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Wednesday, February 26, 2014

Advantages and Disadvantages of Filament Winding Process

The most important advantage of filament-winding is its low cost, which is less than the prepreg cost for most composites. The reduced costs are possible in filament-winding because a relatively expensive fiber can be combined with an inexpensive resin to yield a relatively inexpensive composite. Also, cost reductions accrue because of the high speed of fiber lay-down. Other advantages of filament-winding compared to other compacting and curing processes are:

Highly repetitive and accurate fiber placement (from part to part and from layer to layer). The accuracy can be superior to that of fiber placement and automated tape-laying machines.
The capacity to use continuous fibers over the whole component area (without joints) and to orient fibers easily in the load direction. This simplifies the fabrication of structures such as aircraft fuselages and reduces numbers of joints for increased reliability and lower costs.
Elimination of the capital expense (and size restrictions) of an autoclave and the recurring expense for inert gas. Thick-walled structures can be built that are larger than any autoclave can accommodate.
Ability to manufacture a composite with high fiber volume
Mandrel costs can be lower than other tooling costs because there is usually only one tool, the male mandrel, that sets the inside diameter and the inner surface finish.
Lower cost for large numbers of components because there can be less labor than many other processes. It is possible to filament wind multiple small components, such as up to 20 golf shafts at once (Fig. 8), leading to sharply reduced costs compared to flag rolling. Costs are eliminated for bagging and disassembly of the bagging materials, as well as the recurring costs of these materials.
Costs are relatively low for material since fiber and resin can be used in their lowest cost form rather than as prepreg.

Disadvantages

Need for mandrel, which can be complex or expensive
Necessity for a component shape that permits mandrel removal. Long, tubular mandrels generally do not have a taper. Unless nonuniform shapes are capable of mechanical disassembly, mandrels must be made from a dissolvable or frangible material. Different mandrel materials, because of differing thermal expansion and differing composite materials and laminate lay-up percentages of hoops versus helical plies, will demonstrate varying amounts of difficulty in removal of the part from the mandrel.
Difficulty in winding reverse curvature
Inability to change fiber path easily (in one lamina)
Poor external surface finish, which may hamper aerodynamics or aesthetics.

It is important to note that most of the disadvantages are application-specific and, in many cases, have been circumvented by innovative design and equipment modifications.

Wednesday, February 19, 2014

Type of Mandrel in Filament Winding Process

In mandrel design and material selection, the following criteria should be considered:
1. cost
2. mandrel reusability (durability)
3. production quantity
4. mandrel material thermal characteristics
5. mandrel strength/ability to resist deflection during winding and cure
6. final part tolerances required
7. dimensional stability

To ease part removal, mandrels may be constructed from water-soluble materials (sand), plaster, or an assemblage of metal shells that is collapsible or segmented. Tube mandrels constructed with a high-quality surface finish and a slight taper are often used for cylindrical parts.

The mandrel, which determines accurate internal geometry for the component, is generally the only major tool. Low-cost mandrel materials such as cardboard or wood can often be used when winding low-cost routine parts.
For critical parts requiring close tolerances, expensive mandrels designed for long-term use may be required. For high-temperature cure 315°C (600"F), graphite mandrels with low thermal expansion may be advantageous, however some attention should be paid to the potential difficulties of mandrel removal. Gas containment pressure vessels often require metal liners because composites are porous; these metal liners can also serve as mandrels.

Mandrels can be group to four types of mandrels. First is non-removable, which the mandrel remaining as a part of the wound structure. The other three are removable and classified according to the removal technique as:
-entirely removed (for example, tubular mandrels with or without taper and with release agent);
-collapsible (the mandrel is disassembled or removed piece by piece)
-Breakable or soluble (plaster, sand or salts).

The selection of a mandrel involves several trade-offs. These include part size and complexity, size of openings, resin system and its curing cycle and the number of components to be fabricated. The basic requirements for a mandrel, whether it will be removed from the part after winding or remain as a part of the structure, are:
-It must be stiff and strong enough to support its own weight and the weight of the applied composite while resisting the fiber tension pressure from winding and curing.
-It must be dimensionally stable and should have a thermal coefficient of expansion greater than the transverse coefficient of the composite structure.