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Laser
Welding of Thermoplastics
A study of
Controllability of Melt Depth
Researched,
compiled and written by World Lasers, Inc. © 2007
Thermoplastics are plastic
compounds which, in their final state as finished articles, are capable of being
repeatedly softened by an increase of temperature and hardened by a decrease of
temperature. New development of these plastics and their applications are on the
increase in several key industries worldwide. Consequently, development of
plastics joining (welding) technology has assumed great importance for further
advancement in industries like the car manufacturing, telecommunication,
construction, medical technology and others where there is ever expanding use of
different types (and varieties) of thermoplastic materials. Hence, there is a
growing need for finding more sophisticated ways for joining a large variety of
thermoplastic materials, in different combinations, shapes and sizes and at a
variety of locations (and situations) within the manufacturing facility.
Plastics joining can not be a straight forward approach as each plastic has a
very complicated absorption spectrum for the infrared laser beams and their
weldability greatly depends upon the types of thermoplastics and also the type
of laser to be deployed. This is so because the capacity to absorb heat
is different for different plastics materials.
Researchers at Electronic and
Engineered Materials Laboratory, Mitsui Chemicals, Inc in Chiba, Japan are
active developing new methods for welding thermoplastics that are more refined,
less cumbersome, quick to set up and use, are economical overall, and, most
importantly, those that achieve the desired weld results without affecting the
plastic surface adversely.
Overlapped thermoplastic welding
can be done in two ways using infrared laser techniques. The first method
utilizes ‘near infrared lasers’ absorption technique using sources like the
Diode Lasers. In this method one of the two overlapped plastics to be joined
needs to be transparent (allowing laser beam to pass through without generating
heat) and the other thermoplastic material has to be absorbent at the weld
point. The absorbed laser energy is converted into thermal heat which melts the
interface of the two plastic parts. In this method the absorption of beam energy
is significantly enhanced through use of dyes and pigments at the weld point.
The second method known as
‘Infrared Laser Penetration Welding Technique’ uses direct infrared laser
energy absorption in the overlapped plastics to be joined. This method being
direct will have much greater appeal (and applications) as it can deal with any
combination of thermoplastics, unlike the first method, where, one of the two
plastics needed to be transparent and the other absorbent. This method does not
need any dyes or pigments either. However, in the laser penetration welding
technique, due to highly focused (direct on the spot) laser beams, temperature
of irradiated surface often rises so high that it is not easy to achieve
the desired standard of welding, without causing surface thermal damage such as
shrinkage, bums, perforations, or other undesirable degradations. Extended
research has been conducted concerning this technique by using infrared
radiation heating with transparent heat sinks. The results of welding
experiments using heat sinks show that both visually transparent thermoplastics
and translucent thermoplastics of several millimeters in thickness can be welded
without causing thermal damage.
Sapphire and Silica Glass have been found to be good
materials that can be used as heat sinks. Conductivity of Sapphire is better
than that of Silica Glass. Without heat sinks e.g. performing infrared laser
welding process with only direct contact with air, temperatures at the surface
becoming so high means that the irradiated material will be damaged before
welding is complete.
The temperature profile of the irradiated surfaces is
greatly dependent upon the heat conductivity of the heat sinks, the thickness of
the plastics to be welded together, the capacity of the plastics to absorb heat,
the irradiation power density and the irradiation time. The melt depth needed
for welding process can be controlled by a proper combination of these
parameters.
For its direct applicability in most situations Infrared
Laser Penetration Welding method is expected to become the method of choice in
most thermoplastics joining applications.
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