Polyethylene Melt Flow Rate Test Load Selection Basics
Polyethylene melt flow rate test load selection matches resin density and molecular weight to standard ASTM D1238 masses of 2.16 kg, 5 kg, or 21.6 kg at 190 °C.
Polyethylene Density
Meaning
The mass per unit volume of a polymer resin determines its degree of crystallinity and governs many mechanical properties. A high polyethylene density correlates with increased stiffness and barrier properties, but reduces impact resistance and stress crack resistance. This metric is a primary classification parameter for raw material sourcing.
Moulders select different density grades to match the structural requirements of the end product.
Physical Determination
Gradient columns and buoyancy methods measure this parameter on solid polymer specimens. To establish the polyethylene density, a sample is placed in a liquid column with a graduated density profile, or weighed in both air and a reference liquid. This measurement is conducted at a controlled temperature of twenty-three degrees Celsius according to international test standards.
Achieving high accuracy is important because even a minor shift of one milligram per cubic centimetre indicates a change in material performance. This value is used to verify the consistency of incoming resin lots.
Process Dependency
Cooling rates during injection moulding or extrusion influence the final crystalline structure of the polymer. Fast cooling reduces the achieved polyethylene density by freezing the polymer chains before they can organize into ordered crystals. Conversely, slow cooling allows the material to crystallize more fully, resulting in a higher density and a stiffer part.
This processing variable means the density of the final moulded part can differ from the nominal datasheet value of the virgin resin. Control of mould temperature is therefore necessary to maintain consistent part dimensions.
Material Property
Higher crystallinity increases the resistance to chemical permeation and raises the melting point of the material. This dependency on polyethylene density makes high-density grades suitable for chemical containers, while low-density grades are preferred for flexible films. The density value also affects the shrinkage of the polymer during cooling.
High-density parts exhibit greater shrinkage than low-density equivalents.