Originally Published MDDI February 2004
Cover Story
Chemical and laser processes shine in shaping thin parts.
William Leventon
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| Laser micromachining of a tubular medical device. |
You designed it thin. Now you have to make it thin. How are you going to do it? While considering your options, don't forget about a pair of effective, but sometimes overlooked, techniques for making thin parts: chemical and laser machining. Chemical machining techniques employ acids and masks to form parts, whereas laser machining relies on laser power to cut materials to the desired shape.
Because of their speed, precision, and cost savings, chemical and laser machining techniques appeal to the thin-part manufacturer. But both techniques also have their share of limitations and downsides, which must be weighed against the advantages before deciding whether one of them fits the bill for the application.
Getting Started
The chemical machining process starts with a flat piece of material that has been cleaned and covered with a photosensitive coating. To etch the desired part shape, manufacturers use a masking device called a phototool, which is made by photographing the part image on film. Once created, this phototool is contact printed onto the material coating. In many cases, phototools are used in pairs, one on each side of the material. The phototool includes both opaque and transparent areas. Opaque areas cover material that will be etched away, while transparent areas cover material that will be protected from etching.
When the phototools are in place, the assembly is exposed to light, which reaches only the coating under the transparent areas of the phototools. Exposure prepares these areas of the coating for hardening with a developing solution.
After developing, an acidic etchant is applied to the assembly by spraying or immersion. The etchant dissolves material not protected by the hardened coating, leaving the desired part shape.
Chemical etching turns out burr-free parts without the troublesome heat-affected zones that can be produced by laser cutting. In addition, it won't change material properties, notes Mike Lynch, vice president of operations for United Western Enterprises Inc. (Camarillo, CA). This gives the process an edge over conventional machining techniques such as stamping, which might change magnetic and other properties in certain materials, adds Lynch, whose company etches parts for catheters, pacemakers, and hearing devices.
The medical segment of the company's business has been growing, according to Lynch, who attributes this growth to the advantages of chemical etching. One advantage is that the tooling for a chemical process costs much less than conventional tooling. At United Western, he says, tooling for a typical job costs $250 to $300, while a stamping tool and die might carry a hefty $50,000 price tag.
Made from computer-aided design files, chemical etching tools can be produced within 24 hours, notes Art Long, general manager at Conard Corp., a chemical machining firm based in Glastonbury, CT. As a result, Long says, Conard can deliver finished products in two to three days, much sooner than parts made with conventional tooling.
Design Attractions
Chemical tooling can be especially attractive to product designers. Besides its low cost, a single photographic tool can produce several versions of a medical component, according to Rick Hoppe, engineering manager at Vacco Industries Inc. (South El Monte, CA), which etches a variety of metal and plastic components. Therefore, during a customer's prototyping process, Vacco can make a number of different design variations at the same time. For this reason, Hoppe says, some companies will turn to chemical etching for prototypes even if they plan to use a conventional die to manufacture the product.
In some cases, however, there may be no practical alternative to chemical manufacturing. “Because of the design and geometry of some parts, it would be a nightmare to try to make them any other way,” says Lynch. As an example, he offers a part with a series of very small holes. Using chemical etching, he notes, “you're not trying to get a tool into these small areas. You're dissolving the material you don't want.”
On the downside, chemical etching can limit design flexibility. Because of the physics of the process, for example, hole diameters must equal the material thickness, Hoppe notes.
In addition, chemical machining is less suitable for thicker parts than thinner ones. Consider the crucial matter of tolerances. Generally speaking, the process can hold a tolerance that's 10% of the part thickness, according to Long. This means that tolerances must increase as parts get thicker. As a result, the process isn't able to meet tolerance requirements for many thicker parts.
As parts get thicker, chemical etching becomes less attractive in other ways. For example, the radius of a part loses its sharpness, Lynch notes. The process also gets longer and more expensive. Because of this limitation, Vacco, Conard, and United Western all suggest material thickness limits for etching between 0.060 and 0.090 in. Medical OEMs producing devices with thicker parts should probably explore other machining alternatives.
Customer Demands
Medical customers are particular about their finished parts. Lynch says United Western's medical customers are quite demanding. “Their sampling tends to be very stringent,” he reports. “They want to see that every part they pick out is 100% good.”
To meet such demands for quality, United Western has beefed up its in-process inspection procedure. The company has added an inspection step after the developing stage of the process. “If that step isn't done correctly, the parts may not etch the way you expect, which will cause issues farther down the line,” Lynch explains.
At Conard, Long and his colleagues continually regenerate etchant chemistry to maintain a consistent, specific gravity and acid level. Regenerating improves process consistency and allows Conard to hold tight tolerances for longer periods of time, Long says.
Vacco also pays special attention to the composition of its etchants. For instance, the company relies on statistical process control (SPC) of the chemistry of its ferric chloride etchant. Thanks to SPC, Vacco reports greater etchant longevity and a more-consistent etch rate. “SPC has been a great benefit to the bottom line and to the quality of our products,” Hoppe says.
Another plus for the process is the latest chemical machining equipment. According to Long, etching firms and their customers are benefiting from precise new machines. Unlike older systems, which had just one pressure gauge for dozens of spray tubes, the new machines include a separate gauge for each tube, making it easier to adjust the process. The machines also let users run their chemistry hotter, resulting in faster etch rates that reduce deviation over the entire process.
Looking ahead, Lynch anticipates the development of new light sources for the chemical etching process. Today, the typical light source emits rays that travel straight down to parts directly beneath it. But the light travels at an angle to reach parts on the outer edges of a sheet. According to Lynch, this can cause slight variations in the dimensions of different parts that come from a single sheet.
In the future, though, new light sources may emit rays that travel straight to all the parts on a sheet, eliminating dimensional variation from part to part. “That's what we're all trying to get,” he says.
Making a Mark
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| Components such as stainless-steel axial springs can be produced using chemical machining. |
Like chemical machining techniques, lasers are making a mark in thin-part manufacturing. Laser machining represents a big and growing portion of the business at Norman Noble Inc. (Cleveland), which operates a large laser manufacturing facility. “In certain cases, I think companies are realizing that laser technology can outperform wire electrical discharge machining (EDM) and chemical etching,” says Chris Noble, the company's chief operating officer.
To attract medical device manufacturers, laser machining offers repeatability, consistency, and tight tolerances, Noble says. The technique also requires less setup work than with chemical processes. “In chemical etching, you have to create a mask or image any time you make a change,” he points out. “But in laser cutting, all that's needed is a simple program change.”
In addition, laser machines can cut very small radii that can't be made by wire EDM machines, according to Jeff Miller, Norman Noble's manager of laser research and development. Noble adds that EDM machines require small wires to cut even relatively small corner radii. “But the smaller the wire, the slower the feed rate,” he notes. By contrast, small corners don't slow the feed rate of a laser system, he says.
Noble also maintains that, contrary to a common misconception, laser machining has become faster than cutting stacks of parts with wire EDM. The increased process speed is due to new linear-motion tables and high-speed controls that can keep up with the laser, he explains.
Still, laser machining has its downsides and limitations. For the most part, Noble says, laser cutting is limited to flat, slightly contoured, and tubular parts. In addition, Norman Noble focuses its laser capabilities on parts no thicker than 0.015 in., and the company does not laser cut parts thicker than 0.040 in.
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| A polyimide haptic for use in an intraocular lens can be etched using chemical machining methods. |
What's more, laser machining doesn't always produce high-quality edges. “An edge cut with a diamond saw will be an almost polished edge,” notes Richard Press, president of LPL Systems Inc. (Mountain View, CA), which manufactures laser-cutting systems for the production of medical devices. By contrast, a laser-cut edge can look “like it's been vaporized, melted, and blown away,” Press says. Translation: laser-cut edges can be rough, uneven, and strewn with debris. So laser-cut edges might require postprocessing work such as electropolishing or multistep chemical procedures.
Such procedures aren't always required, however. In fact, Press notes, the roughness of some laser-cut edges can be measured in the tens of microinches—even before cleaning. Although this still isn't as good as mechanically ground edges (which can have surface roughnesses measuring less than 10 µin.), it's by no means bad.
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| Chemical tooling is useful for producing metal components such as the titanium interconnect shown here. |
For better-cut quality, Norman Noble laser systems include electronic equipment that analyzes the beam profile in real time. Beam profiling allows the company to change the beam width and the number of “hot spots” to better suit the material being cut. “That lets us provide cleaner cuts through all kinds of materials,” Noble notes.
That includes platinum, a material that many of the company's competitors won't tackle, according to Miller. Not only will Norman Noble's lasers cut platinum, Miller says, but “we've raised the bar to the point where we can cut it dross free.”
Rather than buy off-the-shelf laser systems, Norman Noble purchases the laser, controller, and other components and uses these components to build its own systems. For increased accuracy, the company's systems include linear stages that move the workpiece, as well as direct-drive rotary motors for tube cutting.
Stent-Cutting Systems
In high-volume manufacturing of round products such as stents, lasers offer high throughput and fast cutting time, according to Press. LPL's systems can cut tubes in “tens of seconds,” he says, which compares favorably with EDM systems that can take three minutes to cut through a tube wall. Depending on the material to be cut, LPL can provide lasers with a variety of different wavelengths and output configurations.
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| A high-precision stage and YAG laser system creates a flexible platform for diverse applications. |
Scheduled for introduction in the first half of this year, LPL's new stent cutter is designed to hold tolerances of 200 µin. per inch of tubing. The machine will also offer a feed rate of 3.2 in./sec—four times faster than the company's current system. Press attributes the increased speed and higher accuracy to a commercially available motion controller customized with LPL's own algorithms.
For flat-sheet work, LPL will also be introducing a flatbed version of the upgraded system. This machine will include a high-power, low-divergence laser that can maintain a laser spot size less than 0.001 in. in diameter. In addition, the machine will offer the same high accuracy as the stent cutter, along with speeds as high as 6 in./sec, Press claims.
The new systems haven't been priced yet. But sophisticated laser machining equipment can be pricey. For example, LPL's laser systems start at $195,000 and go up to more than $250,000.
Currently, ultrahigh costs are holding back a promising new breed of laser. With pulse widths measured in picoseconds and femtoseconds, these lasers apply cutting energy for such brief periods of time that they don't create troublesome heat-effect zones. This results in much cleaner machining, Press says.
Although the technology exists to produce industrial lasers with very narrow pulse widths, the expenses involved make them cost prohibitive, Press notes. “If you're building a machine that you want to sell for $250,000, you can't put a $200,000 laser into it,” he notes.
But within 5 to 15 years, he believes the lasers may be a popular method of small-dimension, high-precision micromachining. The adoption rate could be boosted by technology improvements that drive down the cost of the systems, as well as demanding new applications. “It's a matter of when the technology is needed to do things that can't be done with existing laser sources,” he says.
Conclusion
Chemical and laser machining processes can be attractive alternatives to conventional machining operations such as EDM. The advantages of chemical machining include low-cost tooling, better prototyping, and burr-free surfaces. Chemical machining can be used to shape stainless steel, titanium, nitinol, and other metals into a variety of components, including mesh, stents, springs, and lead frames. But the process becomes problematic as part materials get thicker.
Laser machining systems cut flat and tubular materials into many different medical parts. The systems offer fast, precise, and repeatable cutting. But they can leave rough edges that need postprocessing work. Laser systems can also be expensive compared with other machining options. Despite the limitations, though, both laser and chemical machining options are worth a look for OEMs who have been less than satisfied with their thin-part manufacturing process.
William Leventon is a New Jersey– based freelance writer who frequently covers the medical device and diagnostic industry.
Copyright ©2004 Medical Device & Diagnostic Industry