Kneading Blocks vs Conveying Elements: How to Choose for Twin Screw Extruders

Conveying elements and kneading blocks are two of the most important components in a co-rotating twin screw extruder. Although both are installed along the same screw shafts, they perform different functions during polymer processing.

Conveying elements are primarily responsible for transporting materials, while kneading blocks are designed to promote melting, mixing and material deformation.

Choosing the right combination is essential for stable extrusion, appropriate mixing intensity and controlled processing temperatures.

So, when should you use conveying elements, and when are kneading blocks necessary?

Conveying elements have continuous helical flights that move material along the screw axis.

They are widely used in feeding, conveying, melting-transition and downstream transport sections of a twin screw extruder.

Their primary functions include:

Transporting raw materials from the feeding section

Moving partially melted or molten polymer between processing zones

Supporting material flow through the screw configuration

Providing appropriate conveying and pressure-development characteristics

The geometry of a conveying element directly influences its behavior.

Important parameters include screw pitch, flight depth, number of flights, element length and rotation direction.

For example, larger-pitch conveying elements may provide greater forward conveying capacity under suitable conditions, while shorter-pitch elements can produce different filling and pressure characteristics.

However, actual material flow also depends on screw speed, material properties, feeding rate and downstream resistance.

A conveying element cannot be selected by pitch alone.

Kneading blocks consist of multiple discs arranged at specific stagger angles.

Unlike continuous-flight conveying elements, their geometry is designed to redistribute material and generate controlled deformation within the processing zone.

Kneading blocks are commonly used for:

Polymer melting and plastification

Distributive mixing of additives and polymer components

Dispersive mixing of agglomerated fillers or pigments

Improving material contact and mixing uniformity

Adjusting local filling and residence conditions

The behavior of a kneading block depends on its disc geometry, stagger angle, orientation and location within the screw configuration.

Not all kneading blocks produce the same mixing intensity or conveying behavior.

A forward-conveying kneading block may support material transport while providing mixing, whereas neutral or reverse-oriented configurations can introduce greater flow restriction.

Neither component is universally better.

The correct choice depends on the function required at each position in the screw configuration.

One of the most important considerations is the relationship between kneading geometry and mixing intensity.

Forward-oriented kneading blocks combine mixing with a degree of forward material transport.

They may be suitable where mixing is required without creating excessive flow restriction.

Neutral configurations generally provide limited net forward conveying action and can increase local filling and mixing.

However, they may also increase mechanical energy input and residence time, depending on the processing conditions.

Reverse-oriented kneading blocks create greater resistance to forward material movement.

They may be used where additional filling or mixing intensity is required, but they can also increase pressure, torque demand and shear heating.

Kneading angle alone is not enough to determine performance.

Disc width, element length, stagger direction, screw speed and material properties must also be considered.

Different materials require different balances between conveying and mixing.

Calcium carbonate, talc and other mineral fillers can increase abrasive wear and create additional mixing requirements.

Conveying elements should provide suitable material transport, while kneading blocks may be required to distribute or disperse fillers within the polymer.

However, excessive flow restriction or mechanical energy input may increase temperature and wear.

Material selection is also important in high-abrasion positions.

For glass fiber reinforced materials, achieving sufficient fiber distribution without excessive fiber breakage is often a major objective.

Aggressive kneading configurations can contribute to fiber shortening.

Depending on the formulation and feeding arrangement, lower-shear mixing strategies and carefully selected conveying elements may help balance fiber dispersion and retention.

The final fiber length is influenced by the complete process, not just one kneading block.

Materials with narrow processing windows may be sensitive to excessive mechanical energy input and long residence times.

In these applications, conveying elements and less restrictive mixing configurations may be appropriate.

The goal is to obtain the required melt homogeneity while limiting unnecessary shear heating.

Color masterbatch production may require both effective polymer melting and pigment dispersion.

Kneading blocks can help provide the mechanical deformation required for dispersion, while conveying elements maintain material transport between processing sections.

The correct arrangement depends on pigment characteristics, carrier resin, additive loading and the required final dispersion quality.

It may seem reasonable to increase the number of kneading blocks when dispersion is insufficient.

However, this does not always solve the problem.

Additional kneading sections may change:

Local material filling

Residence time

Melt temperature

Torque demand

Pressure development

Mechanical degradation risk

For some materials, increased mixing intensity can improve dispersion. For others, it can contribute to overheating, fiber damage or unnecessary energy consumption.

Therefore, mixing performance should be evaluated together with material condition, melt temperature and extrusion stability, rather than simply increasing the number of kneading elements.

A twin screw extruder does not operate through independent screw elements.

The conveying and kneading sections interact continuously.

For example, an unsuitable conveying element before a kneading section may influence local filling, while excessive restriction within a kneading section may affect upstream pressure and downstream material flow.

A practical screw configuration should consider:

Feeding and material intake requirements

Polymer melting behavior

Required distributive and dispersive mixing

Shear sensitivity of the formulation

Venting and devolatilization requirements

Pressure development toward the discharge section

When modifying an existing screw configuration, it is generally better to identify the specific processing problem before replacing multiple elements with a different geometry.

Both conveying elements and kneading blocks may experience wear, but the severity and wear mechanisms can differ by processing position.

W6Mo5Cr4V2 tool steel and selected wear-resistant alloys may be considered for abrasive processing applications.

Other materials may be appropriate where corrosion resistance or specific mechanical properties are required.

For OEM replacement, material selection must also be combined with accurate dimensional matching.

Important parameters include outside diameter, internal spline geometry, element length, end-face dimensions and compatibility with neighboring components.

Not as a general replacement. Although certain kneading block designs provide forward conveying action, their main purpose is mixing and material deformation. They should be selected according to the required function of the processing zone.

No. Mixing depends on the complete kneading geometry, including disc width, stagger orientation and element length, as well as operating conditions.

A combination of suitable conveying and controlled-shear mixing elements is often required. The objective is to achieve adequate fiber distribution while avoiding unnecessary fiber breakage.

Not necessarily. Individual elements or groups can be evaluated for replacement when the remaining screw assembly is in acceptable condition and dimensional compatibility can be maintained.

ZHITIAN manufactures conveying elements, kneading blocks, mixing elements and other modular screw components for twin screw extrusion systems.

We support custom screw element manufacturing, material selection, OEM replacement and dimensional matching based on drawings, existing samples and application requirements.

Whether you are replacing worn elements or evaluating changes to an existing screw configuration, understanding the different functions of conveying elements and kneading blocks is an important starting point.

Send Your Screw Element Drawings or Existing Configuration for Technical Review.