Quality Assurance Criteria in Sheet and Pipe Fabrication

Thermoplastic sheet and pipe find expanding and continuously more challenging applications in a variety of markets and demands for longevity and operational performance of fabricated structures and components vary greatly. The requirement for the quality of a drip pan to catch the possible leak of a washing machine is certainly very different form a tank that holds aggressive chemicals at elevated operating temperatures.

Companies can consider a number of approaches to assure the quality of a fabricated structure. Let us take an example in the fabrication of a structure that does not require a high degree of operation performance. Some basic steps can be good practices for a quality in construction. When starting the fabrication, at least two materials are involved that can come from different sources, sheet and welding rod (and possibly piping or other connections). After visual inspection it should always be verified that the materials match each other, being able to trace the material is an additional assurance. Very helpful here is any documentation or other identifiers that state material type, possibly date of manufacture and type of resin used, unfortunately color is a poor indicator as numerous materials are available in the same colors. A further good practice is to verify compatibility by laying down some hot air welded beads and checking the penetration when trying to pull off the welding rod. Should heated element contact welding (butt welding) be involved in the project, the testing of a sample weld will indicate that the welding parameters (either stored in the memory of modern operating controllers of sheet butt welding machines or available as written guidelines) are adequate (Ref: DVS (German Welding Society) technical code 2201-2; testing of semi-finished products of thermoplastics: Weldability � Test Methods - Requirements). The execution of the work should then be performed by qualified or certified welding personnel whose ability to perform good welds should be monitored. In some areas, the full documentation of the welding and inspection process may be worthwhile the effort for future reference.

With the limited testing processes available for assuring the quality once the structure is completed, companies use visual inspections (Ref: AWS (American Welding Society) G1.10M:2001, Guide for the Evaluation of Hot Gas, Hot Gas Extrusion and Heated Tool Butt Thermoplastics Welds), possibly high voltage spark testing of root welds or filling the tank with fluid once completed to be reasonably assured of a good construction.

The above are good steps to undertake for at lot of applications but thermoplastic materials are very often and increasingly used in processing systems where a much greater degree of safety and thus the assurance of quality is required. The cutting out of a sample weld in a finished structure for mechanical testing can only in very limited circumstances be the method of choice as now reconstruction/ repairs have to be performed which in all likelihood rather decrease than increase operational performance. Thus the industry has the need for welding procedure and performance qualification verification.

An excellent tool for engineers, inspectors, end users or other parties involved in the design and construction of a system involving thermoplastic pipe and sheet is AWS B2:4 �Specification for Welding Procedure and Performance Qualification for Thermoplastics�. This document references industry wide used welding procedures, materials, testing methods and qualification variables. Figure 1 for example shows the angle and ram displacement of the bend test.

This is a short overview describing the kind of considerations and procedures that are being utilized to provide for confidence within the engineering and end using community to choose thermoplastic constructions. Companies may certainly use additional or other procedures pertaining to their field. Not considered here are topics related to systems/ tank design and the short and long term performance to individual materials related to their application.

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.