FEATURE BEARINGS, SEALS & GASKETS
Gaskets have many vital roles to play in industrial applications – including to improve sealing, prevent leaks and produce longer service life through ongoing integrity, no matter how demanding the operating conditions. Bob Orme, senior technology specialist at Henkel, looks into the options
THE VIRTUES of liquid gasketing
M
ost will be familiar with traditional compression gaskets, which are
typically made from rubber, metal, cork or paper, but these have a number of practical disadvantages. Firstly, the compressive load has to be correct – too much or too little can have serious consequences. In addition, gaskets need to conform precisely to the surfaces that need sealing, many of which can be highly complex. The solution to this dilemma is liquid
gasketing, typically using formed-in- place (FIP) gaskets with anaerobic cure, or room temperature vulcanizing (RTV) elastomers.
FIP GASKETS Anaerobic FIP sealants cure in the absence of air and the presence of active metal surfaces. These are best suited to sealing rigid flanges, such as those found in vehicles gearbox housings, as well as bedplate to crankcase, water pump to engine block, and cam cover to cylinder head applications. Anaerobic FIP sealants offer metal-to-
metal contact, ensure correct bolt tension, add structural strength and reduce micro-movement. Further benefits include no gasket relaxation, no re-torquing of bolts, no shimming and no need for super smooth surfaces. They also offer good-to-excellent solvent resistance, automatic dispensing capabilities, and better suitability for vertical flange faces. To ensure these benefits are maximised, some straightforward design guidelines should be observed, not least flange pressure distribution and joint operating load.
RTV ELASTOMERS RTV elastomers cure to a rubbery solid by reacting with moisture in the environment. These products are best suited to seal flexible flanges, such as gearbox covers, timing chain covers, stamped sheet steel parts, thin-walled metal castings and oil pans. Unlike rigid flanges, they do not usually support the
28 MAY 2018 | DESIGN SOLUTIONS
function of the component, therefore micro-movement between the flanges can be tolerated and an optimum clamp load distribution is not necessary. The benefits of RTV elastomers are
wide-reaching, and include high gap- filling characteristics, the capability to seal joints prone to micro-movements, ensuring metal-to-metal contact, and the achievement of correct bolt tension with no setting. In short, RTV elastomers are able to seal joints with poor flange design, as well as critical areas such as T-joints, where using anaerobic sealants or hard gaskets will prove problematic. While most of the same design
considerations that apply to FIP anaerobic sealants are also relevant for RTV elastomers, one of the principal differences is the advised inclusion of an inside chamfer. A chamfer will provide defined flow of product, and no release of product pieces due to squeeze-out. Moreover, good durability will result because of the product layer (in the chamfer).
DISPENSING CONSIDERATIONS With design-for-manufacturing such an important consideration, both FIP sealants and RTV elastomers lend themselves to ease and speed of application. The most flexible and reliable way to
apply any kind of sealant is robotically. Purpose-designed dispensing systems, which can be accommodated by a robot, are able to apply high-viscosity
Loctite 5188: Anaerobic liquid gasketing automatically applied on a mass-production automotive engine line
anaerobics and RTV elastomers with high speed and quality. Systems such as flow monitors or visual inspection systems are recommended to ensure quality remains consistent. Screen printing, which can be used to
apply anaerobics, is especially suitable for medium-scale production and where flexibility is not required. However, screens do wear and therefore require replacement from time to time. Screen printing cannot be used for the application of RTV elastomers.
ASSEMBLY REQUIREMENTS It is essential to understand procedures on assembly lines where FIP gaskets will be used. Assembly conditions, operations and cycle times have a major effect on sealant selection, and later on the quality of the whole process. Importantly, any contamination of the flanges prior to assembly of the parts should be avoided. Furthermore, once the applied product connects with both flange surfaces, any movement relative to the joint surface must be averted. The basic curing chemistry of an anaerobic sealant allows a virtually unlimited open time. Nevertheless, fast-curing sealants can start to pre-cure even before assembly. It is therefore highly recommended to fully torque down all bolts immediately after joining the flange faces to avoid shimming and, later, leakage. With regard to RTV elastomers, once the product is dispensed, the flanges have to be assembled within the skin- over time of the product. Full torque down is not immediately necessary. Ultimately, with the correct design, product and process, the gasketed joint will maintain its sealing capability throughout the life of the vehicle.
Automatic application of a liquid silicone sealant
Henkel
www.loctite.co.uk
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