I’ve been working with plastic materials for more than 30 years now, helping companies design and build products ranging from boogie boards, water basketball games and SCUBA diving equipment to disposable lighters, cell phones and hand-held medical devices. I’ve learned a lot about plastics along the way. While I can’t claim to know everything there is to know about these materials, there is one thing I do know for certain: I despise plastic property data sheets.
Don’t get me wrong, property data sheets can be useful. They allow for a comparison of Material A versus Material B versus Material C, and so forth, by using standard tests, with measurements taken under controlled conditions. When all goes well, you can easily evaluate the materials and make some informed decisions about the materials that are suitable for your application. The problem is, things rarely go well.
Data sheets are a crap shoot
For starters, finding comparable data on two different data sheets is almost a random event. Vendor A might offer some basic mechanical data—tensile stress and strain at yield (and at break), tensile and flexural modulus and so forth, along with some basic impact data at two different temperatures. But Vendor B only gives you tensile stress and strain at yield, along with flexural modulus and Izod impact at 23° C. Then as you dig further, you realize Vendor A and Vendor B are reporting data in different ways. Not only are the units different, this one measures heat deflection at one thickness, this one at another. Then, when you think you have comparable data for a given set of tests, you realize Vendor A has listed the test . . . but there is no data given for the material!
The data Isn’t always useful
Even when you can find comparable data, the test itself may not be all that useful. It’s kind of like the National Football League (NFL) Scouting Combine, where hundreds of college football players who are hoping to play in the NFL perform a series of tests. Each athlete takes each test, whether it's a 40-yard dash or a jump or drill, in exactly the same way.
The problem is, while the test scores can be compared, how can you use the test results to predict the performance of that athlete in a given position during a future NFL football game? In fact, there are those who claim that some of these tests are completely useless. I would make the same claim about some of the test data provided in property data sheets. (As an example, see my earlier article for PlasticsToday on toughness.)
Data sheets are filled with marketing mumbo-jumbo
Another issue I have with property data sheets is that the “data sheet” is often a marketing brochure. Instead of providing data on the material, the supplier is using the data sheet format to deliver a marketing spiel. As a technical professional, I understand that marketing technical products—and services—is a difficult endeavor. All suppliers want to present their offerings in a manner that highlights the advantages of their material. But there is a distinct difference between describing the facts of your product and using marketing mumbo-jumbo. It’s not unlike the description you might see on the back of a bottle of wine: With delicate flavors of blackberries and licorice, Jack Ass Red is the perfect complement to full-flavored meats such as rabbit, lamb and roasted wild game.
How many times have you read a data sheet where the material description reads something like this:
Crapalloy® 727WTF offers high strength and stiffness along with exceptional chemical resistance and toughness over a broad temperature range. It has excellent processability, low warpage and good flow characteristics, making it suitable for a wide range of applications.
Really? But does it go better with chicken or fish?
|Graphics courtesy Eric Larson.|
They don’t tell me what the material is
At the top of every property data sheet, there is usually an entry titled, “material description.” In this box, the resin supplier enters a description of the material, which is usually very brief. It goes something like this: ABS, general purpose, medium flow. That entry is usually coupled with a supplier description, where the resin supplier enters a BRANDNAME™® and PRODUCTCODE.
All of this drives me nuts. Maybe that’s because I spent so many years working in the resin supply industry. Every resin supplier has oodles of brand names and trade names, along with product codes that are used to categorize and differentiate between the various grades and versions of different materials. And not only are these product codes confusing as all get out, the brand names and trade names get mixed and jumbled and combined and hyphenized. Yeah, I know, every resin supplier thinks his brand is superior, but can you just tell me—in plain English—what the hell it is that you are selling?
Something to the effect of:
This material is a nylon blend. It consists of a mix of standard unreinforced PA 6 (50% by weight), along with a customized PA 6/6 that has been reinforced with standard short glass fiber using a commercially available coupling technology, at a glass loading of 30% by weight. By weight, the resulting blend is, thus, 50% PA 6, 34.5% PA 6/6, 15% glass fiber and 0.5% coupling agent. The molecular weight distribution of both resins used in the blend is within industry standards.
Note: an impact-modified version of this material is also available, using 98% of the blended material, along with a proprietary (and totally kick-ass) impact modifier with a proprietary coupling agent.
Is this too much too ask?
Data sheets don’t tell me what the material can do
Have you ever gone to a lumber yard, and asked a salesman:
What's the flexural modulus of quartersawn lumber made from the eastern white pine tree (Latin name Pinus strobus) at 73° F, when moisture conditioned for 90 days at 50% relative humidity?
If you did, they would probably look at you like you were nuts. (Although, someone who really knows this stuff might ask you in return, “Is that from a tree grown on a western facing slope? Or an eastern facing slope?”)
But even if you had this data, that is not going to tell you
- how to work with the wood;
- what kind of saw blade to use;
- how close to an edge you can pound a nail;
- what happens when the hammer misses the nail and hits the wood itself;
- how the wood takes stain;
- or how it flexes and twists when the humidity levels change.
The same is true for plastic property data sheets. In all fairness, this is not what data sheets are intended to provide, but why can’t they?
Part of the problem may be the way I assimilate information. We all learn in different ways. Some of us learn by reading, some by listening, others by watching others and some of us learn by experimenting. I tend to learn best by experimenting, and I often learn the most when my experiments go wrong. (Have you ever taken something apart, and then put it back together, only to find a left over part on your workbench? You learn pretty quickly not to do that again).
Good data is hard to find
Lastly, the main reason I despise data sheets is that good data is so hard to find. I might be able to get a data sheet from a supplier web site, but all it has is basic data and marketing mumbo-jumbo. If I try to dig deeper, I get stonewalled with a “sign-up now for updates and breaking news” pop up, followed by a browser crash. If I call an 800 number, I get grilled on my company, the project, the expected annual volume and then, maybe, if I am lucky, I’ll get someone who tells me, “We’re sorry, but that data is unavailable.”
Maybe it’s because resin suppliers think we’re stupid.
All I know: I despise the data sheets they provide.
Eric R. Larson is a mechanical engineer with over 30 years' experience in plastics design. He has helped develop products ranging from boogie boards, water basketball games and SCUBA diving equipment to disposable lighters, cell phones and handheld medical devices.
Larson owns Art of Mass Production (AMP), an engineering consulting company based in San Diego, CA. AMP provides services to manufacturing companies in the consumer electronics, wireless and medical device industries, helping them to create products that make a difference in people’s lives.
On the blog plasticsguy.com, Larson writes about plastics technology and its effect on people and the planet. His newest book, Plastics Materials Selection: A Practical Guide, can be purchased through his blogsite.