The Production Process of Bottle Caps

Although bottle caps are small, they play a crucial role in the packaging field. Their production process integrates materials science, mechanical engineering, and automation technology. From raw materials to finished products, multiple rigorous processes are required. Below is a detailed introduction to the production process of common plastic bottle caps.
Raw Material Preparation
The raw materials for producing bottle caps typically include thermoplastic plastics such as polyethylene (PE) and polypropylene (PP). These plastic raw materials have good plasticity, chemical resistance, and sealing performance. The raw materials are delivered to the factory in granular form and undergo strict quality inspection to ensure that indicators such as purity and melt index meet production requirements. After passing the inspection, the raw materials are transported to a drying device to remove moisture, preventing defects such as bubbles and silver streaks during subsequent processing due to the presence of water. The dried raw materials are stored in a hopper, waiting to enter the production process.
Injection Molding
Injection molding is the core link in bottle cap production. Raw material particles enter the barrel of the injection molding machine from the hopper. Inside the barrel, the raw materials are heated to a molten state by a heating device—generally, the heating temperature for PP is 180–240°C, and for PE, it is 160–230°C. Meanwhile, the screw rotates inside the barrel, pushing the molten plastic forward and compacting and homogenizing it. When a certain amount of molten plastic accumulates, the injection device of the injection molding machine injects the plastic into the mold cavity at high speed. The shape of the mold determines the appearance of the bottle cap, and the mold is internally equipped with a cavity and cooling channels matching the structure of the bottle cap. After injection, the mold maintains a certain pressure and time to ensure that the plastic fully fills the cavity, guaranteeing the dimensional accuracy and surface quality of the bottle cap.
Cooling and Demolding
After injection molding, the molten plastic in the mold needs to be quickly cooled and shaped. Through the cooling channels in the mold, circulating cooling water takes away the heat of the plastic, causing its temperature to drop rapidly. The cooling time varies according to the size, thickness, and plastic material of the bottle cap, generally ranging from 10 to 30 seconds. When the plastic cools to a certain extent and has sufficient strength and rigidity, the mold opens, and the formed bottle cap is pushed out from the mold through a demolding mechanism. During demolding, demolding components such as ejector pins apply uniform force to avoid damaging the bottle cap.

Post-Processing
Trimming and Deburring
The edges of the newly demolded bottle cap may have excess flash and burrs, affecting the appearance and performance, so trimming and deburring are required. A common method is to use a special trimming machine to cut off the flash with a rotating blade, and then remove burrs through processes such as grinding and polishing, making the edges of the bottle cap smooth and flat.

Printing and Marking
To enhance product aesthetics and brand recognition, many bottle caps need to be printed and marked. There are various printing processes, such as screen printing, thermal transfer, and hot stamping. Screen printing transfers ink to the bottle cap through a screen plate, enabling multi-color printing; thermal transfer uses high temperature and pressure to transfer patterns from a transfer film to the surface of the bottle cap; hot stamping can form metallic patterns and text on the bottle cap. Before printing, the surface of the bottle cap needs to be cleaned and pre-treated to improve the adhesion of the ink.

Assembly and Inspection
For some functional bottle caps, such as those with inner plugs and gaskets, assembly is required. Components like inner plugs and gaskets are accurately installed onto the bottle cap through automated assembly equipment or manual labor to ensure sealing performance and functionality. After assembly, the bottle caps undergo strict quality inspection. Visual inspection equipment is used for appearance inspection to check the color, pattern, size, and surface defects of the bottle cap; sealing performance inspection is carried out through methods such as pressure testing and leakage testing to ensure that the bottle cap does not leak air or water under a certain pressure; torque testing is also conducted to check whether the torque required for screwing on and off the bottle cap meets the standard, ensuring that consumers can easily open and close the bottle cap while it does not loosen spontaneously during transportation and storage.

Packaging and Warehousing
Qualified bottle caps after inspection are packaged in a certain quantity. Commonly used packaging materials such as plastic film bags and cartons are used to neatly arrange and seal the bottle caps, preventing contamination and damage during transportation and storage. The packaged bottle caps are transported to the warehouse, classified and stored according to batch, specifications, and other information, waiting for delivery.
The above is the main process of bottle cap production. The entire production process strictly follows quality standards and process specifications to ensure the production of high-quality bottle cap products that meet requirements.
