CONTROL & MONITORING OF WELDING PARAMETERS FOR BEST PRACTICE IN THERMAL FUSION WELDING OF GEOMEMBRANES
AUTHORS
28th August 2023
By Eddie Weiser1, John Scheirs2 & Attila Marta 3
Affiliations: 1 Leister; 2 ExcelPlas; 3 Red Earth Engineering (a Geosyntec Company)
ABSTRACT
Thermal fusion welding plays a crucial role in ensuring the integrity and effectiveness of geomembrane liners used in geotechnical and hydraulic engineering systems.
This technical paper emphasizes the importance of monitoring all welding parameters during the hot wedge or hot air combi thermal fusion welding process to achieve best practices.
The paper highlights the significance of factors influencing geomembrane fusion welding and emphasizes the need for accurate control and reproducibility of heat/energy, speed, and pressure parameters. It also discusses the importance of squeeze out and the significance of welding device design principles. The paper concludes by emphasizing the importance of adherence to testing standards and the necessity of incorporating data acquisition systems for post-welding analysis.
INTRODUCTION
Geomembranes are widely used in geoenvironmental, geotechnical and hydraulic engineering systems, and their proper fusion welding is critical for ensuring their effectiveness. This paper focuses on the hot wedge and hot-air combi wedge thermal fusion welding technique and highlights the need for monitoring all welding parameters to achieve best practices.
THE IMPORTANCE OF WELDING PARAMETERS
The thermal fusion welding process relies on the precise balance of three key parameters namely: heat/energy, speed, and pressure. When these parameters are correctly balanced, the fusion welded area becomes as strong as the parent material. Any deviation from the optimal parameter range can result in weak welds. Monitoring and controlling these parameters are therefore crucial for achieving consistent and strong geomembrane welds.
The geomembrane welding process is dependent upon the combination of the following 3 parameters in the correct combination or balance as illustrated in the schematic below:
Note the purpose of this diagram above is to illustrate the ‘sweet spot’ for welding parameters to optimize weld quality. The zone where the red color is most intense, indicates the point at which the three welding parameters are closest to being in the correct proportion or optimum balance. All three parameters are inter-dependent upon each other and any change in one will influence the other. The further one or more of the parameters moves outside of this ideal zone, then the weld strength will be weaker than optimum. Different thermoplastics have different welding characteristics meaning that this balance will be different for each type of thermoplastic being welded.
Typical Fusion weld parameters which have been used to provide good results for polyethylene geomembranes are as follows:
- Heat = 280-460 deg.C (hot air combi wedge 380-560°C)
- Pressure = 20-40 N/mm that is per/mm nip roller width
- Speed = 1.5-4.5 m/min
For consistent reproducibility of the welding process, it is necessary for the welding device to have an accurate and reproduceable control and display of all 3 parameters.
SQUEEZE OUT AND ITS SIGNIFICANCE
Squeeze out (refer to figure below) is the lateral melt extrusion that occurs during welding, is an important indicator of the welding process. It signifies that sufficient melt plasticization has taken place for a good weld. However, excessive squeeze out can indicate improper welding parameters such as overheating or excessive pressure.
Additionally, overheating and excessive squeeze out production at the seam may reduce antioxidant levels or induce morphological change making a seam more susceptible to oxidative degradation and stress cracking failures. Understanding the significance of squeeze out helps in assessing the quality of the weld.
WELDING PARAMETERS AND REPRODUCIBILITY
To ensure consistent and reproducible welding, it is essential to have accurate control of all three welding parameters: heat/energy, speed, and pressure. The closer these parameters are to the ideal settings, the stronger the weld becomes. Weld strength should be comparable to the parent material’s strength, which can be verified through destructive testing using a tensiometer and compliance with relevant standards.
CONSIDERATIONS FOR OPTIMUM WELDING PARAMETERS
Welding parameters must be tailored to the specific site conditions, taking into consideration such things as material type, melt flow index (MFI), surface texture, and ambient weather conditions. Welding under the dew point is not allowed as it adversely affects weld quality.
Field welding parameters should be derived from welding tests conducted according to recognized standards such as but not limited to GRI-19a, GRI-19b, German DVS 2225-3 or 4, ASTM D6392, or ASTM D7747, depending on the materials being fusion welded.
It is also important to understand the limitations of these standards and where required adopt project specific welding parameters. The latter has become more relevant over the past few years as a result of new geomembranes with limited welding history being introduced to the market.
WELDING DEVICE DESIGN PRINCIPLES
To ensure consistent and reliable welding results, welding devices should adhere to specific design principles. These principles include a closed-loop control system for maintaining welding temperature, closed-loop control for controlling welding speed, and a calibrated pressure system with a load cell or measuring system to enable reproducible welding pressure. Incorporating a data acquisition system, as per ASTM D8468*, allows for automated data acquisition, welding data log files, and post-welding analysis.
* ASTM D8468-23 – Standard Practice for Data Recording Procedure for Welding Devices Used to Produce Thermal Fusion Welds in Geomembrane Systems
WELDING DEVICE CONSIDERATIONS
All parts of the welding device that come into contact with the material to be welded must be free from sharp edges. Sharp edges can notch the welded material and negatively impact long-term weld strength due to defect introduction such as score lines and grooves and resultant notch sensitivity. The minimum radius of all edges in contact with the geomembrane should be no less than 2.0mm.
TESTING
Final weld parameters must be verified according to recognised standards, relevant to the materials being fusion welded. It’s the last point which is always crucial.
CONCLUSIONS
Proper fusion welding relies on interpenetration and entanglement of polymer molecules of the two geomembrane sheets being joined across the weld interface.
Under-heating, low dwell time and insufficient nip pressure will lead to lack of chain mobility and therefore inadequate interpenetration of the polymer chains across the weld interface resulting in poor bond strength.
Conversely overheating, long dwell times (due to excessive heat and or too slow welder speed) and excessive nip pressure will lead to the melt ‘puddle’ being extruded out laterally forming a larger than normal ‘squeeze out’ and resultant poor weld strength. Such conditions can also lead to excessive thickness reductions on the weld track, an abrupt thick-thin weld geometry and the formation of a severe heat affected zone directly next to the weld track.
Given the multifactorial nature of geomembrane welding the optimum weld performance is therefore a function of striking a balance of the three critical weld parameters.
Standard wedge welding of geomembranes relies on the operator properly adjusting the welding parameters (temperature, speed, and pressure) which are needed to create a strong and reliable weld. Changes in one or more of these parameters during production seaming, whether deliberate or accidental, can lead to poor weld quality. Prior to data acquisition systems, verifying welding parameters throughout the welding process relied solely on periodic visual observations and documentation, leaving substantial room for error.
It is recommended that a data acquisition system be installed on thermal fusion welding devices which allows for the recording of these welding parameters along the entire length of the welded seam at pre-determined intervals, throughout the entire thermal fusion seaming process.
Monitoring all welding parameters during thermal fusion welding of geomembranes is crucial to ensure best practices. By maintaining a balance between heat/energy, speed, and pressure, high quality welds can be achieved. The significance of squeeze out, adherence to testing standards, and the incorporation of data acquisition systems further enhance the quality and reliability of the welding process.
KEY TAKE AWAYS
- Interpenetration and Entanglement: The success of fusion welding relies on the interpenetration and entanglement of polymer molecules from both geomembrane sheets at the interface. This molecular interlocking creates a strong bond.
- Heating and Dwell Time: Proper heating and a suitable dwell time are crucial. If the heating is insufficient and the dwell time is short, the polymer chains might not have enough mobility to interpenetrate effectively, leading to a weak bond.
- Overheating and Long Dwell Times: On the other hand, overheating the material or using a slow welding speed (long dwell times) can result in excessive melting and “squeeze out” of melted material. This can lead to poor weld strength and excessive thickness reductions on the weld track.
- Nip Pressure: Nip pressure refers to the pressure applied between the two sheets during the welding process. Too little pressure can prevent proper interlocking of molecules, while excessive pressure can cause the melted material to squeeze out and weaken the weld.
- Weld Strength and Heat-Affected Zone: Weld strength can be compromised if the optimal balance of parameters is not achieved. Moreover, unfavourable conditions can cause significant reductions in the thickness of the welded area and the formation of a heat-affected zone (HAZ) adjacent to the weld track. The HAZ can have altered material properties due to the heat exposure and may age more rapidly than the sheet leading to an increased potential for stress cracking.
- Optimum Weld Performance: Achieving the best weld performance involves finding the right balance among all 3 parameters namely temperature, speed and pressure. Adjusting parameters such as heating, dwell time, and nip pressure is essential to create a strong and reliable weld between geomembrane sheets.
- Testing: Final weld parameters must be verified according to recognised standards, relevant to the materials being fusion welded.
