Everyone has seen the plastic bottle cap. Every time we open a sealed cap, do you find that there is a circle connecting the two parts between the cap and the plastic bottle. In fact, this small circle is called PET bottle embryo. It is a typical injection molding product, easy to transport, mostly made of plastic material, with uniform texture and good insulation. Do you know how it is molded? It is processed into a bottle blank with a certain thickness and height corresponding to the mold under a certain temperature and pressure, filled with raw materials in the mold, and processed by an injection molding machine. Do you know the characteristics of PET bottle embryo?
The characteristics of PET bottle embryo materials are as follows
1. Pet is the main variety of thermoplastic polyester, commonly known as polyester resin.
2. Pet is a polycondensate of terephthalic acid and ethylene glycol, which is called thermoplastic polyester or saturated polyester together with PBT.
3. Pet is an opalescent or pre yellow highly crystalline polymer with smooth and glossy surface.
4. Pet has good creep resistance, fatigue resistance, friction resistance and dimensional stability, low wear and high hardness, and has the largest toughness among thermoplastic plastics;
5. Pet has good electrical insulation property, little affected by temperature, but poor corona resistance.
6. Pet is nontoxic, weather resistant, chemical resistant, stable, low water absorption, and resistant to weak acids and organic solvents.
Precautions for injection molding of PET bottle embryo:
1. Treatment of plastics
Because pet macromolecules contain fatty groups and have a certain hydrophilicity, the particles are sensitive to water at high temperature. When the moisture content exceeds the limit, the molecular weight of pet decreases during processing, and the products are colored and brittle. The drying temperature is 150 ℃ for more than 4 hours, generally 170 ℃ 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 25%, and the recycled materials should be dried thoroughly.
2. Selection of injection molding machine
Due to the short stabilization time after the melting point of pet and the high melting point, it is necessary to select 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.
3. Mold and gate design
PET bottle embryo is usually molded by hot runner mold. It is better to have a heat shield between the mold and the mold plate of the injection molding machine, with a thickness of about 12 mm, and the heat insulation board must be able to withstand high pressure. The exhaust must be sufficient to avoid local overheating or fragmentation, but the depth of the exhaust port should not exceed 0.03mm, otherwise it is easy to produce flash.
4. Melt temperature
It can be measured by air shooting method. The enhanced gf-pet can be set at 290-315 ℃.
5. Injection speed
Generally, the injection speed should be fast, which can prevent premature solidification during injection. But too fast, high shear rate makes the material fragile. Firing is usually done in 4 seconds.
6. Back pressure
The lower the better, to avoid wear. Generally not more than 100bar. It is usually unnecessary to use.
7. Retention time
Do not use too long residence time to prevent the decrease of molecular weight. Try to avoid the temperature above 300 ℃. If the shutdown time is less than 15 minutes. If it is more than 15 minutes, clean it with viscosity PE and lower the barrel temperature to PE temperature until it is started again.
a. The recycled material should not be too large, otherwise it is easy to "bridge" at the blanking place and affect the plasticization.
b. If the mold temperature is not well controlled or the material temperature is not properly controlled, it is easy to produce "white fog" and opaque. The mold temperature is low and even, the cooling speed is fast, and the product is transparent with less crystallization.
The above is about the PET bottle embryo material characteristics and injection molding precautions, you can learn about it, if you are interested in PET bottle embryo, you can consult our bottle cap manufacturer for more relevant knowledge.