Melt Index Mysteries Solved

One of the most if not the most important property of any resin used in extrusion blow molding is its Melt Index (MI). The MI is a reflection of the molecular weight of the material or the length of its chains. The longer the chains, the higher the molecular weight, the more viscous and tough the material, the lower the MI. ASTM method D1238 regulates how MI is measured. At a temperature of 190º C a weight of 2.16 kg is placed on a material sample that can only escape through a defined orifice of about 2 mm. The amount of material that passes through the orifice in 10 minutes is recorded as the material's MI. For blow molding we use materials with a MI of 0.1 to 1 g/10 min except for very stiff materials.

As materials get stiffer, less and less passes through the orifice and eventually the numbers become so small that errors are more likely. Therefore, another parameter has been created, the so-called High Load Melt Index (HLMI). Instead of a weight of 2.16 kg, a weight of 21.6 kg is placed on top of the sample. The higher force pushes more material through the orifice.

Sometimes it becomes necessary to compare a MI with a HLMI and unfortunately the correlation is usually not given on the Technical Data Sheet (TDS) of the resin. The relationship between HLMI and MI is not linear as placing a higher weight on the sample moves the point in the stress/strain curve of the material to a different location. And of course, as with all plastic curves, this one is not a straight line but a curve of some sort. Molecular weight distribution is a measure of how the material chains are cross-linked and comes into play here. A very ballpark number to get an idea of the relationship between HLMI and MI would be 100. A resin with a MI of 0.1 would have a HLMI of around 10. Resins with a broad molecular distribution might have this relation in the 115 to 120 range, while resins with a narrow distribution would fare around 85 to 90. So this relationship can also be used to assess the molecular weight distribution of a resin.

The MI determines many process characteristics of a resin and processors should pay attention to it especially when they encounter an unusual problem. It is a good idea to have a TDS of the material you are running handy during the production run. Most can be accessed on the Internet on the sites of the resin manufacturers.