Twin Screw Compounding Extruders - Introduction to Screw Design
Let me begin by saying that there is nothing like one “universal” screw design that is ideal for processing all polymers and compounds. To achieve best efficiency, screw design is a critical parameter and just like formulations it must be exactly the same whenever that particular compound is manufactured. This is so because it affects the quality of the product. Polymers are complex molecules and each one has its own unique characteristic which requires different designs and processing conditions to get optimum results. Here are some general tips and guidelines to be kept in mind when designing the extruder:
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Keep the L/D as short as possible. This ensures that the polymer experiences only the minimum required heat and shear. A long L/D is one of the main causes of quality problems. So much so that processing materials like unfilled Polycarbonate may need no more than an L/D of 24. In this case, a longer extruder will only cause yellowing and increase the MFI.
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The design should be as “mild” as possible. This is so because a “strong” design with more kneading blocks or left-hand-elements will subject the polymer to unnecessary excess shear resulting in degradation. In case of Glass Fiber reinforced materials, excess kneading leads to breakage of fibers and subsequently lower mechanical properties.
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Use long pitch elements at all feed openings and at vacuum port. This allows faster conveying of material and a larger free volume which is helpful to ensure proper feeding while also helping in preventing vacuum choking. Often, the use of long flight SKE type of elements are suggested though they may not be the best for conveying melted material due to their non-self-wiping design.
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Create a proper internal melt seal just before the vacuum port. This is done by using a left-hand element so that a pool of melted material is formed which prevents air sucking from upstream openings.
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Use the shortest and mildest possible compression zone before the die. This prevents excessive heat generation and protects sensitive ingredients from heat damage. Keep in mind that by using proper compression elements of the correct length, the backup length can be tailored to suit the product being processed.
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Use of 3-lobe and Fractional Kneading Blocks is an advantage especially when melting and mixing has to be done at lower average shear rates.
Last but not the least, using a good Screw Design Software helps in making correct, reliable and readable screw design drawings. While there are several international software companies specializing in screw design drawings, the only Indian software available in this domain is from Steer Engineering in Bengaluru. This generates realistic screw design images which are easy to understand.
If you have any other questions or would like to suggest topics for us to write about, please feel free to contact us at info@polymerupdateacademy.com
Author
Dr. Pradeep Bakshi
Trainer, Polymerupdate Academy
