Coating processes – Spraying
Reexamination Certificate
1997-11-06
2002-03-26
Beck, Shrive P. (Department: 1762)
Coating processes
Spraying
C427S221000, C427S422000, C427S290000, C427S292000
Reexamination Certificate
active
06361830
ABSTRACT:
The invention relates to a process for manufacturing metal sheet gaskets coated with elastomer, as are often used as cylinder head gaskets or as compressor gaskets in refrigerant circuits.
To date, the coating of such metal sheet gaskets has been carried out by the metal sheetings being transported from roll to roll and having a continuous coating applied to them and by the corresponding gasket sheets then being punched out of the endless material and further processed to produce the finished gaskets.
The problem with this process is that the cut edges resulting from the punching remain uncoated and, therefore,
a) are susceptible to corrosion, and
b) offer at the cut edge possible points of attack for liquids and gases which may cause the elastomeric coating on the surface of the metal sheets to become detached.
This problem was resolved in a different way of manufacturing metal sheet gaskets coated with elastomer wherein the metal sheets for the gaskets were first punched out of the endless material, and the punched-out sheets were coated with an elastomeric substance while passing through a curtain of liquid in a so-called flow coating process. The disadvantage of this way of manufacturing the metal sheet gaskets is that adjacent to edges of the gaskets, in particular, at openings such as, for example, combustion chamber edges of cylinder head gaskets or other passages for liquid in the gasket, and also at beads and shoulders, coating irregularities, for example, in the form of so-called runs or sags, occur on the underside of the sheets. The irregularities easily attain approximately the thickness of the desired coating and more, and so these negatively affect the function of the gasket. Furthermore, with this process, especially if the problem of the coating irregularities is to be kept within reasonable limits, only a relatively thin coating can be applied in one run, and so the coating has to be carried out several times if thicker coatings are desired. This involves not only a delay in the manufacture or manufacturing installations of correspondingly larger dimensions, but also additional operations, for example, if the accumulations of coatings were to cause such disturbances that they had to be eliminated by a separate punching operation. All of this negatively affects the cost factor in the manufacture of such known gaskets to a considerable extent.
A factor which is also not to be overlooked is the intermediate drying of the coating substances which is often necessary when several coats are applied.
The object of the present invention is to provide a process for the manufacture of metal sheet gaskets coated with elastomer wherein the coating with elastomer can be carried out with greater cost efficiency and avoidance of the above-mentioned disadvantages.
This object is accomplished in accordance with the invention in the process described at the outset by the elastomeric coating being applied to the individual metal sheet in a spray coating process.
Surprisingly, it has been found that the coating substances, although they have a distinctly higher viscosity than is usually the case in the spraying of paints, result in a problem-free way in a coating of the metal sheets of a metal sheet gasket with elastomer, in particular, using conventional spraying equipment, which is of excellent quality. Furthermore, with the spray coating technique, excessive accumulations of coatings on the underside of the sheets, in particular, at cut edges, beads or shoulders of the metal sheet layers, are avoided, and so additional machining such as, for example, subsequent punching to remove accumulations of coatings is dispensed with.
Furthermore, with the spray coating process a considerably larger layer thickness can be applied in one run than with the flow coating process used so far. In particular, an elastomeric coating with a layer thickness of distinctly more than 16 &mgr;m is possible with the inventive spray coating process. With this, there are no coating irregularities to impair the function of the gasket, and also a slight loading of the metal sheets with elastomer on the underside thereof due to spray mist has no disadvantageous consequences.
An elastomer coating operation is preferably carried out with a coating substance having a viscosity at approximately 23° C. of 10 to 120 seconds, preferably 30 to 90 seconds, efflux time with a 4 mm German Industrial Standard cup. On the one hand, these viscosities still allow good working with the spray coating technique, and, on the other hand, ensure that considerable layer thicknesses on the metal sheet, if desired, can be achieved in one run.
The equivalents in cps to the viscosity values given in seconds for the 4 mm standard up are:
10 seconds
50 cps
30 seconds
145 cps
90 seconds
435 cps
120 seconds
580 cps
Preferred elastomeric coatings which are manufactured in accordance with the inventive process have a layer thickness of 3 to 60 &mgr;m. It is to be noted that all these layer thicknesses can be applied in a single operation, i.e., in a single run, optionally with two directly successive coating procedures, by the spray coating apparatus.
The elastomer solutions used as coating substance can have a solids content of 5 to 40% by weight, with carbon black admixtures, fillers, auxiliary vulcanizing agents., etc. being understood as solids. It is important that the coating substance used for the elastomeric coating should include an elastomer solvent or solvent mixture so the elastomer is present in the substance in a dissolved form and not as a solid.
For easy handling of the spray coating process, in particular as regards the technical side of the installation, it is desirable for the solvent or solvent mixture to be recoverable. The solvent of the coating substances or the solvent mixture is preferably selected from elastomer-dissolving alcohols, aromatic compounds, esters and/or ketones having a boiling point in the range of 100 to 200° C. These solvents or solvent mixtures meet the requirements stated hereinabove.
Further preferred are solvents or solvent mixtures which include esters with boiling points in the range of 100 to 180° C., preferred up to approximately 150° C., and/or contain ketones with a boiling point in the range of 100 to 160° C., preferred up to approximately 120° C.
The inventive process can be carried out in a particularly economical way if the coating substance is recovered by the transport means with which the metal sheets are conveyed through the spray coating installation. This will often be a conveyor belt, and, in particular, one will recover the portions of the coating substance which pass through the combustion chamber openings in the metal sheets to the belt. This is possible in a particularly simple way by the pot life of the coating substance being adapted so as to be more than one hour.
In this way, only slight amounts of coating substance are lost in the coating of the metal sheets, and the process attains a particularly high cost efficiency.
The auxiliary vulcanizing agents referred to hereinabove are adapted to the overall system of the coating substance and are selected such that they are only activatable by thermal action or by high-energy rays so that a crosslinking of the cross-linkable elastomer systems is only achieved at the dry stage and so that pot lives of the elastomer solution of one hour and more are achievable.
The inventive process thus makes it possible to produce the necessary openings, beads, shoulders, etc. before the coating operation for with the spray coating it is ensured that no unacceptable accumulations of coatings will occur in these problem areas during the elastomer coating operation. This, furthermore, has the great advantage that the punched edges of the metal sheets are also coated with elastomer and, therefore, do not offer any points of attack for streams of lubricant, streams of coolant, etc., which can result in the elastomeric coating becoming detached from the metal sheet.
A coating substance which comprises per 100 parts by we
Buck Reinhold
Schenk Peter
Beck Shrive P.
Calcagni Jennifer
ElringKlinger AG
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