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The elements of pet plastic preform molding and common quality defects
Dec 09,2022
The main elements of pet plastic preform molding include plastic processing, selection of injection molding machines, design of molds and gates, etc. Only by properly handling these elements can we produce high-quality products. However, sometimes there will be some quality defects, such as yellowing of the preform, excessive change rate of wall thickness, etc., which need to be discovered and solved in time to avoid affecting its normal use.
What are the elements of pet plastic bottle preform molding?
1. Plastic processing
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. Generally, the proportion of PET preform recycled materials should not exceed 25%, and the recycled materials should be thoroughly dried.
2. Selection of injection molding machine
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. 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.
4. Mold and gate design
PET preforms are generally molded with hot runner molds. It is better to have a heat shield between the mold and the template of the injection molding machine. Its thickness 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.
What are the common quality defects of PET plastic preforms?
1. The AA value is too high
There are quality problems in raw material performance → check the quality of raw materials before entering the warehouse, the drying effect is poor → check the operation of the dryer (such as: dew point, drying temperature/time), the heating temperature of the barrel is too high → try to reduce the temperature under the premise of ensuring quality reduce.
2. The preform turns yellow
The drying time is too long/the temperature is too high → release the over-dried raw materials, and re-draw and dry the material, the barrel temperature is too high → reduce the temperature (generally, the temperature of the preform does not produce white fog and increase by 2-5 ° C) to normal temperature , The mold temperature is too high → reduce the temperature to an appropriate temperature (275-280°C).
3. The wall thickness change rate is too large
The screw temperature is too high → lower the screw temperature appropriately, the screw back pressure is too large → adjust the screw back pressure to the appropriate pressure, the injection speed is too fast → properly reduce the injection speed (generally 3.5 ~ 4S for injection). The mold cooling water temperature is too high → check the cooling circulating water temperature (9～11°C ±2°C).
4. Surface scratches (blank body/thread)
The mold has sticky material → clean the mold, the mold is damaged → replace the damaged parts or repair, take out the board and the suction cylinder is damaged → replace the damaged suction cylinder or repair. The ejection position/pressure of the die lip is too high → adjust the ejection position/pressure to a suitable state.
