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Reduced Bloom in Extruded PP Film

I recently received this from an extruder manufacturing PP film…

RB
We are processing a 1.1mil 3-layer PP film with a random copolymer.

Initially, we were processing at 210ºC with a kinetic COF (coefficient of friction) = 0.5 initially, and 0.2 after two weeks.

Currently, if we process at 210ºC, we get a COF of 0.8, so we must process at 225 to get a COF of 0.5.

Do you know why we are getting this increase in COF?
note: After further questioning, it was discovered that the quantities of slip, AB, and MB anti-block additives have not changed. Also, they do not currently collect any MFI (melt flow index) or viscosity data from the supplier.
My Response
It is very likely that your material supplier has changed material characteristics on you. The efficiency of your additives relies on their ability to ‘bloom’ or migrate to the surface. Additionally… it appears that overall morphology of the polymer, including crystallinity, has changed since the material no longer exhibits the two week drop in COF as it once did.
If a significant change in either molecular weight, or molecular weight distribution has occurred, then it will change the COF of the base polymer as well as the additive’s ability to ‘bloom’ or migrate to the surface.
You should require the supplier to provide certification for each lot that you receive. This should include some basic data including the Melt Flow Index.
note: the same effect occurs with injection molding with molded-in lubricants and internal release agents.
Additional Thoughts
You should consider performing some basic material tests at your facility… this should include the MFI (Melt Flow Index), but may also include the DSC (Differential Scanning Calorimeter) and capillary rheometer.
Some companies actually purchase a benchtop extrusion line to test and understand the processing characteristics of incoming material lots prior to actual production.
-Andy

Procedure for Measuring Screw Wear

As a follow-up to one of my recent posts, I received this question…

Matt
The problem we are
encountering is the different opinions on how to measure the screw and barrel
wear. Is there a procedure that is widely used throughout the industry to determine screw wear?
My Response
Regarding the screw – Most screw manufacturers will recommend that you use an outside micrometer. Since the flights do not match up, you should lay a block gauge block across a couple flights on one side of the screw and deduct the thickness of that block from the overall measurement. For consistency, you should note the distance, from the end of the screw, that the micrometer contacts the screw. As a result you should have a table with lengths and corresponding diameters.
Regarding the barrel – Using an inside micrometer, most companies will follow a similar procedure as with the screw. The diameters should be take at specific distances down the barrel and be listed in a table of lengths and diameters.
Additional Thoughts
You should avoid using a surface plate with a height gauge to measure the screw wear… This will often mask the areas of the screw with high wear.
-Andy

Applicable Training

During a recent seminar, an extrusion technician recently made this false statement…

Technician
None of the training out there applies to what my plant is doing.
My Response
I first asked him if his extruder had a barrel and screw… He said yes. I then asked about the barrel heaters, cooling fans, and temperature controllers… He again agreed.
He also agreed his extruder has an adapter, die, screw motor, reduced, feed throat, and so on. Then he explained that they used unique downstream equipment on their standard extruder. 
I explained that half of their process is the extruder and the second half is the downstream, in which much training is available. Lastly, I asked if his employees could improve their knowledge and abilities such as in math, quality, and problem solving… He again agreed.
Additional Comments
The purpose here was not to sell training… but to get people to think about training materials as components of the overall training solution.
-Andy

Measuring Screw Wear

A friend of ours recently asked this question about screw wear…

Jim
I can get maintenance to measure the screw, but their philosophy is that
there’s no reason to do so. Typically we replace barrels and check rings, but
do not know what the state of the screw is.  I think we should know, and
measure, but I can’t say technically why.  Can you help?

My Response

The purpose of routinely measuring screw and barrel wear to see the trend over time.  This practice allows you to preemptively correct for wear by scheduling repairs and replacements. for example, let’s suppose a screw is still working, but having some mixing or melting issues, you can document approximately how much wear, and clearance, indicates such complications. If you see a similar situation developing on another machine, you can schedule a screw replacement or repair, rather than react after you produce a bunch of scrap. You can also avoid certain jobs with sensitive materials or strict colorant requirements if you know a machine is suspect.
For comparison… top drag racers completely dismantle and re-assemble the engine after each race, replacing everything from pistons to pushrods (takes about 90 minutes). The components they remove are later measured and a determination is made whether to re-use, refurbish, or discard each component. Each team has its own set of records derived from experience to identify when a component is useful or useless. If the team fields different classes of cars, then the acceptable tolerances would vary from machine to machine. 
Additional Thoughts
There are always rules of thumb, but your mechanics need to think about their jobs more like crew chiefs knowing the driver (mold) is stepping into a machine that is ready to do the job.
-Andy

Explaining Melt Flow Index

I receive this question very often, and feel it would be great for the blog…

Larry
Can you briefly explain melt flow index, and how processors use it?
My Response
Melt flow indexing is the most popular, and yet least accurate way to determine material viscosity. The melt flow index (MFI) is the measure of how many grams of polymer pass through a standardized capillary under a standard load over 10 minutes. The value obtained through the melt flow index test is a single data point. The melt flow index only tests the material at one shear stress, and temperature. 
In general, a higher melt flow index indicates a lower material viscosity. This means that a material with a melt flow index of 20 flows easier than a material with a melt flow index of 5. Melt flow index information from different materials and material grades may be used for a rough comparison of flow characteristics for different materials.
Many processors use this data to qualify incoming materials and to help anticipate changes in the process. For example, if the lot of material you are processing has a MFI of 10, and a new lot has an MFI of 15… you can anticipate issues such as flash, over packing, or overweight product and make the appropriate adjustments.
Additional Thoughts
To obtain more accurate and relevant viscosity data… it is better to perform rheology tests using a capillary rheometer or a parallel plate rheometer. Many companies will also perform in-mold rheology tests using actual production molds.
-Andy

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