Move to Plastic Springs Dictated Design Approach

By: 
March 12, 2010

A Brooklyn, NY company is introducing the first highly engineered helical compression spring made from plastic composites, with an eye on medical, electronics and other demanding markets.

"Plastic springs can never fully match the strength of metal springs, but we wanted to develop a composite spring that would approach the strength of metal and still offer corrosion resistance and other properties engineers are looking for," says Subramanya Naglapura, global product manager for Lee Spring. Those other properties include imperviousness to magnetic fields, retention of properties at elevated temperatures, light weight, and low thermal and electrical conductivity, as well as recyclability.

"We worked very closely with Sabic Innovative Plastics (formerly known as GE Plastics) to develop the right material, and we chose Ultem (R) resin, a very high-strength plastic," he says. Ultem polyetherimide performs in continuous use to 340F (170C) and meets the other requirements established by Lee Spring. It's designed to perform under load with minimum side thrust.

Development of the first highly engineered composite springs ever produced required significant testing and tool development efforts.

Shear Modulus

"Published materials' properties focus on mechanical characteristics such as impact strength, but there isn't data available on what we are most interested in - shear modulus," says Naglapura. Sabic Innovative Plastics conducted significant testing on the materials' properties required for use as a spring, while Lee Spring tested actual springs at its labs.

A contract injection molder did flow analysis to develop data on orientation of the glass fibers in the spring. Initial production quantities will be met from a single-cavity tool, but multi-cavitation tooling will be built as demand grows, says Naglapura. Examples of target applications in the medical field are syringes and fluid delivery devices.

Development of a mechanical design was challenging. "The relatively weak strength of plastic materials, as compared to steel, means that traditional spring designs with round cross-sections would not provide sufficient load for most applications" says Naglapura. Furthermore, injection molding is a difficult process for creating a helical shape due to undercuts that impede withdrawal of a multi-piece rigid molding with squared ends. Surface finish can also be affected by knit lines where melt flows meet in a mold cavity.

The Lee Spring design has a nearly rectangular cross section to maximize the amount of active material used as opposed to the more common round wire spring design. The exact cross section is a slight trapezoid to facilitate manufacturability.

"The design element that proved difficult yet critical is the configuration of the ends to provide for minimal side thrust and maximum flat load bearing surface without creating stress points or increasing the solid height while again accounting for manufacturability," says Naglapura.

Due to difficulties with both design and manufacturing processes, there are very few springs made from plastic available in the market today.

Helical compression springs are widely used because of their simple configuration and high functionality.

They are cylindrical in shape with an outside diameter allowing fit inside a cylindrical bore and an inside diameter that fits over a round rod for secure positioning. They generally have ends

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