3.1.1) The hygroscopicity of Polypropylene (PP) is very small, so it does not need to be dried before polypropylene injection moulding, but should be dried if the humidity exceeds the allowed value.
3.1.2) The Polypropylene (PP) structure contains tertiary carbon atoms, so the antioxidant capacity is very low, and antioxidants should be added during polypropylene injection molding.
3.1.3) The molten material should be avoided to stay in the barrel for a long time during polypropylene injection molding. Polypropylene (PP) will degrade thermally when it exceeds 280 °C Celsius, so the performance will be deteriorated.
3.1.4) Polypropylene (PP) melt fluidity is good, between HDPE and LDPE, the melt flow rate is 2-9g/10min. Melt viscosity decreases with the increase of injection pressure and temperature, Easy to mold thin-walled, large plastic parts.
3.1.5) Polypropylene (PP) is crystalline and has a wide range of molding shrinkage, from 1.0% to 3.0%, and has a more obvious post-shrinkage, so it is easy to get defects, such as sink marks, dents and deformation with strong directionality.
3.1.6) The melting point and melt heat content of Polypropylene (PP) are higher than LDPE, and more heat will be released in the process of crystallization and cooling, so the mould should have a better cooling system to reduce the deformation of plastic parts.
3.1.7) Due to the thermal shrinkage and crystallization of Polypropylene (PP), the specific volume in the pp injection molding process has a large change, and defects such as bubbles and dents are easily produced in the tendons, holes and parts with large wall thickness.
3.1.8) The orientation is obvious when the melt temperature is low, especially at low temperature and high pressure, so the molding temperature should be controlled during polypropylene injection molding .
3.1.9) The shrinkage of Polypropylene (PP) plastic parts is large when demoulding, it should be placed on the shaping device for more than one day after demoulding to fix the shape, and for the plastic parts with high dimensional accuracy, heat treatment can be carried out in time.
3.1.10) Due to the large shrinkage rate of polypropylene injection molding, low temperature is brittle, so the wall thickness of the plastic parts should be uniform, avoid notches, and sharp corners to prevent stress concentration.
3.1.11) If the holding time is too long, there will be a large shrinkage of the plastic parts and quality defects; implement scientific injection molding to determine a reasonable holding time.
3.2.1) Injection machine type selection:
Polypropylene injection molding can be done by screw injection machine or plunger injection machine, and the former is generally used more often.
3.2.2) The moisture content of the injection material should not exceed 0.05%, otherwise it should be dried at 80~l00°C for 3~4Hours before polypropylene injection molding .
3.2.3) Barrel temperature:
A lower barrel temperature can be selected when the injection pressure and speed are low.
The melting point and decomposition temperature of polypropylene are higher than polyethylene, with good thermal stability, the melting point temperature range is 165-170°C, and the decomposition temperature is higher than 350°C. The molding temperature range is wide, generally 205-315°C.
General barrel temperature control at 210-280°C, nozzle temperature is 10-30°C lower than the barrel temperature. when molding thin-walled, complex products, the material temperature can be increased to 280-300°C; production of thick-walled products, the barrel temperature should be reduced to 200-230°C, to prevent the material in the barrel for too long time and cause thermal degradation.
However, the barrel temperature should not be too low, otherwise the macromolecular orientation will increase and the products will be easily warped and deformed.
3.2.4) Mold temperature:
Mould temperature should be determined according to the thickness and performance requirements of the product, usually adjusted at 20-60°C, which is not only conducive to crystallization, but also conducive to the relaxation of macromolecules, thus reducing
molecular orientation and reduce internal stress.
When the mold temperature is lower than 20°C, the plastic parts are not smooth, easily produce welding lines, and welding marks, and above 90°C, easy to warp and deformation.
3.2.5) Injection Pressure
In Polypropylene injection molding, increasing the injection pressure helps to increase the fluidity of the melt. The injection pressure of plunger injection molding machine is higher than screw type.
Generally, it is controlled at 70-100MPa, and the specific value is determined during mold testing, so that the plastic parts do not short-shot, overflow and do not have dents and bubbles or any others defects.
3.2.6) Due to the large shrinkage rate of polypropylene, post-shrinkage often occurs when the product is stored at room temperature.
90% of the total post-shrinkage is completed within 6 hours after the product is removed from the mould, and the remaining 10% occurs within the next 10 days, so the product should be stored on the shaping device for 1 day after it is removed from the mold.
For products with the complex shape or high dimensional stability requirements, heat treatment should be carried out, that is, the products are placed in hot air at 150°C, heat treatment 0.5~1Hours. When the wall thickness of the products is greater than 3mm, the time can be extended appropriately.