Shop Solutions: Machining Plastics for Medical Devices
Devices and instruments machined from high-performance plastics and polymers for the medical industry, as well as other industries, offer many advantages over aluminum or stainless. They are lighter, just as strong and durable, and some are radiolucent (permeable to X-rays or other forms of radiation).
These materials are often machined on the same types of VMCs or HMCs as metal parts, but that doesn't mean the processes are the same. It was the need for precisionmachined plastic parts that led to the founding of Applied Engineering Plastics Inc. (AEP; Earth City, MO) by three veterans of the plastics industry, each with 20 years experience.
"We founded AEP two years ago concentrating on an underserved niche market," says Chris Klope, president. "We have 20,000 ft2 (1858 m2) with just under 20 employees. In our first year we met our sales plan, and we've already doubled that performance in year two."
Plastics continue to make inroads in the medical industry, as well as in the semiconductor, food processing, and aerospace industries. "There are plastic materials today that are designed to withstand 1000°F. Most people are shocked when they hear that," says Klope.
A short list of AEP's products includes plastic gears and sprockets; for custom packaging equipment, star wheels, worm gears, bushings, bearings, and wear pads. AEP's customers manufacture equipment for specific industries. Some customers make machines that test agricultural products, paint samples, and food products, requiring machining of FDA-compliant materials in a controlled environment.
"We've produced instruments for the ear, nose, and throat industry, devices for which our clients charge a good price. One of the complaints from doctors is that the anodization on aluminum instruments would wear off, leaving them looking cheap. The last thing they want is shoddy-looking equipment," says Klope.
The solution involved changing to a plastic that is lighter than aluminum and just as durable. "The color is integral to the material and can't wear off." Klope explains: "You can cut threads in it, you can put a fine surface finish on it, plus, it's cost-effective. In addition, our customers can talk about the environmental benefits of replacing nasty anodized aluminum with a clean material." A bonus: the doctor incurs less fatigue using the lighter device.
AEP also machines plastic for surgical instrument handles, positioning pieces, and fixtures. These materials not only perform as well as or better than stainless, but they are lighter, and X-rays can penetrate them to help the doctor position the components during a surgical procedure. Stainless will shadow out anything that's behind it. Radiolucent plastics will allow the X-ray to penetrate it, and doctors can actually see what's beneath the fixture or the jig.
When Klope and his team started the search for just the right machine for their needs, Rich Pohrer of Zimmerman-McDonald Machinery Inc. (St. Louis), suggested they look at Bridgeport machining centers from the Hardinge Group (Elmira, NY). The XR series of VMCs and HMCs feature high spindle speeds, fourth-axis capability, and the Big-Plus dual-contact spindle system from Big Kaiser Precision Tooling Inc. (Elk Grove Village, IL) for added rigidity. AEP purchased a Bridgeport XR 760 VMC, followed eight months later with a Bridgeport XR 700 HMC.
"For the money, the Bridgeports offered the work envelopes we needed, as well as the 300 psi (2068-kPa) through-the-spindle coolant, high-speed Weiss spindles, and twin rotary pallets on the HMC. Once we got the Bridgeports in house, their rigidity, stability, reliability, and ease of use enabled us to generate critical surface finishes and hold tolerances, which are absolutely required on the plastic components that we produce," Klope says.
"We've machined parts as small as the head of a wooden match," Klope says. "At the other end of the scale, we have a CNC router that allows us to machine nested components from sheets of 5 x 10' (1.5 x 3-m) plastic. The thing you have to keep in mind about plastics is their light weight, which enables us to machine large and small components with basically the same types of equipment. The keys to our success in machining plastics are rigidity and high spindle speeds."
"Having a finished part every cycle is a big issue around here, and that's where the fourth-axis interface plays an important role," says Klope. Currently, AEP is running medical device handles that require machining on all sides. The fourth axis is used to reduce setup time, so that an operator is not turning the part continuously in a vise. Instead, he just rotates the part with the fourth axis, loads it once, and gets a finished part out in a single cycle—all sides machined without operator intervention.
AEP is also using the fourth axis on the Bridgeport XR 700 HMC to allow them to do angles, easier, faster, more precisely, as well as side holes and more, again, reducing setup and increasing part precision without having to physically move the part from fixture to fixture.
"A major concern for us is reducing our setups and cycle times, and making sure that were turning out finished parts with every cycle." Klope explains: "We're not interested in running all of one step [or setup], and then starting all over for step two [another setup], and then doing step three, and so on."
The twin pallets on the XR 700 HMC can be loaded with small or large parts for maximum efficiency. "Often we run multiple jobs on a single fixture. We'll load up two sides of the fixture with one part, and the other two sides with a different part. Then, we'll move to the free pallet, and begin loading that fixture," Klope says.
The 40-taper Weiss spindle has a grease replenishment system along with a dual-contact (Big Plus) flange fit system. "This gives us a much more rigid machine; we get almost no vibration, and vibration really shows up in machining lines when working with plastics—plus, vibration leads to horrible tool life," Klope says. "This spindle nose design removes virtually all machining lines without our having to go to balanced toolholders, and we can run at much higher speeds. We can take a 1.500" [38-mm] depth of cut with a 1" [25.4-mm] end mill with full tool engagement."
AEP uses a water-soluble coolant, and the 300-psi (2068-kPa) through-spindle coolant helps maintain size and surface finish. "If you introduce heat into a plastic, you'll cause it to grow and expand out of tolerance. It's very important to cool the tool and to keep it from building up heat. Further, tool geometry has a significant impact on cycle time and finish quality," Klope points out.
Workholding poses a challenge in machining plastics. "Workholding is based on experience," Klope says. "I know this is not very helpful, but at this stage, workholding of plastic is best guided by experience. A lot is just knowing what works and what doesn't. Many times we build our own fixtures and machine special jaws from special materials. This way we encapsulate parts without distorting their shape. The other thing we do is maintain consistent vise pressure, whether it's on a VMC or HMC. Once we find the pressure that holds the product in position without distortion, we'll set a gage, record the pressure, and consistently use that vise pressure for that particular job."
Klope notes that on some of the higher performance materials they're able to hold ±0.001" (0.03 mm) which, for plastics, is considered tight. The tolerance varies depending upon the material and the size of the component. "When we see a print for a Teflon part with an 8" [203.2-mm] diam, and the customer wants ±0.001" [0.03 mm] well, the material itself will not hold that tolerance. So we spend a lot of time educating our customers about what a material will hold, what that coefficient of linear thermal expansion is, and we show them exactly what the tolerance spread would be over an 8–10°F [4.4–5.6°C] temperature change."
An average volume lot size is about 150 to 250 pieces. "There are parts that we machine in quantities above 250, and also prototype quantities of one and two, which are critical in medicalindustry trials, for example," Klope concludes.
Probing for a Bobsled Edge
Imagine that you are an Olympic bobsled champion, who has to transfer the proven speed of hand-made blades into a new standardized material mandated by the sport's governing body.
The blade project arose following new rules introduced by the FIBT (International Federation of Bobsleigh and Tobogganing) in October 2006. The new rules aimed to remove on-going disputes over the use of various materials and treatments in blade manufacture.
An appeal in a German metalworking magazine by Team Kiriasis, the world's top two-person woman's bobsled team, brought a partnership proposal from world manufacturing leaders Renishaw, Siemens, Sescoi, and Iscar. Renishaw used its latest measurement technologies, including the Revo ultra-high-speed measuring head for CMMs, to deliver precise data capture of the legacy blade geometry that had carried Team Kiriasis to a world championship in 2005 and Olympic gold at Turin in 2006.
Kiriasis prized the competitive edge achieved by her existing blades, but these had been created using manual techniques. There were no drawings or electronic CAD data to allow them to be remanufactured using the new standard specification steel. The first step by the Renishaw-Siemens-Sescoi-Iscar partnership was to send the existing blades to Renishaw's (New Mills, Wotton under Edge, Gloucestershire, UK) research facility.
The Renishaw's Revo five-axis measuring head for CMMs was used to scan the blades, capturing the thousands of data points to enable form geometry to be defined in exact mathematical detail.
Unlike conventional touch-scanning methods, which rely on speeding up the motion of the CMM's three axes in order to scan quickly, the lowmass Revo head combines horizontal and vertical rotary axes to perform high-speed "infinite" positioning of the touch probe. A 3-D measuring device in its own right, Revo does the direction-changing measuring work to minimize CMM motion errors. Revo's low-mass, low-inertia design allows scanning at speeds of up to 500 mm/sec. and capture of 4000 data points/sec. vs. 200–300 data points for conventional scanning.
Once the blade geometry data were captured, both DXF and IGES files were created and sent electronically to Sescoi, a software specialist for tool and moldmaking. It created a CAD/CAM program for a Siemens Sinumerik 840D CNC control and ShopMill HMI fitted to a DMG CNC milling machine located at tooling manufacturer Iscar Germany.
The finishing program for the runner surfaces ran 5 MB and contained about 100,000 lines, producing surfaces almost as polished as a mirror.
Following machining, the finished blades were checked for form while still fixtured on the machine tool, using the Renishaw OMP400 touch probe with industry-leading strain-gage accuracy. A patented strain-gage sensing mechanism and advanced electronics allow lower, highly consistent contact forces with reduced pretravel, enabling submicron 3-D probe measurement and verification of the contoured surfaces.
Sandra Kiriasis was on hand to personally evaluate the machining. She received runners machined to exactly the same geometry as her championship-winning blades. Mounted to her sled, the new blades performed as well or even better than the old ones, continuing her edge over world-class competition.
Success at blade replication won Team Kiriasis both the 2006-2007 FIBT World Cup and World Championships. In fact, running the new blades, the team won the world championship by more than 2 sec, the biggest margin ever in championship history where races are usually decided by hundredths of a second.
After taking the gold medal at the FIBT championships in St. Moritz with brakeperson Romy Losch, driver and team captain Sandra Kiriasis told TV broadcasters, "The blades are the secret of my success." To show her appreciation, she displayed her bobsled in Renishaw's booth at the 2007 EMO machine tool show.
Team Kiriasis' success highlights the impact that engineering technologies can have at the highest levels of competitive speed sports, says Rainer Lotz, managing director of Renishaw GmbH, the company's German subsidiary. "We know about the small margins between success and failure at the highest levels of international sports," he says. "Renishaw is already making significant technical contribution in the world of international motorsport, such as F1 and NASCAR racing, both in engine manufacture and on-car monitoring systems. We have been delighted to add our measurement expertise to the Team Kiriasis blade projects, and look forward to contributing to Sandra's continuing success."
Ingenuity Goes a Long Way in Moldmaking
SHM (Gardnerville, NV) is a moldmaking business for machining compression, transfer, and injection molds for the rubber industry. Owner Jack Latragna thinks molds all of the time—the one he's currently working on, the one just finished, and the one yet to come.
Latragna says that much of the product he, and many other mold makers, used to routinely produce has been swept away to China, Taiwan, Vietnam, and India. "As things have changed in our industry, we don't see many high-volume production jobs any more. It used to be that you could go to a hardware store and buy faucet washers that came out of SHM molds. Years ago, Brasscraft, Crane, and Fluid Master all manufactured plumbing parts. We made molds for all that stuff. Not any more."
These days Latragna is especially focused on the molds yet to come. SHM is a 2500 ft2 (232-m2) shop where just about any mold-related process that one can think of goes on: turning, milling, sawing, grinding, and inspection.
"I don't build molds from mold drawings. I build from part drawings," says Latragna. The difference is that SHM does all the engineering of the mold, so when he ships a mold it's his design, not someone else's. "This gives us a very real sense of the mold as our product, so if the mold doesn't work correctly, if there's an issue, it's our issue."
SHM generally doesn't work on complex molds. Rubber molds are different from plastic molds in that they can be undercut and don't need slides, so the molds tend to be a little simpler. Plastic molds usually require a mold designer to engineer the mold prior to the product/mold being sent to a machine shop. They tend to be more complicated.
SHM's molds are used to produce synthetic elastomer products, such as silicone, fluorocarbon, gaskets, seals, bellows, and diaphragms. The company isn't limited to producing small molds. "I just finished a mold that weighed 500 lb [227 kg]," Latragna says. "The part was 12 x 8 x 6" [304 x 203 x 152 mm] and looked like a slightly shrunken irrigation box, the kind that goes into the lawn where the sprinkler valves hook up. But it was a military product, and I don't know what it ended up being used for. The print title block called it a boot. If the designers haven't thought of a good name for a rubber part, they call it a boot."
Until recently molds were being machined on a couple of older VMCs. One was an 18-year-old unit, which Latragna decided to sell because it had seen its best days. Besides, he needed more capacity, tighter accuracy and repeatability, and greater spindle power.
While walking around SME's annual Westec exposition in Los Angeles, Latragna came across the Feeler VMCs at the GBI Cincinnati Inc. (Cincinnati) booth. "What impressed me about the Feeler was the ability of the VM32SA on the floor to take full horse-power cuts in die steel with ceramic inserts. The VMC wasn't vibrating or shaking, or doing anything weird. I thought, 'if this machine can do this, it can certainly handle my work.'"
An important feature that he was looking for on the Feeler was a mechanically counterbalanced head. Located near Lake Tahoe in Nevada, SHM is likely to be hit with deafening thunderstorms and power anomalies that can briefly interrupt the electrical supply.
The VMC he sold had a counterbalanced head, and Latragna's other VMC didn't. The VMC without a counterbalanced head would scrap a part every time there was a power interruption. So he was searching for a machine with a counterbalanced head, and the Feeler fit the bill. "If there's a power outage, the VM32SA just stops cutting and the head doesn't drop. No scrapped part, no broken tool."
When using a large drill, speeds on the Feeler are fairly low. Typically, the way servos work, without the use of a gearbox, as the speed goes down the torque drops off. Feeler has improved the torque on drilling without a gearbox, and adopted a combination of linear guide ways on the X and Y axes and box ways on the Z axis. "I think the Z-axis boxway is a good compromise for steel-cutting machines. Boxways dampen vibration, whereas linear guides just don't," says Latragna.
Rigidity may not be the real issue regarding vibration-dampening characteristics. "When you get chatter, it could be any number of things. Sometimes it might be similar to positive feedback in an oscillator circuit. It tends to run away, and the chattering gets worse and worse. The oil between the box way and the sliding members dampens that out entirely. This is well known in the industry. It isn't a secret," Latragna explains.
The VM32SA is designed basically for large parts, but it will run small parts, too. The mold sizes range up to 24 x 24" (610 x 610 mm). The Feeler has a 30 x 20" (762 x 508-mm) travel, so Latragna can reach almost the entire plate.
"Often times I'll run a mold base, which is like a die set," Latragna says. "You have match plates, pins, bushings, and all of that. I'll run the bottom on the one machine and the top on the other, and they'll line up. I have 0.001" [0.03-mm) clearance between pin and bushing, and we're holding well within 0.0002" [0.005 mm] on hole location."
Changeover times on the Feeler are measured in minutes rather than in hours. Latragna doesn't have any complicated fixturing, in general, and usually the parts are in a vise or a set of parallels, bolted directly to the table. He uses 100% off-line programming, and often the program is ready to go prior to the part being put into the machine. The changeover is very quick, and while the machine's cutting the part, he'll be doing the next program.
"I design tools for ease of manufacture. When I design the tool, I say, primarily this tool has to function and make this specific part. Secondarily, but almost equally important, we have to be able to build that tool in-house with the machinery that we've got and do so accurately and quickly. In most cases, a mold base will come off the Feeler finished," says Latragna.
ATC capacity allows pre-loading from tools 10 through 24. "We've got 22, 23, 24 set up for 0.5" [12.7-mm] dowel pins and bushings. Tools 10–24 are the same cutters all the time. Those are basically for mold base operations: drilling the holes for the leader pins and bushings, boring, and milling."
Materials vary depending on the job. The lower-cost tools are made of an SAE 1045 material that's sulfurized. The requirements of volume production for the medical field demand stainless. He uses prehardened stainless in the 400 Class. Also used are prehardened chrome moly steels in the 4130, 4140 Class, including P20. And then for Class A tooling, which is heat-treated and quenched, it's A2, but more often S7.
"I don't even know how to machine aluminum anymore; I've done mold work for so long. We had a job not long ago that was to be made of aluminum, and I was dismayed by chips flying all over the place. You sweep the shop and get done, and then you have to sweep it again."
"Feeler put a really great feature on the spindle head, in addition to the four coolant nozzles which are mounted to the spindle nose," Latragna says. "There are four Loc-Line hoses with little valves that are on the side of the spindle, opposite the toolchanger, and the very front one is for a compressed air blast. You call up an M7 code, and you have a compressed air flow from that nozzle. Well, that comes in really handy for certain things.there are some materials and cutter combinations that don't like oil and don't like coolant. They prefer to run dry. It's a tremendous feature, and I was pleasantly surprised to see that. I've not seen this feature on any other mills except Feeler," Latragna concludes.
Chips Can't Fall Where They May
What do heavy-duty truck parts and medical packaging have in common? Not much. Except in the case of the Reading, PA plant run by Brentwood Industries, where tight space requires that these two manufacturing operations happen side-by-side.
A set of rugged curtain partitions from Goff's Enterprises (Pewaukee, WI) blocks off the debris-throwing router used to make the truck parts, to ensure that they don't land near the more environmentally sensitive area.
Brentwood Industries packs a lot of activity into its 205,000 ft2 (19,045 m2) specialty products manufacturing plant. Go to one part of the plant and they are at work on heavy-duty parts for trucks and passenger trains; stop over in another area, and employees are turning out packaging for pharmaceuticals, food, and medical industries. Wheelbarrows are made in a third location.
"We have to play host to customers and potential customers walking through our plant from time to time, and we have to present a tidy operation," explains Mike McHenry, the Brentwood compliance officer.
"We have several Motion Master CNC routers trimming Mack truck parts from plastic sheet, throwing debris where we don't want our medical and pharmaceutical customers to see. Besides, we don't want to have people peppered by chips as they walk past the area."
This is a rather new facility. In its old specialty products plant, Brentwood had set up a makeshift barricade made of 4 x 8' (1.2 x 2.4-m) sheets of plywood slapped up on framing lumber around the routers.
"The barrier did the job of preventing debris from shooting out of the area," says McHenry, "but for this new facility, plywood sheeting wasn't the image we wanted to put across."
Plant management took to the internet for ideas, and came up with curtain walls from Goff's Enterprises (Pewaukee, WI) as the solution. These partitions are designed to withstand tough industrial environments, create a professional looking space for the routers, and, of course, confine the scrap flying off the machines.
The upper and lower opaque PVC sections of the curtain walls are 14 oz per square yard (473 g/m2). The fabric is reinforced with polyester 9 x 9 x 1300 denier weft-inserted scrim, laminated into the fabric to give the curtain wall a high tear and tensile strength without sacrificing flexibility. These reinforced vinyl curtain materials are manufactured to be certified flame retardant by the California State Fire Marshall's office, as well as passing the NFPA-701 test for fire resistance.
The polyvinyl curtain wall material is water repellent and mildew and rot resistant, as well as being resistant to most chemicals. It can withstand a maximum temperature of 180°F [82°C] and contains a cold crack resistance to -4°F [-20°C].
The upper and lower reinforced vinyl curtain wall sections are double lock stitched to a 20 mil double-polished, clear, 52" (1.3 m) high windowed section that withstands temperatures of between -20°F [-29°C] and +150°F [65.6°C].
Brentwood Industries' heavy-parts business has been brisk, and the department is using the routers throughout the workday. Without the curtain walls, the force of the router could spew chips that would be tracked into other areas such as the office and cafeteria. Air nozzles could carry it further. Aside from the aesthetics of the situation, it's crucial to minimize contaminants in manufacturing for the medical, pharmaceutical, and food packaging industries.
The medical/pharmaceutical manufacturing area is about 60' (18.2 m) away from the routers and somewhat separated. In the old facility, the medical/pharmaceutical/food packaging department was well-separated from the router operation. "Blocking the debris is important," says McHenry, "but we are concerned that we don't want it to look like we are just slapping things together. Both the opaque and the window sections on the curtains can resist the spray of chips off the router and still retain their good looks.
Along with restricting the spread of the chips, Goff's curtain walls serve as a sound barrier. "We had an issue with acoustics getting out of control. This building is like a big drum," says McHenry The curtain walls have turned the sound down 5–6 dBA. The curtains form a 10' (3-m) high barrier beneath the 36' (11-m) clear height ceiling, making space for the router, which measures 10' x 25' (3 x 7.6 m).
This article was first published in the April 2008 edition of Manufacturing Engineering magazine.