v FILAMENT WINDING:
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.
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.
Any type of body of revolution which can be withdrawn from the cured part, can be used.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)
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.
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.
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.
h. Simple spherical.
i. Simple Ovaloid.
j. True spherical.
Different equipment are used with different considerations for each pattern.
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.
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.
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.
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.