Precautions when injection molding PET bottle blanks

1. Processing of plastic 

As PET macromolecule contains lipid group, it has certain hydrophilic, the granule is sensitive to water at high temperature when the water content exceeds the limit, the molecular weight of PET decreases in processing, and the products become colorful and brittle. In this case, the material must be dried before processing, and the drying temperature is 150℃ for more than 4 hours, usually 170℃ for 3-4 hours. The air injection method can be used to test whether the material is completely dry. 

The proportion of recycled material should not exceed 25% in general, and the recycled material should be dried thoroughly. 

2. Injection molding machine selection 

PET due to the short stabilization time after the melting point, and the melting point is high, so you need to use more temperature control section, plasticization since the frictional heat less injection system, and the actual weight of the product (including water mouth material) can not be less than 2/3 of the injection volume of the machine. based on these requirements, HERMIDA has developed a small and medium series of PET special plasticizing systems in recent years. The clamping force is greater than 6300t/m2. 

3. Mold and gate design 

PET bottle embryos are generally molded with hot runner molds. It is best to have a heat insulation plate between the mold and the injection molding machine template, the thickness of which is about 12mm, and the heat insulation plate must be able to withstand high pressure. Exhaust gas must be sufficient to avoid local overheating or shattering, but the depth of its exhaust port generally should not exceed 0.03mm, otherwise it is easy to produce a flying edge. 

4. Melt glue temperature 

It can be measured by air injection method, ranging from 270-295℃, and the reinforced GF-PET can be set to 290-315℃, etc. 

5. Injection speed 

The general injection speed should be fast to prevent premature solidification during the injection. But too fast, a high shear rate makes the material fragile. The injection is usually completed within 4 seconds. 

6. Back pressure 

The lower the better to avoid wear and tear. Usually not more than 100 bar, usually no need to use. 

7. Stagnation time 

Do not use too long a retention time to prevent the molecular weight from decreasing. Try to avoid a temperature above 300℃. If the downtime is less than 15 minutes. Only have to make empty shot processing; if more than 15 minutes, then clean with viscosity PE, and the barrel temperature down to PE temperature, until the machine is turned on again. 

8. Caution 

(1) The recycled material should not be too big, otherwise, it is easy to produce "bridging" at the lower material and affect the plasticization. 

(2) If the mold temperature control is not good or the material temperature control is not proper, it is easy to produce "white mist" and opaque. Low and uniform mold temperature, fast cooling speed, less crystallization, then the product is transparent.