PP plastic sheet commonly known as polypropylene sheet, 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 PE.
Treatment of PP plastic sheet
Pure PP is translucent ivory white and can be dyed in various colors. The dyeing of PP plastic sheet 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 plastic sheet generally does not need special drying treatment before injection molding.
PP sheet injection molding machine selection
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%.
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 the wall thickness, the width is twice the wall thickness, and 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, 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).
Melt temperature
The melting point of polypropylene is 160-175 °C, and the decomposition temperature is 350 °C, but the temperature setting of PP plastic sheet during injection processing cannot exceed 275 °C, and the temperature of 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 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.
Melt back pressure
5bar melt adhesive back pressure can be used, and the back pressure of toner can be appropriately adjusted.
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.
Post-processing of PP plastic sheet products
The finished PP plastic sheet extruded by the injection molding machine usually needs to be further processed, paste the layout protective film, cut it into the specified size, product inspection and other steps.
Characteristics of PP plastic sheet
PP sheet is a semi-crystalline material with a higher hard point and melting point than PE sheet. When the temperature is higher than 0 °C, the homopolymer PP sheet is very brittle. In order to solve this problem, many manufacturers are in PP material. rand.
Ethylene with 1%~4% OM copolymer has low heat distortion temperature, low transparency, low gloss and low rigidity, but greater impact strength, the strength of PP increases with the increase of ethylene content because of crystallization High hardness, good surface hardness and material scratch resistance, PP sheet has no environmental stress cracking, PP sheet can be modified by adding glass fiber, metal additives or thermoplastic rubber, under the influence of crystallization, the shrinkage rate of PP sheet It is quite high, generally 1.8%~2.5%.
The uniformity of shrinkage direction is better than that of PE.PP material. It has excellent water resistance, acid and alkali corrosion resistance and solubility. However, PP board is resistant to aromatic hydrocarbon solvents and chlorinated hydrocarbon solvents. Not tolerated.