Advances in detachable and permanent fasteners

As thermoplastic materials constantly improve in technology and cost-efficiency, the need for a low-cost and secure fastening method becomes increasingly important.

As thermoplastic materials constantly improve in technology and cost-efficiency, the need for a low-cost and secure fastening method becomes increasingly important.

The market requirements placed on fastener solutions are extremely complex.

As a general rule such solutions can be classified into two main groups - detachable fasteners and permanent fasteners.

Detachable fasteners are required where they may need to be undone, perhaps in the event of damage or because the application requires it.

Here the market requirements are for low-cost parts and assembly materials, together with a high level of operating and process reliability.

Permanent fasteners are used where components cannot be repaired, and for safety reasons, where they must never be removed (for example the airbag in a car).

Here the market requires short assembly times, a high level of process reliability, and it goes without saying that the fasteners must never get lost.

Detachable fasteners are usually in the form of nuts and bolts.

These are certainly cheap to make, but they require time at the assembly stage.

According to Mario Metzger, Production Manager at Arnold Umformtechnik, using Remform screws to fasten plastic materials is the most cost-effective way of dealing with such a comprehensive challenge.

By using this type of fastener system, producers can keep the cost of both the assembly process and the fastener itself as low as possible.

Indeed, to illustrate the point, VW's internal standard 01128 contains application recommendations for plastics fasteners.

With its asymmetrical flank geometry the Remform screw provides a high level of operating security of the overall fastening, so the need to redo faulty assemblies is reduced and field rejects become a thing of the past.

The screw's asymmetrical flank geometry improves the material flow round the formed plastic thread, and increases the drawing torque compared with conventional symmetrical flanks.

"The benefits for designers are plain to see", said Metzger.

"Maximum security during the assembly process comes from the wide difference between screw-in torque and overturn torque - achieved by increasing the cross-section of the threaded core".

Other advantages are cost-efficient assembly with fewer components, easy recycling, improved component design (more fitting space and better weight ratio) which mean shorter plastic moulding cycle times.

To prove that these statements are not just plain theory, Arnold Umformtechnik has recently implemented Remform screws into the production process of a well-known manufacturer of washing machines.

"The customer needed to fasten a two-piece washing machine drum, and specified the following: to guarantee drum watertightness, the residual clamping force must remain effective throughout the washing machine's service life; temperature resistance of between -10 and +65C; extremely short assembly times (less than 1s per screw); and vibration resistance.

"The Remform screw meets these requirements perfectly", stated Metzger.

Permanent fastenings are generally achieved by means of welding, adhesives, rivets, or by clips.

Permanent fastenings cannot get lost, but the downside is that they are often expensive to install.

That is why Arnold Umformtechnik has developed - in the Arnold-Tripress (ATP) - a fastener that can be either permanent or detachable, or a combination of the two.

This is a clinch stud available in various versions and combinations.

As against conventional products with a cylindrical shank, its cross-section is trilobular (triangular).

The stud clinches into a round core hole, and in the process the hole takes on the trilobular shape of the stud.

This ensures positive locking, and minimises the clinching force required.

Once the shank has clinched, high torsion and drawing forces are generated, exceeding the failure moment of a conventional fastener.

The ATP clinches into ductile light alloys and plastic synthetic materials.

Although there are very many applications where this fastener can be implemented, its benefits are particularly apparent for fastening electrical connections in the automotive industry.

Many of these connections have traditionally been made using standard hexagonal head screws in order to absorb the torques caused when the connection lead is fastened, via a counter-plate inside the component.

The fastener or the connection pin is fitted separately.

To carry out the assembly, expensive operating materials are required, such as screwdrivers and power supply, not to mention extras such as nuts, washers and locking elements.

So it is an expensive process to fit the connection pin, the fastening technology is subordinate and therefore very cost-intensive.

However, by using the ATP fastener system the cost of operating materials is considerably reduced; the fastener clinches right up to the head - no need for expensive operating tools and extra parts.

Once the clinch operation is done, the threaded section of the ATP fastener is now jutting out of the component.

The part that needs to be fastened on is placed over it, and then fastened with a nut.

The ATP clinch stud thus forms a removable or a permanent fastening, or a combination of the two.

With the press-fit operation, its triangular cross-section form locks into a round hole.

The thread section can then be used to fasten on other parts in the conventional way.

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