Main factors affecting plasticizing quality
The main factors affecting plasticizing quality are: length-to-diameter ratio, compression ratio, back pressure, screw speed, and barrel heating temperature, etc.
- Length-to-diameter ratio: the ratio of the effective working length of the screw to the diameter of the screw.
a.A larger length-to-diameter ratio makes feeding more uniform;
b.Plastics with better thermal stability can use longer screws to improve compounding without burning, while plastics with poor thermal stability can use shorter screws or screws without threads at the end. Considering the plastic characteristics, the length-to-diameter ratio is generally as follows: 14-16 for thermosetting plastics, 17-18 for rigid PVC, high viscosity PU and other thermosensitive plastics, 18-22 for general plastics, and 22-24 for high-temperature stable plastics such as PC and POM.
- Compression Ratio: the ratio of the last depth of the screw in the feeding section to the first depth of the screw in the metering section.
a. Considering the compressibility of the material, filling degree, backflow and other factors, the product should be compact, with good heat transfer and ventilation.
b. Appropriate compression ratio can increase the density of plastic, make the binding between molecules more tight, help reduce the absorption of air, lower the temperature rise caused by pressure, and affect the difference in output. Improper compression ratio will destroy the physical properties of plastic.
c. The higher the compression ratio, the higher the temperature rise of the plastic during the plasticization process in the barrel, and the better the mixing uniformity of the plastic in the plasticizing process. The relative output will be greatly reduced.
d. High compression ratio is suitable for non-melting plastics, especially those with low melting viscosity and thermal stability; low compression ratio is suitable for melting plastics, especially those with high melting viscosity and thermal sensitivity.
- Back Pressure
a. Increasing the back pressure can increase the work done by the screw on the melted resin, eliminate unmelted plastic particles, and increase the density and uniformity of raw materials in the barrel.
b.Back pressure is applied to increase the temperature of the barrel, and its effect is most significant.
c.Excessive back pressure can cause decomposition of plastics with high thermal sensitivity, and may cause drooling phenomenon in plastics with low viscosity. If the back pressure is too low, the molded product may have bubbles.
- Screw Speed
a.The rotation speed of the screw directly affects the shear of the plastic in the helix groove.
b.Small screw grooves are shallower and absorb heat sources quickly enough to soften the plastic in the compression stage. The frictional heat energy between the screw and the barrel wall is lower, which is suitable for high-speed rotation to increase the plasticization capacity.
c.Large screws should not rotate quickly to avoid uneven plasticization and excessive frictional heat.
d.For plastics with high thermal sensitivity, if the screw speed is too high, the plastic will easily decompose.
e.Usually, each size of screw has a certain range of rotation speed, generally 100-150rpm; if the speed is too low, the plastic cannot be melted, and if the speed is too high, the plastic will burn.
- Electric Heating Temperature Setting
a.To melt the cold and hard plastic remaining in the barrel and the screw in order to facilitate the rotation of the screw and provide part of the heat required for the plastic to melt.
b.Set the temperature 5-10℃ lower than the melting temperature (partly provided by frictional heat).
c.The adjustment of the nozzle temperature can also be used to control issues such as drooling, condensation of cold materials (blocking the nozzle), and stringing.
d.The temperature of the molten plastic in the barrel is usually higher than the temperature controlled outside the barrel.