Plastic Compounding in a Twin Screw Extruder – Part 3

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Plastic Compounding in a Twin Screw Extruder

Plastics compounding is the process of adding functionality and value to polymers or polymer systems through melt blending or mixing of additives into a polymer matrix or melt mixing two or more polymers together to make an “alloy”. Extruders provide unparalleled performance and durability that make the difference to manufacturers in every industry.

Why is it necessary for plastic material to go through a compounding step? Many applications need enhanced or tailored properties to meet specific requirements…a compounding step where additions can be made helps to satisfy this need. The compounding stage allows for the precise addition of additives, pigment, anti-microbial, scents and more.

Leistritz-twin Screw extruder image from brochure

Source: Leistritz

There are three common compounding type machines, they include Twin screw, Single screw, and hi-intensity mixers. Twin screw compounding is the most popular and versatile compounding method.

As the name denotes, twin screw extruders have two screws. They can be classified into two general categories; co-rotating and counter-rotating screw configurations. By numbers, co-rotating machines are generally used more than counter-rotating machines for compounding purposes as they operate at higher speeds, provide higher shear rates, and have more mixing capability. Co-rotating machines also provide the mixing advantage provided by “lobal blending”. In co-rotating machines, several (usually 3) lobal pools provide areas where intense dispersive mixing takes place.

Twin screw extruders also allow for control of additive at different stages in the compounding process including, injecting liquids and gasses into the mix, more specifically, property boosting additives for plastics. Twin screw extruders can often operate at a higher speed than similar machines with only a single screw. This aids in increase production capacity and decreasing overhead costs for suppliers. The capability of twin screw extruders have boundaries.  Material blends in the machine will function best within a certain viscosity range. The blend needs to be small in size as the feed size is limited by the available space in the screw.

Leistritz-Screw elements from an extruder

Source: Leistritz. Examples of various screw elements

It is important to note that each set of screws are made up of smaller sections to provide different types of mixing and conveying conditions at various zones in the barrel.  The screw sets are tailored and optimized for each product. Each of the sections contributes to a different type of mixing element: dispersive and distributive. Screw sets typically possess a balance of dispersive and distributive properties. Narrower sections are typical of distributive mixing, while wider sections are common among the dispersive mixing sections. There are three basic screw elements types: forwarding, mixing and zoning. The screw sections are selected based on the element type and combined to create a tailored screw set.

Twin screw compounding is used to great advantage in the manufacturing of styrenic block (SBC) thermoplastic elastomers.  The rheology characteristics of SBC TPEs, as well as the additional needs of these materials, are a great fit for the mixing advantages that twin screw compounding provides. As an experienced compounder of styrenic thermoplastics elastomers (TPEs) we utilize the compounding stage to enhance various features of our material.

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Check out the other parts to this series: Part 1- Why are Plastics Compounded? and Part 2-The Anatomy of a Compounding Line.

To learn more: Comparison of the Flow in Co-rotating and Counter-rotating Twin-screw Extruders, Hot Melt Extrusion, and Plastic Behavior in the Twin Screw Exturder (video).

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