METHOD OF TESTING THE STRENGTH OF SEAMS IN PLASTIC SHEETS

METHODE D'ESSAI DE LA RESISTANCE DES SOUDURES DE FEUILLES DE PLASTIQUE

English Abstract

ABSTRACT OF THE DISCLOSURE
Current state-of-the-art requires welded seams between
adjacent sheets of material, e.g. polyethylene to be tested for
leaks using vacuum, ultrasonic or low pressure air techniques. In
general, the devices used are somewhat complicated and consequently
expensive. A relatively simple method of determining the strength
and integrity of a welded seam between layers of plastic, which are
interconnected by welding to form a sealed passage including the
step of injecting air or another gas into the passage under
sufficient pressure that any weakness in the seam results in
visible distortion or total separation of the plastic in the area of
such weakness.

Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:
1. A method of testing a dual welded seam between
adjacent, overlapping layers of a plastic material, the
layers being interconnected by welds to form a sealed
passage, said method comprising the step of injecting gas
into said passage under sufficient pressure that any
weakness in the seam results in distortion and/or total
separation of the plastic in the area of such weakness.
2. A method of testing parallel seams between overlapping
layers of a plastic material comprising the steps of
sealing the ends of the passage defined by the area between
the seams forming an opening through one of the layers into
the passage; attaching, by welding an inlet duct to the
sheet in operative relation with the opening, and injecting
a gas into said passage through the inlet duct under
sufficient pressure that any weakness in the seams results
in distortion of the plastic in the area of such weakness.
3. A method according to claim 2, wherein the plastic
material is polyethylene and the seams are welded.
4. A method according to claim 3, wherein the gas is air,
which is injected into the passage under a pressure of at
least 1 psi per mil thickness of each layer of polyethylene
per inch of passage width between welds.

Description

" ~33~
This invention relates to a method of testing seams
between adjacent layers of plastic material, and in
particular to a method of testing parallel, welded seams
between adjacent layers of polyethylene.
During the past few years, because of environmental
concerns, there has been a tremendous increase in the use
of impervious sheet material or containing waste. The
sheet material most suitable at this time for containing
waste liquids is polyethylene which is extremely inert to
most chemicals
Because polyethylene is inert, the material cannot be
glued, and all seams must be welded, i.e. the overlapping
plastic sheets are partially melted and thus fused
together. Obviously, the finished product is only as good
as the welded seams.
There are two basic welding techniques currently in
use, extrusion welding and heat fusion welding. Both
welding techniques are capable of producing quality welds
having strengths equal to that of the basic plastic
material. By the same token, incompetent operators,
inferior equipment or adverse conditions can result in
1awed welds which are weaker than the plastic.
It is currently required that welded seams between
adjacent sheets of material, e.g. polyethylene, be tested
for leaks using vacuum, ultrasonic or low pressure are
techniques
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2~3~2
In the conventional method of testing, the quality of
the weld is determined by cutting out coupons from the weld
seam ~or "peel" testing on a tensometer. There are however
two major flaws to this method. Firstly, by cutting out
the coupon, a hole in a good seam is created and future
leakage at the resulting patch is a distinct possibility.
Secondly, the quality of the weld is only checked at the
point where the coupon is removed, the rest of the seam is
assumed to be flawless.
A need therefore for a relatively simple method of
determining the strength and integrity of a welded seam
along the entire length o~ the seam.
The object of the present invention is to meet the
above defined need by providing a relatively simple method
stress testing a seam between adjacent layers of a plastic
material, without removal of sections of the seam and
subsequent patching.
Accordingly, the present invention relates to a method
of testing a dual welded seam between adjacent~ overlapping
layers of a plastic material, the layers being
interconnected by welding to form a sealed passage, said
method comprising the step of injecting yas into said
passage under sufficient pressure that any weakness in the
seam results in distortion of the plastic in the area of
such weakness, or total separation of one of the welds with
subsequent total loss of pressure.
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The invention will be described in greater detail with
reference to the accompanying drawings, which illustrate a
preferred embodiment of the invention, and wherein:
Figure 1 is a schematic cross-sectional of a welded
coupon cut from a seam undergoing a "peel" test in
accordance with the prior art;
Figure 2 is a longitudinal sectional view of a dual
welded seam of plastic incorporating an apparatus for
performing the method of the present invention; and
Figure 3 is a schematic, cross-sectional view of the
seam shown in Figure 2, prior to pressurizing;
Figure 4 is a schematic, cross-sectional view of a
pressurized seam with no defects; and
Figure 5 is a schematic, cross-sectional view of a
pressurized seam, partially separated.
Conventional stress testing of weld seams is normally
carried out on small samples. The so-called "peel" test is
performed on samples or coupons which are one inch wide in
J accordance with A.S.T.M. D 882. A quick check is normally
done in the field using a pair of locking pliers. A more
accurate test is done using a tension testing machine (not
shown) designed to pull the ends 1 of a pair of plastic
layers o~ sheets 2 and 3, which are welded together by
parallel spaced apart seams 4 and 5. The test is continued
until the material fails. The test o proper welds is that
failure of the material occurs outside of the weld area,
i~e. the weld is stronger than the material. ~ ;

"-` 2~33~
The high pressure test of the present invention is
intended for use with two elongated sheets, e.g. one
thousand feet long plastic sheets 7 and 8 which are welded
together along their lengths by two transversely spaced
apart weld seams 10 and 11 (Figs. 3 to 6).
In order to perform the method of the present
invention, the ends 12 and 13 (Fig. 2) of the sheets 7 and
8 in the area between the welds 10 and 11 are welded
together to form an elongated closed passage 15. An
opening 16 is formed in one sheet 7 near one end 12
thereof, and a polyethylene inlet duct 18 is welded to the
sheet 7 around the opening 16. The outer end 19 of the
inlet duct 18 is internally threaded for receiving th stem
21 of a T-coupler 22 which connects the inlet duct 18 via
pipe 23 to a pressure gauge 25, and via pipe 27 to a source
of air (not shown) under pressure.
Prior to the stress test, the sheets 7 and 8 in the
area of the passage 15 are generally parallel to each other
(Fig. 3). When air under pressure is introduced into the
passage 15 via the inlet duct 18, stress is created across
each of the weld seams 10 and 11, simulating a i'peel"
test. As best shown in Figure 4, during testing, the
sheets 7 and 8 in the area between the welds 10 and 11 bow
outwardly to define a generally cylindrical passage. Under
~S a pressure of at least 1 psi/mil thickness/inch channel
width, a sub-standard weld can either blow out completely
or partially separate causing a lump 28 (Fig. 5) in the air
passage which can easily be detected by visual inspection.
The above-defined method can be applied to virtually
any length of uninterrupted weld seam.
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