DESIGN FOR CLEANING
Introduction
In the present context, cleaning means the removal of contaminating or unwanted liquid,
solid, or semisolid matter from the surfaces of a work-piece. During manufacturing process,
cleaning of work-piece is recommended to keep the component in proper condition for
subsequent events like machining, painting, plating, or assembly. In fact, cleaning is
performed prior to storage to remove corrosives.
Work-pieces are generally cleaned one or more times either by chemical or mechanical
cleaning process.
The type of cleaning need to be carried out depends on the following factors:

The soil to be removed

The degree of cleanliness required

The cost of cleaning
In addition to these, part design is also another factor might play a role (might not be
significant) in selecting the proper cleaning agent.
Soils to be removed
In industries, the following basic types of soil are generally removed

Rust and scale (formed due to surface oxidation of hot worked mill products like

forgings, castings, and weld fillets)

Chips and metal-cutting fluids (these are residues from machining operations)

Oil and grease (un-pigmented)

Pigmented drawing compounds (mostly used in cold-finishing steel bars, white lead,
molybdenum disulfide, graphite, and soaps are the most sticky pigments)

Polishing and buffing compounds

Miscellaneous soils such as lapping residue or residue from magnetic-particle
inspection
In-process cleaning operations
Industrial cleaning processes are recommended under the listed situation.
1. Prior to various production operations like machining, forming, assembly etc. or prior
to gauging to remove grease or oil that might interfere with handling.
2. Before carrying out any surface treatment operation like phosphating, sufficiently
clean work-piece is required for the process. Various factors which affect the
phosphate coating are contaminants like drawing compounds, rust preventives, cutting
oils, and coolants.
3. Prior to painting operation, cleaning is important as paint films will not adhere
permanently if the work piece surface contains oil, dust, grease, rust, scale, water, or
salts.
4. Prior to bonding cleaning is important as foreign matter present on the work-piece and
will prevent molecular contact between the adhesive and the substrate materials
thereby reducing the strength of the joint.
5. Prior to brazing cleaning is required. Surfaces must be free of oil, grease, dirt, and
oxides. Depending on the metal to be joined, degree of cleanliness required is
decided.
6. Prior to electroplating, a higher level of cleanliness in comparison to other
applications is required. In such case, multistage cleaning process is carried out. For
example, solvent, alkaline, electrolytic, and acid cleaning comprise a typical four-step
sequence.
Cleaning processes and their applications
Mechanical processes
Brushing: Brushing is used to remove solid material such as rust, loose paint and caked dirt.
This process involves use of power driven wire or fiber brushes. In industrial products, a
wide variety of abrasive action is required and this is achieved by using different brush
materials, bristle thicknesses and lengths. Abrasive action can be mild, moderate and strong
depending on the type of bristles.
Abrasive blasting: This is a process to remove rust, scale, and other corrosion products, dry
surface dirt, and paint. Working principle of this process involves bombardment of various
abrasive particles namely, aluminum oxide, sand, silicon carbide, steel shot (hard material)
rice hulls, corncobs, nut shell, plastic beads (soft materials) etc., to the work-piece surface at
a high velocity. However, removal of grease in this process is difficult.
Steam-jet cleaning: It is a method of removal of oil, grease, and dirt from the
object/equipment which is too large or bulky (difficult to immerse in a cleaning tank). Flame
jets are preferred for the removal of scale or old paint from large iron or steel components.
Tumbling: This method is used for removing rust and scale and is generally considered to be
a low-cost method. This method can be applied for in-process cleaning and as part of a prepainting, pre-plating sequence.
Chemical processes
Solvent cleaning: In this process of cleaning, liquid hydrocarbons are used. Cleaning is
carried out with one or a combination of three application methods namely, immersion or
soak-tank cleaning, spray degreasing and vapor degreasing. Commonly used solvents are
petroleum solvents (Stoddard solvent, mineral spirits, or kerosene) and chlorinated
hydrocarbons (trichloroethylene or perchlorethylene).
Immersion cleaning: It is considered to be the most effective in-process cleaning method.
This method removes oil, grease and oil-borne dirt. However, the level of cleanliness is not
high due to rapid solvent contamination.
Vapor degreasing: This is another effective method for removal of oil and grease. Sufficient
flushing (immersion and spraying) are to be done prior to this vapor-degreasing cycle. The
following steps are followed for this type of cleaning operation. A small amount of solvent is
allowed to vaporize and the vapor is contained in the tank by the tank walls and a cooling
coil. When a work-piece is lowered into the vapor, the vapor condenses on its surface and
drips back to the reservoir below. When the work-piece is heated by the warm vapor,
condensation stops and the work-piece dries.The work-piece is then taken out from the vapor
warm and is completely free of oil and grease. Various problems associated with solvent
cleaning process are solvent disposal, toxicity and fire prevention.
Ultrasonic cleaning: It is the most convenient method for thorough cleaning of small,
intricate parts which may not be even possible with other cleaning methods. In this method,
cleaning solution agitated by ultrasonic vibrations. Commonly used cleaning solutions are
chlorinated and alkaline solvents.
Emulsion cleaning: When the work-piece contains heavy buffing compounds (cakes
materials) or grease loaded with solid particles, this method is preferred. The liquid cleaner
used for this purpose is a mixture of water, a hydrocarbon (petroleum) solvent, and
emulsifying agents (soaps, glycerol, and polyether). Usually, the cleaner is applied by spray
or immersion at a temperature of 54 to 60°C and often upto 82°C. If work-piece is to be
immersed then agitation of the cleaning liquid is recommended. After cleaning of the workpiece, rinsing in hot water is required.
Alkaline cleaning: It is used for the removal of oil, grease, shop dirt, and compounds from
polishing, buffing, and drawing operations. In this type of cleaning, the cleaner used is the
combination of a solution of certain alkaline salts (caustic soda, tri-sodium phosphate,
silicates, borates, or carbonates) and detergents in water. Cleaning process involves pressure
spray in soak tanks, often with agitation at a temperature of 60 to 93°C and water rinsing of
the work-piece after the alkaline. This method is considered to be the least expensive and
most popular for mass production.
Acid cleaning: It is similar to alkaline cleaning process except that, the cleaner is a solution
of acids or acid salts instead of alkaline salts. Acid cleaning is used for removal of light rust,
tarnish, scale, drawing compounds, oil, and grease. Acid cleaning does not work efficiently
when heavy coatings of oil and grease are there on the work-piece.
Pickling: It is used for the removal of surface oxides, scale, and dirt from metals.This process
is stronger than acid cleaning. The cleaning solution is prepared by mixing an aqueous
solution of acid with a wetting agent. The type of acid to be considered depends on the metal
to be cleaned. For example, in ferrous metals sulfuric, hydrochloric, and phosphoric acids are
used. It is essential to neutralize the effect of pickling acid and hence an alkaline rinse is
required to be performed after the process
Salt-bath cleaning: It is a suitable method for the removal of scale, oxides, sand, carbon, and
graphite from a various metals. Cleaning is done by immersing the work-piece in a bath of
the molten salt at a temperature about 440 to 524°C. The salt in the bath reduces the scale and
loosen it to some extent from the work-piece surface. Further, the work-piece is placed in a
water rinsing bath and due to heat of the work-piece the water turns to steam and blasts the
scale from its surface.Often this is called sodium hydride de-scaling.
Electrochemical processes
Among various cleaning methods, alkaline cleaning, acid cleaning, pickling, and salt-bath
cleaning can be performed by using an electrolyte (electrochemical). For example, in
electrolytic-alkaline cleaning, the work-piece is taken as one electrode and the tank or a steel
plate is as other. The alkaline solution works as the electrolyte. With the supply of current,
hydrogen gas is released at the cathode and oxygen at the anode. The produced gas bubbles
provide a scrubbing action at the work-piece surface, causing the soil to break up rapidly.
Various cleaning processes and their suitable materials are summarized in Table M5.1.1.
Suitable materials
Table M5.1.1:Suitable Materials for Common Cleaning Processes. (Source: Design for
Manufacturability Handbook by James G Bralla, 2nd Ed)
Process
Suitable
materials
Marginal, difficult or unsuitable
materials
Brushing
Steel, cast iron, and
most other metals,
ceramics, and glass
Plastics and rubber require soft brushes. Stainless
steel and aluminum require stainless brushes.
Magnesium requires dust control.
Abrasive
blasting
Steel and all harder
metals
Ductile metals require special abrasives. Plastics
and other soft materials may entrap abrasive.
Stainless steel, copper, brass, zinc, aluminum,
lead, and tin require nonmetallic abrasive.
Magnesium can be hazardous.
Design recommendations
1. The cleaning surface must be easily accessible for cleaning and full flow of cleaning
fluid as shown in Figure M5.1.1.
Not this
This
Figure M5.1.1: Wide path in the right side image for full flow of cleaning fluids.
2. Parts subjected to liquid cleaning operations should have some provision of hole for
hanging or a rack for supporting the part easily. (See Figure M5.1.2.)
Hook or hole for racking
Not recommended
Recommended
Figure M5.1.2: provision of hole for hanging
3. When parts are subjected to liquid cleaners, drain holes must be provided. Generally,
the part configuration or method of hanging traps the cleaning solution as shown in
Figure M5.1.3 and Figure M5.1.4.
Figure M5.1.3: Drain hole and wide space for unimpeded flow of coating material
Figure M5.1.4: Drain holes and air vent.
4. Cleaning and finishing smaller parts and then assembling is more economical than
cleaning and finishing a large single component.
5. Nest table parts may cause difficulties in some cleaning operations which involve
immersion in a bath. Advantages and disadvantages of handling nest-able parts need
to be understood.
Cleanliness specifications
The following methods can be used to specify cleanliness requirements:
1. Specifications of the maximum allowable weight of soils on a part
2. Specifications of the maximum particle size of allowable soils on a part
3. Various specifications based on some kind of performance test, e.g.,

Water-break test

Acid-copper test (electrolysis copper-plating coverage)

Atomizer test

Wiping test