What waxes are used for linear polyesters (PET)?

Polyethylene wax plays an important role in improving the mold release and flow of filled PET in the injection molding process of linear polyesters (PET). New engineering plastics are very difficult to process, some of them are very viscous soon after processing, some have very high relative molecular mass and therefore very poor flowability.

PET chemical name is polyethylene terephthalate ethanol ester, also known as polyester. Currently the most used in the customer is GF-PET, mainly for playing bottle blanks.

PET has better rheology in the molten state, and the pressure has more influence on the viscosity than the temperature, therefore, the melt flowability is changed mainly from the pressure.

1, the processing of plastic due to PET macromolecules containing lipid groups, with a certain hydrophilic, granules at high temperatures are more sensitive to water, when the water content exceeds the limit, in the processing of PET molecular weight decline, the product with color, 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, the choice of injection molding machine PET because of 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 generation less injection system, and the actual weight of the product (including water mouth material) can not be less than 2/3 of the machine injection volume. based on these requirements, huamida in recent years developed a small and medium series of PET special plasticization system. The clamping force is greater than 6300t/m2.

3, mold and gate design PET bottle embryos are generally molded by hot runner mold, and it is better to have heat insulation plate between mold and injection 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 should not exceed 0.03mm in general, otherwise it is easy to produce flying edge.

4, The temperature of melt glue can be measured by air injection method. 270-295℃ is different, and the enhanced grade GF-PET can be set to 290-315℃, etc.

5, Injection speed is generally fast, which can prevent premature solidification during injection. But too fast, high shear rate makes the material fragile. Injection is usually completed within 4 seconds.

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

7, Do not use too long retention time to prevent the molecular weight from falling. Try to avoid the temperature above 300℃. If the stop is less than 15 minutes. Only have to make empty shot processing; if more than 15 minutes, it should be cleaned with viscosity PE, and the barrel temperature down to PE temperature, until the machine is turned on again.

8, Cautions

(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 improper material temperature control, easy to produce “white mist” and opaque. Low and uniform mold temperature, fast cooling speed, less crystallization, then the product is transparent.