pet injection process
pet injection process
Pet chemical name is polyethylene terephthalate, also known as polyester. At present, gf-pet is the most used in customers, mainly bottle embryo.
The rheology of pet in molten state is better, and the influence of pressure on viscosity is greater than that of temperature. Therefore, the fluidity of PET melt is mainly changed from pressure.
Pet properties:
The glass transition temperature of pet is about 165 ℃, and the crystallization temperature range is 120 ~ 220 ℃. Pet has strong hygroscopicity at high temperature. For Glass Fiber Reinforced PET materials, bending deformation is also very easy to occur at high temperature.
The crystallization degree of the material can be improved by adding crystallization enhancer. Transparent products processed with pet have gloss and heat distortion temperature. Special additives such as mica can be added to pet to minimize bending deformation. If a lower mold temperature is used, transparent products can also be obtained by using unfilled PET materials.
Injection molding process:
Injection molding is mainly used for the molding of Reinforced PET. Screw injection machine is usually used. Generally, the screw shall be hardened to avoid abrasion after long-term use. The length of the nozzle hole of the injection machine shall be as short as possible, and its diameter shall be controlled at about 3mm.
The melting point of Reinforced PET is as high as 260 ℃. In order to prevent nozzle blockage, a heater with high power should be installed. In addition, the tip of the nozzle hole is preferably machined into a reverse cone type as shown in figure-1, so that the molten material in the flow channel and nozzle can be easily cut off.
Injection molding machine:
Injection molding is mainly used to enhance the molding of pet. Generally, pet can only be formed by screw injection molding machine.
It is better to select the mutant screw with check ring on the top, which has high surface hardness and wear resistance, the length diameter ratio is not L / D = (15 ~ 20): 1, and the compression ratio is about 3:1.
If the L / D is too large, the material stays in the barrel for too long. Excessive heating is easy to cause degradation and affect the product performance. Compression ratio is too small, shear heat generation is less, easy to plasticize, poor product performance. On the contrary, it will cause more fracture of glass fiber and reduce the mechanical properties. When processing Glass Fiber Reinforced PET, the inner wall of the barrel is severely worn, and the barrel is made of wear-resistant materials or lined with wear-resistant materials.
The nozzle shall be short, the inner wall shall be polished, and the aperture shall be as large as possible. Hydraulic brake valve nozzle is better. The nozzle shall be provided with thermal insulation and temperature control measures to ensure that the nozzle will not freeze and block. However, the nozzle temperature should not be too high, otherwise it will cause salivation. Low pressure PP material must be used to clean the barrel before forming.
Main injection molding conditions of PET:
1. Barrel temperature. The molding temperature range of pet is narrow, and the temperature will directly affect the properties of products. If the temperature is too low, the plasticization is not good, resulting in defects such as depression and shortage of plastic parts; On the contrary, if the temperature is too high, it will cause edge overflow, nozzle salivation, darker color, lower mechanical strength, and even degradation. Generally, the barrel temperature is controlled at 240 ~ 280 ℃, the barrel temperature of glass fiber reinforced pet is 250 ~ 290 ℃, which shall not exceed 300 ℃, and the nozzle temperature is generally 10 ~ 20 ℃ lower than the barrel temperature
2. Mold temperature. The mold temperature directly affects the cooling rate and crystallinity of the melt. The properties of plastic parts are different with different crystallinity. Generally, the mold temperature is controlled at 100 ~ 140 ℃. When forming thin-walled plastic parts, a small value can be taken; When forming thick wall plastic parts, the large value can be taken.
3. Injection pressure. PET melt has good fluidity and is easy to form. It usually adopts medium pressure, with the pressure of 80 ~ 140MPa, and the injection pressure of glass fiber reinforced pet is 90 ~ 150MPa. The determination of injection pressure should consider the viscosity of pet, the type and quantity of filler, the position and size of gate, the shape and size of plastic parts, mold temperature, type of injection molding machine, etc.
