Polypropylene injection molding is a cost-effective and versatile manufacturing process that produces high-quality plastic parts and components. It offers excellent design flexibility, allowing for the creation of complex shapes and geometries. The resulting parts have excellent resistance to heat, chemicals, and impact, making them ideal for use in various industries.

PP(Polypropylene)

1.PP performance

PP is a crystalline polymer, the lightest of commonly used plastics, PP density is only 0.91g/cm3 (smaller than water). Among general plastics, PP has the best heat resistance, and its thermal deformation temperature is 80-100 ° C, which can be boiled in boiling water. PP has good stress cracking resistance and high bending fatigue life, commonly known as "folding glue".

The comprehensive performance of PP is better than that of PE material. PP products light weight, good toughness, good chemical resistance. The disadvantages of PP: low dimensional accuracy, insufficient rigidity, poor weather resistance, easy to produce "copper damage", it has a post-shrinkage phenomenon, after stripping, easy to aging, brittle, easy to deformation. PP has always been the main raw material for the manufacture of fibers, because of its coloring ability, wear resistance, chemical resistance and favorable economic conditions.

PP is a semi-crystalline material. It is harder than PE and has a higher melting point. Because homopolymer PP is very brittle at temperatures above 0 °C, many commercial PP materials are random copolymers with 1 to 4% ethylene or clip-on copolymers with higher ratios of ethylene content. Copolymer type PP materials have a lower thermal distortion temperature (100 °C), low transparency, low gloss, low rigidity, but have a stronger impact strength. The strength of PP increased with the increase of ethylene content.

The Vicat softening temperature of PP was 150℃. Due to the high crystallinity, the surface stiffness and scratch resistance of this material are good.

There is no environmental stress cracking problem in PP. Usually, PP is modified by adding glass fiber, metal additives or thermoplastic rubber. The MFR of PP mobility ranges from 1 to 40. PP materials with low MFR have better impact resistance but lower tensile strength. For materials with the same MFR, the strength of copolymer type is higher than that of homopolymer type.

Due to crystallization, the shrinkage rate of PP is quite high, generally 1.8 to 2.5%. And the directional uniformity of the shrinkage rate is much better than that of materials such as HDPE. The addition of 30% glass additive can reduce the shrinkage rate to 0.7%.

Both homopolymer and copolymer PP materials have excellent hygroscopic resistance, acid and alkali corrosion resistance and solubility resistance. However, it is not resistant to aromatic hydrocarbon (such as benzene) solvents, chlorinated hydrocarbon (carbon tetrachloride) solvents, etc. PP also does not have oxidation resistance at high temperatures like PE.

2. Process characteristics of PP

PP has better fluidity at melting temperature, good molding performance, PP has two characteristics in processing:

First, the viscosity of PP melt decreased significantly with the increase of shear rate (less affected by temperature);

Second: the degree of molecular orientation is high and the shrinkage rate is large. The processing temperature of PP is 220~275 °C, pay attention not to exceed 275 °C or so, it has good thermal stability (decomposition temperature is 310 °C), but at high temperature (270-300 °C), there is a possibility of degradation if it stays in the barrel for a long time. Because the viscosity of PP decreases significantly with the increase of shear speed, increasing injection pressure and injection speed will improve its fluidity, improve shrinkage deformation and depression. Mold temperature (40~80℃), 50℃ is recommended.

The degree of crystallization is mainly determined by the mold temperature and should be controlled within the range of 30-50℃. PP melt can pass through very narrow mold gaps and appear. PP in the melting process, to absorb a lot of melting heat (specific heat is larger), the product is hot after the mold.

PP material processing does not need to dry, PP shrinkage and crystallinity lower than PE. Injection speed Usually using high-speed injection molding can minimize internal pressure. If there are defects on the surface of the product, then low-speed injection molding at higher temperatures should be used. Injection pressure: up to 1800bar.

Runner and gate: For cold runner, the typical runner diameter range is 4~7mm. It is recommended to use the injection port and the flow channel with the whole body rounded. All types of gates can be used. Typical gate diameters range from 1 to 1.5mm, but gates as small as 0.7mm can also be used. For edge gates, the minimum gate depth should be half of the wall thickness; The minimum gate width should be at least twice the wall thickness, and PP materials can fully use hot runner systems.

PP has always been the main raw material for the manufacture of fibers, because of its coloring ability, wear resistance, chemical resistance and favorable economic conditions.

3. Typical application range:

The automotive industry (mainly using PP containing metal additives: fenders, ventilation pipes, fans, etc.), equipment (dishwasher door liners, dryer ventilation pipes, washing machine frames and covers, refrigerator door liners, etc.), consumer goods (lawn and garden equipment such as lawn mowers and water sprinklers, etc.).

Injection molding products are the second largest market for PP homopolymers, including containers, sealers, automotive applications, household goods, toys and many other consumer and industrial end-uses.

If you are interested in PP products or are looking for plastic mold manufacturers, please do not hesitate to contact me.