Frederick C. Weissert

Flow patterns in carbon black filled polyethylene at the entrance to a die

Abstract Flow patterns in the entrance region of dies with various entrance geometry configurations have been investigated for low density polyethylene filled with carbon black. The experiments were carried out using samples with titanium dioxide markers in the barrel of an Instron capillary rheometer using dies with varying entrance geometries. The material in the barrel was cooled under pressure, removed and longitudinally sectioned. Samples with 0–5 volume percent carbon black exhibited in 180° entrance angle dies corner vortices typical of low density polyethylenes. At higher volume loadings, no vortices were observed. All longitudinal sections of these systems showed streamline motion directed towards the die entrance. This was found to be the case for blacks of varying particle size and different die entrance geometries. Purging experiments using initially white (CaCO 3 or TiO 2 ) filled regions at the die entrance revealed only very small stagnant regions in the corners of 180° entrance angle dies, 270° diverging entrance angle dies were also shown to exhibit small stagnant regions in the diverging parts of the die.

Flow Patterns in Elastomers and Their Carbon Black Compounds during Extrusion through Dies

Abstract A basic study of flow patterns in elastomers in the entrance region of a die has been carried out for various gum elastomers including emulsion and solution butadiene—styrene copolymers, polybutadiene, and natural rubber. All exhibit streamline flow into the entrance with the exception of a cold mastication degraded natural rubber which gave evidence of vortices in corners. A study of a die with a sharp diverging region showed dead spaces for all the elastomers. Carbon black compounds all exhibited converging streamline flow in a 180° entrance angle die and stagnant regions in the sharply diverging die. Evidence based on marker motions has been presented for slip in elastomer compounds in the entrance region.

Modelling of shear viscosity behavior and extrusion through dies for rubber compounds

Abstract This paper marks a first effort to develop a fundamental basis for die flow analysis and design for rubber compounds. We have accomplished a modelling of the shear viscosity function and its application to one- and two-dimensional shearing in die cross sections. There are major limitations in what we have done, much of which is apparent even in the early work of Mooney. In particular, we have not considered (i) slip phenomena on die walls, (ii) die entrance and exit pressure losses associated with converging and diverging dies, nor (iii) extrudate swell. We have an active research in our laboratories investigating these problems. In the future, we are seeking to generalize the procedures described in this paper to more complex die designs. Inclusion of entrance and exit effects and rigorous analysis of coathanger dies requires the handling of elongational flow contributions, a still unsolved problem.

Air-Lubricated Die for Extrusion of Rubber Compounds

Abstract The flow of a rubber compound through a die is an essential step of a rubber extrusion operation. The die forms the compound into the desired shape. In many cases, the output of the extruder is limited by the resistance of flow in the die. Too high pressure losses in the die may additionally result in an excessive increase of the temperature, and unwanted crosslinking of the rubber compound may occur. Various possibilities are available to reduce the resistance of flow in the die. The simplest one is elevation of the processing temperature, which leads to reduction of viscosity. This can be done, however, only within some limit. Reduction of the pressure losses in the die may also be achieved by decreasing the die length, but this option produces larger swelling. Other more innovative procedures may be suggested. Evidence exists that the pressure losses in the die can be reduced by introducing sound or ultrasonic waves of high intensity into the die zone. In this paper, we present what we believe...

Comparison of Screw Extrusion of Rubber Compounds with Different Extruder/Screw Combinations

Abstract The extrusion of rubber compounds has long been an area of industrial importance. There have, however, been few basic studies on the flow characteristics of rubber compounds. Considerably more extensive investigations have been reported on the mechanisms of screw extruders for thermoplastics. The latter researches are summarized in the monograph by Tadmor and Klein and the textbook of Tadmor and Gogos. Maddock, and others carried out experimental studies which led to basic flow mechanisms of thermoplastics in screw extruders. Maddock fed pigmented pellets to screw extruders and pulled screws from the barrel with the plastic stips in it, which were then unwound and cross-sectioned. Few screw-pulling investigations have been published for rubber compounds. We are only familiar with the work of Menges and Lehnen and our own recent studies. In the present paper, we describe a fundamental study of flow mechanisms in screw extruders for rubber compounds. We expand our studies based on the Maddock screw...

An experimental and theoretical study of heat conduction and vulcanization of rubber compounds in molds

Abstract A fundamental study on curing of rubber compounds in molds is presented. We have measured the thermal conductivity of a range of rubber compounds determining the influence of carbon black, other fillers, and oil. The heats of reaction associated with the curing kinetics of model compounds were measured. A mathematical model is proposed to predict the temperature profiles for curing a reactive slab. This involves inclusion of an energy generation rate, which depends on time and temperature. This is expressed through a Damkohler number. Solutions of the heat conduction equation are interpreted in terms of the Fourier number and the Damkohler number. Calculations are carried out using experimentally determined thermal conductivities and curing kinetics. Thick parts are shown to heat up more slowly (associated with the Fourier number) and to show greater overshoots of cure temperature (associated with the Damkohler number).

An Experimental Study of Flow and Stress Fields in a Pressurized Mooney Viscometer

Abstract The boundary conditions of rubber compounds on solid surfaces during extrusion has long been a subject of question. The concerns of Mooney on this topic date back 60 years. He continued to return to this question throughout his career. In more recent years Vinogradov and his coworkers have called attention to slip phenomena occurring in the extrusion of elastomers through dies. This is associated with fluctuating pressure losses which occur at the onset of extrudate distortion. Such behavior has also been observed with rubber-carbon-black compounds. More recently, Turner and Moore have developed a pressurized rotational rheometer to characterize this type of behavior in rubber compounds. We have recognized the importance of this problem in our own laboratories. Marker experiments have been used in extrusion of rubber compounds to detect slippage. In the present paper, we describe an experimental study of the response of gum elastomers and their carbon black compounds in a pressurized Mooney visco...

Experimental and Theoretical Study of the Flow Characteristics of Rubber Compounds in an Extruder Screw

Abstract We describe an experimental and theoretical study of the mechanisms of flow of rubber compounds in a screw extruder. Two compounds, a typical synthetic rubber (butadiene-styrene copolymer-cis-1, 4 polybutadiene) based passenger tire tread (PTT) and a natural rubber based truck tire tread (TTT), were investigated in a 1½ inch NRM cold feed screw extruder using a screw with a constant channel depth and constant channel pitch along the whole screw length. The rubber was cured in place and the screw removed. The screw exhibited starvation behavior except near the die. The length of the rubber strip in the screw is proportional to the pressure developed at the die. Marker studies showed transverse circulating flows beginning in the region immediately downstream from the hopper. These observations indicate that cold feed extruders exhibit a metering region flow, albeit sometimes starved, from entry port to die. Under these conditions the extruder is independent of the pressure developed in the screw channel. A simple non-Newtonian isothermal mathematical model for the metering zone is described. The rubber compound is considered as a pseudoplastic material with a yield value. A comparison of theory and experiment shows that a one dimensional isothermal model quite accurately predicts the flow rate pressure development of the rubber compound in our screw (with a shallow prismatic channel) when starvation is considered. The yield value of the rubber compound influences screw characteristic curves only for very high values of the dimensionless pressure gradient.

Experimental Study of the Flow Characteristics of Rubber Compounds in Extruder Screws

Abstract The application of screw extrusion to rubber compounds probably dates back over a century. Rubber extrusion has been an area of industrial importance throughout this period. Technological innovation continues in this area, as may be seen in a recent review by Johnson. However, there have been only a few basic studies on the flow characteristics of rubber screw extruders. Many more extensive investigations on the working of plastics screw extruders have been carried out. The latter researches are summarized in the volumes by Tadmor and Klein and Tadmor and Gogos. The basic mechanisms of flow of plastics in screw extruders were first revealed in experimental studies by Maddock. He fed the screw with pigmented pellets, pulled it from the barrel with the plastic strips in it, which were then unwound and cross-sectioned. There seem to be few such similar published investigations for rubber compounds. We are only familiar with the work of Menges and Lehnen. It is our purpose in the present paper to ini...

A Rheo-Optical Study of the Influence of Additives on Crystallization Rates in Cis-1,4 Polyisoprene

Abstract A rheo-optical study of the influence of additives on crystallization rates in synthetic cis-1,4 polyisoprene (IR2200) at room temperature is presented. Nine different additives at a 1% concentration were examined. This study has found many additives which seem to be effective in enhancing the rate of stress-induced crystallization in IR2200. We have found that there is a good relation between enhancement of green strength and fraction of stress-induced crystallinity, ϕc, in IR2200 caused by additives.