IMM Focus: Automotive:Long-fiber-reinforced nylon comes through in the clutch

Automotive clutch slave cylinders are moving inside the transmission, and changing from metal to plastics. It’s hot in there, and corrosive, too. LFRT nylon makes it happen.

Metal replacement has long been a prime focus of materials suppliers, particularly in automotive applications. Is it slowing down? On the contrary, it is getting more interesting. The applications increasingly are under the hood, even on the engine—places where plastic “can’t go.” Now we have one inside the transmission, which until recently would have been extremely hostile territory for plastics.

Hydraulic actuators have been used for automotive brake and clutch systems for some years now. Unlike mechanical systems that require more or less direct linkage, hydraulic tubing can be routed over a distance to apply force where needed. The external slave cylinder that used to operate the clutch is now being replaced in many applications by a concentric slave cylinder that can reduce part count—and weight—by eliminating release levers, mounting brackets, and pivots. It also increases travel and load efficiency, eliminates side load on the bearing and clutch, and simplifies assembly.

Moreover, the concentric slave cylinder can be mounted inside the transmission, thereby freeing up space. However, inside the transmission, the slave cylinder has to withstand higher temperatures than does an external unit, and it is exposed to hydraulic fluids and lubricants at those temperatures. For the cylinder to function efficiently and reliably over time, tight dimensional tolerances must be held—regardless of temperatures and chemicals.

Given such a tough environment, metals have been used in both master and slave cylinder assemblies, but things are changing. Automotive Products (USA) Inc. of Auburn Hills, MI, which supplies clutch actuation systems to vehicle makers on every continent where cars are made, led the way to internal concentric slave cylinders. The company even owns a trademark on it: CSC. If you look in its catalog, you will see that it is now offering its CSCs in either engineered thermoplastics or diecast aluminum.

Strong, Tough, and, Well, Pretty

The engineering thermoplastic that Automotive Products chose for this tough job is Ticona’s Celstran LFRT (long-fiber-reinforced thermoplastic). More specifically, the grade is a black nylon 6/6 with 50% long-glass-fiber reinforcement. Ticona has been advocating LFRT materials in general for metal replacement because of their high stiffness and low weight, as well as a specific modulus that can often exceed that of lightweight metals such as aluminum. In addition, nylon as a base resin offers excellent resistance to the automotive fluids and can handle high temperatures, such as in the CSC.

Dimensional stability is high because the fibers are randomly dispersed, and since they are completely wet out in the polymer matrix and have fewer ends, an excellent glossy surface is no problem. A level of long glass fibers as high as 50% provides almost metal-like thermal expansion coefficients. Ticona says that if the nylon is carefully burned off a molded part, the glass fiber skeleton left behind retains its integrity, showing the multidirectional orientation that supports uniform shrinkage and suppresses warpage.

The surface finish of the Celstran CSC improves on the diecast metal version and helps extend the cylinder’s seal life. Celstran LFRT PA66-GF50-02 passed all long-term and burst-testing requirements, aided by a room temperature tensile strength of almost 50,000 psi, high fatigue strength, and superior creep resistance. Tensile strength remains as high as 22,000 psi up to 300F (149C), enabling the slave cylinder to stand up to the temperatures in the clutch mechanism. A density of 1.56 for the 50% LFRT nylon helps reduce weight, and the ability to injection mold a complex part contributes to cost savings over diecast and machined metal.

The grade used in this application is part of a range of LFRT products from Ticona. Celstran LFRT technology may employ glass, stainless steel, carbon, aramid, or other reinforcing fibers, depending on the use. Stainless fibers can be used for static dissipation or EMI/RFI shielding, and carbon or aramid fibers can provide even lower density than glass. Hybrid fiber compositions are available, and typical fiber lengths are 10 to 15 mm. Feedstocks range from polypropylene and nylon to polyphenylene sulfide, acetal copolymer, polyester (PBT), high-density polyethylene, and polyurethane.