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Laser Machining Fiber-Reinforced Plastics (FRP)
Researched, compiled and written by World Lasers, Inc. © 2007
What are Fiber-Reinforced Plastics (FRPs)?
Fibre-reinforced plastic (FRP) is a composite structural
material containing high-strength fibrous embedded reinforcements which develop mechanical properties
greatly superior to the base polymer (called ‘matrix’ in this context). The fibers are
usually fiberglass, carbon, or aramid (a family of nylons), while the polymer is usually an epoxy, vinyl
ester or polyester thermosetting plastic (thermosetting plastics are those that can not be
recycled). FRPs are commonly used in the aerospace, automotive, marine, and construction industries. The
popular Carbon fiber reinforced plastic is a strong, light and very expensive composite material
or fiber reinforced plastic. Similar to glass-reinforced plastic, which is sometimes simply called
fiberglass. The composite material is commonly referred to by the name of its reinforcing fibers.
Some FRPs’ contain both carbon fiber and fiberglass reinforcement. Less commonly, the term
graphite-reinforced plastic is also used. FRP is becoming increasingly common in small consumer
goods as well, such as laptop computers, tripods, fishing rods, racquet sports frames,
stringed instrument bodies, classical guitar strings, drum shells etc.
Role of Lasers in FRP based Products Fabrication:
Since FRPs have to be put to use in virtually infinite number of
ways (their new applications are emerging everyday in ever growing list of industries), the
material has to be processed, cut, shaped, machined and finished before acquiring its final form.
Machining of the fiber-reinforced plastics (FRP) composites differs significantly from machining
of conventional metals and their alloys. In the case of FRPs the material's behavior depends on
diverse fiber and matrix properties, fiber orientation and relative volume of the matrix and fiber.
The tool continuously encounters alternate matrix and fiber materials, whose response to
machining can vary greatly. For example, in a glass epoxy composite, the tool encounters a low
temperature soft epoxy matrix and brittle glass fibers. It is this variation in the requirements of
cutting that makes the composite difficult to machine in traditional ways.
In view of high tool wear and high cost of tooling with
conventional machining, a noncontact material removal process like laser machining offers an
attractive alternative. Laser machining is a thermal process and material is removed
predominantly by melting and evaporation. They can also minimize dust and noise, extensive
plastic deformation and consequently heat generation associated with conventional
machining of FRP composites.
Experiments using laser technology are constantly being
conducted with encouraging results. The models developed so far and the experimental results presented
are all directed to straight line cut with variations in different parameters. However, further
experimental investigations are needed to understand the material removal mechanism. Fiber-reinforced
plastics laser cutting has been compared with different cut-ting techniques such as water jet
and abrasive water jet, punching, and milling, sawing, and cutting with knives.
It is shown that laser cutting is well suited if high feed rate, cutting contours and three-dimensional shapes,
minimum waste and narrow cut kerfs are required.
Laser Types Commonly Used for FRP Composites:
Two commonly used laser types in the industry today are the CO2
(carbon dioxide ) laser and the Nd-YAG (neodymium yttrium-alumina garnet) laser. The former is a
gas laser and the latter a solid state laser. The type of laser to be used for machining a
given composite depends on the work material properties and characteristics of the beam. The
important requirements of the laser for machining include adequate power available at the work,
reliability and initial and running costs.
Benefits of Laser Machining of Fiber-Reinforced Plastics:
Benefits of laser machining include minimum material waste,
minimum set-up time, no tool wear, low overall distortion or part damage, parallel-sided cuts
possible. Sharp contoured surfaces can be generated independently of work material hardness and
strength.
Simulation of Laser Cutting Parameters:
Significant amount of preparatory work prior to laser cutting
process is needed. This involves simulation of laser cutting parameters not only to reduce the
heat-affected zone but also to improve the laser cut quality with respect to surface quality
and dimensional accuracy. It is recommended that research effort be directed towards contour
cutting and sharp corner cutting to make the laser more applicable in industries.
Associated Problems:
Problems associated with laser cutting of composites materials
are due to the two or more chemically distinct materials (forming the FRP) that are not in
thermodynamic equilibrium. The properties of the phases used in the material are usually
significantly different, which makes their machining difficult as it takes a lot of experimentation to
arrive at the best laser parameters balance for a given composite.
The quality evaluation of laser cuts is not a simple task. The
damage of the material involves the thermal alteration of the fiber and matrix as well
as interface failures for a certain depth from the cut surface, which cannot be easily
detected. Moreover, the roughness is difficult to measure and scarcely representative of the
surface finish owing to the presence of chars, fuzzes (mixture of charred matrix and
burnt fibers) and scattered fibers.
The morphology of the cut edge is strongly influenced by the
cutting velocity. At low velocities, the surfaces are characterized by the fibers
leaning out of the matrix coupled with material loss. With increasing velocity, this structure tends to
disappear and is strongly influenced by the nature of the constituents of the composites. In case of
glass fiber and carbon fibers, negligible improvements in the cut quality with increased
cutting velocity are observed. Material damage also occurs owing to thermal stresses which arise during
laser cutting due to high thermal gradients near the cutting zone and the particular structure of
the composite laminate.
Conclusion:
Research efforts should be directed towards cutting of sharp
comers and contours to make laser cutting more applicable with higher flexibility. Laser is very
sensitive to process disturbances which call for sophisticated process monitoring and control.
Combinations of fiber matrix ratio may need close scrutiny so that modeling can address more
appropriate fiber matrix combination for prediction of composite parameters.
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