Analysis and control of sagging factors of blow molding parison

(1) Factors affecting the sagging of the parison

In the process of parison extrusion, the parison is prone to sag due to its own weight, resulting in uneven wall thickness of the parison. The more severe the sagging, the greater the unevenness of the parison wall thickness. Therefore, during the molding process, the sagging of the parison must be strictly controlled.

In the process of extruding the parison, there are many factors that affect the parison's sagging, such as the quality of the parison, the extrusion time, and the melt viscosity, which also affect the uniformity of the parison thickness to varying degrees.

①The weight of the parison. The weight of the parison increases, the parison sags due to its own weight, and the wall thickness of the parison becomes thinner.

②Melt viscosity. As the melt viscosity decreases, the sag of the parison increases, but for materials with higher melt viscosity, the sag of the parison is smaller (the sag is less affected by the extrusion time).

③Squeeze out time. The squeeze time increases and the sag increases.

④When the weight and extrusion time of the parison are the same, the sag of the parison increases with the increase of the melt flow rate

(2) Control of parison sag

In the extrusion blow molding process, the factors that affect the sagging of the parison are often not single, but mutual influence. Therefore, during production, tests should be conducted according to different grades of materials to determine the best molding process conditions to reduce the weight of the parison and improve the uniformity of the parison. When the parison sags severely, the following measures can be taken according to the situation.

①Choose a resin with a relatively small melt flow rate to fully dry the material and reduce the fluidity of the melt.

②The temperature of the machine head and die can be appropriately reduced to reduce the viscosity of the melt.

③Increase the extrusion rate to reduce the extruded parison, thereby reducing the time for the parison to sag.

④ Speed up the mold closing speed and reduce the time for the weight of the parison to sag

⑤Appropriately increase the width of the die gap and increase the wall thickness of the parison.