Physics of metal spraying

From: Greg Gibbons (esskc@atcmail.wmg.warwick.ac.uk)
Date: Tue Nov 23 1999 - 11:08:21 EET


In answer to the question 'How is a molten metal powder applied to a
plastic ssurface without melting or burning the plastic?', the simple
answer is due to the fact that the small volume (and thus mass) of
the individual particles results is each particle containing a very
small amount of thermal energy. Q=mc theta (Q=heat, m=mass,
theta=temperature of body) illustrates this (low m=low Q).

When incident upon the surface of a pattern material the heat is
conducted through into the pattern and the splat cools and
solidifies.Some heat is also lost as convection and radiation, the
contribution of each heat loss process will depend on the thermal
conductivity of the pattern and the metals emissivity.
 
Provided the mass of the pattern material is large c.f. the metal
spray droplets (which it undoubtly will be) then the heat rise of the
pattern will be relatively small. Of course, the feed rate of metal
through the metal spraying apparatus could theoretically be increased
to a point where the combined mass of metal hitting the surface of
the pattern per unit time would be sufficient to heat the pattern to
a temperature sufficient to cause its degradation but this is
controllable. Using a fast enough traverse speed of the spray gun
over the pattern surface is often sufficient to prevent localised
heating of the pattern to damaging temperatures. Also, the spray
process is often performed with intervals of cooling, often
consisting of application of atomising air to the pattern.

Ultimately, the amount of cooling required and the traverse rate
required will depend on the through rate of the metal through the
spray system and the type of metal applied. Zn, with a low melt
point, can be applied at a faster rate than say steel, which
obviously has a much higher Mpt.

Hope this has cleared things up a little.

That's what I call magic!

Greg
Dr G.J.Gibbons,
Rapid Prototyping & Tooling,
Warwick Manufacturing Group,
ATC,
University of Warwick,
COVENTRY,
CV4 7AL
ENGLAND

Phone: +44(0)1203 524722
FAX: +44(0)1203 524878
email: esskc@warwick.ac.uk

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