Manufacturing Processes & Tube Fabrication Techniques

Fabrication Methods:

There are three fundamental methods used to fabricate carbon fiber tubing; pultrusion, filament winding and roll-wrap.

Pultrusion

Many carbon fiber tubes on the market are made using the “pultrusion” process.  Pultruded tubes are less expensive but do not possess optimum performance characteristics.  The mandrel and the carbon filaments are pulled through a machine and cured at the same time.  The tubes that result have little torsional strength since all of the fibers are run in one perfectly parallel direction.   This configuration has little torsional load capacity.  The slightest torsional loading can split the tube.  Additionally, pultruded tubes are typically heavier (more resin rich) which degrades the stiffness.  Pultruded tubes are not pressure cured and typically hold looser tolerances than a roll- wrapped tube.  The most common example of this construction is in the sport of archery where small diameter carbon fiber arrow shafts represent the bulk of the current market.

Filament Winding:

Just like winding fishing line on a fishing reel, this method provides a great deal of design flexibility for a variety of applications. 

Carbon tow is wound helically at various angles to the tube axis.  Tubes have slightly less stiffness torsionally than axially and are somewhat less stiff than aluminum, although stronger.  Bending stiffness will be a function of helix angle and compared to uni-directional axially-oriented fibers, these tubes exhibit less bending strength, and less bending stiffness and less tensile strength.  This method is ideal for tubing that will be used to carry the large hoop stresses caused by internal pressure. 

Filament winding is an expensive manufacturing process and produces a product generally more brittle than the other methods.  Thus, it is not the best choice for applications that will receive impact loads such as in model rocketry, automotive or other structural applications.

Roll-Wrap:

This method starts with a precision-machined mandrel around which pre-impregnated carbon fiber material is wrapped. 

The dictionary definition suggests that sheets of fiber, pre-impregnated with resin, are wrapped at various angles onto a mandrel to form the parts.  Though this is sometimes done, in actual practice, most roll-wrap tubing is made from pre-woven carbon fiber sleeves that resemble the classic Chinese handcuff material in appearance.  The weave angle can be altered simply by selecting a different nominal diameter of sleeve and pulling or compressing it to fit a particular size mandrel. 

In addition to the weight savings, this design flexibility is the major benefit of using composite tubing versus metal tubing.  It gives the designer the ability to design stiffness where it is desired.  Contrast this with a metal tube where the only parameters you can alter are diameter and wall thickness.  In the roll-wrap process, carbon fibers can be oriented appropriately, tailored to each application to handle specific loading.

The hand wrapping of the base material is followed by a winding process where a cellophane wrap (typically polyester Mylar shrink tape) is carefully wound at a specific tension around the diameter of the tube.  This step is the key to producing a high quality, high performance tube. 

Once the tube is wound it goes into an oven for curing.  The elevated temperature causes the shrink wrap to shrink against the mandrel/sleeve which is trying to slightly expand.  The compaction pressures created are enormous which maximizes density and fiber to resin ratio.

Tubes with less than a 3/8” diameter are made with interspersed layers of 4 oz. fiberglass veil for added toughness and deflection, and have a 2” long fiberglass reinforcement wrap on each end. 

 The various tube types and their characteristics are summarized in the table below.

Roll-Wrap Layup Schedule:

The layup used is a combination of uni-directional carbon fiber and woven fabric.  Tubes are laid up with the bulk of the fiber in the layers running in an axial direction to maximize bending stiffness and tensile strength.  The true optimized structural integrity of the tube is due to the uni-directional portion of the layup.  By popular demand, a woven fabric layer is sometimes added to provide the expected carbon fiber esthetic appearance.

Surface Finish & Tolerances:

Roll-wrap tubing is manufactured using precision ground mandrels around which the prepreg is wrapped.  The roll-wrap process will produce an outer diameter tolerance of  about ±0.008”.  The outer diameter has a slight texture from the manufacturing process.  Once cured and the cellophane removed, a slight stepped texture remains in the epoxy resin.  This imprint is only about .002"-.003" thick and can be removed easily with a light sanding if cosmetics are important.  Once sanded, if the tube is to be clear coated to a smooth glossy finish, the use of polyurethane will provide the necessary UV resistance.   

The inside of roll-wrapped tubing is very smooth reflecting the surface finish of the precision-machined mandrels.  These mandrels are machined to their respective diameters with a tolerance of ±0.001.”  This provides an inside diameter tolerance of the cured tubing of +.002/-.000”.

Load Ratings:

Since some applications may involve threat to life and limb and since we cannot control how our tubes are used and what safety factors are held for a given application, we do not offer specific load ratings.

We do offer basic information on our data sheets to provide a relative guide for estimating performance but each application should be tested by the end-user to assure final performance reflects estimated predictions.

Precautions:

Carbon fiber is a relatively safe material to work with and requires the same protection you would expect when cutting anything fibrous.  A face mask to protect the lungs from the fibers and safety glasses to protect the eyes are highly recommended.  While carbon fiber/epoxy composite is not harmful to the

human body, the dust particles are very small and stiff and can irritate your skin just like fiberglass.  Latex gloves and/or cutting sleeves will minimize the risk of discomfort.

Resistance To The Elements:

One of the benefits of using carbon fiber tubing is that it will withstand weather better than most other materials.  Moisture and temperature changes do not have any adverse effects. 

Ultraviolet radiation, while having no adverse effects on carbon fiber itself, will degrade the epoxy and weaken the structure if the structure is not coated properly. 

To fully protect the tubing it is advisable to paint any portion of the tubing to be exposed to UV rays for long periods of time.  Even when left outside in a very high index UV area of the country you would not notice any significant changes for many months.  As the UV light degrades the epoxy the tube may become slightly more golden in color and may look faded. 

If you want to maintain the look of the carbon fiber just use a clear coat paint such as one of the polyurethanes that contain UV inhibitors. 

There are companies that manufacture a clear spray-on protectant to inhibit the effects of UV radiation exposure.  The downside to these coatings is they need to be applied every 4-8 weeks depending on exposure.   One such company is www.303products.com

Temperature Resistance:
Carbon fibers by themselves can withstand very high temperatures but when used in an epoxy resin matrix the laminate is limited by the resin in its ability to withstand heat.  The mechanical properties of all materials begin to change when exposed to heat or cold.  Sometimes this change is severe and sometimes the change is barely noticeable.  The material we use to fabricate our tubing is designed to be used at temperatures less than 160F.  This does not mean the tube will fail at temperatures greater than 160F.  It does however mean that the tubing will begin to loose strength and stiffness beyond this temperature.  You may not see any visual change in the material until you reach 350-400 degrees Fahrenheit.  At that temperature the tubing will begin to break down and may turn ashen in color.  There are specialized resins that can be used at elevated temperatures but even with specialized resins, 400F is pushing the limit.  You may be aware of carbon fiber clutch or brake disks being used in race cars which would see temperatures well beyond 400F.  In this case a carbon fiber/resin laminate is created and then undergoes a coating/curing process in which the part is super-heated to burn out the resin.  Once the resin is burned out it is replaced with a liquid silicon based compound and cured again to become a silicon carbide laminate.

Cold Working/Bending:
Carbon fiber tubing is built using a thermoset epoxy resin.  This means that once cured the epoxy never returns to a liquid state.  If you tried to bend this cured tubing it would break with enough applied force but it will not bend.  Carbon fiber/epoxy composite is very stiff.  There are resins out there under the classification of thermoplastics that can be heated and formed over and over but we do not use thermoplastic resins.

Cello Lines:
The manufacturing process leaves small cello lines that leave a very small imprint in the top layer of resin. Lines are evidence of the extreme pressures these tubes are cured under. These lines can be sanded smooth by removing a few thousandths of an inch from the outer diameter. After sanding the tubes may be clear coated to return the shine.

Appearance:

If you are used to seeing carbon fiber as a woven fabric with a pattern these tubes may look a little foreign to you but trust us, uni-directional gives a much better strength-to-weight ratio when compared to woven fabrics. Uni-directional fibers are laid up in a single direction but layers can be rotated to give strength in any direction without the added weight and weakness of a woven fabric.  Fabric is used in molded parts for many reasons not the least of which it is easier to form into a complex mold.  With tubes we don't need to use it. 

All of our large tubes have an outer layer of cosmetic fabric specifically to achieve that pattern look but underneath that top layer they are all uni.  Uni-directional carbon is often used by the aerospace industry due to its performance.

Carbon Fiber Legal Disclaimer
There are no implied guarantees as to the fitness of these tubes, nor are they guaranteed to work for your application.  There are so many possible applications and loading scenarios, it is impossible to provide a tube that will work for all situations. 

The customer will be responsible for any and all calculations of material fitness.  In some applications life and limb may depend on these calculations and a given safety factor.   It is strongly recommended that you check with a qualified engineer before subjecting anyone to a potentially dangerous situation. 

Testing of material fitness is the sole responsibility of the customer.  The material properties for a given material are estimates only. 

All bonding procedures described are for demonstration purposes only, therefore the customer must be responsible to assure correct steps are taken to create a reliable bond. 

We will not be responsible for any injury incurred through the use of power tools while working with our tubing.  Always wear eye and airway protection when cutting carbon fiber tubing.  Follow all instructions that came with your tools before using them.  Any damage to property or human life through the use of carbon fiber tubing is the sole responsibility of the customer

GLOSSARY OF TERMS

Tensile Modulus :
Also called modulus of elasticity, tensile modulus is measure of the stiffness of a material.  It is derived as the proportion of observed strain (deflection or stretch) relative to applied tensile stress. 

It is generally constant before the material approaches the point at which permanent deformation will begin to occur.  It is most easily observed as the slope of the stress-strain curve prior to the yield point.  In our chart the tensile modulus is shown as (MSI), or million pounds per square inch.  Tensile modulus can also be shown as (10^6 PSI).

Tensile Strength:
The ultimate tensile strength is defined as the maximum stress that a material can withstand before failure in tension. Values are determined by an extension test. A simple example of tension would be the rope used in tension during a tug-of-war. Tensile strength tests are a common way to compare the strength of two materials. In the chart above tensile strength is displayed as (KSI), or thousand pounds per square inch. Carbon fiber is used most efficiently when loaded in tension. When a tube is loaded in bending some of the fibers experience tension while others experience compression.

Specific Tensile Modulus:
Specific tensile modulus can best be described as the stiffness to weight ratio of a given material. In our chart this number is determined by dividing the tensile modulus by its specific gravity (weight). It is an easy way to tell which material gives you more stiffness with the least weight penalty. Kevlar is the only material that even comes close to carbon fiber.

Specific Tensile Strength:
Specific tensile strength is the strength to weight ratio of a given material. In our chart this number is determined by dividing the tensile strength by its specific gravity (weight).

Specific Gravity:
Specific gravity is the heaviness of a substance compared to that of water, and it is expressed without units. If something is 7.82 times as heavy as an equal volume of water (such as 1090 steel is) its specific gravity is 7.82. Now you can really see how light carbon fiber is compared to other materials.

Density:
One of the fundamental properties of any material is its heaviness. In solids we think of materials like cork or styrofoam as being very light and lead and iron as being heavy. Actually we have to consider the volumes in this discussion. We could have a ton of styrofoam, or a half-gram of lead. It depends on how large a sample we are using. To calculate heaviness or density we divide the mass of material in pounds by its volume in cubic inches. Density could also be expressed as grams per cubic centimeter (g/cm3)