Pultrusion Technology Realizes Commercial Production of Automotive Parts

 

Pultrusion is one of the most traditional forming processes for thermoset composites, and today it is used to produce products ranging from utility poles to window frames as well as ladders, rebar, and wind turbine components. Traditional pultrusion is easily automated with low labor and can produce highly structured parts at reasonable cost and speed, and can currently extrude almost any shape (hollow or solid, symmetrical or asymmetrical) through a die.

 

However, that shape must have a constant cross-section/thickness. In addition, unlike thermoplastic extrusion or extrusion of metal tubing (where pressure and heat are applied to the finished parts during or after production to bend them into new shapes), it is difficult to produce anything other than straight, linear contours using pultrusion.

 

That is starting to change, thanks to Thomas GmbH + Co. Technik + Innovation KG (TTI, Bremen, Germany), which has developed a patented process to overcome many of the limitations of traditional pultrusion.

 

In fact, the first curved pultruded automotive part went into commercial production with the launch of the 2020 Chevrolet Corvette sports car by General Motors (Detroit, Michigan, U.S.). Tier 1 supplier Shape Corp. (Grand Haven, Mich., U.S.) is using the TTI radius pultrusion kinematic molding process of carbon fiber-reinforced polyurethane-acrylate to produce curved rear bumper beams for automotive applications.

 

Shape Corp. is the first company in the Americas to have a TTI radius pultrusion line

 

Mobile vs. Stationary

In traditional linear pultrusion, a reciprocating pulling system with grippers is used to pull dry reinforcements from their creels through a bath where they are impregnated with a liquid thermosetting resin. Next, the impregnated reinforcements are pulled into a stationary heated die where they are contoured and fully cured. After leaving the die, the moving profile cools (still pulled by the pull-out unit) and is then cut to length and packaged.

 

Initially, fiber reinforcement was strictly limited to unidirectional rovings, that is, continuous glass, carbon, or other fibers oriented at 0 degrees to the axis of pull. Over time, however, a variety of multiaxial fabrics (woven and non-crimp fabrics (NCF)) have been incorporated into pultruded profiles, allowing for a variety of fiber orientations without deformation/fiber movement.

 

Although a large number of applications use polyurethanes, nearly any thermosetting resin system can be used to impregnate reinforcements in pultrusion. Pultrusion has even been improved in recent decades to allow the use of pre-polymerized thermoplastics.

 

Many features of conventional pultrusion are reversed in TTI’s radius pultrusion process, but the most important aspect to understand is that with radius pultrusion, the die is not fixed. Instead, it moves and pulls the material to produce either a linear constant cross section or a constantly curved profile.

 

Not only does the die or series of dies move back and forth over the profile, shaping and solidifying the material, both along the track (in this case, the profile moves in one direction and the die moves in both directions) but the die also pulls the dry reinforcement material through the resin pool and the solidified profile, toward the clamps and cutters. To produce a linear cross section, both the die and profile move along the straight portion of the track. To produce a curved cross section, the die and profile move along the curved portion of the track.

 

“In this technology, the die effectively replaces the reciprocating puller, which is replaced by a fixed gripper that opens or closes but does not move,” explains Sebastian Mehrtens, TTI’s head of sales. “When the gripper has reached the predetermined length and is ready to be cut, the gripper closes and supports the solidified profile, otherwise it remains open and moves the profile towards the cutting unit.” To reduce the risk of pulling fibers out of the hot material, the heated die is chrome-plated.

 

In addition, TTI’s radius pultrusion equipment allows the machine axis to rotate horizontally or vertically, making it possible to produce profiles with continuous curves (hoop structures) and constant 2D or 3D radii, or alternating straight and curved sections, or, more obviously, profiles that look like coil springs. “While the curved sections occur parallel to the long [pulling] axis, the rotation and linear [pulling] movement are perpendicular, that is, they curve up or down like a bumper,” explains Jens Bleke, head of composite materials and new technologies at TTI. “On the other hand, when the curve occurs parallel to the short [transverse pull] axis, the rotation and linear motion are horizontal, meaning it bends left or right like the window frame of a glass-top skylight system.”

 

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, and in the case of helical spring-like structures, the maximum radius is currently 40 mm. A few years ago, the company stated that future work in radius pultrusion would focus on producing curved wave spring structures and possibly moving into thermoplastic profiles via reaction polymerization

 

Advantages and Application Possibilities

 

There are many advantages to radius pultrusion:

 

It runs at about the same speed as conventional linear pultruders (up to 2 m/min).

 

It produces almost no scrap.

 

It can produce solid or hollow structures with high fiber volume fractions and 2D or 3D curvatures.

 

There is almost no shrinkage; the profile exits with low residual stresses and is able to hold tight tolerances.

 

Many different resin systems are available – from polyurethanes and epoxies to vinyl esters and unsaturated polyesters.

 

A wide range of reinforcement types are available, including unidirectional rovings as well as various fabrics – with or without veils for improved surface finish and drape.

 

In addition to the straight or curved bumper beams, which are already in production, other potential automotive applications include roof bows, driveline and chassis components, anti-roll bars, door side impact beams and coil springs. Target markets also include building, construction, agriculture, marine, infrastructure, medical components and sporting goods. TTI reports that other commercial applications for its technology include use as lost cores in the production of bicycle rims.

 

TTI currently manufactures and sells radius pultrusion equipment, licenses and produces conventional linear and radius pultruded parts for customers. TTI’s latest innovation is its pullCUBE system, which was originally set to be launched at the 2020 JEC World Congress and will now be demonstrated at a live online conference in May. Arguably the world’s shortest pultrusion machine at 3.5 meters (compared to conventional pultrusion machines, which can be 15 to 20 meters longer), the compact system can produce both straight and curved profiles on the same machine and is equipped with dies, clamping units and cutting saws.

 

“Being 75% shorter than conventional pultrusion machines, pullCUBE consumes significantly less energy during operation and generates 75% less dry fiber waste during initial setup,” adds Bölke. “Also, unlike conventional pultrusion systems, which require purging of the material approximately every 15-30 minutes during production, our new machine requires no purge cycles at all during normal production runs.

 

 

The latest technology from Thomas GmbH + Co. Technik + Innovation KG is the company’s pullCUBE compact pultrusion machine, which is said to be the world’s shortest pultrusion system. At 3.5 meters long, it is 75% shorter than conventional pultrusion lines that can be 15 to 20 meters long, and can produce straight (conventional) or curved (radius) pultruded parts.

 

The unit’s small size offers significant savings in energy, dry fiber waste, shipping costs, and warehouse space. It does not require regular cleaning throughout the production process, further reducing resin and fiber waste; and the fully enclosed design helps improve worker safety.

 

In addition, the small system is easy to transport, requiring only a forklift to move it, is quicker to install than conventional lines, and requires much less space on the factory floor. “This could have interesting benefits in certain applications,” Mehrtens says. “For example, a machine could be installed near a bridge renovation to produce structural elements as needed.

 

Or it could be set up while a new highway is being laid and produce rebar that can be cut to custom lengths on the job site.” Another advantage of the pullCUBE is worker safety, as the equipment is fully enclosed and operators cannot burn their hands on hot mold surfaces, get pinched by hoes or get cut by saws.