Patent Application
20110182133
The invention relates to twin screw extruders and particularly to a method of operation of a twin screw extruder with shafts rotating in the same direction.
Twin screw extruders having a feed opening at one end and a discharge opening at the other end are well known and extensively used for specific purposes of mixing materials which includes kneading, dispensing, homogenizing, distributing a material with or without increase in temperature or heat input in a continuous and controlled manner. Material is fed to the extruder by a hopper connected at the inlet and the extruder is typically operated on the so called ‘starve feeding’ methodology.
The material may consist of a single constituent or ingredient or multiple constituents in one phase or more phases. Some constituents may need to undergo phase transformation during processing. The work that is done by the extruder is the result of applications of forces that cause shearing action in various planes, compression, elongation and folding. It is desirable that these forces are applied uniformly on every particle or molecule only for a specific period of time in order to limit the work done as excessive work will cause fragile molecules to disintegrate resulting in a degraded product. The extent of work done is dependent on a number of factors including nature of the material being processed, amount of material that the extruder can receive as input at various ports, amount of torque available at any given speed, length of the extruder, and configuration of the extruder processing zone elements and components.
It is believed that an increase in shear rate enhances the quality of mixing and homogenization, though an increase in shear rate results in an increase in the specific energy requirements and temperatures. To balance this increase in the specific energy requirements and temperatures, it is essential to reduce the dwell time within the extruder and increase the through put. One approach has been the increasing of the volumetric ratio of the extruder and also by significantly increasing the minimum specific torque requirements. The volumetric ratio of an extruder is defined as the ratio of the extruder diameter [D] divided by the root diameter of the screw [d]. An increase in the volumetric ratio increases the fill capacity of the extruder though adversely affects the torque carrying capacity. The specific torque [T/a3] of an extruder refers to the ratio between the torque [T] and the third power of the center distance between the two axes [a] of the twin screw extruder. To increase the specific torque requirements a higher powered motor is required which requires corresponding strengthening of other components inside the gearbox and the torque transmitting system. The increase in volumetric ratio and specific torque requires various modifications involving a combination of design, materials, treatment and higher levels of safety.
The invention relates to a method of extruder operation; the extruder comprising a housing having at least two cylindrical housing bores, each housing bore having an axis disposed parallel to the other axis and the first and second axes separated by a distance “a”; at least a first screw shaft and a second screw shaft being disposed in the first and second housing bores; the first and second screw shaft being provided with a first and second screw respectively; the first and second screw each having an extruder diameter D and a screw root diameter d; the first and second screw shaft each having a volumetric ratio of at least 1.4 wherein the volumetric ratio is defined by the extruder diameter D divided by the screw root diameter d; at least two inlet hoppers being provided in the vicinity of the housing bores and opening into said housing bore with at least one of the inlet hopper having a side inlet into the housing bore; the method comprising: feeding material to be processed through the at least two inlet hoppers into the housing bores; conveying the material through the extruder length from the inlet hoppers to the extruder outlet; driving each of said first and second screw shafts at a speed of at least 1000 rpm and at a torque density of at least 8.5 Nm/cm3; the torque density defined by the shaft torque at each first and second shaft divided by the third power of the distance ‘a’ of the first and second axis.
The invention also relates to a method of extruder operation; the extruder comprising a housing having at least two cylindrical housing bores, each housing bore having an axis disposed parallel to the other axis and the first and second axes separated by a distance “a”; at least a first screw shaft and a second screw shaft being disposed in the first and second housing bores; the first and second screw shaft being provided with a first and second screw respectively, each screw including at least one element; the first and second screw each having an extruder diameter D and a screw root diameter d; the first and second screw shaft each having a volumetric ratio of at least 1.4 wherein the volumetric ratio is defined by the extruder diameter D divided by the screw root diameter d; a first intake element coupled to the first shaft and including a first flight, a second intake element coupled to the second shaft adjacent the first intake element and including a second flight, wherein the first flight and the second flight are adapted to co-operatively create a positive conveying effect to convey material when the first intake element and the second intake element are rotated in the same direction; at least two inlet hoppers being provided in the vicinity of the housing bores and opening into said housing bore with at least one of the inlet hopper having a side inlet into the housing bore; the method comprising: feeding material to be processed through the at least two inlet hoppers into the housing bores; conveying the material through the extruder length from the inlet hoppers to the extruder outlet; driving each of said first and second screw shafts at a speed of at least 1000 rpm and at a torque density of at least 8.5 Nm/cm3; the torque density defined by the shaft torque at each first and second shaft divided by the third power of the distance ‘a’ of the first and second axis.
The accompanying drawings illustrate the preferred embodiments of the invention and together with the following detailed description serve to explain the principles of the invention.
In the following description, for purpose of explanation, numerous specific details are set forth in order to provide an understanding of the various embodiments. It will be evident, however, to one skilled in the art that the various embodiments may be practiced without these specific details. It will be appreciated that the various embodiments discussed herein may or may not be the same embodiment, and may be grouped into various other embodiments not explicitly disclosed herein.
It will be understood by those skilled in the art that the foregoing general description and the following detailed description are exemplary and explanatory of the invention and are not intended to be restrictive thereof. Throughout the patent specification, a convention employed is that in the appended drawings, like numerals denote like components.
A method of extruder operation is disclosed for a twin screw extruder. With reference to
The extruder in accordance with an aspect of the invention has at least two hoppers for feeding material to the extruder. At least one side hopper is provided to feed additional material to the extruder. With reference to
In accordance with an embodiment, one hopper may be force fed while one hopper is gravity fed. Alternatively, both hoppers may be force fed to achieve the desired feed rate into the extruder. In the embodiment illustrated in
The method of extruder operation provides for side feeding the extruder at least at the inlet end of the extruder. Side feeding the extruder at the inlet allows the extruder to run at the feed rate determined by the available torque in the extruder. The method of side feeding the extruder in the manner as described herein allows complete control over the amount of material that the extruder can receive at lower diameter ratios and lower specific torque levels.
The method of extruder operation as described requires the extruder to have a volumetric ratio of at least 1.4 wherein the volumetric ratio is defined by the extruder diameter D divided by the screw root diameter d. The extruder includes at least two inlet hoppers being provided in the vicinity of the housing bores and opening into said housing bore with at least one of the inlet hopper having a side inlet into the housing bore. Material is to be fed to the extruder through these hoppers including the at least one side hopper. The method also requires that each of said first and second screw shafts are driven at a speed of at least 1000 rpm and at a torque density of at least 8.5 Nm/cm3.
In accordance with another aspect of the invention, the extruder uses elements for the twin screws that provide a positive conveying effect.
With reference to
Undercuts (32) are defined by the flights (f) of the first and second elements as illustrated by
The method of extrusion in accordance with an aspect requires elements as described above to be used in the extruder that is run at a speed of at least 1000 rpm. In addition the volumetric ratio of the extruder is at least 1.4 and the extruder is fed by at least one side inlet. This method of operation significantly reduces operating costs while providing the desired shear rates and through put.
A 21.1 mm center distance extruder with a number of D/d ratios ranging from 1.27 to 1.89 is studied for volumetric capacity and maximum possible power at prescribed specific torque, and results of the study are tabulated in Table 1. Based on this information, mean residence time is calculated at 50% degree of fill and the results of this calculation are tabulated in Table 2. Generally applications are torque limited and residence time is calculated for D/d=1.4 and 1.49 and compared with D/d=1.55 and 1.71. With an increased mean shear intensity (D/f), the residence time is comparable at lower specific torque.
By the use of special elements as explained in the previous section, further control over the extrusion can be achieved for extruder with D/d>1.4, by overcoming feed limitation as well as achieving higher torque limited outputs that substantially exceed outputs of D/d>1.5 extruders as shown in Table 3.
The method provides for higher throughput machines with greater torque carrying capability. The method does not demand design restrictions and allows for smaller sized machines to deliver higher throughput without comprising on mixing efficiency, thereby providing reduction in floor space requirement and ease of operability. The elements as described above along with the additional side feeding overcome feed limitations of the extruder and allow for greater throughputs without requiring an increase in volumetric ratio or in the torque density requirements.
The method also allows to use extruders with the highest torque densities with specific torque of 17 Nm/cm3 and higher. As a result of this, higher outputs, a better product quality and better utilization of resources will result.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2383/CH3/2008 | Sep 2008 | IN | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/IN08/00861 | 12/24/2008 | WO | 00 | 3/29/2011 |
