The Relationship Between the Manufacturing Process of Bottle Preforms and the Definition of Injection Molded Parts
1. Core Characteristics of the Injection Molding Process
Injection molded parts are plastic products produced using injection molding technology. The process involves injecting molten plastic into a mold, cooling it to set the shape, and then demolding it. According to the "Plastics Molding Process Handbook" (2021 edition), injection molded parts account for 34% of the global plastic product output (data source: Plastics Europe), making it one of the most widely used molding methods.
2. Typical Injection Molding Properties of Bottle Preforms
Bottle preforms are intermediate products for bottle containers (such as mineral water bottles and beverage bottles). They are typically formed using an injection molding machine from materials such as PET. Their production process fully conforms to the characteristics of the injection molding process:
- Molten PET is injected into the mold using a screw injection molding machine;
- After cooling, a tubular preform (i.e., the bottle preform) is formed;
- Subsequently, a blow molding process is used to process the finished bottle.
Therefore, based on the nature of the process, bottle preforms are considered injection molded parts.
Differentiation between Bottle Preforms and Other Injection Molded Parts
1. Special Functions and Structures
Bottle preforms must meet the requirements of blow molding expansion, resulting in stricter specifications for wall thickness uniformity and neck thread accuracy than standard injection molded parts. For example, the industry standard "GB/T 17931-2018" stipulates that the wall thickness tolerance of PET bottle preforms must be within ±0.05mm, while standard injection molded parts allow ±0.1mm.
2. Limitations in Material Selection
Bottle preforms are primarily made of crystalline plastics such as PET and PP, while injection molded parts use a wider range of materials (such as ABS and PC). This difference stems from the specialized properties that bottle preforms must possess, such as high transparency and pressure resistance.
Controversy and Industry Consensus
1. Some believe bottle preforms are considered "semi-finished products"
Some scholars have suggested that bottle preforms require blow molding to become final products and therefore should not be classified as injection molded parts. However, the Society of Plastics Engineers (SPE) clarified in its "Injection Molding Technology White Paper" (2022) that the definition of an injection molded part is independent of subsequent processing steps and is based solely on the molding process.
2. Classification Practices in Actual Production
The financial reports of leading domestic companies such as Zhuhai Zhongfu and Zijiang Enterprise include preforms in the "Injection Molding Business" segment, further confirming their industry affiliation.