Themoplastic Sheet Butt Fusion

Butt fusion, also referred to as heated element welding or heated element butt welding, is a widely used technology. The fusion method is used in high volume production applications of molded parts as well as in the field of thermoplastic piping and sheet products. The butt fusion method generally provides a very high quality weld when the material’s specific parameters are adhered to. In principle, the process requires that the materials to be welded are held against a heated plate, the heated plate is removed and the materials are pressed together. The parameters that need to be observed are time, temperature and pressure. These vary depending on type of material and size of weld surface. A graph (Figure 1) explains the process as follows:

• The high pressure melting cycle assures that a bead is formed all along the heater plate to guarantee material contact to the element.
• The low pressure heating cycle permits the heat to be absorbed into the material.
• The change over time, i.e. the time it takes for the element to be removed and to bring the two materials into contact with each other, needs to be kept to a minimum to avoid cooling or even oxidation of the surfaces to be welded.
• The welding time allows the material to cool down under pressure to form the appropriate molecular bond. An important step here is the “Pressure built-up time,” which should be performed as a ramp, rather than smashing the materials together and squeezing the heated molecules out of the weld zone.

The German Welding Society (DVS) has comprehensive guidelines for this process and materials and DVS 2208- 1, in particular, spells out in detail the requirements for butt welding machines for both piping and sheet. A few of these cover:

• Machine frame, construction and guide/travel elements.
• Material clamping fixtures and their impact on machine base.
• Machine table travel parallelism and table top alignment.
• Heating element positioning, temperature accuracy and surface condition.
• Machine controls

Operator Expertise

While CNC controlled machines facilitate the task of the operating personnel by calculating the machine settings based on material type and overall surface to be welded, operators need to pay attention to some principles to assure good welds. These include:

• Welding Surfaces: All welding surfaces need to be clean, especially when welding sheet, as sheet butt fusion machines do not incorporate surface preparation which is performed with the facing/planning operation in pipe welding.
• Condition of Heating Element: Element surfaces should have clean, anti-stick coating, free of residue and easily release hot melt. Elements should be cleaned and recoated when necessary. Residue on the element can contaminate the weld zone and lead to weld failure.
• Verification of Welding Parameters: When changing parameters, welds should be tested and validated. It is also good practice to validate welds with newly received sheet lots.

Benefits of Butt Welding

The benefits of butt welding versus manual processes such as extrusion or hot air welding are multifold. The process is faster and less dependent on operator expertise and attention for every inch of weld, it is fully repeatable and the welding parameters are much more precise. Manual processes further require a welding rod, introducing a secondary material into the weld zone and using hot air which causes higher stresses. Combined with process accuracy, the butt welding technology produces weld qualities which allow butt welded materials to be used as regular sheet.

Applications

In sheet fabrication, butt welding is performed to obtain larger sheet, to easily fabricate cylindrical structures, to recover scrap and, with 90º welding technology, to efficiently make corners for U channels or tanks. Today’s applications not only include PP, PE and PVC/CPVC as well as PVDF materials but customized specialty butt welders are available for UHMWPE, fluoropolymers, various types of olefin or vinyl foams of numerous densities and materials with different geometries such as corrugated/ hollow wall or anchor sheet.

The wide range of standard and customized butt fusion equipment available today allows companies in the thermoplastic welding and fabricating industry to effectively produce structures and fabrications in the sheet, piping and molded parts markets to assure quality, gain efficiency and pursue new markets to expand their business.

Written by Dagmar Ziegler, Vice President, Sales, WEGENER Welding, LLC, distributors of thermoplastic welding and fabrication equipment including hand welding systems, extrusion welding systems, sheet bending and fusion machines, pipe butt and socket fusion tools and equipment, pipe band saws, AC and DC spark testers and a wide variety of specialty welding equipment.

For more information, contact WEGENER Welding, LLC, 16W231 S. Frontage Road, Suite 12, Burr Ridge, IL 60527, 630-789-0990, Fax: 630-789-1380, E-mail: info @wegenerwelding.com, Web: www.wegnerwelding.com.