Polypropylene (PP) plastic is a versatile material used in various applications due to its excellent properties such as resistance to chemicals, high melting point, and durability.
- Slurry Process: Polymerization in a diluent such as hexane, which is the earliest industrialized method and, to date, the one with the largest production volume.
- Bulk Liquid Phase Process: Polymerization at 70°C and 3MPa within liquid propylene.
- Gas Phase Process: Polymerization under conditions where propylene is in a gaseous state. The latter two methods do not use a diluent, have shorter processes, and lower energy consumption. The bulk liquid phase process has now demonstrated its advantages, surpassing the others.
PP Plastic Molding Characteristics
(1) Physical Properties: Polypropylene (PP) is a non-toxic, odorless, milky white highly crystalline polymer and one of the lightest varieties among all plastics. It is exceptionally stable in water, with a water absorption rate of only 0.01% after being submerged for 14 hours. Its molecular weight ranges between approximately 80,000 to 150,000, and it has good moldability. However, due to its high shrinkage rate, original wall products are prone to denting. The surface of the products has a good gloss and is easy to color.
(2) Mechanical Properties: PP has a high degree of crystallinity and a regular structure, which results in excellent mechanical properties. Its strength, hardness, and elasticity are higher than those of high-density polyethylene (HDPE). A notable feature is its resistance to bending fatigue; it can withstand 70 million cycles of bending without showing damage. Its dry friction coefficient is similar to nylon, but its performance under oil lubrication is not as good as nylon.
(3) Thermal Properties: PP has good heat resistance with a melting point between 164-170°C. Products can be sterilized at temperatures above 100°C. Without external forces, it does not deform at 150°C. It becomes brittle at -35°C, and below this temperature, brittleness occurs. Its heat resistance is not as good as PE.
(4) Chemical Stability: PP has good chemical stability. Apart from being eroded by concentrated sulfuric acid and nitric acid, it is relatively stable against various other chemical reagents. However, low molecular weight aliphatic hydrocarbons and aromatic hydrocarbons can soften and swell PP. Its chemical stability improves with increased crystallinity. Therefore, PP is suitable for making chemical pipelines and fittings in Russia, with good anti-corrosion effects.
(5) Electrical Properties: Polypropylene has excellent high-frequency insulation properties. Since it hardly absorbs water, its insulation is not affected by humidity. It has a high dielectric constant, and with rising temperatures, it can be used to make heat-resistant electrical insulation products. It also has a high breakdown voltage, making it suitable for electrical accessories, etc. It has good resistance to voltage and electric arc, but it has a high static charge and can age quickly when in contact with copper.
(6) Weatherability: Polypropylene is very sensitive to ultraviolet light. Adding antioxidants such as zinc oxide, dilauryl thiodipropionate, carbon black, or similar milky-white fillers can improve its aging resistance.
Molding Process of PP Plastic
Injection Molding Machine Selection: There are no special requirements for the selection of injection molding machines. Due to the high crystallinity of PP, it is necessary to use a computer-controlled injection molding machine capable of high injection pressure and multi-stage control. The clamping force is generally determined by 3800t/m², and an injection capacity of 20%-85% is sufficient.
Drying Treatment: Drying treatment is not necessary if storage is appropriate.
Melting Temperature: The melting point of PP is 160-175℃, and the decomposition temperature is 350℃. However, during injection molding, the temperature setting should not exceed 275℃. The temperature of the melting zone is best maintained around 240℃.
Mold Temperature: The mold temperature should be between 50-90℃. Higher mold temperatures should be used for parts with high dimensional requirements, with the core temperature being at least 5℃ lower than the cavity temperature.
Injection Pressure: Use higher injection pressures (1500-1800bar) and holding pressures (approximately 80% of the injection pressure). Switch to holding pressure at about 95% of the full stroke and use a longer holding pressure time.
Injection Speed: To reduce internal stress and deformation, high-speed injection should be chosen. However, some grades of PP and molds are not suitable (bubbles, gas marks may occur). If a patterned surface exhibits alternating light and dark stripes spreading from the gate, use low-speed injection and higher mold temperatures.
Runners and Gates: The diameter of runners should be 4-7mm, the length of pin-point gates should be 1-1.5mm, and the diameter can be as small as 0.7mm. The length of edge gates should be as short as possible, about 0.7mm, with a depth of half the wall thickness and a width of twice the wall thickness, increasing with the length of melt flow in the cavity. The mold must have good venting, with vent depths of 0.025mm-0.038mm and a width of 1.5mm. To avoid sink marks, use large, round gates and rounded runners, and the thickness of reinforcing ribs should be small (e.g., 50-60% of the wall thickness). Products made of homopolymer PP should not exceed a thickness of 3mm to avoid bubbles (thick-walled products should only be made with copolymer PP).
Melt Back Pressure: A melt back pressure of 5bar can be used, with higher back pressure for color powder materials if appropriate.
Post-Treatment of Products: To prevent shrinkage deformation caused by post-crystallization, products generally require treatment by soaking in hot water.