Designing Successful Rocker Covers in PA66

Transcription

1 Abstract Rocker covers (or valve covers) historically have been made out of metal. This paper will discuss the history, challenges and opportunities of designing successful rocker covers in nylon 66. by Charles Taylor, Solutia, Inc. Automotive Development Specialist Charles Taylor holds a bachelor of science in plastics technology from Eastern Michigan University. TM Introduction Rocker covers were first used in the 192s and 193s at the advent of overhead valve engines (Figure 1). These early models were typically either stamped steel or cast in aluminum or iron. In the 193s and 194s, automotive design and engineering began to standardize across the industry. Almost all rocker covers during this period were made out of stamped steel, and designs changed very little until the 198s. The one notable exception to the stamped steel trend involved overhead cam (OHC) and double overhead cam (DOHC) engines. Because of their more complex designs, a stamped steel rocker cover could not be formed to meet all the contours, and cast aluminum was most often used. In the early 198s, the first nylon 66 rocker covers were introduced. All major OEMs were working with nylon 66 rocker covers at the time, and nearly all had incorporated the technology into one of their engines. Still part of Monsanto during this period, Solutia used Vydyne R4G a nylon 66 with 4 percent glass fiber and mineral fill that is still available for rocker cover design today. The initial design was in partnership with AMC now owned by Chrysler (Figure 2), and went into production as part of the six-cylinder engine on the CI. Weight reduction was a major goal of the partnership, and the Vydyne R4G reduced the weight from 3.91 pounds (with stamped steel) to 1.96 pounds. The fact that this part and many others were direct copies of their metal predecessors proved detrimental to these Above, Figure 1. Below, Figure 2. 1

2 early PA66 rocker covers. The elevated temperatures and creep of these materials allowed the seals to relax, causing oil leaks and, in extreme cases, fires and recalls. As a result, by 1986, nylon rocker covers were all but absent from the market. The concept of the plastic rocker cover did not die, however. The next stage of development involved bulk molding compound/sheet molding compound a material that minimized creep and allowed designs to be further revised. In the mid-199s, renewed effort was put into the development of the PA66 rocker cover. Learning from past mistakes made success possible and led to today s startling growth in the use of PA66 for this application. Part Function Though some may have unique characteristics, all rocker covers share some common functions. The first and most basic of these is to cover and seal the cylinder head, keeping the oil in and contaminates such as dirt and moisture out. It sounds like a simple task, but this aspect can often be the most challenging. The second function rocker covers perform is oil and air separation. As the engine runs, a mist of oil forms. The cool inner surface of the rocker cover collects this mist, causing the oil to condense and run back down into the oil sump. The rocker cover is also responsible for crankcase ventilation. The movement of the pistons within the cylinders builds up pressure in the engine. If left unchecked, this pressure will cause the various seals to leak, lowering the efficiency of the engine. To prevent this, a tube connects from the rocker cover to the intake tract to combust the vented air. The tube can be connected with a hose bard, screwin fitting or quick connector. A baffle or filter within the rocker cover is used to minimize oil transfer to the intake tract. Serving as the oil fill is another function of the rocker cover. The mechanism can be as simple as a hole that the oil fill cap screws into, or it could be a more complex fill tube. The rocker cover also serves as a sensor mount, containing sensors for cam position and cam timing. Of course, the inclusion of these sensors depends on the complexity of the engine. Design Considerations A variety of considerations must be taken into account when designing PA66 rocker covers. The following paragraphs will discuss materials-related considerations, sealing, noise vibration harshness and head design all of which impact successful rocker cover design using PA66. Material-Related Considerations The characteristics of the chosen material are the first major design considerations. PA66 is a thermoplastic; therefore, its properties vary with the environment. The use of reinforcements can reduce the effects of these environmental changes, but they will still occur to some extent. The most important of these environmental considerations is temperature. A typical rocker cover material, such as Vydyne R533H (PA66, 33GF), will lose half its strength as it moves from 23 C to 15 C. This reduction in properties is normal for all polymeric materials and should be taken into account at the beginning of the design process. Two other environmental considerations are aging and oil contact. The stabilizer system in products such as Vydyne R533H is such that the properties do not change significantly after long-term exposure at elevated temperatures. Nylon 66 is very stable in oil as well, but it does have a plasticizing effect. This will typically reduce the properties 1 percent to 2 percent depending on the specific exposure. Creep is normally experienced as deformation under load or stress relaxation. This is a major concern for clamping and bolt loads and becomes more of a factor as temperature is applied as well. Material-related considerations should be addressed early in the design phase. When putting information into the finite element analysis, properties can be considered at adjusted levels. If the data sheet properties are used without taking the adjusted levels into account, it will lead to failure in the end part. Sealing When designing a sealing system for the rocker cover, compression set and sealing force retention are two primary considerations. The compression set of the sealing system is important to maintain the clamping force on the fasteners. The seal force retention is the ability to maintain a seal with the movement of the rocker cover in relation to the cylinder head. There are four materials currently used to seal rocker covers. The first of these, room temperature vulcanized (RTV), is limited in its use today. In the early days of PA66 rocker covers, this was the most common system. RTV is a silicone rubber applied as a liquid. Exposure to the air cures the material to form a gasket. RTV has excellent compression set and seal force retention. The main drawbacks are the volatile organic compounds (VOC) emitted during the curing and the need to apply RTV during engine assembly rather than before. The most commonly used system today is a VMQ elastomer. The VMQ is a molded silicone rubber with a very broad temperature range. However, it is known to have poor seal force retention and will commonly weep oil. To offset the disadvantages of VMQ, acylic ethylene elastomer, or AEM, 2

3 is used. AEM has excellent sealing and high-temperature properties. At low temperatures, however, it is known to stiffen for a poor seal. Polyamide rubber, or PA, is sometimes used as well. PA has a low oil swell and balanced properties compared with VMQ. PA also has difficulties at low temperatures and can leak. Noise Vibration Harshness Today s customers are expecting less noise from their automobiles, and regulations demand lower drive-by noise levels. Therefore, noise vibration harshness (NVH) is an important issue in designing a rocker cover. PA66 has lower stiffness and density than the metals it replaces, so sound damping can be decreased. An acoustical analysis will demonstrate the areas with noise issues, and these can usually be addressed by adding ribbing or contours to the part to increase its apparent modulus. The other way to decrease noise is to isolate the rocker cover. Typically we think of mounting a rocker cover tightly to the cylinder head. This will transfer all the noise from the engine to the rocker cover. Isolating it with shoulder bolts and rubber grommets can greatly reduce the noise transfer. Figure 3 shows the isolated system. Rocker Cover Seal Head Design Shouldered Bolt Grommet Motor Figure 3. The sealing surface or head design is a critical design consideration. In the days of stamped steel rocker covers, manufacturers used between four and 1 bolts to clamp the sealing surface due to the high modulus of steel. PA66 also has a high modulus. However, it is not comparable to steel. The finite element analysis will help determine how many bolts are needed to achieve an even clamping force. The rocker cover pictured in Figure 4 has 14 bolts to provide proper sealing. Another key issue with bolt placement is the need for corner bolts. They will provide an even clamping force and eliminate a leak path. For decades, the makers of Vydyne have been aware of this design consideration, as shown in Figure 5, from the Vydyne Rocker Cover Design Manual, circa 198. Manufacturing Processes Consider the following manufacturing processes as well as the comparative strengths and weaknesses of each. Molding Techniques The most widely used manufacturing method for rocker covers is injection molding. This method allows for a finished part right out of the mold, but it is important to make sure the tool has adequate and balanced cooling to ensure that the part has the lowest level of molded-in stresses. Also, it is important to take advantage of mold filling simulations. Compression molding is another method, which Figure 4. is often overlooked. However, for rocker covers with a box-like form, compression molding is well suited to the application. Post-mold finishing can sometimes increase the cost beyond that of an injection molded part, but if lower levels of post-mold finishing are required, this method can, in fact, reduce overall part cost. Transfer molding, a variation of compression molding, is also well suited to rocker cover applications. This method uses a closed mold similar to injection molding but has the quicker cycles of compression molding. Vydyne PA66 has been used in these processes and is formulated to work well with them. Separately Molded Parts Rocker covers sometimes have internal baffles for oil fill or crankcase ventilation. It is typically not possible, due to die lock situations, for these components to be molded in. Therefore, these parts are molded separately and attached to the rocker cover in one of three ways. The Figure 5. 3

4 simplest method is snap fits. Snap fits are inexpensive and quick, but the main concern is failure that can cause improper function or engine damage. A variation of snap fits is mechanical fasteners. Due to their complexity, they are not often used. They also take up quite a bit of space in the rocker cover. The most common method is welding. The configuration of the parts will dictate which welding method to use spin, vibration or induction. Welding offers good sealing and little complexity for the part. Because it is, in effect, a single piece, there is not the concern of failure associated with a snap fit. One downside to welding is the cost of the necessary equipment and fixtures. One other form of attachment to consider is bonding. New epoxy-based adhesives give the benefits of welding without the added cost. There are time concerns with this method, as most epoxy-based adhesives require an hour or more to cure. Adhesive bonding is often combined with a snap fit to create a clamping method and additional protection against failure. Material Properties Effective rocker cover design requires material properties to function together. The first of these is material stiffness. The stiffness of the material transfers the clamping force from the fasteners to the seal. PA66 has excellent stiffness over temperature, enabling transfer of clamping force. There is an incorrect perception that PA66 does not have the same properties at high temperatures Flexural Modulus Kpsi Stiffness at Temperature 5 C 1 C 15 C 2 C PPA 33GF PPA 35GF PPS 4GF Figure 6. of other high-temperature resins. However, Figure 6 shows that at 15 C PA66 has similar stiffness to polyphenylene sulfide (PPS) and polyphthalamide (PPA). Typically 15 C is the maximum temperature to which a rocker cover is exposed. The other issue to consider for good clamping force transfer is warpage. If the part is distorted due to warpage, it will not transfer the clamping force evenly. This is usually addressed by proper mold design and gating. For some long rocker covers, this is still not enough to solve the problem. In such cases, a low warpage material is needed. A typical rocker cover would be made out of a PA66 with 33 percent glass fiber such as Vydyne R533H. If reduced warpage is needed, a glass fiber and mineral combination such as Vydyne R4G would be well suited. The part must not only provide adequate clamping force, it must also maintain clamping force over time. A common culprit in degrading the clamping force is creep (Figure 7). As mentioned before, most creep in rocker covers can be dealt with in the design phase. The material chosen can have a dramatic effect in this area. Figure 7 shows the creep of PA66 is higher than PPS and PPA at 15 C, Strain % Flexural Modulus MPa Creep Resistance Stress 4psi@15 C , 1, Time (Hours) PA66 33GF PPA 35GF PPS 4GF Thermal Aging Vydyne R533H Air@15 C 9,8 9,7 9,6 9,5 9,4 9,3 9,2 5 Figure 7. 1, 1,5 2, 2,5 3, Exposure (Hours) Figure 8. though not near a point of failure. When properly accounted for during the design phase, this should not pose a threat of failure in the final part. The final material-related issue is long-term exposure to hot air and oil. PA66 is easily heat stabilized to handle the long-term exposure to hot air. The data in Figure 8 show that PA66 has no issues with exposure to 15 C air for long periods. Flexural Modulus MPa Thermal Aging Vydyne R533H Used Motor Oil@15 C 9,8 9,6 9,4 9,2 9, 5 1, 1,5 2, 2,5 3, Exposure (Hours) Figure 9. The exposure to hot oil is the other side of this issue. In an engine, the oil is exposed to acidic combustion byproducts that go into solution with hot oil. Acids are known to affect PA66, but the testing shown in Figure 9 indicates that this will not be an issue. When taken together, the data presented clearly indicate that PA66 is a material well suited to rocker cover applications. Industry Trends A notable emerging industry trend in the automotive sector is part integration, and rocker covers are no exception. Generally this integration involves the rocker cover and either the air cleaner housing or air intake manifold. The most obvious integration is the intake manifold. The rocker cover and intake manifold are both attached to the cylinder head, making them a logical choice for integration. In some cases, the air cleaner housing is a more obvious choice to be integrated into the rocker cover. The air cleaner can be a better choice for integration because it allows for more flexibility in packaging and simpler 4

5 Figure 1 Figure 11. tooling. Figure 1 illustrates both this integration as well as that of the air intake manifold and rocker cover. A second emerging trend is the concept of the radial seal. This concept was initiated by Mann and Hummel. All current rocker covers use the same sealing concept developed in the 192s, but the radial seal concept minimizes machining of the cylinder head to reduce cost. Additionally it improves the performance of the sealing system (Figure 11). With radial seals no fasteners are needed, due to exceptional sealing performance. The only job of the fastener is to hold the rocker cover in position so that the seal is not disturbed by accidental lifting of the rocker cover. Figure 12 shows how the seal has multiple elements dissimilar to the conventional rocker cover with only one sealing element. Cover Elastic connection and decoupling Sealing system Figure 12. Conclusion Though rocker covers (or valve covers) have typically been applications where metals have dominated, PA66 is a viable alternative with many benefits. By taking into account the considerations outlined in this paper, performance as well as both cost and weight savings are achievable. Additionally, when designing successful rocker covers in PA66, one should keep in mind the design considerations, manufacturing processes, material properties and industry trends. North America Europe Asia TM 575 Maryville Centre Drive St. Louis, MO USA Rue Laid Burniat, 3 Parc Scientifique-Fleming B1348 Louvain-la-Neuve (Sud) Belgium +32 (1) Unit 118, Ocean Towers No. 55, Yanan Road E Shanghai P.R. China Solutia, Inc. Vydyne is a registered trademark of Solutia, Inc. All other trademarks are the property of their respective owners. [ jct-1/7] WHITEPAPERRC-EN7 5