Because of its long chain structure and the flexibility of molecular chain, generally, when the polymer is placed at a certain temperature, from the action of external force, the complex of macromolecule undergoes a series of intermediate states to the equilibrium state suitable for external force, which is a relaxation process.

Taking advantage of the temperature dependence of the relaxation time of polymer delaying high elastic deformation, the semi-finished polymer products (tubes, hollow profiles, etc.) are rapidly deformed near TF above the polymer glass transition temperature, and then maintained in deformation, cooled to below the glass transition temperature or crystallization temperature in a short time, so that the deformation of the formed product is frozen, This is the viscoelastic principle of hollow forming. The process flow of extrusion blow molding includes: ① the formation of pipe blank: it is extruded by extruder and hung in the pre separated cavity directly below the head; ② When the sagging parison reaches the qualified length, the mold shall be closed immediately, and the parison shall be cut off by the notch of the mold; ③ The compressed air blowing pipe inserted from the small hole on the parting surface of the die sends compressed air to make the parison expand and form close to the die wall; ④ Keep the air pressure so that the product can be demoulded after cooling and shaping in the mold cavity.

The key factor is the temperature (parison). When the parison temperature is too high, the molten material is easy to deform due to low viscosity, resulting in uneven thickness during the movement of the parison, which will affect the quality of blow molded products. When the temperature is too low, more internal stress will accumulate in the products, which is easy to produce stress rupture in use. If the mold temperature is too low, the plastic extensibility at the clamp mouth will be reduced and it is not easy to blow, The part is thickened and difficult to form; The mold temperature is too high, the cooling time is prolonged, and the production cycle is long. The cooling time is too short and the cooling is insufficient, which is easy to lead to product deformation, shrinkage increase and dull surface.

There are many air inlet forms of hollow molding blow molding in Dalian, including needle blowing method, top blowing method and bottom blowing method. As for which one to adopt, it can be selected according to the requirements of equipment conditions, finished product size and wall thickness distribution. However, no matter which form is adopted, it shall be based on the principle that the compressed air does not contain oil and water droplets, and its pressure is sufficient to blow up the parison to obtain products with obvious contour and clear lettering pattern.