PET plastic bottles
have better rheology in the molten state. Pressure produces a greater influence than temperature on the viscosity. Therefore, it is mainly on the pressure that attention is put in order to change the melt fluidity. Plastic bottles have different usage conditions in different environments, so the rheology and viscosity are more effectively controlled when plastic bottles are in the molten state.
1. Choice of injection molding machine. PET plastic bottles have a short stability time after melting. The melting point is high. Therefore, it is important to use an injection system with more temperature control segments and less self-heating during plasticization, and the products (including reground material) shall not be less than 2/3 of the injection volume of the machine. The injection molding machine should be properly selected and controlled according to the injection volume and the injection system of the machine. In light of these requirements, a small and medium-sized PET special plasticization system has been developed in recent years in China. The injection mold machine shall be selected to have a clamping force greater than 6300 t/m2.
2. Treatment of plastic. As PET plastic bottles contain macromolecules with a lipid base, showing a certain degree of hydrophilicity, the pellets are sensitive to water at high temperatures. In processing, once the moisture content exceeds the limit, the molecular weight of PET plastic bottles drops and the bottles discolor and become brittle. For this reason, before processing the material must be dried at a temperature of 150°C for more than 4 hours; in practice, the combination is usually 170°C and 3-4 hours. Air injection method can be utilized to check if the material is completely dry. Before using material, the raw material must be dried. Care must be taken of different drying temperatures and time limits to make sure the produced plastic bottles have good properties. The proportion of recycled material, which should be thoroughly dried, should not exceed 25%.
3. Detention time. Detention time shall be limited in order to avoid the drop of molecular weight. Temperatures above 300°C shall be avoided if possible at all. If the stoppage is less than 15 minutes, only air injection treatment is required; if the stoppage exceeds 15 minutes, it is necessary to clean using viscosity PE and lower the barrel temperature to PE temperature until the machine is turned on again. Strict control must be applied to the detention time of plastic bottles. Different temperatures call for different processing, so time limits must be strictly managed.
4. Mold and gate design. PET plastic preforms are generally shaped through a hot runner mold. An insulation plate shall preferably be installed between the mold and the injection molding machine template. This insulation plate, about 12mm thick, must be able to withstand high pressure. Vent must be sufficient to avoid local overheating or breaking, but generally the depth of the vent port should not exceed 0.03mm, otherwise it is likely to produce flashes.
5. Back pressure. The lower the better, for the sake of avoiding wearing or damage. Generally, back pressure shall not exceed 100bar. Normally, it is not necessary to apply back pressure.
6. Injection speed. Generally speaking, the injection shall be quick, because quick injection prevents premature coagulation. However, too quick an injection speed drives up the shear rate and makes the material fragile. Material shot is usually completed within 4 seconds.
7. Precautions. (1) Recycled material should not be too large in size. Otherwise, it is vulnerable to "bridging" at the blanking area and plasticization may be affected. If recycled plastic is used, its quantity shall be tightly limited in order to guarantee the plasticization quality. (2) If the mold temperature or the material temperature is not well controlled, "cloudiness" is likely and the material may become opaque. The mold temperature must be tightly controlled; otherwise undesirable events may occur. A low and uniform mold temperature, a fast cooling rate, and less crystallization contribute to transparent products. Plastic bottle production processes shall be subject to a strict quality system inspection to ensure that plastic bottles are of good quality. The production processes are complex, demanding strict control over different production processes to make sure that each process is performed in strict accordance with the standards.