Raw material preparation and pre-treatment:
Common materials: The 28mm injection molded cover is mainly made of PP (polypropylene) or PE (polyethylene). These materials offer good cost-effectiveness and comply with food contact standards.
Drying treatment: If the moisture content of the material is high (for example, for some PE materials), it is usually necessary to dry at 55℃ for approximately 2 hours to prevent the formation of bubbles or silver streaks after molding.
The core process of injection molding:
Molding and Injection: The pre-treated plastic pellets are heated in the injection machine barrel (the temperature of the PP barrel is approximately 180-280℃) and undergo screw shearing (the screw rotation speed ranges from 1 to 150 rpm) to melt. Then, they are rapidly injected into the mold cavity at a speed of up to 484 g/s.
Pressure retention and cooling: After the melt fills the cavity, pressure retention (with the pressure generally being 60-80% of the injection pressure) is required to replenish the volume of material that shrinks due to cooling, preventing the product from shrinking. Subsequently, the mold is cooled through the cooling water channels inside the mold (commonly using warm water as the medium). The mold temperature usually needs to be controlled at 60-90℃, which is crucial for product quality.
Demolding and ejection: After cooling is complete, the mold is opened. The injection molding machine’s ejection system (with an ejection force of approximately 70.6 kN) pushes the ejection rod or push plate, thereby removing the molded bottle cap from the mold.
Post-production and quality control:
Post-processing: After demolding, the bottle caps need to have the gates trimmed and undergo necessary post-processing (such as annealing the PE material parts, which is done by immersing them in hot water for 2-4 hours).
Quality Inspection: This is a crucial step to ensure product quality. Various inspections need to be conducted, including the size of the bottle cap (for example, the bottle opening size of a 28mm cap must comply with the drawing requirements), sealing performance (such as no leakage after a 0.2MPa internal pressure test for 30 minutes), and the opening force of the anti-theft ring (such as a force of 25-30N needs to be applied for the initial opening).
?? The crucial role of molds
Molds are the heart of injection molding. Their design and manufacture directly affect the quality of the lids and the production efficiency.
Number and layout of cavities: To enhance efficiency, a multi-cavity design (such as one cavity producing two pieces, one cavity producing four pieces, or even more) is commonly adopted. The 28mm bottle caps, due to their small size, are particularly suitable for a multi-cavity layout.
Pouring system: The common types of gates include:
Gate point: Suitable for lids with high appearance requirements. It can be automatically separated, but the mold structure is more complex (requiring a three-platen mold).
Inverted gate: It can automatically cut off the gate when the mold is opened, and it is a common choice.
Hot runner system: Widely used in high-speed production, it can reduce runner blockage, increase material utilization rate, and shorten the molding cycle (can support rapid molding cycles as short as <2 seconds).
Cooling system: An efficient cooling circuit (such as a wrap-around cooling system) is crucial for enhancing production efficiency and product uniformity.
Demolding and thread processing: For bottle caps with threads, in addition to forced demolding (applicable to soft materials such as PP) and slider core ejection (used for handling side holes or side depressions), a screw ejection mechanism can also be adopted (by driving the core to rotate through gear racks or motor-driven mechanisms, to achieve thread demolding).