PET chemical name is Polyterephthalate alcohol ester, also known as polyester. At present, the most used by customers is the GF-PET, mainly for the bottle preform.
The rheological property of PET is better in the melting state, and the influence of pressure on the viscosity is larger than that of the temperature. Therefore, the property of flow of melted material is changed mainly from the pressure.
2.PET features & property:
The glass conversion temperature of PET is about 165℃, and the temperature range of the material is 120~220℃. PET has a strong hygroscopicity at high temperature. For glass fiber reinforced PET materials, bending deformation is very easy to occur at high temperature.
The crystallinity of the material can be improved by adding a crystalline reinforcing agent. The transparent products made with PET have glossiness and heat distortion temperature. Special additives such as mica can be added to PET to minimize the bending deformation. If lower mold temperature is used, transparent products can be obtained by using non - filled PET materials.
3.Injection molding process:
The injection molding method is mainly used to enhance the molding of PET. A screw type injection machine is usually used. The screw usually needs to be hardened to avoid wear after long term use. The length of the nozzle hole of the injection machine should be as short as possible, and the diameter of the injection machine should be controlled around 3mm.
The melting point of the enhanced PET is up to 260 ℃. In order to prevent the nozzle clogging, a higher power heater should be installed. In addition, the tip of the nozzle hole is best processed as a reverse cone as shown in figure -1, so that the molten material in the flow channel and the nozzle can be cut off conveniently.
4.The main injection molding conditions of PET:
1, the temperature of the barrel. The forming temperature range of PET is narrow, and the temperature will directly affect the performance of the product. If the temperature is too low, it is not good to make plastic plastic parts, depression, lack of material defects; on the contrary, the temperature is too high, will cause the spill, nozzle salivation, deep color change, the decline in mechanical strength, even cause degradation. Usually, the barrel temperature is controlled at 240~280℃, and the barrel temperature of glass fiber reinforced PET is 250~290℃. The temperature of nozzles should not exceed 300 ℃, and the temperature of nozzles is generally lower than that of barrel temperature.
2, mold temperature. The mold temperature has a direct effect on the cooling rate and crystallinity of the melt, and the crystallinity is different, and the properties of the plastic parts are different. Usually, the mold temperature is controlled at 100~140℃. When the thin-walled plastic parts are formed, it is advisable for small value; when the thick wall plastic parts are formed, it is advisable to have a large value.
3. Injection pressure. The fluidity of PET melts is good and easy to be formed. Medium pressure is usually used, pressure is 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 location and size of gate, the shape and size of plastic part, the mold temperature, the type of injection molding machine and so on.