How to Control the Thickness and Length of Parison for Extrusion Blow Molding? What Are the Requirements for Parison Quality?
（1）Control of parison thickness and length
When the parison is extruded from the die, the expansion phenomenon will occur, which makes the parison diameter and wall thickness larger than the die gap. The parison hanging on the die will droop due to its own weight, causing elongation and thinning of the extrusion end wall thickness. In the extrusion process, the common ways to control the parison size are as follows.
1. Generally, the conical die is designed to adjust the die gap, which is driven by the hydraulic cylinder to move up and down to adjust the die gap as a variable of parison wall thickness control.
2. When the extrusion rate is changed, the faster the extrusion rate is, the larger the parison diameter and wall thickness will be due to the die swelling.
3. The wall thickness of the parison can be controlled by changing the drawing rate of the parison periodically.
4. Pre-blown molding method, when the parison is extruded, the parison is cut off by a special cutter to seal the bottom, and the air is blown into the parison before it enters the mold. The wall thickness of the bottom-molded parison can be controlled by automatically changing the air volume of the pre-blown molding when the parison is extruded.
5. The control program control of parison thickness is a method to control the thickness and weight of blow-molded products by changing the wall thickness of the cross-section of the extruded parison.
（2）Requirements for parison quality
The main requirements for parison quality in extrusion blow molding production are as follows.
① The size, melt viscosity, and temperature of each batch of parisons are uniforms.
② The appearance quality of the parison is better because there are defects in the parison, and the defects will be more obvious after blowing.
③ The extrusion of parison must be synchronized with the time required for die closing, blowing, and cooling.
④ The parison must be extruded at a steady speed with uniform thickness.
⑤ The parison should be extruded at low temperature as far as possible, and the temperature should be stable.