New flow hole forming screw to extend the application limits

With the Flowform®, ARNOLD UMFORMTECHNIK GmbH & Co. KG developed and established a joining solution on the market several years ago that meets the increasing demands of sheet metal joining. The company is now presenting the further development.

Text: Annedore Bose-Munde

Compared to the conventional Flowform®, the developers at ARNOLD changed the dimensions of the Flowform® Plus from a diameter of 5 mm to a diameter of 4 mm. In addition, the geometry of the flow form tip has been optimized and the heat treatment process adapted so that the performance remains virtually the same.

The Flowform® flow-hole and thread-forming screw enables a fully automated joint that is accessible from one side. With its polygonal tip geometry, the Flowform® screw forms a flow hole and a thread that can accommodate a metric screw in the event of a repair.
With the Flowform® Plus, a further development of the established Flowform®, the company is now responding to growing customer requirements. The automotive industry is focusing on weight reduction against the backdrop of ever-decreasing CO2 emissions. Thinner and higher-strength steels or aluminum are increasingly being used and mixed construction is on the rise. But the screw must also guarantee a reliable connection under changed boundary conditions.
A customer approached the Forchtenberg-based fastener specialist with a corresponding request. In short: the Flowform® had to be further developed to meet these new requirements.


Flowform® Plus

Three-layer sheet metal joint with 2.5 mm thick aluminum sheets, joined with the Flowform® Plus.


 Three-layer sheet metal joint with 2.5 mm thick aluminum sheets, joined with the Flowform®.

Lightweight construction requires significant weight optimization

Heiko Miller

"Depending on the sheet thickness, the application limit for joining steel applications with the conventional Flowform® was a maximum of 600 MPa, with the Flowform® Plus it is 1,000 MPa," says Heiko Miller, Application Sales Engineer Flowform® at ARNOLD UMFORMTECHNIK.

Compared to the conventional Flowform®, the developers at ARNOLD changed the dimensions of the Flowform® Plus from 5 mm in diameter and 20 mm in length to 4 mm in diameter and 20 mm in length. Depending on the head and length, the conventional Flowform® 5.0 weighs around 4.0 grams, while the new Flowform® Plus 4.0 weighs around 3.0 grams. This means a weight saving of 25 percent. Extrapolated to the total number of screws used in the body, this adds up to quite a bit of weight. For example, the weight of 500 elements of the conventional Flowform® 5.0 per vehicle is 2 kilograms, with 500 elements of the Flowform® Plus 4.0 per vehicle it is 1.5 kilograms. This alone means a saving of 500 grams.
In addition, the geometry of the Flowform® tip has been optimized and the heat treatment process adapted so that the performance of the connection remains almost the same.

Joining point validation provides safety for the joint

But when is the Flowform® Plus the right fastener?

Nadine Schmetzer

"The customer's ideas for a particular connection point are usually quite specific. Our job is to investigate whether a Flowform® or a Flowform® Plus is the better solution for the specific application," explains Nadine Schmetzer, Application Sales Engineer Flowform®

Accordingly, we carry out feasibility studies with original material in the sheet metal joining laboratory, for example. Here, the strength values for the bond and, of course, the limits for the failure case are determined. A joining point test goes into a little more detail. Here, the customer has already defined certain material pairings that he wants to use. Accordingly, they also expect a recommendation and specific values on the strength values they can achieve at this joining point," explains Nadine Schmetzer. She has been working in research and development for sheet metal joining technology since 2017, including for Flowform® Plus.

Over the past few years, ARNOLD UMFORMTECHNIK has built up a great deal of expertise, particularly with regard to testing, which customers are happy to draw on. In the new laboratory for sheet metal joining technology in Dörzbach, which is integrated into the research and development department, various test set-ups are available, as well as all common system technologies, including a robot cell for near-series testing.

Classic applications of the Flowform® Plus

In principle, the Flowform® Plus is designed for higher-strength steel sheets and thicker sheet combinations. "By using aluminum components, thicker joint combinations with several layers can also be created. In the past, this meant that the top and middle layers of three-layer joints were pre-punched. According to ARNOLD UMFORMTECHNIK, the use of Flowform® Plus now allows thicker material combinations of up to 7.5 mm to be joined without pre-punching, depending on the joining materials," explains Miller.
This is due to the lower material displacement caused by the diameter reduction. This results in a smaller gap between the sheets and less friction surface due to the smaller dimensions. This means that the tightening torque is lower than with the conventional Flowform® 5. As the torque level is generally lower here, the joint can also be tightened with a lower tightening torque. According to ARNOLD UMFORMTECHNIK, the joint strength is slightly lower than with Flowform® 5, but good enough for the application.
With regard to the fastener, higher strength is achieved by selecting a suitable material in combination with a suitable heat treatment process. Due to the smaller diameter of the Flowform® Plus and therefore also the smaller head diameter, the screw is also suitable for applications where only a small flange width is available.
The developers at ARNOLD have used an example to illustrate what is possible with the Flowform® Plus. "We joined a three-layer sheet metal joint with 2.5 mm thick aluminum sheets with the conventional and with the new Flowform® element. The result clearly showed less gap formation with the Flowform® Plus. It was also possible to join without a pilot hole. In applications with adhesive, which is usually the case, the adhesive is also trapped if a pilot hole is avoided and therefore cannot escape to the pilot hole," says Heiko Miller. "Joining without a pilot hole is much more cost-effective in terms of the cost efficiency of the process. The user does not need expensive camera technology to check whether the screw hits the center of the pilot hole during the joining process," adds Schmetzer.
The new flow form is used, for example, in car body shell construction in aluminum-steel mixed construction, for profile-intensive construction methods, for one-sided accessibility, for hybrid connections or for battery packs.

Multiple savings effects for the overall process

The smaller head diameter of the Flowform® Plus means that the design of the customer's assembly can already be adapted. This means, for example, that flanges can be made narrower and therefore more weight-optimized.
Due to the lower material displacement in the screw, fewer axial forces are also required in a comparable material combination compared to the conventional Flowform®. This means that the process load can be reduced. This means that the robot can be dimensioned smaller if necessary. If the robot remains the same, there is less stress on the screw during the joining process.
"Another advantage is that users can continue to use their existing systems for screw fastening. No major conversions are required," says Nadine Schmetzer.
It is fundamentally important to the developers at ARNOLD that an optimum joining solution is realized for the customer component. This is why customer service is just as important as coordination with the system supplier. "Involving everyone involved in the process at an early stage ultimately supports the implementation of optimized and cost-efficient processes," explains Miller.

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