Joining method

Rigid thermoplastic articles having sections of different thickness are extrusion welded together, the part of the thicker section to be contacted by the extruded bead being preheated before being thus contacted, to a temperature higher than that of the thinner section. The thicker section may be heated above its softening point, e.g. by the tip of the extruder, while the thinner section remains below its softening point until contacted by the extruded bead. The method is useful for welding thin sectioned profile extruded board to thicker sectioned header pipes during manufacture of heat exchangers such as solar panels.

The invention relates to the joining together of rigid shaped articles 
(including both completed and partially completed objects, and parts 
therefor) formed of thermoplastic materials, by extrusion welding. 
In its broad aspects, extrusion welding is a known technique which 
comprises bringing into juxtaposition the parts of the articles to be 
welded together and extruding onto their adjacent surfaces a molten bead 
of a compatible thermoplastic material at a temperature above the melting 
temperatures of the articles, so that some of the thermoplastic material 
of both articles is melted by the bead. The molten bead being compatible 
with the material it melts, fuses with it, and on cooling forms a welded 
joint. Extrusion welding can be a very effective technique for use where 
the articles are of a similar thickness. However problems can arise when 
self-supporting, thin, rigid sections are to be welded to thick sections, 
in that extruded beads having sufficient heat to adequately melt the 
surface of the thicker section with its higher heat capacity, may also 
have sufficient heat to melt the thinner section to too great a depth and 
cause distortion. These problems can arise, for example, in the 
manufacture of heat exchangers such as the solar panels described in 
German Offenlegungsschrift No. 2,505,015. Illustrative examples are 
described therein which are manufactured from polypropylene profile 
extruded packaging board whose outer sheets are typically only 0.4 mm or 
less thick, and whose header pipes are much thicker, typically about 6 mm 
thick. Thus whereas it may be possible to extrusion weld these 
satisfactorily as described above, the margin of error is not large, and 
too cool an extruded bead will give a weak bond to the header whereas too 
hot a bead will distort the board. 
According to the invention, a method of joining two rigid thermoplastic 
articles having sections of different thickness, comprises extruding into 
contact with adjacent parts of both articles a molten bead of compatible 
thermoplastic material at a temperature above the softening points of the 
thermoplastic articles so as to fuse with the parts contacted and on 
cooling form a weld; wherein the part of the thicker sectioned article to 
be contacted by the extruded bead is preheated before being thus 
contacted, to a temperature higher than the temperature of the thinner 
section. 
Preferably the part of the thicker sectioned article to be contacted by the 
extruded bead is preheated above its softening point while the thinner 
section is maintained below its softening point until contacted by the 
extruded bead. Particularly preferred is a method in which there is no 
substantial increase in the temperature of the thinner section during the 
preheating of the thicker section. 
The method of the invention is useful in that it enables the bead to be 
extruded at a lower temperature without reducing the strength of the weld 
to the thicker section. This lower bead temperature is advantageous in 
reducing any tendency for the bead to distort the thinner section. 
Moreover, where the thermal stability of the extruded material is low, 
less degradation occurs. 
There are several methods for carrying out the preheating, some being more 
readily adaptable to the differential application of heat than others. 
Thus for example, preheating of a workpiece can be effected by blowing hot 
gas onto it, and indeed extrusion welders have been described (e.g. in 
British Pat. No. 1,187,136) in which hot gas preheating is incorporated. 
However, with hot gas preheating it is difficult to preheat one article 
without the other. A preferred method is one in which the thicker section 
is preheated by heat applied to its surface by a heated tool. The tool may 
simply be held close to the thicker section to direct radiant heat onto 
the parts of the surface to be welded, but preferably the tool should 
actually be brought into contact with the surface. This latter preferment 
generally enables the position of the parts preheated to be more 
accurately defined. The heated tool may conveniently be a heated tip of 
the extruder adjacent the orifice through which the bead is extruded.

The heat exchanger illustrated in FIGS. 1 and 2 comprised a polypropylene 
profile extruded packaging board 11 connected at either end to a 
polypropylene header 12, 13. (These are shown spaced apart in the exploded 
view of FIG. 1.) The board was about 1 m wide 2 m long and 4 mm thick, 
comprising two spaced-apart sheets (each about 0.38 mm thick) connected by 
a plurality of webs (also about 0.38 mm thick) spaced apart by about 5 mm. 
The webs divided the space between the two sheets into a plurality of 
parallel closed channels extending from one header to the other. The webs 
and the two sheets had been extruded as an integral extrudate of a medium 
ethylene content (i.e. about 6-10 weight %) polypropylene having a melt 
flow index (230.degree. C./2 kg) of 0.6. 
The headers were formed from 25 mm outside diameter polypropylene tubing 
having a wall thickness of about 6.4 mm, and milled in the side of the 
tube was a slot 14 sufficient just to receive one end of the board, the 
outer lips of the slot being chamfered. Assembly of the heat exchanger 
entailed inserting the ends of the board into their respective slots in 
the headers, and applying a bead 15 to weld the two together as described 
hereinafter. 
The extruder used to extrude the bead was a small hand-held extruder as 
shown in FIGS. 3 and 4. The extruder comprised a base board 31 having two 
hand grips 32, 33. Mounted on the base board were two bearings 34, 35 out 
of the latter of which extended a hopper 36 and a barrel 37 of outside 
diameter 25.4 mm. Around the barrel were two cuff heaters, and the barrel 
ended in a 45.degree. aluminum nose cone 38 with an orifice for extrusion 
of the bead at the apex of the cone. Within the barrel was a simple 9.5 mm 
diameter screw, about 152 mm long. The screw had a drive shaft 39 coupled 
to a flexible drive shaft 40 by a flexible coupling 41, the flexible shaft 
being supported by the other bearing 34 to which a flexible torque tube 42 
surrounding the flexible shaft was attached. 
In forming the heat exchanger, the hopper of the extruder was filled with 
powder of a medium ethylene content polypropylene of melt flow index 
(230.degree. C./2 kg) of 0.8, stabilised against degradation by heat and 
ultra-violet radiation and kept cool by compressed air while in the 
hopper. On rotation of the screw by a motor (not shown) via the flexible 
drive, the powder was transported along the barrel where it was heated to 
a melt temperature of about 270.degree.-290.degree. C. The temperature of 
the nose cone was about the same as that of the melt, and the bead which 
extruded had a diameter of about 5 mm. The extruder was mounted on a lathe 
carriage so that it could be moved at a steady rate along the lathe bed. 
The headers were each provided with a chamfered slot 14 to receive the 
board, and then a transverse V-shaped slot was cut into each header at 
both ends of the slot. The ends of the board were then inserted into the 
slots of the two headers, one at either end, and transverse welds made 
using the V slots, the ends of the welds being trimmed in line with the 
sides of the board. The assembled board and header unit was then mounted 
in a jig so as to lie parallel to the lathe bed. The extruder was 
positioned so that the nose cone touched the header but not the board, and 
with the bead being extruded, the carriage was moved along the lathe bed, 
the heated nose cone remaining in contact with the header as it moved 
along. Orientation of the extruder was such that the extrudate on emerging 
from the die, was forced against the surfaces of board and headers, into 
the chamfered slot. The extruder was angled to the board, with the orifice 
trailing, to provide smoother flow of extrudate into the slot. The 
direction of movement was such that the molten bead was deposited 
immediately behind where the nose moved in contact with and melted a strip 
of the header immediately adjacent the slot, but without touching the 
board. A single bead was deposited on both sides of the board for each 
slot, and on cooling a strong watertight join was made between the board 
and each header without any noticeable puckering of the board. 
The invention has been illustrated above by reference to the welding of 
polyolefins as it is of particular utility in connection with these and 
other materials for which solvent welding does not readily provide an 
alternative method. However, the method is, in fact applicable generally 
to thermoplastic materials suitable for manufacturing moulded articles, 
even to those having high softening points and hence requiring welding 
beads extruded at still higher temperatures. To illustrate this there will 
now be described an operation in which a reinforcing hub was welded onto 
the surface of a thin sheet, both articles being moulded from a 
polysulphone having a softening point of about 174.degree. C. The thin 
sheet had a thickness of 0.38 mm, and the hub was a disc about 5 mm thick 
and 25 mm diameter. 
The extruder had an aluminium nose cone with a 60.degree. apex, and the 
hopper was supplied with polysulphone of the same composition as that used 
to mould the articles. Extrusion was commenced with a barrel temperature 
reading 340.degree. C. and the nose cone temperature reading 350.degree. 
C. A bead of the molten polysulphone was extruded around the perimeter of 
the disc into contact with both the disc and the surrounding sheet, while 
the tip of the nose cone was pressed against the disc immediately ahead of 
the bead to provide preheat. The nose cone tip appeared to melt the disc, 
digging into the surface slightly, while avoiding contact with the 
surrounding thin sheet. When a bead had been extruded right round the disc 
and allowed to cool, the disc was found to be firmly welded to the sheet. 
With the disc appropriately supported during welding so as to ensure 
evenness of conditions, a weld could be obtained with little or no 
distortion of the thin sheet. 
Several repeated and comparative experiments were carried out respectively 
with and without preheating of the discs, and the strengths of the welds 
tested by manually holding the parts using pliers and pulling apart 
(having first removed parts of the sheet or provided a bead around only 
part of the disc perimeter, to enable the disc to be held). The poor welds 
gave a smooth disc edge while the good welds gave a jagged edge to the 
disc. It was found that under these conditions of extrusion, good welds 
were obtained where preheating was applied as described above, but there 
was generally very poor adhesion between the extruded bead and the disc 
when there had been no preheating. The welds between the bead and the thin 
sheet were found generally to be strong.