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Advances in silicon engine gasket sealing.

OEM requirements for engine sealing have evolved to include 150,000 mile/10-year durability. These new requirements exceeded the capabilities of existing silicone rubber materials and presented a significant materials development challenge. Dow Corning worked with an automotive OEM and a tier one supplier as partners to address the material performance, processing and design aspects, quickly commercializing a new generation of high-consistency silicone rubber technology called high performance in oil (HPO) materials. HPO compounds exhibit significant improvements in sealability and durability and are currently being used on production automobile engines.

In this article, we will discuss our systems approach to engine sealing, as well as the development and use of functional tests that better simulate the actual service environment and sealing performance.

Background

Silicone rubber gaskets began replacing cork-rubber composites for automotive engine sealing during the 1980s. Since that time, high-consistency silicone rubber (or HCR) compounds have seen widespread use in molded automotive gaskets, primarily in static sealing applications such as cam/valve covers and oil pans. Benefits include long-term flexibility, exceptional temperature stability and resistance to engine compartment fluids.

Early HCR silicone compounds for engine gaskets offered sealability improvements and longer service life over previous non-silicone materials. After several years of use, however, it was found that volatile components of these formulations oxidized to amorphous silica in the exhaust stream. Over time, silica deposits can cause fouling of the zirconia elements in exhaust oxygen sensors.

To address the problem, Dow Coming first developed a new test method for measuring the low molecular-weight components of the silicone materials, which has since become an OEM standard. A second generation of HCR compounds was subsequently developed, which delivered low volatility and improved compression set properties. Processability was also a key facet of materials development. Traditional physical property requirements were retained, including durometer, tensile strength, elongation, modulus and hot air compression set.

Hierarchy system

In the past, OEM/supplier relationships have typically been a top-down hierarchy in the design and development of gaskets and sealing materials. The order went from OEM to components supplier to rubber fabricator to the materials supplier. Gaskets were considered a separate entity from the two sealing surfaces and at times were almost an afterthought when' engine components were designed. This frequently resulted in less-than-optimum gasket performance.

New systems approach

Under the current materials development system, Dow Coming and its OEM/tier one partners have refined their view to consider the gasket as part of die engine sealing system's total design. This represents a shift for most companies, one that more fully integrates the efforts of OEMs and their tiered supply base.

The systems approach to engine sealing recognizes that gasket design, material and fabrication all have an effect on performance. The method takes advantage of the unique skills contributed by each sealing team member: OEM;, material supplier,and fabricator/tier one supplier.

No individual element of the entire product design and manufacturing process takes precedence over another, resulting in a critical balance of all three areas. This cooperation produces better designs and materials, raising product quality while expediting the commercialization cycle.

New material requirements

As gasket performance requirements and fabricating goals continued to escalate, it became apparent that the second generation of silicone materials would not meet emerging vehicle performance goals of 150,000 mile/10-year durability. The team had the following design objectives as they worked on a new family of high-consistency silicone compounds:

* Provide effective sealing for 150,000 miles.

* Maintain the modulus range of the 2nd generation materials for NVH (noise, vibration and harshness) issues.

* Optimize compression set in oil vs. air.

* Improve compression stress relaxation properties.

* Contain cost as indicated by the OEM.

* Optimize processing characteristics for gasket fabrication.

Throw out the current OEM specification if necessary.

In developing the new family of high-consistency silicone compounds, engineers utilized a series of designed experiments to optimize crosslink density, tear strength and hot oil compression set. Because improved compression set resistance directly affects sealing force, it is viewed as an indicator of longer gasket life.

Physical property testing

Traditional tests for evaluating silicone rubber molding compounds for gasket applications include:

* Durometer

* Tensile strength

* Elongation

* Tear strength

* Modulus at 100% elongation

* Compression set in hot air

* Volatility

* Heat resistance

* Fluid resistance

While the existing methods provided important data, materials engineers were convinced that they were not specifically representative of the application. It was determined that functional testing was necessary to more closely simulate the actual service environment under hot oil conditions and speed new product commercialization.

Accordingly, new tests were developed and integrated with existing tests to evaluate material candidates more quickly and accurately. Dow Corning, tier one and OEM engineers all contributed to the new-procedures, which were used for both materials screening and validation.

Functional testing

A brief description of each test:

* The hot oil compression set test was designed as a tool for material development and performance testing, because the traditional "hot air" compression set test method (ASTM D-395) was found to be a poor indicator of actual gasket performance.

The new procedure is performed as follows: a square, cross-section o-ring (25 mm O.D. x 17 mm I.D. x 4 mm thick) is compression molded 15 minutes at 171[degrees]C. The o-ring is placed into a steel fixture, compressed 20% and submerged in Mobil Super HP 5W-30 SH motor oil at 150[degrees]C. Every two weeks the fixtures are cooled to room temperature, removed from the oil, disassembled and measured for height loss. Compression sets are then calculated from this data. The 6il is changed at every inspection before restarting the test. The cycles continue until 100% compression set is reached.

Compared to first- and second-generation materials, the new family of HPO materials show dramatic improvement in compression set resistance (see figure 1).

* The accelerated functional test was developed to evaluate the sealability of specific rocker cover assembly designs (including gasket, cover and fastener). In this case, the cover assembly is mounted onto a test fixture and placed in an environmental chamber. Oil lines are then connected and oil heated to 120[degrees]C is circulated through the cover assembly at 2 1/min and 7 kPa. In addition, the fixture and chamber are heated to 150[degrees]C, to simulate hot engine conditions.

The test is continued until the sealing system fails, unable to retain the 7 kPa pressure. This functional test provides an opportunity to observe accelerated gasket/cover/fastener performance under conditions closely simulating those in actual service and allows engineers to more accurately predict sealing system reliability.

Results of the HPO products' performance in accelerated functional testing shows nearly a fourfold increase in durability over previous commercial materials (see figure 2).

* The heat age/pressure test was developed to test the durability of sealing system components with exposure to heat and motor oil. A decay of sealability over time is the eventual result. Test fixtures are prepared by bolting components, with gaskets to be tested, to metal fixture plates. An air pressure test is performed to verify the integrity of the system and the fixtures are then filled with Mobil Super HP 5W-30 SH motor oil and placed in a vented oven. The fixtures are vented to the atmosphere to prevent the buildup of internal pressure. The oven is heated to 150[degrees]C and the test fixtures are heated for three- or four-day intervals.

At the end of the specified period, the fixtures are cooled and removed from the oven, the oil is drained and an air pressure test is performed. The pressure required to cause a leak (if less than the maximum test pressure of 35 kPa) is recorded. Fresh oil is added and the test cycle repeated until the sealing system is unable to maintain 7 kPa. The test shows that the third-generation HPO materials outperform previous commercial materials (see figure 3).

* The compression stress relaxation (CSR) test is conducted using 19 mm O.D. x 12.5 mm I.D. x 2 mm thick o-rings die-cut from slabs according to ASTM procedures. The rings are placed in Shawbury-Wallace test jigs, compressed 25% and then immersed in IRM-903 oil. A hot air circulating oven is used to heat the fixtures and oil to 150[degrees]C. Sealing force measurements are taken after one and three days, then weekly throughout the balance of the six-week test period (1,008 hours), after which the percentage of sealing force retention is determined. The new HPO materials exhibit significantly improved CSR retention over the previous production material (see figure 4).

* As part of the design/material/processing triangle, the fabricator performed molding tests of the new HPO materials using the most severe mold available. These tests allowed the new materials to be examined for demolding, deflashing and scrap rates compared to existing production materials. Several material variations were evaluated to find the optimum balance of physical properties and processing characteristics. Key mold parameters were identified to make the materials less process-sensitive and the new formulations have demonstrated equal or improved processing when compared to 1st- and 2nd-generation products.

* During die dyno test phase, the HPO materials were evaluated by the OEM on 300 hour, 500 hour and other engine durability cycles. They were found to provide the necessary longevity to survive all dyno tests and subsequently received OEM approval.

* The HPO materials were also subjected to extensive fleet testing, using taxi, limousine and police department fleets as the test bed. These were chosen intentionally to expose the test vehicles to high mileage accumulation and extreme temperatures. The fleet evaluations have demonstrated that the HPO materials deliver increased service life over previous materials and as a result a major U.S. OEM began using the new compounds in production in Q2, 1995.

Summary

The new, 3rd-generation HPO materials are currently being used in rocker cover and cam cover applications. They are also being evaluated for timing chain cover gaskets and oil pan gaskets. These materials improve long-term durability of engine sealing systems without a sacrifice in processing characteristics.

Along with its-partners, Dow Coming has adopted a systems approach to engine sealing, as it continues rapid development and commercialization of new silicone materials. The cooperation between OEM, materials supplier and fabricator/tier one supplier capitalizes on the strengths of all the parties involved in engine sealing to optimize design, processing and performance characteristics. Future material goals include continuous improvement of compression set values, compression stress relaxation properties and functional performance in hot oils.

[Figures 1-4 ILLUSTRATION OMITTED]

by Lawrence D. Fiedler, Thomas J. Hebda, Edward M. Kucinski, Mark H. Lee, Dow Coming Corp. and Jeffery K Zawadzke, Dow Coming STI
COPYRIGHT 1996 Lippincott & Peto, Inc.
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 1996, Gale Group. All rights reserved. Gale Group is a Thomson Corporation Company.

Article Details
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Author:Zawadzke, Jeffery K.
Publication:Rubber World
Date:Mar 1, 1996
Words:1770
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