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Twin screw extrusion production of profiles
This article is taken from the German magazine Kunststoffe plast europe. The author is German Christian Stuetzinger.
Cost pressures and increasing demands on production quality and yield have driven the development of high-output extruders and various special designs. The purpose of improving speed while improving product quality has been achieved.
In the past decade, the output of twin-screw extruders has almost doubled, and the pitch of the screw shaft has not changed. In the development of die heads and sizing systems, the increase in output is even more compelling. At the same time, the possible operating speed has tripled. These output growth are achieved through the following measures:
â—† Increase the torque with a more powerful drive.
â—† Lengthen the processing device (screw and sleeve).
â—† Increase specific output.
â—† Trim the common screw shape.
â—† Increase the screw diameter and keep the screw shaft spacing constant.
Applying greater torque over smaller areas With the new drive concept, torque can be increased by 25-50% at constant speed. As a result, the specific throughput of the extruder can be increased without increasing the screw speed. Therefore, the peripheral screw speed remains the same as that of the previous common models.
In addition to the larger torque, the new drive concept significantly shortens the drive design. The total length of the installation of the drive and the floor space of the entire machine are greatly reduced. Factors such as floor space often play an important role in the procurement of extruders. With a more compact drive design, higher yields can be achieved with smaller machine dimensions.
Special care must be taken to ensure that the material is plasticized as uniformly and gently as possible. With an advanced processing device with an L/D ratio of 27, the material can be very finely plasticized. The required input heat is obtained by the torque applied to the plasticized material by the screw and the sleeve heating system. To ensure that the plasticization is as uniform as possible, it is also helpful that the screw cooling system supplies heat to the preceding section of the discharge section. In this case, a dead-end internal cooling system that actively supplies heat to the molten plastic is particularly suitable.
The elongation of the processing device naturally also makes it possible to modify the shape of the screw. For short processing devices, PVC is primarily plasticized by mechanical energy. In long processing devices, the heat does not have to be provided primarily by shear because the larger sleeve surface can be used for heating. By increasing the pitch of the feed section, lengthening the preheat section is the solution. The length of the discharge section and the metering section remain the same as the length of the 22D design.
If in the screw design, the pitch and clearance of the short device (screw clearance and thread clearance) are not changed and the device is elongated, the consequence is that the melt temperature will be higher at the same speed, which will significantly limit the maximum output range. Excessive melt temperatures cause serious problems at the inlet of the die. Displacement of the fluid front will occur and a clearly visible abnormality will occur in the finished product of the profile.
The trade-off between flexibility and high yield
However, the high-volume concept in the development of extruders and molds is hampered by the need for maximum flexibility and reliability of the production process. Advanced high-volume extruders must eventually blend with existing old machines. There is an inevitable conflict between old machines with L/D values ​​of 19 or 22 and new-generation extruders with L/D values ​​above 27. The biggest difference between extruder types is the residence time of the material in the sleeve, which is determined by the apparent difference in the length of the process. Therefore, when the 27D extruder and the 19D machine are integrated in the factory, problems often occur in screw design. The plasticization of short processing devices is mainly provided by mechanical energy, but in advanced long processing devices, the dominant factor of plasticization is not the shear of the material, but the thermal energy provided by the longer heatable surface of the sleeve. In order for the 19D processing device and the 27D device to have the same degree of plasticization, a more compact screw or a pre-plasticized PVC formulation is required.
If a 27D extruder uses a recipe designed specifically for shorter extruders, this can lead to excessive plasticization, causing surface and color problems in the final product of the profile. A possible way to overcome this problem is to use a screw with a relatively moderate shape, which introduces less mechanical energy into the PVC. However, if the speed and output of such an extruder is extremely low, it will eventually have to supply a large amount of heat, which is due to insufficient shear at low speeds and insufficient mechanical energy. In the opposite case, if the same formula is used, then the PVC blended dry material that is adjusted for the 27D extruder will produce difficulties in the short extruder. This will have the opposite effect on plasticization unless the screw is properly adjusted.
Coextrusion
To meet the needs of the market, the maximum flexibility in the design of the extruder is overwhelming. For this reason, the production of window profiles from co-extrusion is the latest technology. Re-abrasives can be used on surfaces that are masked and invisible, and in the outer layer, new materials that are co-extruded can be used to cover the profiles.
There are several possible variations when installing a co-extrusion line. In the general case, a parallel twin-screw extruder was used as a master and a conical twin-screw extruder was used as a co-extruder. Another option for increasing throughput on the same footprint is twin-strand extrusion. However, at present, the internal cooling dies of the twin-screw extrusion are too complex and the cost is too high, so it must be said that it is not popular in the market. Because the extruder size is designed for high throughput, twin extrusions do not provide the manufacturing flexibility required by this many processors.
In order to meet the requirements of processors to achieve double-twist extrusion in the smallest space, the German Weber Machine Company has developed a machine design that has been further improved with traditional twisted-squeeze extrusion. In a twin production line, two parallel twin screws are placed in a frame, as close as possible to each other. In this way, the material can be extruded at a rate of 1000 kg/hr on a single machine. Two extruders can be independently controlled. Downstream devices such as calibrating tables, cutters, etc. can also be controlled individually. Compared to previously known double extrusion extrusion in a single extruder, the operational problems at startup can be significantly reduced. The increase in production flexibility is the main reason for developing this method.