How much do you know about the processing of PET plastic?

The rheological behavior of PET in molten state is better. 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. Use the air - shot method to check whether the material is completely 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 injection system, the friction heat generating less when products and actual weight not less than two-thirds of injection machine. Based on these requirements, ramada has developed the PET special plasticizing system for small and medium-sized series in recent years. The clamping force is selected by more than 6300t/m2.

3. Mould and gate design

The PET bottle embryo is usually formed by hot runner mould, and it is best to have insulation board between mould and 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 fragmentation, but the outlet depth should not exceed 0.03 mm, otherwise it is easy to produce the edge.

4. Melt temperature

Available air - shot measurement. 270-270 ℃, enhance the level of GF - PET can be set to 290-315 ℃, etc.

5. Injection speed

The general injection speed is fast, can prevent injection premature coagulation. But too fast, the shear rate high makes the material fragile. The shot is usually done in 4 seconds.

6, back pressure

The lower the better, so as not to wear out. Usually no more than 100bar. It's not usually used.

7. Detention time

Do not use long lag time to prevent molecular weight loss. As far as possible avoid temperature above 300 ℃. If the stop is less than 15 minutes. It is only required to be airshot; For more than 15 minutes, clean the PE with viscosity and reduce the temperature of the cylinder to the PE temperature until it is switched on again.

8. Precautions

If the mold temperature control is not good or the material temperature control is improper, easy to produce "white fog" and not transparent. Mold temperature is low and even, cooling rate is fast, crystallization is less transparent.