Oscillating Flow in Capillary Dies for a Hdpe Melt: Effects of Extrusion Parameters

Abstract

The study of the instability phenomena of polymer melts in capillary flows is of great technical relevance since, during an industrial die forming operation, these instabilities can give origin to a transition from a defect-free to a highly-defective extrudate. In this work, the instability phenomena exhibited in capillary flow by a blow moulding grade of high-density polyethylene (HDPE) were analyzed. In particular, by means of a capillary rheometer, which mimics polymer extrusion, the effects of specific extrusion parameters (temperature and imposed flow rate) on the oscillating flow, that takes place when "stick-slip" instabilities occur, were investigated.

Experimental study of the melt fracture behavior of filled high-density polyethylene melts

“…This result indicates that, at a given temperature, the piston velocity has an influence on the kinetics of the processes responsible for the achievement of the hydrodynamic boundary conditions able to give rise to the macrofracture of the flow; by contrast, as outlined above, the critical levels of pressure involved ( P o,min and P o,max ) are practically piston velocity independent. This was already observed by the authors for an unfilled HDPE with molecular mass distribution broader than the HDPE object of this study , and it is confirmed also for melts filled with glass beads or fibers. Interestingly, it can be observed from Fig.…”

“…It comes out also that, for each material, both v p,stick‐slip,min and v p,stick‐slip,max increase by increasing the process temperature. This, for the unfilled HDPE melt, was expected on the basis of the results obtained by the authors in a previous work . Although the piston velocities at the extremes of the “stick‐slip” region generally increased with the process temperature for the unfilled and the filled melts, the temperature effects were more contained for the latter melts than for the former.…”

“…it emerges also that, for both the unfilled and the glass‐filled melts, the period of oscillation decreases as the reservoir is emptying. The decrease in the period of oscillation as the volume of polymer melt in the rheometer diminishes, already observed in the oscillating flow of unfilled polymer melts , and here confirmed also for melts filled with glass beads or fibers, has been related to the material compressibility, which is considered one of the essential factors governing the “stick‐slip” instabilities . During a “stick” phase, the polymer melt is compressed by the piston in the reservoir of the rheometer, and the lower the amount of compressible material, the lower is the period of oscillation.…”

An Experimental and Theoritical Study on the Pressure Drop of Fluid at Extrusion Process

“…In the work of Baldi, the effects of extrusion parameters (temperature and imposed flow rate) on the oscillating flow exhibited by a HDPE melt were analyzed by capillary rheometry. All the tests were performed using a fixed die geometry (Baldi, F., Franceschini, A., Briatico-Vangosa, F., Locati, D. and Riccò, 2010). The results put in evidence that the imposed flow rate (piston speed) has an influence on the kinetics of the processes that govern the oscillating flow but not on the extrusion pressure levels characteristic of the instabilities, which are strongly dependent, in a complex way, only on the temperature.…”