Zeroing in on the best plastic for a new product is no easy task. Not only do engineers have at least 50,000 grades of commercial plastics from which to choose, they must also consider that many factors affect plastic performance. While engineers can use data sheets from suppliers on the Web to compare mechanical and physical properties, this is only a starting point. They must take into account that performance characteristics hinge not just on stress and strain conditions but also on strain rate, temperature, time and the environment. As a result, it's tricky to come up with the list of needed mechanical and physical properties for a product from application requirements.

Engineers can end up oversimplifying factors. Even the experienced ones often rely too heavily on the single-point property descriptions that typically appear on data sheets despite the warnings and disclaimers printed on them. And the consequences aren't limited to part failures. Picking the wrong plastic can impede the design process early on, resulting in over-engineering and wasted time and money. Fortunately, engineers can avoid these negatives and pick the best plastic for the job by following some simple tips from plastic pros:

1. Look beyond the data sheet. Instead of regarding data sheets as a definitive source of information, engineers should think of them as resumes, says Andrew Poslinski from GE Plastics' global application technology group. He recommends using these information sheets for screening and comparing materials only—and perhaps inputting the values into simple, back-end calculations. "Data sheet properties can give you a quick feel for the material's ability to meet deflection, impact and processing requirements," he tells Design News. "But only in a comparative sense." Engineers should keep in mind that information on data sheets is based on lab tests conducted under ideal—not real-world—conditions using test specimens that are dissimilar to real-world parts. And they contain the averages from several tests, not the spread of data. "The point is that this data is just a single point," notes Poslinski.

2. Do some digging. To access multipoint information, engineers just have to search a little deeper—not much deeper in many cases. In fact, some information is readily available on the Web. For example, the CAMPUS database provides standardized materials data from dozens of suppliers and much of that information is multipoint, says Ranganath Shastri, a materials scientist and global knowledge management steward for Dow Plastics in Michigan. From this source, engineers can get the creep data for many materials in tension in at least three temperatures and four stress levels. Another good source of both single-point and multipoint information is a proprietary database from IDES of Wyoming. Additionally, material suppliers themselves can provide multipoint information on their websites.

3. Consult materials suppliers. If researching doesn't yield all the necessary information, then it's time to ask materials suppliers for help. Their expertise is valuable, as they have gathered a wealth of data on impact, creep and acceptable design values—information they provide to customers or prospective customers. And while they may not have data, say, in one-degree increments at a range of strain rates, they can usually propose a good alternative grade for which there is such data. And they may be able to develop such information for a customer if they expect to sell a large volume or are otherwise strongly motivated.

4. Pay extra attention to five basic properties. According to materials experts, engineers should heed five basic properties in order to avoid many design errors and even facilitate the more advanced work of later design phases. These properties include impact resistance, moduli, creep, thermal properties and anisotropy. Engineers should be careful to look beyond data derived from common tests for these properties, as such tests often do not reflect end-use conditions.

5. Perform some testing yourself. Unfortunately, obtaining extra information early in the design cycle won't attend to all of the more complicated concerns that come up later on. Thermal issues, high-rate impact and fatigue, to name just a few, often demand sophisticated computer simulations and thorough testing. And performing such tests can provide huge payoffs. For example, Delphi Safety and Interior Systems of Ohio was able to save millions by pursuing a more rigorous method of analyzing flexural modulus and creep performance to find the right polypropylene for its instrument panel lower carrier. According to David Boch, a materials engineering supervisor for Delphi, they molded the most promising eight material candidates into sample parts and then cut test strips from matching locations from each sample to account for any localized deviations in mechanical properties. They then conducted a bench test on the pieces, hanging weights on them at room and elevated temperatures for about 10 days. They chose the best-performing material and achieved 43% in resin-cost savings. "This savings is applied over 1.6 million parts annually at 5 lb/part," Boch tells Design News.

Source:

The Misunderstood Material
Joseph Ogando
Design News, February 2, 2004
www.designnews.com