As we all know, "screw plasticization component" of injection molding machine is main core unit of injection molding machine, which directly reflects performance of injection molding machine. Screw, known as "heart", is a vital part, and its performance determines quality and efficiency of processed products.

 

With rapid development of plastics industry, injection molding equipment has matured. As designers and users of injection molding equipment, it is necessary to clearly understand which screws are suitable for processing of which materials, especially in today's diverse materials, types of screws on the market are also changing day by day, but in actual use it is rather chaotic. Users often use a certain screw to process plastics with unsuitable physical properties.
There is also a misunderstanding of concept: "Thinking that a screw is omnipotent", but it is not. In this way, a brief introduction of several commonly used screws with main structure is provided to provide some reference when processing products.

1. Amorphous plastic, no obvious melting point, glass transition temperature 140℃-160℃, melting temperature 215℃-225℃, molding temperature 230℃-320℃;
2. High viscosity, high processing temperature, narrow range, easy to decompose, avoid overheating when selecting screw;
3. Sensitive to moisture, resin is easily hydrolyzed under high temperature, drying temperature is 120℃-130℃, and time is about 4 hours.
Parameter selection:
1. In view of its characteristics of good thermal stability and high viscosity, L/D chooses a large aspect ratio as much as possible in order to improve plasticizing effect;
2. Because of its wide melting temperature range, long feeding section, short homogenization section, and moderate compression ratio, barrier type over-rubber heads can be used to increase plasticizing effect;
3. If you add flame retardant or glass fiber reinforced PC material, you can choose to use corrosion-resistant and wear-resistant high-quality stainless tool steel screw and three small pieces.

1. Glass transition temperature is 105℃, melting temperature is greater than 160℃, decomposition temperature is 270℃, and molding temperature range is 180-250℃;
2. Viscosity is large, fluidity is poor, and shear rate is more sensitive, so temperature must be strictly controlled during processing;
3. Temperature of barrel is 240℃-250℃, and residence time will not exceed 8 minutes when it exceeds 260℃;
4. Strong water absorption, drying temperature 80℃-90℃, time over 3 hours. To
Parameter selection:
1. L/D ratio is 20-22, and gradual screw is generally L1=35-45%, L2=35-40%;
2. Compression ratio ε is generally 2.3-2.6;
3. Because it has a certain degree of hydrophilicity, a mixing head structure is adopted at the front end of screw.

1. There are many types of crystalline plastics, different types, and different melting points. Upper melting point range is narrow. Generally, PA66 has a melting point of 260℃-270℃;
2. Low viscosity, good fluidity, relatively obvious melting point, and poor thermal stability;
3. Water absorption rate is high, temperature of the dryer is 80℃, not more than 120℃, and time is 16 hours. If raw materials are exposed to air for 8 hours, use a vacuum oven at about 105℃ for 12 hours.
Parameter selection:
1. Select a mutant screw with a length ratio of 18-20 for L/D;
2. Compression ratio is generally 2.5-3.5, in which to prevent overheating decomposition h3: 0.067-0.089D;
3. Because of its low viscosity, gap between rubber ring and barrel should be as small as possible about 0.05-0.08, gap between screw and barrel should be 0.08-0.12;
4. Nozzle can be equipped with a spring self-locking nozzle.

1. Melting point is 250℃-260℃, molding temperature of blow-molded PET is relatively wide, about 255℃-290℃, and reinforced grade GF-PET can reach 290℃-315℃, etc.;
2. High melting point, low viscosity, strong adhesion to metal, easy to hydrolyze;
3. Hydrophilic, viscous material is more sensitive to water at high temperature, drying temperature is 150℃-160℃, and time is more than 4 hours.
Preferences:
1. L/D is generally taken as 20-22, three-stage distribution L1=50-60%, L2=20-30%;
2. Use a low-shear, low-compression ratio screw compression ratio ε=1.8-2.2, because shearing overheating causes discoloration or opacity h3=0.09-0.10D;
3. When using recycled materials (sheets), increase conveying capacity of feeding section and adopt a forced feeding plasticization system.

Heat-sensitive materials are generally divided into hard and soft. Difference lies in amount of plasticizer added to raw materials. Less than 10% is hard, and more than 30% is soft.
Features:
1. No obvious melting point, soft at 60℃, viscoelastic state at 100℃-150℃, narrow melting temperature range at 140℃, poor thermal stability, easy to decompose, rapid decomposition at 170℃, softening point close to decomposition point, decomposition release For HC1 gas.
2. Due to high viscosity of UPVC melt, poor thermal stability, temperature and time will cause decomposition, so it has a corrosive effect on general steel;
3. Temperature control is strict, screw design should be as low as possible to prevent overheating, and runner must be smooth, without dead corners, to prevent accumulation of material from decomposing;
4. Screw and barrel must be protected from corrosion;
5. Injection molding process needs to be strictly controlled.
Preferences:
1. L/D=18-21, h3=0.067-0.08D, ε=1.6-2, L1=40-45%, L2=35-45%, in order to prevent storage material from running through rubber ring, head is 20℃ -30℃, using hard chrome-plated screw;
2. Choice of temperature control;
①Multi-stage air-cooling device for glue cylinder;
②UPVC special screw and barrel device for low shear and high mixing.