Polypropylene (PP) is a rigid and crystalline thermoplastic used widely in everyday objects. Read this article and learn everything you need to know about Polypropylene plastics include the characteristics, usages, properties, process, causes & solutions to bubbles in Polypropylene injection molding products.
Characteristics of Polypropylene Plastics, What It’s Used For?
Polypropylene has no toxic, tasteless, has low density, strength, stiffness, hardness, heat resistance are better than low pressure polyethylene, can be used in about 100 degrees. It has good electrical properties and high-frequency insulation, not affected by humidity, but brittle at low temperature, not wear-resistant, easy to aging. It is suitable for making general mechanical parts, corrosion-resistant parts and insulating parts. The common organic solvents of acid and alkali have little effect on it, and the proper brand can be used for tableware.
Physical Properties of Polypropylene PP
Polypropylene is usually a translucent colorless solid, odorless and nontoxic. The melting point of polypropylene is about 164 ~ 170 ℃, and the density is 0.91g/cm. High strength, high hardness, wear resistance, bending fatigue resistance, moisture resistance and chemical resistance are good, easy processing molding, low price, so it is a general polymer variety with large output and wide application. The disadvantage is poor low temperature toughness and aging resistance. But it can be overcome by modifying and adding antioxidant respectively.
Mechanical Properties of PP
Polypropylene has excellent mechanical properties because of its high crystallinity and regular structure. However, the impact strength is poor at room temperature and low temperature due to its high molecular structure regularity. The most outstanding property of polypropylene is bending fatigue resistance.
Thermal Properties of Polypropylene PP
Polypropylene has good heat resistance, and the product can be sterilized and sterilized at temperature above 100 ℃. Under the condition of no external force, it will not deform at 150 ℃. The embrittlement temperature is – 35 ℃ and embrittlement occurs below – 35 ℃. The cold resistance of Polypropylene is not as good as that of polyethylene.
Chemical Stability of Polypropylene PP
The chemical stability of Polypropylene plastic is very good. In addition to being eroded by concentrated sulfuric acid and nitric acid, polypropylene is relatively stable to various chemical reagents. However, low molecular weight aliphatic hydrocarbons, aromatic hydrocarbons and chlorinated hydrocarbons can soften and swell polypropylene. At the same time, its chemical stability is also improved with the increase of crystallinity. Therefore, polypropylene plastic is suitable for making various chemical pipes and fittings and has anti-corrosion effect Good.
Electrical Properties of Polypropylene PP
The high frequency insulation of polypropylene plastic is excellent. Because it hardly absorbs water, it is not affected by humidity. It has a high dielectric coefficient, and with the rise of temperature, it can be used to make heated electrical insulation products. Its breakdown voltage is also very high, suitable for use as electrical accessories. It has good voltage resistance and arc resistance, but it has high static electricity and is easy to be aged in contact with copper.
Molding Characteristics of Polypropylene PP plastics
● Crystalline material, with small wettability, is easy to crack and decompose when contacting with hot metal for a long time.
● The fluidity is great, but the shrinkage range and value are large, and the shrinkage cavity is easy to occur. Dents, deformations.
● The cooling speed is fast, the pouring system and cooling system should slow heat dissipation, and pay attention to control the molding temperature. When the material temperature is lower than 50 ℃, the plastic parts are not smooth and easy to produce welding defects and flow marks. When the temperature is higher than 90 ℃, it is easy to produce warping deformation. Therefore, the temperature should be controlled at 80 ℃.
● The thickness of the plastic wall must be uniform to avoid the lack of glue and sharp corners to prevent stress concentration.
Injection Molding Process Conditions: PP Plastic Processing Technology
● Selection of injection molding machin: There are no special requirements for the selection of injection molding machines. Because PP has high crystallinity. Computer injection molding machine with high injection pressure and multi-stage control is required. The clamping force is generally determined by 3800t / m2, and the injection volume is 20% – 85%.
● Drying: Drying is not required if stored properly.
● Melting temperature: The melting point of Polypropylene (PP) is 160-175 ℃, and the decomposition temperature is 350 ℃. However, the temperature setting can not exceed 275 ℃ during injection processing. The best melting temperature is 240 ℃.
● Mold temperature: mold temperature 50-90 ℃, for size requirements of high mold temperature. The core temperature is 5 ℃ lower than the cavity temperature.
● Injection pressure: higher injection pressure (1500-1800bar) and holding pressure (about 80% of injection pressure) are used. Turn to hold pressure at 95% of the whole stroke for a long time.
● Injection speed: in order to reduce the internal stress and deformation, high-speed injection should be selected, but some grades of PP and mold are not applicable (bubbles, air lines). If there are light and dark stripes diffused by the gate on the surface of the engraved pattern, low-speed injection and high mold temperature should be used.
● Runner and gate: the diameter of runner is 4-7mm, the length of needle gate is 1-1.5mm, and the diameter can be as small as 0.7mm. The shorter the gate length is, the better it is, about 0.7mm. The depth is half of the wall thickness and the width is twice of the wall thickness, and increases with the melt length in the mold cavity. The die must have good exhaust performance. The depth of the vent hole is 0.025mm-0.038mm, and the thickness is 1.5mm. To avoid shrinkage marks, it is necessary to use a large and round nozzle and a circular runner, and the thickness of the reinforcing rib should be small (for example, 50-60% of the wall thickness). The thickness of products made of homopolymer PP should not exceed 3mm, otherwise there will be bubbles.
– The raw material is not dry enough and contains moisture.
– The plastic stays in the barrel for too long.
– Poor mold exhaust.
– The injection pressure is too small and the injection speed is too high.
– The melt temperature is too high and the mold temperature is too low.
1. Increase mold temperature and reduce injection time and cycle, because Polypropylene PP bubbles are formed by internal shrinkage due to long cooling time.
2. Increase back pressure, prolong injection time, use high pressure and low speed.
3. Polypropylene PP material generally will not have bubbles, it may be that there is a problem with the filling material during PP granulation.
If the solution in the process is:
1 add spare material post drying material.
2. Injection back pressure increases, injection rate slows down.
3. The barrel temperature should not be too high.P