An extrusion-forming jig for extruding a ceramic batch to regulate a sectional shape and external dimensions of a honeycomb structure is formed by the extrusion. The jig includes electric discharge coating layers on surfaces of the jig contacting the ceramic batch. The electric discharge coating layers are made of a cemented carbide such as tungsten carbide. The jig is able to be used for many hours without necessitating replacement.

BACKGROUND OF THE INVENTION 
This invention relates to an extrusion-forming jig for use in extruding a 
ceramic material to form ceramic honeycomb structures. 
Extrusion-forming jigs such as masks, rings and the like have been used, at 
a location downstream of dies for extruding ceramic materials to form 
honeycomb structures. As disclosed in, for example, Japanese Utility Model 
Application Laid-open No. 49-8,559, such jigs are used to form outer walls 
of honeycomb structures made of ceramic batches, or the like, passing 
through dies, or to regulate sectional shapes and external dimensions of 
honeycomb structures. 
The surfaces of such extrusion-forming jigs contact ceramic batches and are 
likely to be abraded. However, honeycomb structures are required to be 
formed within extreme dimensional tolerances. Therefore it has been 
necessary to treat at least the surfaces of the jigs which contact ceramic 
batches by surface treatments such as plating, to obtain wear-resistant 
surfaces. 
However, even if the surfaces of the jigs contacting ceramic batches are 
treated by surface treatments such as plating for the purpose of improving 
the wear-resistance of the jigs, the improved wear-resistance is not 
always sufficient. Therefore, extrusion-forming jigs having much higher 
wear-resistance have been desired for many years. 
In the case of applying a surface treatment to jigs in the conventional 
manner, however, the jigs are generally treated at high temperatures. 
Therefore, strains were produced in the surfaces of jigs which were 
subjected to such a surface treatment thus detrimentally affecting their 
dimensions and accuracy. As a result, it is disadvantageously required to 
correct the altered dimensions and accuracy of the jigs. Moreover surface 
treatments such as plating increase the man-hours necessary for producing 
the jigs. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide an extrusion-forming 
jig which eliminates all the disadvantages of the prior art and has a much 
higher wear-resistance which is improved in a simple manner. 
In order to achieve this object, the extrusion-forming jig for use in 
extruding a ceramic batch to regulate a sectional shape and external 
dimensions of a honeycomb structure formed by the extrusion according to 
the invention comprises electric discharge coating layers on surfaces of 
the jig contacting the ceramic batch. 
The electric discharge coating layers are preferably made of a cemented 
carbide. 
According to the invention, electric discharge coating layers preferably 
made of a cemented carbide such as tungsten carbide of a thickness of 
10-60 .mu.m are applied onto inner diameter portions of the jigs which 
contact ceramic batches while forming honeycomb structures by extruding, 
thereby obtaining high wear-resistance enabling the jig to be used for 
many hours without requiring that the jig be replaced. 
The electric discharge coating layers have a diffusion property to exhibit 
a strong attaching force to the jigs. Moreover, the electric discharge 
coating layers can be locally applied to the jigs with ease. Therefore, 
such coating layers are best for the extrusion-forming jigs. 
In order that the invention may be more clearly understood, preferred 
embodiments will be described, by way of example, with reference to the 
accompanying drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 is a sectional view illustrating one embodiment of a die assembly 
equipped with the extrusion-forming jig according to the invention for 
extruding a ceramic material to form honeycomb structures. In the 
embodiment shown in FIG. 1, the die assembly 3 for extruding the ceramic 
material comprises an extruding die 1 and an extrusion-forming jig 2 
provided downstream of the extruding die 1. With this arrangement, a 
ceramic batch is supplied into material supply apertures 4 of the 
extruding die 1 and forced through slits 5 in the form of a grid to form 
cells of a honeycomb structure. The ceramic batch is then forced through 
the extrusion-forming jig 2 to form outer walls of the honeycomb structure 
so as to complete the honeycomb structure. 
As a result, an inner diameter portion 6 of the extrusion-forming jig 2 
contacting the ceramic batch regulates the sectional shape of the 
structure and external shape and dimension of the outer walls. According 
to the invention, electric discharge coating layers 7 are provided on the 
inner diameter portion to improve the wear-resistance of the inner 
diameter portion. Namely, as shown in detail in FIG. 2, the inner diameter 
portion 6 is provided with the electric discharge coating layers 7 having 
a thickness of 10-60 .mu.m by the use of a commercially available electric 
discharge coating apparatus. The coating layers are preferably of a 
cemented carbide such as tungsten carbide. The "electric discharge coating 
apparatus 720" manufactured by Stanley-IJR is preferably used for this 
purpose. The electric discharge layers 7 could be simply formed by causing 
electric discharge at the relevant surfaces of the extrusion-forming jig 2 
from an electrode made of a cemented carbide in the apparatus. 
An actual example will be explained hereinafter. 
EXAMPLE 
Three kinds of extrusion-forming jigs 2, such as shown in FIG. 1, were 
prepared. They were the jigs of the prior art made of an untempered steel 
of JIS (Japanese Industrial Standard) SKD-61, the jigs of the prior art 
made of the untempered steel of JIS SKD-61 whose inner diameter portions 
were subjected to a nitriding treatment, and the jigs made of the 
untempered steel of JIS SKD-61 whose inner diameter portions were provided 
with electric discharge coating layers 7 of tungsten carbide according to 
the invention. These extrusion-forming jigs 2 were mounted on an extruding 
end of a ceramic batch extruding apparatus and used for actually extruding 
ceramic batches. Relations between amounts of extruded ceramic batches and 
amounts of wear at the inner diameter portions of the jigs were measured. 
The amounts of wear were determined by differences in diameters before and 
after extruding predetermined amounts of the ceramic batch. Results are 
shown in FIG. 3. 
As can be seen from the results in FIG. 3, the jigs of the prior art made 
of the untempered steel of JIS SKD-61 were worn 0.2 mm when the 20 tons 
ceramic batches had been extruded through the jigs, and even the jigs of 
the prior art whose surfaces at the inner diameter portions were treated 
by the nitriding treatment were worn 0.15 mm when 70 tons of ceramic batch 
had been extruded therethrough. On the other hand, the jigs provided with 
electric discharge coating layers 7 according to the invention were worn 
only 0.05 mm when 150 tons of the ceramic batch had been extruded. 
As can be seen from the explanation as above described, by applying the 
electric discharge coating layers preferably made of cemented carbide such 
as tungsten carbide, to the surfaces of the jigs contact the ceramic 
batches in this simple manner, the extrusion forming jigs according to the 
invention exhibit high wear-resistance which enables the jigs to continue 
extruding ceramic batches for many hours without requiring replacement of 
the jigs. 
It is further understood by those skilled in the art that the foregoing 
description is that of preferred embodiments of the disclosed jigs and 
that various changes and modifications may be made in the invention 
without departing from the spirit and scope thereof.