Function and Role of Twin Screw Thread Elements for Extruders

Generally a screw in a twin screw extruder is divided into five sections: conveying section, melting section, mixing section, exhaust section and homogenization section.

1. Conveying section: conveying materials to prevent overflow;
2. Melting section: through heat transfer and friction shearing, the material is fully melted and initially homogenized;
3. Mixing section: the size and components in the material is further compounded and uniformified, forming an ideal structure, taking into account the distribution and dispersion;
4. Exhaust section or venting section: impurities such as water vapor and low molecular weight substances are discharged;
5. Homogenization (metering) section: through conveying and pressurizing, establishing a certain pressure the material at the die mouth has a certain density, and at the same time, it is further mixed, and finally the purpose of smooth extrusion and granulation is achieved.

The main difference between the two is that distribution is achieved by strain, while dispersion is achieved by stress.

1. Distributed mixing: splitting and recombining the melt, and making the spatial distribution of each component uniform, which is mainly achieved by displacement flow under the action of separation, stretching (alternating compression and expansion), twisting, and reorientation of fluid activity;

2. Dispersing and mixing: breaking the components into particles or making the size of the incompatible two-component dispersed phase reach the required range, mainly achieved by shear pressure and extension stress.

1. Model code: for instance, "56/56" conveying block, the former "56" means the stroke is 56mm, and the latter "56" means the length is 56mm;
2. Large lead, refers to the pitch of 1.5D~2D;
3. Small lead, refers to the pitch of about 0.4D;
4. Law of its use: as the lead increases, the extrusion volume of the screw increases, the material residence time decreases, and the mixing effect decreases;
5. To select large-lead threads, conveying is the main application, which is beneficial to increase the output; heat-sensitive polymers can shorten the residence time and reduce degradation, volatilization, etc.;
6. To select medium-lead thread, which gradually narrows the combination of different working sections, it is used for conveying and pressurization;
7. To select a small lead thread, it is generally gradually reduced in combination, and is used in the conveying section and the homogenizing metering section to increase the pressure, improve the melting, improve the degree of mixing and the extrusion stability.

There are two main categories, "K" series (kneading) and "M" series (dentate).

Model code: For example, K45°/5/56 (KB45/5/56) belongs to the shear block. "K" refers to the sliced shearing block, "45°" refers to the angle formed by the pieces, "5" means a total of 5 pieces, "56" means the length is 56MM, and the width of screw flight is 56/5=11.2mm).

Main Parameters:

Direction: forward and reverse. Reverse (left-handed), hinders the conveying of materials, prolongs the time, increases the filling pressure, and greatly improves the mixing effect.

Angle: it is generally divided into "30 °, 45 °, 60 °, 90 °".

Function and Effect:

In the positive direction, increasing the stagger angle will reduce the conveying capacity, prolong the residence time, and improve the mixing effect, but it will be easier to flow over. For distributive mixing and dispersive mixing, distributive mixing is more effective as the angle increases, and dispersive mixing is the best at an angle of 45° followed by 30°. 60° is the last.

In the reverse direction, increasing the angle will reduce the effective confinement of the polymer, but it's more likely to flow over.

Width of Screw Flight: it's generally sorted with 7mm, 11mm, 11.2mm, 14mm, 19mm, etc., which is among one of the most important parameters to measure the shear force and mixing force. The larger the width, the larger the shear and the smaller the mixing; the smaller the width, the smaller the shear, the greater the mixing. For distributive mixing and dispersive mixing, the effectiveness of distributive mixing decreases as the width increases, and the effectiveness of dispersive mixing increases as the width increases; the smaller the width, the greater the ratio between the effective axial flow and the effective radial flow of the material.

The number of screw lobes is generally divided into single head, double head and triple head.

Effect:

In the positive direction, the smaller the number of heads, the greater the extrusion conveying capacity, the greater the torque, and the better the mixing characteristics, but the less the shearing effect.

In the reverse direction, the smaller the number of heads, the smaller the extrusion conveying capacity and the better the mixing characteristics.

The two-lobe thread features uniform and quick heat. It has a good self-cleaning performance and is popularly used for extruders.

The three-lobe thread can flexibly select the pressure and temperature distribution of the material in the machine corner. The exhaust effect is good, but the output is low.

The tooth-shaped mixing element mainly disturbs the material flow and accelerates the homogenization of the material. The more teeth, the stronger the mix, but be careful when using for high shear damage. There are mainly three types: ZME, TME, and SME.

Generally speaking, TME is the tooth-diameter disc element, which is the earliest tooth-shaped mixing element. It has no positive and negative conveying ability. However, through the positive, negative and straight angles of the tooth shape, different mixing modes of materials can be designed. In other words, the TME element provides the most specific surface area for different materials, and the lateral mixing ability of the TME element is very good, which is the same as the ZME element, but the disadvantage is that it has no self-cleaning function;

The ZME element is an upgrade of the TME element. It has a certain reverse conveying ability, but it is very weak. The function is basically the same as the TME element, but it has a self-cleaning function. The disadvantage is that it is more troublesome to install.

The SME element is actually an ordinary conveying element with a leaking groove on the screw, which has a positive conveying capacity and has the advantage of having an axial distribution and mixing capacity.

To design a combination of different screw elements for a "perfect" screw profile requires a good knowledge of both polymers and extruders. We have good experience from our customers to help acheive a best solution to polymer extrusion process.