Volume 7, Issue 2

A visual study was performed to determine the mixing abilities in simple conveying sections of the partially-filled, corotating, twin screw extruders (PFCoTSE). The amount of mixing that occurs in the PFCoTSE is heavily dependent on the degree to which the channels are filled. A quasi-quantitative method of measuring the growth of an injected tracer is used in determining mixing. It was determined that the PFCoTSE's mixing sections are linear mixers as can be seen from the fact that an injection of tracer quickly transforms into a line and stays as a line throughout the machine and does not break apart while going through the nip (i.e., no reorientation).

By combined usage of a high speed intermeshing twin screw extruder and an injection molding press, a direct reactive extrusion and injection molding process has been developed for improving toughness of the thermoset unsaturated polyester (UP) by blending with an elastomer. The unsaturated polyester and elastomer were homogenously kneaded with each other and a radical initiator dissolved in a liquid monomer was added to the molten mixture. This was carried out at a relatively low temperature from the final feeding port of the twin screw extruder to allow not the curing reaction to proceed during the twin screw extrusion stage. The molten UP-elastomer alloy was accumulated, injected into the hot mold, and cured. The products obtained from these experiments were evaluated with regards to their physical properties and their morphologies. The elastomer, NBR produced the greatest improvement in toughness. The physical properties of the UP-NBR alloy obtained by this reactive extrusion process were generally equivalent to those of polypropylene except for thermal characteristics.

Mechanical data obtained on the blends described in a previous paper together with some new data have been correlated with the theory derived for the mechanical properties of polymer composites, to gain insight into the nature of domain interfaces. Thermoplastic polyurethanes are blended in the entire composition range, with styrene-acrylonitrile copolymers. Blending with thermoplastic polyurethanes improves the toughness. Improved toughness is indicated by the increase of elongation at break and by decrease of hardness.

A new approach to assessing mixing in single screw plasticating extruders is proposed. It is based on image analysis of microtomed sections obtained from the polymer helix formed in a suddenly cooled split barrel extruder fed with a mixture of black and white polymer granules. The observations suggest that most of the mixing occurs on melting with little improvement thereafter. A simple laminar mixing model is drawn in the melting zone the results of which compare favourably with experimental data. In particular, it is shown that improved mixing can be achieved with increasing screw speed in complete contradiction with the classical strain theories which consider mixing in the melt conveying zone only.

Slip at the wall is often encountered during the flow of polymeric materials and during the mixing of multiphase systems, such as concentrated suspensions. This article is concerned with the development of a numerical method to simulate flows that show such slip. The flow of a generalized Newtonian fluid with Bingham-like viscosity curve is considered and a linear relation between the slip velocity and the shear stress at the wall is used. A novel method based on a penalty parameter is developed to implement the slip boundary conditions into the finite element method. The flow in the conveying elements of the extruder is used as an example of the application of this method. It is shown that the wall slip decreases the pressurization capability of the extruder and increases the leakage flow occurring over the flight tips, as well as its mixing efficiency by reducing the rate of recirculation and the stress levels. The technique developed in this article can be applied to many flows that show slip phenomena and may be used to gain better understanding of the dynamics of mixing of commercially important materials.

Plastic pipe is produced by extruding molten polymer through an annular die. As the pipe leaves the die, it passes through a sizing sleeve and then a cooling tank where it solidifies slowly. For thick walled pipe, the inside of the pipe remains molten for as long as ten hours causing downward melt flow called sag. Sag can cause serious non-uniformity in pipe wall thickness. The conventional way to reduce sag is by manually adjusting the die eccentricity, until an acceptable wall thickness profile is achieved. This tedious trial and error procedure can take up to four attempts to get the right profile. In this paper a new way has been proposed to reduce sag, by rotating the pipe during cooling. A numerical simulation of sag flow while pipe is rotating, predicts that for rotational speeds between 0.1 to 10 rad/s, sag is virtually eliminated for thick walled, large diameter HDPE pipe extrusion.

Two-dimensional convective heat transfer in a non-isothermal spinline is analyzed. A rigorous derivation of the macroscopic, one-dimensional energy balance describing the cooling of a spinline leads to an equation that differs subtly from the commonly used one-dimensional equation; this difference has practical implications for the correct computation of convective heat transfer coefficients from experimental data obtained from spinlines exhibiting significant radial temperature gradients and, in turn, for the prediction of correct spinline temperature profiles. The errors involved are illustrated with computed results and a method is proposed for the determination of the correct heat transfer coefficients in the presence of radial temperature gradients. Further, it is shown that the radial convective heat flux resulting from the inward radial velocity of an attenuating spinline is important and its omission can lead to significant errors in the computed temperature.

The formation and characterization of complex chain orientation of polycarbonate skew stretched films is described. The directions of symmetry axes of orientation are represented in the form of refractive index and infrared absorption ellipsoids. The chain orientation is evaluated in terms of White-Spruiell biaxial orientation factors based on the principal axes of orientation. The anisotropic mechanical properties of skew stretched films are correlated with their orientation states. The major symmetry axes of skew stretched polycarbonate films tend to move toward the second stretching direction as increasing second stretching ratio. The chain orientation behavior of these films exhibit increasing uniaxial orientation character at high second stretching ratio. The anisotropic characters of tensile modulus and yield stress of skew stretched films follow a linear relation with the magnitude of chain orientation within our stretching range.

The molecular orientation process in injection molding of polypropylene copolymers with ethylene was theoretically analyzed from a view point of growth of melt orientation (recoverable shear strain) at the gate and its relaxation in the mold cavity. Regardless of the characteristics of raw resin (copolymerization type and ethylene content) and molding conditions (cylinder temperature), there exist unitary correlations between calculated degree of molecular orientation and experimentally measured degrees of molecular orientation such as the thickness of skin layer and crystalline c-axis orientation function. According to this analysis, the fact that molecular orientation is decreased by copolymerization with ethylene is due to decrease in melt orientation (recoverable shear strain), decrease in the relaxation time and reduction of crystallization temperature.

Talc-filled polypropylene injection moldings show a peculiar crystal orientation, in which the plate planes of talc particles are aligned parallel to the molding surface, the c- and a*- axes of polypropylene crystals are bimodally oriented to the flow direction, and the b-axes are oriented to the thickness direction. If they are melt-recrystallized, although the orientation of talc and the b-axis orientation of polypropylene crystals are restored, the c- and a*-axes become unoriented in a plane parallel to the molding surface. This peculiar crystal orientation cannot be explained by any effect of an epitaxy caused by lattice matching between the cleavage plane of talc and (040) plane of polypropylene crystal, a crystallization nucleation effect, a concentration of stress or melt orientation, or their synergisms. From the facts that, even if talc-filled polypropylene injection moldings are melt-recrystallized, the b-axis orientation to the thickness direction is restored and that an injection molding of polypropylene filled with only 0.5 wt.-% talc already shows the peculiar crystal orientation, it is assumed that talc influences a remote interaction on the crystallization of polypropylene so that the (040) planes of polypropylene crystals pile on or the b-axes are oriented perpendicular to the plate planes of talc particles.

While there are several alternatives for producing thermoplastic sheet composites, only two have the characteristics needed for large scale manufacturing; continuous melt impregnation, and deposition of a polymer-impregnated, fiber-dispersed mat from an aqueous slurry. The most important factor affecting the melt impregnation process was found to be the rate of penetration of the molten polymer into the fiber structure. While Darcy's Law, using the Kozeny-Carmen permeation coefficient, provides an initial estimate of penetration rate, it does not adequately consider the slow flow around closely spaced fibers. The slurry deposition process incorporates the polymer directly within the fiber structure, and is not flow rate limited. The most important parameter affecting the consolidation of slurry formed mats was found to be the rate of heat transfer prior to consolidation. In both processes the time of contact between the molten polymer and glass fiber was shown to be important to the development of good adhesion.