Improve Pinch-off result of multi-layer blow molding

Focus on HDPE PE/EVOH based COEX bottle having drop test failure, Specially in Agrichemical / Pesticide bottles : For reference PA (Polyamide) Material from BASF taken and truly incorporated with new mold design :-

Focusing on PE-HD/Recycled/Tie/PA/ EVOH multi-layer blown molding only. Polyamide here provides good resistance to various chemicals therefore this multi-layer structure is widely used for agro-chemicals e.g. liquid pesticides which contain organic solvents.
[Determination of a “good pinch-off” or a “bad pinch-off” result]
A good pinch-off should to create a welding line which is rather smooth on the outside and forms a flat elevated line or a low bead inside, not a groove.

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, mold design, or a combination of these factors. Developing the optimal material shape inside the part at the pinch-off is a key to build parting-line integrity.
Selection of polymers

• Ultramid B40L and C40L/C40LX are recommended for multi-layer blow molding containers or bottles, C40L and C40LX provide better dart impact strength therefore suitable for finished product volume bigger than 500ml, while B40L is usually used for no more than 500ml bottles.
• Various tie materials are available in market, but to match low MI and high viscosity PA, strongly suggest to select low MI (no higher than 2.0) MAH-grafted LLDPE based tie material.
• Lower MI HDPE (higher molecular weight) definitely increases the impact strength but anyway its melt viscosity needs to match with polyamide under processing temperature. For example a MI-0.25 is just well matched to Ultramid C40L(X).
Parameters optimization
• Typical temperature profile for Ultramid B40L and C40L(X): 230C/250C/250C for extruder and 195-200C for the die, too low melt temperature of polyamide may cause melt fracture, poor thickness distribution and “bad” grooved pinch-up Weld line.
Mold design
Because of the comparatively high pressure and mechanical stress extered on the mold bottom when (in the closing step) it pinches one end of the parison together, the pinch-off in a nonferrous metal mold is frequently an insert made of hard, tough steel. The effect on the blown part always shows in the so-called weld line.
The pinch-off section does not cut off the excess parison tail. Its protruding edges cut nearly through, creating an airtight closure by pinching the parison along a straight line which makes it easy later to break off or otherwise remove the excess tail piece. A high quality pinch-off of a thick-walled parison is more difficult to obtain than that of a thin-walled parison. However, much depends on the construction of the pinch-off insert.




 The pinch-off should not be knife-edged, but, according to some molders, should be formed by lands about 0.1-0.5mm long. The total angle outward from the pinch-off should be acute, up to 15 degree. These two features combine to create a high quality welding line. A groove, which weakens the bottom along the seam, may be formed when these two features of the pinch-off are missing.
One method of obtaining more uniform weld lines is to build “dams” into the mold halves at the parison pin-off areas. These dams force some of the molten resin back into the mold cavities to produce strong, even weld lines.