Key Influencing Factors in the Blow Molding Process

To balance inflation speed with efficiency and molding quality, the inflation speed should be as high as possible to shorten blowing time and ensure uniform product wall thickness. However, excessive speed can cause a vacuum at the air inlet, leading to the parison collapsing inward and forming a diaphragm, or tearing the parison at the mold opening, resulting in defective products. In such cases, the problem can be resolved by increasing the diameter of the blow tube or appropriately reducing the inflation speed.

03Blowing ratio

The blow-up ratio, also known as the parison expansion ratio, directly affects material consumption and product performance. Typically, it ranges from 2 to 4; for large thin-walled products, it is set between 1.2 and 1.5, while for small thick-walled products, it can reach 5 to 7. An excessively high blow-up ratio can reduce wall thickness and save raw materials, but it increases the difficulty of the blowing process and diminishes the strength and rigidity of the product. Conversely, an excessively low ratio increases material consumption and wall thickness, which not only reduces the effective volume of the product but also extends cooling times and raises production costs.

04Mold temperature

The mold temperature must be uniformly distributed and is generally controlled between 20–50°C. Its setting is related to the glass transition temperature of the plastic: for plastics with high glass transition temperatures, such as polycarbonate, the mold temperature can be appropriately increased; for plastics with low glass transition temperatures, such as polyethylene and polypropylene, the mold temperature should be lowered. If the mold temperature is too low, the extensibility of the plastic at the pinch-off area will decrease, leading to molding difficulties, blurred profiles or patterns, and thickening of the pinch-off site. Conversely, excessively high mold temperatures will extend the cooling time, increase the production cycle, and cause demolding deformation or increased shrinkage of the product. For thick-walled products, in addition to mold cooling, internal auxiliary cooling can be achieved by injecting liquid nitrogen, carbon dioxide, or other media into the interior of the product.

05Cooldown time

Cooling time directly affects the molding effect and comprehensive properties of the product. After the parison is blown, it needs to be fully cooled, otherwise deformation may occur due to plastic elastic recovery, or defects such as stress and voids may appear. However, excessively long cooling times will increase the crystallinity of the product, reduce its toughness and transparency, and at the same time prolong the production cycle and reduce production efficiency. Therefore, while ensuring the product is fully cooled and molded, it is necessary to accelerate the cooling rate as much as possible, balancing quality and efficiency.