Band 6, Heft 3

Automotive side protection moldings, which consist of polymer and an inserted metal core, are produced by profile extrusion coating. The molding process is similar to wire coating process except using a cross-head profile die because the cross-sectional shape of the side protection molding is complex. The polymer is extruded from the profile die covering the metal core, which is fed to the die simultaneously. Then, there are two flow regions within the profile die. One is pressure flow region where the polymer is flowed by pressure gradient. Another is drag flow region including pressure flow. The drag flow appears in the area where the polymer contacts the moving metal core in the die. Finite element analysis was developed for simulating the combined pressure and drag flow in a complex profile die. The flow is assumed to be isothermal planner flow for a non-Newtonian fluid. Power law model is used to represent viscosity of the polymer. Distributions of pressure and velocity are simulated by the analysis. Polymer melt flow through a flow channel of a die for producing the side protection molding was analyzed to achieve uniform velocity at the outlet of the die. A suitable shape of the flow channel was designed by several iterative simulations.

In recent years, many applications of process control to extruders have concentrated on the control of temperature and pressure in the die. This study investigates the simultaneous control of the two-dimensional characteristics of a tubing. An identification and control package is used. The system is shown to be non-interacting.

A ‘Stick-Slip’ mixed-boundary model is postulated to support the mechanism of transition from no-slip to slip at the polymer/die interface during capillary extrusion of polymeric materials at and above a critical shear rate at a fixed throughput. The predicted shear stress singularity explains the occurence of gross extrudate distortion known as melt fracture.

This research was concerned with the effect of two different plasticizers on the plasticating extrusion behavior of polyvinylchloride (PVC). Diisodecyl pthalate (DIDP) and dihexyl pthalate (DHP) were the the two plasticizers used in this study. First some differences in the extrusion performance of the two differently plasticized PVC compounds were identified. In particular, it was observed that pressure build-up and power requirement were affected by the plasticizer type with the DIDP plasticized materials generating higher pressures and requiring more power than the DHP plasticized materials. Also the DIDP plasticized material fused earlier in the screw channel and in general exhibited slightly higher flow rates. The differences in extrusion characteristics were observed for two different dies attached to the extruder.

Flow-induced crystallization of high-density polyethylene and polypropylene films has been studied in a slit die equipped with optical windows to allow direct visual observation of the process. Mechanical and thermal property measurements and morphological analyses verify the presence of self-reinforced, extended-chain structures in the extrudate. By controlling the location of the crystallization front inside the die but near the exit, continuous extrusion was possible at lower pressures than previously reported. Birefringence measurements of the melt flow indicated the feasibility of an optimal die geometry to maximize oriented structure development during continuous extrusion while minimizing pressure buildup.

An off-line experimental simulation of cooling and warpage of rotationally molded parts is described. Slabs of polyethylene rotationally molded parts containing thermocouples and monitored by displacement transducers are placed on a metal surface representing a mold wall. This is cooled by varying methods such as simulated water showers, fan blown air, and natural convection cooling. Temperature profiles in the mold wall and through the polymer are determined as a function of time. The position of crystallization and the development of warpage are observed. Warpage is attributed to thermal quench stresses and release of the polymer from the mold surface.

Extrusion blow molding is utilized in the production of plastic hollow containers. One of the critical steps in the process sequence is the formation of an annular extrudate, called the parison. Extrusion of the parison through an annular die is associated with varying degrees of swell and sag along the length. Moreover, the design of many commercial blow molding products requires variations in wall thickness along the length of the parison. The present work describes the design and operation of an on-line computerized parison thickness profile measurement technique for the purpose of implementing closed loop control of the parison thickness profile. The proposed thickness measurement technique was evaluated by comparing it to the conventional pinch-off mold technique. The control strategy employed involves a simple on-line adaptive control scheme. Programmed set point profiles, such as constant, linear and oscillatory profiles were implemented to within a 5% error. Moreover, the adaptive controller compensates for system disturbances.

Dynamic viscoelastic properties of two samples of co-polyesters, PHB60 and PHB28, were measured over a wide range of temperatures. The copolyesters had been synthesized from polyethylene terephthalate (PET) and p-hydroxybenzoic acid (PHB). PHB60 and PHB28 contain 60 and 28 mol.-% PHB as the comonomer, respectively. PHB60 is well known as a thermotropic liquid crystalline copolyester. The effect of thermal histories on viscoelastic behavior of the copolyesters are discussed in terms of structure and liquid crystallinity of the copolyesters. A rheological transition in viscoelastic properties of the copolyester PHB60 was observed at about 270°C in the fluid state. The viscoelastic properties of the copolyester PHB60 strongly depend on thermal histories below the transition temperature, but not above the temperature. The transition behavior was attributed to a crystal-nematic transition by wide angle X-ray diffraction and differential scanning calorimetry. The attainable crystallinity was fairly low. Thermal history behavior found below the transition temperature is caused by slow crystallization and supercooling of the sample. PHB28 forms no liquid crystal phase, and is thermorheologically simple in the molten state: the sample showed no effect of thermal history in viscoelastic properties.

The relationship among processing parameters, molecular orientation, and modulus of a thermotropic liquid crystalline copolyester synthesized from poly(ethylene terephthalate) and 60 mol.-% p-hydroxybenzoic acid was systematically investigated so that the dominant processing factors to result in high orientation and high modulus were elucidated. Solid filaments were prepared by shear flow in capillaries and elongational flow by spinning from the liquid crystalline phase of the copolyester at various processing conditions. Superstructure, especially molecular orientation, of the filaments was analyzed by wide angle X-ray scattering (WAXS), scanning electron microscopy (SEM), and polarizing optical microscope (POM). Also, tensile modulus of the filaments was determined. Although filaments prepared by means of shear flow in capillaries had the Hermans orientation function of about 0.3, tensile modulus and morphology by SEM and POM did not exhibit the evidence of high orientation of the filaments even at high ratios of length over diameter and high shear rates. In the elongational flow by spinning, the dominant processing parameters for molecular orientation and increasing of tensile modulus are a spin-draw ratio and an initial diameter of the filament. The small diameter results in high orientation and high modulus. Enhancement of modulus was independent of the processing conditions related to a relaxation process of molecular orientation, such as spinning temperature, strain rate, and cooling. These results imply that there exists a size effect for the orientation process.

By investigation of adhesion following the harmonic mean method after Wu a suitable pair out of seven liquid crystalline (LCP) and conventional polymers was established in order to produce a LCP-matrix compound. Using a bidirectional piston mixer with a steel wire mesh with mesh size of 63 μm, 5% of the picked out PHB-PET copolymer was finely dispersed into the PET matrix polymer in the molten state. As a result a dispersion of the initial volume of the LCP granules of 1 mm 3 into small particles of 0.5 to 1.0 μm 3 as well as a very uniform local distribution was achieved which can be considered as an argument for the compatibility of the selected pair and as a confirmation of the usefulness of the preceding investigations of adhesion.

Rheological and mechanical properties, structural characteristics, extrusion and injection molding behavior of blends of polycarbonate (PC) with a thermotropic, wholly aromatic copolyesteramide have been investigated. The processability of polycarbonate is remarkably improved by addition of small contents of liquid crystal polymer. All the flow curves of the blends lie between those of the pure components. In the processing conditions adopted, the LCP particles are easily elongated in the flow direction. In this direction the elastic modulus rises remarkably with LCP content. The modulus-composition curve is, however, lower than that expected on the basis of an additive rule due to the incompatibility and poor adhesion between the two phases. Finally, injection molding of this extrusion grade PC has been easily performed by adding small contents of LCP. The impact strength values of the injection molding samples of these blends are only slightly lower than that of the pure PC.