Cleanroom construction begins with the floors, walls, and ceilings. This article focuses on the necessary specifications for these cleanroom components to ensure maximal cleanliness and minimal contamination.
In building a cleanroom, the starting point is usually neglected. Looking at the floors, walls, and ceilings, a “target” specification isas follows:
1. No joints or cracks shall be exposed
2. Porosity shall be as close to 0.0% as possible
3. All areas shall be easily cleanable
4. Dust due to wear shallbe minimal as defined in terms of wear
Looking at item 1, the material to meet this specification would possibly be a vinyl plastic that is structurally weldable, eliminating all cracks and joints. Figure 1 shows treatment of an outside corner. The transmission from floors to walls is a molded outside corner, the back of which is relieved as shown in Figure 2. The radius used was 1.25 inches to fit a standard vinyl cove-stick. This eliminates welding the two sides of the corner at the nose,resulting in a non-structural weld. It is necessary to cut away 60% of the material thickness to prevent puckering. The material is heated and then stretched. Most plastics cannot be successfully shrunk. The material shown in Figure 2was conductive.
Figure 1. Outside corner invisible weld
Figure 2. Molded outside corner relief
The wall material is also relieved at the corner by cutting away 60% of the thickness to prevent the material from puckering.
The wall, floor, and ceiling materials are joined by grooving the joints to a depth of 60% of the material and then welding with a welding rod of the same material. Some companies have produced triangle welding rods of matching materials, producing an almost invisible weld.
Figure 3 shows an inside corner that is either heated and formed in place or molded and welded in place. The radius used was 1 inch, which is the standard cove-stick.
Figure 3. Inside clean corner
Referring back to the initial list, item 2, cleanroom materials would like a specification of 0.0% porosity. There is no such material. Even the most solid materials have some degree of porosity, allowing the material to breathe with environmental changes.
Looking at item 3, a sharp inside corner cannot be clean. A 1.25-inch radius in transition from floors to walls and walls to ceilings was adopted by the cleaning discipline, which is a very good compromise.
Welding floors is a very critical operation that should be done by a certified welder with an automatic welding tool. A technician using a hand gun must change position every 2.5 feet, which causes a heat problem at that point of the change in position. A faulty weld cannot be detected for 6-12 months and appears as a black line. An automatic welder is three times faster than a hand welder gun and does a very uniform job.
Materials roughene
d or grooved are not easily cleanable. Porosity is also a very large factor in cleaning.
Most cleaning personnel like to use solvents in their water (acetone-based), which can, over time, damage the ability of the surface of the material to resist absorption. For personnel safety, a high coefficient of friction is desirable to prevent slipping. The semiconductor industry originally adopted Mipolam products because of their resistance to solvent-based cleaners. Later, there was concern about dust, static, and contamination.
In terms of item 4, all materials have a “wear” factor. This is primarily a concern for floors. Particles are released into the environment from both the floor covering and the item that rubs the covering. The usual method to establish floor wearability is the standard abrasion test.
In conclusion, any controlled environment should be looked at carefully regarding contamination from floors, walls, and ceilings.
Lee Baldwin has worked in the applications of sanitary clean-rooms for over 50 years as a designer and engineer.He was awarded a Dynamite Nobel private gold metal stamp for his work in applications to sanitary rooms.He can be reachedat 818-366-1596;[email protected].
The author would like to thank Dave McLaury, Edward Baldwin, and Jerry Graff for contributing to the information described here.
