Heat Cured Silicone Rubber for Engine Coolant Sealing
|Publication Date:||1 December 2015|
This specification covers the materials and performance of silicone rubber sealings in the cooling circuit.
Note: The words must, shallorwill as used in this document mean a mandatory requirement.
Material Description. Commercial compounds are divided into the following types: 1.1.1
Type A. Compression set resistant silicone (VMQ) gasket or o-ring compound(s) in contact with engine coolant for application temperatures with a functional temperature range of (-40 to +175) °C (continuous) as defined by ASTM D1329 TR10 and SAE J2236 respectively.
Type B. Tear resistant silicone (VMQ) gasket or o-ring compound(s) with a functional temperature range of (-40 to +175) °C (continuous) as defined by ASTM D1329 TR10 and SAE J2236 respectively. Recommended for higher coolant temperature applications (see Table 2 "Coolant Resistance").
Symbols. Identify the elastomer nomenclature using ASTM D1418 or ISO 1629.
Applicability. Engine coolant temperatures rarely exceed +120 °C. Heat cured silicone rubber described in this standard is used where the joint flange temperature up to +175 °C continuous occurs due to the close proximity to exhaust gas recirculation, turbo, heat exchangers and other heat generating components. Consult GM Materials Engineering for excursion conditions beyond +175 °C.
Other Coolant Standards. Use GMW14747 for coolant joints not exposed to hydraulic fluids during initial assembly or service. Use GMW14751 for joints sealing oil and coolant simultaneously. Use GMW17474 for joints with incidental exposure to hydraulic fluids during assembly or service.
High compression set occurs in soft silicone compounds. Compounds where the minimum hardness is 50 Shore A or less are not recommended.
Properties for Tables 2 and 3 were developed from heat cured silicones with a hardness range from (50 to 80) Shore A.
Due to its high gas and fluid permeability, silicone gasket design shall consider a wider contact footprint at the mating surface, increased sealing contact pressure and width (e.g. no 3 mm by 5 mm groove) compared to EPDM or HNBR compounds. Type B is recommended with nominal hardness starting at 70 Shore A or 80 Shore A especially joints subject to pressure up to 517 kPa (75 psi), with flow and immersion consistent with the normal application environment.
O-rings. Some prints/drawings identify "D" or race track shapes as o-rings. To avoid confusion, testing and table data only apply to the ASTM D1414 definition of an o-ring.
Tensile and elongation properties of o-rings are measured by spool separation, however during this step the elongation across the specimen width is not uniform. To minimize data variation, the recommended ratio of ring ID to cross section width should be ≥ 9 : 1. Of greater interest is tensile stress at (20 to 50) % elongation because it is closer to the performance range of the o-ring. Note: The ratio for SAE AS568-214 size is 7 : 1.
Compression Set. Testing a whole o-ring for compression set is not permitted. Air trapped in the o-ring inside diameter results in a different thermal condition between the inside and outside surface of the specimen.
Rubber Adhesion to Carrier (Edge Molded Design). Rubber shall be secure to the carrier over the entire contact length. On a fully cured part, attempts to remove the rubber using pliers or other gripping tools in a 90° or 180° pull (to the carrier) shall result in tearing of the rubber. A cut (up to 12 mm) between the rubber and carrier is permitted to facilitate gripping of the rubber. A clean adhesive failure (rubber to carrier) is not acceptable. Nondestructive test alternative using an interference fit, post die for small openings or a straight segment rule die that deflects or bends the edge bonded rubber is permitted. For open segment tests, the carrier must be securely clamped to prevent movement during deflection of the edge bonded elastomer. The interference of the die edge to the elastomer profile must make contact at the mid-point of the flexible extension from the bonded edge. One pass (downward and upward) is sufficient. There must be no loss of elastomer to metal adhesion or elastomer cracking under 2x magnification.
Elastomers used in part of assembly (POA) are usually selected by supplier specific criteria and often remain unspecified or unidentified on end user prints and drawings. As the original equipment manufacturer (OEM) end user, GM reserves the right (through directed buy) to define common rather than source specific requirements. Use this document in the appropriate Statement of Requirements, Appendix C and corporate prints and drawings using paragraph 8 "Coding".