To seal two flat surfaces as gasket material of some sort may be used. The primary purpose of most gasket materials is to mate to any irregular surfaces including waviness, pitting, scratches or poor tolerancing. If two perfectly flat surfaces are maintained, a seal can be created without any gasket material. In truth most gasketing is their to lower the cost of production by allowing use of non-perfect parts. Gaskets fix poor tolerancing. The worse the surface conditions are the thicker the gasket matieral needs to be.

It is a common missconception that a thicker gasket material is better (most likely started by gasket salesmen to up the sale). A thinner gasket will always provide a better seal. Of course the catch twenty two is to have a thinner gasket the parts have to have a closer tolerance. If you want to know what manufacturer makes a better car, or washing machine, just look at the thickness of the gasket material used in their product.

To identify what what gasket material you need to use in your gaskting application several things must be considered. Most importantly, what are your sealing? The gasket material must be compatible with the fluid or gas your are sealing. Secondly what temperature is the fluid or gas, and not just the running temperature, but what is the worst case scenerio. High or low, if the temperature is not compatible with your gasket material, your will have a inexpensive part causing major expenses. Thirdly, what type of pressure is invloved? If there is very little pressure a material with very little tensile strength and minimal pressure will hold up. The higher the pressure, the higher the tensile strength of the material and the higher the bolt load is required to maintain the seal. Also pressure and temperature combined must be considered. A gasket material can be fine for either a given pressure or a given temperature, but if you have that same application with both high pressure and high temperature, be aware something can blow. Many materials are rated with a P x T rating (usually non-asbestos gasket material). Some gasket materials like Flexible Graphite can be combined with metal to increase pressure resistance.

Over compression of gasket material is a common problem with metal gaskets which have mechanically designed recovery built into the gasket. Over compression removes the gasket materials ability to recover. This is also true of expanded PTFE once over compressed it will have no recovery and therefore will not longer adapt to maintain a seal. The common strategy of "The more compressive load exerted on the gasket, the longer it will last" is generally true of elastomeric materials since elastomers (rubbers) are not compressible but deflect compression. Many materials such as non-asbestos compressed gasket materials and beater addition materials contain elastomers in the mix of material they are produced from, making them gasket materials that are difficult to over compress.