A conveying element, kneading block or mixing element is useful only in the right position, on the right shaft and beside the right neighbors. This guide turns an assembled screw into a record that can be checked, discussed and quoted.
A screw element is not a generic spare
In a modular parallel twin screw, separate elements slide onto two splined shafts to build the process section. The sequence can contain conveying elements, kneading blocks, mixing elements and special shapes. The current EJS screw element page lists conveying, mixing, kneading, transitional, profile, Schubkanten, elongational, Igel, Camelback and blister elements.
That list describes manufacturing scope, but the name alone does not define a replacement. "Conveying element" still leaves pitch, length, handedness, lobe profile, bore, spline and material unanswered. "Kneading block" leaves disc width, number of discs, offset angle, direction and axial position unanswered.
The same caution applies to uncommon or manufacturer-specific names. EJS lists Schubkanten, Igel and Camelback elements, but its visible product page does not define their geometry or process effect. A buyer should send the drawing, part code or sample instead of assigning a function from the name.
Map the element sequence to the process
A Thermo Fisher Scientific application note divides a parallel twin screw compounding line into feeding, plasticizing, conveying, mixing, venting and extrusion zones. Its example uses partially filled conveying sections, full mixing sections, another conveying section for venting or split feeding, then an extrusion section that builds pressure.
| Process position | Main job | Element record to preserve | What can go wrong after a casual substitution |
|---|---|---|---|
| Main feed | Accept solids, move air and material away from the inlet, and establish stable fill. | First element position, feed opening relationship, pitch progression and shaft orientation. | Material can back up or feed unevenly even when the new element fits the shaft. |
| Plasticizing | Apply heat and mechanical work so the polymer begins to melt. | Conveying-to-kneading transition, block widths, angles and exact zone length. | Melting can shift upstream or downstream, changing torque and temperature. |
| Primary mixing | Disperse and distribute the formulation at the intended fill level. | Kneading and mixing element type, direction, offset, sequence and neighbors. | A stronger restriction may add residence time or heat; a weaker section may leave agglomerates. |
| Side feed or liquid addition | Create the open volume and conveying condition needed for the added stream. | Port location, element type directly below the port and downstream sealing section. | Powder can flood the port, or liquid can move toward the wrong zone. |
| Venting | Expose melt surface and move released air or volatiles toward the vent. | Vent position, conveying geometry under the opening and restriction downstream. | A filled vent zone can discharge melt through the opening. |
| Discharge | Build and stabilize pressure for the die or downstream unit. | Final conveying or pressure-building elements, adapter and die relationship. | Pressure, temperature and output can change despite an unchanged die. |
This is why a row of loose parts is not a screw design. The sequence connects each geometry to feed ports, temperature zones, vents, barrel sections and the die. The EJS plastic compounding twin screw barrel page identifies feeding, melting, mixing, venting and pressure development as distinct compounding functions. Record them as distinct hardware positions.
Identify the element family before comparing dimensions
| Element family | What it generally does | Details that change its behavior | How to describe it in an RFQ |
|---|---|---|---|
| Forward conveying element | Moves material downstream and provides free volume between more restrictive zones. | Lead or pitch, number of flights, length, handedness, outer profile and fill condition. | State the exact lead, width, direction, shaft side and axial position. |
| Reverse conveying element | Creates a backward-pumping restriction that can raise fill and pressure upstream. | Reverse lead, length, downstream restriction and the formulation's sensitivity to residence time. | Mark it as reverse and photograph both faces. Do not infer direction after cleaning. |
| Kneading block | Applies mixing and shear through offset discs while also affecting forward conveying. | Disc count, disc width, stagger angle, forward or neutral orientation, and block length. | Give the block drawing or every disc and angle value. "Kneading block" is incomplete. |
| Distributive mixing element | Rearranges melt streams to improve spatial uniformity with a geometry designed for that mixing job. | Cutouts, channels, tooth or lobe geometry, open area and position. | Use the original part code, drawing or sample. Names are not consistent across machine systems. |
| Transition or profile element | Connects unlike geometries or completes a defined change between zones. | Both mating faces, bore, lobe orientation and neighboring elements. | Photograph the element between its two neighbors and mark the feed-facing side. |
| Special element | Performs a manufacturer-specific job such as elongational mixing or another controlled flow pattern. | The complete geometry and intended process effect. | Do not substitute from a similar trade name. Send the approved drawing or physical sample. |
The table describes the decision logic, not a universal configuration. Screw speed, throughput, formulation, feed method, venting and die resistance all change how a given sequence operates. Thermo Fisher notes that backward mixing elements can materially change residence-time behavior, while screw speed and feed rate affect melt temperature through mechanical energy input in mixing zones.
Read kneading blocks as a mixing and conveying trade-off
Disc angle is not a cosmetic feature. Coperion publishes a direct comparison of 45-degree and 90-degree kneading blocks in its extrusion research material. In that comparison, the 45-degree block has more conveying effect but less mixing effect. The 90-degree block produces stronger mixing but does not convey the melt forward.
That does not make 90 degrees "better." A block that restricts forward flow can increase fill, residence time, pressure and product stress in the surrounding section. Whether that helps depends on the process job and material. The correct angle belongs to a complete sequence, not a ranking chart.
| Record | Why it matters | Common documentation failure |
|---|---|---|
| Stagger or offset angle | Changes the balance between forward conveying and mixing. | Writing "KB" without the angle. |
| Forward, neutral or reverse orientation | Changes pumping direction and the fill condition around the block. | Photographing only one face with no feed-direction arrow. |
| Number and width of discs | Defines the block length and the amount of working geometry in that position. | Measuring total length but not recording the disc construction. |
| Position from a fixed datum | Connects the block to the correct barrel zone, port and temperature section. | Counting from whichever shaft end happens to be nearest. |
| Pairing on the second shaft | Preserves the intermeshing relationship and intended phase. | Mixing all left- and right-shaft elements into one tray. |
Use the EJS material list without turning it into a ranking
The current EJS screw element page lists 38CrMoAlA (1.8509), Steel 45, W6Mo5Cr4V2, 40CrNiMoA, 40Cr and HYM4 as popular materials. It lists full-body nitriding and bimetallic alloy coating as surface-treatment routes. The page also places screw elements in color, additive and filler masterbatch service, along with film, pipe, profile, fiber and other extrusion applications.
Those facts show the available manufacturing directions. They do not establish one grade as the default for every position. A feed element handling clean polymer, a kneading block working on mineral-filled compound and a discharge element exposed to corrosive chemistry can face different wear mechanisms on the same machine.
| Selection input | What to send EJS | Why the grade name alone is insufficient |
|---|---|---|
| Abrasive loading | Filler or fiber type, amount, particle contamination and wear map by axial position. | Wear is usually concentrated where fill, shear and hard particles meet. |
| Corrosive duty | Polymer and additive package, actual temperatures, shutdown history and attack photos. | Hardness and corrosion resistance are different material properties. |
| Mechanical load | Machine model, shaft interface, torque history and any spline damage. | The working surface must be supported by a body and shaft connection that suit the load. |
| Existing specification | Original material, treatment, hardness report, drawing and service history. | A part can look interchangeable while using a different heat-treatment route. |
| Mating barrel | Barrel bore material or liner, measured bore wear and position of the element inside it. | Element and barrel surfaces work as a wear pair with a controlled clearance. |
The separate EJS segment barrel page lists full-body nitriding, a full-piece bimetallic carbide liner and a two-piece bimetallic carbide liner. It also tells buyers to identify whether a barrel segment is at the front, middle or tail because the connections differ. The same position discipline should be used for the elements inside that barrel.
Preserve both shaft sequences before removal
Cleaning a mixed pile of elements is the wrong first step. Oil, polymer and color can be removed later. Lost order, orientation and shaft identity may be impossible to reconstruct from the loose parts.
- Choose one datum. State whether position zero is the gearbox face, first process element or discharge end. Use the same datum on both shafts.
- Mark the shafts. Identify left and right as viewed from a stated direction. Add a feed-direction arrow to every photo sheet.
- Photograph the assembly. Take an overall image and overlapping close views. Keep a scale in frame without covering the element interfaces.
- Number before sliding. Mark each position on a durable tag. Pair the two elements that worked at the same axial station.
- Record the faces. Photograph feed-facing and discharge-facing sides, especially for transition, reverse and special elements.
- Measure after cleaning. Record geometry and wear without sanding away edges or polishing the evidence that engineering needs to see.
Build a replacement specification that another person can check
The EJS product page says buyers normally send drawings or samples. A strong drawing package separates machine identity, shaft fit, working geometry and process position. It also preserves the original units instead of relying on a converted screenshot.
| Specification group | Fields to include | Evidence format |
|---|---|---|
| Machine identity | Extruder maker, model, series, year if known, screw diameter and rotation arrangement. | Nameplate photo plus current machine manual or process-section drawing. |
| Shaft interface | Spline standard or profile, tooth count, major and minor dimensions, engagement length, bore profile and any key or locking detail. | Dimensioned drawing, calibrated measurement report or unworn sample. |
| Element envelope | Outer diameter, total width, number of flights or lobes, both end faces and any relief or chamfer. | Front, rear and side views tied to drawing dimensions. |
| Working geometry | Lead or pitch, handedness, kneading disc count and offset, mixing cutouts, transition profile or original part code. | Original OEM drawing or a complete reverse-measurement record. |
| Sequence position | Left or right shaft, axial start, feed-facing side, paired element and both neighbors. | Two-shaft sequence map with numbered photos. |
| Material and finish | Base material, heat treatment, working surface, hardness requirement and inspection document required. | Existing certificate, approved drawing and current wear evidence. |
| Operating duty | Polymer, filler or fiber, regrind, output, speed, torque and actual temperature window. | Stable production log and formulation summary. |
A physical sample can resolve geometry that is difficult to measure, but a worn sample is not a master dimension. Mark the worn zones and provide the machine or OEM reference so engineering can separate original geometry from material loss.
Inspect the shaft and barrel at the same axial position
Replacing an element does not repair a worn shaft spline or an enlarged barrel bore. A new element on a damaged spline can move under torque. A new outer profile inside a worn barrel may still leave excessive working clearance. Localized wear on one element can also point to a restriction, poor pairing or contact at the corresponding barrel section.
At each suspect station, record the element bore and shaft spline condition, outer-profile wear, matching element on the other shaft, and both lobes of the barrel bore. The current EJS screw barrel quality inspection guide provides a broader pre-payment inspection framework. For an element-only order, keep the acceptance record specific to the drawing and agreed inspection points.
This check is also where the buyer decides the scope. Replacing one position may be reasonable when the damage is isolated and the mating parts remain serviceable. A repeated pattern across several stations calls for a sequence, material or process review rather than a bag of identical replacements.
Prepare a twin screw element RFQ
Use this form to organize the request, then send the full drawing, sequence map and photos through the approved EJS inquiry channel. Machine model and element diameter alone are not enough for manufacturing.
Frequently asked questions
What are the main types of twin screw elements?
The common functional groups are conveying elements, reverse elements, kneading blocks, distributive mixing elements, transition elements and special process elements. EJS also lists profile, Schubkanten, elongational, Igel, Camelback and blister elements. Use a drawing or sample for uncommon names because terminology and geometry can differ by machine system.
Can I order a kneading block by diameter and angle?
No. Also specify the shaft bore and spline, total width, number and width of discs, offset sequence, forward or reverse orientation, material, treatment, shaft side, axial position and neighboring elements. A sample or approved drawing is preferable.
Why must I record the order of the screw elements?
The sequence connects each element to feeding, melting, mixing, side feeding, venting and pressure building. Moving one element can change fill, residence time, shear, temperature or pressure even when every part still fits the shaft.
Does a 90-degree kneading block always mix better?
Coperion's published comparison shows stronger mixing and no forward conveying contribution for its 90-degree block, while its 45-degree block conveys more and mixes less. That does not make 90 degrees the universal choice. The correct angle depends on the complete process sequence and material.
Which materials does EJS list for screw elements?
The current EJS screw element page lists 38CrMoAlA (1.8509), Steel 45, W6Mo5Cr4V2, 40CrNiMoA, 40Cr and HYM4. It lists full-body nitriding and bimetallic alloy coating as treatment routes. Final selection should use the formulation, wear mechanism, mechanical load and mating barrel condition.
Can I replace only one worn screw element?
Sometimes, if the wear is isolated and the shaft spline, paired element, neighboring elements and barrel bore remain serviceable. Measure all mating parts at the same axial station before deciding. A repeated wear pattern needs a sequence, material or process review.
What should I send EJS when no original drawing is available?
Send the machine nameplate, both shaft sequences, clear orientation marks, an unworn sample if available, the worn sample with wear zones marked, calibrated measurements, spline details, material and treatment records, process duty and photos of the matching barrel station.
Sources checked for this guide
- EJS Industry: Screw Element for EJS element names, materials, treatment routes, applications and drawing/sample inquiry route.
- EJS Industry: Segment Barrel for barrel materials, liner routes and position-specific connection note.
- EJS Industry: Segment Screw Barrel for modular parallel twin screw product context.
- EJS Industry: Plastic Compounding Twin Screw Barrel for EJS compounding applications, materials and surface treatments.
- Thermo Fisher Scientific: Relevant Process Parameters for Twin Screw Compounding for process zones, segmented element roles, fill conditions and residence-time context.
- Coperion: Extrusion research and development for the published 45-degree and 90-degree kneading-block comparison and screw-element process effects.