Twin screw extrusion production of profiles
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Twin screw extrusion production of profiles
This article is taken from the German Kunststoffe plast europe magazine author German Christian Stuetzinger
Cost pressures and increasing demands for production quality and production have contributed to the development of high-volume extruders and various special designs. The goal 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 and sizing systems, the increase in output is even more compelling. At the same time, the possible operating speed has tripled. These production increases are achieved through the following measures:
â—† Increase torque with a more powerful drive.
â—† Lengthening the processing device (screw and sleeve).
â—† Increase specific production.
â—† Trim the common screw shape.
â—† Increase the screw diameter and keep the pitch of the screw shaft constant.
Applying greater torque over a smaller area is driven by a new drive concept that increases torque by 25-50% at constant speed. Thereby, the specific yield of the extruder can be increased without increasing the screw speed. So the peripheral screw speed remains the same as the speed of the previous common models.
In addition to the large torque, the new drive concept significantly shortens the drive design. The total length of the drive installation and the footprint 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 throughput can be achieved with smaller machine sizes.
Special attention must be paid to ensuring that the material is plasticized as evenly and gently as possible. With advanced processing equipment with an L/D ratio of 27, the material can be plasticized very finely. 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 to supply heat from the screw cooling system to the previous section of the discharge section. In this case, an infinite loop 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 equipment, PVC is mainly plasticized by mechanical energy. In long processing devices, heat does not have to be provided primarily by shearing because a 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 gap and thread gap) are unchanged and the device is elongated, the result will be a higher melt temperature at the same speed, which will significantly limit the maximum output range. Excessive melt temperatures create serious problems at the inlet end of the mold. Displacement of the fluid front occurs and clearly visible anomalies in the finished profile.
The tradeoff between flexibility and high yield
However, the high-volume concept in extruder and mold development is hampered by the need for maximum possible flexibility and reliability of the production process. Advanced high-volume extruders must eventually be integrated with existing vintage machines. There is an inevitable conflict between older machines with L/D values ​​of 19 or 22 and a new generation of 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 length of the process. Therefore, when the 27D extruder and the 19D machine are used in combination at the factory, problems often arise in the screw design. The plasticization of short processing equipment is mainly provided by mechanical energy, but in advanced long processing equipment, the dominant factor of plasticizing is not the shearing of the material, but the thermal energy provided by the longer heatable surface of the sleeve. In order to make the 19D processing device and the 27D device the same degree of plasticity, a relatively strong screw or a pre-plasticized PVC formulation is required.
If a 27D extruder uses a formulation designed for a shorter extruder, this can result in excessive plasticization, which can create surface and color problems on the finished profile. A possible way to overcome this problem is to use a relatively moderately shaped screw that introduces less mechanical energy into the PVC. However, if the speed and output of such an extruder is extremely low, then a large amount of heat energy will eventually have to be supplied, which is due to insufficient shear at low speeds and insufficient mechanical energy. In the opposite case, if the same formulation is used, then the PVC blend dry material adjusted for the 27D extruder will create difficulties in the short extruder. This will have the opposite effect on plastication unless the screw is properly adjusted.
Coextrusion
In order to meet the needs of the market, the maximum flexibility in the design of the extruder is overwhelming. To this end, the production of window profiles by coextrusion is the latest technology. The regrind may be applied to the surface that is hidden and not visible, while the outer layer may be overlaid on the profile with a new coextruded material.
There are several possible variations when installing a coextrusion line. In the general case, a parallel twin-screw extruder was used as a main machine, and a conical twin-screw extruder was used as a co-extruder. Another option to increase throughput on the same footprint is twisted pair extrusion. However, at present, the internal cooling die of twisted pair extrusion is too complicated and the cost is too high, and it must be said that it is not popular in the market. Because the extruder size is designed for high throughput, double extrusion does not provide the production flexibility required by this multi-processor.
In order to meet the requirements of the processor to complete the twisted pair extrusion in the smallest space, the German Weber Machine Company has developed a machine design that has been further improved with the traditional twisted pair extrusion. In a twin 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 stand-alone floor. Two extruders can be controlled independently. Downstream devices such as sizing tables, cutters, etc. can also be individually controlled. The operational problems at start-up can be significantly reduced compared to the double propulsion extrusion in a previously known single extruder. Increased production flexibility is the main reason for developing this approach.