By this method it is possible to produce very effective continuously GRP profiles of various cross section shape with high reinforcement volume (up to 80%). Reinforcement, mostly glass, but also carbon or other rovings, in some cases combined with fabric or mat tapes giving the transversal strength, is pulled through the resin bath, impregnated and subsequently the excess resin is squeezed out. Impregnated reinforcement is then pulled into the dies. Cross section of die matches the final profile shape. Modification of this process is pressure impregnation of dry reinforcement in the first section of die.
Curing takes place in the die either by means of conductive heat (the dies are heated electrically or by heat media) or by heat caused by means of high frequency field. Cured composite profile is pulled by hydraulic clamps or towing belts with controlled speed and it is cut by the saw to the required length on the end of the line.
Pulforming is the variant of simple pultrusion. It is semicontinuous process – pulled out impregnated reinforcement is placed into two part heated mould where the final shape is reached and composite is cured. Leaf springs with changing cross section along the length are manufactured by this method as an example.
Reinforcements: Primarily glass rovings, less often carbon rovings, weaved tapes and bands made of various fibres or mats with heavily solved binder, surfacing mats, sometimes printed.
Resins: Low viscosity polyester, vinylester and epoxy resin systems quickly cured by elevated temperature (80-160°C). The matrix usually contains internal release agent, additives for better surface smoothness and better pigmentation, pigments and fillers for instance to reach fire retardation properties.
Pultrusion is suitable for continual production of profiles from very thin and simple ones (1 mm reinforcing strips for hockey sticks) to complex shaped and large size ones (width and height in the range tents of centimeters, wall thickness up to 15 mm, rods and tubes of various diameter.
Other continuous methods
Flat laminate sheets or corrugated (roof or decorative) sheets are also produced using continuous lines. Starting material can be either weaved reinforcements and mats or chopped strands spread onto supporting foil and subsequently impregnated in resin bath. The upper release foil is then applied and material is pulled into shaping and heated curing chamber. Pulling and cutting sections make the end part of the line.
Reinforcements: Rovings, mats, fabrics, possible their combination, mainly of glass fibres.
Resins: Polyester, rarely epoxy resins