The blow molding process is characterized by the presence of inner and outer air streams between the molten parison extruded from the die head. A cooling and sizing die is located at the front of the die head, and some designs also include a rubber plug at the front of the sizing die. The positive pressure generated by the outer air stream inflates the outer parison, causing it to adhere to the mold cavity and form the outer corrugations. The inner and outer air streams are kept in balance, and the inner parison is pressed against the cooling and sizing die, resulting in a smooth inner wall of the pipe.
The single-stage stretch blow molding method for automotive corrugated pipes uses a lower processing temperature and results in a lower degree of crystallization. Therefore, the maximum hot-filling temperature it can withstand is lower than that of the two-stage stretch blow molding method.
Although the stretching, blow molding, and heating processes are completed in one step, cooling is still required, which actually extends the total molding time. In the secondary stretch blow molding process, the reheated automotive wire harness corrugated tube is stretched and blow molded to a volume 1-2 times larger than the final product. Then, the automotive wire harness corrugated tube is heated to approximately 200 degrees Celsius, at which point the heated corrugated tube visibly shrinks in volume. The heat-shrunk automotive corrugated tube is then stretched and blow molded again to achieve the final shape before demolding. The secondary stretch blow molding method has the following characteristics.
The higher temperature used during the heating process results in a higher degree of crystallization in the automotive bellows, allowing them to withstand higher hot-filling temperatures compared to those produced by a single-step stretch blow molding process. Furthermore, the heating treatment of the bellows can be performed independently, thus shortening the overall molding cycle time and increasing production efficiency.
