Same Supplier… Same Grade… Bigger Dimensions…

I received a follow-up question regarding a blog entry last week…

We switched our product line over to 4710 PE 100 extruder grade PE about 2.5 years ago. The material was produced in Belgium. We sized all of our new tooling to this material.In February they started making the material in Texas. The supplier says the material is the same. All of my processes were established with Decoupled II process using cavity pressures sensors.

Machines, mold, melt, and barrel temperatures are the same.

When I match the templets from before the dimensions of the parts are too big. I think they have changed the carbon black. I have suspected that a Nucleating Agent has changed. If it has changed the crystalinaty it will affect the shrinkage. Can this be in the Carbon Black? Or can the Nuculating Agent be some where else?
My Response
Although a change in nucleating agent may have occurred, it is more likely that there is a change in the polymer. During polymerization, high density polyethylene can grow, branch, and propagate in many ways. Different material manufacturing conditions can result in a change in Average Molecular Weight, Molecular Weight Distribution, as well as the amount of branching even though the melt flow index is the same.
First, I would request some rheological data from the material suppliers from both facilities. If you can compare some detailed shear rate and viscosity data, it can help you better determine the differences between the grades.
You may wish to investigate the difference in rheological characteristics between both the old and new. A Differential Scanning Calorimeter, or DSC, can be used to determine the relative degree of crystallinity between the samples from your past and current production lines. This may also give you some data regarding any differences in additives.
Additional Thoughts
If you really want to delve into the material, you can use a capillary rheometer or parallel plate rheometer to measure the viscosity characteristics across multiple decades of shear using different temperatures. These types of tests will provide very accurate information regarding the behavior and characteristics of the material.

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