Everything under control: the importance of post welding test

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Product Insights

The sealing of welding of thermoplastic materials is essential. Satisfying the need for validation of welds requires appropriate tools: as is logical to expect, these are part of the offer of Leister.

Author: Alberto Pecchio, Marketing Specialist, Leister Italy

The sealing of welding of thermoplastic materials is essential, especially when this concerns the sealing of waste disposal sites or civil engineering works such as dams, reservoirs or tunnels. For this type of construction, a work can only be said to be definitively completed when it is also possible to complete an accurate quality check of the welds according to well-defined parameters. Satisfying the need for validation of welds requires appropriate tools: as is logical to expect, these are part of the offer of Leister, a Swiss multinational leader in providing solutions for plastic welding.

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Leister's experience in this segment is an excellent prerequisite for the careful design of suitable systems for assessing the quality of welds. We talked about these systems with Klaid Mastora, Leister's technical expert in machine maintenance and quality control.

What types of welds are tested?

For quality work it is essential to frequently check the quality of the welds carried out. Welding checks and documentation become the norm when working on sites such as landfills where quality must be at its highest. Errors in the waterproofing phase would in fact put the water table at risk with potential negative consequences in terms of public health. Depending on the type of work, the client may be more or less demanding and the waterproofer must be prepared to check and document the quality of its work. All welds can be tested, both those performed with automatic machines, such as our GEOSTAR G5 or G7 version, TWINNY T5 or T7 version or COMET, and those performed using extruders. The first phase of the test is the realization of a test weld on the materials that will then be welded. It is necessary to prepare test pieces and the perfect tool for this operation is the COUPON CUTTER, the mechanical punching machine signed Leister which, thanks also to the long ergonomic lever, allows to easily obtain welding samples in line with DVS regulations (directives on plastic materials processing, editor's note). Once the sample has been obtained, it can be subjected to the tensile strength test.

How is the tensile strength measured?

It's simple, with EXAMO USB. This device basically consists of a control and measuring unit, test tube clamping jaws and a jaw movement mechanism. A test speed must be set, for example the speed at which the jaws move away causing the test sample to be subjected to stress due to progressive elongation. At the end of the test, of destructive type, through digital display it is possible to visualize maximum load (F peak) and breaking load (F tear) measured together with the corresponding elongation values. 
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The breakout force values must remain within a precise minimum limit established according to the materials and the intended use of the weld. The appearance of what remains of the sample after tearing also provides the expert technician with useful information on the quality of the weld. Handy and light, Examo is a small portable laboratory that was created to be easily used on site.

What other methods can be used to assess the quality of a weld?

Very common tests are done directly on site and are based on measuring the air pressure inside the welded membranes. The most typical example takes advantage of the particular conformation of the so-called double track welds, close welds that create a test channel into which air can be injected.

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A compressed air pressure gauge and a TEST NEEDLE are used to test the seal of the weld. The needle is inserted into the weld where air is blown in until a certain pressure, measured with the pressure gauge, is reached. Then the pressure trend is monitored at specific intervals; the test is passed if the pressure drop remains within a range considered acceptable. The needle with pressure gauge is available in two different models according to the hardness of the material to be tested and a simple protection system avoids any risk to the operator.

How to accurately locate any leaks in the welds?

For this requirement, Leister offers a system consisting of a VACUUM PUMP and bells which can be of different shapes (square = VACUUM BELL or rectangular = VACUUM BOX). The weld to be checked is previously sprayed with a solution of soapy water. Thanks to the vacuum created by the pump inside the bell, a passage of air is generated that immediately highlights any leaks. If the problem is not too extensive, the area can easily be repaired - for example, with a new extrusion weld - without the need to redo the entire weld.

Are the tests carried out using these methods reliable?

Absolutely, yes. The preventive check on the initial sample, carried out with EXAMO, and the subsequent tests with the TEST NEEDLE and the Vacuum system, allow the correct parameters to be set for the entire welding process. In addition, Leister's most modern welding machines such as GEOSTAR G5 or G7 version or TWINNY T5 or T7 version are equipped with LQS (Leister Quality System), a system equipped with GPS that makes them able to constantly record all welding parameters together with the geolocation of the machine.