CLEANING
AND DEGREASING
What is cleaning?
Cleaning is nothing more than the
displacement of dirt from a place
where it is disturbing to a place
where it is not disturbing, for
example from a product to a washing
agent.
With cleaning, all dirt is removed
that has a disadvantageous effect.
What is degreasing?
With degreasing, only the organic
contamination is removed, such as oils and
greases. Together with the oil or the grease,
the attached dust particles are also
removed. The degree to which this
happens depends strongly on the
degreasing method used.
Purpose of cleaning
The purpose of cleaning can be to give the
products a visually attractive (cosmetic)
appearance. This class of cleaning is mostly
not critical.
Often, however, functional requirements are
placed on the cleanness of products that are
required for a following operation.
Examples of this are:
- assembly in cleanrooms
- application of electroplating layers
- anodising or chromium plating of aluminium
- phosphorating of steel
- application of lacquers
- heat treatments.
What is clean?
In a production process, the cleanness is
often described as so clean that the following
processes are not disturbed.
Some simple methods that can be used are:
- visual observation
- weighing
- water refraction test
- contact angle measurement
- copper sulphate test
Steps in the cleaning process
-
pre-cleaning
intermediate cleaning
fine cleaning
final cleaning
rinsing
drying.
Pre-cleaning
In many cases, it is necessary that a large part of the
pollution is first removed by pre-cleaning, after which fine or
final cleaning takes place.
If products must be stored for a certain time before they are
cleaned according to the requirements, manual pre-cleaning is
often applied.
Pre-cleaning prevents the dirt from drying on the product,
because it is then very difficult to remove.
The requirements that are placed on pre-cleaning are not as
high as those placed on fine or final cleaning.
Intermediate cleaning
In a series of mechanical operations, products
sometimes have to be cleaned before the
following operational step.
The requirements on intermediate cleaning
are not high. The products must be
sufficiently clean to be able to do the
following operational step.
Fine cleaning
High requirements are placed on fine
cleaning. After this, processes often follow
that demand the highest cleanness, such as
gluing, lacquering, soldering, welding or
electroplating.
Final cleaning
The highest requirements are placed on final
cleaning. Examples are reflective metal
surfaces (cosmetic) or printed circuit boards
for military purposes that must satisfy the
heaviest specifications.
With the exception of assembly, mostly no
more operations are carried out after final
cleaning.
Rinsing
The intention of rinsing in a rinsing tray is that the
attached cleaning liquid is divided through the whole
amount of rinsing liquid, so that a strong dilution is
obtained.
Movement of the products in the bath helps here. After
the rinsing, we no longer have a pure rinsing liquid,
but a strongly diluted cleaning liquid. This diluted
cleaning liquid is also on the cleaned products. If
necessary, the rinsing must be repeated.
If one has a rinsing system with several
compartments after each other, we speak of a
cascade system. Here, the baths flow over into each
other. The result is that much less rinsing liquid is
required.
Drying
With the use of aqueous cleaning agents, drying must be
carried out as an extra operational step after rinsing.
The least expensive manner of drying is when a product is
heated by a previous operational step, for example rinsing in
hot water. If this is not the case, blowing with cold air or with
nitrogen can dry the product. Here, the liquid present on the
product surface is blown out of blind holes and seams.
Blowing with hot air, drying in a drying oven or with infra-red
is more effective, whereby the drying effect is obtained more
through evaporation.
Another method is with the aid of water
repellent or soluble liquids. Vacuum drying is
also applied. As a result of the pressure drop,
the vapour pressure drops so that water or
solvent evaporates earlier.
This process is accelerated when it is done
at a higher temperature.
Factors that have an effect on the cleaning process
- chemistry
- time
- temperature
- movement (agitation).
Cleaning agents
Organic solvents
Organic solvents can be subdivided into
combustible and (mostly) non-combustible solvents
Mineral oil products such as petroleum ether,
paraffin and white spirit belong to the group of
combustible solvents.
Other organic solvents are acetone and alcohols.
These solvents are used for cold degreasing.
Cold degreasing is used for degreasing on a small
scale, such as manual cleaning with a cloth or
cleaning products in a component cleaner.
Hot degreasing with solvents is used in vapour
degreasers. Here, the objects that must be
degreased are hung in a solvent vapour.
Aqueous cleaning agents
Aqueous cleaning agents must be subdivided into
three groups on the basis of their acid content, i.e.:
- acid
- neutral
- alkaline.
Neutral and alkaline cleaning agents are especially
used for degreasing. Aqueous cleaning agents are
preferably used at increased temperature.
Cleaning method
The following methods are often used:
-
manual
immersion
spraying
high pressure cleaning
vapour degreasing.
Manual
Manual cleaning is
done almost solely with
solvents.
Besides cleaning small
objects with a cleaning
cloth or tar brush on
the workbench, a
component cleaner is
widely used.
Immersion
The cleaning of metal
products in
immersion machines
is a much-used
method. This method
is used for both
solvents and for
aqueous cleaning
agents.
The products are placed in a bath with a cleaning agent for a
certain time.
The time required depends on the choice of cleaning agent and
the possibilities that one has of movement and increasing the
temperature.
the positioning of the products in the bath is also of
great importance. The products must be so
positioned or separated from each other that they
are sufficiently rinsed.
Spraying
High pressure cleaning
Vapour degreasing
Agitation methods with immersion
In combination with the immersion as a cleaning
method, different ways of agitation (movement) are
used, i.e.:
- immersion rinsing or rotation
- boiling-off
- air agitation
- liquid injection
- ultrasonic cleaning.
Immersion rinsing or rotation
Boiling-off
Air agitation
Liquid injection
Ultrasonic cleaning
Cleaning with alkaline agents
Depending on the material that must be cleaned,
alkaline cleaning is subdivided into three categories,
i.e.:
- Strong alkaline, for steel and magnesium and
heavy pollution
- Weak to moderate alkaline, for light metals,
copper, aluminium, zinc and reasonable pollution
- Neutral, for sensitive metals and light pollution.
pH
Composition
Strong
alkaline
pH 11 –
14
Surfactants
Sodium (potassium) hydroxide
Carbonates
Silicates
Phosphates
Corrosion inhibitors
Complexing agents
> 50
Weak
alkaline
pH 8 - 11
Surfactants
Phosphates
Silicates
Borates
Corrosion inhibitors
Complexing agents
> 40
Surfactants
> 20
Sensitive metals
Low degree of pollution
Weak
acid
pH 3 - 6
Surfactants
Organic acids
Corrosion inhibitors
> 50
Steel
Phosphorating
Oxide
Strong
acid
PH 1 - 3
Surfactants
Inorganic acids
Corrosion inhibitors
ca. 20
Neutral
Temp. (C)
Conditions for use
Steel
Heavy pollution
Light metals, copper,
aluminium, zinc
Reasonable pollution
pH 6 - 8
Etching removal
Rust removal
Safety
When degreasing with combustible
solvents, we are involved with:
-
danger of fire and explosion
-
toxicity of the solvents used.
Fire and danger of explosion
In general, the vapour of the solvent starts to burn. A fire in a
tray with solvent, above which the vapour has started to burn,
can be put out by simply covering the tray. This is not the case
if the liquid itself is also heated by a fire that has been burning
for a long time and has even started to boil.
If there is a lot of solvent vapour present in the air, then we
get an explosive mixture that can be ignited by an electrical
spark, tool sparks or friction.
Toxicity of solvents
If you work in surroundings where the air can be
polluted with harmful or toxic vapours, it is
important to know how harmful these vapours are.
To express the toxicity and the danger of various
substances in a number, the MAC value has been
determined. MAC is the abbreviation of:
Maximum Acceptable Concentration.
With a low MAC value, the substance is therefore
very toxic and with a higher MAC value less toxic.
The MAC value is the concentration of the harmful
substance that may be in the air during an eight-hour
working day.
MAC value
Flash point
Methanol
200 ppm
11 C
Ethanol
1000 ppm
12 C
Acetone
750 ppm
-19 C
Hexane
25 ppm
-22 C
Benzene
10 ppm
-11 C
1-methyl-2-Pyrrolidon
(NMP)
100 ppm
96 C
Safety with alkaline cleaning is aimed at the chemical
properties of these agents. Alkaline agents are very
dangerous for the eyes, they cause immediate
damage to the cornea.
Therefore, always wear special goggles that are
completely closed or a facemask when working with,
filling and mixing alkaline cleaning agents.
Concentrated alkalines soften the skin. Serious burn
wounds are the consequence.
The more diluted bath liquids have a strong
degreasing action on the skin, skin cuts. A strongly
reduced resistance to infections is the consequence.
Therefore, always wear special, protective clothing
and gloves when working with these agents.
If the (diluted) bath liquids come into contact
with the skin, rinse thoroughly immediately
and, if possible, neutralise the skin with a
soap or soda solution. After drying the skin,
apply a protective cream.