Under certain temperature and pressure, the mold is filled with raw materials, and under the processing of the injection molding machine, it is processed into a preform with a certain thickness and height corresponding to the mold. PET preforms are reprocessed by blow molding to form plastic bottles, including bottles used in cosmetics (Sunscreen Spray Bottle / Spray Perfume Bottle / Foaming Dispenser Bottle, etc). , medicine, health care, beverages, mineral water, reagents, etc. A method of forming PET plastic bottles through blow molding.
1). Characteristics of PET Raw Materials
The transparency is as high as more than 90%, the surface gloss is excellent, and the appearance is glassy; the aroma retention is excellent, the air tightness is good; the chemical resistance is excellent, and almost all organic drugs are resistant to acids; the hygienic property is good; it will not burn Toxic gas is generated; the strength characteristics are excellent, and various characteristics can be further improved by biaxial stretching.
2). Dry Moisture
Because PET has a certain degree of water absorption, it will absorb a lot of water during transportation, storage and use. High levels of moisture will exacerbate during production:
- Increase of AA (Acetaldehyde) acetaldehyde.
Odorous effect on bottles, resulting in off-flavors (but little effect on humans)
- IV (IntrinsicViscosity) viscosity drop.
It affects the pressure resistance of the bottle and is easy to break. (The essence is caused by the hydrolytic degradation of PET)
At the same time, make high temperature preparations for PET entering the injection molding machine for shear plasticization
3). Drying Requirements
Drying set temperature 165℃-175℃
Dwell time 4-6 hours
The temperature of the feeding port is above 160°C
Dew point below -30℃
Dry air flow 3.7m⊃3; /h per kg/h
The ideal moisture content after drying is about: 0.001-0.004%
Excessive dryness can also exacerbate:
- Increase of AA (Acetaldehyde) acetaldehyde
-IV (IntrinsicViscosity) viscosity drop
(Essentially caused by oxidative degradation of PET)
5). 8 Factors in Injection Molding
① Disposal of Plastic
Since PET macromolecules contain lipid groups and have a certain degree of hydrophilicity, pellets are sensitive to water at high temperatures. When the moisture content exceeds the limit, the molecular weight of PET decreases during processing, and the product becomes colored and brittle.
Therefore, before processing, the material must be dried, and the drying temperature is 150°C for more than 4 hours; generally 170°C for 3-4 hours. The complete dryness of the material can be checked by air shot method. Generally, the proportion of PET preform recycled materials should not exceed 25%, and the recycled materials should be thoroughly dried.
② Injection Molding Machine Selection
Due to the short stable time of PET after the melting point and the 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 machine injection. 2/3 of the amount.
③ Mold and Gate Design
PET preforms are generally formed by hot runner molds. It is best to have a heat shield between the mold and the injection molding machine template. The thickness of the heat shield is about 12mm, and the heat shield 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 generally not exceed 0.03mm, otherwise flashing will easily occur.
④ Melt Temperature
It can be measured by air injection method, ranging from 270-295°C, and the enhanced grade GF-PET can be set at 290-315°C, etc.
⑤ Injection Speed
Generally, the injection speed should be fast to prevent premature coagulation during injection. But too fast, the shear rate is high, making the material brittle. Injection is usually done within 4 seconds.
⑥ Back Pressure
The lower the better to avoid wear and tear. Generally not more than 100bar, usually do not need to use.
⑦ Residence Time
Do not use too long residence time to prevent the decrease of molecular weight, and try to avoid the temperature above 300°C. If the machine is shut down for less than 15 minutes, it only needs to be treated with air injection; if it is more than 15 minutes, it must be cleaned with viscosity PE, and the temperature of the machine barrel should be lowered to the PE temperature until it is turned on again.
Recycled materials should not be too large, otherwise, it is easy to cause "bridge" at the cutting place and affect plasticization; if the mold temperature control is not good, or the material temperature is not properly controlled, it is easy to produce "white fog" and opaque; the mold temperature is low and uniform, The cooling speed is fast, the crystallization is less, and the product is transparent
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