By Vishu Shah, Consultek
Plastics part failure is generally related to one of four key factors including Material Selection, Design, Process and Service Conditions.
Failures arising from hasty material selection are not uncommon in plastics or any other industry. In an application that demands high-impact resistance, a high-impact material must be specified. If the material is to be used outdoors for a long period, a UV-resistant material must be specified. For proper material selection, careful planning, a thorough understanding of plastic materials, and reasonable prototype testing are required. The material selection should not be solely based on cost. A systematic approach to material selection process is necessary in order to select the best material for any application. The proper material selection technique involves carefully defining the application requirement in terms of mechanical, thermal, environmental, electrical and chemical properties. In many instances, it makes sense to design a thinner wall part taking advantage of the stiffness-to-weight ratio offered by higher-priced, fast cycling engineering materials. Many companies, including material suppliers, have developed software to assist in material selection simply by selecting application requirement in the order of importance.
Proper material selection alone will not prevent a product from failing. While designing a plastic product, the designer must use the basic rules and guidelines provided by the material supplier for designing a particular part in that material. One must remember that with the exception of a few basic rules in designing plastic parts, the design criteria changes from material to material as well as from application to application. Today, design-related failures are by far the most common type of failure. The importance of adequate fillet radius cannot be overemphasized. The following figure shows how dramatically the stress concentration factor increases as the fillet radius thickness ratio decreases.
After proper material selection and design, the responsibility shifts from the designer to the plastic processor. The most innovative design and a very careful material selection cannot make up for poor processing practices. Molded-in stresses, voids, weak weld lines, and moisture in the material are some of the most common causes for premature product failures. The latest advancement in process control technology allow the processors to control the process with a high degree of reliability and also help in record keeping should a product fail at a later date. Such records of processing parameters are invaluable to a person conducting failure analysis. Any assembly or secondary operation on processed part must be evaluated carefully to avoid premature failures. Failures arising from stress cracking around metal inserts, drilled holes and welded joints are quite common.
In spite of the built-in safety factor warning labels, and user’s instructions, failures arising from service conditions are common in the plastics industry. Five categories of unintentional service conditions are as follows.
- Reasonable misuse.
- Use of product beyond its intended lifetime.
- Failure of product due to unstable service conditions.
- Failure due to service condition beyond reasonable misuse.
- Simultaneous application of two stresses operating synergistically.
Most of the stresses imposed on plastics products in service can be grouped under the headings of thermal, chemical, physical, biological, mechanical, and electrical.
Types of Failure
Mechanical failure arises from the applied external forces which, when they exceed the yield strength of the material cause the product to deform, crack, or break into pieces. The force may have been applied in tension, compression, and impact for a short or a long period of time at varying temperatures and humidity conditions.
Thermal failures occur from exposing products to an extremely hot or extremely cold environment. At abnormally high temperatures the product may warp, twist, melt, or even burn. Plastics tend to get brittle at low temperatures. Even the slightest amount of load may cause the product to crack or even shatter.
About the Author
Consultek Consulting Group
|Vishu H. Shah is President of Consultek Consulting Group, a technical and management consulting firm for plastics professionals. His 35 years of extensive practical experience in plastics Industry includes positions as president and cofounder of Performance Engineered products, Senior Plastics Engineer of Rain Bird Corporation and Nibco Inc. His areas of expertise include product design, processing, automation, materials, rapid prototyping, tooling, failure analysis and testing. His industry related experience ranges from developing and training personnel, writing standard operating procedures, obtaining certifications, negotiating major contracts, and developing business plans for growth and expansion as a custom Injection Molder, Providing technical services to engineering and quality assurance staff in the area of material selection, product development, processing, tooling design, and inspection techniques, setting up and running PVC compounding operation, developing formulations, and automation projects to serving as expert witness for legal community.|
The views, opinions and technical analyses presented here are those of the author, and are not necessarily those of UL, ULProspector.com or Knowledge.ULProspector.com. While the editors of this site make every effort to verify the accuracy of its content, we assume no responsibility for errors made by the author, editorial staff or any other contributor. All content is subject to copyright and may not be reproduced without prior authorization from Prospector.