“Plastic performance is the foundation of injection molding technology”, mastering the process performance and characteristics of various plastics is the basic professional knowledge that every injection molding worker must understand. The performance of plastics is the basis for setting “injection molding process conditions”. One of the factors that must be considered when analyzing quality problems and abnormalities in the injection molding process.
PP, commonly known as polypropylene, is also known as “100% glue” because of its good fracture resistance. PP is a translucent, semi-crystalline thermoplastic with high strength, good insulation, low water absorption, high heat distortion temperature, low density, and high crystallinity. Modified fillers usually include glass fibers, mineral fillers, thermoplastic rubber, etc.
The fluidity of PP for different purposes is quite different, and the flow rate of PP generally used is between ABS and PC.
1. Handling of plastics
Pure PP is translucent ivory white and can be dyed in various colors. The dyeing of PP can only use color masterbatch on the general injection molding machine. On some machines, there are independent plasticizing elements that enhance the mixing effect, and can also be dyed with toner. Products for outdoor use are generally filled with UV stabilizers and carbon black. The use ratio of recycled materials should not exceed 15%, otherwise it will cause strength reduction and decomposition and discoloration. PP injection molding generally does not require special drying before processing.
2. Selection of injection molding machine
There are no special requirements for the selection of injection molding machines. Because PP has high crystallinity. Computer injection molding machines with high injection pressure and multi-stage control are required. The clamping force is generally determined by 3800t/m2, and the injection volume is 20%-85%.
3. Mold and gate design
The mold temperature is 50-90 °C, and high mold temperature is used for higher size requirements. The core temperature is more than 5°C lower than the cavity temperature, the diameter of the runner is 4-7mm, the length of the needle gate is 1-1.5mm, and the diameter can be as small as 0.7mm. The shorter the length of the edge gate, the better, about 0.7mm, the depth is half of the wall thickness, the width is twice the wall thickness, and gradually increases with the length of the melt flow in the mold cavity. The mold must have good venting. The vent hole is 0.025mm-0.038mm deep and 1.5mm thick. To avoid shrinkage marks, it is necessary to use large and round nozzles and circular runners, and the thickness of the reinforcing ribs should be small ( e.g. 50-60% of the wall thickness). The thickness of products made of homopolymer PP should not exceed 3mm, otherwise there will be bubbles (thick-walled products can only use copolymerized PP).
4. Melt 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, and the temperature of the melting section is preferably 240 °C.
5. 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 gas lines appear). If the patterned surface appears light and dark streaks diffused by the gate, low-speed injection and high mold temperature should be used.
6. Melt back pressure
5bar melt adhesive back pressure can be used, and the back pressure of toner can be appropriately adjusted.
7. Injection and pressure holding
Use higher injection pressure (1500-1800bar) and holding pressure (about 80% of injection pressure). Switch to holding pressure at about 95% of the full stroke, and use a longer holding time.
8. Post-processing of products
In order to prevent shrinkage and deformation caused by post-crystallization, the products generally need to be soaked in hot water.
