How does the crystallinity of a 30mm PET preform affect the transparency of the bottle?

PET (polyethylene terephthalate) is a polymer material widely used in food packaging and beverage bottle production. Its physical properties have an important impact on the quality of the final product. Among them, the crystallinity of PET preforms directly determines the transparency of the bottle, affecting its market competitiveness and consumer experience.

Relationship between PET crystallinity and transparency
PET has semi-crystalline characteristics, and its molecular structure can be either amorphous (non-crystalline) or form regularly arranged crystalline regions. In the process of producing 30mm PET preforms, the degree of crystallinity will significantly affect the transparency of the bottle.

When PET is in a high crystallinity state, the molecular arrangement inside the material is more orderly. This ordered structure causes light to scatter inside the material, thereby reducing the transparency of the bottle and making it appear foggy or milky white. On the contrary, PET materials with low crystallinity are close to amorphous structures, and light can pass through more smoothly, making the bottle more transparent. Therefore, in the manufacturing process, controlling the crystallinity of PET is the key to ensuring that the bottle has high transparency.

Main factors affecting PET crystallinity
Cooling rate
During the preform injection molding process, the cooling rate plays a decisive role in crystallinity. Rapid cooling will inhibit the orderly arrangement of PET molecules, keep it at a low crystallinity, and thus improve the transparency of the bottle. Too slow cooling will promote the formation of crystallization zones, resulting in a decrease in bottle transparency. Therefore, reasonable control of cooling time and cooling water temperature is an important means to optimize transparency.

Injection molding temperature
Excessively high injection molding temperature will increase the fluidity of PET molecules, which may lead to increased crystallinity in some areas and affect transparency. Lower injection molding temperature helps to reduce crystallization and keep the preform in good optical properties. Therefore, when producing 30mm PET preforms, the melt temperature of the injection molding machine needs to be accurately set to take into account both molding quality and transparency requirements.

Raw material purity
The purity of PET raw materials will also affect crystallinity, and thus transparency. High-purity PET has a more stable molecular structure, which can reduce the interference of impurities on the crystallization process and improve the transparency of the bottle. On the contrary, PET containing impurities or a high proportion of recycled materials may lead to uneven material structure, which in turn affects optical properties.

Influence of additives
In PET materials, some additives (such as crystallization promoters or inhibitors) can regulate the crystallization rate of the material. For example, crystallization inhibitors can effectively reduce the crystallinity of PET and keep the bottle highly transparent, while crystallization promoters may enhance the heat resistance of the material but reduce transparency. Therefore, in the production formula of 30mm PET preforms, it is necessary to reasonably select additives to meet different application requirements.

Influence of bottle blowing process on transparency
During the bottle blowing process, the heating temperature and stretching ratio of the preform will also affect the crystallinity of PET, thereby affecting the transparency of the bottle. If the heating temperature is too high or uneven, local crystallization may occur in some areas, resulting in a decrease in the transparency of the bottle. In addition, during the stretching process, a reasonable stretching ratio can reduce the orderly arrangement of molecular chains, reduce crystallinity, and improve transparency. Therefore, in the bottle blowing process of 30mm PET preforms, the heating temperature and stretching ratio need to be strictly controlled to ensure that the bottle has uniform transparency.