How to implement glue into production? Adhesive standard DIN 2304

DIN 2304 and ISO 21368 – what is current today?

DIN 2304-1 was originally introduced as a cross-industry quality standard for adhesive bonding processes. The 2020 edition, DIN 2304-1:2020-04, is now marked by DIN Media as withdrawn.

The current German reference is DIN EN ISO 21368:2024-10, which DIN Media explicitly identifies as the document replacing DIN 2304-1:2020-04. Its title is Adhesives — Guidelines for the fabrication of adhesively bonded structures and reporting procedures suitable for the risk evaluation of such structures.

At international level, the currently published standard is ISO 21368:2022. ISO describes it as a framework for adhesive user-companies that need quality requirements for fabrication and reporting procedures in workshops and on site. The standard is independent of a particular adhesive manufacturer or a particular bonded structure.

Why is adhesive bonding considered a special process?

Fraunhofer IFAM describes adhesive bonding under DIN EN ISO 9001 as a special process because important characteristics of the finished joint cannot be fully verified by non-destructive testing.

This does not mean that every production joint must be destroyed or that every application requires a separate ageing test. It means that final inspection alone cannot guarantee quality. The process therefore needs defined and controlled inputs such as:

• joint geometry and design loads,
• substrate material and surface condition,
• surface preparation,
• adhesive batch, storage and shelf life,
• mixing and dispensing parameters,
• application quantity and bond-line geometry,
• open time and assembly time,
• curing temperature and duration,
• inspection and verification methods,
• personnel competence and documentation.

Destructive testing, ageing studies, witness specimens or other verification methods may form part of validation or process control where required by the application, but the required test programme should come from the actual risk, specification and quality plan.

What should be controlled before adhesive bonding enters production?

The most common implementation error is to treat adhesive selection as the entire technology. ISO 21368 takes a much broader view of the production process.

This is why the same adhesive can produce very different results in two plants. The difference is often not the product itself, but the stability of the process around it.

For substrate-specific examples, see our guides to bonding plastics and bonding stainless steel instead of welding.

Safety classes S1–S4

The DIN 2304 quality framework classifies bonded joints according to the consequences of failure, not according to adhesive strength alone. This risk-based approach remains central to the current implementation guidance used around ISO 21368.

The class should be assigned from the consequences of failure of the specific joint in the specific product. A particular adhesive, application type or component family is not automatically S1, S2, S3 or S4 in every project.

Personnel competence: EAB, EAS and EAE

A controlled bonding process requires clearly defined responsibilities and suitable technical competence. Fraunhofer IFAM lists three established European qualification levels:

These are personnel qualifications, not job titles imposed by ISO 21368. The company should assign competence to the actual tasks and risk level of its bonding processes.

How to implement adhesive bonding in production step by step

1. Assess the application. Define the substrates, geometry, loads, environment, required life and consequences of failure.
2. Assign the risk or safety classification. Document why the joint has been assigned to the chosen class.
3. Select the bonding technology. Define adhesive, surface preparation, dispensing method, assembly sequence and curing conditions.
4. Validate the solution. Test representative production materials and joint geometry under conditions relevant to the actual application.
5. Create a production instruction. Convert technical knowledge into parameters that operators can perform and the plant can measure.
6. Assign responsibilities and competence. Define who performs bonding, who supervises it, who evaluates tests and who approves technical changes.
7. Control production. Record critical parameters, material status, equipment condition and process deviations.
8. Manage change. Define what happens if the adhesive, substrate, coating, supplier, dispenser, surface treatment or curing parameters change.

Who is responsible: the plant or the adhesive supplier?

ISO 21368 is deliberately independent of individual adhesive manufacturers’ product recommendations. The adhesive supplier can know the product in great detail, but normally does not control all process variables at the customer’s site.

A supplier or technical partner can support:

• product selection,
• interpretation of technical documentation,
• surface-preparation trials,
• dispensing and mixing concepts,
• joint testing,
• troubleshooting and process optimisation.

The adhesive user-company must, however, control the way the technology is actually implemented in production: materials, equipment, people, application parameters, curing, inspection and documentation.

What does a company gain from a controlled bonding process?

The biggest benefit is not additional paperwork. It is reducing the number of situations in which joint quality depends on one operator’s experience or on whether “it worked today”.

A controlled process makes it easier to:

• identify the cause of a nonconformity,
• train new personnel consistently,
• manage shift work and staff rotation,
• compare production batches,
• control subcontractors,
• approve changes without starting development from zero,
• demonstrate how the required joint quality is achieved.

FAQ – DIN 2304 and ISO 21368

Is DIN 2304 still current?

DIN 2304-1:2020-04 has been withdrawn. DIN Media identifies DIN EN ISO 21368:2024-10 as its replacement. DIN 2304-2 remains a separate current document for adhesive bonding in shipbuilding.

What is the current international standard?

ISO 21368:2022 is the current published international edition for guidelines on fabrication quality and reporting procedures for adhesively bonded structures.

Does ISO 21368 apply only to one industry?

No. The international standard is designed as a cross-industry framework and is independent of a particular type of bonded structure or adhesive supplier.

Why is adhesive bonding called a special process?

Because the finished joint cannot be fully verified by non-destructive inspection alone. Quality therefore has to be created and controlled throughout the production process.

Does every bonded joint need destructive testing?

No. The validation and verification plan should reflect the application, safety class, customer requirements and quality system. Destructive testing can be important, but it is not a universal requirement for every production joint.

What do S1–S4 mean?

They classify a bonded joint according to the consequences if that specific joint fails, from high safety relevance in S1 to no safety requirement under this classification in S4.

Are EAB, EAS and EAE job titles required by ISO 21368?

No. They are recognised personnel qualification levels used in European adhesive-bonding training. A company still needs to map competence to the actual responsibilities in its process.

Can an adhesive supplier take responsibility for the complete bonding process?

A supplier can provide technical support, product information and trials, but the user-company must control its own production process. The final allocation of contractual or legal responsibility depends on the specific project and agreements.