Why Do Plastics Fail?

[Dr. Dick]

From ThomasNet.com Industrial Market Trends Vol 8, issue 3, 11/13/07

November 13, 2007
Why Do Plastics Fail?
By Fred White
To better foresee and thus avoid failure, we must understand how a part will behave in relation to its larger product. Polymers may have come a long way over the past century, but it still pays to ensure high quality.

Knowing what properties are important and what testing must be completed to qualify a part fully can enable us to better foresee — and avoid — part failure.

Plastics failure — whether mechanical, thermal, chemical or environmental — typically relates to one of the following factors: material selection, design, processing or service conditions, according to Vishu H. Shah, president of plastics consulting firm Consultek Consulting Group and author of Handbook of Plastics Testing Technology, an excerpt of which is at IDES. A look at each of these factors can help prevent defective products from reaching customers.

1) Material Selection
“There are hundreds of different plastics, with hundreds of different additives and reinforcements that can be used,” notes plastic consulting engineers The Madison Group. While this allows engineers to create unique material with unique mechanical properties tailored to a specific product, it also “creates a larger window for the incorrect decisions to be made, which may eventually lead to part failure.”

As such, there are many questions to ask before settling on the optimum choice. Some of these include:

• What is the desired stiffness-to-weight ratio?
• How much impact resistance is needed?
• Will the product be used outdoors — with subsequent exposure to UV radiation?
• What will be the maximum and minimum temperatures at which the product must serve?
• Will the product be exposed to caustic or acidic liquids (such as solvents) and gases?
• Will the product carry deionized or ultra pure water or other liquids?
• Are there specific electrical properties the polymer must possess?
• How much creep is acceptable?
• What maximum tensile strength is required?
• Will the polymer be expected to exist within mammalian anatomy in contact with blood and tissue?

There are also a number of software offerings available that can help in systematically choosing the appropriate material (polymer, metal, ceramic or composite) simply by selecting application requirements in order of importance.

2) Design
Because design-related failures rank highest among all causative factors, “the importance of adequate fillet radius cannot be overemphasized,” notes IDES. Designers will be well rewarded by referring to curve plotted with stress-concentration on the Y axis and the fillet radius-to-thickness ratio on the X axis.

Moreover, it is critical that designers adhere to basic guidelines provided by the material supplier, remembering that aside from a few basic rules in designing plastic parts, design criteria typically changes from material to material and from application to application.

“Materials suppliers’ know-how and their practical experience gathered in the use of plastics […] is therefore all the more important,” DuPont itself has charged.

3) Process
Today’s processing control equipment advancements minimize the chance of inadvertently creating defects such as molded-in stresses, voids and moisture — “some of the most common causes for premature product failures,” according to Shah. If there is a glitch due to some unexpected event during processing, it makes good sense to keep records. This way, you can refer to them so you can carry out a root cause analysis.

Also, ensure strong weld lines, as another potential process problem involves polymer products that are welded. For example, if you’re welding fluoropolymer sheets inside a tanker trailer, test every millimeter of welds to ensure no liquid contacts the metal. Some liquids can corrode the metal, and the liquid itself can become contaminated. It is well known that many plastics and chemicals are not compatible — that under the influence of chemicals, the properties and behavior of plastics can change.

Processing speed also influences a polymer’s mechanical properties, according to plastic consultancy The Madison Group, while density affects impact resistance.

Molders and other polymer processors must keep all of this — in addition to customer requirements and expectations — in mind before adjusting settings.

4) Service Conditions
Further, unintentional service conditions can also lead to failures, as Shah explains. Such factors can include the following:

• Reasonable misuse;
• Use of product beyond its intended lifetime;
• Unstable service conditions;
• Failure due to service condition beyond reasonable misuse; and
• Simultaneous application of two stresses operating synergistically.

For plastic parts that will see service in combination with other materials (such as metals), often “typical failures occur due to the following:

• Differences in the coefficient of thermal expansion of the materials;
• Bad adhesion of components;
• Wrong molding conditions;
• Stress concentrations due to sharp edges; and
• Poor design of interfacing surfaces.

When failure occurs, you owe your customer an explanation. It seems unfair to take the blame for assembly that occurred beyond your control, but customers hate finger pointing. It is better to ask too many questions before you start filling an order and solve any problems before they emerge than to be party to a product failure. If you prevent problems, you show concern for your customer’s success and they’ll appreciate that. And, of course, you’ll want to prevent any future failures through better selection, design and processing.

DuPont recently added a series of articles, including Choosing Plastics to Match the Application to its library of online documentation. They are free of charge and accessible by everyone.


Resources

Why Plastics Fail … and What Can Be Done About It?
by Vishnu Shah, Consultek Consulting Group
excerpt of Handbook of Plastics Testing Technology (3rd edition, 2007) via IDES, April 11, 2007

Plastic Failure Analysis
The Madison Group

What Can Plastics Do?
DuPont Engineering Polymers

[Will Call]

Great information!! Thanks.

[kerstenpretty21]

wow thats a whole new idea for me. i lerned a lot from your posted thread thany so much for posting it. it gave me more knowledge about plastics.

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