Volumes of production
Batch production to medium-level production.
Unit price vs. capital investment
Molds can be manufactured from a range of materials, including modeling clay, which allows reasonably cheap tooling to be produced for short-batch production.
Speed
Although the laying together of the resin and reinforcement can be automated, the process requires manual labor and the material must pass through a number of stages. The time the material spends in the autoclave can be up to 15 hours.
Surface
Gel coats are sometimes used on the surface of the mold to provide a higher quality surface finish. Without this gel, the surface would have a fibrous texture.
Types/complexity of shape
Although the process is versatile in terms of being adaptable to different shaped molds, it is nevertheless limited to fairly simple shapes.
Scale
Part sizes are only limited by the size of the autoclave.
Tolerances
Shrinkage does occur, so tolerances are hard to control.
Relevant materials
Suited to various advanced fibers, such as carbon fiber, and thermoset polymers.
Typical products
Widely used in the aerospace industry to fabricate high strength-to-weight ratio parts for aircraft, spacecraft, and missile nose cones.
Similar methods
All forms of contact molding (p.152), the vacuum infusion process (VIP) (p.154), and filament winding (p.158).
Sustainability issues
The combination of intense heat and pressure applied over several hours during autoclave molding results in high energy use and increased emissions. However, the use of heat improves the performance and surface quality of the material, which may prolong the life of the product and prevent it entering the waste stream. Unfortunately, composites are difficult to recycle as it is very hard to separate the combined materials.
Further information
www.netcomposites.com
Filament Winding
Product |
spun carbon chair |
Designer |
Mathias Bengtsson |
Materials |
carbon fiber and polymer resin |
Country |
UK |
Date |
2003 |
This chair is made using a helical winding technique, though the desired effect is more gappy than is usual for components made from filament winding. This highly decorative spun structure firmly establishes filament winding—a process most often associated with engineering composites—as a design application.
Imagine impregnating the thread on a cotton reel with resin and then being able to pull the wound thread off its reel to form a rigid plastic cylindrical part: this is the essence of filament winding.
In filament winding, a reinforcement fiber combined with a polymer resin is used to form strong, hollow composites. It involves a continuous length of tape or roving (in other words, fiber) that is pulled through a polymer resin bath. The sticky fiber is wound over a preformed mandrel in a process that is allowed to continue until the required thickness of material is built up. The shape of the mandrel determines the internal dimensions of the finished product. If the end product is likely to be used in pressurized conditions, the mandrel may be left inside the winding to add strength.
There are various forms of filament winding that differ only in the configuration of the winding. These include circumferencial winding, where the threads are wound in parallel like the cotton thread on a spool; helical winding, where the threads are wound at an angle to the spool (which gives a woven surface pattern that is instantly recognizable); and polar winding, where the threads are run almost horizontally to the axis of the spool.
1 Three composite tubes being formed on a three-spindle filament-winding machine.
2 The yellow control arm feeds the resin-impregnated fibers onto the tube-shaped mandrel. (The resin bath is out of frame.)
3 The helical winding pattern of the fibers is clearly visible.
– Produces components with a very high strength-to-weight ratio.
– Filament-wound components will always have a woven surface pattern unless they are post-finished.
Volumes of production
Equally suited to one-off and high-volume production. Economical mass-production starts at approximately 5,000 units and can range up to hundreds of thousands of units.
Unit price vs. capital investment
Foam tooling can be used for small runs or one-off production, as can existing aluminum bar stock, so costs can be kept down.
Speed
The speed is dependent on the shape and desired wall-thickness of the final component. However, by using a “pre-preg” system, in which the fibers are precoated in resin, the process eliminates the need for a resin bath. Speed is also affected by the number of “tows” of fiber that are used, so that multiple tows result in a faster covering of the mandrel.
Surface
The internal surface depends on the finish on the mandrel, while the external surface can be finished in a number of ways, including with machining.
Types/complexity of shape
Produces very strong, thin- or thick-walled hollow components, including asymmetrical shapes.
Scale
Machines can be built to produce filament windings to a massive scale. An all-plastic, 1,300-foot-long motor case for a NASA rocket with a 174-foot diameter was produced in the 1960s.
Tolerances
Tolerance is controlled by the internal diameter, which is determined by the size of the mandrel.
Relevant materials
Generally used to reinforce thermosetting plastic with glass or carbon fiber.
Typical products
This is a process that is often used for closed-pressure vessels such as aeronautical components, tanks, and rocket-motor housings. Because of the high strength-to-weight ratio of these parts, they are used as “stealth” materials to replace metals in military hardware. The process is also used for its more decorative capabilities in expensive “designer” pens made from composite materials (as well as in the chair pictured).
Similar methods
Pultrusion (p.99) and hand or spray lay-up (see contact molding, p.152).
Sustainability issues
Filament winding is largely automated so requires electrical energy to power the motors. The high speeds at which the machines can operate help to make efficient use of this energy consumption through large volume production. The high strength-to-weight ratio is also significant and offers weight savings.
Further information
www.ctgltd.co.uk
www.vetrotexeurope.com
www.composites-proc-assoc.co.uk
www.acmanet.org
Centrifugal Casting
including true- and semicentrifugal casting, and centrifuging
Centrifugal casting is a process that is based on a specific use of gravity. The same force that is at work when lettuce leaves are spun in a salad spinner, or when people are rotated in a waltzer at a fairground, is employed to thrust a heated liquid material horizontally against the inside of a mold. Once the liquid has cooled, the finished part is taken out of the mold. In industrial manufacturing, centrifugal casting is most often used to make large-scale metal cylinders that require specific surface properties within the metal component.
Centrifugal casting for metals can be broken down into three main variants: true centrifugal casting, semi-centrifugal casting, and centrifuging. As you may well have guessed, each process uses a centrifugal force to throw molten metal against the inside wall of a mold to produce a variety of shapes.
True centrifugal casting is used to make pipes and tubes, and it involves molten metal being poured into a rotating cylindrical mold. The mold defines the outside surface of the final component, while the wall thickness of the final tube or pipe is determined by the amount of material that is poured in. This type of casting solves one of the problems traditionally associated with metal, because the outer surface of the component is of such a fine grain that it is resistant to atmospheric corrosion (which is a common issue with pipes), while the internal diameter is rougher, with more impurities.
In semicentrifugal casting, either permanent or disposable molds are employed for making symmetrical shapes such as wheels and nozzles. It involves a vertical spindle around which the mold is held, like a spinning top. It also involves a slower rotation than true centrifugal casting and parts can be “stacked”—in other words, more than one part can be made at a time because multiple molds can be attached to the spindle. Because the material nearest the center (that is, nearest the spindle) rotates at a slower rate than the material farthest away, small air pockets can occur in the component.
Centrifuging is similar to semi-centrifugal casting in as much as the spinning occurs around a vertical spindle, but it is used to produce small multiple components. The metal is forced into the various mold cavities (which are only a short distance from the spindle) to produce fine details.
– Parts can be produced with good mechanical properties in all directions, because the process results in nondirectional grain orientation.
– The strength of centrifugal castings is close to that of wrought metal.
– With true centrifugal casting, the outer surface has a fine grain, which makes it more resistant to corrosion.
– Can achieve economical production over short runs.
– Limited production base.
– Limited shapes achievable.