Process and apparatus for coating metal strips on both sides with coats of enamel

A process and apparatus for coating metal strips on both sides with an enamel base coat and on at least one side thereof with an enamel top coat by continuously passing such strips in a contactless manner through baking furnaces. For the heat treatment following the coating step, the metal strips are vertically admitted into a baking furnace, and vertically guided through the baking furnace.

BACKGROUND OF THE INVENTION 
The invention relates to a process and apparatus for coating metal strips 
on both sides with an enamel base coat and, at least on one side, with an 
enamel top coat applied over the base coat, by continuously passing the 
strips in a contactless manner through furnaces with heating and cooling 
zones. 
A process is known in which the metal strips, which are coated on both 
sides, preferably with a slurry for producing an enamel base coat, are 
continuously passed through a horizontal furnace with a preheating zone, a 
baking zone and a subsequent cooling zone. As a rule, an enamel base coat 
is applied first to both sides of the metal strip in a first furnace and 
an enamel top coat made of slurry or powder is subsequently applied to one 
side of the strip in a second baking furnace of the same type. 
Due to the fact that the metal strips are horizontally guided at baking 
temperatures in the furnace ranging from 700.degree. to 900.degree. C., 
the freely suspended loop in the furnace sags considerably. Thus, the 
maximum length of the baking furnace is limited by the tensile load on the 
strip in the freely suspended loop. Furthermore, this results in different 
spacings relative to the burners or burner tubes, which are arranged in 
the furnace in the longitudinal direction of a fixed, predetermined curve 
of sag. As a result, a constant baking temperature and uniform enamel 
quality cannot always be assured at high operating speeds of the 
equipment. Furthermore, the inlet and outlet openings on the baking 
furnace have to be dimensioned relatively large, so that such furnaces 
have high heat losses. Moreover, the efficiency of such a plant is 
relatively low due to the limited length of the furnace. 
Accordingly, it is an object of the invention to provide a more efficient 
and qualitatively enhanced process and apparatus for coating strips on 
both sides with an enamel base coat and at least on one side with an 
enamel top coat. 
SUMMARY OF THE INVENTION 
This object is achieved according to the invention by the provision of a 
process and apparatus for coating metal strips on both sides with an 
enamel base coat and an enamel top coat to be applied to at least one side 
thereof by continuously passing the strips in a contactless manner through 
baking furnaces with heating and cooling zones. The process is 
characterized in that for the heat treatment following the coating step, 
the metal strips are vertically admitted into the baking furnace and 
vertically fed therethrough. 
According to a preferred embodiment of the invention, such a process is 
characterized in that the strips, on coating with a slurry or powder for 
producing the enamel base coat, are passed through a first 
vertically-disposed baking furnace, advancing from the bottom to the top 
of the furnace, and following their path reversal and subsequent coating 
with a slurry or powder for producing the enamel top coat, through a 
second vertically-disposed baking furnace, advancing from the top to the 
bottom thereof. 
By vertically admitting the metal strips after the coating into the baking 
furnace and passing them vertically through the furnace, the coating 
remains free from contact until it is cured after passing through the 
cooling zone, so that damage of any type is avoided. By guiding the coated 
strips vertically in the baking furnace, a constant spacing can be 
maintained between the strip and the burners of the furnace, assuring 
uniform baking of the coating. In a vertical furnace, the temperature can 
be controlled more favorably for optimizing the temperature/time curves 
for the baking process. The cross section of the inlet and outlet gates of 
such a vertical furnace can be kept much smaller than with horizontal 
furnaces, so that the heat losses are lower. Furnaces with a greater 
passage length can be used in order to achieve higher strip speeds and 
thus higher output rates of a plant for carrying out the process according 
to the invention. As no sagging of the strip to be treated occurs in the 
process of the invention, the tensile stress of the strip can be adjusted 
to much more favorable values, which has a favorable bearing on the 
flatness of the strip. If the strip should tear, the lead end of the strip 
can be admitted in a faster and safer way as compared to the known 
horizontal equipment. 
Other objects and features of the present invention will become apparent 
from the following detailed description considered in connection with the 
accompanying drawing, which discloses one embodiment of the invention. It 
is to be understood that the drawing is to be used for the purpose of 
illustration only, and not as a definition of the limits of the invention.

DETAILED DESCRIPTION OF THE DRAWING 
In the drawing, a horizontally advancing pretreated metal strip is directed 
downwardly via the reversing drive roll 2 and cooperating pinch wheel 21, 
and guided into the slurry bath 3 where it receives an enamel base 
coating. Strip 1 is guided around roll 4 to reverse its advancement 
direction and metal strip 1 (now coated on both sides) is subsequently 
admitted in the vertical direction into the first baking furnace 5. 
Stripper rolls 9 are mounted ahead of the baking furnace 5 to guide the 
strip and remove any excess coating. 
First, preheating takes place in the preheating zone 51 of the furnace 5, 
which is followed by the baking zone 52, in which the closed, glassy 
enamel base coating is fused at the suitable baking temperatures. In the 
cooling zone 53 following the baking zone, cooling takes place preferably 
by a countercurrent of cool air which is admitted via the inlet 531 and 
exhausted by way of the outlet 532. 
The metal strip 1 so provided with the cured enamel base coat is reversed 
controlled by tensile stressing via the roll device 8 and admitted into a 
second baking furnace 6. This time, however, strip 1 travels from the top 
to the bottom of the baking furnace, again in the vertical direction, via 
a second drive roll 2 and pinch wheel 21. Before entering this furnace 6, 
strip is coated on one side with slurry or powder for producing the enamel 
top layer, for example by roll application, spraying, or electrostatic 
application. This process permits the production of coatings on one or 
both side(s). 
While the strip is guided in the first baking furnace 5 from the bottom to 
the top, metal strip 1 is guided in the second baking furnace 6 from the 
top downwardly and initially into the preheating zone 61. Subsequently, it 
is vertically guided downwardly into the baking zone 62 and, subsequently, 
again free from contact, into a cooling zone 63 having a cooling air inlet 
631 and a cooling air outlet 632. 
After its direction has been reversed on another roll device 8 with control 
of the tensile stress, the coated metal strip 1 is passed on via another 
controlled drive roll 2 cooperating with a pinch wheel 21. 
The basic process of the invention permits the production of enamel 
coatings on one or both sides of the strip in one single vertical baking 
furnace as well. For example, the strip can be provided with both the 
enamel base and enamel top coats before it is admitted into the single 
baking furnace. 
Thus, while only one embodiment of the present invention has been shown and 
described, it is obvious that many changes and modifications may be made 
thereunto, without departing from the spirit and scope of the invention.