In-Process R eometry Studies of LDPE Compounds

Abstract

The second part of a comprehensive study into the investigation and validation of in-process measurements on a low density polyethylene compound during extrusion is reported. A commercial grade of branched low density polyethylene was used in the study, compounded with various levels of magnesium hydroxide¯ame retardant ®ller, up to 50 % by weight. In-line and on-line rheometry were used to monitor melt¯ow behaviour during single screw and twin screw extrusion; off-line capillary rheometry was performed on the compound before and after extrusion, to examine any change in rheology. Morphological observations were made using scanning electron microscopy, and molecular characterisations carried out using gel permeation chromatography. In-process rheometry was found to be able to detect the presence of ®ller at low and high loadings, but was unable to discriminate between intermediate levels. Offline rheometry indicated that twin screw extrusion altered the rheology of the compound, and capillary wall slip was found to increase after extrusion. Morphological observation indicated that ®ller in the compound was better dispersed after extrusion, especially at high volume fractions. Molecular weight and polydispersity of the LDPE were shown to increase after extrusion, more so in the case of low ®ller loadings. Overall, in-process measurements provided useful rheological data, and the compound was found to exhibit complex, process-dependent rheology.

In‐process Measurements for Reactive Extrusion Monitoring and Control

“…On-line capillary or slit rheometers, either coupled to the die or between extruder and die, are preferred for industrial compounding and reactive extrusion. Comparisons between in-line, on-line, and off-line [90] and between on-line and off-line [38] rheometry have generally shown good agreement, but were performed with thermally stable polymers. The design, complexity, and compactness of these devices vary, and so do the range of attainable viscosities and shear rates (in some instruments, above 10 5 s −1 ), the time delay between sample collection and measurement, and whether the sampled material is wasted or returned to the main flow (sampling rates vary typically between 0.5 to a few kilograms per hour).…”

The effect of copper alloy mold tooling on the performance of the injection molding process

“…More advanced methods of improving mold temperature control and reducing cycle time have included conformal cooling channels [7–9] and cyclic cooling techniques [10–12]. Conformal cooling channels have complex geometries which closely follow the geometry of the mold cavity, and can be specifically designed to optimize heat transfer to the cooling fluid.…”