Why a two-stage method is recommended for making PET bottles
Why PET plastics are molded using high temperatures compared to another form of plastics. This is because PET materials have a high melting point of about 270 °C which can sustain high-temperature injection molding. Apart from melting point, PET products are preferred due to their fluidity. In most cases, PET molding uses hot runners where gate valves are selected to ensure management of fluidity is in point. PET molding machine operates in two ways; one is through the hot preform while another one is cold preform where it involves two steps.
Hot preform method (one-step method)
In the hot preform method, one integrated machine incorporates all processes converting the PET granules into complete bottles. The injection molding usually happens by channeling the preform through the injection cavity and then finally stretch blown to come up with a bottle. In this method, there is no heating required as the preforms are molded directly without being stockpiled. Hot preform method is most recommended for small-medium industries that require minimal production scale.
In the two-step method, it uses two different molding machines. The preform first goes through injection molding first before it is heated again and blown. This process requires an injection molding machine and blow molding machine. The injection machine produces the preform while the other one reheats the preform before it blows. When acquiring the heating system for the preform, you should first ensure it has low thermal efficiency. This two phase method is suitable for both medium and large scale producers.
Why is the two-stage method recommended?
Here are the reasons why a two-phase process is preferred and widely used for PET bottles:
- Drying of PET material is done before the injection molding. PET has to be free from moisture, and this is done by dehumidifying drying.
- Plasticizing the PET- during the molding process, you have to compress and rotate the PET pellets in a rotational screw.
- The injection of a molded PET is usually done through an injection cavity and has to be cooled rapidly to make a preform.
- When heating the preform, the temperature level has to be adjusted to produce the correct profile ideal for blowing.
- During the PET bottle production, the hot preform has to be stretched and blown simultaneously to form the required bottles.
- PET container ejector is required where the finished bottle is ejected.
When PET is subjected to high temperatures, its chain-like molecules become mobile making the preform to stretch. With the help of PET molding machine, biaxial orientation can be achieved; in this case, PET is stretched from two different directions at right angles. This makes the molecules to align themselves in the direction of stretch. PET products are usually reliable and can sustain chemicals, impact and different barriers.