Composite material usually consists of a woven fiber material, something like cloth, made of fiberglass, graphite, carbon-fiber, or other fiberous material. This material may be pre-impregnated with a glue or resin. The material can be very flexible, or can be relatively stiff.
Typically the material is applied to a form or mold that represents the shape of the final piece. The material is applied in layers with each layer applied at a different angle to the previous layer. Each composite layer has an isotropic stress characteristic (i.e. is structurally strong in one direction). The layering process makes the finished piece structurally strong in all directions. After the layers are applied the piece is in an uncured or « green » condition.
During material application a bonding-agent (glue or resin) is added to each layer, or it is already pre-impregnated in the material. The « green » piece is then cured to harden the bonding-agent. The curing often happens in a oven, also called an autoclave. During oven « baking » the bonding agent liquefies and flows through the layers of material, creating a single coherent structure.
After curing or baking the piece must typically be cut or trimmed to the final shape, and possibly other machining operations (milling, drilling, turning, etc) must be done to complete it. Composite cutting is also done using waterjet or ultrasonic knife technology.
Composite material can produce very strong and light structures. In many cases they are cheaper to manufacture than metals or plastics, especially where strength and lightweight are required. But composite manufacturing requires complex processes involving chemical, mechanical, and thermal technologies and expertise. And the raw material itself often has critical storage requirements (temperature, humidity, shelf-life, etc). Hence use of composite materials is not wide-spread and can be considered a « new » technology, even though it has been around for many years.
Composite components are used in aerospace, aircraft, and automotive products. They are also present in sports and recreation products such as tennis rackets, golf clubs, bicycles, motorcycles, and various high-end racing components. Composite components are frequently not covered with paint or other coatings, so the composite fibers are visible and create fear and envy in fellow competitors.
Material application processes:
The following processes can be used to create a composite piece.
This process consists of applying a wide strip of composite « tape ». The tape is typically 1″ to 12″ wide (25 to 200mm), depending on the curvature of the mold or form the material is being applied to. Wider tape can can only be applied to relatively flat surfaces, narrower tape can follow more curved surfaces.
The tape is applied from a spool of material. It travels over a « compaction roller » that presses the material onto the form, or onto the previous layers of material, typically with a few hundred pounds of pressure. The tape usually has a « sticky » side that makes it stay where it is placed until the curing process.
The compaction roller is part of a tape head attached to a CNC machine, often similar to a 5-axis head, but with the tape and compaction roller instead of a milling cutter. The tape is pulled onto the form by the compaction roller being rolled over the surface of the form, very similar to apply packaging tape from a « tape gun » onto a box.
The tape can be a simple rectangular strip, or the sides can be trimmed to a specified contour. The contour trimming can be done before the tape is put on the spool, or while it is being deposited from the spool to the form. The tape is cut at any angle.
Machine companies producing tape-laying machines include Ingersoll, Cincinatti, Forest-Line’, and Mtorres.
This process has been around at least since the mid-1980’s in advanced aerospace manufacturing. It is limited to very regular, smooth, and relatively flat shapes. The tape has a tendency to wrinkle or tear if applied to a surface with high curvature. Current applications include wing surfaces and cylindrical rocket and missle sections.
Similar to tape laying, material moves from a spool over a compaction roller onto a form. But in Fiber-placement the tape is typically narrow, between 1/8″ and 1/2″wide (3 to 12mm). There are typically several strands of fiber (called « tows ») that pass over the roller parallel to each other. The CNC machine is capable of cutting and re-starting each tow individually. The individual narrow strips allow the material to be applied over contoured surfaces without wrinkling, tearing, or warping.
This process is newer than Tape-laying, and is much more complex and prone to process problems and errors.
Hand-layup is the traditional process of cutting flat patterns of composite material which is then manually placed or pressed onto a form. The shape of each layer of material must be converted from the 3-dimensional object into a flat pattern. The shape is then cut out using knife cutting machines (either CNC or manual).
The flat material is manually oriented to the form or mold using measurement marks or a laser alignment system.
This process has been around for decades.
Filament winding is a process where individual strands of fiber feed from spools through a loom or weaving machine and are wrapped around a form. I have only seen this process used to completely encase objects in composite material, such as ski’s, tennis rackets, etc. This is typically a high-production process with dedicated machines producing thousands and thousands of single product.
This process has also been around for decades.
Cutting composite material is a dirty process. The resulting chips or swarf is dust, which is often toxic. Frequently the only way to achieve the desired form and feature is to mill, drill, turn, etc.
However, other cleaner processes are used to cut composite material as well: ultrasonic knife and waterjet. The advantage to each of these cutting technologies is they do not produce dust.