Converting Magazine

Eldridge M. Mount

Melt velocity calculation
May 28, 2008

The other day while discussing a question about draw down, I said that one way to define was the ratio of the melt speed exiting the die to the film speed after the film is quenched. This is sometimes called the haul off speed.

So how do you estimate the melt velocity?  Basically you divide the volumetric flow rate by the area of the die opening through which the melt is flowing. But usually you only have the mass flow rate in weight/hr (lbs/hr or kgm/hr) or at least you can measure it with a weight check. Once you have the weight of melt you divide this by the melt density to obtain the volumetric flow rate (cm³/sec or in³/sec).

Melt density is not the solid density and for many polyolefins you can use 0.76-0.78 gm/cm³ as a good starting point. So say we have a line producing 100 lbs/hr of HDPE through a 12 inch wide die with a gap of 0.015 inches, what is the melt velocity?

100 lbs/hr = 1.6 lbs/min = 0.0278 lbs/sec

0.76 gm/cm³ = {0.76/27.7}= =.0274 lbs/in³

Volumetric flow rate is 0.0278/0.0274 = 1.01 in³/sec

If we divide this volumetric flow rate by the area of the die gap (12 * 0.015) = 0.18 in², we find the average melt velocity to be:

1.01/0.18 = 5.6 in/sec for the average melt velocity

This is the basic way you would estimate melt velocity from the output and die dimensions. The critical material property you need is the melt density for the polymer you are using.

Posted by Eldridge M. Mount on May 28, 2008 | Comments (1)