This article titled “How to Fix Pinch-off Failures” by Lew Ferguson from Parisons was reprinted with persmission by the author.
Extrusion blow molded parts often fail at the parison pinch-off seam of the mold parting line. Common forms of part failure at the pinch-off are cracking from impact, fatigue failure from flexing, or chemical stress cracking. Once the mode of failure is identified, the appropriate processing changes or pinch-off design modifications can be selected to optimize part performance and appearance.
Part failure along the parting line is related to material processing conditions, parison geometry, molding conditions, mold design, or a combination of these factors. Developing the optimal material shape inside the part at the pinch-off is a key to building parting-line integrity.
The pinch-off mold section is designed to cut through the excess extruded material (flash) that extends beyond the part and separate it along the mold parting line. When the mold closes on an extruded parison, three segments are formed: the top flash (parison between the die and the part), the part, and the bottom flash/tail (parison below the part). In some cases, the whole part is surrounded by flash and is considered fully flashed (part plus flash).
The array of pinch-off designs is endless, often selected from experience or designed for use in specific applications. For instance, multilayer barrier pinch-offs require bonding or manipulation of the barrier layer to maintain the part’s barrier properties. Some mold designs, including pinch-off designs, are patented, so it is always best to check before redesigning.
