PET chemistry is called polyethylene terephthalate, or […]
PET chemistry is called polyethylene terephthalate, or polyester. The rheological properties of PET in molten state are better, and the effect of pressure on viscosity is greater than that of temperature. Therefore, the fluidity of melt is mainly changed from pressure.
1. Plastic processing
Because the PET macromolecular contains aliphatic, has certain hydrophilicity, aggregate at high temperature is more sensitive to water, when the moisture content is more than the limit, in the processing of PET molecular weight drops, products with color, crisp. Trapped here, in front of the processing must be carried out on the material drying, the drying temperature is 150 ℃, more than 4 hours, generally is 170 ℃, 3-4 hours. Can be used to test whether the material is completely dry. The proportion of recycled materials should not exceed 25%, and the recovery material should be thoroughly dry.
2. Selection of injection molding machine
PET since the stable time is short, after melting point and melting point is high, so need more temperature control section and plasticizing the friction heat generating less injection system, and products (water cut materials) actual weight not less than two-thirds of injection machine. Based on these requirements, ramada has developed a series of PET Preformizing systems in recent years. The clamping force is selected as greater than 6300t/m2.
3. Mold and gate design
The PET bottle embryo is usually formed by hot runner mold, and it is best to have insulation board between the mould and the injection molding machine. The thickness is about 12mm, and the insulation board must be able to withstand high pressure. Exhaust must be sufficient in order to avoid local overheating or rupture, but the vent depth is generally not more than 0.03mm, otherwise it is easy to produce a flying edge.
4. Melt temperature
The airborne method is used to measure. 270-270 ℃, enhance the level of GF - PET can be set to 290-315 ℃, etc..
5. Infusion speed
General injection speed is fast, can prevent the premature coagulation of injection. But too fast, high shear rate makes the material fragile. The shot is usually finished in 4 seconds.
6, back pressure
The lower the better, so as not to wear out. Usually no more than 100bar. Usually not used.
7. Retention time
Do not use long detention time to prevent the molecular weight from falling. As far as possible avoid temperature above 300 ℃. Stop for less than 15 minutes. It is only required for air shooting; If it is more than 15 minutes, clean with the viscosity PE and reduce the cylinder temperature to PE temperature until the machine is turned on again.
The recovery material should not be too large, otherwise it will be easy to produce the "bridge" at the bottom and influence the plasticization.
If the mold temperature control is not good or the material temperature control is improper, it is easy to produce "white fog" and not transparent. The mold temperature is low and uniform, the cooling speed is fast, the crystal is less transparent.
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