Plastics have designs on springs

One of the last metal holdouts against replacement by plastic is the spring, an application, it seems, that would be one of the most difficult to ply with polymer. More than ever, though, not only is it possible, it's the way to go. But if you think you can simply mold a plastic spring in the image of its metal predecessor, think again. Fortunately, the cost-effectiveness and flexibility of plastic over metal give you lots of options when it comes to designing and molding springs.

IMM talked to three people who have spent many of their waking hours thinking about the potential for plastic in spring applications. They all work for Ticona (formerly Hoechst Technical Polymers): Zan Smith, staff engineer; Maribeth Fletcher, program executive; and Dennis Sopka, senior business specialist. They've developed some handy guidelines for spring applications. With them, you can decide when plastic is appropriate for a spring, what material properties are best, and what you can expect in performance.

When To Say When
The benefits of converting to or designing a plastic spring are numerous. But before you do, Smith has this recommendation that comes before all others: "The general rule is that plastics should be used in springs if immediate recovery is not necessary." Ideally, plastic springs work best in intermittent service, when the spring generates a specified force for a short time, but otherwise relaxes. If you're designing a new spring, or replacing an existing spring, recovery time should be at least equal to the time under load, says Smith.

If this seems like a rather limiting requirement, think about it. Few springs require uninterrupted use. Consider the products pictured throughout this story. The cord lock is used only to adjust the tension on the cord. The well-known snapfit buckle is sprung only to lock and release, hardly a full-time job.

If you do choose to go with plastic, there are several spring types from which to choose. They include leaf springs, coils, zig zag springs, and curved beam springs. In fact, says Fletcher, a lot of designers don't realize that plastic can be a spring. "Others," she says, "are designing springs, but don't know it. Most people think in terms of coils."

However, says the Ticona trio, there are properties metal brings to the party that plastic can't match. The flex modulus of steel, for example, is 30 to 100 times that of resin commonly used to mold springs. And as a result, plastic springs have to be larger to compensate.

Still, plastic springs have their place. Consider these benefits:

Watch the Creep
If you do apply plastics to a spring function, as mentioned earlier, the flex modulus of metal is 30 to 100 times greater than that of resin used to mold springs. Resin choices includes PE, PP, polyester, ABS, nylon, acetal, and polyphenylene sulfide, according to Smith, Fletcher, and Sopka. The material you choose should be partly determined by its resilience.

Compared to metal, the stiffness of the best resin for springs erodes quickly under sustained loads. Acetal copolymer loses 50 percent of its spring force after 1000 hours (about six weeks). It loses 60 percent of its original force after 10,000 hours (about a year), and two-thirds of its force after 100,000 hours (about 11 years). In general, crystalline acetal and polyphenylene sulfide have better resistance to creep than polypropylene and ABS. On the processing end, says Smith, remember that the creep rate goes up considerably in crystalline materials if the mold temperature is too low.

Material Options
Your material choices for springs vary, depending upon the properties you're looking for. The factors to consider are resistance to creep, load bearing, fatigue response, and resistance to chemical corrosion. Acetal copolymer is the material of choice for Ticona, says Smith. Below are some candidate materials and their associated characteristics:

The use of fillers will change a spring's characteristics. Glass fibers increase stiffness and strength, but they limit deflection. Impact modifiers reduce modulus and make the material more flexible, but less resistant to creep.

The operating temperature also affects plastic spring performance. Most perform best at room temperature, and the higher the glass transition temperature (the point at which a certain material changes from a hard state to a softened state), the longer the spring properties will be retained with rising heat.

Another consideration is ultraviolet (UV) exposure. If your plastic spring is buried in a product, or otherwise shielded from the sun, UV radiation is not a problem. However, for products that spend a lot of time outside, such as garden tools and sports equipment, UV light can deteriorate spring action. When selecting a material, Smith, Fletcher, and Sopka recommend looking beyond the typical resin data sheet to determine if the material you're considering can do the job under UV exposure.

Finally, in the mold, Fletcher says to watch your gates and melt fronts. "Avoid gate or weld lines where the spring or the spring actuator is," she says. Weld lines, of course, introduce weaknesses that could imperil the spring's performance.

On the design front, remember that plastic coil springs are not your only option. Consider a zig zag or other design whenever possible; or try a cylindrical spring instead. This design replicates the action of a coil spring, but uses a high-modulus resin and flexible beams to achieve greater stiffness than molded coil springs.