Each material has its own characteristics. To achieve the desired output and quality in molded products, a single screw type cannot meet all the needs of plastic molding. The screw must be chosen based on the properties of the raw material. The selection of a screw's compression ratio, length-to-diameter ratio, and form should be based on the melting temperature, stability, viscosity, and flow rate of the extruded material.

1.   Types of Screws: To improve the extrusion efficiency and plastification capacity of extruders, the screw type is chosen based on the type of extruded material. For crystalline polymers, screws with equal pitch but varying diameter, such as those used for PE, PA, POM, should be selected. For amorphous polymers like PVC, PS, ABS, PC, etc., screws with equal pitch and gradual variation are suitable. Various new types of screws have also been developed for selection.

   a. Separating Screw: This type of screw features an additional thread in the compression section, dividing the original thread's groove into two. One groove communicates with the feeding section for transporting solid materials, and the other with the homogenizing section for transporting viscous materials. This design prevents temperature fluctuations caused by the coexistence of solid and viscous states in the same groove.

   b. Barrier Screw: A barrier screw is designed with a barrier in a certain part of the screw to prevent unmelted solid materials from passing through, thus promoting the melting of solid materials.

   c. Diverting Screw: This screw has multiple protrusions, grooves, or channels at certain parts to split the flow in the groove, altering the material's flow state, improving the melting state, and enhancing mixing and homogenizing. An example is the pin screw, which is typically placed in the compression section for uniform mixing and low-temperature extrusion.

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3. Selection of Screw Length-to-Diameter Ratio: The length-to-diameter ratio of the screw is a critical parameter. When combined with the screw speed, it indicates the screw's plastification capacity and quality. Common ratios include 11:1, 15:1, 20:1, 25:1, and 30:1. Increasing the ratio extends the screw length, allowing materials more time inside the barrel for more uniform and thorough plastification, thus enhancing output and quality. However, a higher ratio complicates the manufacturing and assembly of the screw and barrel, increases costs, and requires more space for the extruder. Additionally, a larger length-to-diameter ratio may cause uneven gaps between the screw and barrel due to bending, potentially scraping the barrel and reducing the extruder's lifespan. Therefore, it is essential to achieve high quality and high output under a smaller length-to-diameter ratio. The required length-to-diameter ratio for common plastics is as follows

4. Plastic Name	L/D Ratio	Plastic Name	L/D Ratio
   RPVC	(16~22):1	ABS	(20~24):1
   SPVC	(12~18):1	PS	(16~22):1
   PE	(22~25):1	PA	(16~22):1

   Selection of Compression Ratio: The purpose of determining the compression ratio is to compress and exhaust the material and establish the necessary pressure. The choice of compression ratio is related to the properties of the material and the requirements of the product. For an extruder that only extrudes one type of raw material, a specialized screw should be selected. If the extruder is used to extrude a variety of materials with different properties, it is also necessary to consider having several screws suitable for different materials for extrusion molding or to choose a general-purpose screw.