HK1125593B - Mixing and kneading machine for continual compounding and operating method thereof - Google Patents

Description

Mixing kneader for continuous mixing and method for using same

Technical Field

The invention relates to a mixing kneader for continuous mixing, comprising: a screw shaft rotating and simultaneously axially translating within the housing. The invention also relates to a method for achieving continuous mixing using said mixing kneader.

Background

The mixing kneader which is currently concerned is used in particular for large-scale flowable plastics and/or pasty materials. For example: they are used for processing viscous plastic masses, homogenizing and plasticizing plastics, mixing fillers and reinforcing additives and for food, chemical/pharmaceutical industries and generally also for raw material production involving a uniformly carried out continuous degassing, mixing and expansion process. But does not include the treatment of carbon electrode materials for aluminum production and the treatment of hydrofluoric acid. In some cases, mixing kneaders may also be used as reactors.

The working parts of the mixing kneader are usually arranged as so-called screws which axially advance the material to be processed.

In conventional mixing kneaders, the working parts only produce a rotating action. In addition, it is also known that the working parts within a mixing kneader rotate with a translational movement. A particular feature of the motion profile of the working element is the sinusoidal motion performed by the spindle during rotation. This motion profile allows for nesting of suitable components such as stirring pins or stirring teeth. For this purpose, the screw is threaded to receive discrete kneader blades. The screw flights located on the main shaft interact with the appropriate parts of the set-up and thus establish the required shearing/mixing and kneading functions in the various different processing zones. Such trade name of the above type is BussMixing kneaders of (a) are well known to those skilled in the art.

The screw shaft diameters in known mixing kneaders of the above-mentioned type are up to 700mm, and the throughput is in each case determined primarily by the screw shaft diameter. In general, the ratio of the outer diameter (Da) of the screw shaft to the inner diameter (Di) of the screw shaft is about 1.5, while the ratio of the outer diameter (Da) of the screw shaft to the stroke (translation portion) (H) is about 6.7, and the ratio of the pitch (axial pitch of the screw blades) (T) to the stroke (H) is about 2. Depending on their volume, the mixing kneader can be operated at speeds of 5 to 500 rpm.

Mixing kneaders are generally designed according to the principles of geometric similarity. This principle is always valid, whatever the size, as long as the ratios Da/Di, Da/H and T/H are constant.

Factors that determine the quality of the processed product being dispersed, mixed and homogenized are the melt temperature, the residence time of the product in the machine processing zone, the shear rate and the number of shear cycles in the screw channel/processing zone.

In many processing applications, the more harmonious the transfer, shear rate levels and packing between successive processing zones such as the feed, melt, mix, disperse and discharge zones, the better the product is mixed, dispersed and homogenized. In the current state of the art of mixing kneaders, a common indicator for standard mixing is an average shear rate in the melting range of between 151/s and 1501/s and an average product residence time over the entire screw extension of between 30s and 600 s.

In conventional mixing kneaders, the average shear rate is defined maximally by the rotational speed of the screw and the ratio Da/Di. However, increasing the shear rate also results in a greater amount of specific energy input and thus unacceptably high melting temperatures. As well as the average residence time of the product in the mixing kneader being too long, too high a shear rate may also lead to degradation (thermal degradation or crosslinking) of the product and thus to a reduction in quality.

Disclosure of Invention

The invention is based on the object of improving a mixing kneader in such a way that the efficiency, defined by the throughput of material per unit time, can be increased without a significant reduction in the quality of the processed product.

This object is achieved by a mixing kneader, i.e. a mixing kneader for continuous mixing, comprising a screw shaft rotating in a housing and simultaneously axially translating, the screw shaft having a plurality of helically arranged screw blades, the mixing kneader being provided with stirring pins fixed to the housing and extending into a processing space; the method is characterized in that: the geometry of the mixing kneader is selected such that the ratio Da/Di of the external diameter Da of the screw shaft to the internal diameter Di of the screw shaft is between 1.5 and 2.0, the ratio Da/H of the external diameter Da of the screw shaft to the stroke H is between 4 and 6, and the ratio T/H of the screw pitch T to the stroke H is between 1.3 and 2.5, whereby the essential requirements for optimizing the efficiency of the kneader in relation to the maximum throughput can be achieved, wherein the screw shaft can be operated at rotational speeds of more than 500 rpm. A mixing kneader designed according to this defined geometry is particularly suitable for operation at rotational speeds of more than 500rpm, which is basically understood to mean that the higher the speed, the greater the throughput.

The determined geometry furthermore ensures that the processing zones, in particular the feeding zone, the melting zone, the mixing zone and the discharge zone, arranged in succession along the axial direction, can now be optimized, each being suitable for the operating capacity of the other zones, the shear rate level packing to allow obtaining an average shear rate range that improves the quality while shortening the effective duration of the peak temperature of the product.

By selecting the geometry according to the invention, the mixing kneader can be operated directly at high screw speeds in order to increase the throughput per unit time without unacceptably high specific energy input.

A further object of the invention relates to the provision of a method for achieving continuous mixing using the mixing kneader, by means of which the throughput of material per unit time can be increased.

To achieve this, the screw shaft is operated at a speed of more than 500rpm, in particular at a speed of more than 800 rpm.

Increasing the rotational speed of the screw shaft in turn makes it possible to greatly shorten the product residence time.

Short product residence times of 1 to 20 seconds due to the high rotational speed and high throughput of the screw while reducing the tendency of the product to thermally degrade or crosslink.

The mixing kneader designed according to the invention enlarges the range of applications of such machines.

Drawings

The invention will now be described in detail with reference to the accompanying drawings, in which:

FIG. 1 is a schematic representation of a longitudinal section through a mixing kneader;

FIG. 2 is a perspective view showing the geometry of a portion of a screw shaft according to the present invention;

FIG. 3 is a schematic view showing the action of the stirring pin relative to a conventional screw blade;

FIG. 4 is a graph illustrating throughput as a function of average residence time within a mixing kneader.

Detailed Description

Referring now to FIG. 1, a longitudinal section through a mixing kneader 1 is schematically shown. The mixing kneader 1 includes: the working part, which is enclosed by the housing 2, is designed in the form of a screw shaft 3 with a plurality of helically arranged screw blades 4. Such a mixing kneader 1 is also referred to as a single-screw extruder, since the machine has only one screw shaft. The screw blades 4 of the screw shaft 3 are circumferentially spaced to form axial holes provided in the housing 2 for the stirring pins 5, so that the screw shaft 3 can perform an axial action, i.e. a translational action, in addition to its present rotational action. Formed between the inside of the housing 2 and the screw shaft 3 is an actual machining space 6 which usually comprises a plurality of machining zones 8-11 in succession. In the mixing kneader 1 of the present example, there are represented, for example, a feed zone 8, a melt zone 9, a mixing/dispersing zone 10 and a discharge zone 11. The mixing and kneading machine 1 is provided at its feed end with a hopper 12 and at its discharge end with a discharge opening 13, through which discharge opening 13 the mixed material can be discharged in the direction of arrow 14. The basic structure of such mixing kneaders is known, for example, from the Swiss patent CH 278,575. Although the stirring pin 5 is shown only in the mixing/dispersing area 10 in the present embodiment, the stirring pin 5 may of course be provided in other areas, if desired.

Referring now to fig. 2, there is shown a perspective view of the geometry of a portion of a screw shaft according to the present invention. It should be noted that the shaft geometry of the screw shaft module 3a shown in the figures is not true to scale in this case. The screw shaft 3 is intended to be used in the mixing kneader 1 in the form of a so-called single-screw extruder, wherein the screw shaft 3 is provided as a working part capable of simultaneous rotational and translational movement, as mentioned above under the trade name BussIn the case of the mixing kneader of (4). The screw shaft module 3a is provided with 8 sets of screw blades, 6 sets 4a-4f of which are visible. Between the two sets of screw blades 4a, 4b, in turn, a through-hole 16 is provided along the circumference, which remains open, into which through-hole 16 a stirring pin (not shown) provided on the housing can project. The inner diameter of the screw shaft 3 is denoted Di while the outer diameter of the screw shaft 3 is denoted Da. The inner diameter Di is determined by the cylindrical housing outer surface 7 of the screw shaft 3 while the outer diameter Da is determined by the radial spacing between the highest or outermost portions of the axially staggered screw blades 4a, 4b which are diametrically opposed. The pitch, i.e. the average distance between two sets of axially successive screw blades 4b, 4e, is denoted T, and the screw blades determining the pitch T may also be radially staggered, if desired. The stroke, i.e. the distance over which the screw shaft 3 axially spans, is marked H.

In the present embodiment, the main surfaces of the screw blades 4a-4f are designed as freely formed surfaces. Preferably, the main surfaces of the stirring pins (not shown) are likewise designed as freely formed surfaces. A free-formed surface is a surface whose three-dimensional geometry does not have any natural origin. In this way, because the main surfaces of the screw blades 4a-4f and/or the main surfaces of the stirring pins are provided as at least partially freely formed surfaces, a completely new possibility for influencing the static and dynamic geometrical properties of the screw shaft is opened up, for example with regard to the clearance remaining between the screw blades and the associated stirring pins. The size and orientation of this gap can be varied to virtually any degree, particularly while taking into account the axial motion of the screw shaft that accompanies the rotational motion. This ultimately makes it possible to optimize the mechanical energy input and/or to optimize the changes in the shear and extensional flow zones formed in the process space and acting on the product being processed.

The relevant ratios for a screw shaft 3 designed according to the invention are as follows:

-Da/Di is 1.5 to 2.0, that is to say the ratio of the external screw shaft diameter Da to the internal screw shaft diameter Di is between 1.5 and 2.0;

-Da/H-4 to 6, that is to say the ratio of the external diameter Da of the screw shaft to the stroke H is between 4 and 6;

T/H is 1.3 to 2.5, i.e. the ratio of the pitch T to the stroke H is between 1.3 and 2.5.

The screw shaft designed according to the invention was tested on a Buss Ko kneader (single screw extruder rotating and simultaneously translating) with the same machine configuration (setting of the processing zone) as was previously used for the mixing of plastics at normal rotation speeds of 100 to 500 rpm.

Test results surprisingly found that when the screw speed was well above 500rpm, the material temperature, i.e., the temperature of the processed product in the machining zone, was still not significantly increased, as was the case with the harmonised transport, shear rate levels and packing.

Thus, in actual operation, such screw shafts are preferably operated at speeds in excess of 500rpm, which can be as high as 800rpm and even as high as 2000rpm, without nevertheless damaging the product being mixed.

Preferably, the stroke of the screw blades 4a-4f is adapted to the length of the processing space 6 (fig. 1) so that the residence time of the product in the machine does not exceed 20 seconds at the maximum when the screw shaft 3 is operated at a rotational speed exceeding 500 rpm.

Referring now to fig. 3, a simplified sequence of movement of the screw shaft in a translational motion is shown, showing the respective housing surfaces inside the housing and throughout the length of the working space, only the screw blades 4a, 4b, 4c being labeled. For simplicity, the stirring pin 5 is shown as a circular member. The action of the individual screw blades 4a, 4b, 4c relative to the respective edge stirring pins 5 is evident from the figure. For greater ease of viewing, the sequence of actions is shown kinematically reversed, that is to say the screw blades 4a, 4b, 4c are assumed to be stationary while the stirring pin 5 moves along a sinusoidal path formed by the rotary action and the concomitant translational action of the screw shaft. As is evident from the figure, between the two main surfaces of the screw blade 4c and the passing stirring pin 5 there remains a free space S in the form of a gap whose width and orientation are determined by the geometry of the screw blade 4c, the axial displacement of the relevant stirring pin 5 and the rotating working member. The pitch T is also marked which corresponds to the distance between two axially juxtaposed stirring pins 5 and the distance between the screw blades 4c, 4f, respectively. The stroke H of the screw shaft is also marked.

Referring now to FIG. 4, throughput G (kg/h) is shown as a function of residence time t (seconds) of the product being processed in the mixing kneader. It is evident from the figure how the duration of the product at high temperature is significantly reduced with increasing throughput.

Tests carried out have shown that even if high temperatures would inevitably lead to a reduction in quality according to past experience, it is now safe for quality as long as the time of the high temperature effect is sufficiently short. However, it is only possible to achieve sufficiently short residence times with increased throughput.

Among these considerations, the throughput and quality of the mixed product depend on the geometry of the screws used, the rotation speed and the transport characteristics of the various processing zones of the machine.

The purpose of any mixing is to obtain a homogeneous end product, usually mixed with additives. This is why the additive and any inhomogeneous material need to be broken up and distributed mixed in the machine. In order to break down the particles, the shear stress needs to be changed during the process in which the material is converted into particles by the surrounding matrix.

The shear stress τ is given by the following equation:

where eta is the viscosity of the matrix medium and the shear rate isAnd (4) showing. Thus, in addition to melt temperature and residence time, another factor that affects how well the processed product is dispersed, mixed and homogenized is the shear rate in the melt-fill screw flight(l/sec).

Taking into account this simplified formula, the quotient of the peripheral screw speed and the shear gap is taken asAverage value of (assuming that the screw grooves are 100% filled)

Many processes are controlled by the following factors:

the stable shear rate level results in optimum mixing, dispersion and homogenization. In the current state of the art of mixing kneaders, a common indicator for standard mixing is an average shear rate in the melting range of between 20l/s and 150l/s and an average product residence time over the entire screw extension of between 30s and 600 s.

It is evident from equation (2) that the average shear rate in conventional mixing kneaders is maximally defined by the screw speed and Da/s.

However, because:

increasing the shear rate also results in a greater amount of specific energy input, which in turn results in an unacceptably high melting temperature, since the increase in melting temperature is given by:

wherein Cp is specific heat. In other words, too high a shear rate can also lead to degradation (thermal degradation or crosslinking) of the product and thus to reduced quality, as well as too long an average residence time of the product in the mixing kneader.

The mixing kneader according to the invention can be operated at a screw shaft speed of 500 to 2000rpm for both rotary and translatory action, since the average shear rate increases the product quality which is currently achievable and at the same time the peak temperature duration of the product is shortened by the adjustment of the ratios Da/Di, Da/H and T/H as described above.

The symbols used in the formula:

e_spec: average specific energy input [ KWh/kg]

t: average residence time of product in extruder [ s ]

ρ: melt density [ kg/m 3]

γ: average shear rate [ l/sec ]

Eta: average dynamic viscosity [ Pa sec ]

Da: screw shaft external diameter [ mm ]

Di: screw shaft inside diameter (mm)

S: average shear gap between screw blade and stirring pin/tooth

ns: screw rotation speed [ rpm ] or [ l/s ]

vu: peripheral speed of screw shaft [ m/s ]

τ: shear stress [ N/mm ^2]

cp: specific heat [ kJ/kg K ]

G: throughput [ kg/h ]

Δ T: increase in temperature of material [ K ]

Claims (12)

1. A mixing kneader (1) for continuous mixing, comprising a screw shaft (3) rotating and simultaneously axially translating within a housing (2), the screw shaft having a plurality of helically arranged screw blades (4), the mixing kneader (1) being provided with stirring pins (5) fixed to the housing (2) and projecting into a processing space (6) formed between the inside of the housing (2) and the screw shaft (3), characterized in that: the ratio of the external diameter of the screw shaft to the internal diameter of the screw shaft Da/Di is between 1.5 and 2.0, the ratio of the external diameter of the screw shaft Da to the stroke H Da/H is between 4 and 6 and the ratio of the pitch T to the stroke H T/H is between 1.3 and 2.5, wherein the screw shaft (3) can be operated at a rotational speed of more than 500 rpm.

2. The mixing kneader (1) as claimed in claim 1, characterized in that: the screw shaft (3) can be operated at a rotational speed of more than 800 rpm.

3. The mixing kneader (1) as claimed in claim 1, characterized in that: the mixing kneader (1) comprises a plurality of regions which are arranged one behind the other in the direction of transport and form a processing space (6).

4. A mixing kneader (1) according to claim 3, characterized in that: the processing space (6) is formed by at least one feed region (8), a melting region (9), a mixing/dispersing region (10) and a discharge region (11).

5. The mixing kneader (1) according to claim 3 or 4, characterized in that: the rotational speed of the screw shaft (3) is adapted to the length of the processing space (6) such that the product residence time in the machine is kept between 1 and 20 seconds.

6. The mixing kneader (1) according to claim 3 or 4, characterized in that: the pitch of the screw blades (4) is adapted to the length of the processing space (6) such that the product residence time in the machine does not exceed 20 seconds at maximum when the screw shaft (3) rotates above 500 rpm.

7. Mixing kneader (1) according to any of the claims 1 to 4, characterized in that: the main surfaces of the screw blade (4) and/or the stirring pin (5) are at least partially provided as freely formed surfaces.

8. The mixing kneader (1) as claimed in claim 7, characterized in that: the three-dimensional geometry of the main surfaces of the screw blade (4) and/or the stirring pin (5) is at least partially arranged such that it does not have any natural origin.

9. A method for achieving continuous mixing using a mixing kneader (1) according to claim 1, characterized in that: the screw shaft can be operated at rotational speeds in excess of 500 rpm.

10. The method of claim 9, wherein: the rotational speed of the screw shaft is adjusted such that the average product residence time in the machine (1) is between 1 and 20 seconds.

11. The method of claim 9 or 10, wherein: for the production of large batches of flowable plastic and/or pasty masses.

12. The method of claim 9, wherein: the shaft can be operated at speeds in excess of 800 rpm.