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Study of flowhole-forming screws: Zinc flake makes it more efficient






Study of flowhole-forming screws: Zinc flake makes it more efficient


Lightweight construction is very much the trend - and not just in the car industry. In the scope of a current research project headed by the Laboratory for Material and Joining Technology (LWF) of the University of Paderborn well-known partners – including DÖRKEN – have investigated opportunities for optimising the use of flow punch forming screws in high-tensile steel sheeting and profiles. With success.

Joint research into flowhole-forming screws for lightweight construction

Lightweight construction is in higher demand than ever in the car industry. For bodywork construction in particular it saves valuable resources and also enables a reduction in vehicle weight - and therefore fuel consumption. In view of this, the use of high-tensile steel sheeting and profiles is also increasing further. And this ultimately also has an influence on fastening. The flowhole-forming screws currently in use – these form their own hole in the material in the course of being screwed in (no chip formation) – frequently come up against their limits. As a result, in the scope of a broad-based joint study, sponsored by the Federal Ministry for Economic Affairs and Energy, of the European Research Association for Sheet Metal Working (EFB) the goal was to investigate options for process-reliable flow punch forming fastening of high-tensile steel sheeting and profiles. Dörken and the screw specialist and long-term partner Arnold Umformtechnik also applied their expertise to the project.      

Investigation of various materials and coatings

In the course of the extensive study both the materials of the flowhole-forming screws used (steel types) and the coating applied were variegated and examined precisely in diverse tests and test structures, Martin Juros, Global Sales Manager at Dörken, explains: “So far, flowhole-forming screws have mostly been used in electroplated form, in combination with a lubricant. However, the coating is relatively temperature sensitive and the screw-in torque is high. We therefore joined with Arnold Umformtechnik and the other partners to seek a solution that would allow the new screwing requirements to be met and thus further optimise the joining and assembly time of parts in production.”

Zinc flake coating offers numerous advantages

In the investigation of suitable coating solutions the zinc flake system with DELTA-PROTEKT® KL 120 as basecoat and DELTA-SEAL® without added lubricant (topcoat) with the addition of DELTA-LUBE® 50 proved to be a particularly effective combination. Applied to the high-tensile steel flow punch forming screw, this highly heat resistant system (up to 900°C) enables a clear reduction in the maximum tapping and screw-in torque, as well as damage to the thread during fastening. As a result, process-reliable joining is feasible beyond the current operational boundaries. For car production, this could mean a higher cycle rate for parts assembly and therefore sustainable savings of time and costs. This also represents a key advantage of the zinc flake system in the coating of screws made from high-tensile steel (>1,000 Mpa): No hydrogen is generated in the coating process, with the result that no hydrogen-induced stress corrosion cracking occurs.

Order the detailed professional article of the study now

The complete professional article for the study “Development and qualification of flowhole-forming screwing for the joining of high-tensile steel sheeting and profiles” (by Prof. Dr.-Ing. Gerson Meschut and Martin Ivanjko, M. Sc., Laboratory for Material and Joining Technology/University of Paderborn) is published by the in-house publisher of the EFB and can be ordered directly online at

Car body:

Lightweight parts are a major trend in the automobile sector.


Flowhole-forming screw with zinc flake coating after 720 hours neutral salt spray test.

Thanks to the zinc flake system from Dörken the flowhole-forming screws are joined swiftly, efficiently and reliably to high-tensile steel.