The materials can be tailored to meet any performance requirement. No single inherent set of properties rules. Engineers can modify the physical and chemical characteristics by specifying different polymer formulations or different proportions of materials. For example, high glass fiber reinforcement structures produce maximum physical strengths; high resin content structures produce maximum chemical resistance. Resin characteristics and performance can also be modified with the addition of a number of additives, such as fire retardant, thixotropes, pigments, vapor suppressants and others.
These elements are combined through the pultrusion process, a continuous method for the manufacture of products having a constant cross section, such as rod stock, structural shapes, beams, channels, pipe, tubing, fishing rods and golf club shafts, in addition to window frame profiles. Pultrusion produces profiles with extremely high fiber loading, thus pultruded products have high structural properties.
As the name suggests, pultrusion involves pulling the raw materials (rather than pushing as is the case in extrusion) through a heated steel forming die using a continuous pulling device. Two distinct pulling systems are used: a caterpillar counter-rotating type and a hand-over-hand reciprocating type.
To produce fiberglass profiles, continuous glass filaments are formed by drawing molten glass resting on platinum/rhodium bushings through thousands of holes of the appropriate fiber diameter (5 to 25 microns), quenched, sized and wound into strands of either 102 or 204 filaments. The sizing acts as a coupling agent to bond the resin to the glass filament during resin impregnation.
The resin impregnator then saturates (“wets out”) the reinforcement with a solution containing the resin, fillers, pigment and catalyst plus any other additives required. The interior of the resin impregnator is carefully designed to optimize the complete saturation of the reinforcements.
As the reinforcements are saturated with the resin mixture in the resin impregnator and pulled through the shaping die, the gelation (or hardening) of the resin is initiated by the heat from the die and a rigid, cured profile is formed that corresponds to the shape of the die. In certain applications an RF (radio frequency) wave generator is used to preheat the composite before it enters the die. When in use, the RF heater is positioned between the resin impregnator and the preformer. RF is generally only used with an all roving part.
On exiting the resin impregnator, the reinforcements are positioned for the eventual placement within the cross section form by the preformer. The preformer is an array of tooling which squeezes away excess resin as the product is moving forward and gently shapes the materials prior to entering the die. In the die the thermosetting reaction is heat activated (energy is primarily supplied electrically) and the composite is cured (hardened).
On exiting the die, the cured profile is pulled to the saw for cutting to length. It is necessary to cool the hot part before it is gripped by the pull block (made of durable urethane foam) to prevent cracking and/or deformation by the pull blocks.
Pultruded shapes can be chemically welded together, resulting in strong, yet light weight structures. Newer technologies have made possible the continuous production of curved FRP profiles and the inline pultrusion of very wide panel structures. Continuing new developments are improving physical properties while also enabling increased line speeds.
The combination of engineered materials and the pultrusion process yields a number of well-known benefits for fiberglass structural shapes:
- Design Flexibility. Pultruded profiles are pigmented throughout the thickness of the part and can be made to virtually any desired custom color. Special surfacing veils are also available to create special surface appearances such as wood grain, marble, granite, etc.
- High Strength. Stronger than structural steel on a pound-for-pound basis.
- Lightweight. Pultruded products are easily transported, handled and lifted into place. Total structures can often be preassembled and shipped to the job site ready for installation.
- Corrosion/Rot Resistant. Pultruded products will not rot and are impervious to a broad range of corrosive elements. This feature makes pultrusions a natural selection for corrosive environments such as coastal areas.
- Non-Conductive. Glass reinforced pultrusions have low thermal conductivity and are electrically non-conductive.
- Electro-magnetic Transparency. Pultruded products are transparent to radio waves, microwaves and other electromagnetic frequencies.
- Dimensional Stability. The coefficient of thermal expansion of pultruded products is less than that of most metals.
- Low Temperature Capabilities. Glass fiber reinforced pultrusions exhibit excellent mechanical properties at very low temperatures, even down to -70°F. Tensile strength and impact strengths are actually greater at -70°F than at +80°F.