Pultrusion

Pultrusion is used for producing continuous lengths of reinforced composite with a constant cross-section. During pultrusion a continuous pulling device (such as grippers or pads) pulls the material in line through the process. Most pultrusion processes run horizontally, however specialised vertical processes also operate.

Although the mechanical properties are strongest in the machine direction (due to reinforcement fibre alignment) they can be improved in the cross wise direction by the introduction of continuous filament mat or fabrics. See also Pullwinding.

allnex products commonly used:

  • Resin
  • Catalyst
  • Additives (pigments)
  • Single end roving
  • Fabrics
  • Continuous filament mat
  • Mould release

Pull Winding

Pull winding is a variation of pultrusion where winders introduce fibres in a cross wise direction (out of alignment with the machine direction), hence increasing the out of plane mechanical properties of the final product.

Steps

  1. A combination of fibre reinforcements (typically continuous roving, continuous filament mat, cloth or veil) are pulled from creels into either a catalysed resin bath or injection die where it is thoroughly wet out by the resin. Alternatively catalysed resin may be injected at the front of the die.  The resin may contain fillers and/or specialised additives.
  2. The resin saturated reinforcement then proceeds through bushings where excess resin is removed.
  3. The material passes through a heated steel forming die, where it forms into the exact geometry of the die, while the heat of the die initiates the gelation and curing of the finished part.
  4. The material is cut in line to length.  Most resins and reinforcements can be used with pultrusion.  The curing system for polyester and vinyl ester resin usually consists of highly reactive 'kicker' peroxide for initial cure, in combination with medium or low reactivity peroxides to give a more gradual cure. 

Hints & Comments

  1. Parts produced by pultrusion have a constant cross-section, although some variation in cross-section is possible with variations of the process.
  2. Since parts are produced with straight fibres and glass content as high as 75%, they have very high mechanical properties in the longitudinal direction.
  3. The resin system can be enclosed to limit volatile emissions.
  4. Once the intensive set-up is complete, Pultrusion is a medium speed, economical process with a relatively low labour component.  
  5. It is best suited for high volume (long) continuous runs of the same product.
  6. The disadvantages of pultrusion are that parts are limited to constant or near constant cross-section, and the equipment and die costs can be high.