Finding Root Causes of a Material Problem

Periodically, we see papers and articles that at some point assign root causes in these neat categories, stating for example that 40% of failures are due to poor design, 20% are due to poor material selection, etc. Usually these percentages are skewed according to the expertise or bias of the person or people performing the assessment. Designers will tend to blame designs (unless it is theirs) because they are attuned to the things that can go wrong in this area. People with specialization in materials will similarly point to deficiencies in this area because they know the details of what can go wrong. 

Chemically, PP is a hydrocarbon and therefore burns readily unless some modifications are made to the formulation. A variety of substances can be added to PP in order to achieve some level of flame-retardant properties. But these substances need to be added at significant levels and they frequently reduce toughness. So the prospects for finding a material in the PP family that could fulfill the complete list of requirements was not good.

However, a closer look at the mold showed that this was far from just a material-selection issue. The part had been gated in such a way that a weld line formed in the same area where the impact test was conducted. Many years ago, I went to a Glenn Beall design seminar and one of the slides that really stuck with me was the one that advised against allowing weld lines to develop in areas of high stress.

Weld lines are unlikely to be as strong as the parent material. The strength that can be obtained at a weld line is partly a matter of the material being molded and partly a matter of processing conditions. Under optimal molding conditions, amorphous polymers such as polycarbonate and polysulfone can produce relatively strong weld lines. Semi-crystalline materials such as PP and nylon tend to produce weaker weld lines because of the rapid transition that they undergo as they cool from molten to solid. Fillers tend to erode weld-line strength to an even greater degree because they do not melt with the polymer and therefore they do not form a good bond when the flow fronts meet. They also displace the polymer that does fuse. Even though flame retardants are not thought of as fillers, they essentially influence the properties of the compound in a manner similar to common fillers such as talc or glass. And it can require a significant amount of flame retardant to keep PP from burning.

Processing conditions also affect weld-line integrity. Mold-filling rates, melt and mold temperatures, and pack pressures and times can all influence the integrity of a weld line. The angle at which the flow fronts meet when the weld forms will also have an effect on the strength of the material. But even assuming optimal processing conditions, it is improbable that a weld line in a flame-retardant PP will have more than 70% of the integrity of the surrounding material. Once the mold-design consideration of gate location is included in the problem-solving exercise, the simple conclusion that this is a “material problem” is not so satisfactory.

And there was a third problem. When the part was analyzed in the lab, it was found that the material had been contaminated with nylon 66 at a level of about 5%. Very few polymers mix well. Most of the time, contamination results in delamination, cosmetic problems, and reduced performance. In this case there was the additional problem that typical melt temperatures used to process PP are below the melting point of nylon 66. So there will be evidence of unmelted material within the part. While strictly speaking this is a material problem, it is not a reflection of the inherent ability of the selected material to perform. Instead it is a material-handling problem, which is the responsibility of the processor.

So when diving into a problem-solving exercise, it is useful to keep in mind that there is rarely one cause. More often than not, multiple influences converge to create failures. They all need to be captured in order to come up with the most efficient and lowest cost solution to the problem. 

ABOUT THE AUTHOR

Mike Sepe is an independent, global materials and processing consultant whose company, Michael P. Sepe, LLC, is based in Sedona, Ariz. He has 40 years of experience in the plastics industry and assists clients with material selection, designing for manufacturability, process optimization, troubleshooting, and failure analysis. Contact: (928) 203-0408 mike@thematerialanalyst.com.