The molding process of food-grade PET plastic packaging cans has a decisive impact on their structural strength. This conclusion can be analyzed from four aspects: material properties, process type, process parameters, and structural design. As a food contact material, the molecular structure and processing adaptability of PET directly determine its mechanical properties after molding. The differences in molecular chain arrangement, crystallinity, and defect control caused by different processes further amplify the impact of process selection on structural strength.
From the perspective of material properties, the benzene ring structure in the PET molecular backbone gives it high rigidity, while the flexible ether bonds provide toughness. This combination of rigidity and flexibility means that sufficient heat is needed during molding to promote molecular chain movement, while overheating and degradation must be avoided. Food-grade PET must meet the FDA and other regulations regarding leachate limits, thus requiring a narrow processing temperature window. If the temperature is too low, the molecular chains cannot fully extend, resulting in insufficient crystallinity and making the can prone to brittle fracture; if the temperature is too high, thermal degradation may occur, producing low molecular weight substances that weaken intermolecular forces and reduce impact resistance. This sensitivity to temperature makes precise control of the molding process a prerequisite for ensuring structural strength.
The choice of process type directly affects the molecular arrangement of PET. Injection molding injects molten PET into a mold cavity under high pressure, suitable for producing cans with complex structures. However, rapid cooling may lead to insufficient orientation of molecular chains, forming amorphous regions and reducing tensile strength. Blow molding, on the other hand, stretches the molecular chains to align them along the direction of force, increasing crystallinity and thus enhancing the axial strength of the can. For example, injection stretch blow molding can produce PET cans with precise neck dimensions, good rigidity, and excellent appearance quality, exhibiting significantly superior structural strength compared to ordinary injection-molded cans. Furthermore, extrusion-stretch-blow molding improves transparency and barrier properties through longitudinal stretching, but the stretch ratio must be balanced to avoid stress concentration caused by excessive orientation.
Optimizing process parameters is crucial for controlling structural strength. Cooling rate significantly affects the crystallinity of PET. Rapid cooling inhibits crystallization, forming an amorphous structure, suitable for cans requiring high transparency; slow cooling promotes crystallization, increasing hardness and rigidity, but may lead to localized stress concentration due to uneven crystallization. Pressure control is equally important. Excessive pressure can lead to uneven can wall thickness, reducing pressure resistance; insufficient pressure can cause flash or shrinkage marks, affecting sealing and structural integrity. For example, in the production of 500ml carbonated beverage preforms, optimizing the cavity layout and hot runner system can shorten the molding cycle while improving the uniformity of preform wall thickness, significantly reducing the scrap rate in subsequent blow molding.
Structural design must be coordinated with the molding process to maximize structural strength. The can wall thickness distribution must be uniform to avoid reduced pressure resistance due to localized thinning; the bottom structure should employ a castle-like design or reinforcing ribs to distribute stress and prevent breakage upon drop. Furthermore, the design of draft angles and corner radii can reduce stress concentration. For example, designing a large radius of curvature transition at the junction of the bottle body and bottom can prevent insufficient thickness and stress concentration during blow molding.
In practical applications, the impact of the molding process on structural strength has been widely verified. By optimizing the cavity layout and equipping it with a high-precision hot runner system, the daily output per mold was increased. Simultaneously, improvements to the cooling system significantly reduced the wall thickness variation of food grade pet plastic packaging cans preforms, resulting in a substantial decrease in subsequent blow molding scrap rates. These cases demonstrate that precise molding processes not only improve production efficiency but also significantly enhance the structural strength of food grade pet plastic packaging cans by optimizing molecular arrangement and structural uniformity.
