Face sealing fundamentals

To seal the two faces voids formed between the two mating surfaces because of irregularities – technically surface roughness and flatness errors – needs to be filled completely. The requirement on the chosen sealant is to resist the internal pressure, thermal expansion and contraction, resist the fluids being sealed at the operating temperature of the system.

Face Sealing – Chemistries Deployed.

Two Systems are Popular in the Automotive Industry- RTV Silicone and Anaerobic Adhesives. Depending on the mating surface and other production requirements either can be used based on the below mentioned considerations.

• Substrate material

Anaerobic adhesives or sealants require metal as one of the surface. Essential condition.

RTV Silicone do not have any such requirement for polymerization, so substrate is no bar.

• Temperature

For temperature up to 150 °C, both technologies may be suggested. Some grades of anaerobic adhesives will withstand temperatures up to 200oC also.

Above 150 °C and till 365 °C only RTV Silicones will work.

• Gaps Sealed.

If gap between the two mating surfaces is 0.25 mm and below, anaerobic or silicone both will work but if it is above 0.25 mm only silicone is recommended.

• Adhesive strength

Some bonding strength is required to resist seepage of products beneath the surfaces. Sometimes the sealant also improves structural rigidity and for this higher bonding strengths are necessary.

• If Strengths of 4 -9 N/mm2 is required then only anaerobic will work.
• RTV Silicones will give bonding strength up to 2.5 N/mm2 only. But for most applications this is enough.

• Open time or Before assembly time

Anaerobic Sealants will not start to cure unless confined between two surfaces so as to exclude the oxygen of the air which retards the reaction. Thus for all practical purpose a bead of Anaerobic Adhesive applied can stay uncured for up to 60 minutes. This is a useful property when the worker suddenly leaves the work station for lunch or some other reason and comes back and completes the assembly.

On the other hand RTV Silicones will start to cure immediately up on dispensing and hence the assembly must be completed with 5-10 minutes to avoid ‘shimming’ action due to partial cure.

• Intrinsic Shimming.

The reason liquids gaskets have replaced paper or cork gaskets is because they provide metal to metal contact between mating flanges. This results in retention of the bolt tension and joint integrity. Loss of bolt tension due to the ‘Compression Set’ factor of solid gaskets has been the major cause of leakage/seepage/sweating.

-Anaerobic Adhesives and sealant will offer 100% metal to metal contact and hence are the preferred Sealants of choice for machinery assembly.

– RTV Silicone Sealants may not offer 100% metal to metal contact as they are highly filled and very high viscosity materials.