Introduction: PET bottles have strong applicability and are widely used in daily necessities, daily chemical packaging and other fields. The preform is the intermediate semi-finished product of the subsequent two-way stretch blow molding. Some factors of the preform affect the quality of the finished injection molded product. Below, I will share with you the precautions for the PET preform, and the content is for your reference:
Under a certain temperature and pressure, the mold is filled with raw materials and processed by the injection molding machine to form a bottle blank with a certain thickness and height corresponding to the mold. PET preforms are reprocessed by blow molding to form plastic bottles, including bottles used for packaging cosmetics, medicines, health care, beverages, mineral water, reagents, etc. This bottle making method is called a two-step method, that is, the preform is formed through injection molding. A method of forming PET plastic bottles through blow molding.
PET raw material characteristics
The transparency is as high as 90% or more, the surface gloss is excellent, and the glass-like appearance is excellent; the fragrance retention is excellent, the air tightness is good; the chemical resistance is excellent, and it is resistant to almost all organic drugs, led by acid; good hygiene; Poisonous gas is generated; excellent strength characteristics, and various characteristics can be further improved by biaxial stretching.
Because PET has a certain degree of water absorption, it will absorb a lot of water during transportation, storage, and use. High content of water will aggravate during production:
-AA (Acetaldehyde) increase in acetaldehyde.
The effect on the bottle’s odor, which leads to a change of flavor (but has little effect on the human body)
-IV (IntrinsicViscosity) viscosity drop.
It has an impact on 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, prepare the high temperature for PET into the injection molding machine for shearing and plasticizing.
Dry setting temperature 165℃-175℃
Residence time 4-6 hours
The temperature of the feed opening is above 160℃
Dew point below -30℃
Dry air flow 3.7m⊃3; /h per kg/h
Degree of dryness
The ideal moisture content after drying is approximately: 0.001-0.004%
Excessive drying can also increase:
-AA (Acetaldehyde) increase in acetaldehyde
-IV (IntrinsicViscosity) viscosity drop
(The essence is caused by the oxidative degradation of PET)
8 factors in injection molding
Because PET macromolecules contain fat base and have certain hydrophilicity, the pellets are more sensitive to water at high temperature. When the moisture content exceeds the limit, the molecular weight of PET decreases during processing, and the product becomes colored and brittle.
Therefore, the materials must be dried before processing, and 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 PET preforms should not exceed 25%, and the recycled materials should be thoroughly dried.
Injection molding machine selection
Because PET has a short stabilization time after 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 cut material) cannot be less than the machine injection 2/3 of the amount.
Mold and gate design
PET preforms are generally formed by hot runner molds. It is better to have a heat shield between the mold and the injection molding machine template, the thickness of which is about 12mm, and the heat shield must be able to withstand high pressure. Exhaust must 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.
It can be measured by air injection method, ranging from 270-295℃, and for enhanced GF-PET can be set at 290-315℃.
Generally, the injection speed should be fast to prevent premature coagulation during injection. But too fast, the shear rate is high, and the material is fragile. Injection is usually completed within 4 seconds.
The lower the better, so as to avoid wear. Generally, it does not exceed 100bar and usually does not need to be used.
Do not use too long a residence time to prevent the molecular weight from falling, and try to avoid temperatures above 300°C. If the machine is shut down for less than 15 minutes, only air injection is required; if it exceeds 15 minutes, it must be cleaned with viscosity PE and the temperature of the barrel is lowered to the PE temperature until the machine is turned on again.
The recycled material should not be too large, otherwise it will easily cause “bridging” at the blanking site and affect plasticization; if 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, If the cooling rate is fast and the crystallization is less, the product will be transparent.