Flexo Graphic Printing
Doctor Chamber Seals. Commonly overlooked as an inconvenient, but necessary consumable item. “End Seals” it could be said are widely taken for granted.
However these components are vitally important in maintaining extended Doctor Blade life, reduced ink wastage due to leakage, and possibly most important is the elimination of score lines and impression marks made on anilox rollers. An enclosed chamber can potentially be a large investment for any organisation and one of the determining factors in its continued and effective operation is the quality and functionality of the chosen End Seals.
Felt is Best
A “Felt End Seal” manufactured from high quality wool felt containing minimum 95% wool, provides one of the most effective liquid seal barriers available in the market.
Due to its structure of naturally interlocking hollow fibres, felt can absorb many times its own weight in fluid and retain that fluid within its matrix until it is demanded by another medium.
A good quality felt will not flood, nor leak. It will absorb to its maximum potential then effectively self seal and absorb no more. This effect can be further enhanced by priming the felt first in a light/medium engineering oil for around 1 minutes, then allowing the seal to drain of excess surface liquid and finally allowing it to prime itself over approximately 24 hours prior to insertion into the chamber housing.
The oil priming of the seal helps operation in a number of ways, the most significant being:
- The oil will be released to the surface contact point between the felt seal and the anilox roller providing a seamless lubricating impenetrable membrane in which the anilox is allowed to ride. This skinless contact area is the essence of all lubrication systems that involve a felt seal and a rotating shaft.
- Due to the felt already reaching its optimum absorption point during priming, it neither allows ink to enter it, or releases oil into the chamber so cross contamination is eliminated.
- Wool felt provides, even without oil, a near constant co-efficient of friction and this docs not change even as the seal ages within the chamber. The normal reason for changing a seal would be to replace an older seal that no longer correctly sits in its position due to wear.
Various types/grades of felt are available, all in a wide thickness range. Grey felts and White felts operate to exactly the same levels of absorption and endurance given that they contain the same wool contents, the only difference between them being personal colour choice.
Softer felt seals tend to swell more in a liquid environment and so will conform to surface irregularities better than harder seals, however harder felt seals can be more accurately machined and in most cases offer enhanced life expectancy.
Many sites promote felts containing wool contents below 95% that are said to enhance sealing characteristics and wear potential.
This information is misguided and the only reason a wool felt is not manufactured from a minimum of 95% wool is cost benefits to the manufacturer.. There is no evidence available that suggests by blending natural wool fibres with other (normally synthetic) fibres that this in some way enhances the felt. Synthetic fibres are non absorbent; they interfere with the friction characteristics of the felt and have lower melt points, which can cause hard spots and caking at the contact point.
All things considered if you buy a lower quality of felt you have to expect a lower performance and reduced life with the very real potential of seal failure.
Alternative materials. Not everyone believes a felt seal is the best for their systems and many new and exotic materials have been used in the recent past with varying success.
Polyethylene Foam This closed cell synthetic plastic foam material is available in a number of grades of hardness and even in some cases some rather bright colours. It is impervious to liquids and as such cannot absorb lubricants. As with all polyethylene foams it tends to soften at relatively low temperatures and can deform in its softened state.
This would prove disastrous to any sealing potential the foam may have and as such it is recommended that the Anilox/Seal contact point is either felt lined, or waxed in some way (Vaseline or grease is appropriate) to prevent any heat build up due to friction.
Although these seals tend to be relatively low in unit cost they do not perform or last the length of a felt seal and can be more difficult to produce without the right cutting equipment (normally water jet). These seals possibly have the lowest life expectancy of all seals.
Cellular Rubber This is another closed cell synthetic product and normally manufactured from a blend of Neoprene and Epdm.
This can offer the sponge seal reasonable resistance to solvent/oil rich environments. Again, even though the product is available in a range of grades and thickness, it also requires special cutting techniques to ensure perfect concentricity and alignment in the chamber and it requires lubricating particularly at the contact point with the anilox to ensure friction free running. These seals tend to be more expensive than Polyethylene Foam seals and have a medium life expectancy.
Your End Seals. Our Custom Service
In order to offer the best End Seal for your application all we need is either a sample or drawing of the seal required and we will offer our best engineered solution. If your seals are leaking or you feel like they are being changed too often due to failure maybe its time for a change.
We can provide samples for extended trials and can manufacture seals for all the major Doctor Systems out in the field including among the more popular types, Schiavi, Bobst, Comexi, Tresu, F & K and all after market chambers available.