In the field of injection molding, the screw of the injection molding machine can be called the "heart" of the equipment. Its performance directly affects the quality of plastic products, production efficiency and the overall operation stability of the equipment. A key point that is often overlooked but extremely critical is that different plastic raw materials, such as PP (polypropylene), ABS (acrylonitrile-butadiene-styrene copolymer), and PC (polycarbonate), require screw designs that are precisely adapted to them due to their unique physical and chemical properties. Choosing the wrong screw may not only lead to frequent product defects, but also greatly reduce production efficiency and increase production costs. So, what are the differences in screw design for these common raw materials?
PP raw materials and adapted screw design
PP is a crystalline plastic with good fluidity and a high crystallization rate. Its melting point is relatively low, at around 160-170℃. During the injection molding process, PP is sensitive to shear, and excessive shear will cause molecular chain degradation and affect product performance.
Based on these characteristics, screws suitable for PP raw materials usually have the following design features:
High compression ratio: generally between 3.0 and 4.0. High compression ratio helps to quickly compact and melt PP raw materials in a shorter screw stroke, making full use of its crystallization characteristics and improving plasticization efficiency. For example, when producing PP plastic pipes, high compression ratio screws can quickly plasticize PP raw materials and extrude them evenly, ensuring that the wall thickness of the pipe is uniform and the texture is tight.
Gradient thread: The thread depth of the screw gradually becomes shallower from the feeding section to the metering section. This gradient design allows the PP raw material to smoothly transform from a solid state to a molten state during the rotation of the screw, avoiding the degradation of the raw material due to sudden pressure and shear. When producing PP injection molding trays, the gradient thread screw can make the PP raw material evenly plasticized, ensuring that the surface of the tray is smooth and there is no obvious weld mark.
Appropriate aspect ratio: Usually the aspect ratio (L/D) is between 20 and 25. Sufficient aspect ratio provides sufficient plasticization path for PP raw materials, so that they can be evenly heated, melted and mixed in the screw, thereby improving the quality of the product.
Screw characteristics suitable for ABS raw materials
ABS is an amorphous polymer with excellent comprehensive performance and certain fluidity, but compared with PP, its fluidity is slightly worse and it is more sensitive to temperature. The processing temperature range of ABS is relatively wide, generally between 200 and 250℃.
The screw design suitable for ABS raw materials is as follows:
Medium compression ratio: The compression ratio is generally between 2.5 and 3.0. This range can ensure that the ABS raw materials are fully compacted and melted, and will not generate excessive shear heat due to excessive compression ratio, causing ABS decomposition. For example, when producing ABS plastic shells, medium compression ratio screws can make ABS raw materials uniformly plasticized, fill the mold cavity, and ensure the surface finish and dimensional accuracy of the shell.
Universal gradual screw: The thread depth changes gradually, and the transition zone is relatively long. This helps the ABS raw material to slowly heat up and plasticize evenly in the screw, reducing product defects caused by rapid temperature changes, such as bubbles and silver threads. When producing ABS toy accessories, the universal gradient screw can fully mix the ABS raw material with the masterbatch to ensure uniform product color and exquisite appearance.
Surface treatment: Since ABS may produce certain corrosive gases during processing, the screw surface is often chrome-plated or nitrided to enhance the corrosion resistance of the screw and extend its service life.
Key points of screws adapted to PC raw materials
PC is a high-viscosity, non-crystalline plastic with excellent mechanical properties, heat resistance and impact resistance. The melting temperature of PC is relatively high, generally between 220 and 260°C, and it is extremely sensitive to moisture, so it must be fully dried before processing.
The screw design requirements for PC raw materials are as follows:
Low shear design: PC is sensitive to shear, and excessive shear will break its molecular chain and reduce the performance of the product. Therefore, the screw adopts a low shear design, such as a larger thread groove depth and a smaller compression ratio (usually 1.8 - 2.2) to reduce the shear effect on the PC raw material. When producing PC optical lenses, the low shear screw can ensure that the PC raw material maintains the integrity of the molecular chain during the plasticization process and ensure the optical properties of the lens.
High wear-resistant material: PC has a high hardness and causes greater wear on the screw during the conveying and plasticization process in the screw. Therefore, the screw is often made of wear-resistant materials such as high alloy steel and hardened to improve the wear resistance of the screw. For example, in the long-term production of PC products, a high wear-resistant screw can effectively reduce wear and reduce the frequency of equipment maintenance.
Special anti-return structure: In order to prevent the PC melt from flowing back during the injection process, the screw head needs to be equipped with a special anti-return structure to ensure the stability and accuracy of the injection process. When producing high-precision PC plastic products, a good anti-return structure can ensure the accuracy of each injection volume and improve the product qualification rate.
It can be seen that the three common plastic raw materials, PP, ABS, and PC, have different characteristics, and the screw designs of injection molding machines that are suitable for them have significant differences in compression ratio, thread type, aspect ratio, material, and surface treatment. As a professional injection molding machine manufacturer, we are well aware of the subtle relationship between different raw materials and screw designs. We can provide customized screw solutions according to the specific needs of customers to ensure that your injection molding production process is efficient and stable, and produce high-quality plastic products. Choosing us means choosing professionalism, efficiency, and quality assurance. Let us work together to empower your injection molding business.
PP raw materials and adapted screw design
PP is a crystalline plastic with good fluidity and a high crystallization rate. Its melting point is relatively low, at around 160-170℃. During the injection molding process, PP is sensitive to shear, and excessive shear will cause molecular chain degradation and affect product performance.
Based on these characteristics, screws suitable for PP raw materials usually have the following design features:
High compression ratio: generally between 3.0 and 4.0. High compression ratio helps to quickly compact and melt PP raw materials in a shorter screw stroke, making full use of its crystallization characteristics and improving plasticization efficiency. For example, when producing PP plastic pipes, high compression ratio screws can quickly plasticize PP raw materials and extrude them evenly, ensuring that the wall thickness of the pipe is uniform and the texture is tight.
Gradient thread: The thread depth of the screw gradually becomes shallower from the feeding section to the metering section. This gradient design allows the PP raw material to smoothly transform from a solid state to a molten state during the rotation of the screw, avoiding the degradation of the raw material due to sudden pressure and shear. When producing PP injection molding trays, the gradient thread screw can make the PP raw material evenly plasticized, ensuring that the surface of the tray is smooth and there is no obvious weld mark.
Appropriate aspect ratio: Usually the aspect ratio (L/D) is between 20 and 25. Sufficient aspect ratio provides sufficient plasticization path for PP raw materials, so that they can be evenly heated, melted and mixed in the screw, thereby improving the quality of the product.
Screw characteristics suitable for ABS raw materials
ABS is an amorphous polymer with excellent comprehensive performance and certain fluidity, but compared with PP, its fluidity is slightly worse and it is more sensitive to temperature. The processing temperature range of ABS is relatively wide, generally between 200 and 250℃.
The screw design suitable for ABS raw materials is as follows:
Medium compression ratio: The compression ratio is generally between 2.5 and 3.0. This range can ensure that the ABS raw materials are fully compacted and melted, and will not generate excessive shear heat due to excessive compression ratio, causing ABS decomposition. For example, when producing ABS plastic shells, medium compression ratio screws can make ABS raw materials uniformly plasticized, fill the mold cavity, and ensure the surface finish and dimensional accuracy of the shell.
Universal gradual screw: The thread depth changes gradually, and the transition zone is relatively long. This helps the ABS raw material to slowly heat up and plasticize evenly in the screw, reducing product defects caused by rapid temperature changes, such as bubbles and silver threads. When producing ABS toy accessories, the universal gradient screw can fully mix the ABS raw material with the masterbatch to ensure uniform product color and exquisite appearance.
Surface treatment: Since ABS may produce certain corrosive gases during processing, the screw surface is often chrome-plated or nitrided to enhance the corrosion resistance of the screw and extend its service life.
Key points of screws adapted to PC raw materials
PC is a high-viscosity, non-crystalline plastic with excellent mechanical properties, heat resistance and impact resistance. The melting temperature of PC is relatively high, generally between 220 and 260°C, and it is extremely sensitive to moisture, so it must be fully dried before processing.
The screw design requirements for PC raw materials are as follows:
Low shear design: PC is sensitive to shear, and excessive shear will break its molecular chain and reduce the performance of the product. Therefore, the screw adopts a low shear design, such as a larger thread groove depth and a smaller compression ratio (usually 1.8 - 2.2) to reduce the shear effect on the PC raw material. When producing PC optical lenses, the low shear screw can ensure that the PC raw material maintains the integrity of the molecular chain during the plasticization process and ensure the optical properties of the lens.
High wear-resistant material: PC has a high hardness and causes greater wear on the screw during the conveying and plasticization process in the screw. Therefore, the screw is often made of wear-resistant materials such as high alloy steel and hardened to improve the wear resistance of the screw. For example, in the long-term production of PC products, a high wear-resistant screw can effectively reduce wear and reduce the frequency of equipment maintenance.
Special anti-return structure: In order to prevent the PC melt from flowing back during the injection process, the screw head needs to be equipped with a special anti-return structure to ensure the stability and accuracy of the injection process. When producing high-precision PC plastic products, a good anti-return structure can ensure the accuracy of each injection volume and improve the product qualification rate.
It can be seen that the three common plastic raw materials, PP, ABS, and PC, have different characteristics, and the screw designs of injection molding machines that are suitable for them have significant differences in compression ratio, thread type, aspect ratio, material, and surface treatment. As a professional injection molding machine manufacturer, we are well aware of the subtle relationship between different raw materials and screw designs. We can provide customized screw solutions according to the specific needs of customers to ensure that your injection molding production process is efficient and stable, and produce high-quality plastic products. Choosing us means choosing professionalism, efficiency, and quality assurance. Let us work together to empower your injection molding business.