For blow molding, melt strength influences parison sag to control wall thickness. With film, melt strength impacts bubble stability and the ability to draw the film in the machine and transverse directions (MD, TD) to get balanced properties. In the case of sheet, melt strength is important to know because it influences drawdown and sag from the die to the rolls. That’s unfortunate because this data point is vital. But all extrusion processes depend to some degree on the melt strength of the polymer, and this property is generally a no-show on data sheets. Many even have some thermal properties that are necessary for screw design and cooling calculations. Spec sheets typically show the solid density, melt flow, and a variety of physical properties of the solid polymer. There is an important data point for extrusion-grade polymers that is almost never found in the data sheets.
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