In this two-part article, we cover the products, materials, and features that designers and engineers should specify when choosing motion components for cleanroom environments.
In part 1, we looked at how to reduce particle generation due to friction between moving components. In this second part, we’ll look at another source of contamination — outgassing — and how to minimize it.
Objective #2: Minimize outgassing
Silicon wafer processing is extremely sensitive to contamination, including both particulates and outgassing.
Image credit: WaferPro
In cleanroom applications that involve the manufacture of LEDs, optics, or glass, or the processing of silicon wafers, a second type of contamination can damage the process or the product: outgassing.
Outgassing is the desorption of vapors or gasses, either from within a material or from the surface of a material. ISO standard 14644-8 addresses outgassing in cleanroom environments and specifically refers to airborne molecular contamination (AMC) as:
The presence in the atmosphere of a cleanroom or controlled environment of molecular (chemical, non-particulate) substances in the gaseous or vapor state that may have a deleterious effect on the product, process, or equipment in the cleanroom or controlled environment.
These types of airborne molecular contaminants take the form of molecular vapor. They’re smaller than particles and easily pass through HEPA and ULPA filters, making them particularly difficult to control once released.
Minimize outgassing through material selection
The first criteria when choosing motion components for cleanroom environments where outgassing is a concern is to ensure the material has a smooth surface, which makes it more difficult for airborne molecular contaminants to adhere to the surface. Stainless steel and anodized aluminum are the preferred materials, although some epoxy paints are suitable for environments where low-outgassing is required.
Fortunately, numerous motion control components, such as linear guides, motors, and gearboxes, can be made of stainless steel. When possible, plastic end caps on linear guides and screws should also be replaced with stainless steel versions to further minimize the use of plastics. And standard steel fastening hardware, such as screws or pins, should be replaced with stainless steel versions.
Although epoxy paint is sometimes suitable for motor and gearbox housings, stainless steel versions are better options.
Image credit: Wittenstein
When the required material has a high outgassing tendency, other solutions exist. For example, epoxy paint is suitable in some cleanroom environments and can be applied to the housings of linear actuators, motors, and gearboxes. And when steel is required — to maintain hardness, durability or load-carrying capacity — nickel plating can reduce the tendency of steel to outgas. (Note: Some nickel-plating formulas include a PTFE, or Teflon®, top coat. The use of Teflon is generally advised against, so be sure to choose a nickel plating formula that does not include Teflon.)
- Good: Nickel-plated steel, epoxy coated aluminum
- Better: Stainless steel or anodized aluminum; minimal use of plastics
- Look for: Smooth surfaces (no textured paints)
Reduce outgassing from lubricants
A wide variety of cleanroom-rated lubricants are suitable for bearings, motors, gearboxes, and other motion components.
Image credit: NSK
When it comes to outgassing, lubricants are one of the major offenders, readily spewing molecules of water and oil vapor into the atmosphere. Unfortunately, in the majority of motion systems, the use of components that require lubrication simply can’t be avoided. When lubrication is required, ensure the lubricant is suitable for cleanroom environments. Cleanroom compatible formulas that minimize outgassing while protecting bearing surfaces from rust and friction are widely available.
- Good: Cleanroom-rated lubricants
- Look for: Opportunities to minimize the need for lubrication through component selection
Feature image credit: Lin Engineering, Inc.
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