Keeping it clean

Figure 1. Added chamber on head end of conventional cylinder contains vacuum to remove any possible leakage past rod seal.

Figure 2. Rodless cylinder with slotted tube design has multiple sliding surfaces, and may require vacuum in locations to avoid introducing particles into a clean room.

A clean room is a room in which the particulate in the atmosphere is controlled. Usually, the air conditioning systems in these rooms contain high-efficiency particulate air (HEPA) filters and direct air flow from the ceiling to the floor in a laminar mode. These rooms may house laboratories, manufacturing processes, sensitive equipment, or other events which need a particulate-controlled environment.

Clean rooms are rated by Federal Standard FS 209D, "Federal Standard Clean Room and Workstation Requirements, Controlled Environment," dated June 15, 1988, and their environments classified as indicated in the Table. These ratings range from those with very large particles to those as small as 0.1-µm diameter. For comparison, 5 µm is the approximate diameter of the cross section of a grain of talcum powder.

To certify a clean room for a classification, samples of air are continouosly monitored by a particle counter and the number and size of identified particles evaluated. A room with or without equipment can be certified as a clean room, but a piece of equipment or machinery cannot be certified. That equipment is evaluated in the clean room and a statement made that under a given set of conditions with the machinery operating, the classification of the room was maintained at an XX classification level.

Automation within clean rooms
Products generally manufactured in clean rooms include space vehicles, pharmaceuticals, and electronics, to name a few. Depending on the process or operation that takes place within a clean room, cleanliness certification or evaluation is only one of the important parameters. Particle composition also is important in many processes.

For example, in the electronic and semiconductor industries, any particles that exist in the clean room must be non-conductive and inert. This is one reason why a lot of ultra high molecular weight (UHMW) plastic bearing materials are used in clean room machinery for that industry.

These industries also are concerned about ionic contamination, as well as particulate contamination. The release of metallic ions during many stages of semiconductor production can cause a short circuit on the semiconductor because of the submicrometre conductor widths now used in the newer devices and the conductivity of copper.

The metal industry has a different set of important parameters and, consequently, the solutions for problems in medical equipment clean rooms are different.

As clean rooms began being used for product assembly, the nature of the equipment operating in them became important. This machinery performs all the normal assembly functions but has an additional list of design parameters because of its clean room use. They include:

Machine size — Clean room space is much more expensive — by factors ranging from 10 to 500 times — than regular assembly space. Thus, the larger the machine, the more costly the room and installation.

Speed of movement — Air in a clean room flows from top to bottom in a laminar flow mode. Machinery element movement may disturb this laminar flow and create turbulent flows. This turbulence can disturb particulate that has collected on surfaces and can cause particulate problems. This limitation is counter to the machine-size problem in that after setting production goals, a greater number of machines are required if they must move more slowly to avoid creating turbulence problems.

Product placement — The closer the product is to the top of the clean room, the cleaner the air it will encounter. The product should be located at the top of the room with the machinery and any particle-generating components located underneath.

Typical operation
Uses of pneumatic systems in clean rooms are not that much different from those in general industrial pneumatic applications. Typical functions include operating doors, lifting or positioning products, material transfer, shuttling, and operating valves. Invariably, when building automated machinery for clean room use, pneumatic systems often are the low-cost choice to drive these machines.

To keep the often lubricated compressed air of these systems from co-mingling in the clean room environment, the pneumatic system is isolated from the room. In addition to the pressure piping from the compressor to the valving and the actuators, all air that normally would exhaust at the valve is piped into a return manifold which exhausts outside the room.

The greatest potential for a piston-rod type cylinder to cause environmental contamination occurs in the rod-seal area. Because of the sliding contact between the rod and the seal, there is a potential for contaminant particles to be propelled into the atmosphere by the ever-present low-level rod-seal leakage. Add this to the lubricant typically used to prelubricate pneumatic cylinders and the normal materials found in the rodbushing area of the cylinders, such as impregnated bronze, SAE 660 bronze, or other copper-bearing alloys, and standard pneumatic cylinders present a real contamination issue in clean rooms.

Special pneumatic cylinders are available that can operate within a Class 10 clean room. To ensure that contaminated air leakage past the seal does not get into the clean room, an additional chamber, Figure 1, is added to the rod-gland area of the cylinder. A vacuum applied to this chamber ensures that any leakage past the rod seal is pulled away by the negative pressure in the chamber. The vacuum is then vented into a vacuum manifold that exhausts outside the room.

To remove any particulate build-up from the seals and bearing, an orifice in the wall of the additional chamber controls air velocity of the vacuum evacuation system so that any particulate from the seal or bearing also is removed from the cylinder. All bearings and seals are made of inert material to eliminate potential problems in case any particulate gets into the clean room.

Rodless cylinders have advantages that make them the component of choice in many applications, such as:

• shorter than normal cylinders for a given stroke,
• integral load-supporting and guiding system, and
• low cost.

Rodless cylinders offer these same advantages when used in clean rooms. Again, particular attention should be given to the sealing method used on the cylinder.

As shown in Figure 2, a slotted tube design has several different sliding surfaces, such as piston seals and the full-length seal used to seal the slot of the tube. Additionally, there are wipers and guide bars used in this design, which may also generate particles. It may, or may not, be necessary to introduce vacuum at certain locations to meet a Class 10 environment — this depends on both the design and the materials used within the cylinder.

Thanks to Randy Deforge of Norgren, Inc., for his assistance with this article. Contact him at [email protected]