Plastic plasticizing screws are classified into three sections according to their geometric shapes:
1. Feeding zone: The depth of the fixed screw groove is determined, and its function is to be responsible for preheating, solid plastic conveying and pushing. It is necessary to ensure that the plastic begins to melt at the end of the feeding section – that is, it needs to be preheated to the melting point.
The general lengths are AS follows: for amorphous types (AS, ABS, ps), approximately 48 to 58%L; for crystalline types (PA, POM, PE, PP, CA), approximately 48 to 58%L; and for fiberglass types, approximately 45 to 55%L.
2. Compression zone: Gradually reduce the depth of the screw groove teeth. Its functions include melting, mixing, shearing compression and pressurized exhaust of plastic raw materials. The plastic will completely dissolve in this section and its volume will shrink, so the design of the compression degree is very important.
The general lengths are AS follows: amorphous (AS, ABS, PS) approximately 25 to 35%L, crystalline (PA, POM, PE, PP, CA) approximately 22 to 32%L, glassy approximately 28 to 40%L, heat-sensitive (PVC) 100%. 3. Metering zone: The screw groove has a fixed groove depth. Besides its main functions of mixing, melt adhesive transportation and metering, it must also provide sufficient pressure to maintain a uniform temperature of the melt adhesive and a stable flow rate of the molten plastic.
The general length is: about 15 to 58%L for non-crystalline (AS, ABS, PS) types, about 48 to 58%L for crystalline (PA, POM, PE, PP, CA, etc.) types, and about 45 to 55%L for glassy types. Although plasticizing screws can be classified by function or geometric shape, in the actual plasticizing process, the functions of each section overlap, making it difficult to distinguish and interval them. The research on plasticizing screws is nothing more than the intention:
1. Enhance the shearing and mixing effect.
Second, improve uniform mixing.
Iii. Enhance plasticizing capacity.
4. Improve the uniformity of the melt adhesive temperature.
The deeper the tooth depth of the feeding section is, the greater the conveying capacity will be, but the torque required by the screw will be larger. The tooth depth of the feeding section is too shallow, the conveying capacity is insufficient, and the compression ratio is inadequate. When the tooth depth of the metering section is too deep and the compression ratio is insufficient, the required feeding force is relatively large. When it is too shallow, it is prone to overburning and charring. The general tooth depth is approximately 0.03 to 0.07 times the screw diameter. Therefore, plastic plasticizing screws have the functions of conveying, melting, mixing, compression and metering, and play a very important role in plasticizing quality.
Material of plastic plasticizing screw
1. ACM2 (West German material, commonly known as black and white Cross) : ACM2 is an aluminum-molybdenum-nickel-molybdenum alloy steel. After quenching and tempering, it can reach HB280~320. After processing and nitriding for 72 hours, it can achieve a hardness of HV800. It has good wear resistance, but the effect of hard chromium plating is not good. It is used for plastic plasticizing screws and melt tubes in general material tube groups.
2. MAC24 (Mitsubishi material, quenched and tempered nitrided steel) : It does not contain aluminum, so there is less white layer after nitriding. The nitriding hardness is approximately HV800 to HV950, and it has excellent resistance to acid corrosion and hard chromium plating.
3. Double alloy: Its manufacturing methods are divided into PTA top spray welding and HPHVOF melt injection. The selection should be based on the characteristics of different plastics. Its surface hardening hardness reaches HRC74, making it a super-hard alloy with excellent wear resistance. The film density is as high as 98 to 99.8%, and the surface adhesion strength is over 10,000 psi. It can be known that the plastic plasticizing screw is the heart component of the injection molding machine, responsible for the transportation, melting, mixing, metering and other functions of plastic raw materials. Therefore, the plastic plasticizing screw is closely related to the quality of the molded products.