v FILAMENT
WINDING:
Definition:
Roving
or single strands of glass, metal or other reinforcement are wound in a
predetermined pattern on a suitable mandrel. The pattern is so designed as to
give maximum strength in the direction required. The strands can either be run
from a creel through a resin bath before winding or pre-impregnated mat
(Randomly distributed interlocked felt of glass fiber) can be used. When the right
number of layers have been applied the wound mandrel is cured at room
temperature or in an oven.
Material:
Epoxy
resins are usually used with fiber glass, graphite, or Kevlar (aromatic
polyamide fiber) roving reinforcements. When graphite is used, a pre-pegged
fiber (reinforcing material containing or combined with full complement of
resin) or tape is used because it can be made with closely controlled resin
content. Certain angles of winding which cannot be done with ‘wet’ fiber
winding due to slippage, can be done using pre-pegged tapes.
Equipment:
Any
type of body of revolution which can be withdrawn from the cured part, can be
used. Mandrels are made of steel, Aluminum and plaster. Some tools are made
collapsible to facilitate their removal. Some tools are made of breakaway
plasters, or of material which can be dissolved out. Inflatable mandrels are
also used. The winding machine acts like a lathe. (Machine for turning wood,
metal etc. or with revolving disc for throwing and turning pottery)
Process:
Continuous strands (roving) are fed
through a catalyzed resin bath and then wound over a mandrel to form a part. OR
Woven or unidirectional pre-peg tapes are used.
Advantages:
a.
Higher
production rates.
b.
Certain
wind patterns.
c.
Possibility
of more uniform resin content.
d.
Parts
with highest strength in one direction or in several direction can be made
because the ratio of reinforcement to resin is very high.
e.
Mostly
the parts are made using automated equipment, hence very little operator skill
is required.
f.
Parts
can be very uniform.
g.
High
quality parts can be made very economically because reinforcement material is
cheap.
Disadvantages:
a.
Higher
costs.
b.
The
use of a woven tape does not result in a part with the maximum content of
unidirectional fibers, hence in some application this does not result in the
highest strength.
Extremely high strengths can be obtained depending
on the angle of wind, particularly if all the winding is done in the
circumferential direction.
The winding machine lays the strands perpendicular
to the axis of the part or the machine is programmed to wind the strands at
some helix angle to result in more or less reinforcement in the axial and
circumferential direction. Some of the wind patterns are:
a.
Hoop
or circumferential.
b.
Helix
with wide ribbon.
c.
Helix
with narrow ribbon and medium or high angle.
d.
Helix
with low winding angle.
e.
Zero
or longitudinal.
f.
Polar
wrap.
g.
Cone.
h.
Simple
spherical.
i.
Simple Ovaloid.
j.
True
spherical.
k.
Miscellaneous.
Different
equipment are used with different considerations for each pattern.
Application:
Corrosion
resistant pipe, high pressure resistant tank, rocket motor nozzles, tank
closures, metal fittings, metal threaded sections, it is excellent adhesive
because epoxy resin is used. It is used when metal inserts are required. Used
in small volume project connected with the space program or for the military,
this include very large filament wound tank, rocket motor nozzles, entry cones
etc.
Ingenious
techniques:
In
order to accelerate the normal filament winding process and/or to eliminate the
need for very large sized processing equipment used for application of pressure
to the part, number of ingenious techniques are developed.
1.
Snap
cure method for tape winding.
2.
Cable-clave
process.
3.
Hydro-clave
process.
4.
Deep
submergence technique.
Snap
cure method:
a.
The
pre impregnated tape is heated with a quartz lamp, just before placing it on
the mandrel.
b.
This
quickly sticks the resin and starts curing.
c.
The
tape is then pressed down on the mandrel with a hydraulically actuated roller
at a pressure of 400-800 psi.
d.
To
assure good density, several rollers are used at a time.
e.
This
method, mainly using phenolic impregnated tape, result in a fast cure,
eliminates the volatiles and produces very dense and uniform parts.
Cable-clave
process:
a.
Large
part is wound either with filament roving or tape.
b.
Two
form fitting rubber bags are placed on the wrapped part and then a number of
loose, thin metal strips are placed over the bags to cover the total bag area.
c.
A
steel cable is then wrapped completely around the entire structure.
d.
Fluid
at pressure up to 2000 psi is pumped into the space between the two rubber
bags.
e.
The
steel strips prevents the bag extrusion between the cables and the cables
restrains the bag hence very dense structures can be made.
f.
Heating
is done either by heating the structure in an oven or heating the pressurized
fluid.
Hydro-clave
process:
Filament
wound parts are placed in a rubber bag and the entire assembly is then placed
in a pressure vessel which is filled with a liquid as a pressurizing agent. By
choosing a suitable liquid, pressures up to 1000 psi at reasonable temperature
are expected. However, requirement of an enormously costly pressure vessels for
a very large part is the major disadvantage of the process.
Deep
submergence technique:
The parts are enclosed in a
rubber bag and are surrounded by electric heating coil. Another bag surrounding
the assembly will completely seal the structure which is then submerged in the
ocean up to a sufficient depth to get satisfactory pressurization. However,
transportation of the assembly to the sea, application of the large power
requirement, and to hold the parts steady during the cure, make the process
impractical for the fabrication. Still no parts are made by this process.
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