Your processing questions answered

Q. I am working with a high-speed BOPP line and facing problems of “delta” marks on heat-seal films of 15 to 35 microns. What could be going on?
Indian BOPP processor

A. Based on your comments, I would assume that a melt disturbance is causing the defect. Another possible cause is MDO temperatures. But a small melt disturbance would be my first thought. You need to examine with a microscope and check your materials and process conditions to determine the most likely cause.
Eldridge M Mount III
EMMount Technologies LLC, Canandaigua, NY
(585) 223-3996 • emmount-technologies.com

Q. In our plant, different lots of the same PC/ABS resin can get mixed in the hopper. These different lots have wildly different melt-flow rates. What is the effect of this variation on injection molding conditions for the part? Do the processing parameters need to change when a new and different MFR lot enters the barrel?
West Coast electronics molder

A. You raise two issues. The first is how to control the process when MFR varies. I know of no other option than to use Delta P—having extra injection pressure available to ensure uniform filling. (I explained this concept in the January issue of Plastics Technology.) Some will tell you the machine is closed-loop, so it does not matter. But they’re wrong: The closed loop will not work if you don’t have a proper Delta P.

The second issue is to check for degradation. If you run with a Delta P, you can watch the pressure at transfer or the injection first-stage pressure, or the pressure integral, any of which will tell you about the material viscosity. If you have degradation, the machine can “talk” to you by showing a change in pressure that tells you viscosity is changing. If you are not using Delta P, you can tell degradation by comparing MFR tests on the granules and on ground-up parts. MFR should not change by more than 30% from pellets to part for unfilled resins. Part weight may or may not indicate degradation. There are also cavity-pressure measurements and infrared spectroscopy that can spot degradation. You also should do physical testing periodically to check part strength, etc.

John W. Bozzelli
Scientific Molding, Midland, MI
(989) 832-2424 • scientificmolding.com