hot melt extrusion process

With the rapid development of the polymer materials industry, the development and application of reciprocating kneading and extruding system (co kneader) has become increasingly attractive. The performance of many aspects of the single-screw and twin-screw extruders that dominate the extrusion industry can no longer meet the requirements for blending, filling, reinforcement, toughening and other modifications. It has gradually become a trend to use co kneader to produce some high-tech and high value-added products.

During the processing of polymers, in order to impart certain characteristics to the polymer or specific performance properties to the product, it is often necessary to blend different polymers or mix the polymer with various additives and pigments. The co-kneader happens to have the characteristics of uniform shearing, high dispersion, high filling, and melt stretching. And co-kneader combines the advantages of single and twin-screw extruders with a unique working principle, plus a complete set of thread combination components and supporting equipment, making it more widely used. On one machine, mixing, kneading, plasticizing, dispersing, shearing, stretching, degassing, and granulation can be accomplished, making the interface area of the melt flowing in the machine much larger than that of normal shear flow.

Co Kneader Working Principle


The structure is different from single-screw and twin-screw extruders.

The co-kneader has a unique building block screw design on the screw core shaft, which is broken three times within one pitch, called the mixing screw block. Corresponding to these gaps, there are three rows of mixing pins arranged on the inner sleeve of the barrel. During the radial rotation of the screw, it also performs axial reciprocating motion. Each rotation makes an axial movement. Due to this special movement method and the action of the mixing screw block and pin, the material is not only be sheared but also transported back and forth between the mixing pin and the irregular trapezoidal mixing block. The countercurrent motion of the material adds a very useful axial mixing motion to the radial mixing. The melt is constantly cut, turned, kneaded and stretched, regularly interrupting simple lamellar shear mixing.


Can prevent material from degradation.

Due to the simultaneous mixing in the radial and axial directions, the mixing effect is enhanced and the best dispersion and distribution mixing are ensured, so the homogenization time is short. In addition, the mutual engagement between the mixing pin and the screw block also improves the self-clearing ability of the barrel. Through the appropriate combination of screw blocks, stable working pressure can be ensured without uncontrollable pressure and temperature fluctuations, and degradation of materials in the barrel can be prevented.


Screw torque deformation is small.

Since the thread of the kneading element is interrupted, the pressure acting on the polymer is very small and no significant temperature rise occurs. Single- and twin-screw extruders shear thin-layer polymers under high shear and high radial pressure, which will inevitably cause a large temperature rise in the melt and affect the principle of low-temperature extrusion of materials. Compared with other machines, under the same diameter condition, its aspect ratio is small, and torque is transmitted over a shorter distance. Therefore, the torsional deformation of the screw is small.

Co Kneader Structure and application

The main machine barrel and screw are assembled like The barrel is composed of multiple groups of open and closed-hole cylinders. The clam-shell barrel can be opened quickly and conveniently for cleaning and maintenance. The screw consists of various mixing screw blocks and conveying screw blocks mounted on a mandrel. The barrel and screw can be flexibly formed into an ideal form according to different types of materials and different process requirements.

The unique design of the gear box and swing box enables the screw to perform axial reciprocating motion while rotating. Each time the screw rotates once, it reciprocates once and the thread is interrupted three times, thus producing a strong mixing effect. The mixing effect is axial rather than radial, and occurs between the thread and the pin. All materials in the screw channel are subject to uniform shear stress, rather than a thin layer of material being sheared.

The threaded components are arranged in a building block style on the shaft and can be arranged and combined according to different requirements. There are three rows of pins installed on the inner liner, and the pins are divided into six types. The solid long pins are used to fix the inner liner sleeve and enhance the mixing effect; the hollow long pins are used to fix, mix, install the temperature measuring head, and inject liquid additives. The long flat-head pin fixes the inner liner; the flat-head pin is used to block the spare hole; the solid pin plays the role of mixing; the short solid pin crushes the material at the feed port.

As the screw reciprocates, the relative motion trajectory of the pin and each screw block forms a ∞ curve. When the kneading pin passes through the thread gap, axial material exchange occurs. During one revolution of the screw, the relative movement of the pin with the side of the screw shuttle causes shearing of the material, and at the same time, the screw and barrel are cleaned in the axial and radial directions. This is the basis for the thorough mixing, kneading, and self-cleaning action of a reciprocating compounding extruder.

These pins act like a series of agitators to redistribute the material. The interrupted threads cause the material to flow back, allowing the material to undergo a continuous process of shearing, orientation, cutting, folding, and stretching. Part of the melt is recombined, and part of the melt is further divided. In each L/D channel, the melt has to be divided four times. From this, we can see why the reciprocating extruder can operate within a very short aspect ratio. It achieves effective mixing of the melt, and the inner surface of the barrel is swept by thread elements. There are no dead ends in logistics. It has good self-cleaning ability, high conveying efficiency and stable and uniform material residence time.

The function of the melt pump is to pressurize the high-temperature polymer melt from the extruder and send the flow rate to the extrusion die stably after clearing the pressure. No matter how the pressure inside the extruder changes or the flow rate fluctuates, the melt pump can always supply a certain amount of material to the die. The gear is the core of the melt pump. On the one hand, the rotation and meshing of the gear can squeeze the melt between the teeth and transport the material from one end to the other. On the other hand, the meshing of the gear can close the low-pressure cavity, reduce backflow and lower the processing temperature, which is conducive to establishing higher output pressure. We use involute helical gears, which can increase the overlap of gear meshing. During meshing, as the gears continuously inhale and discharge, the material can be squeezed continuously and smoothly, reducing pressure fluctuations and achieving stable melt conveyance. At the same time, the helical gear also has a certain self-cleaning effect to avoid the deposition and decomposition of materials.

A heating and cooling device is installed outside the melt pump to control the temperature within a certain range to maintain the best process state. The heating methods include electric heating and oil heating. In addition, the working principle and characteristics of the co-kneader determine that the machine is not suitable for plastic molding processing. However, a melt pump is installed after the extruder and is connected to the molding die. This new process combines mixing and modification with molding processing, which reduces the granulation process, and means reducing one-time investment and lowering production costs , shortening the production cycle, saving energy, and the melt omits the thermal history of secondary processing and plasticization. The advancement and economy of this technology are self-evident. The melt pump is equipped with a set of pressure sensors that can accurately measure its pressure changes, which is beneficial to process adjustment.

Accurate measurement and reasonable feeding methods are the key to strictly implementing the formula and are also the first step to ensure product quality. According to the performance of the material and the needs of the users, we are equipped with a variety of feeding methods, such as volume measurement, dynamic weight loss measurement, etc., to meet the needs of different products.

Accurate measurement and reasonable feeding methods are the key to strictly implementing the formula and are also the first step to ensure product quality. According to the performance of the material and the needs of the users, we are equipped with a variety of feeding methods, such as volume measurement, dynamic weight loss measurement, etc., to meet the needs of different products.

This machine adopts Siemens frequency conversion speed control device, which has excellent performance and reliability. The barrel adopts electric heating and water cooling (or oil heating and oil cooling), equipped with a secondary pressure sensor, six sets of RKC intelligent temperature control devices, two-way PID adjustment, and four sets of melt temperature sensors deep inside the barrel.It truly reflects the actual temperature of the melt in each zone. In addition, it has a multi-point temperature inspection instrument, so the equipment has accurate temperature control and small errors. Advanced mechanical and electrical interlocking devices have dual mechanical and electrical overload protection, which can effectively protect equipment and personal safety. The equipment control cabinet is equipped with an analog display board, and the rotation status of each part of the unit is intuitively and clearly displayed.

The system is equipped with several cutting methods such as drawing water-cooled pelletizing, hot-cut water-cooling, hot-cut air-cooling, etc. It can be configured according to different materials and user requirements.

The system is equipped with a two-stage granulation unit, which uses equidistant and equal-depth single-screw extruders to transport the melt. Its function is to complete plasticization and longitudinal mixing and establish stable extrusion pressure. Because of its low speed, the shear is low, which avoids shear heat in the material and provides the best temperature control. With this configuration, for some low-foaming, heat-sensitive products (especially PVC products), the system separates the two-step processes of mixing and pressure building. After the melt is plasticized, it will be transported to the extrusion die at a constant pressure, temperature, and quantity, to prevent the degradation of heat-sensitive and shear-sensitive materials. It has the characteristics of high output and stable product quality. The mixing process can be optimized and is not affected by the material-making process. During the process of the material entering the screw, a vacuum exhaust device is provided. This method of exhausting has a larger exhaust area, a larger change in the material surface area, and sufficient exhaust time, so it has a better exhaust effect than a single-stage exhaust extruder.

The main machine barrel is equipped with a vacuuming device and a natural exhaust device, and the forced feeding hopper and side feeding are equipped with natural exhaust devices to ensure the complete elimination of moisture in the material and various volatile gases generated during the processing.

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