Key hazardous factors and control measures in the entire process of blow molded injection of blow molded products are discussed. In the injection process of blow molding products using blow molding molds, factors that can lead to hazards in the blow molding product forming and processing technology mainly include blowing pressure, blowing speed, blowing ratio, and mold temperature.
1. Blowing pressure in blow molding process
During the injection process, air compression is utilized for two purposes: first, using the work pressure generated by compressed air to inflate and push the semi-molten and precisely machined tube against the mold cavity, and then creating the required shape; second, cooling down the blow molding products. The blowing work pressure is also related to the capacity of the product. A higher blowing work pressure should be adopted for large capacity products, while a lower blowing work pressure should be employed for small capacity ones. The suitable blowing work pressure should also ensure that the appearance designs and patterns of the product are clear after forming.
2. Blowing speed in blow molding process
To shorten the air blowing time and quickly obtain blow molded products, a large amount of air needs to be injected into the mold within a short period to ensure uniform and fast expansion of the mold cavity and reduce the cooling time within the mold cavity.
3. Blowing ratio in blow molding process
The blowing ratio refers to the proportion of the plastic part’s diameter to that of the preform. When the specifications and quality of the preform are the same, the blow ratio of the blow molded product increases as the product size increases, causing the product thickness to become thinner. The size of the blowing ratio should be determined based on the type, characteristics, appearance, specifications, and preform specifications of the plastic. As the blowing ratio increases, the thickness of the product becomes softer, and the compressive strength and bending stiffness decrease. Additionally, the forming process becomes more challenging.
The mold temperature has a significant impact on the appearance of the blow molded product. Generally, the temperature of blow molding molds should be evenly cooled as much as possible. The mold temperature is related to the type of plastic, product thickness, and size. If the mold temperature is too high, the cooling time is prolonged, the production cycle increases, and the production efficiency decreases. Moreover, if the cooling is insufficient, it can cause product deformation, increase shrinkage rate, and result in a poor surface gloss. Generally, for plastics with larger molecular chain rigidity, the mold temperature should be higher, while for plastics with greater molecular chain flexibility, the mold temperature should be lower.
5. Cooling time of blow molding products
To ensure sufficient cooling and prevent deformation upon demolding, the cooling time of blow molding products is relatively longer. The cooling time is generally related to the thickness, size, appearance, and plastic type of the plastic product. The thicker the wall of the blow molded product, the longer the cooling time. The cooling time for PE products with a large specific heat at constant pressure is longer than that of PP products with the same thickness and a small specific heat at constant pressure.
6. Cycle time for blow molding products
The cycle time for blow molding products includes extrusion of the preform, mold locking, preform extraction, air blowing inspection, air release, demolding, and product removal. The standard for selecting the cycle time is to minimize the time while ensuring that the blow molded product can maintain its shape and not deform, thereby improving production efficiency.