PET plastic bottles are not only widely used for packaging carbonated beverages, drinking water, fruit juices, enzymes and tea beverages, etc., they are the most used beverage packaging today, but also widely used in food, chemical, pharmaceutical packaging and many other fields. PET plastic has a wide range of applications in the packaging field. Both packaging film and roll materials use PET plastic.

What is PET?

Polyethylene terephthalate is the most important type of thermoplastic polyester. The English name is Polythylene terephthalate, abbreviated as PET or PETP, and is commonly known as polyester resin. It is a polycondensate of terephthalic acid and ethylene glycol. Together with PBT, it is collectively referred to as thermoplastic polyester, or saturated polyester. PET is a milky white or light yellow highly crystalline polymer with a smooth and shiny surface. Creep resistance, fatigue resistance, good friction resistance and dimensional stability, low abrasion and high hardness, have the greatest toughness among thermoplastics.

Selection of injection molding machine

Since PET has a short stabilization time after the melting point and a higher melting point, it is necessary to select an injection device with a higher injection pressure and a higher injection speed that has more temperature control sections and less heat generated by self-friction during plasticization, and the actual product The weight cannot be less than 2/3 of the injection volume of the machine. Based on these requirements, LISONG has developed a special injection molding machine PET series. When selecting the clamping force, it is determined by multiplying the average stress of the 63MPa cavity by the projected area of the cavity.

Process points

💡Melting temperature

It can be measured by air-fire method. 270-295℃ range, enhanced GF-PET can be set to 290-315℃ and so on.

💡Injection speed

Generally, the injection speed should be fast to prevent premature coagulation during injection. But too fast and high shear rate make the material fragile. Injection is usually completed within 4 seconds.

💡Back pressure

The lower the better, so as to avoid wear. Generally, it does not exceed 1MPa.

💡Detention time

Do not use too long stagnation time to prevent molecular weight drop. Try to avoid temperatures above 300℃. If the machine is stopped for less than 15 minutes, only air injection is required; if the machine is stopped for more than 15 minutes, PE should be used to clean the screw barrel. At this time, the temperature of the barrel should be lowered to the PE temperature until the machine is turned on again.

💡Matters needing attention

The recycled material should not be used too much, otherwise it will easily cause "bridging" at the feeding place, which will affect the feeding and prevent the normal progress of plasticization.

If the mold temperature is not well controlled or the material temperature is not properly controlled, it is easy to produce "white fog" and opacity. The mold temperature is good and uniform, the cooling rate is fast, and the product is transparent with less crystallization.

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