Introduction: As one of the widely used general plastics, PP can be seen everywhere in daily life. It has higher purity than ordinary pc. Although it does not have the high coloring of ABS, PP has higher purity and color rendering. In the industry, PP material is often used in packaging materials such as plastic bottles, bottle caps, cream bottles, etc. I am sorted out by RB PACKAGE and shared with the supply chain for reference:
Chemical name: Polypropylene
English name: Polypropylene (referred to as PP)
PP is a crystalline polymer. Among the commonly used plastics, PP is the lightest, with a density of only 0.91g/cm3 (less than water). Among the general-purpose plastics, PP has the best heat resistance. Its heat distortion temperature is 80-100°C and can be boiled in boiling water. PP has good stress cracking resistance and high bending fatigue life. It is commonly known as “100% plastic”. The comprehensive performance of PP is better than that of PE material. PP products have light weight, good toughness and good chemical resistance.
Disadvantages of PP: low dimensional accuracy, insufficient rigidity, poor weather resistance, easy to produce “copper damage”, it has post-shrinkage phenomenon, after demolding, it is easy to age, become brittle, and easy to deform.
01
Molding characteristics
1) The crystalline material has low hygroscopicity and is prone to melt fracture, and it is easy to decompose in long-term contact with hot metal.
2) The fluidity is good, but the shrinkage range and shrinkage value are large, and shrinkage holes, dents, and deformation are easy to occur.
3) The cooling speed is fast, the pouring system and the cooling system should slowly dissipate heat, and pay attention to controlling 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, flow marks, Prone to warping and deformation above 90 degrees
4) The plastic wall thickness must be uniform to avoid lack of glue and sharp corners to prevent stress concentration.
02
Process characteristics
PP has good fluidity at melting temperature and good molding performance. PP has two characteristics in processing
One: The viscosity of PP melt decreases significantly with the increase of shear rate (less affected by temperature)
Second: The degree of molecular orientation is high and the shrinkage rate is relatively high.
The processing temperature of PP is around 200-300℃. It has good thermal stability (the decomposition temperature is 310℃), but at high temperature (270-300℃), it may degrade 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 increase its fluidity and improve shrinkage deformation and depression. The mold temperature should be controlled within the range of 30-50℃. PP melt can pass through a very narrow mold gap and appear front. In the melting process of PP, it has to absorb a large amount of heat of fusion (larger specific heat), and the product is hotter after being ejected from the mold. PP material does not need to be dried during processing, and the shrinkage rate and crystallinity of PP are lower than that of PE.
03
Points to note in plastic processing
Plastic processing
Pure PP is translucent ivory white and can be dyed in various colors. PP can only be dyed with color masterbatch on general injection molding machines, but some models have independent plasticizing elements that strengthen the mixing effect, and they can also be dyed with toner.
Products used outdoors are generally filled with UV stabilizers and carbon black. The use ratio of recycled materials should not exceed 15%, otherwise it will cause strength drop and decomposition and discoloration. Generally, no special drying treatment is required before PP injection processing.
Injection molding machine selection
There are no special requirements for the selection of injection molding machines. Because PP has high crystallinity. A computer injection molding machine with higher injection pressure and multi-stage control is required. The clamping force is generally determined by 3800t/m2, and the injection volume is 20%-85%.
Mold and gate design
The mold temperature is 50-90℃, and the high mold temperature is used for the higher size requirements. The core temperature is more than 5℃ lower than the cavity temperature, the runner diameter is 4-7mm, the needle gate length is 1-1.5mm, and the diameter can be as small as 0.7mm.
The length of the edge gate is as short as possible, about 0.7mm, the depth is half of the wall thickness, and the width is twice the wall thickness, and it gradually increases with the length of the melt flow in the cavity.
The mold must have good venting. The vent hole is 0.025mm-0.038mm deep and 1.5mm thick. To avoid shrinkage marks, use large and round nozzles and circular runners, and the thickness of the ribs 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 (thick wall products can only use copolymer PP).
Melting temperature
The melting point of PP is 160-175°C, and the decomposition temperature is 350°C, but the temperature setting cannot exceed 275°C during injection processing. The temperature in the melting section is preferably 240°C.
Injection speed
In order to reduce internal stress and deformation, high-speed injection should be selected, but some grades of PP and molds are not suitable (bubbles and air lines in the human mantle). If the patterned surface appears with light and dark stripes diffused by the gate, low-speed injection and higher mold temperature should be used.
Melt back pressure
5bar melt adhesive back pressure can be used, and the back pressure of the toner material can be adjusted appropriately.
Injection and pressure holding
Use higher injection pressure (1500-1800bar) and holding pressure (about 80% of the injection pressure). Switch to holding pressure at about 95% of the full stroke, and use a longer holding time.
Post-processing of products
In order to prevent the shrinkage and deformation caused by the post-crystallization, the products generally need to be soaked in hot water.
Shanghai Rainbow Industrial Co., Ltd is the manufacturer, Shanghai rainbow package Provide one-stop cosmetic packaging.If you like our products, you can contact us,
Website: www.rainbow-pkg.com
Email: Bobby@rainbow-pkg.com
WhatsApp: +008613818823743
Post time: Oct-04-2021