Materials Matter -- The Material Selection Process

The title “Materials Matter” pun-ishly describes the dual character of this new, fairly technical white paper from Proto Labs, the company behind the Firstcut and Protomold services for custom CNC machined parts and injection-molded parts, respectively. This paper serves both the engineer who needs to get down with quantitative analyses of the plastic material options for a part -- say an application that someone’s life rides on -- and designers or technicians who need parts that will survive their product’s projected lifecycle without giving them servicing headaches. For that matter, it also serves the guy like me who just wants a part and some consultant will not recommend a material.

This 10-page PDF could have been titled “Material Matters.” See, it covers the A-to-Z of the material selection process beginning with the elephant in the room: The process is often an educated guessing game. And why shouldn’t it be? Your typical commercial material database offers data on 85,000 or so plastic materials sorted in 45 or so families. Then, material data sheets provide somewhat idealized data. For example, performance characteristics are measured at room temperature. And standard metrics like elongation at break, while nice to know, are not necessarily the performance characteristics you’re after. So you end up digging around application notes, blogs, and design manuals to fill in the gaps.

This is where this paper will come in handy. It breaks down the data you encounter in data sheets and strives to relate that data to general application usage. The effect here is to build up your knowledge base by enhancing your ability to sort through data sheets, to eliminate polymers from consideration, and to hone in on the optimal material for your application. It covers a broad spread of data reported on materials, ranging from yield and tensile strength to the relationship between temperature and aging, and from modulus to melt flow rates.

The approach used is to focus on key parameters, such as deflection temperature under load (DTUL) and the Vicat softening temperature when you’re looking at a polymer’s maximum short-use temperature data. You get a few quick insights on what that means, then some tips to keep in mind. For example, after explaining that you can determine the upper temperature limit for filled or unfilled amorphous polymers by looking at DTUL data, you’re advised that DTUL values should never be used for long-term performance characteristics but you can estimate short-term heat resistance using it and Vicat values.

The paper devotes a lot of its discussion, as you no doubt guessed already, to the data on the modulus of a material. In this dialog you’ll find the majority of the paper’s nine figures and tables. Among the modulus-related topics explored are how modulus is calculated, stress cracking, the effect of strain rate on modulus and yield stress, temperature effects, and impact resistance. The paper also addresses other material properties such as dielectric constant and strength, surface and volume resistivity, and coefficient of thermal expansion.

And what about you people who just want a part that is made with an “it’ll do” material? On page 10 you’ll find a section called “Don’t Make Me Do the Math.” It offers a few rules of thumb to try if you want to skip a full numerical analyses. It also includes the best rule of thumb of all: When in doubt, do a full materials selection process. With the “Materials Matter -- The Material Selection Process” white paper at hand, you’ll have a good guide to help you along that road. Hit the link over there and download your copy.

Thanks, Pal.  – Lockwood

Anthony J. Lockwood

Editor at Large, Desktop Engineering

Download “Materials Matter " The Material Selection Process”