In the Plastics News story titled “Valve covers latest push from industry” (March 29, Page 1), reporter Rhoda Miel states: “The plastics industry is … trying convince North American automakers to use thermoplastics in valve covers in place of the existing metal components.”
In a virtual sales pitch for engineered nylon, PN goes on to substantiate its headline with quotes from both producers and consumers of nylon materials. While mentioning the use of nylon in applications such as intake manifolds, PN fails to illustrate the environmental differences between applications (manifolds vs. valve covers). Nor does PN touch upon the complications forecasted with thermoplastics and the impending move to hotter-running, 42-volt engines. Most absent from PN's effort, however, was input from those that have supplied the valve cover industry for years: the manufacturers and processors of fiber-reinforced thermosets.
Bulk molding compound is the only composite material that has met the automotive industry's performance/durability benchmark for valve covers (10 years/150,000 miles). BMC has been molded into more than 60 million valve covers without one material-related failure. Everyone knows that manufacturers and processors of nylon maintain a tremendous marketing presence in Detroit. These people have certainly presented their offerings to engineers responsible for valve cover design and material specification. So I would like to ask: Why are fewer than 1 percent of the valve covers made in the United States nylon and 37 percent thermoset? The answer: properties and price!
In metal-replacement applications such as valve covers, the rigidity or modulus of the material used is critical. Valve cover test protocol often requires durability testing at 150° C (302° F). BMC maintains flexural modulus properties greater than three times those displayed by nylon when tested at these temperatures. Nylon covers often require more mounting bolts to offset this modulus loss. These bolts are relatively expensive and increase general assembly costs when compared to BMC.
Material shrink is another important factor when considering a sealed engine system. Nylon shrinks several times more than BMC. Nylon also displays variation in shrink transverse to flow. The manufacture of long, flat parts (such as valve covers) is difficult when materials behave in this manner. Adjusting for these shrink idiosyncrasies contributes to the expense of tooling revisions and overall project start-up. BMC eliminates these shrink issues.
Nylon displays hydroscopic tendencies and will absorb water and engine fluids more than BMC. The physical properties of nylon are affected by this moisture absorption. Additionally, post-consumer recycling of nylon becomes less likely if engine fluids have been absorbed by the substrate. Recyclability has been touted as a major selling feature for nylon over thermoset. BMC 685 (used in GM's 3800 valve cover) contains up to 9 percent reground BMC product. Thermo-sets are being recycled — don't be misled.
The reason less than 1 percent of the valve covers produced in the United States are nylon is not because automotive engineers have refused to study the option. Rather, U.S. automotive engineers refuse to compromise the integrity of their sealing systems because they understand the limitations that accompany nylon materials. Engine designers will not compensate for a material's shortcomings (especially when the more advanced polyamides cost three times their BMC equivalent). The 42-volt engine is on its way and with it even greater operating temperatures. For price and performance, BMC will continue to be the answer.
Bulk Molding Compounds Inc.
West Chicago, Ill.