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You know there are tools out there to help you pick the right material for your part, but are you up to speed on the terms and technology needed to use them? Here’s a place to start.
Injection molders are being asked by their customers to become more involved in resin selection. Unfortunately, many processors don’t have this expertise in-house, especially for demanding engineering applications. Getting the resin right is a cornerstone for successful processing and for the overall success of the plastic part. Today, there are many resources to assist molders with this task; tomorrow, there will be even more.
A number of years ago you could contact a resin supplier, who usually had a design or engineering center, and ask for help with the development of your plastic part concept. Experts there would spend time understanding your application and recommend the right resin choice. Although suppliers obviously still get involved in resin specification, they generally don’t have the internal resources that were once available. This shift has put more pressure on the OEM and the processor to sort out the selection of the right material for the job. As we have more recently seen, many jobs at OEMs are being eliminated. In the past, an OEM may have had a strong core of plastics expertise. Now within even a large OEM, it isn’t surprising to see a sole plastics engineer that is taxed with handling the entire resin function.
Ideally, an OEM should begin looking at resin choice early in the development process when part design and processing method all begin to take shape. Quite often, however, OEMs tend to stick with what they know and have used. In automotive, for example, ABS or PC/ABS are used extensively. Many argue that polypropylene is a better, more economical choice for many of these applications, but companies tend to get stuck in their ways.
These changes at both the resin supplier and the OEM have put more of the resin selection pressure on the injection molder. Generally speaking, molders have not been staffed with a materials engineering team. What usually happens is that experienced plant managers or processing managers “just know” what the right material is for the job. They know because of years on the job running perhaps thousands of different parts. They may also have a relationship with a rep from a distributor, so after a quick phone call, the resin may be spec’d in. The trouble is, these “old timers” retire or leave the company, sometimes resulting in a huge gap in capability.
What you can do
So, how can a molder sharpen its resin selection skills and thus become a more important resource for its customers? The most important initial consideration is to narrow the choice down to a product family based on key application requirements, all the while pushing lower on the resin cost scale. Many websites exist that provide guidelines. One notable site is the European distributor, Distrupol (click on “Useful Stuff”). It has a library with all sorts of helpful information.
Once a potential resin family, or a handful of families, has been chosen, it is time to begin looking at specific grades from suppliers. Of course, Google can yield interesting results in seconds. Just be careful when reviewing the resulting information because it can often be biased toward a particular manufacturer. Also, IDES’s website has supplier-agnostic information for identifying distributors and resin grades.
Plastics are chosen based on the properties that are tested by the resin suppliers. Because many materials engineering groups at OEMs tend to deal with several material types, they tend not to fully understand material properties that are specific to plastics. Even if a materials engineer fully understands the application’s requirements, he or she may have difficulty translating these into the property requirements needed to find a particular grade of plastic.
For example, an engineer may realize that his or her part needs to withstand impact and high heat. What may not be clear is that attributes like Izod impact, heat deflection temperature, continuous use temperature, and glass transition temperature are the key properties. (See box below, for more definitions.) And, to find resin grades for the application requires some indication of the range of values for each of these. Expect some tools to emerge on the Web within the next few years that will simplify this situation. There is a massive number of plastics grades on the market today, and the number continues to increase. As much of the brain trust in plastics seems to be tending toward retirement, it is extremely important for companies to be able to access the resources they need, when they need them. Tomorrow’s online tools will help to reduce the complexity, taking some of the pain out of finding the right resin for the job.
Definitions to help you when choosing a resin:
Here’s a primer on the basic terms you should know when conducting your materials search. For a comprehensive list, visit our Property Descriptions page.
Glass transition: The reversible change in an amorphous polymer or in amorphous regions of a partially crystalline polymer from (or to) a viscous or rubbery condition to (or from) a hard and relatively brittle one. The glass transition generally occurs over a relatively narrow temperature region. Not only do hardness and brittleness undergo rapid changes in this temperature region, but also other properties, such as thermal expansion and specific heat, change rapidly. The glass transition temperature range is generally provided by material suppliers on their data sheets.
Heat deflection temperature: The temperature at which a standard test specimen deflects 0.010 inch under a stated load of either 66 or 264 psi. This temperature is one indicator of how a resin might behave at elevated temperatures.
Impact resistance: Relative susceptibility of plastics to fracture by shock—e.g., as indicated by the energy expended by a standard pendulum-type impact machine in breaking a standard specimen in one blow. The most common type of test is Izod impact (see below).
Izod impact: A test designed to determine the resistance of a plastic material to a shock loading. It involves the notching of a specimen, which is then placed in the jaws of a machine and struck with a weighted pendulum.
Tensile strength: The pulling force per area required to yield or break a given specimen. The area used in computing strength is usually the original, rather than the necked-down area.
Toughness: The resistance to fracture of a plastic when stressed. |
September, 2007 -
Reprinted with permission from Injection Molding Magazine. Copyright © Canon Communications LLC.
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