The Transfermix degassing extruder.

Degassing or vacuum extruders are frequently used foer profile extrusion in the technical rubber goods manufacturing industry. The compounds to be extruded often contain volatile gases which must be transported away from the actual rubber so as to prevent porosity, The gases are usually low molecular weight substances or moisture originating from fillers.

The basic layout of a degassing extruder consists of a conveyance section towards the degassing section and a conveyance section after the degassing section. The screw section downstream of the degassing section must convey the rubber at a higher rate per revolution than the upstream screw section so as to prevent overfilling of the degassing section and allow for some free surface area of the compound within that section. Different conveyance properties for the two sections are usually accomplished by introducing different flight depths. The upstream depth will be less than the downstream depth to ensure that the upstream conveyance performance is lower.

Directly ahead of the degassing section, the screw is no longer flighted, but instead consists of a solid cylinder containing capillaries aligned in the approximate direction of extrusion. The extrudate thus enters the degassing section as many small cross-sectional area strings of rubber. This provides for the necessary large compound surface area to enable the entrapped gases to escape.

The reduction of flight depth and the capillaried cylindrical section therefore act as a fixed position throttle. Process optimization and cross-sectional area variation can only be performed by exchanging the screw or modifying the existing screw. In practice, the screw is specified according to the most difficult to process compound. When an easier compound is extruded, the compound is excessively throttled, thereby reducing performance and economy of the system.

Effective degassing is also critically dependent on compound temperature (ref. 1), where compound temperature is determined by screw and barrel geometry for a given machine setting. Conventional vacuum extruders do not feature a mixing section, so that homogenous heating of the compound and compound temperature cannot be regulated. The compound temperature is determined by the throttling action of the capillaried cylindrical section. Since the extruder is usually configured for the most difficult to extrude compound, easier to extrude compounds are likely to be over-throttled, so that temperature generation is excessive and throughput for such compounds reduced.

The drawbacks of the conventional vacuum extrusion screw can thus be summarized as:

* Reduction of potential throughput for easy to extrude compounds;

* possible excessive temperature elevation due to fixed throttle action; and

* mechanical optimization of the degassing process is only possible with great effort.

The numbered components of the degassing section shown in figure 1 are termed as follows: 1--degassing section; 2--overflow section or blister to create large compound surface area; 3--reduction of flight depth.

[FIGURE 1 OMITTED]

Principle of operation of the degassing extruder

A sectional view of the Transfermix degassing extruder (Patent EP 0345687) is presented in figure 2. The extruder consists of the following main components: 1--feed; 2--Transfermix section with adjustable throttle; 3--hydraulic actuator for throttle setting change; 4--degassing section; 5--conveyance (pressure) section.

[FIGURE 2 OMITTED]

After ingestion (1), the compound is conveyed to the Transfermix section (2). The compound is then forced from the screw into the barrel in the Transfermix section, where the opposite flighting of the screw and barrel create intensive shearing and hence heating and homogenization of the compound. An annular slit throttle (3), [figure 3], is located directly behind the transfermix section. The annular slit throttle consists of two slitted discs that can rotate in opposing directions and thereby adjust the effective slit width/compound flow area. The throttle section is followed by the actual degassing section. A vacuum pump is connected to this section to ensure the removal of the compound gases. The final section of the extruder (5) is designed to rapidly convey material away from the degassing section and to produce enough pressure to force the compound through the die and extrusion head (6).

[FIGURE 3 OMITTED]

The key to the flexibility of this extruder design is therefore the variable throttle. The throttle itself consists of a fixed section together with a variable section, where the axis of rotation is the direction of extrusion. As compound passes through the throttle, it must pass through the rectangular sectioned slits therein, so that it then enters the degassing section as many small rectangular sectioned strings of rubber. By varying the degree of rotation of one of the throttle sections, the flow area of each rectangle is varied. Figure 4 contrasts the degassing section with a standard (non-degassing) design.

[FIGURE 4 OMITTED]

When adapting the system to a new compound, the extruder is initially run with fully open throttle. Should the degassing performance prove insufficient at the desired throughput, the throttle is adjusted in the closing direction. This will have two effects, both of which tend to increase the degassing capability of the system:

* The surface area relative to compound volume is increased in the degassing section, since the "strings" produced are now thinner. The actual degassing process is determined by the available compound surface area: The transfer of the volatile substances out of the rubber compound to the gas is an action which can only occur on the compound surface (ref. 1). The degassing extruder therefore enables the operator to adjust the amount of compound surface area within the degassing section according to a particular compound.

* Thermal homogeneity and compound temperature also play a critical role in determining the degassing effectivity of the extruder (ref. 1). At open throttle, the Transfermix section of the degassing extruder will provide for a certain minimal level of plastification or homogenous heating of the compound; during the initial stages of a project, tests are performed to determine the screw/barrel configuration to best suit a customer's general requirements.

However, by closing the throttle, this effect can be increased, due to consequent greater compound residence time within the Transfermix section of the extruder. By closing the throttle, the temperature of the compound entering the degassing section is therefore increased, so that the degassing performance is in turn increased. The operator can therefore determine the compound temperature entering the degassing section to suit a particular application.

The degassing extruder with gear pump 

The conveyance/pressure section of a conventional degassing extruder has the dual function of transporting compound away from the degassing section and of overcoming extrusion head/ die resistance to force the compound through these. It is critical that the screw is only partially filled with compound in the area following the degassing section, so as to ensure a large compound free surface area. To avoid rubber flooding into the degassing section, the conveyance/pressure section is configured very long, so that the overall length of degassing extruders is usually much greater than that of standard cold-feed extruders.

If a gear pump is used to induce the pressure required to the degassing section may be considerably shortened, as shown in figure 5. Such a system requires the shortened conveyance/pressure section to merely feed the gear pump at a certain minimum gear pump inlet pressure of 20 to 40 bar. The extruder/screw is thereby greatly unloaded, so that the throughput per screw revolution is greatly increased. The pressure required to overcome the extrusion head/die is now induced only by the gear pump, which, due to its positive displacement characteristics, is far more suitable to this requirement.

[FIGURE 5 OMITTED]

The danger of flooding the degassing section due to excessive head pressure is therefore completely eliminated. The extruder can now be configured for optimal degassing performance, with virtually no consideration of any pressure inducing function. Much improved overall system performance for both throughput and degassing capability is therefore achieved.

References

(1.) Der Einschneckenextruder, VDI Verlag, 2nd ed., 2001, Gerhard A. Morton. author.