Pultrusion is one of the most traditional molding processes for thermoset composites. Today, it is used to produce products ranging from telephone poles to window frames and ladders, steel bars and wind turbine components. Traditional pultrusion is easy to automate with low labor, and can produce highly structured parts at a reasonable cost and speed. Currently, almost any shape (hollow or solid, symmetric or asymmetric) can be extruded through the mold. However, the shape must have a constant cross-section / thickness. In addition, unlike thermoplastic extrusion or metal pipe extrusion (pressure and heat are applied to finished parts during or after production to bend them into new shapes), pultrusion is used to produce anything other than straight and linear profiles The materials are very difficult.
Thanks to Thomas GmbH + Co. Technik + Innovation KG (TTI, Bremen, Germany), this situation has begun to change. The company has developed a patented process to overcome many of the limitations of traditional pultrusion. In fact, after the introduction of the 2020 Chevrolet Corvette sports car by General Motors (Detroit, Michigan, USA), the first curved pultruded car parts were already in commercial production. First-tier supplier Shape Corp. (Grand Haven, Michigan, USA) is using carbon fiber-reinforced polyurethane-acrylate TTI radius pultrusion motion forming technology to produce curved rear bumper beams for automobiles.
Move and fix
In traditional linear pultrusion, a reciprocating traction system with grippers is used to pull dry reinforcement from its creel through a bath and immerse it in liquid thermosetting resin. Next, the impregnated reinforcement material is drawn into a fixed heated mold, where the contour is shaped and fully cured. After leaving the mold, the moving profile is cooled (still pulled out by the pull-out unit), then cut to a certain length and packed.
Initially, fiber reinforcement was strictly limited to unidirectional rovings, that is, continuous glass fibers, carbon fibers, or other fibers oriented at 0 degrees to the tensile axis. However, over time, a variety of multiaxial fabrics (woven and non-crimped fabrics (NCF)) have been added to pultruded profiles to achieve multiple fiber orientations without deformation / fiber movement. Although polyurethanes are used in a large number of applications, almost any thermosetting resin system can be used as a reinforcement material in dip pultrusion. In recent decades, pultrusion has even been improved, and pre-polymerized thermoplastics can be used.
Many features of traditional pultrusion are reversed in TTI's radius pultrusion process, but the most important aspect to understand is that for radius pultrusion, the mold is not fixed. Instead, it moves and pulls the material to produce a linearly constant cross-section or a constant curved profile.
Not only does the mold or series of molds move back and forth on the contour to shape and solidify the material, but both advance along the track (in this case, the contour moves in one direction and the mold moves in both directions) but The mold also pulls the dried reinforcement material through the resin melt pool and the solidified profile toward the fixture and cutter. To produce a linear cross-section, both the mold and the profile move along the linear part of the track. To create a curved cross-section, the mold and contour move along the curved portion of the track.
Sebastian Mehrtens, head of sales at TTI, explained: "In this technology, the mold effectively replaces the reciprocating puller, and the puller is replaced by a fixed gripper that opens or closes but does not move." "When the clamp When the predetermined length is reached and ready for cutting, the gripper will close and support the solidified profile, otherwise it will remain open and move the profile toward the cutting unit. "In order to reduce the risk of pulling fibers from the hot material, heat the mold Chrome treatment.
In addition, TTI's radius pultrusion equipment allows the machine axis to rotate horizontally or vertically, which can produce contours with continuous curves (hoop structure) and constant 2D or 3D radii, or alternating straight and curved sections, or more obviously The outline looks like a spiral spring. "When the curved part appears parallel to the long [pull] axis, the rotation and linear [pull] movement are vertical, that is, they bend up or down like a bumper," explains Jens Bölke, TTI Composites and New Technical director. "On the other hand, when the curve occurs parallel to the short [horizontal] axis, the rotation and linear motion are horizontal, which means that it bends left or right like a window frame of a glass roof skylight."
Currently, the main design limitation of radius pultrusion is that it cannot yet produce profiles that are both curved and have variable cross-sections. Some other practical limitations are that although there is no theoretical limit to the maximum radius, the maximum TTI diameter produced is 1500 cm. In the case of a coil spring-like structure, the maximum radius is currently 40 mm. A few years ago, the company pointed out that the future work of radius pultrusion will focus on the production of curved wave spring structures, and possibly into thermoplastic profiles through reactive polymerization.
Advantages and application possibilities
Radial pultrusion has many advantages:
· Its running speed is about the same as traditional linear pultrusion machine (up to 2m / min).
· It produces almost no waste.
· It can produce solid or hollow structures with high fiber volume fraction and 2D or 3D curvature.
· There is almost no shrinkage; the profile exits with low residual stress and can maintain tight tolerances.
· Many different resin systems can be used-from polyurethane and epoxy to vinyl ester and unsaturated polyester.
· Various types of reinforcing materials can be used, including unidirectional rovings and various fabrics-with or without veil to improve surface finish and drape.
In addition to the straight or curved bumper beams that are already in production, other potential automotive applications include roof arches, transmission systems and chassis components, anti-roll bars, door side impact beams and coil springs. Target markets also include construction, construction, agriculture, marine, infrastructure, medical components and sporting goods. TTI reports that other commercial applications of its technology include use as a lost core when manufacturing bicycle rims.
TTI currently manufactures and sells radius pultrusion equipment, is licensed and produces conventional linear and radius pultrusion parts for customers. TTI ’s latest innovation is its pullCUBE system, which was originally scheduled to be launched at the JEC World Congress in 2020 and will now be demonstrated at a real-time online conference in May. This compact system is arguably the shortest 3.5-meter pultrusion equipment in the world (compared to traditional pultrusion machines, it can be stretched by 15 to 20 meters). The compact system can produce straight and curved shapes on the same machine Contour and equipped with molds, fixture units and cutting saws. “Because it is 75% shorter than traditional pultrusion equipment, pullCUBE consumes significantly less energy during operation, and the dry fiber waste generated during initial setup is reduced by 75%,” Bölke added. "In addition, unlike traditional pultrusion systems, the material needs to be purged approximately every 15-30 minutes during the production process, and our new equipment does not require a purge cycle at all during normal production operation."
The latest technology of Thomas GmbH + Co. Technik + Innovation KG is the company's pullCUBE compact pultrusion machine, which is said to be the shortest pultrusion system in the world, with a length of 3.5 meters and a length of 15 to 20 than that of Coco Rice's traditional pultrusion line is 75% shorter and can produce straight (traditional) or curved (radius) pultruded parts. The small size of the device can significantly save energy, dry fiber waste, transportation costs and warehouse space. It does not require regular cleaning throughout the production process, which further reduces the waste of resin and fibers; the fully enclosed design helps improve worker safety.
In addition, this small system is easy to transport, can be moved using only forklifts, is faster to install than traditional production lines, and requires much less space on the factory floor. "In some applications this may bring interesting benefits," Mehrtens said. "For example, a machine can be installed near the refurbishment of a bridge to produce structural elements as needed. Or, it can be set up when laying a new highway, and it can produce reinforcing steel, which can be cut to a custom length on the job site." One advantage is worker safety, because the device is completely enclosed, and the operator will not be hot on the hot mold surface, will not be caught by the child or cut by the saw.