Understanding Granulation Forms in Twin Screw Extruders

Introduction

When two or more chemicals are combined or compounded, the traditional twin screw extruder is utilized for the plastic extrusion process. Reactive polymeric materials are the ideal candidates for this extrusion method. A twin screw extruder is especially helpful for producing stiff mixtures of wood fiber and PVC. Using a kneading block, forward and reverse capabilities, and other unique mixing features, two screws rotate either in tandem or against one another in this approach.

twin screw extruder

Understanding Granulation Forms

By definition, granules are collections of basic particles. They are extensively employed in several sectors. Granules for pharmaceutical applications can be produced using a variety of techniques.

One type of granulation technology is wet granulation, which involves agglomerating powders using solvents or binder solutions. The fluid bed granulation, high shear granulation, and spray drying procedures are examples of wet granulation techniques. Conversely, there are dry granulation techniques like roller compaction and slugging that use mechanical pressure to create agglomeration.

Melting materials facilitate the increase of particle sizes for melt granulation procedures, which can be executed, for example, in high-shear mixers. Powder particles are expanded into granules with a particle size range of 0.1 to 2.0 mm in each of these procedures. Granules are now typically utilized as fillers or intermediates for capsule filling or tablet compression—the most common dosage form. Granules were first given in a multiple-unit dose form in sachets. 

Co-Rotating Twin Screw Extruders

You must first comprehend the extrusion process to comprehend what an extruder is. The process of extrusion involves heating and forcing any material—typically pellets, plastic, metal bar stock, dry powder, or rubber—through a die.

In essence, a die is a mold that forms the material into the required size and shape as it is pushed through a tiny opening and out the other side. It’s a widely used method for creating sheets and strips of rubber, plastic, and standard metal forms.

A machine used in the extrusion process is called an extruder. The machine warms the product and forces it through the die to take on the required shape using a series of barrels and cylinders.

Importance of Co-Rotating Twin Screw Extruders:

An extruder can be used to shape several materials into a wide range of useful items. Through precise control of processing parameters, the extrusion process attempts to physico-chemically change viscous polymeric medium continuously, producing high-quality structured products.

The raw ingredients for our twin screw extrusion method might be liquids or solids like slurries or powders, granulates, and flours. Products that are extruded include cellulose pulps, food and feed products with texture, polymeric compounds, and chemically modified polymers. We have been providing the plastics sector with thousands of recycled plastic goods in a variety of categories since 2014.

Advantages and applications:

To meet diverse mixing needs, the twin screw extruder machine’s combination design may organize and combine screw and barrel components in a variety of geometrical configurations. The design of the co-rotating twin screw extruder is very adaptive since it is tailored to fit the requirements of various process formulas. It primarily shows up in the following ways:

Processing Flexibility

The Co-Rotating Twin Screw Extruders typically use metered feeding, meaning that at a given speed, the amount fed determines the output. As a result, you can handle various processing tasks in a machine, such as melting, mixing, exhausting, and reacting, with flexibility.

High Production Efficiency

A screw with positive displacement can handle a greater quantity of raw materials and combined formulas with little downtime.

Process Parameter Controllability

The small dwelling time distribution of the co-rotating twin screw extruder facilitates accurate temperature curve control and convective heat transfer. To achieve greater stability in the product quality, it can also achieve superior extrusion shearing-time-temperature courses and input mechanical energy in a wider range.

Increased Economy

Owing to the high productivity and process flexibility, it can create a broader range of finished products, deliver highly consistent product quality, and change the screw speed to account for wear on the screws.

Improved Mixing Performance

Improved mixing performance is achieved because the two screws mesh with one another and because there are many different threaded components with rich functionalities, like an anti-threaded, meshing disc, tooth disc, and so on. Process engineers may precisely regulate the shearing/mixing process by adjusting the screw element combination based on the material’s processing requirements. This allows them to successfully control the mixing strength and mixing quality, regardless of whether they are combining dispersions. This is more than a single screw extruder machine can handle.

Counter-Rotating Twin Screw Extruders

Twin-screw extruders that counter-rotate are employed in a multitude of plastic processes and products. It’s noteworthy to note that materials can be pumped in locked C-shaped chambers in a non-drag flow manner using closely spaced, counter-rotating twin-screw extruders. Comparing this device to the drag flow single-screw extruder and the semi-drag flow co-rotating twin-screw extruder, only ram extruders can convey by positive displacement.

Unique Features and Use Cases:

The counter-rotating dual screw extruder’s geometric specifications allow for the closure of its longitudinal flow channel, which causes the material to be driven forward by the screw—a process known as positive displacement.

As a result, a counter-rotating twin screw extruder’s ability to produce stable molding extrusion is essential. Consequently, the direction of rotation for twin screws ought to be marked.

The two screws revolving in the same direction are precisely the same from the standpoint of the screw shape, as are the thread directions. The counter-rotating twin-screw extruder has two rotational states: inward rotation and outward rotation. The two screws rotate in opposing directions.

There are currently few instances of inward rotation because, upon the addition of material from the feeding port, the material first enters the meshing area between the radial gaps of the two screws under the push of the screws, forming a material pile above.

As a result, there is less available space in the screw groove for usage, which eventually hinders the ability to receive materials from the feeder. This makes it difficult to fill the screw groove quickly and move the material ahead. Bridge formation is simple, yet feeding performance is poor.

In addition, the material that enters the radial gap between the two screws exerts a force that pushes the screws against the barrel wall and separates them, hastening the deterioration of both the screws and the barrel.

These drawbacks are not present with outward rotation. The two screws work swiftly to divide the material to both sides, filling the screw groove as they go. The material then quickly comes into touch with the hot barrel to absorb heat, helping to heat and melt it. The two counter-rotating screws have symmetrical appearances, with the right-handed screw having a different direction of threading than the left-handed screw.

Side-Feeding Twin Screw Extruders

Extruders that use twin screw side feeders can feed cut glass fibers or fillers and additives in the form of powder or pellets into the twin screw extruder’s processing area. The side feeder twin screw extruder has a dual screw profile that cleans itself and feeds the product into the extruder’s screw flights without creating stagnant zones. The side feeder’s small size means that it takes up very little room. It can be installed anywhere on the process portion of the extruder using a combi barrel or side feed. The barrels’ swivel connectors enable quick installation and removal.

Our extensive selection also includes side feeders, which are used to forcefully feed minerals, such as calcium and glass, from the side to the main channel.

The feeder is powered by an AC inverter and has a high DO-DI volume and efficiency output. These co-rotating/counter-rotating twin-screw side feeders are made especially to handle higher rates and accomplish the necessary extrusion goal.

Applications and Benefits:

Glass fiber barrels with integrated wear-resistant steel sleeves are used to increase abrasion resistance and extend service life. Rather than using an integrated type with equal screw pitch for the srcew, we used modular assembly through the shaft and intermeshing segments with equal gaps and varying pitches. To guarantee precision and output, it enhances the force-carrying effect.

Additionally, we apply several humanistic features on the side feeder, which make it more practical for users. These features include the glass pane, the splined sleeve, the flat screw head, the adjustable support, and the drawer for cleaning up leaks. For certain unique needs, we are also capable of designing.

Intermeshing and Non-Intermeshing Twin Screw Extruders

Intermeshing Twin Screw Extruder:

The “building block” concept is applied in the design of barrels and screws alike. Various material qualities and process requirements can be taken into account when determining the best screw arrangement and barrel setup.

The extruder is compatible with the “building block” design for a range of uses. Furthermore, the unique design permits the replacement of a single part, increasing the extruder’s overall lifespan and significantly lowering repair costs.

One screw operates to draw material into the intermeshing area, while the second screw pushes material out of it because the two screws rotate in the same direction at different speeds. As a result, as the material travels from one screw to the next, it is forced forward in a “∞” shape.

Intermeshing counter-rotating extruders:

The two screws in an intermeshing counter-rotating extruder are identical. The material spiral ahead on one screw is obstructed by the screw flight of the other screw because of the difference in rotation direction. As a result, “∞”-shaped material movement cannot be formed by intermeshing counter-rotating extruders.

The material is moved forward in the solids conveying section in the approximate shape of a closed “C”-shaped chamber. To allow the material to flow through, the design does, however, leave a little amount of space between the outer diameter of one screw and the root diameter of the other screw.

Counter-rotating extruders with intermeshing perform well in conveying and extrusion. Compared to a co-rotating extruder of the same diameter, the extrusion volume is approximately one times larger. In addition, a homogenous temperature, reduced heat, thoroughly dispersed materials, and a shorter material residence time are attained.

Non-intermeshing counter-rotating twin screw extruder:

Material is conveyed by this two-parallel extruder using viscosity and friction. Due to the relatively big distance between the non-intermeshing screws, there is a significant leakage flow. Because of this, there are more uses for the intermeshing type than the non-intermeshing kind.

Non-intermeshing screws promote material exchange between screws but decrease positive displacement flow when compared to single screws. They work well for chemical reactions, mixing, venting, and de-volatilizing.

Comparative Analysis

Applications of Intermeshing co-rotating twin screw extruder

The intermeshing region’s small gap causes the threads and screw grooves to move in the opposite direction, which produces a high shear rate and a good self-cleaning effect. This means that any accumulated material adhering to the screw can be scraped off, significantly reducing the material’s residence time.

Consequently, mixing and granulation are the primary uses for the co-rotating twin-screw extruders of the intermeshing type. Most fully intermeshing twin-screw extruders are used for polymer reaction extrusion; the most popular type of these extruders is the intermeshing co-rotating parallel twin-screw extruder.

Both low-speed and high-speed varieties of intermeshing co-rotating twin-screw extruders are available. High-speed co-rotating twin-screw extruders are utilized for particular compounding and reactions, whereas low-speed extruders are utilized for profiles.

Applications of intermeshing counter-rotating extruders

Strong shear, mixing, and compact forces are applied to the material as it passes through the radial clearance of the two screws, speeding up the plasticizing process. The screw flight spacing can be adjusted to obtain the required compression ratio. Process operations such as granulating and extrusion are frequently performed by intermeshing counter-rotating extruders.

Applications of non-intermeshing counter-rotating extruders

When the melt content is up to 50% and 4500–6750 kg of material needs to be dried per hour, the non-intermeshing twin-screw extruder is typically utilized in the preparation of reactive extrusion, melt or emulsion production of polymers, coloring, glass fiber reinforcement, and hot melt adhesives.

Factors Influencing Granulation

Temperature Control

The twin screw extruder’s construction determines the feeding section’s temperature, which ranges from 50 to 90°C. To reach the necessary melt temperature and have the heat flow profile take on the desired shape, the extruder cylinder temperatures are adjusted under the observed heat flow profile, which is assessed by a smart sensor. The subordinate control concept regulates the temperatures of the cylinders.

Screw Configuration

To enable the feed to fall into the screw’s root and be conveyed into the extruder, the screw needs to be built with lengthy, open flights. Following the feeding of the material into the extruder, the solids are moved by the screws into the extruder and down its length to the melting section, where the polymer is melted.

Feed Rate

A 2.8″ (71 mm) twin screw extruder, for instance, may have a screw speed N of 300 rpm and a feed rate Q of 900 pph. 3.0 pph/rpm would be the Q/N. With the same 3.0 pph/rpm, higher screw speeds—even up to 1000 rpm—would result in proportionately higher rates.

There are two screws in a twin screw or multi-screw extruder. These screws are extremely effective Twin Screw Extruders that can process many tons of plastic per hour, despite their diminutive size. Twin screw extrusion equipment’s versatility enables it to be specially tailored for the formulation being processed. To attain certain mixing qualities, the screw configurations themselves can also be changed utilizing forward conveying elements, kneading blocks, reverse conveying elements, and other designs.

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