Introduction To The Segmentation of The Extruder Screw

When the material moves forward along the screw, it experiences changes in temperature, pressure, viscosity, etc. This change is not the same throughout the entire length of the screw. According to the changing characteristics of the material, the screw can be divided into a feeding section, a compression section and a homoge0nization section.

①. Plastics and three states of plastics

There are two categories of plastics: thermoset and thermoplastic. After thermoset plastic is molded and solidified, it cannot be heated and melted for molding. Products formed from thermoplastic plastics can be reheated and melted to form other products.

As the temperature changes, thermoplastic plastics produce three state changes: glassy state, high elastic state and viscous flow state. As the temperature changes repeatedly, the three states change repeatedly.

a. Different characteristics of polymer melt in three states:

Glassy state - the plastic appears as a rigid solid; the thermal kinetic energy is small, the intermolecular force is large, and the deformation is mainly contributed by bond angle deformation; the deformation recovers instantaneously after the external force is removed, which is a general elastic deformation.

Highly elastic state - the plastic appears as a rubber-like substance; the deformation is contributed by the conformational stretching of macromolecules caused by chain segment orientation, and the deformation value is large; the deformation can be recovered after the external force is removed but is time-dependent, which is a highly elastic deformation.

Viscous flow state - plastic appears as a highly viscous melt; thermal energy further intensifies the relative slip motion of chain molecules; the deformation is irreversible and belongs to plastic deformation.

b. Plastic processing and three states of plastic:

Plastics can be cut when they are in the glassy state. In the high elastic state, it can be stretched and processed, such as wire drawing, extrusion, blow molding and thermoforming. In the viscous flow state, it can be processed by coating, rotational molding and injection molding.

When the temperature is higher than the viscous flow state, the plastic will thermally decompose, and when the temperature is lower than the glassy state, the plastic will embrittle. When the temperature of the plastic is higher than the viscous flow state or lower than the glassy state, the thermoplastic plastic will tend to deteriorate and be damaged seriously. Therefore, these two temperature areas should be avoided when processing or using plastic products.

② Three segmentations of the extruder screw

There are three physical states of plastic in the extruder - glassy state, highly elastic state and viscous flow state. Each state has different requirements for the screw structure.

c. In order to adapt to the requirements of different states, the screw of the extruder is usually divided into three sections:

Feeding section L1 (also known as solid conveying section)

Melting section L2 (called compression section)

Homogenization section L3 (called metering section)

This is what is commonly called a three-stage screw. The extrusion process of plastic in these three stages is different.

The function of the feeding section is to send the material supplied from the hopper to the compression section. The plastic generally remains in a solid state during the movement and is partially melted due to heat. The length of the feeding section varies with the type of plastic, and can start not far from the hopper and end at 75% of the total length of the screw cup.

Generally speaking, extruded crystalline polymers are the longest, followed by hard amorphous polymers, and soft amorphous polymers are the shortest. Since the feeding section does not necessarily produce compression, the volume of the screw groove can remain unchanged. The size of the helix angle has a greater impact on the feeding capacity of this section, which actually affects the productivity of the extruder.

The function of the compression section (migration section) is to compact the material, convert the material from solid to molten, and eliminate the air in the material. In order to adapt to the characteristics of pushing the gas in the material back to the feeding section, compacting the material and reducing the volume when the material melts, the screw in this section should produce greater shearing and compression of the plastic. For this reason, the volume of the screw channel is usually gradually reduced, and the degree of reduction is determined by the compression rate of the plastic (specific gravity of the product/apparent specific gravity of the plastic). The compression ratio is not only related to the compression rate of the plastic, but also related to the shape of the plastic. The powder has a small specific gravity and a lot of entrained air, so a larger compression ratio is required.

The length of the compression section is mainly related to the melting point and other properties of the plastic. For plastics with a wide melting temperature range, the compression section is long; for most polymers with a very narrow melting temperature range, the compression section is even only one pitch in length.

The function of the homogenization section (measuring section) is to feed the molten material into the machine head at a constant volume and pressure to form it in the die. The channel volume of the homogenizing section is as constant as that of the feeding section. In order to prevent materials from being retained in the dead corner of the screw head and causing decomposition, the screw head is often designed to be conical or semicircular; the homogenizing section of some screws is a rod body with a completely smooth surface, but some are also engraved with grooves.

d. According to the melt transport theory, there are four forms of melt flow in the screw homogenization section. The flow of molten material in the screw groove is a combination of these four flows:

Forward flow - the flow of plastic melt between the barrel and the screw along the groove direction toward the machine head.

Counter flow - the flow direction is opposite to the forward flow, caused by the pressure gradient caused by the resistance of the machine head, porous plate, filter plate, etc.

Cross flow - the flow of melt along the direction perpendicular to the thread wall, affecting the mixing and heat exchange of the melt during the extrusion process.

Leakage flow - the backflow formed in the gap between the screw and barrel due to the pressure gradient, along the axial direction of the screw.

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