Washers

Rings with a square or rectangular section are called "washers". Washers are usually only used for static seals and in the presence of high pressures since, unlike O-Rings, they offer a very wide sealing surface, which may efficiently counter all degeneration of the seal due to the straining action of the pressure on the rubber molecules with which they are made.

Washers are suitable for numerous conditions of use, whether under minimum or considerable pressure and squashing. The choice of the exact compound and hardness is of the utmost importance in the use of washer seals, in that the sealing surface is usually very broad and widely differing results may be obtained even with just a minimum difference in hardness.

Elastotech can make this particular type of seal in three ways, each of which has advantages and disadvantages that are specified in more detail later on:

• by moulding;
• from moulded hose;
• by extrusion.

By moulding
Elastotech has over 3,000 moulds that may be used for the production of rubber washers. A Washer Table is available for consultation with the list of moulds for existing washers in order of outside or inside diameter or thickness. Our Sales Office should always be asked for confirmation before placing an order, since the mould of a particular size, although existing, may not be useable for all the compounds and hardnesses, or the costs for making the washer in a compound other than the one for which the mould was first made may be prohibitive. Using the moulding system for obtaining a washer guarantees the tolerances required both from an aesthetic and a dimensional point of view, thereby keeping an excellent quality to price ratio To sum up, the advantages in using this system to obtain washers are:

• guarantee of maintaining required and confirmed tolerance;
• excellent aesthetic and surface appearance;
• good quality to price ratio;
• possibility of obtaining items with very restricted tolerances;
• limited cost of moulds;
• very small minimum quantities required.

From moulded hose
An alternative method for making washers is with the "moulded hose". First of all a hose is moulded with inside and outside diameters the precise size of the washer to be made, but with a length of 50/80 mm, which in some cases may reach 400 mm. Having obtained this "tube", it is cut with special machines into the required thickness, thereby obtaining the required washer. The entire working process, including the cutting of the hose, is carried out inside Elastotech so that the whole production process is carefully controlled. The confirmed tolerances are slightly greater than those that can be obtained with the moulding system, but are in any case deemed acceptable by most customers. This system also gives perfectly flat washers that may be used for fitting with automatic machinery. A minimum quantity is normally necessary to make items with this system. Furthermore, the cost of the hose moulds is slightly higher than the cost of the moulds for making the finished piece. This system has many advantages, including:

• relatively limited tolerances on diameters, since the hoses are obtained from a mould and are not extruded;
• tolerance on thickness +/- 0.10;
• exceptional quality-price ratio, especially for large quantities and medium-large diameters;
• perfect flatness of the pieces, which may also be used for fitting in an automatic mode;
• possibility of making washers in several thicknesses with the same hose, without having to incur expenses for several moulds;
• greater precision of the shearing and hollow punching system with lower costs.

By extrusion
The last method for making a rubber washer consists of obtaining it by continuous extrusion of a hose even several hundreds of metres long. This system is undoubtedly the most economic, but the final article has the same limitations and problems already present on the extruded hose before cutting, such as: very broad tolerances, inconstancy of values which may easily fluctuate several times between the minimum and maximum, poor quality of the compound cross-linking and possible signs of foreign matter incorporated in the extruded piece. Also the quality of the cut is worse compared to the excellent quality of that to be found on washers obtained from the moulded hose, due to the different method of production. For the above reasons and other minor ones, the washers obtained by continuous extrusion may only be used where poor quality is required and attention is mainly focussed on the economic aspect. This process is also carried out entirely inside Elastotech factories, which allows the above-mentioned problems to be limited to the minimum. The greatest advantages in choosing this production system are:

• extremely limited costs;
• easiness of obtaining several thicknesses with the same extruded piece without having to produce many moulds.

No standardised regulations exist fixing the tolerance limits of washers. They are considered a part designed to specific customer request and the dimensional tolerances should therefore be given on the actual design drawing. Should the tolerance limits not have been communicated or are unknown, Elastotech applies the same dimensional and aesthetic tolerances and controls to washers obtained from moulding as used for the O-Rings. For further information, please refer to the section regarding the tolerances for O-Rings.
It is also obvious that due to their particular type of static seal, washers may also have greater tolerances without this jeopardising the duration of the actual seal.
With regard to the diameters of the washers obtained from a hose, the tolerances are normally + 20% greater than for those of washers obtained by direct moulding, while the tolerance is +/- 0.1 mm on the thickness, which is obtained with a cut. Lastly, the tolerances are much greater on the diameters of washers obtained by continuous extrusion and depend on the type of compound and hardness of the extruded hose. For example, the tolerances on an extruded piece with an outside diameter of 19 mm are +/- 0.6 mm and both the minimum and the maximum value of the allowed tolerance are frequently found in the same production run. With regard to tolerances on the thickness, determined by the cutting precision, Elastotech guarantees +/- 0.15 mm, with the possibility of achieving even +/- 0.10 mm on request by the customer.

As already mentioned, Elastotech has over 3,000 moulds available that can be used for the production of rubber washers by direct moulding and about 1000 moulds for making moulded hoses from which the washers are then obtained. The size of the extruded piece instead creates no dimensional restriction due to equipment. A Washer Table is available for consultation with the list of moulds for existing washers in order of outside or inside diameter or thickness. This table includes only the moulds with an adequate productivity level and those which are not the property of customers Our Sales Office should always be asked for confirmation before placing an order, since the mould of a particular size, although existing, may not be useable for all the compounds and hardnesses, or the costs for making the washer in a compound other than the one for which the mould was first made may be prohibitive.