双螺杆挤出机熔融段螺杆配置

In the previous article, we detailed the basic screw configurations for the feeding section. This chapter will introduce the screw configurations for the melting section.

The melting mechanism in co-rotating twin-screw extruders is unique. Single-screw processes rely on heat from the barrel and friction between the barrel wall and the polymer conveyed between the screw flights to melt the polymer. In contrast, twin-screw extruders use kneading blocks to mechanically work the polymer, stretching, shearing, and folding it to generate heat. In this article, we will discuss how kneading blocks melt polymers and how the design of the melting section affects the melting method, melt temperature, and quality.

1、Function of the melting section

All compounding operations are performed in the molten state, but configuring the screw elements to melt the polymer is not straightforward. An overly aggressive melting section can lead to excessively high melt temperatures, potentially causing polymer degradation or scorching if the polymer’s degradation temperature is exceeded. Conversely, a too mild melting section may fail to melt the polymer sufficiently, resulting in an inhomogeneous product.

Figure 1 shows a picture of the assembled melting section. Solid material is conveyed from the feed port (starting from the right side in the figure). As the material is pushed from conveying elements onto the kneading blocks, the shearing action of the discs stretches and pulls the material. The solid feed particles are mixed, while the work applied to the polymer causes the temperature of the material itself to rise. This temperature rise, in turn, causes the polymer to soften and then melt.

2. Kneading Elements

Kneading elements consist of a series of discs or paddles, where each disc is offset from the previous one by a fixed angle. This stagger angle and the width of each disc determine the intensity of the work imparted by the kneading element. The direction of the stagger also determines whether the kneading element conveys material downstream (forward conveying) or upstream (reverse conveying). Kneading elements are used not only for melting but also for mixing; mixing and melting actions occur simultaneously in the extruder’s melting section.

a. Stagger Angle

Figure 2 shows a forward conveying kneading element. Looking at the end of the kneading element, each disc on this element is offset by 60° from its predecessor. Furthermore, the general movement from one disc to the next is clockwise. This is the same as the rotation direction of a conveying screw. When this kneading element rotates within the screw, its rotation pushes the polymer forward—hence, a forward conveying kneading element.

The stagger angle influences the energy imparted to the polymer, partly depending on how much material is conveyed forward. For conveying elements, the screw rotation pushes the polymer forward. Similarly, a forward conveying kneading element also pushes most of the polymer forward. Some polymer flows past the end of the discs and is sheared against the barrel wall, some transfers to the adjacent kneading element on the other screw, and a small amount flows back upstream. The combination of these three actions causes the polymer temperature to rise while conveying the polymer forward.

The shorthand coding method used by some manufacturers to label kneading elements can be seen in the etching on the side of the kneading element in Figure 2. The shorthand code for a kneading element uses the following format:

KB XX/YY/ZZ, where:

KB — Indicates Kneading Block

XX — The first number is the stagger angle. In Figure 2, the angle is 60°. In Figure 3, it is 90°.

YY — The second number indicates how many discs the kneading block has. In Figure 2, there are 6 discs.

ZZ — The third number is the element’s length in millimeters. In Figure 2, the shown kneading element is 60 mm long.

A kneading element with a 30° stagger angle provides minimal mixing and conveys material forward optimally. A 45° kneading element imparts more energy to the polymer while increasing mixing but conveys less forward. A 60° kneading element has poorer forward conveying capability but is more efficient at mixing and imparts even more energy to the material.

As the stagger angle increases, the forward pumping capability of the kneading element gradually decreases until the angle reaches 90°. At 90°, the kneading element is neutral; polymer is mixed but not significantly conveyed forward or backward. Figure 3 shows a neutral kneading block with a 90° stagger angle. As conveying capability decreases, more polymer flows past the end of each disc, resulting in more energy being imparted to the polymer and increased mixing.

b. Disc Width

Another factor significantly influencing melting effectiveness in twin-screw extruders is the width of the discs. Kneading elements typically consist of as few as three or as many as seven discs, commonly having four or five discs per element. Often, the number of discs remains constant, and only the length of the kneading element is adjusted to increase or decrease the disc width. Wider discs impart more energy to the polymer. When the kneading element rotates in the extruder, polymer can only flow in one of two directions: flow past the tip of each disc or flow around a disc to the next one.

Increasing the width of the discs causes more polymer to flow past the disc tips. A narrow disc kneading element essentially slices through the polymer, while a wide disc acts more like a plough. The space between the disc tip and the barrel wall is where the shear rate is highest; polymer passing through this zone experiences the highest shear forces in the extruder. A wide disc kneading element will impart more energy to the polymer than a narrow disc element. This increased energy input will result in a higher melt temperature.

3. Reverse Conveying Kneading Elements

The final type of kneading element used in the melting section is the reverse conveying kneading element. This type is the most severe, as it pumps molten polymer back upstream. In this case, a smaller stagger angle imparts more intense action on the polymer.

Figure 4 shows a reverse conveying kneading element. For most manufacturers, forward conveying elements rotate to the right. The rotational direction from one disc to the next in a kneading element is clockwise. Therefore, a reverse conveying element has paddles or discs that rotate counterclockwise. These manufacturers specify reverse conveying elements using the same nomenclature as forward conveying elements, followed by an “L”. The “L” stands for left-hand, as these elements are left-hand elements, while forward conveying elements are right-hand elements.

4. Melting Section Screw Configuration

The design of the melting section depends on the type of material being processed. When setting up the screw configuration for the melting section, factors such as whether the polymer is crystalline or amorphous, whether the melt viscosity is high or low, or whether it is processed near its decomposition temperature must be considered.

The condition of the formulation is also critical. Is only one polymer or a blend of several polymers fed through the feed port? What types of fillers and additives are added with the polymer? Does one or more additives have a melting temperature significantly lower than the polymer? Is a melting zone consisting of a continuous series of kneading blocks suitable, or is a configuration alternating kneading blocks and conveying elements a better design for the material being compounded? These factors all influence the design of the screw configuration.

The principles behind screw element action are a science; screw configuration design is an art. In actual production, engineers may design different screw configurations to achieve the same function. This means there isn’t necessarily one design that is wrong and another that is right; they may simply be different, but if the product quality meets requirements, both can be acceptable.

Figure 1 shows the melting section of a 50 mm twin-screw extruder. Solid material is fed by conveying elements from the right side of the photo. The first element in the melting zone is a 30° kneading element, followed by three 60° kneading elements, and the melting section is completed by three 90° neutral kneading elements.

As material enters the melting zone, it transfers from the conveying element to the 30° kneading element. This helps pull the material into the melting zone and transition from conveying elements to kneading elements while minimizing polymer backflow. The 60° kneading elements then provide stronger work input to initiate the melting process while still conveying material forward.

The 90° kneading elements serve two functions. The first is to apply higher energy to the polymer to melt it and raise the melt temperature to the desired processing temperature. The second function is to act as a restriction, preventing material from passing through the melting zone too quickly. This ensures the local residence time of the polymer in the melting zone is sufficient to guarantee all polymer passing through is molten. The restriction created by the 90° kneading elements acts as a “melt dam,” increasing the fill level of molten polymer in that area of the screw.

Some alternative designs include the following:

Reverse Kneading Elements: Left-hand kneading elements can be used to increase the amount of restriction at the end of the melting section. Pumping material upstream increases polymer residence time, improving melting, especially for crystalline polymers and high-temperature polymers.

Reverse Conveying Elements: Left-hand conveying elements are more severe than left-hand kneading elements. This provides more backflow for better mixing, increased melt homogeneity, and imparts a higher melt temperature. Some formulations even require the use of a left-hand kneading element followed by a left-hand conveying element.

Separating Kneading Elements with Conveying Elements: For certain polymers, a single long series of kneading elements is not optimal and can lead to issues like excessively high melt temperatures or extruder surging due to uneven flow or melting. One approach is to design several short melting segments separated by conveying elements. This could be configured, for example, as two 45° kneading elements followed by a 90° kneading element, repeated once or twice, followed by a reverse 45° or 60° kneading element to ensure complete melting.

Now that the polymer is molten, we will discuss next time how to add other materials, such as liquids, low-melting-point additives, and fibers, to the melt.

At Nanjing Granuwel, we specialize in designing and manufacturing high-performance twin-screw extruders and screw elements to meet diverse compounding needs. To learn more about our solutions or discuss your application requirements, feel free to contact us – your trusted partner in extrusion technology from China.