INTRODUCTION

Roll-wrapped composite tubes offer unique characteristics that are not achieved through other construction methods. While other ways of producing tubes are usually limited to a single design or layup throughout, roll-wrapping composite tubes give the creator a much greater level of customization with fiber orientation. Because of these different fiber orientations, the finished tube can be engineered to have strength in multiple load paths or strength in specifically desired load paths.

Roll-wrapped composite tubes are ideal components in a variety of industries. From gun barrels to hiking sticks, from the NASA Mars Helicopter to guitar truss rods, wrapped composite tubes can enhance the durability, strength, weight, and style of your product.

In this paper, you will learn the process by which roll-wrapped composite tubes are created, the characteristics of wrapped composite tubes, and the benefits to using wrapped composite tubes.

CONSTRUCTION

The construction of roll-wrapped composite tubes begins with frozen composite prepreg from which flags (a cut piece of prepreg that will be used to make the tube) are cut to a specific size and dimension. This takes place on an automated cutting table to maintain precision over many different flags, ensuring that each finished tube is identical. These patterns are cut to the length, diameter, fiber orientation and wall thickness desired.

After the flags are cut, the uncut prepreg is returned to the freezer to avoid premature curing.

To obtain a specific inside diameter on each tube, a steel tool called a ‘mandrel’ is used. These precision-machined mandrels are prepared with a mold release agent which chemically prevents the prepreg from bonding itself to the metal surface. The release agent allows the tube to extract from the steel tool.

The composite flags are rolled onto the prepared mandrel using a rolling machine, which is designed to roll the flags with perfectly even pressure along the entire length of the tool. The consistent compaction imparted by the table is something that cannot be achieved with human hands.

After this rolling procedure is finished, the prepreg-wrapped mandrel is processed through a wrapping machine to wrap shrink tape around the flags. The mechanical compaction applied by the tape is necessary to obtain the final exterior dimension of the tube. The tape also helps to remove any air within the rolled flags as well as express any excess resin during the cure cycle. Enough resin will remain within the prepreg to transfer the various loads from one fiber to the next, but not enough to add any unnecessary weigh to the final product.

Once the mandrel is wrapped with tape along its entire length, it is ready for curing, a process which takes place in a large oven where many mandrels can cure during the same oven cycle. Because the prepreg flags are still cold, the oven is slowly brought up to curing temperature to allow the resin to fully permeate the fibers.

After the lowest viscosity point is achieved, the resin immediately sets and hardens. The remainder of the oven cycle ensures that the final mechanical and thermal properties of the resin are reached. After this curing cycle is complete, the tube is allowed to cool down to room temperature before being handled.

When the mandrel is cool enough, the tube can be extracted from the mandrel. This is achieved by simply pulling the composite tube away from the steel tool, by hydraulic or pneumatic force is sometimes necessary, depending upon the length and taper of the tube.

The final step to creating a roll-wrapped composite tube is removing the shrink tape by simply pulling it away. This reveals what is almost a finished tube, but a variety of secondary processes can be undertaken to finalize it to the required specifications.

SECONDARY MACHINING PROCESSES

After the tube has cured, you might want to perform a variety of secondary machining processes to turn the tube into a finished product. These secondary processes include:

• Cutting to specification
• Grinding to a specific outer diameter
• Sanding for a bonding surface or to give the tube a matte finish
• Drilling holes
• Milling notches
• Chamfering ends

ADVANTAGES

Composites offer distinct advantages in product design or even re-design. You can immediately reduce the weight of your product and make gains in stiffness, strength, durability, and customization when choosing carbon fiber composites over steel or aluminum.

Stiffness & Strength.

Carbon fiber composites are between 2 and 6 times more rigid than steel and aluminum for a given weight, depending on the orientation of the carbon fibers within the composite and the orientation of the stress. For example, a roll-wrapped carbon tube made of high modulus woven prepreg will be much stiffer than a steel tube of the same weight or just as stiff for a fraction of the weight.

Durability.

Carbon composites have low thermal expansion, high fatigue level, and are corrosion resistant, making them ideal materials for harsh environments. As opposed to metal, wood, or even concrete, carbon fiber composites have near neutral coefficient of thermal expansion, meaning that the material does not expand or contract as the temperature fluctuates. Carbon composites can be used in space, on Mars, on airplanes, in the desert, and other temperature extremes without deforming, depending on what resin is used to bind the carbon fibers. Resins can be made to withstand remarkable temperature fluctuations and resist combustion as well. Carbon fiber composites keep their mechanical properties under dynamic loads, rather than deteriorating slowly over time. This high fatigue level means that carbon composites make great components in systems with active, quantified stresses.

Carbon composites perform well in acidic, basic, or otherwise chemically challenging environments. Additives in the resin used can enhance corrosion resistance.

CUSTOMIZATION

Each roll-wrapped composite tube is designed from the ground up and because of this, each tube can be engineered to have specific, exceptionally customizable characteristics. Here are many ways in which a roll-wrapped composite tube can be customized:

Prepreg fabric can be:

• Carbon fiber, fiberglass, kevlar, or any combination of these
• Resins can have various attributes including temperature resistance,
• UV protection, or impact resistance, among others
• Fabric can be woven, straight, or chopped fibers

Layup of fabric

• The prepreg fabric can be unidirectional or twill and can be cut with or against the fiber direction and flags can be layered over one another during the manufacturing process
• Adjusting the fiber layup controls the characteristics of the tube, which are:
  • Flexibility
  • Weight
  • Impact resistance
  • Hoop strength
  • Wall thickness
  • Appearance

The mandrel can be cylindrical or tapered

• The diameter of the mandrel controls the inside diameter of the finished tube

Secondary processes:

• Centerless grinding to a perfectly uniform outside diameter
• Hole drilling
• Notching
• Sanding for a bonding surface
• Milling to specification
• Cutting to specified length

GLOSSARY

Automated Cutting Table

Machine using CAD/CAM software to create exact cuts on the prepreg fabric.

Composite

Made up of two or more parts or elements. In this case, a composite refers to a fiber (carbon) or filament (fiberglass) combined with a resin and cured to make a finished product – a carbon or fiberglass roll-wrapped
tube.

Cure

The process of heating the tube in an oven until the resin hardens to its final state.

Flag

The cut pattern of prepreg material. Often, the flags are layered over one another in a series of “wraps” or “plies.”

Mandrel

Steel tool machined to an exact outer diameter and taper around which composite flags are wrapped.

Prepreg

Carbon or other fiber fabric infused with resin and then frozen. The material is thus “PRE-imPREGnated” with resin.

Release Agent

Chemical spread in a thin layer over the mandrel, designed to allow the cured composite tube to release from the steel mandrel.

Rolling Machine

Pneumatic machine designed to apply consistent force over the length of the tube to evenly roll prepreg flags around a mandrel.

Shrink Tape

Cellophane tape that delivers compaction to the wrapped carbon tube, keeping the prepreg wrapped around the mandrel during the cure cycle.

Wrapping Machine

Wraps compaction tape with a designated pressure and distance between wraps over the length of the tube.

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