Agitating – Rubber or heavy plastic working – Stirrer is through-pass screw conveyor
Reexamination Certificate
2002-12-05
2004-03-23
Cooley, Charles E. (Department: 1723)
Agitating
Rubber or heavy plastic working
Stirrer is through-pass screw conveyor
C366S081000
Reexamination Certificate
active
06709147
ABSTRACT:
TECHNICAL FIELD
The present invention relates generally to machines for extrusion of materials and more particularly to screw extruders adapted for use with plastics and plastic-like materials. The inventor anticipates that primary application of the present invention will be for the manufacture of color concentrates, polymer blends, and polymer alloys or articles produced by mixing polymers with concentrates, fillers, other polymers, additives, and the like.
BACKGROUND ART
A screw extruder is a machine in which material, usually some form of plastic, is forced under pressure to flow through a contoured orifice in order to shape the material. Injection molding machines utilize extruders to force materials under pressure into a mold cavity. Screw extruders are generally composed of a housing, which is usually a cylindrical barrel section, surrounding a central motor-driven screw. At a first end of the barrel is a feed housing containing a feed opening through which new material, usually plastic particles, is introduced into the barrel. The screw contains raised portions called flights having a larger radial diameter than the screw's central shaft and which are usually wrapped in a helical manner about the central shaft. The material is then conveyed by these screw flights toward the second end of the barrel through a melting zone, where the material is heated under carefully controlled conditions to melt the material, and then passes through a melt-conveying zone, also called a pumping zone. The melted plastic is finally pressed through a shaped opening or die to form the extrudate.
Besides conveying material toward the die for extrusion, the screw is depended upon to perform mixing of the feed material. Very generally, mixing can be defined as a process to reduce the non-uniformity of a composition. The basic mechanism involved is to induce relative physical motion in the ingredients. The two types of mixing that are important in screw extruder operation are distribution and dispersion. Distributive mixing is used for the purpose of increasing the randomness of the spatial distribution of the particles without reducing the size of these particles. Dispersive mixing refers to processes that reduce the size of cohesive particles as well as randomizing their positions. In dispersive mixing, solid components, such as agglomerates, or high viscosity droplets are exposed to sufficiently high stresses to cause them to exceed their yield stress, and they are thus broken down into smaller particles. The size and shape of the agglomerates and the nature of the bonds holding the agglomerate together will determine the amount of stress required to break up the agglomerates. The applied stress can either be shear stress or elongational stress and generally, elongational stress is more efficient in achieving dispersion than is shear stress. An example of dispersive mixing is the manufacture of a color concentrate where the breakdown of pigment agglomerates below a certain critical size is crucial. An example of distributive mixing is the manufacture of miscible polymer blends, where the viscosities of the components are reasonably close together. Thus, in dispersive mixing, there will always be distributive mixing, but distributive mixing will not always produce dispersive mixing.
In some extrusion processes, the need for good dispersive mixing is more important than for distributive mixing. This is particularly true in the extrusion of compounds which contain pigment agglomerate that must be reduced in size.
In screw extruders, significant mixing occurs only after the polymer has melted. Thus, the mixing zone is thought of as extending from the start of the melting zone to the end of the extrusion die. Within this area there will be considerable non-uniformities in the intensity of the mixing action and the duration of the mixing action, both in the barrel section and in the extrusion die. In molten polymer, the stress is determined by the product of the polymer melt viscosity and rate of deformation. Therefore, in general, dispersive mixing should be done at as low a temperature as possible to increase the viscosity of the fluid, and with it, the stresses in the polymer melt.
Fluid elements are spoken of as having a “mixing history”, which refers to the amount of elongational and shear stress to which it has been exposed, and the duration of that exposure. A polymer element that melts early in the melting zone process will have a more significant mixing history than one that melts near the end of the melting zone.
Generally, in an extruder with a simple conveying screw the level of stress or fraction of the fluid exposed to high stresses is not high enough to achieve good dispersive mixing. Distributive mixing is easier to achieve than dispersive mixing, but unmodified screws have also been found to produce inadequate distributive mixing for many applications. Therefore, numerous variations in screw design have been attempted in prior inventions to increase the amount of distributive or dispersive mixing in screw extruders. These devices usually contain a standard screw section near the material input hopper, and one or more specially designed sections to enhance mixing. These mixing sections naturally fall into the categories of distributive and dispersive mixing elements although some mixing devices achieve both distributive and dispersive mixing.
Prior mixers that have attempted to improve distributive and dispersive mixing are shown in
FIGS. 3-6
(prior art). Three mixers, the Cavity Transfer Mixer (CTM), the Twente Mixing Ring (TMR), and the Kneader, are discussed below.
The Cavity Transfer Mixer (CTM)
FIG. 3
(prior art) shows the geometry of the CTM. It consists of a screw extension with hemi-spherical cavities and a barrel extension that also contains hemi-spherical cavities. The screw rotates and the barrel is stationary. The fluid passing through the mixer flows from a screw cavity to a barrel cavity and back to another screw cavity. This action repeats itself several times as the fluid passes through the mixer. The CTM was a significant development because it was able to improve the mixing capability of single screw extruders (SSE) significantly. The reason for the efficiency of the CTM is the multiple reorientation events that occur when the fluid moves from a cavity in the screw to a cavity in the barrel.
The CTM suffers from several practical drawbacks that have limited the commercial success of this mixer. Some of these disadvantages are:
1. The mixing section has no forward pumping capability; as a result, it is a pressure consuming element of the extruder and this will tend to reduce the extruder output and increase the polymer melt temperature.
2. The barrel has hemi-spherical cavities in the CTM section. This means that a separate CTM barrel section has to be installed—this increases the cost of the mixer substantially and also complicates the installation of a CTM.
3. The barrel surface is no longer completely wiped by the screw. Polymer melt will enter the barrel cavities and the polymer melt flow in the bottom of the cavities can be very slow. As a result, when a change in material is made (e.g. from white to red) it can take an inordinately long time for the old material to disappear in the extruded product. Therefore, in many cases the mixer has to be physically cleaned when a material change is made. This cleaning can be quite time consuming and results in lost production. This can be a distinct disadvantage when frequent material changes are made.
The Twente Mixing Ring (TMR)
The TMR (
FIG. 4
, prior art) was developed at Twente University by Semmekrot. The TMR uses the same principle of mixing as the CTM; however, instead of using cavities in the barrel it uses a floating annular mixing ring or sleeve with holes bored into it. The mixing sleeve rotates with the screw but at a lower rotational speed and this provides the relative velocity between the screw cavities and the sleeve cavities. Thus, the TMR eliminates an important drawback of the CTM, the cavi
Cooley Charles E.
Rauwendaal Extrusion Engineering, Inc.
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