Originally Published MDDI June 2004

Metals

For small, precision metal parts, electroforming offers many benefits over machining and stamping.

Paul Hazlitt

Medical designers have used electroforms to satisfy unusual requirements, meet short deadlines, and achieve small lot sizes without high tooling costs.

Medical product designers are often tasked with making precision mechanical or structural metal parts for prototypes or small production lots. In recent years, electroforms have solved a variety of problems for medical equipment designers seeking strong, thin-walled, metal components.

Electroformed or electrodeposited parts are made of thin layers of metal formed on a dissolving mandrel. Electroforms have been used as flexible joints, hermetic seals, electromagnetic shields, and other special functions, and have long provided designers with unusual shapes. With knowledgeable technical support, medical equipment designers can use electroforms to meet unusual requirements and short deadlines without high tooling costs. Medical engineers can also use electroforms to achieve shapes, properties, and functions that are not otherwise practical. Often, one-piece electroforms can replace stampings, machined parts, complex assemblies, or brazements and weldments.

Part-for-part, short-run electroforms naturally cost more over time than stampings mass-produced in dies. Each electroform needs its own sacrificial aluminum mandrel, costing $100 to $200 apiece in lots of, for example, 10 pieces. That mandrel cost drops by about half for 100-piece lots, where the cost per electroform is usually about the same as a stamping. In high-volume production, stamping spreads one-time tooling and setup costs over thousands of identical parts, so unit part costs will be lower. This is not true for electroforms. However, for prototype quantities or low-volume production with running charges, electroforms can be made and modified at a fraction of the cost of stamping. 

Electroforming can also produce shapes and tolerances that are too difficult or expensive to machine. Medical device designers have used electroforms to provide cost-effective production solutions to very different design problems, including

• A long, thin nozzle for a blood analyzer that needed a precise orifice profile. 
• A thin-walled cover for operating room instrumentation that must shield electronic circuitry from electromagnetic interference (EMI). While the part could be made by stamping, the short production run could not justify the cost of stamping tools.
• A thin-walled retainer for a diagnostic imaging probe that required tolerances too tight to machine affordably. The precision part had to protect electronics from EMI and moisture, and withstand repeated sterilization with harsh chemicals.

Thin, Strong, and Flexible

So how do electroforms pay off for medical equipment manufacturers? The benefits are in size and weight savings, simplified assembly, and geometries and functions not economically available in metal parts fabricated by other means. Electroformed parts preserve all the desirable mechanical properties of the metal in almost any shape. They emerge with a shiny finish that is easy to clean and tolerates common sterilizing and autoclaving chemicals. 

Medical devices are getting smaller, so designers are looking for strong, low-mass parts to fit into tight spots. In the preceding examples, engineers used a proprietary nickel that is ideal for miniature medical parts. It has 125,000-psi minimum tensile strength, 110,000-psi minimum yield strength, and 270 Vickers hardness; its density is 0.321 lb/cu in. This combination of strength and ductility makes it uniquely suitable for flexible bellows as well as rigid electroformed components. 

Medical equipment designers commonly need unusual shapes made to tight tolerances. Electroforming mandrels made of aluminum are CNC-machined; electrocoated with nickel, copper, or gold; then chemically dissolved away to leave a thin, strong shell. 

Internal dimensions of electroforms can be verified before the parts are made, and electroforming can reproduce dimensions far more accurately than stamping. Unlike annealed stampings, fully hardened electroforms need no heat treating—a process that can distort finished shapes. In addition, unlike metal cutting and bending, electroforming can make precise parts without the induced machining stresses that can warp thin components. 

Electroformed parts can be used in many medical applications and can be formed to specific, tight tolerances. Hollow electroformed parts can have walls just 0.0003 in. thick, one-tenth the thickness limit of common stampings. Thin-walled electroforms capture fine details and make parts of many sizes. Electrical bellows contacts, with walls only 0.0005 in. thick, have been electroformed for circuit boards. Flexible nickel bellows with diameters of only 0.020 in. have been electroformed for laboratory instruments, while shapes less than 0.2 in. across have been produced for minimally invasive surgical instruments. Although maximum wall thickness for electroforms is about 0.025 in., varying wall thickness at different locations of the same part can maximize rigidity and flexibility. 

Flexible Shapes

Flexible electrodeposited nickel bellows are widely used as sensing elements in air regulators, switches, gauges, actuators, and pressure compensators. The electroforming process makes bellows walls just one-quarter the thickness possible with mechanical hydroforming. The sharp radii of the bellows segments can also be made with three times the wall thickness—far smaller than radii produced by mechanical forming.

Thin walls and ductile nickel give electroformed bellows just one-fifth to one-tenth the spring rate of hydroformed brass bellows of the same size. Thus, the force required to extend and compress their convolutions is very low and stays consistent from part to part and over a wide range of motion. In addition, the dynamic properties of nickel can give electroformed bellows an essentially infinite life of 1016 cycles.

For some critical sealing applications, seamless, nonporous bellows are tested to be leak-tight to 10–9 cm3 of helium per second. Engineers have also integrated flexible bellows into rigid parts, eliminating welds and other potential leak points. Flexible sections in rigid parts can accommodate component misalignment and eliminate welding, soldering, and other costly assembly steps. 

Fine Details

When designers of a blood analyzer required a nozzle to aim a precise fluid stream, they designed a 5-in.-long probe with a 0.023-in. orifice. The nozzle had to taper down with a carefully defined internal shape to dispense pressurized reagent at the correct angle. 
Initial attempts to machine the probe from steel tubing failed. The orifice was nearly impossible to manufacture to the required tip configuration, and the thin walls deformed under machining pressure. Spinning and mandrel drawing were ruled out because the geometry made it impossible to withdraw the mandrel after forming. Investment casting and plunge–electrical discharge machining operations, followed by manual deburring, would have been prohibitively expensive. Likewise, molded plastic parts would have required injection molds, the costs of which could not be amortized with low-volume parts. Moreover, the finished moldings would not have provided the parts' required strength and rigidity.

Electroforming produced a rigid, low-mass probe in small lots at about a third the cost of precision machining and manual finishing. The probe had walls only 0.008 in. thick, and an ID of exactly the correct profile for proper nozzle function. To guarantee the purity of the reagent flow, the electroform was isolated from the fluid by an internal polymer coating applied in a secondary operation. In addition, electroformed nickel will not oxidize in air and is unaffected by alkaline liquids. 

Multiple Layers

An electroformed rigid, low-mass blood analyzer probe (center) has a 0.023-in. orifice and was made at about a third the cost of precision machining and manual finishing.

When a design team needed an instrument cover to protect electronics from operating room interference, they had to incorporate electromagnetic shielding into an existing space. The already-final design of the instrument would not allow a greatly increased wall thickness for the 2 × 2 × 2-in. cover. Because the instrument was to be produced in small quantities, the cost of stamping dies for a conductive metal cover would be too high. 

An electroform provided an answer within the existing footprint, at about one-tenth the cost of a stamping. A layer of conductive copper electrodeposited between inner and outer layers of nickel created leak protection as well as forming an effective EMI shield. The instrument makers were thus able to achieve EMI shielding in walls less than 0.01 in. thick.

Assembly Advantages

In another example, a leading manufacturer of diagnostic imaging equipment sought to reduce the size of its solid-state in vitro camera probe, but faced a complex manufacturing challenge. The miniaturized camera retainer at the tip of the probe had to be made to precise tolerances to keep the device properly aimed and focused. With a diameter of less than 0.2 in., the 0.5-in.-long retainer required walls 0.003 in. thick with tolerances of ±0.0005 in. The housing had to withstand temperatures from 75° to 250°F, and protect the camera during EtO sterilization. In addition, the lot sizes were very small.

Electrodeposited microminiature nickel bellows as small as 0.020 in. in  diameter can be used as sensing elements in air regulators, switches, gauges, actuators, and pressure compensators.

Stamping the new retainer was out of the question because of the deep draw and required accuracy of the part. Machining the camera jacket was beyond the capability of most machine shops, because the process would deform the walls. Project engineers also concluded that manual machining, if possible, would be too expensive. But in the small lots, electroforming provided a strong precision camera housing at one-tenth to one-twentieth the cost of precision manual machining. The nickel electroform met the harsh environmental requirements of the application. The accuracy of the camera retainer eliminated clumsy alignment steps and reduced assembly time and cost.

Conclusion

The rigorous development and testing associated with medical devices often requires frequent, rapid design iterations during prototyping. To speed development and reduce cost, prototype electroforms can be supplied within weeks rather than the months required to tool new stampings. Based on rough drawings, CAD/CAM files, or stereolithography shapes, electroforms often yield parts that are impractical or impossible to machine. 

However, for all their advantages, electroforms demand special design and manufacturing facilities, as well as special expertise. A close working relationship and early involvement between the design team and the electroform vendor are essential. Medical designers should seek out vendors with experience in medical parts and the in-house CAD/CAM capabilities, CNC machines, and electroforming and subassembly capabilities to produce quality parts with short turnaround. They should also seek vendors who offer comprehensive engineering support to maximize the payoffs of electroformed components. 

With the right process and application insight, the shapes and potential of electroforms are limited only by the imagination of the designer. 

Copyright ©2004 Medical Device & Diagnostic Industry