How does injection blowing achieve high-precision molding with no flash or parting lines through "injection before blowing"?
Publish Time: 2025-10-28
Blow molding is a mainstream technology in the manufacturing of plastic hollow containers. Due to its exceptional molding precision and superior product quality, injection blowing is widely used in applications such as pharmaceutical bottles, cosmetic bottles, baby bottles, and precision measuring bottles, where cleanliness, sealing, and appearance are crucial. Unlike conventional extrusion blow molding, injection blowing utilizes a two-step process: "injection before blowing." By integrating injection molding and blow molding in separate stations, it successfully achieves high-precision molding with no flash, no parting lines, precise dimensions, and a smooth finish, making it the preferred process for high-end container manufacturing.
1. "Injection before blowing": Step-by-step molding, precise control
The core of injection blowing lies in the "injection before blowing" process. The entire process is divided into three key stations: injection molding, blow molding, and removal. First, in the injection molding station, molten plastic resin is injected under high pressure into a closed parison mold, precisely shaping a preform with a complete bottle mouth and threads. At this point, the parison remains warm and securely wrapped around a hollow metal core rod. The core rod then rotates to the blow molding station, where the hot parison is fed into the blow mold and closed. Finally, compressed air is introduced through the core rod into the parison, causing it to expand evenly within the mold and adhere to the mold walls. The mold cooling system quickly sets the shape, ultimately creating a complete hollow container.
2. No flash: Injection molding eliminates flash
Flash is a common defect in plastic products, typically caused by a loose mold closure or excessive melt pressure, leading to plastic overflow. In injection blowing, the parison is formed entirely in the injection molding station, a process characterized by extremely high pressure control and mold closure precision. The parison fills the closed mold cavity under high pressure, with the mold parting surfaces tightly aligned, virtually eliminating flash. Because the parison is fully formed before blow molding, the subsequent inflation process does not involve new plastic injection. Therefore, the entire bottle body requires no trimming after final molding, resulting in a clean appearance and no residual flash, making it particularly suitable for automated filling and aseptic packaging.
3. No parting line: The blow mold does not contact the bottle neck or parting surface.
The "parting line" is the seam left at the parting surface of traditional blow or injection molded products. It not only affects the appearance but can also cause contamination and stress concentration. Injection blowing cleverly avoids this problem. First, the bottle neck is formed in one step during the injection molding process, completely eliminating the need for secondary compression by the blow mold. This ensures high precision and smoothness in critical areas such as threads and sealing surfaces. Second, in the blow molding process, the blow mold is solely responsible for the expansion and cooling of the bottle body, with the parting surface located in the middle of the bottle. Because the plastic adheres evenly to the mold wall during the inflation process and the mold closes with high precision, even if a parting line exists, the seam is extremely subtle and far less noticeable than in extrusion blow molding. More importantly, critical areas such as the bottle mouth, shoulder, and base are completely free of parting lines, significantly enhancing the product's sealing and safety.
4. High-Precision Molding: Mandrel Transfer and Temperature Control
The high precision of injection blowing is also due to the precise design and temperature control of the mandrel. The mandrel serves as both the core of the parison molding process and a carrier for heat transfer and the blowing channel. Its surface finish directly affects the quality of the bottle's inner wall, while its temperature control influences the uniformity of the inflation. Precisely controlling the parison temperature between injection and blow molding ensures optimal inflation of the plastic, avoiding uneven thickness and stress cracking. Furthermore, the high repeatability of the multi-station turntable ensures that each parison is accurately placed into the blow mold, ensuring consistent mass production.
In summary, injection blowing, through its "injection-first, then blow" step-by-step molding process, leverages the high precision of injection molding and the hollow-forming capabilities of blow molding to achieve a flawless bottle appearance with high dimensional accuracy, free of flash and parting lines. It is not only a model of precision in plastics processing technology, but also a reflection of the relentless pursuit of quality and safety in modern high-end packaging.
