Injection molding machine screw

I. Types and Characteristics of Injection Molding Machine Screws

Gradual-Type Screw

Features: The compression section is long, accounting for approximately 50% of the total screw length. Energy conversion during plasticization is gentle, making it suitable for plastics with poor thermal stability, such as PVC.

Function: The long compression section allows for gradual melting and plasticization of the plastic, avoiding intense shear heat and protecting the plastic's thermal stability.

Sudden-Type Screw

Features: The compression section is short, accounting for approximately 5% to 15% of the total screw length. Energy conversion during plasticization is more intense, making it suitable for crystalline plastics such as polyolefins and PA.

Function: The short compression section allows for rapid melting and plasticization of the plastic, improving production efficiency.

Universal-Type Screw

Features: Highly adaptable, suitable for processing a wide range of plastics.

Function: The design of the universal screw takes into account the melting and plasticization characteristics of various plastics, ensuring stable performance under varying injection molding process requirements. II. Working Principle of an Injection Molding Machine Screw
The working principle of an injection molding machine screw is to heat, melt, and plasticize plastic pellets through the screw's rotation and propulsion, and then inject the molten plastic into the mold. The specific process is as follows:

Plastic Pellet Delivery: Plastic pellets enter the injection molding machine's barrel from the hopper and are gradually moved forward by the screw's rotation and the propulsion of the screw grooves.
Plastic Heating and Melting: During the screw's rotation, the plastic pellets are subjected to shear and friction forces, gradually heating and melting them.
Plastic Plasticization and Homogenization: In the screw's compression and homogenization sections, the plastic is further mixed, compressed, and pressurized to form a uniform molten plastic.
Molten Plastic Injection into the Mold: When the molten plastic reaches the front end of the barrel, the injection system of the injection molding machine injects it into the mold, forming the desired plastic product.
III. Key Parameters of an Injection Molding Machine Screw
Screw Diameter (D)
Influencing Factors: Related to the required shot volume, inversely proportional to the maximum injection pressure, and directly proportional to the plasticizing capacity. Calculation method: Injection volume = 1/4 × π × D² × S (injection stroke) × 0.85.
Conveying Section
Function: Responsible for conveying, extruding, and preheating the plastic.
Design Requirements: Preheating should be ensured to the melting point. Crystalline plastics should be long, amorphous materials should be short, and heat-sensitive materials should be short.
Compression Section
Function: Responsible for mixing, compressing, and pressurizing the plastic to exhaust.
Design Requirements: As the plastic gradually melts, the screw channel volume must decrease accordingly to correspond to the decrease in the plastic's geometric volume. The compression section length is determined by the type of plastic and process requirements.
Homogenizing Section
Function: Responsible for mixing, melt conveying, and metering.
Design Requirements: Provide sufficient pressure to maintain a uniform melt temperature and stabilize the flow rate of the molten plastic.
Screw Channel Depth
Influencing Factors: The deeper the feed channel depth, the greater the conveying capacity; the shallower the metering channel depth, the higher the plasticizing heat generation and mixing performance index. However, if the metering groove depth is too shallow, shear heat and self-generated heat will increase, leading to excessive temperature rise, potentially causing discoloration or burning of the plastic.

Calculation method: Metering groove depth = KD = (0.03-0.07) × D. As D increases, select a smaller value for K.

IV. Injection Molding Machine Screw Operation and Maintenance

Precautions

Do not start the machine before the barrel reaches the preset temperature.

Prevent foreign matter from falling into the barrel and damaging the screw and barrel.

When using new plastic, clean any remaining material from the barrel.

Avoid mixing POM and PVC in the barrel simultaneously to prevent chemical reactions and serious industrial accidents.

Maintenance

Regularly check the screw for wear and replace any severely worn screws promptly.

Clean and maintain the screw regularly to ensure internal cleanliness and no blockages.

Check the screw and barrel for tight contact and no leaks. 5. Development Trends in Injection Molding Machine Screw Technology
As injection molding technology continues to advance, so too does the innovation and improvement of injection molding machine screw technology. In the future, injection molding machine screw technology will place greater emphasis on high efficiency, energy conservation, environmental protection, and intelligent development. For example, new materials and technologies will be used to improve the wear and corrosion resistance of the screw; screw structure and parameters will be optimized to enhance plasticizing quality and production efficiency; and intelligent control systems will be developed to achieve precise control and automatic adjustment of screw operation.