Apparatus for forming unevenness on an outer surface of a synthetic resin-coated steel pipe

In order to improve the bonding strength between a synthetic resin-coated steel pipe and concrete applied around the synthetic resin-coated steel pipe for weight coating or protection, unevenness formed of groove portions and ridge portions arranged alternately is formed on the outer surface of the synthetic resin-coated steel pipe by pressing the synthetic resin coating layer with a working tool having an uneven inner surface under a condition where at least one member of the working tool and the synthetic resin coating layer is kept at a temperature at which the synthetic resin is melted.

BACKGROUND OF THE INVENTION 
The present invention relates to a method and an apparatus for forming 
"unevenness" on an outer surface of a synthetic resin-coated steel pipe. 
The term "unevenness" used herein means unevenness formed of groove 
portions and ridge portions alternately arranged as shown in FIGS. 3 and 4 
by numerals 20 and 21. 
Synthetic resin-coated steel pipes, for example, are provided for marine 
bottom use by surrounding the outer surface of these pipes with concrete 
for the purpose of weight coating or protection. However, the synthetic 
resin-coated steel pipes are relatively hard and smooth on the outer 
surface, so that the bonding strength between the synthetic resin outer 
surface and the concrete surface is low. Accordingly, there is the 
possibility that the concrete surface and the synthetic resin outer 
surface may become separated owing to the external force acting in an 
axial direction of the steel pipe. 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide a method and an apparatus 
for forming easily unevenness on an outer surface of a synthetic 
resin-coated steel pipe. 
The above described object can be attained by keeping the temperature of a 
working tool having an inner surface having the above defined unevenness 
and/or a synthetic resin coating layer at a temperature capable of melting 
the synthetic resin and pressing the synthetic resin coating layer with 
the working tool to form the uneven surface on the synthetic resin coating 
layer. 
An apparatus for forming the unevenness on the outer surface of the 
synthetic resin-coated steel pipe includes a working tool having the 
uneven inner surface formed of groove portions and ridge portions, which 
is provided with a heater therein and an urging means which presses the 
synthetic resin coating layer with the working tool.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 is a side view showing an entire apparatus for forming the 
unevenness according to an example of the present invention and FIG. 2 is 
a cross-sectional view of said apparatus taken along line II--II in FIG. 
1. A synthetic resin-coated steel pipe 11 is held on a trolley 13 movably 
mounted on rails 12. At a middle position of the rails 12, a gate-formed 
frame 14 is arranged. At each of four positions in the frame 14 in the 
longitudinal direction of the steel pipe 11 transferred together with the 
trolley 13, three compressing cylinders 15 as an urging means are arranged 
in the peripheral direction of the steel pipe 11 at equal interval. A 
piston rod 16 of each compressing cylinder 15 orients to the center of the 
steel pipe 11 as shown in FIG. 2 and a top end portion of the piston rod 
16 is connected to one end of a connecting member 17 with a pin and 
another end of the connecting member 17 is connected to a working tool 18 
by which the uneven outer surface is formed on the synthetic resin coating 
layer, with a pin. The working tool 18 is supported with supporting 
members 19A and 19B made of leaf springs fixed to both sides of the 
connecting member 17 and the inner surface thereof is commensurate with 
the outer surface of the steel pipe 11. The working tool 18 is provided 
with a heater therein and heated to a temperature at which the synthetic 
resin coating layer 11A is melted, and has an uneven inner surface 20 as 
shown in FIG. 3. 
Explanation will be made with respect to a manner for forming the uneven 
surface, characterized by groove portions and ridge portions, on the 
synthetic resin coating layer by means of the above described apparatus. 
The uneven inner surface 20 having groove portions and ridge portions of 
the working member 18 is kept at a temperature, for example, 200.degree. 
C.-250.degree. C., at which the synthetic resin coating layer 11A of the 
steel pipe 11 is melted, by means of a heater housed in the working tool 
18. The steel pipe 11 is transferred together with the trolley 13 and 
stopped in such a position that the outer surface of the steel pipe is 
opposite to the working tool 18. Then, a compressing cylinder 15 is driven 
and the uneven surface 20 of the working tool 18 is urged (for example, 
about 2 kg/cm.sup.2) to a surface of the synthetic resin coating layer 11A 
for several seconds and after the surface of the coating layer 11A has 
been softened, the projected portions on the working tool are pressed to 
the coating layer into a given depth to partially melt the synthetic resin 
coating layer 11A, whereby the form of the uneven surface 20 of the 
working tool 18 is transferred to the outer surface of the synthetic resin 
coating layer 11A. Thereafter, when the working tool 18 is returned back 
to the original position, the uneven surface on the coating layer 11A is 
solidified to form the uneven surface 21 on the outer surface of the 
synthetic resin-coated steel pipe. 
In this example, the uneven surface 21 can be easily formed by pressing the 
synthetic resin coating layer 11A of the steel pipe 11 with the working 
tool 18 provided with a heater therein. The frictional force of the 
synthetic resin-coated steel pipe to earth ground (where the steel pipe 11 
is piled), and the bonding strength of the synthetic resin-coated steel 
pipe to concrete (by which the outer surface of the steel pipe 11 is 
surrounded), and the like can be increased. Furthermore, the above 
described bonding strength increases in proportion to the increase of the 
total area of the uneven surface 21 as shown in FIG. 5. 
In the above described example, explanation was made with respect to the 
case where a heater is provided in the working tool 18 and the working 
tool is kept at a temperature at which the synthetic resin is melted and 
the unevenness on the outer surface of the synthetic resin-coated steel 
pipe (according to the present invention) may be formed. However, the 
latter may be obtained by pressing the synthetic resin coating layer 11A 
having the given high temperature, as immediately after coating the 
synthetic resin on the steel pipe with a working tool having the uneven 
inner surface at room temperature. 
As mentioned above, the method for forming the unevenness on the outer 
surface of the synthetic resin-coated steel pipe according to the present 
invention includes pressing the synthetic resin coating layer with the 
working tool (having the uneven inner surface) under the condition that 
the temperature of the working tool and/or the synthetic resin coating 
layer are kept at a temperature capable of melting the synthetic resin, 
whereby the uneven shape of the inner surface of the working tool is 
transferred to the synthetic resin coating layer. 
The apparatus for forming the unevenness on the outer surface of the 
synthetic resin-coated steel pipe according to the present invention 
includes a working tool having the uneven inner surface which is provided 
with a heater therein and an urging means by which the working tool is 
pressed against the synthetic resin-coated steel pipe, so that the 
unevenness is easily formed on the outer surface of the synthetic 
resin-coated steel pipe by a simple apparatus.