If you're eating lunch, you may want to skip down a few paragraphs.
Still reading? Very well, you've been warned: The air in your assembly plant is filthy. Drilling, grinding, welding, staking, soldering, bonding, finishing and other manufacturing and assembly processes spew all sorts of particulates into the air. This is in addition to the particulates generated by forklifts, conveyor belts, vibratory feeders, contact bearings, assorted pneumatic devices and the office carpet.
Not that air enters your plant in a virgin state. Far from it. It's loaded with pollutants, soot, pollen, mold, bacteria, viruses and dust. Worse, your colleagues are lending their own personal touch to the miasma in the form of skin flakes, lint, cosmetics and respiratory emissions. According to the Environmental Protection Agency, a cubic foot of air in a typical home contains 700,000 respirable particles.
As gross as all this sounds, the stuff floating around in the air isn't much of a problem for most assembly plants. If, however, you're assembling medical devices, airborne contaminants are a serious concern. Indeed, for some products, they are literally a matter of life and death. That's why syringes, cannulas, test cartridges, inhalers, catheter assemblies and other medical devices are assembled in clean rooms.
A clean room is an enclosed environment in which the concentration of airborne particles is strictly controlled. The rooms are classified according to the number and size of particles that are allowed to float in the air inside them. In the most basic facility, a Class 100,000 clean room, a cubic foot of air cannot contain more than 100,000 particles 0.5 micron in diameter. In a Class 1 clean room, the cleanest of the clean, a cubic foot of air cannot contain more than one particle 0.5 micron in diameter or 35 particles 0.1 micron in diameter.
Most medical devices are assembled in Class 10,000 clean rooms, but some are produced in Class 100 facilities. For comparison, Class 10,000 clean rooms are also used for precision machine assembly, high-speed video duplication and glass lamination. Compact discs and various optical products are manufactured in Class 1,000 facilities, while hard drives and microchips are assembled in Class 10 and Class 1 facilities.
How Clean Rooms Get Clean
A key aspect of clean rooms is laminar airflow. In an ordinary room, airflow is turbulent. Dust floats randomly through the room and collects freely on any surface. With laminar airflow, filtered air passes at a constant rate from one side of the room to the other with minimal turbulence. This gets all the dust moving in the same direction-away from assemblies and out of the clean room. Laminar airflow is usually vertical, flowing down from the ceiling to the floor, but it can also be horizontal.
Outside air first enters a clean room through a climate-control system, which adjusts the air's temperature, humidity and pressure. The air then goes to a fan system mounted in a deck above the room. After basic filters remove gross particulates, the air passes through high-efficiency particle arrestors, or HEPA filters, mounted in the ceiling. These filters remove particulates less than 0.5 micron in diameter.
After flowing through the room, the air enters vents in the floor or along the bottoms of the walls. Ducts in the walls carry the air back to the fan system, where it gets recirculated.
To save money and allow for expansion, clean rooms are usually constructed of modular components. These include the air-conditioning system, top deck, filters, blowers, air-return walls and raised floor.
Assembly equipment for clean rooms should be selected carefully. In some cases, equipment can be used in clean rooms with little or no modifications. In others, it requires special materials and adaptations. The extent of the modifications depends on the class of the clean room.
Overall, equipment should be streamlined, says Kevin D. Gingerich, marketing services manager with the Linear Motion and Assembly Technologies Div. of Bosch Rexroth Corp. (Buchanan, MI). Because floor space inside clean rooms is extremely valuable, assembly lines should occupy as little space as possible. Laminar airflow is critical for maintaining a clean environment, so workstations and machine bases should provide adequate clearance with the floor, walls and ceiling. To prevent dust-trapping turbulence, horizontal surfaces can be perforated. Panels, arms and other elements should be mounted flush with their frames to avoid trapping particles. Structural framing should have rounded edges, and T-slots should be covered.
Plastic components should be made from emission-free materials, such as polypropylene, polyamide and thermoplastic elastomer. Outgassing plastics, such as polyvinyl chloride, should be avoided.
Workstations can be used in clean rooms with a few simple alterations, says Ray Gottsleben, vice president of sales and marketing at Arlink (Burlington, ON, Canada). For example, workstation materials should resist corrosion from cleaning agents. Operators should be able to relocate the workstation frequently without generating particulates, and open areas that might collect dust should be covered. Accessories and supply bins should be located so that workers aren't reaching over the workpiece.
The work surface is another area of concern, explains Gottsleben. On most workstations, the work surface is plywood with a plastic laminate covering the top and four edges. To cut costs, the bottom surface is typically covered by a paper backing sheet. In a clean room, the paper backing is a no-no, because it wears easily and creates dust. Thus, the work surface for a clean room application has laminates on the both the top and the bottom. In a higher class of clean room, the work surface might be an even more durable material, such as stainless steel.
Work chairs also need special modifications. For example, clean room chairs from Bevco Ergonomic Seating (Waukesha, WI) are cushioned with resilient, high-density foam and upholstered with durable, seamless vinyl. The vinyl is attached with stainless steel staples, and a filtration system catches any dust that might be produced when workers sit down. The base is made from five-star aluminum or heavy-gauge, chrome-plated tubular steel.
Gottsleben advises engineers to use only products that have been certified by an independent laboratory for use inside clean rooms. "A lot of companies say their products can be used in clean rooms, but there aren't many that have actually had their products tested," he says.
Of course, even the cleanest equipment in the world is useless if operators don't follow good clean room procedures. "I have seen clean room practices that are frankly deplorable," says Gottsleben. "Companies spend all this money on beautiful clean rooms and certified clean room products, but then they run the place poorly. It's like controlling electrostatic discharge. There are a lot of standards out there, but it's hard to find two companies that practice them in the same way."
Automatic dispensing equipment is one assembly technology that requires little or no modification for use in clean rooms, says David Titone, life sciences business development manager at EFD Inc. (East Providence, RI). Valves, barrels and tips need not be sterile to apply adhesive to a catheter or to pot the leads of a pacemaker.
However, dispensing gear for pharmaceuticals, biologics and reagents is another story. These materials should be dispensed with an aseptic valve, says Titone. Constructed of 315 stainless steel with Teflon diaphragms, aseptic valves accommodate frequent cleaning and flushing. There are no "dead spaces," and fluid passages are electrically polished for smooth flow.
If designing a high-speed, multistation automated assembly system for an ordinary environment can be challenging, designing a system for a clean room is even more so.
"When designing a system for a clean room, the footprint becomes a high-priority consideration, but one must be careful not to sacrifice reliability or performance in the process," says William E. Bodine, president of Bodine Assembly and Test Systems (Bridgeport, CT). "Often, portions of the assembly system can be split off and passed through the clean environment for critical final assembly processing. This helps minimize the square footage of clean room space and its related expense."
The requirement for a minimal footprint can sometimes create problems with machine access. And, because the ceiling of a clean room is lower than the rest of the plant, the height of the assembly system can be an issue, adds Paul Beduze, business development manager at Mikron Corp. (Aurora, CO).
Noise is another concern. "It's generally more difficult to keep the noise level within 80 decibels because of the limited volume of the room," says Beduze.
Assembly systems don't have to be built in clean rooms, but they are thoroughly cleaned before and after delivery. Systems are first "blown down" with compressed air to clear away dust or burrs leftover from machining and assembly. Then, all surfaces are cleaned with alcohol to remove oil and grease. Machine components need not be sterilized, though the product itself is often sterilized in a separate process after packaging.
"In some cases, it is advantageous to perform final system assembly and factory acceptance tests in a clean room at the machine builder's location, so that the system can be evaluated under normal operating conditions," says Steven D. Hoenig, strategic marketing manager for medical devices and diagnostics with ATS Automation Tooling Systems (Cambridge, ON, Canada).
Machine components are built from materials that are easy to clean and won't generate particulates, says Hoenig. Hard, anodized aluminum and 304 stainless steel are common. Other metal parts are usually plated rather than painted, to avoid putting paint flecks into the air. Wire ways and pneumatic lines run below the system, so tabletops can be cleaned easily.
Depending on the application and the clean room, automated assembly systems may require special actuators and sealed, permanently lubricated bearings. If sealed lubricating systems cannot be employed, food-grade lubricants can be used sparingly.
"You wouldn't be able to stick just any actuator into a Class 100 clean room," says Danielle Collins, product manager for linear modules at Bosch Rexroth.
In general, ballscrew-driven linear actuators are preferred over belt-driven actuators for clean room applications. "A belt will generate more particles than a ballscrew," says Collins. "But, we do have one customer in the semiconductor industry that uses a belt-driven module in a Class 1 clean room application."
Bosch's CKK and PSK ballscrew-driven linear modules are certified for use in Class 10 clean rooms. A sealing strip keeps particulates from entering or escaping the module. In addition, each module has a hole through which a vacuum can be applied to pull particulates away from the assembly. To further reduce particulate generation, the actuators can be lubricated with DuPont Krytox, a special low-outgassing grease.
Collins warns engineers that an actuator's cleanliness rating is usually tied to a specific speed, and exceeding that speed will make the device less clean. "The faster you run a module, the more particulates you're likely to generate," she explains. "As long as you stay at or below the rated speed, we can guarantee a certain level of cleanliness."
Some assembly processes require special attention in clean room operations. "Ultrasonic welding could produce particulates, so additional exhausting and flushing with ionized air may be necessary," says Howard Speiden, customer dialog section head at Sortimat Technology (Schaumburg, IL). "Adhesives and solvents also may require additional venting outside the clean room."
"Metal forming, welding and any processes prone to producing particulate matter need to be isolated as separate operations outside of the clean environment, or steps need to be taken to control and evacuate the contaminants," adds Bodine.
One such step would be to isolate or shield certain parts of the machine. "Exhaust air from pneumatic devices is manifolded and expelled from the clean room," says Bodine. "In some cases, we completely shroud the assembly machine in a Lexan enclosure, which is pressurized with filtered air, creating a clean room within a clean room."
In addition, stations should be designed to minimize the amount of moving parts above product assembly areas, says Speiden. Holes and slots that could collect dust or stray parts should be covered, and traps can be built into feeder bowls to collect particulates generated by plastic parts vibrating against each other.
How Would You Handle It?
Many parts for medical devices are challenging to feed and handle in high-speed automated assembly systems. We asked William E. Bodine of Bodine Assembly and Test Systems and Steven D. Hoenig of ATS Automation Tooling Systems how they'd handle various difficult parts.
Fragile parts, such as needles: "Small, micropolished grippers will minimize surface damage," says Hoenig. "For needles, you can use a hopper with rotating singulation device. It's like a coin changer. It keeps the product contained and safe, yet you can pull them out one at a time. For glass parts, you might use vacuum grippers. Fixtures and pallets should be polished to prevent marring plastic parts."
Cannulas should be fed radially oriented, says Bodine. Machine vision should be used before and after handling to verify that the angle and quality of the tips have not been damaged.
Odd-shaped parts: "Standard syringes are made up of cylindrical, mostly symmetrical components, which makes them ideal for continuous-motion assembly systems," says Bodine. "The new safety syringes, which are designed to prevent accidental sticking, have more components than standard syringes, and they're not symmetrical. For that reason, indexing assembly equipment is much more practical and reliable."
Soft, sticky elastomeric parts, such as the stopper inside a syringe: "These parts can be fed in trays, or they can be palletized," says Hoenig. "You can also use a rotating drum feeder. However, it's important to keep the level of components as low as possible, to reduce friction."
Floppy parts, such as tubing: "Tubing is normally contained on reels, and push-pull systems are highly effective for feeding and dereeling," says Hoenig. "Concentric, micropolished grippers can handle tubing without stretching, scarring or crushing it. At the same time, you want to handle tubing and floppy parts for as long as possible, until it's fully assembled and bonded."
Compressible parts, such as filter media: "These parts are typically fed on a web, die-cut on the machine, and then handled with a vacuum gripper," says Hoenig. "You want to minimize handling, to prevent abrading the material."
Court to Hear Protective Clothing Dispute
The U.S. Supreme Court agreed in February to consider whether factories must pay workers for the time they need to change into protective clothing and walk to their workstations.
The justices will review a pair of lower court rulings examining workers' rights under federal labor law. One ruling ordered IBP Inc. (Pasco, WA) to pay $3.1 million to 815 workers for their changing time. The other ruling said 44 employees of Barber Foods (Portland, ME) were not entitled to payment.
Although both cases involve meat processing plants, the Supreme Court's ruling could have ramifications for many occupations that require protective gear, including assemblers in clean rooms.
At the IBP facility, workers are required to gather their protective gear, don it in the locker room, and then prepare their work-related tools before entering the shop floor. The gear typically consists of a sanitary outer garment, boots, hardhat, goggles and gloves. However, workers are not considered clocked in until they show up, fully equipped, at the processing line. They also aren't paid for the time spent changing out of the heavy gear for a 30-minute lunch break and at the end of the workday.
Under the Fair Labor Standards Act, employers aren't required to pay workers for the time spent "changing clothes," but the 9th U.S. Circuit Court of Appeals said that doesn't matter. Donning protective gear in a hazardous profession is different from changing clothes because the safety protection is "integral and indispensable" to the job.
However, in ruling against employees in Maine, the 1st U.S. Circuit Court of Appeals said that time spent changing gear and walking to workstations shouldn't be compensated, in part because donning the equipment at Barber Foods only takes a few moments.