Electrically weldable sleeve for joining thermoplastic pipeline parts

The electrically weldable sleeve has a tubular sleeve member, heating elements arranged on the inner wall of the sleeve member and terminals for an energy supply. A material recess in the form of slots is provided in the edge zone of the boundary area of the heating element. This makes it possible to completely utilize the shrinkage reserve imparted to the sleeve member during the production of the sleeve, so that it is effective up to the solidification of the core of the melting zone. It also completely compensates for the volume contraction of the material linked with the solidification of the melting zone, so that the occurrence of cavities is reliably avoided. Corresponding to the axial flux of force represented by lines, the material recesses must be located at a point, which leads to no weakening of the sleeve member portions decisive for assembly and operation.

BACKGROUND OF THE INVENTION 
The present invention relates to an electrically weldable sleeve for 
joining thermoplastic pipeline parts, which has a tubular sleeve member 
made from a thermoplastic material, an electrical heating element embedded 
into the inner wall of the sleeve member and terminals for the supply of 
electrical energy to the heating element. The electrical energy supplied 
to the electric heating element is used for producing heat for the purpose 
of forming a melting zone on welding the sleeve member to the portions of 
the pipeline parts located in the melting zone. 
Electrically weldable sleeves made from the same or a similar thermoplastic 
material are known for joining thermoplastic pipeline parts. The term 
pipeline parts is understood to cover straight and curved pipeline 
sections, shaped pipe portions and pipeline fittings. For the purpose of 
joining to other pipeline parts, said pipeline parts have tubular 
connection pieces, on to which is placed a sleeve of the aforementioned 
type and is electrically welded therewith to form a drip-proof and gas 
tight joint. Electrically weldable sleeves of the aforementioned type are 
known in numerous different constructions, reference being made e.g. to 
U.S. Pat. Nos. 3,943,334 and 4,117,311. 
If such known electrically weldable sleeves are used for joining pipeline 
parts, the attaining of a reliable, tight connection is dependent on 
different characteristics of the sleeve. A particularly important 
characteristic of a weldable sleeve is its capacity to shrink by releasing 
latent stresses during its heating occurring during the welding process, 
in order to remove the clearance between the weldable sleeve and the 
connection pieces of the pipeline parts to be welded. For this purpose it 
is known to impart a shrinkage reserve to the thermoplastic sleeve member 
during the production of the sleeve. This is released during the heating 
taking place during the welding process for eliminating the existing 
clearance between the sleeve and the connecting pieces of the pipeline 
parts. 
Simultaneously with the release of the shrinkage reserve and the removal of 
the clearance between the sleeve and the connection piece of the pipeline 
part, a volume contraction occurs during the cooling of the welded joint 
in the welded zone when the molten material passes into the solid state. 
This volume contraction can be considerable and is e.g. approximately 20% 
in the case of polyolefinic materials. If the shrinkage reserve imparted 
to the sleeve member is not sufficient, to completely compensate for the 
volume shrinkage through the shape change occurring during shrinking, 
cavities form in the solidifying melting zone. Since as a result of the 
heat gradient occurring on the sleeve member, cooling advances from the 
edges of the melting zone towards the center thereof and there is an 
increase in the proportion of the sleeve member which is supported on the 
connecting pieces projecting into the sleeve. This supporting action of 
the connecting pieces acting counter to the shrinkage of the sleeve means 
that in the final area of the not yet solidified melting zone, the sleeve 
member completely follows the volume shrinkage occurring, and can 
compensate same by a shape change. Thus, cavities impairing the strength 
of the sleeve occur in the center of the melting zone. Such cavities can 
in particular occur in the case of relatively thick-walled sleeve members, 
such as conventionally occur when joining pipeline parts which are to be 
operated under pressure. 
SUMMARY OF THE INVENTION 
The problem of the present invention is to so further develop an 
electrically weldable sleeve of the aforementioned type that a complete 
welding of the sleeve member to the connection pieces in the vicinity of 
the solidified melting zone is reliably insured, without there being any 
weakening of the sleeve member in the portions vital for assembly and 
operation. Simultaneously, the sleeve member is to be used as an 
indicating means for the completion of the welding operation. 
According to the invention this problem is solved in that one or more 
material recesses are provided in the edge areas of the sleeve member 
extending from the sleeve faces into the boundary area of the melting zone 
for insuring an adequate radial freedom of movement of the sleeve member 
during the welding process for compensating for the clearance between the 
sleeve member and the pipeline parts on the one hand and the volume 
reduction during the solidification of the melting zone on the other. The 
present invention includes an improved sleeve member, composite and method 
for forming same. 
The invention also includes a method for utilizing the sleeve according to 
the invention for indicating a functionally correct performance of the 
welding process, in which a change to the location of the material 
recesses of the sleeve member is established.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The electrically weldable sleeve shown in FIG. 1 has a sleeve member 1 and 
a heating means subdivided into two heating elements 2, 3 arranged in the 
vicinity of the inner circumference of the sleeve member 1. Sleeve member 
1 is centrally provided with a web 4, which acts as a stop for two 
connection pieces 5, 6 on not shown pipelines. 
Heating elements 2, 3 are located in the vicinity of the connection pieces 
5, 6 and extend so far that the outer part of an edge zone 9 and an inner 
zone 10 remain unheated. Heating elements 2, 3, which are represented here 
as thick unbroken lines, are mostly constructed as a winding of a 
resistance heating wire, whose turns are arranged in spaced juxtaposed 
manner. However, the construction of the heating elements 2, 3 is not 
essential for the invention. 
If the heating elements 2, 3 are supplied with electrical energy by means 
of terminals located in sleeve member 1 shown schematically in FIG. 1 as 
terminals 2a and 3a, heat is produced in heating elements 2, 3 and 
consequently a melting zone 11 is produced shown in dotted line form, in 
which the sleeve member 1 is joined to the connection pieces 5, 6. When a 
sufficient quantity of material of sleeve member 1 and connection pieces 
5, 6 is liquefied for good welding purposes, the energy supply is cut off. 
The evolution of heat in melting zone 11 is accompanied by the release of 
the shrinkage reserve imparted to the sleeve member 1 during the 
manufacture of the sleeve, so that the sleeve member shrinks and engages 
against the connection pieces 5, 6, which now oppose the further shrinkage 
of sleeve member 1. This supporting effect represented by arrows 12, 13 in 
FIGS. 1 and 2 is relatively small at the end of the heating process, when 
the melting zone 11 has its greatest extension, but increases in size at 
the end of the energy supply, cf. arrows 12, 13 in FIG. 2, because as a 
result of the heat gradient heat is removed from melting zone 11 to the 
outside, so that there is a constant reduction in the extension of melting 
zone 11. As a result of the increased support action, further shrinkage of 
sleeve member 1 is inhibited. The consequence of this is that in the 
center of the melting zone cavities can form as a result of the volume 
contraction of the solidifying melt, and these reduce the quality of the 
weld. 
The invention is based on the idea that said cavity formation can be 
avoided, if a measure is taken in the sleeve member 1, so that at the end 
of the solidification of the melting zone, said member retains its radial 
freedom of movement and consequently its shrinkage capacity to such an 
extent that the volume shrinkage can be made good up to the complete 
solidification of the melt. This measure is achieved through the 
construction of the sleeve member according to FIGS. 3 to 13. For reasons 
of simplicity, these drawings only show part of the electrically weldable 
sleeve and a connection piece. 
This measure, which leads to a greater radial freedom of movement, 
essentially comprises providing one or more material recesses, which 
reduce the stiffening effect of the edge zones, but this is at a point of 
the sleeve member, which cannot lead to a strength reduction for portions 
which are decisive for assembly and operation. 
In FIG. 3, this reduction of the stiffening effect is achieved by a slot 14 
on the outer circumference of sleeve member 1 and which is located in the 
edge portion 9 in the vicinity of heating element 2. In FIG. 4 a notch 15 
is provided at the same point in place of the slot 14 and one wall thereof 
extends from the edge zone 9 so as to slope over part of heating element 
2. 
In FIG. 5, apart from slot 14, the wall thickness of edge zone 9 is reduced 
by a portion of the slot depth, while in FIG. 6 it is reduced to the 
bottom of slot 14. In FIG. 6 the transition from the original slot 14 to 
the circumference of sleeve member 1 is an edge, but in the manner 
indicated in broken line form can also be bevelled. 
FIG. 7 has a multiple slot 16 with three slots 16', 16", 16'" and different 
widths and depths. The multiple slot 16 can also have a different number 
of slots. FIG. 7 has lines 17 which, in the case of axial tensile 
stressing, represent the path of the flux of force between the sleeve 
member 1 and the connection piece 5. It is important that the slots 16 
used for reducing the sleeve stiffness do not lead to any strength 
reduction having an unfavorable influence on operation. Thus, the depth 
and position of the recesses are to be matched to the location of the 
heating element 2 and/or melting zone 11. It must also be insured that the 
full thickness of sleeve member 1 comes to bear in front of the inner edge 
of heating element 2 and/or melting zone 11 This is appropriately made at 
least as thick as the wall thickness of the connection pieces 5, 6 to be 
welded. 
In the construction according to FIG. 8, there are inside wall recesses 18 
in edge zone 9 of sleeve member 1, which extend into the vicinity of 
heating element 2. Between the individual recesses 18 in edge zone 9 are 
provided guidance webs 20. Recesses 18 according to FIG. 8 can also be 
constructed in the form of outside wall recesses 21 according to FIG. 9. 
If no webs 22 are provided, the construction according to FIG. 6 is 
obtained. Finally, the recesses can be in the form of slots 23, which 
extend through the entire wall thickness of sleeve member 1 and are 
separated from one another by web portions 24 as in FIG. 10. The slots 23 
can be relatively narrow, while the web portions 24 extend over a 
correspondingly larger part of the circumference. 
In the construction according to FIG. 11 a slot 25 is provided in the 
vicinity of edge zone 9 of sleeve member 1. Unlike in the construction 
according to FIG. 3, the slot 25 does not extend over the entire sleeve 
circumference and instead forms partial slots, which are separated from 
one another by the slot-free portions 26. 
FIGS. 12 and 13 show constructions, in which the resilient point can be 
simultaneously used to provide an optical indication of the welding 
process. For this purpose, in the construction according to FIG. 12, slot 
27 is made sufficiently deep to insure that during the supply of welding 
energy, melt 28 can pass out of welding zone 11 into slot 27. The escape 
of melt 28 indicates that an overpressure exists in melting zone 11 
through the shrinkage of sleeve member 1 and this is a prerequisite for 
completely satisfactory welding. 
In the construction according to FIG. 13, there is a multiple slot 29, e.g. 
two slots 29', 29" of the same width, whose shape changes when a pressure 
occurs in melting zone 11, as is e.g. shown in the upper half of FIG. 13. 
The two originally identical slots 29', 29" diverge from one another, e.g. 
in that slot 29" becomes narrower than slot 29'. 
In the described constructions according to FIGS. 3 to 13, a distinction 
can be made between two groups. The first group covers the constructions 
according to FIGS. 3, 4, 7, 11, 12, and 13, which all have a frontally 
solid edge zone 9. This solid face can e.g. be used as a force application 
point during the displacement of sleeves during assembly. The other group 
of construction according to FIGS. 5, 6, 8, 9 and 10 achieves the increase 
in the resilience of the sleeve member in that the circumferential 
cross-section of the edge zone 9 is wholly or partly reduced and in the 
case of the constructions according to FIGS. 8 and 10 the axial extension 
of the recesses 18, 23 extends approximately up to the edge of heating 
element 2. However, in all the constructions the flux of force is insured 
in an unimpeded form through the welding point. 
It is to be understood that the invention is not limited to the 
illustrations described and shown herein, which are deemed to be merely 
illustrative of the best modes of carrying out the invention, and which 
are susceptible of modification of form, size, arrangement of parts and 
details of operation. The invention rather is intended to encompass all 
such modifications which are within its spirit and scope as defined by the 
claims.