PULTRUSION

The pultrusion process is a continuous production method in which reinforcing fibres or mats are drawn through a heated die and cured. The shaping takes place here whilst the resin is cross-linking. The pultrusion process ensures maximum flexibility in the design of GRP profiles. Parts up to 1200 mm wide and 350 mm high can be produced. Due to the continuous, endless production process, length dimensions are limited only by shipping or

transport capabilities. The roving spools and glass mats are arranged on a spool rack. The roving can be drawn off from either the inside or the outside. When drawn off from the outside via ball-bearing-mounted spool holders with mechanical braking devices, precise roving guidance with pre-tensioning is ensured. When drawn off from the inside, the rovings are guided immediately behind the spool rack via a pair of fixed rods arranged in a staggered configuration.

Friction applies pre-tension to the roving; this breaks up the sizing and fans out the rovings. If the rovings are drawn off without pre-tension, they become twisted, which hinders resinimpregnation and a loss of strength in the finished profile. For feeding flat structures, such as non-wovens, mats or fabrics, a further rack is used. The reinforcement material, which is still dry, is gradually brought together by several deflection units.

To this end, the various materials are guided through a series of plates with perforated patterns arranged one after the other. These perforated patterns progressively approximate the desired profile shape. This ensures that the various textile structures, without coming into contact with one another, are arranged in the correct order; once the different reinforcing materials are in the correct arrangement, they pass through the impregnation unit. They are then brought together into their final position and fed into the mould.

A distinction is made between three different impregnation processes. In the tank process, the reinforcing fibres are introduced into the resin impregnation bath from above and drawn out. The actual impregnation takes place via several deflection rollers in the resin bath. This process, which is also commonly used in winding technology, is arguably the most widespread impregnation method for the production of pull-moulded profiles, particularly those with simple cross-sections.

The pull-through process is used primarily in the manufacture of profiles with complex cross-sections. The reinforcing material is guided through the resin bath without any deflection. Perforated plates, matching the profile shape, are fitted at the inlet and outlet sides of the bath. The reinforcement materials are fed into and out of the resin bath through these openings. The resin that flows out during this process is collected in a trough and pumped back into the resin bath.

In the injection process, the rovings and mats are fed through the impregnation mould without being diverted. This mould has the shape of the profile to be produced and widens internally. The reaction resin is injected into this cavity from both sides at a pressure of 2–15 bar. To ensure better distribution of the resin and to prevent so-called ‘infeed’ – which refers to guide plates that feed the impregnated fibres and mats into the mould in an orderly manner –

At this stage, excess resin is removed from the reinforcing materials and returned to the resin bath. After the reinforcing materials have been impregnated with resin, they are drawn through a heated, moulded steel die. Several separately controllable heating and cooling zones are integrated into the mould. The resin-impregnated material is heavily compacted at the mould inlet for shaping. The resin enters the mould in a cold state.

The rise in temperature within the steel mould, which is maintained at a constant temperature, triggers the resin’s reaction. The thermosetting matrix system becomes gel-like. During this phase, the profile is shaped, calibrated to its final dimensions and cured. The energy released during curing must be dissipated via a cooling section. Plastics are very poor heat conductors. For this reason, it must be ensured – particularly in the case of thick-walled profiles – that the temperature inside the profile does not rise to an impermissibly high level as a result of the exothermic reaction, thereby compromising the mechanical properties.

The most important process parameters in this phase are the drawing speeds and the curing temperature. Other factors influencing the quality of the semi-finished products include the shrinkage behaviour of the matrix systems, the resin viscosity and the reaction kinetics, as well as the fibre content, which can be as high as 70 per cent by mass. The pull-out device is responsible for pulling the profile out of the mould. Pairs of grippers are usually used for this purpose, which alternately grip the profile, pull it out and then return to the starting position (reciprocating pull-out).

The pulling forces generated as the laminate is drawn through the impregnation bath and the mould can be very high; therefore, there must be a sufficient number of rovings present in the laminate. The rovings effectively act as tension members. The tensile forces depend primarily on the cross-sectional area and circumference, the target fibre content, the geometry, the position and extent of the gel phase within the mould, the drawing speed and the resistance caused by deflection points.

The profiles are cut to the desired length using a synchronously driven saw. Resin (gel time) Reinforcing material