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October 12, 1998

7 Min Read
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.

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Spring-loaded acetal toy links can be easily twisted open by a child's hands to connect and disconnect links, but still spring shut naturally.

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.

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Like most springs that use plastic, this well-known buckle design is used intermittently to engage and disengage the lock, giving the spring time to recuperate in between.

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:

  • Parts consolidation: "To compete with metal springs," says Smith, "you have to look to consolidation to make up the difference." This means that engineers and designers have to be creative in the use of plastic springs. It will probably mean eliminating several parts and completely redesigning the assembly to accommodate the springs of plastic. The payoffs are lower cost, lighter weight, and easier assembly.

  • Recyclability: Parts with metal springs, says Smith, are often scrapped if they do not meet spec, especially if the part is composed of plastic and metal. For some companies, he says, scrap rates approach 5 percent. Plastic springs allow the entire part to be recycled.

  • Corrosion resistance.

  • Natural lubricity.

  • Reduced fabrication cost vs. machining metal.

  • Molded-in color.

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This cord lock is an example of the advantage of parts consolidation that plastic springs can bring. Three parts were reduced to two by molding an acetal resin spring into the top of the cord lock. The plastic spring eliminated a separate metal piece.

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.

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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:

  • Polyethylene and polypropylene are inexpensive, but exhibit poor resilience and poor resistance to creep. They should be used for loads of short duration. Fatigue response is moderate, but chemical resistance is good.

  • Acetal copolymer is more expensive, but shows good load bearing characteristics. Fatigue response is good, but it is not compatible with acid environments of less than 4 pH.

  • Linear polyphenylene sulfide is more brittle than acetal copolymer and better for high-load, low-deflection applications. It has good creep resistance and retains its toughness at high temperatures. Fatigue response is good. It also performs well when exposed to aggressive chemicals, especially at elevated temperatures.

  • Nylon is tough when dry, but stiffness can decrease up to 25 percent when exposed to 50 percent relative humidity.

  • ABS is good for single use, or occasional use, but it has poor fatigue response.

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The input pinion of a dual drive washer transmission has two radial antibackup leaf springs molded into its sides. The springs compress and slip when the agitator oscillates, but lock when the motor reverses. Lower loads on the gear teeth and the resilience required made acetal the resin of choice.

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.

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One-piece razor blade dispensers like these replaced a three-piece assembly with a separate metal spring. Such consolidation often requires a reassessment of the design of the product.

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.

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