Volume 11, Issue 3

In this paper we define the extruder analysis in the Eulerian frame. Then, we convert the analysis to the Lagrangian frame to avoid the difficult moving boundary problem. Finally, unlike the classical analysis we convert the solution back to the Eulerian fame before calculation of velocities and flow rates. The single screw pump or extruder has classically been evaluated by leaving the analysis in the Lagrangian reference frame and the flow in the pump is described as due only to down channel drag flow. Since velocity is not frame indifferent, we complete the analysis by transforming the Lagrangian analysis back to the Eulerian solution. The pumping mechanism is quite different when the velocities are properly converted back to the Eulerian frame of reference. This new analysis demonstrates that, if the problem is approached using superposition, the pumping mechanism in a screw pump is due to the “piston like” action of the helix x-component velocity, and that the down channel z-component velocity, drag flow, from the helix and screw core reduce the maximum pump efficiency of the helix by about 50 %.

A theoretical and experimental study of the behavior of a viscous non-Newtonian polymer melt in a List/Buss Kneader is presented. The modelling study for viscous non-Newtonian behavior considers the dynamics of flow including the influences of screw oscillations and interrupted screw flights. Calculations are made using different methods for various individual Kneader elements which consider (i) pumping capabilities, (ii) leakage flows through the slices in interrupted flights, (iii) pressure distributions and flow fluxes. Predictions of pressure profiles, fill factors, and positions of starvation in modular machines are made. An experimental study which uses polypropylene considers the distributions of polymer in modular screws with different screw configurations. The experimental results are compared to the flow simulation.

A new type of pin to be installed in mixing section of screw extruder is proposed based on the results of numerical calculation of flow field and local mixing efficiency. Two problems presented in general pin mixing section, i.e., the lack of randomization in mixing mechanism and the formation of dead spots, can be solved through diagonally-channeling pins from top to bottom. From the results of numerical analysis of local mixing efficiency it is known that in the cross-section of standard screw (in this study called “cavity”) there are only small regions with high mixing efficiency which is mainly near the two upper corners of the cavity. If two pins are put into the cavity, large high mixing efficiency regions occur in the scope from cavity top to about 1/3 flight height. But, in the case of inserting two holed pins into the cavity, high mixing efficiency regions are distributed in nearly all over the cavity. It is impossible to get such useful information by experiments. Even though the calculation was confined in cross-section of pin mixing section, it has directive significance in designing the practical pin mixing section in the light of theory of distributive mixing.

It has been observed previously [1] that under the specific conditions prevailing in a reflux flask reactor, the overall rate of the transesterification reaction between an ethylene and vinyl acetate copolymer and 3-phenyl 1-propanol can be enhanced appreciably by the introduction of boiling inert hydrocarbon solvents. In the present study, the same reaction carried out under a controlled removal of vapors in a simulator emulating a non-intermeshing counter-rotating devolatilization twin screw extruder, a significant acceleration of the reaction rate has also been observed. In agreement with the previous work [1], the intrinsic reaction mechanism and the chemical equilibrium do not appear to be affected by boiling of the inert hydrocarbon solvents, nor does the additional mechanical agitation due to the rapid bubble growth contribute to the increase of the overall reaction rate. Theoretical analysis of the results confirms that the acceleration of the reaction rate originates from the localized mass transfer effect connected to the presence of the bubble vapor-liquid interface films induced by devolatilization.

The morphology of the dispersed phase in immiscible polymer blends can have a significant effect on the final may be omitted physical properties. Laminar dispersion of an ethylene vinyl alcohol copolymer (EVOH) with high oxygen barrier properties was obtained in the matrix of low density polyethylene (LDPE) when extruded through an annular blown film die. A large range of morphology was observed and indicated that the main factors determining the morphology of the blends were the composition, the viscosity ratio of the components, the processing conditions, and the elongational fields at the exit of the die. Increasing amount of EVOH (from 5 to 20 vol %) resulted in increasing number of stacked layers. Highly elongated laminar structure was formed when the viscosity ratio (η d(EVOH) /η m (LDPE) ) was close to one and slightly lower. Large predeformed domains in the extruder outlet (before drawing and blowing) with short residence time under high screw rpm and lower viscosity ratio in the die temperature favored the formation of well-developed laminar structure. Oxygen permeation tests have shown high barrier effects due to the formation of the laminar dispersion which acted as barriers to the permeant molecules by providing a long tortuous path.

SCORIM (shear controlled orientation in injection moulding) is a new technology using pairs of reciprocating pistons connected to the runner system to impose shear in the mould during the holding and packing stages; this provides important control over part properties. A computer simulation is set up, and solved numerically to assist with development and exploitation of this novel process. The one-dimensional transient model considers the cooling and solidification of polymer melt, contained between infinite plane parallel plates, whilst subjected to reciprocating shear flow. A non-Newtonian, temperature-dependent viscosity is used, together with temperature-dependent thermal properties and latent heat of solidification. Two operating modes are considered: (1) flow driven by a fixed pressure gradient; (2) an initially fixed flowrate, which falls after a limiting pressure gradient is reached. Results are shown, based on data for a moulding grade polypropylene; these include details of temperature and velocity fields, pressure and flowrate profiles, and frozen-in material strains, which are related to material alignment. In mode (1) operation, flowrate falls rapidly as solidification proceeds. This is not hindered significantly by viscous heating. Frozen-in strain has its maximum value close to the wall, and falls in a series of oscillations to zero on the central plane. In mode (2), if the limiting pressure gradient is too high, viscous heating can prevent cooling, leading to a dynamic thermal equilibrium. In other cases, where cooling is completed, frozen-in strain can show a complex, oscillatory profile, corresponding to a layered structure in the moulding with alternating regions of higher and lower alignment.

The effect of processing conditions on the mechanical properties of an injection molded polypropylene/liquid crystalline polymer (PP/LCP) blend was studied. The LCP consisted of a copolymer based on poly(ethylene terephthalate) and poly-(hydroxybenzoic acid). Two PP matrices were used to vary the viscosity ratio of the blend. It was found that the highest flexural moduli were obtained for blends with the PP matrix that provided the highest viscosity ratios (closest to unity). These optimal properties were obtained only at one processing temperature, 250°C. At this temperature, the storage (G′) and loss moduli (G″) of the neat LCP were approximately equivalent. The optimal properties at temperatures where G′ ∼ G″ is in agreement with a prior study of another PP/co-polyester LCP blend [15]. Futhermore, the optimal properties were sensitive to the filling time. This was related to heat transfer via the Graetz number. For thinner molds, higher properties are obtained over a broad range of Graetz numbers (temperatures) while for thicker molds higher properties are obtained only at lower Graetz numbers (temperatures). The dependence of the flexural modulus on the Graetz number was related to the ability to deform the LCP at the advancing front and the ability to freeze the morphology possibly even before cessation of flow.

The β-crystal nucleating effect on polypropylene has been compared between γ-quinacridone and quinacridonequinone. While the β-crystal does not form at γ-quinacridone content below 1 ppm where red coloring is recognized on the molded article, much β-crystal forms even at quinacridonequinone content below 1 ppm where brown coloring is not recognized on the molded article. This means that the β-crystal nucleating ability of quinacridonequinone is superior to that of γ-quinacridone. However, both nucleators show a similar change in the physical properties of injection molded article against nucleator content.

Analysis of injection moulded parts made of polystyrene reinforced with spherical glass particles of various sizes reveals the existence of a number of interesting microstructural features. These include the segregation of particles towards the centre-plane of the cavity, their accumulation close to the free surface and the subsequent occurrence of short-shots, the formation of particle-rich layers adjacent to the perimeter of the moulding and the formation of monolayers of particles attached to the cavity surfaces. Some of these features are consistent with and can he explained by the flow patterns associated with the mould-filling process, namely a Hele-Shaw flow on the plane of the cavity and a fountain flow near the advancing free surface. A mechanism is proposed, according to which the segregation of particles at the cavity mid-plane and their accumulation towards the free surface is the result of the combined action of convection, fountain flow and lateral particle migration. The configuration of the gate is shown to have a marked effect on the observed microstructures in edge-gated mouldings, depending on the ratio of the gate size over the particle size, as is clearly illustrated through X-ray radiography in polydisperse systems. As a result of particle accumulation at the free surface, the formation of particle-rich weld lines in mouldings with inserts is also observed through X-ray radiography. The observed microstructures are shown to be strongly influenced by the size of the particles.

This paper examines the development and consequence of flow structures and possible defects arising in multiple shot injection or multiple-gated injection of two-component resins mixed in-line into a mold with and without fiber preforms. This work may also be relevant to the packing stage. Experiments were run where a premixed batch of vinyl ester resin with peroxide and tracer dye, displaces another older batch of premixed resin in a mold. Polymerization of resin as it flows through the preform during the filling stage sets up an increasing viscosity profile along the fluid filled region and viscous fingering is observed as expected. The types of flow patterns associated with this phenomenon have been explored with random and woven fibrous media packed in a clear mold. The molded plaques were cut up and tested in 3-point bend so that the flexural moduli of specimens with various flow structures could be compared. The flow structures observed include fingers with rounded ends and fingers with multiple tips. Such flow structures lead to regions in the plaques where ‘packets’ of fluid with different extents of polymerization due to differences in age are juxtaposed prior to gelation of the resin. It was found that finger structures with several sharply defined interfaces tended to decrease the value of the flex modulus by up to 32 %, depending on the preform characteristics. Multiple finger tips with diffuse boundaries do not seem to be detrimental to the flex modulus, but multiple tips with sharp boundaries have lowered the flex modulus by 32%.