The long awaited ban on CFCs is finally with us. But while it may provide
the opportunity for a cleaner environment, it also poses an economic danger if
the wrong seal material is used .
Since the automotive industry began grappling with the design issues of new sealing materials for R134A a few years ago, material suppliers have been busy promoting their solutions. Japanese manufacturers traditionally used nitriles (NBRs) and, more recently, saturated nitriles (HNBRs) as their seal material of choice. European counterparts initially joined this movement to HNBRs, as did two of the Big Three U.S. automakers.
Another manufacturer, however, plans to improve the sealability of its air-conditioning systems by changing the material and the design of the seal itself.
Tests conducted at Parker Hannifin's certified seal and applications testing labs show that permeation of R134A through elastomeric seals always increases with temperature, given expansion of the refrigerant and pressure rise.
Permeation is a key performance characteristic to consider when selecting a material. Neoprene's relatively low permeation is of real interest. But Neoprene has difficulty standing up to the high underhood temperatures of today's automobiles. HNBR, on the other hand, can withstand the high temperatures but has higher permeation. Recent tests, however, have shown that HNBR has a major flaw. When exposed to repeated high and low temperatures, it begins to lose its sealability. Inability to seal at low temperatures makes HNBR questionable for air-conditioning systems.
Several European manufacturers and at least one domestic manufacturer of automobile air conditioners have realized this. As a result, they are changing to ethylene propylene (EP) as the material of choice in sealing applications.
EP was not a suitable material for R12 applications. As a result, it was an unlikely first choice for use in an alternate refrigerant. However, EP demonstrates good permeation resistance to R134A and is fully compatible with the various PAG oils used as the system lubricant. Under the same test conditions (high-low thermal cycling) as the HNBR, EP has not shown leakage at either pressure. Thus, EP demonstrates superior permeation resistance and sealability to low temperatures.
Given the stable physical properties and permeation resistance, EP is a very good seal material for consideration in automotive air-conditioning systems. It is also the material of choice for both refrigeration and heating systems. This would allow standardization of seal materials.
Traditional sealing methods for heating and air-conditioning systems have used molded rubber seals. These may take the form of O-rings or other similar type parts. This similarity in appearance makes it very difficult to determine the elastomer without testing. As we have shown, various elastomers will perform differently, thus the potential for wide variations in performance should materials be used improperly. Additionally, many of these seals rely on radial sealing for tube fittings.
* The need for lubrication;
* Potential to "skive cut" the seal;
* Difficult assembly due to the high installation force;
* Potential contamination problems ;
* Difficulties with tolerances.
In a typical 0.070-inch cross section O-ring, a 0.001-inch variation in tolerance (including gland dimensions) is equal to a 1.4% change in compressive loading--or, squeeze--on the O-ring. Taking into account the tolerance stack-up, the performance of an O-ring may vary greatly. As an example, a 0.070-inch cross-section O-ring with a ˝0.003-inch tolerance would have a squeeze variation of ˝4.2% (without including the additional variation caused by the groove tolerance). This could lead to a large variation in performance of individual seal locations.
Advanced seal technology is now available to change these radial-type fixtures to face seal designs. Using a combination metal retainer with a bonded elastomer seal (molded-in-place) offers many advantages in face seal design. The elastomer is confined in a very controlled environment. This allows for a much higher pre-loading of the seal, which means better permeation resistance.
The higher squeeze allowed by the molded-in-place seal will naturally outperform an O-ring-type seal.
The molded-in-place seal's squeeze ranges from the high 20s to the mid-40s. An uncontrolled, unconfined seal would not tolerate that high of a load. However, with the molded-in-place seal, as the load increases, the permeation decreases. This leads to one of the main advantages of molded-in-place seals. They eliminate tolerancing problems associated with seal design. The metal retainer not only carries the load for the seal, it takes the tolerancing away from the seal. The design engineer no longer needs to be concerned about seal tolerancing issues.
As the seal location moves to the face, no lubrication is necessary for installation. This eliminates a costly, messy operation during assembly. The high loading of the seal also allows it to overcome some of the contamination that is sometimes present in an assembly plant, which can cause O-ring failure. Our testing has shown the most impressive aspect of this seal is its performance. Molded-in-place seals on advanced air-conditioning applications have achieved a leak rate of 0.00016 grams of refrigerant lost per hour, the equivalent of less than one pound lost in 300 years.
The new refrigerants cost more than three times as much as the old R12. However, reductions in warranty claims, easy installation, and the potential for standardization can lead to gigantic cost savings.
As a cautionary note, it should be recognized that EP is not recommended for R12 systems. The introduction of this material into the supply network for automotive air-conditioning systems must be carefully controlled to avoid incompatibility. Using alternate designs, like the molded-in-place seal, reduces the likelihood of mixing materials and applications. The retainer can even be uniquely identified using markings, color, or plating variations.
The molded-in-place technology produces a virtual-zero-leak air-conditioning system. The rewards for the consumer can offset the increased cost of using new refrigerants.The consumer and the environment come out winners.
Bowman is materials manager and Golden is marketing manager at Parker Hannifin's O-Seal Division.