystal structures with slow cooling. 

HDPE/PA-6 methacrylic acid/isobutyl acrylate lamellas, post extrusion calendering/ elongation Gonzalez-Nunez et at, 1993... 

Figure 9 is a plot of post-extrusion swelling versus residence time, as calculated from data given in Table V. As can be seen, the... 

With respect to Figure 7, it is clear that with short dies, the post-extrusion swelling includes the entrance effect contribution and the shear response in the viscometric region. Consequently, the extrudate swell not only depends upon the residence time within the die, but also upon the transit time within the entrance zone. Therefore, it is possible to define a correcte residence time, t, according to ... 

When plotting the extrudate swell versus the corrected residence time tj.j, an excellent fit is obtained for the four dies (x), as can be seen in Figure 10. This demonstrates that with the short dies of practice, the entrance converging flow region plays a very important role in post-extrusion swelling. 

Post-extrusion swelling in fact, is not really a processing problem in itself, as long as it remains constant since, in this case, one can always manage it to control extrudate dimensions. The real problem is the extrudate swell variations arising during the process. 

The above considerations explain why with the short dies of industrial practice, rubber extrudates frequently exhibit swelling variations which are actually compensated for by adjusting the takeaway conveyor speed. This is however not always satisfactory and the results presented here suggest that a suitable design of the entrance geometry could reduce post-extrusion swelling variations. 

The increasing volume of polymer production in the chemical industry and the wide range of items being manufactured require an increase in the capability for the production and quality of the products produced. It has been established that an imderlying problem is the distortions introduced as the melted polymer flows through the extrusion channels. Analysis of the extrusion heads, has allowed identification of certain parameters which can positively be controlled to improved performance reduction of the pressure required leading to a decrease in cost, increase of output of the flow, increase in the hardness of extrudate and decrease the post-extrusion swelling. 

As a melt is subjected to a fixed stress (or strain), the deformation vs. time curve will show an initial rapid deformation followed by a continuous flow (Fig. 1-6). The relative importance of elasticity (deformation) and viscosity (flow) depends on the time scale of the deformation. For a short time, elasticity dominates over a long time, the flow becomes purely viscous. This behavior influences processes when a part is annealed, it will change its shape or, with post-extrusion (Chapter 5), swelling occurs. Deformation contributes significantly to process flow defects. Melts with small deformation have proportional stress-strain behavior. As the stress on a melt is increased, the recoverable strain tends to reach a limiting value. It is in the high-stress range, near the elastic limit, that processes operate. 

Levy, S., Processes and Equipment for In-line Post-Extrusion Forming, PM E, pp. 26-29, Aug. 1978. 

It is noted that the specific phase morphology of the injected molded tensile specimens used in the present study may differ from the compression molded film morphology. In the case of extruded samples of polymer blends displaying macrophase separation. Van Oene (25) indicates that the dispersed phase may appear as either ribbons (stratification) or droplets independent of shear strain rate but dependent of the post-extrusion thermal history. A study of the effect of morphology and phase inversion on the mechanical properties of the incompatible PPO blends is presently in progress. 

PS/epoxy-amine CORI Production and stabilization of the fibrillar structure helped via post-extrusion stretch Fenouillot and Perier-Camby 2004... 

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