Article source: admin. Time: 2017-05-22 04-37-46
Overview of polypropylene materials:
PP plastic, chemical name: polypropylene
English name: Polypropylene (referred to as PP)
Specific gravity: 0.9-0.91 g / cm3 Molding shrinkage: 1.0-2.5% Molding temperature: 160-220 ° C
PP is a crystalline polymer. PP is the lightest in common plastics, with a density of only 0.91g / cm3 (smaller than water). Among general-purpose plastics, PP has the best heat resistance. Its heat distortion temperature is 80-100 ° C and it can be boiled in boiling water. PP has good stress cracking resistance and high bending fatigue life. It is commonly known as "percent glue". The overall performance of PP is better than that of PE. PP products are light weight, good toughness, and good chemical resistance. Disadvantages of PP: low dimensional accuracy, insufficient rigidity, poor weather resistance, and easy to cause "copper damage". It has a phenomenon of post-shrinkage. After demolding, it is easy to age, become brittle and deform easily.
In daily life, the commonly used fresh-keeping box is made of PP material.
1.Crystal material, low hygroscopicity, easy to melt rupture, easy to decompose in long-term contact with hot metal.
2.Good fluidity, but the shrinkage range and shrinkage value are large, and it is easy to cause shrinkage. Dent, deformation.
3.The cooling speed is fast, the pouring system and cooling system should slowly dissipate heat, and pay attention to control the molding temperature. The material temperature is easy to be oriented at low temperature and high pressure. When the mold temperature is lower than 50 degrees, the plastic part is not smooth, and it is easy to produce poor welding and flow marks. Warp deformation easily occurs above 90 degrees
4.The plastic wall thickness must be uniform to avoid lack of glue and sharp corners to prevent stress concentration.
Process characteristics of PP
PP has good fluidity at the melting temperature and good molding performance. PP has two characteristics in processing:
One: the viscosity of the PP melt decreases significantly with increasing 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 better around 200-300 ℃, it has good thermal stability (decomposition temperature is 310 ℃), but at high temperature (270-300 ℃), it may be degraded if it stays in the barrel for a long time. As the viscosity of PP decreases significantly with increasing shear rate, increasing the injection pressure and injection speed will increase its fluidity, improve shrinkage and deformation. The mold temperature should be controlled within the range of 30-50 ° C. PP melt can pass through a very narrow die gap and appear to be frontal. During the melting process of PP, a large amount of melting heat (larger specific heat) must be absorbed, and the product is hot after being ejected from the mold. PP materials do not need to be dried during processing. The shrinkage and crystallinity of PP are lower than those of PE.