Factors Affecting the Service Life of the Injection Molding Machine's Plasticizing System
What is the Plasticizing System of an Injection Molding Machine?
The plasticizing components of an injection molding machine include the screw, the melt barrel, the screw tip, check ring and check seat.
During the operation of the injection molding machine, the plasticizing components are subjected to pressure, impact, friction, and extrusion.
- When the injection molding machine is in good condition, and operated by skilled technicians, it can reduce wear on the plasticizing components, extending their service life.
- However, if the injection molding machine is not professionally and smoothly controlled, the result will be the opposite.
Therefore, to ensure that the injection molding machine is regularly in good condition, it is necessary to clearly understand the performance of the injection molding machine being used.
Additionally, awareness of the properties and quality of the plastic materials being used is crucial, along with strengthening the management of injection molding machine usage.
This is done to achieve the goal of reducing failure rates, minimizing maintenance costs, and prolonging the service life.
Factors Affecting the Service Life of the Plasticizing System in Injection Molding Machines and Solutions
Generally, there are six aspects that impact the service life of the plasticizing system in injection molding machines:
1. Mechanical Wear and Tear
Some modified plastics contain minerals, glass fibers, metal powders, etc.
The presence of these materials leads to cumulative mechanical wear and tear on the screw, small components, and the melt barrel during the plasticizing and injection molding processes.
This wear can remove nitrided layers and chromium plating. Increased clearances between the screw and the melt barrel, resulting from wear, reduce plasticization efficiency, increase injection molding leakage, decrease injection efficiency, and lower processing precision.
To minimize wear and extend the service life of plasticizing components, it is advisable to appropriately increase temperatures and reduce screw speeds during processing.
Choosing chromium plating or adopting a bimetallic approach can be more effective in preventing wear and tear.
2. Mechanical Fatigue and Overload Operation
Setup personnel habitually configure low-temperature operations and set high speeds and pressures, causing gradual degradation of the plasticizing components.
For instance, when processing PC or PA plastics, if the temperature is below the required level, the plastic viscosity is high. Forcing the solenoid action at this point necessitates an increase in solenoid pressure and torque, thereby increasing the screw’s stress fatigue.
Simultaneously, due to the high viscosity of the plastic melt at this stage, increased injection pressure and speed are required for injection molding, amplifying the impact and load on small components. This accelerates their wear and stress fractures.
3. Human Factors (Including Operational Errors or Violations)
● When metal impurities mix with the plastic and enter the barrel together, the extrusion action causes varying degrees of wear on the screw’s threads, grooves, check ring, and check valve gasket.
This results in unstable injection molding processing, prone to black spots and streaks.
● Human errors, such as adding the wrong plastic, introducing high-temperature plastic into a barrel set to low temperature, lead to excessive torque during solenoid action, causing stress fatigue in the screw.
● Cold start: During a cold start, when the barrel temperature has not reached the set temperature or has just reached it, the remaining material in the pipe, with its outer layer absorbing heat from the heating coil, becomes hotter, while the inner layer remains cold.
During a cold start, the screw torque is high, leading to stress fatigue in the screw. In severe cases, the screw may break quickly, along with the check ring and check valve gasket.
Most of the metal impurities entering the barrel come in with the crushed material. Therefore, it is essential to regularly inspect the condition of the crusher blades. If any wear is detected, the blades should be replaced immediately.
On the other hand, it is crucial to regularly inspect and clean the magnetic separator in the material hopper.
When the metal shavings around the magnet become saturated, the magnetic force on the outer iron shavings weakens. Even if they are captured, they can easily be washed away by the continuously flowing plastic, entering the barrel together.
4. Proper Assembly, Debugging, and Replacement of Parts
This aspect is also crucial. If the screw barrel is not assembled tightly enough, there may be instances of the screw hitting the barrel during the molding or injection process, causing wear on the screw or barrel.
Therefore, it is necessary to regularly inspect the technical condition of the equipment and pay attention to any abnormal phenomena that occur in the parts during processing.
5. Damage Caused by Improper Processing
- Prolonged use of high back pressure during molding accelerates the wear of the plasticizing components.
This situation typically arises when using color powder, and due to the difficulty of dispersing color powder, higher back pressure is applied.
- For plastics with high viscosity, using rapid molding introduces stress fatigue to the screw.
- For high-temperature plastics, especially those with added glass fiber, high-speed molding methods should not be employed.
6. Chemical Corrosion
The metal material susceptible to corrosion is iron-based. Common corrosive plastics include flame-retardant plastics, acidic plastics, PVC plastics, etc.
After corrosion of the screw, barrel, and flange, the surface develops pits and becomes rough, causing increased flow resistance of the molten material during the operation of the injection molding machine.
Some materials easily adhere to the surface, leading to decomposition and carbonization. Severe corrosion results in an enlarged gap between the screw and the barrel, increased leakage, and a decrease in injection efficiency.
Whether it is flame-retardant plastics or acidic adhesives, plastics processed at high temperatures will decompose acidic gases, and the plastic melt easily carbonizes and sticks to the metal.
(1) The plasticizing components should be made of stainless steel or utilize a chrome-plating solution.
(2) In production, efforts should be made to use low back pressure, low temperature, and low shear processing to reduce plastic degradation.
(3) Due to the thermal sensitivity of the plastics mentioned above, high temperatures or prolonged exposure to heat can lead to plastic decomposition, degradation, and carbonization. Therefore, unnecessary and prolonged machine stops should be avoided during the production process.
If a stop is necessary, the temperature should be lowered first, the material gate closed, and after completing the molten material in the barrel, switch to cleaning the barrel with PP or PS material before stopping the machine.