Getting it right... qualifying plastics welders for manufacturers?

Formalisation of training for makers of non-metalic vessels, tanks and pipework

TWI Bulletin, March - April 2010

Thermoplastics, such as polypropylene, polyethylene, polyvinyl chloride, and polyvinylidene fluoride are being used more and more for the storage, containment and conveyance of both toxic and non-toxic fluids. This is because they have excellent resistance to most chemicals, compared with metals, and do not corrode. The plastics vessels, tanks and pipework used in these applications are normally manufactured using manual welding techniques, where the quality of the weld is very much dependent on the skill of the welder. As Mike Troughton explains in the UK, although there has been a certification scheme for welding personnel for many years, until recently there was no scheme for plastics welders to prove their competency.

In 2003 a European standard was published, which specifies the qualification testing of the skill and knowledge of welders involved in the construction of thermoplastics structures. EN 13067 entitled 'Plastics welding personnel - qualification testing of welders - thermoplastics welded assemblies' applies to the welding of sheet, pipe, fittings and lining membranes using the hot gas, extrusion, wedge, heated tool and electrofusion welding processes. Application of this standard guarantees that examinations are carried out according to a uniform procedure, and qualification to this standard is recognised throughout Europe. Following the publication of this standard, a European Welding Federation guideline, EWF 581-01, was published in 2004, which provides a harmonised scheme for the education and training of plastics welders in preparation for the examinations defined in EN 13067.

Scope

EN 13067 covers the welding processes, materials and products given in Table 1.

Table 1 Welding processes, materials and products covered in EN 13067.

There are a total of 39 different examination categories. For example, the categories for PE are given in Table 2.

Table 2 Examination categories for PE

Examination entry requirements

Only individuals whose previous training and/or experience suggest that they are likely to pass are allowed to take the examination. It is stated in the standard that evidence of such training/experience should be at least one of the following:

• Having at least two years welding experience in the chosen technique.
• Completed an appropriate theoretical and practical training course.
• Completed an apprenticeship as a plastics fabricator.
  

The examination

The examination itself consists of two parts: a theoretical test and a practical test. Both tests are supervised by a Plastics Welding Examiner and both parts must be passed before the candidate is awarded a qualification test certificate. If one of the parts is failed, the candidate must undergo further training before re-taking that part of the examination.

The questions cover such subjects as:

• Characteristics of the materials being welded
• Correct preparation of the work pieces
• Safe operation of the welding equipment
• Welding procedures
• Types of possible weld imperfections
• Relevant destructive and non-destructive tests
• Health, safety and quality assurance requirements
  

In order to pass the theoretical test, at least 80% of the questions must be answered correctly.

The purpose of the practical test is to prove that the candidate can produce a welded test-piece, relevant to the welding category being examined, that meets the required standards for visual appearance and mechanical strength. Once the candidate has prepared the examination test piece the PWE carries out a visual inspection of the weld and, if this is acceptable, sends the test piece to a qualified test house, where it is mechanically tested. The type of mechanical test carried out depends on the examination category (Table 3).

Table 3 Mechanical tests used for assessing examination test pieces

The number of test specimens cut from the test piece varies depending on the test and examination category.

Approval certificate and identity card

Candidates that pass both the theoretical and practical tests are awarded a qualification test certificate and (optionally) also an identity card. These show the examination categories in which the welder is qualified and the date that the certificate/ID card expires.

The welder's qualification begins from the date when the overall assessment pass is awarded and lasts for a maximum of four years. After two years, the qualification is only continued for a further two years if the certificate holder provides proof of welding quality to the certificate issuing authority. After four years, a new qualification test is required.

EWF 581-01: Minimum requirements for the education, examination and certification of European plastics welders

For each of the 39 examination categories given in EN 13067 the required minimum theoretical education and practical training are defined in EWF 581-01, both in terms of content and teaching hours. For the welding of pipes two levels of education and training are defined: Entry Level 1 for individuals with no prior experience in plastics welding and Entry Level 2 for individuals meeting the examination entry requirements given in EN 13067 (see Tables 4 and 5).

Table 4 Practical exercises for heated tool welding of pipes

Table 5 Instruction units of the theoretical training relevant to heated tool welding of pipes

The situation in the UK

TWI has been involved in certifying welding personnel for over 35 years, through its CSWIP certification scheme. However, in the mid-1990s this did not include plastics welders. Therefore, in order to implement EN 13067 in the UK, work was started in 1996 by TWI Certification Ltd, which operates the CSWIP scheme, to develop a certification scheme for plastics welders.

In order to ensure that the scheme would meet the needs of UK industry, a Management Committee was also formed, comprising representatives from fabrication companies, industry associations, welding equipment suppliers and end users.

One of the main things to come out of the Plastics Welder Certification Scheme Management Committee was the requirement for two levels of certification: a Standard Level, in accordance with EN 13067, for experienced welders and an Entry Level for individuals just starting their career as a plastics welder. The reason for this was that, in the past, new starters in the industry would receive their practical and theoretical training on-the-job from a more experienced welder, which would invariably lead to the passing on of bad practice. Ensuring that new starters are properly trained in the current best practice, and that they can then prove their welding competence by passing an examination and obtaining a certificate reduces the passing on of bad practice and improves the quality of the UK workforce.

The requirements for Entry Level certification are different for different industries. For example, the main objective of the tank and vessel fabrication industries is to test the practical skill of the welder, to ensure the welds they produce are of a satisfactory quality. For this reason, the Entry Level examination for the hot gas and extrusion welding of sheet only consists of a practical test. On the other hand, for the welding of pipes it is considered that some basic knowledge of the welding equipment and procedures for welding on-site is important, and therefore the Entry Level examination for these categories consists of both a theoretical and practical test.

Current developments

Since countries have now had a number of years' experience in operating certification schemes in accordance with EN 13067 and EWF 581-01, a number of modifications have been proposed to both documents. These are currently being discussed in a new CEN Working Group and EWF sub-committee, respectively.

Summary

In manual welding of thermoplastics, the quality of the joint is very much dependent on the skill of the welder. In order to avoid the passing on of bad welding practices from 'experienced' welders to new starters it is important that all personnel are properly trained.

The most effective way for companies to prove that their plastics welders are competent, for example to customers, regulatory bodies or insurance companies, is to ensure that their workforce is independently certified to an internationally recognised standard, ie EN 13067.

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