The Role of The Twin Screw Compounder in Plastic Compounding and Pelletizing

Various machines modify plastics in many ways at the production level. The twin screw compounder is one of these machines used in plastic compounding and pelletizing. These are processes that change the standard properties of plastics. In this article, you will learn how this machine facilitates these processes.

What is Compounding?

The term compounding is derived from the act of mixing many compounds. Compounds are materials that are made of multiple chemical elements. On the other hand, plastics are made of more than one compound. These compounds include polymers and other components, depending on the specific type of plastic.

Compounding is the term used to describe the process used to mix all the ingredients needed to manufacture plastic. This process requires heat, pressure, and force to mix these ingredients completely. If there is incomplete mixing, the end product tends to have bubbles that affect the structural integrity of the plastic. This is why manufacturers prefer the twin screw compounder for the best results.

The Working Principle of the Twin Screw Compounder

To understand how this machine works, you need a basic understanding of its main structure, including its components. As you can tell from its name, the twin-screw compounder is made of two screws. This is the main structural feature that describes its working principle.

The main components of the compounder are screws, barrels, feeding systems, heaters, and coolers. However, this does not disregard the importance of the other components of the twin screw compounded.

The process of plastic compounding and pelletizing starts when plastic is fed into the barrel containing the twin screws through the feeding bins. This plastic is subjected to high pressure by the rotating action of the twin screws.

As the screws rotate to granulate the plastic, the heater melts the plastic down to facilitate mixing with the additives used in plastic compounding.

Plasticization and pelletizing

Plastics have thermal and mechanical properties. These properties can be improved or lowered either by modifying the plastic at the molecular or physical level. This process is defined as plasticization. The materials used to alter these properties are known as plasticizers.

Pelletizing on the other hand is a technique used to shred or break down plastic into small pieces. This is done to reduce the surface area of the plastic. Pelletizing is among the necessary steps that enable the twin screw compounding process.

Intermeshing principle

The twin screw compounder is further categorized into three types based on the design and rotation sequence of the screw. The three categories are:

  • Co-rotating – In this type, the threads of each screw interlock completely and rotate in the same direction.
  • Counter-rotating type. Screws are designed to rotate in different directions relative to each other.
  • Non-intermeshing – Screws in this type of twin screw compounder rotate independently but still manage to granulate the plastic.

The co-rotating and counter-rotating screws can either be fully or non-intermeshing. This design consideration will depend on the manufacturer and intended use case for the machine. The full intermeshing type of compounder is the most effective for plastics. For this reason, our twin screw compounder/extruder is the full intermeshing type.

The Compounding Process

The plastic modification compounding process follows a series of steps from selecting the starting material to assembling the finished product. To demonstrate how this process works, we assume that we are recycling used plastic to make another plastic product. 

The compounding process begins with shredding the plastic using the rotating screws. It is imperative to ensure that the plastic is clean to avoid affecting the properties of the end product. As the screws intermesh and granulate the plastic, the heater melts the plastic to make sure it mixes with other components.

In the third step, additives such as stabilizers, fillers, colorants, and other polymeric components are added. The aim of this is to make the plastic more durable, especially when the target product is intended for heavy-duty applications.

Finally, after mixing these elements, the melted plastic needs to be cooled for it to form back into a solid piece. This is what is extruded as pellets that can later be melted and mixed to form one large solid or used as pellets.

Advantages of the Twin Screw Compounder

The twin screw compounder has dominated several polymer compounding lines due to its many benefits. These advantages include the following;

Efficient screw design
Twin screw compounders are highly reliable due to their screw design. All aspects of design including the pitch, channel depth, flight width, and helix angle are engineered with precision and so much consideration. 

High shear force
Shear is the force that screws exert against each other as they rotate. This force is essential in facilitating shredding and mixing operations inside the barrel. High shear force also leads to a short lead time which means more productivity.

All-in-one approach
Unlike other plastic compounding systems, the twin screw compounder is an all-in-one approach. This means that all the processes take place within one system from start to finish. The benefit of this is more value from a single system.

Vents to facilitate degassing 
Provided that plastic produces lots of fumes, degassing is essential. The compounder has vents that release the fumes. Ensuring that fumes do not circulate in the barrel is also meant to ensure that the final product is not contaminated. 

Quality control features
The machine has features such as pressure and temperature control to ensure that the final product is as expected. Cooling and drying are also performed after the process to facilitate the forming of the pellets.

Applications of the Twin Screw Compounder

The applications of the twin screw compounder span across industries. Most of these industries deal with polymeric compounds and other chemical substances. Compounders are used to process, mix, and compound plastics. The specific use cases might vary depending on the industry.

Some of the applications include;

  • Polymer compounding applications – Use cases here include modifying blends, fiber enhancement, and alloying.
  • Reactive extrusion – Applications include modifying polymers by melting, blending, and shaping.
  • Rubber and plastic processing – Applications can include recycling or manufacturing plastic and rubber products from raw rubber and plastics.

Conclusion

Plastic compounding is essential in various industries as its uses are ever-growing. With machines such as the twin screw compounder, this process has become much more streamlined. Moreover, this is expected to improve as better iterations of these machines are developed.

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