How To Configure A Twin Screw Extruder

The two main types of mixing in compounding are dispersion and distribution.

When we really want the additive to be evenly distributed throughout the polymer matrix, we would claim that we need to improve dispersion. Or we design the screw to have a distribution mixing element, but the material is easy to agglomerate and needs to be dispersed first before being distributed.

This article explores the differences in the mixing of these types and how to enhance one type over the other.

As material is fed into a twin-screw extruder, the material falls from the feeder onto the feed screw. For this discussion, we will assume that each material is separate and independent, as if fed individually. In practice, materials can be fed individually through separate feeders, or materials can be pre-mixed into a single batch.

Dispersion is the act of breaking larger particles into smaller particles until they reach the final minimum particle size. Dispersion is needed to break down sticky materials that tend to clump.

Distribution is the uniform mixing of particles throughout the polymer matrix.

In a twin-screw extruder, these two mechanisms mostly occur simultaneously, but can be adjusted to enhance one or the other as needed.

When to disperse

Many compound operations generally do not require scattering. Many additives and fillers are inherently fed as free-flowing, unique individual particles that need to be distributed throughout the polymer to be fully incorporated.

But for materials that tend to stick together and form lumps or clumps, they need to be dispersed.

Carbon black is one of the best examples of a sticky material with a high affinity for agglomerates. Individual particles of carbon black fuse togther to form aggregates during the combustion reaction. These aggregates are the primary particles of carbon black and are the smallest size to which carbon black can be broken down during the compounding process. Carbon black aggregates are held together by weak forces to form agglomerates. The agglomerates must be dispersed within the polymer to produce a compound that exhibits the desired black color completely and uniformly.

Mixed in twin-screw extruder

The main element used for mixing in a twin-screw extruder is the kneading block. The kneading block applies shear forces to the materials being compounded to mix them by dispersing, distributing and homogenizing them.

Three key attributes of a kneading block in terms of mixing are the length of the elements, the number of disks and the staggered angle of the disks relative to each other. If the kneading block is shorter, say 30mm long, each disc will only be 5mm wide. Of course, if the kneading block is twice as long, each disk will also become proportionally wider, assuming the number of disks remains the same.

The width of the disc has a direct effect on the shear force exerted on the polymer. As the wide disk rotates around the screw axis, a pool of molten polymer forms in front of the disk. The disc exerts pressure on the polymer, "slapping" the material against the inner wall of the barrel as it creates grooves in the material. The shear forces here are quite high, promoting dispersion.

As the disc width narrows, the polymer flows around the disc, and the narrow disc of the kneading block cuts through the polymer in a shearing action. This function mainly promotes distributive mixing by stirring polymers and additives. The melt is divided by one disk and then covered by another disk.

Therefore, the primary consideration in screw design regarding mixing is the width of the kneading block disc. If dispersion is required, then a wide pan is beneficial. Narrow disks are preferred if the material only needs to be distributed throughout the polymer matrix.

Effect of staggered angle

Stagger angle is another factor to consider when blending. The smaller the stagger angle, the more polymer is transported downstream in the extruder. As the stagger angle increases, the forward motion of this area of the screw decreases, causing backflow of the polymer in the extruder. When the material is blocked, the work on the polymer increases because the forward motion of the compound decreases with each revolution of the screw, allowing for more intense mixing.

When the staggering angle reaches 90 degrees, the material no longer moves at all due to the rotation of the screw, so the 90-degree kneading block is a neutral conveying element. The forward movement that actually occurs in this case is due to material being forced from the upstream element after being fed, pushing the compound along the extruder.

Counter-pumping elements (also known as left-hand elements) pump material upstream of the extruder to resist forward motion throughout the extruder. This balance between the upstream forward conveying elements and the intermediate or reverse conveying elements results in the mixing section being highly filled with material. In contrast, a mixing section consisting only of forward conveying kneading blocks (perhaps all 45-degree angle kneading blocks) will have the least amount of material in that section because it will be pushed out as quickly as it arrives.

It's tempting to aim for a strong mix in all situations, where you can rest assured that everything is well dispersed. However, doing this in all cases would be a mistake.

How much is the right amount to mix?

The more vigorous the mixing, the more work is imparted to the compound. As the workload increases, the melt temperature will increase significantly.

1. A wider disc will produce higher shear forces, resulting in an increase in melt temperature.

2. A more intense mixing section (many reverse or neutral kneading blocks) will result in greater processing of the compound, which in turn will result in a higher melt temperature.

3. The restricted mixing section must also be designed to ensure sufficient forward motion so that speed is not impeded.

4. Likewise, being too gentle in the mixing part may result in uneven mixing and poor product quality.

The screw is stronger and therefore has more dispersive properties. This mixing section uses a wider disk and two kneading blocks with a larger staggered angle, followed by a neutral kneading block and a reverse kneading block. Designs such as these can be used with high levels of pigments or fillers that are difficult to mix. The design below shows only the forward conveying kneading block, followed by a single neutral kneading block. The idea here is to gently stir the additive into the polymer with minimal pressure, a design that can be used when mixing fiberglass or glass beads.

One thing to note about the "end" of the blending part. As I mentioned in my previous article, screw design is a combination of science and art. Every screw designer will approach the design differently. I prefer to end most of the mixing portion (>90%) with a neutral kneading block, a reverse kneading block or even a reverse delivery element (the strictest pumping element). The purpose of this is to ensure that the mixing portion is filled to the point that the mixing is quite efficient.

Some designers will place a distributed mixing section consisting of all conveying elements. While the purpose of this design is to gently stir the mixture, my experience has shown that this results in a relatively hollow mixing section that is inefficient. Additionally, these kneaded blocks tend to "whip" the polymer like a stirrer due to the lower filling level of this mixing section. This whipping action can actually increase the melt temperature and compromise the aspect ratio of some additives, such as fibers. When the mixing section has a higher fill level, it seems to work more efficiently and more gently.

This must be balanced against severe backflow that can lead to overprocessing of the material. Balance is key to screw design.

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