Working with standard materials are tried and true ways to create what you want and being able to know the outcome. There is little risk but with that comes little innovation. Delving into the use of composites can widely open your possibilities of creation but it comes with a lot of possibility to fail if you don’t know how to use composites properly.
Boeing 787 Dreamliner has a huge amount of Composite Construction to save weight.
What are Composites?
Composites, simply put, are multiple materials with different physical and chemical traits that work together to create a stronger, lighter, or sometimes more flexible product. Composite materials are combined, but never fully fused into one material; so while they do work in tandem, they are separate and distinct from each other at certain levels. One of the most well-known composites on the market is reinforced plastic. Now, in most products today plastic is used in a pure form like in toys and water bottles, but it can be reinforced with fibers from other materials to make some of the strongest, lightest, and most versatile composites to date.
Carbon Fiber and other composites are popular in a variety of non-aerospace applications ranging from automobiles (especially racers like Formula One) to Golf Clubs.
Composites are popular for a variety of reasons in a plethora of businesses. Notably, the Air and Space manufacturing business uses composites for their light weight features not only to launch rockets into space but also to create many modern aircraft. These businesses account for around 50% of composite material sales while sporting goods, such as golf clubs and tennis rackets, account for another 25%. Automobile manufacturing accounts for the remaining 25% of composite material sales and the obvious pull of stronger, lighter, materials makes composites the perfect building block not only for street-legal but also Formula One and professional racing automobiles.
Composites have had a serious shoe in the door since the 1990’s introduction of Polymer Matrix Composites not only in basic modeling of body paneling, drive shafts, and leaf springs in automobiles, but also medical implants, reciprocating industrial machinery, and safer storage and transportation of corrosive chemical material. Composite reinforcement has seriously revolutionized the way we create heat resistant, conductive, light, flexible, and strong products that last longer than their previous models made from pure materials.
Composites are often referred to by multiple names. You may have heard CFRP, or Carbon-Fiber-Reinforced-Polymer, “Black Aluminum,” or even Matrix Composites. These names all refer to the same type of product, although they may be made with different materials. Usually a more specific name for a Composite will tell a lot about what the composite is made out of and what matrix, if any, is suspending those materials.
Carbon Fiber Cloth.
A common use of composites is to reinforce a purer material with a fiber of another pure material or a composite material. Most commonly, Carbon or Graphite fibers will be added to a composite. Carbon fibers are conductive, have an excellent combination of high modulus and high tensile strength, have a very low (slightly negative) CTE, or Coefficient of Thermal Expansion, and offer good resistance to high temperatures. These traits make Carbon a great fiber for a variety of businesses and an easy fuse with multiple materials.
In addition to carbon, Fiberglass is a fairly common fiber reinforcing material. Fiberglass is not as strong or stiff as carbon fibers, but it has characteristics that make it desirable in many applications. Fiberglass is non-conductive (i.e. an insulator) and it is generally invisible to most types of transmissions. This makes it a good choice when dealing with electrical or broadcast applications. While there are many different types of Fiberglass(i.e. alkali, chemical, structural, electrical, and dielectric), they are all used in a variety of businesses and can be easily fused into most composite materials.
While carbon fibers and fiberglass are the most common reinforcements in thermoplastic composites, there are other options. Aramid fibers (such as Kevlar ® and Twaron ®) and boron fibers have been used in composites and offer some beneficial properties (excellent toughness and compressive strength, respectively). However they have characteristics that have limited their use (susceptibility to light/difficulty machining and brittleness, respectively). Still others include ceramic fibers like SiC or aluminum oxide. These may be attractive for their compression, insulating, or high-temperature properties.
Resins are an important part of composites. Resins are the matrices which hold the separate material together without them being completely fused into one pure materials. There are a variety of resin types; Epoxy, which is a high-quality standard in composite machining, Phenolic, which is fire resistant, BMI cyanate, which has a naturally high temperature, Polyester or Vinylester, which is a low cost substitution for most resins, and Thermoplastic, which has a high impact resistance. All of these resins have different traits that make them popular in different businesses, but they all complete a composite and all livability to them.
Fiber Orientation and Structure
Fibers can be introduced into composite in multiple ways:
- Unidirectional resistance, in which maximum strength and stiffness are obtained in the direction of the fiber which results in the highest strength in direction of fiber orientation, bad handling features, and critical machining due to high delamination risk.
- Planar reinforcement, which is a two dimensional woven fabric that results in uniform strength in all directions, better handling features, and lower delamination risk. Often, different layers will consist of fabric laid with the fibers in different direction to maximize this advantage.
Along with being oriented in different ways, you have to manage which type of fiber you are going to be orienting; continuous or chopped fibers. Continuous fibers can be combined with virtually all resins. They are used for braiding, weaving, filament winding applications, unidrectional prepreg tapes, and prepreg tow for fiber placement. Chopped fibers are used in compression and injection molding compounds to produce machine parts. the finished products have excellent corrosion, creep and fatigue resistance, and high strength and stiffness characteristics. Lastly, there are composites that consist of embedding particles rather than filaments of any length.
Fiber orientation is also important to be aware of when machining. Where possible, the material is damaged the least if you can machine parallel to the fibers. Of course it’s often impossible to do so exclusively, but it’s worth being aware that machining parallel gives the least amount of fraying, chipping, and delamination.
Use of Composites in Commercial Airliners
The Boeing 787 Dreamliner is 50% composite materials. Graphic courtesy of Boeing.
Composites are popular in machining for their various chemical and physical traits, but they have economic advantages as well. In December 2009 Boeing flew the 787 Dreamliner for the first time. One of the major design features of the Dreamliner was its lightweight, a result of the use of composite materials. 50% of the Dreamliner’s structure is made up of composite. Since June 2013 Airbus is flight testing the A350XWB. The latest Airbus now boasts a 53% usage of composite material among its long lists of new features. The use of composites not only saved their companies money on fuel and paneling, they also cut back on fossil fuel emissions by decreasing flight time in response to the ACARE’s goal of 50% reduction in CO2, a 50% reduction in perceived noise and an 80% reduction in NOx. Composites have been a part of aircraft since the 1950’s, but having the traits they do, composites have increasingly been added over the years so we see a jump from 12% to 70% in the A350 XWB.
Challenges and Differences When Machining Composites
Carbon Fiber Relaxes When Cut
One of the key problems with composites is that machined holes and pockets will tend to be undersized because the material relaxes when cut. The effect is difficult to impossible to predict because the fibers in the material lie in different unpredictable directions. Dealing with this problem requires extensive inspection and adjustment, a process which is most efficient when automated with probing. Maintaining the required tolerances is much easier if all machining and inspection can be completed on the same machine tool in one setup. If the work requires moving between multiple machines and fixtures, maintaining tolerances is going to be much harder.
Composite Parts are Costly and the Cost of Scrapping is Enormous
In other words, there’s little room for mistakes is most machining on composites. OTOH, composite parts are near-net-shape parts. They’re custom molded to shape and tolerance that are fairly close to what the finished part will require.
Composites are Abrasive and Accelerate Tool Wear
Composites react differently to regular machining tools than metals because rather than chipping away at the material you are machining, working with a composite consists of moving through the different layer of the composite that are all different materials. For example, resins often used in the matrix layers tend to break off at the edge, while the reinforcing carbon fibers may be cut or merely fractured. Moreover, just as machining high-temperature alloys requires different tooling, cutting speeds and feed rates than more common metals, each type of composite material requires special consideration before machining.
To use composites in machining in the first place you need specific tools because most composites are extremely abrasive and wear out tools quickly. This calls for an extremely sharp edge to prevent delamination, which also contributes to rapid tool wear. Lastly, the tools and materials tend to heat up because heat isn’t carried away in chips like most pure metals or materials. Heat is another enemy of Tool Life.
Composites Create Considerable Dust and Mess
Typical machining create a reasonable amount of mess from chips but composites in general create a huge amount of dust and mess from only one session of machining a typical composite.
When machining composites, dust extraction is strongly required for the operator’s safety, and for machine maintenance; carbon dust is electrically conductive so it may effect the electrical parts and wear down the spindle at a highly increased rate.
Composites have “Grain” Like Wood
Fiber-wound products are analogous to wood, because both have a grain. The grain is usually formed from the carbon layer of the composite which is more brittle than glass and makes a much more hazardous mess to clean up for a machinist that isn’t familiar with composites. This grain is formed from how the fibers are layered within the composite. The grain direction is important to know when machining a composite that has a specifically grained material. As mentioned, it is usually desirable to machine “with the grain”, in other words, parallel to the fibers.
- What are the two basic technologies for machining composites? (http://www.mmsonline.com/articles/machining-carbon-composites-risky-business)
- How do we choose which one is better?
Two Basic Technologies for Machining Composites
CNC Machining composites with Datron.
There are two basic technologies to choose from in machining composites: rotary machining and abrasive waterjet machining. Rotary machining employs a cutting tool attached to a high-speed spindle driven by an NC-programmed machining center. The tool has a generally circular but fluted cross-section. The flutes have sharp edges and are typically arranged in a slow spiral configuration, which makes the spaces between the flutes useful for the spinning tool to channel broken material away from the work area.
A water jet being used to cut out a logo at the Automobili Lamborghini Advanced Composite Structures Laboratory
In waterjet machining, a granular silicate or similar material is mixed with water and then emitted at an extremely high pressure(usually around 60,000 to 100,000 psi) moving at around Mach 3(2,200 mph) speeds. It does so without heat generation or dust emission and causes no delamination in the composite, even at a microscopic level. Waterjet systems can be had with 5-axis heads, which enhance manufacturing flexibility and have a built-in probe for measurement work.
Both of these tools are viable options for machining composites, but the choice is completely up to the shop and its manager based solely on the technological requirements for each and the techniques and knowledge needed for each. Dust, mess, heat, tool corrosion, and many other factors will usually decide which tool is best suited for your shop, but both need to be considered when machining composite materials.
Next Installment: Tooling and Machine Considerations for Composites
In our next installment on Milling Composites, we’ll talk about Tooling and Machining Considerations. Make sure you don’t miss out on the future installments by signing up for our Weekly Digest of Blog Posts below.
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