In recent years, the socio-economic development has become more and more rapid, and PET cosmetic preforms are used more and more in daily life. Whether it is beverages, sugar cans or cosmetic bottles, many of them use PET as raw materials. So what should be paid attention to when injection molding PET cosmetic preforms?
1. Treatment of plastics
Since the macromolecule of the PET cosmetic bottle contains a fat base, it has a certain degree of hydrophilicity, and the pellets are more sensitive to water at high temperatures. When the water content exceeds the capacity, the molecular weight of the PET cosmetic bottle will decrease during processing, and the product will become discolored and changed. brittle. In this case, the materials should be dried before processing. The drying temperature is 150°C for more than 4 hours, generally 170°C for 3-4 hours. The air shot method can be used to check whether the material is completely dry. Generally, the proportion of recycled materials should not exceed one-fourth, and the recycled materials must be thoroughly dried.
2. Selection of injection molding machine
Since the PET cosmetic bottle has a short stabilization time after the melting point and high melting point, it is necessary to choose an injection system with more temperature control sections and less self-friction heat generation during plasticization, and the actual weight of the product (water-containing material) should not be less than 2/3 of the injection volume of the machine. Based on these requirements, Ramada has developed a special plasticizing system for small and medium-sized PET cosmetic bottles in recent years. The clamping force is selected as greater than 6300t/m2.
3. Mold and gate design
PET cosmetic preforms are generally formed by hot runner molds. There should be a heat insulation board between the mold and the injection molding machine template, the thickness of which is about 12mm, and the heat insulation board must be able to withstand high pressure. The exhaust should be sufficient to avoid local overheating or chipping, but the depth of the exhaust port should generally not exceed 0.03mm, otherwise it is easy to produce flash.
4. Melting temperature can be measured by air shot method.
The temperature ranges from 270-295°C, and the enhanced GF-PET cosmetic bottle can be set at 290-315°C.
5. Injection speed
Generally, the injection speed should be fast to prevent premature coagulation during injection. But too fast and high shear rate make the material fragile. Injection is usually completed within 4 seconds.
6. The lower the back pressure, the better to avoid wear. Generally not more than 100bar. Usually don't need to be used.
7. Detention time
Do not use an excessively long residence time to prevent the molecular weight from decreasing. Try to avoid temperatures above 300°C. If the downtime is less than 15 minutes. Only need to do air injection treatment; if it exceeds 15 minutes, clean with viscosity PE, and reduce the barrel temperature to PE temperature until the machine is turned on again.
8. Matters needing attention
⑴Recycled material should not be too large, otherwise it is easy to cause "bridging" at the feeding place and affect plasticization. ⑵If the mold temperature is not well controlled or the material temperature is not properly controlled, it is easy to produce "white fog" and opacity. The mold temperature is low and uniform, the cooling rate is fast, and the product is transparent with less crystallization.