1) Factors affecting parison sagging
In the process of parison extrusion, the parison is easy to sag due to the effect of self-weight, resulting in uneven parison wall thickness. The more serious the sag is, the greater the non-uniformity of parison wall thickness is. Therefore, in the process of forming, the sag of parison should be strictly controlled.
In the process of extrusion, there are many factors affecting the parison sag, such as the quality of parison, extrusion time, melt viscosity, and so on, which also affect the uniformity of parison thickness to varying degrees.
① The weight of the parison. With the increase of the weight of the parison, the weight of the parison is sagging and the wall thickness of the parison becomes thinner.
② Melt viscosity. With the decrease of melt viscosity, the sag of parison increases, but for the material with higher melt viscosity, the sag of the parison is smaller (the sag of the parison is less affected by extrusion time).
③ Extrusion time. With the increase of extrusion time, the sag increased.
④ When the weight and extrusion time of parison is the same, the sag of parison increases with the increase of melt flow rate.
(2) Control of parison sag
In the process of extrusion blow molding, the factors influencing parison sag are not single, but mutual. Therefore, in order to reduce the self-weight sag of the parison and improve the uniformity of parison, experiments should be carried out according to different grades of materials in production. When the parison sags seriously, the following measures can be adopted according to the situation.
① Choose the resin with a low melt flow rate, dry the material sufficiently, and reduce the melt fluidity.
② The temperature of the die and mouth mold can be reduced to reduce the viscosity of the melt.
③ Increasing the extrusion rate can reduce the time of parison weight sag.
④ Speed up the die closing speed and reduce the time of parison weight sag.
⑤ The width of the die gap should be increased and the wall thickness of the parison should be increased.