The Physics of Polishing
Physics (from Ancient Greek: φύσις physis "nature") [: a natural science that involves the
study of matter and its motion through space and time, along with related
concepts such as energy and force.]
Backing Plate / Pad Motion
The benefits of a large
orbit or elliptical offset is that more centripetal force is created as the
backing plate orbits, so the backing plate will rotate more than a similar
machine featuring a smaller elliptical offset. A machine featuring a large
stroke delivers increased speed of backing plate motion using the same orbits
per minute. A large stroke elliptical offset increases movement of the baking
plate / pad; therefore levelling is more consistent. This type of pad movement
helps to remove residue (oxidized paint, spent abrasives, etc.) more readily
than a small elliptical offset
A majority of
random orbital machines use an elliptical offset between 1/8 -5/16 - inch
(approximately 3.0-8.0 mm). It is generally accepted that a smaller stroke
leaves a more refined finish, but experience shows that this type of movement
doesn’t readily clear (oxidized paint, spent abrasives, etc. ) polishing
debris, thereby blocking the pores of the pad and placing debris between the
pad and the paints surface negatively impacting the abrasive
Surface (Contact) Area
The distance around the circle is a
circumference. The distance across the circle is the diameter (d). The radius (r) is the distance
from the centre to a point on the circle. (π = 3.14), d = 2(r), c = π d = 2 (p) r, A = π(r) 2
Even a minor change in pad diameter makes a big difference in surface area.
A pad should be designed to
efficiently use its surface area. Foam pads that have lines, squares, circles,
or dimples cut out of the pad face, means there is less actual surface area in
contact with the paint surface. Area = 
(r2) 6-inch pad area = 18.842 sq.ins a
4-inch pad 12.46 sq.ins.
Kinetic (Heat)
and machine energy (Speed) and surface
pressure applied over a smaller area, which results in faster correction. A
further consideration of pad diameter has to do with distribution of the
machine weight and applied pressure.
Another design parameter that
determines how much surface area actually contacts the paint when using foam
pads is the amount of pores per inch it features (commonly referred to as PPI).
More pores, larger pores, thinner walls between the pores, or how stiff the
walls are all affect how much foam contacts the paint during the buffing
process
Block wet sanding (finishing
paper and a sanding block) which ensures a consistent pressure over the surface
contact area, this is the most effective method for paint defect removal
because of its linear process; you abrade the paint surface flat until the
defects are removed.
Pad Velocity (Speed)
The larger the pad, the greater the
pad velocity at an identical RPM; V = RPM (Area) A - 6.5-inch = 20.423 sq. ins
V= 24,507 inches per minute
(IPM) Pad velocity is also substantially increased with a larger
diameter pad, which increase its abrasive ability at the outer edge; 8-inch =25.136 sq. ins V= 30,163 RPM
Pressure / Pad Compression
Depending on the types of
surface abrasions you're dealing with, increase pressure is necessary;
otherwise most of the kinetic energy of the machine will be absorbed by the pad
(especially foam) and not transferred to the paint surface.
Just remember that more
pressure equals more aggressive, so be careful around ridges and raised
surfaces Maintain the same pressure and work the product in, it may take three
or four passes to complete before the residue can be removed. Once you see the
desired results move on to the next area, or repeat the process as necessary.
The required pressure
applied to obtain optimum results to adequately compress the pad (50%) and
obtain uniform abrasion is usually in the range of 10 – 15 lbs. (a random orbital buffer will stall at
approximately 20 pounds of applied force) use just enough pressure to keep the
pad rotating at 1-2 rotations per second.
To compress a 6-inch pad 50%
requires you increase the total force by the ratio of its surface areas; Ratio = [π (radius2)] / [π (radius2)]
= 2.25 as much force, almost 34 pounds).
With the smaller pad you're
applying the same force, at a constant speed but over a smaller, more
concentrated area, which will induce friction and greater abrasion abilities to
the polish, both these abilities require a certain amount of caution as it’s possible
to abrasion burn the paint.
Constant
Pressure™ Foam Pads have a layer of engineered, instant rebound foam between the
pad and the backing plate. This layer acts as a cushion or shock absorber
between the machine, the operator and the surface being worked on. It absorbs
off-axis motion while maintaining a constant and uniform pressure on the
surface; Lake Country Mfg. Constant Pressure™" technology allows even a
neophyte detailer to achieve professional-like results.
Foam Pad Size (Area and Applied Pressure)
Different pad sizes can have
an impact on how the buffer breaks down a polish, as it applies its dynamic
friction over less area, control, better manoeuvrability, and how fast you can
cover an area.
Smaller pads in general will
offer you more control with any machine polisher, as it can reduce the tendency
for the buffer to hop or skip on the paint. Smaller pads also make it easier to
manoeuvre buffers in tighter areas and closer to trim pieces.
The low profile 5.5 inch
buffing pads pack the same CCS technology and performance into a compact,
highly effective size that works best with dual action polishers and air
sanders. Use with a 5 - inch moulded urethane backing plate for excellent
flexibility and balance by Lake Country (LC) manufacturing
Assuming equal speed, radius
and foam compression (50% - 15 pounds of force applied) the difference between
4- inch and 6 - inch pads is their different surface area = π (r2) (4-inch = 12.46 sq.ins
/ 6-inch = 28.26 sq.ins) and therefore surface kinetic (or dynamic) friction applied and
surface pressure applied; 4-inch = 3.75 lbs per sq.ins - 6-inch
= 2.5 lbs per sq.ins. Even a minor change in pad diameter makes a
big difference in surface area.
Actual Speed (RPM)
Formula - RPM (C) = V
Speed = Revolutions per minute, C = circumference (2(π) (r)), V
= velocity
Area 7.5 -inch at 1,800 rpm
(usable area 6- inches)
Speed - 1,800 (rpm) (6"
diameter times pi = 6” x
3.14) x 18.84 = 33,912.
So, using the same formula,
and a problem of: X (unknown rpm) x 25.12
(8" diameter times pi = 8” x 3.14) = 33,912) then; 33,912 / 25.12 = 1,350
actual speed rpm
Kinetic Friction
[ : when contacting
surfaces move relative to each other, the friction between the two surfaces
converts kinetic energy into thermal energy, or heat]
Friction is the force
resisting the relative lateral (tangential) motion of solid surfaces, fluid
layers, or material elements in contact. It is usually subdivided into several
varieties:
Dry friction is also
subdivided into static friction between non-moving surfaces, and kinetic
friction (sometimes called sliding friction or dynamic friction) between moving
surfaces.
Arrows are vectors
indicating directions and magnitudes of forces. W is
the force of weight, N is the normal force, F is an applied force of unidentified
type, and Ff is the force of kinetic friction which is equal to the coefficient
of kinetic friction times the normal force. Since the magnitude of the applied
force is greater than the magnitude of the force of kinetic friction opposing
it, the block is moving to the left.
Heat from Kinetic (or dynamic) Friction
[Energy in a system may take on various
forms (e.g. kinetic, potential, heat, light). Kinetic friction, or surface
resistance induced heat; an often misunderstood concept of polishing /
compounding; abrasives require friction to breakdown, not heat; heat is just a resultant
of friction between two surfaces. Kinetic friction is required to ‘level’
paint, which is simply the removal of paint to the lowest point of the paint
defect] [1]
Energy in a system may take
on various forms (e.g. kinetic, potential, heat, light). Kinetic friction, or
surface resistance induced heat; an often misunderstood concept of polishing /
compounding; abrasives require friction to breakdown, not heat; heat is just a
resultant of friction between two surfaces, besides the most commonly used abrasives
include varieties of aluminium oxide, silicon carbide, diatomaceous earth,
clay, and silica, to produce enough heat to cause a reduction in size would
harm the paint. Kinetic friction is required to ‘level’ paint, which is simply
the removal of paint to the lowest point of the paint defect.
A finishing pad will not
provide as much friction as a cutting foam pad (less surface resistance)
although they will both produce friction induced heat, whereas a wool pad, due
to their composition, creates less friction induced heat but more kinetic
friction (due to its fibrous structure) than most foam pads.
Polishing a paint surfaces
transfer’s kinetic friction induced heat to the paint surface, thermoplastic
polymers have both tensile strength (a linear stress-strain relationship) and
elongation (elasticity) which allow the surface to flex, expand and contract in
accordance to surrounding temperatures, solvents, resins and other ingredients
in polishes will expand causing the paint film surface to expand, temporarily
‘masking’ paint surface defects.
As the metal
substrate expands the paint moves with it, due to its elasticity, thereby
becoming elongated (thinner) this is part of the cause of friction induced
‘burn’, you’re applying pressure and an abrasive to a less dense (‘thinner’)
paint surface, excess friction induced heat can cause the paint surface to
burn, blister, haze, and cause excessive swirls.
[: A thermosetting resin (polymer) is
a material that irreversibly cures (hardens) usually by heat, generally above
200 °C (392 °F), through a chemical reaction, or suitable irradiation]
Clear coat
are formulated from a thermosetting polymer, when subjected to high ‘spot’
kinetic energy (friction heat) from polishing will cause thermal stress, which
causes the paint film to stretch and this negatively affects its structural
integrity. Once a polymer has been heat-set it changes change irreversibly to
become hard and therefore cannot
re-flow. An elevated
temperature will cause the lubrication system (oil, wax and and/or the solvents)
in the abrasive to evaporate, which will result in dry polishing, driving
defects deep into the paint matrix
Kinetic Friction induced
heat can cause a rapid surface temperature rise; (i.e. initial surface temp 80.oF,
friction heat attained with the polisher stationary and a cutting foam pad at
1,100 RPM for approx. ten seconds the friction induced heat attained would be
around 104.oF) the paint temperature can be checked by utilizing an
instant read-out infra-red ‘gun type’ digital thermometer, paint surface ‘spot’
temperature should be limited to 110.oF <
Be cognizant that plastic
has a much lower rate of thermal conductivity than metal, so it absorbs heats
at a far greater rate and is therefore subject to thermal degradation (burning
/deforming).
Polishes and compounds do
not need heat per se for the abrasives to polish a surface, wither they be
diminishing or non-diminishing abrasive, they require both pressure and
friction
In accordance with the
Society of Automotive Engineers (SAE) a localized (spot) temperature of > 115.oF will
cause the urethane clear coat to soften and the foam pad will cause scratching
that is forced deep into the clear coat.(See also the first law of thermodynamics et al) [/I]
The Ideal Gas Laws
Polishing Freshly Applied Paint
When a urethane clear coat
is sprayed its outermost surface, measuring a few nanometres in thickness,
sustains microscopic fractures when it comes into contact with air. These
fractures are microns or nanometres in width and thus too small to be seen with
the unaided eye.
Freshly applied paint that
in the outgas stage, is still full of evaporating solvents, and is usually less
dense (soft) despite the additives used (hardener) once a catalyst, kinetic
energy (friction heat) is added, it causes the paint film to expand,
temporarily hiding scratches, this is often the reason for a body-shops bad
reputation of returning vehicles that have sanding scratches in newly applied
paint that should have been removed.
Be cognizant when polishing
newly applied paint the kinetic energy (heat) from a foam pad can also cause
solvent engorgement, which causes the paint film to thin due to the expansion
of the evaporating gases, applied rotational force may also cause the paint to
tear Kinetic friction (heat) is transferred to a solvent (isopropyl alcohol (IPA)
or fresh paint solvents) causing it to both expand (Charles'
law; also known as the law of volumes) the paint film and soften
it.
Automotive paint is
classified as a semi-permeable membrane; it has both tensile strength and
elongation (elasticity) newly painted surfaces are soft and full of out gassing
solvents, resin binders and additives, as well as and water.
Polish contains solvents,
which soften the paint film, kinetic surface friction and applied downward
pressure transfers its energy into heat / torque (force to rotate an object
about an axis), which could result in the alteration of the paint films bond
between its substrate, causing it to delaminate or tear?
The heat makes the gasses
expand (pV = nRT) the
expanding gases go through a phase transition (change in density) and to relive
this increased pressure they (a) rupture the paint film surface, causing small
fissures (similar to solvent pop) Theheat may
cause the gaseous vapours to expand, but not enough to break through the
hardening clear coat.
Once the vapour has
evaporated, it may leave a void between the basecoat and the clear. Therefore you have a cloudy spot where the
clear and base is no longer adhered together. If this is the case, the clear
coat will delaminate in the future.
Once the outgas process is
complete automotive coatings (paint) becomes a semi-solid permeable membrane,
Being a polymer (elastomers) it remains flexible while retaining its tensile
strength, to enable it to expand and contract to follow temperature
fluctuations of the substrate (elongation). Kinetic friction and its associated
heat can cause a rapid temperature rise (i.e. initial surface temp 80.oF,
heat attained with a cutting foam pad at 1,100 RPM for approx. ten seconds is
approx. 104.oF) the paint temperature can be checked by utilizing an
instant read-out infra-red ‘gun’ thermometer, paint surface ‘spot’ temperature
should be limited to 110.oF <
Applied
Pressure
The pad needs
to have an even distribution of pressure applied to it; depending on the types
of surface abrasions you're dealing with, increase pressure as necessary. Just
remember that more pressure equals more aggressive, so be careful around ridges
and raised surfaces
Maintain the same pressure
and work the product in, it may take three or four passes to complete before
the residue can be removed. Once you see the desired results move on to the
next area, or repeat the process as necessary.
The required pressure
applied to obtain optimum results to adequately compress the pad (50%) and
obtain uniform abrasion is usually in the range of 10 – 15 lbs. (a random
orbital buffer will stall at approximately 20 pounds of applied force) To
compress a 6-inch pad 50% requires you increase the total force by the ratio of
its surface areas
Ratio = [π (radius2)] / [π (radius2)] =
2.25 as much force, almost 34 psi). With the smaller pad you're applying the
same force, at a constant speed but over a smaller, more concentrated area,
which will induce an increase in
friction and greater abrasion abilities to the polish / pad combination, both
these abilities require a certain amount of caution as it’s possible to ‘strike
through’ (friction burn) the paint.
Hardness
So how can a dense (hard)
clear coat be so easily scratched?
It’s a matter of physics, not material
density (material
hardness). Force acts through a body that has a surface area; if the surface
area is really small while maintaining an equal force, the pressure becomes
astronomical and the object under pressure capable of penetrating the surface
of an otherwise tough material.
Newton's third law of motion [:
when a first body exerts a force F1 on a second body, the second
body simultaneously exerts a force F2 = −F1 on the first
body. This means that F1 and F2 are equal in magnitude
and opposite in direction]
That’s why a
micro fine thread that is twice as fine as silk and a 100 times finer than a
human hair, in an otherwise soft towel will scratch your paint. And the same
reason a mosquito can penetrate a rhino hide with its proboscis (stinger).
If you press down on your
paint finish with your palm it feels really hard and tough, but that’s because
the surface area of your palm is relatively large and what you’re actually
feeling is the resistance of the steel underneath the paint. Try pressing your
thumb nail into the paint with the same amount of force you used with your
palm, if you dare.
Relevant Articles
3.
TOGWT® Autopia Detailing Wiki articles
I hope these TOGWT Detailing
Wiki articles will become an asset to anyone who is new to detailing and to the
professionals; enthusiast detailer’s and industry experts who seek to advance
their knowledge of detailing entry level enthusiast, but to professionals and industry
experts as well.
