#2 | 2019
SYN] Magazine
15/21

Key Factor: Filling in the Hem

Hem flange bonding is one of the most challenging joining methods in car body construction. Here, the focus is placed on the ideal distribution of material to achieve an optimal filling of the seam area and thus ensure structural stability and prevent corrosion. The application process of the adhesive has a significant impact on this.

There are three major application processes in car body construction: structural bonding, stiffening and hem flange bonding. Hem flanges are used for closure parts, such as doors, front hood, engine hood or trunk. The closures are manufactured in a separate process step within the body-in-white manufacturing process. Hem flange bonding is one of the most challenging joining methods in the body shop. The adhesive fulfills important structural properties, improves crash safety, and protects against corrosion. Other joining techniques, such as spot welding or riveting, are not suitable here, as they leave visible joints and compromise the aesthetic appearance.

Hemming process:

In hem flange bonding, two sheets are nested inside each other. The adhesive – often a one-component or two-component epoxy or a rubber-based material – is applied to the outer sheet. It is then bent around the inner sheet and folded over. During hemming, the adhesive is pressed, thus filling the hem area.

Quality when using hem flange bonding:

The long-term stability of this bond depends on the material distribution in the hem. The following criteria are relevant: 

  • A: 100-percent bonding between the inner and outer sheet
  • B: Defined X-percent bonding between the inner and outer sheet
  • C: Complete filling of the hem with adhesive
  • D: Sufficient distribution of adhesive on the inside 

If the hem is insufficiently filled with adhesive, this leads to the formation of cavities and air channels. During the subsequent e-coating process, these cavities fill with the liquid. This liquid then dries during the curing in the oven, resulting in air bubbles and wetting defects. In addition to visual defects, the hem flange also becomes susceptible to corrosion. The vehicle body has to be discharged for reworking.

If, on the other hand, too much adhesive has been applied, or the adhesive bead was positioned incorrectly, this leads to unwanted material leaks. The component has to be reworked before painting to ensure that the e-coat bath is not contaminated or to avoid problems during cosmetic sealing – as a consequence, the material consumption and quality cost increase.

Material distribution: focus on the application

Im gesamten Fügeprozess gibt es zahlreiche Faktoren, welche die Materialverteilung im Falz beeinflussen können – etwa die Wahl des Klebstoffs oder des Falzverfahrens. Einer der Haupteinflussfaktoren ist allerdings der Applikationsprozess an sich. In der Regel werden dafür automatisierte, robotergestützte Klebe- und Dosiersysteme eingesetzt, um Parameter wie Materialfluss, Temperatur, Druck und Auftragsgeschwindigkeit präzise und wiederholgenau steuern zu können. Dabei eignen sich grundsätzlich zwei Auftragsmuster: eine klassische Raupenapplikation und eine sogenannte Swirl-Applikation, bei der das Material in einer Kreiselbewegung aufgetragen wird. Beide Applikationen ermöglichen unter idealen Voraussetzungen eine hochwertige Falzverklebung.

Improved process reliability

Nevertheless, the challenges in modern production and current trends, such as multi-material design and light-weight construction, put increasingly higher demands on the industry. Increasingly complex vehicle designs and part geometries require more dynamic robot movements and make access to the components more difficult. Application solutions must be flexible. The swirl application technique can offer several benefits here. While with bead application, the application distance to the component should correspond to the bead diameter, the swirl method allows larger distances to the component of up to 50 millimeters. Changing the distance does not affect the application pattern. This makes robot programming easier, allows higher speeds and facilitates accessibility, especially for complex geometries. 

At the same time, swirl applications ensure a material distribution optimized for hem flange applications – the same amount of material is distributed over a larger surface. The application is still precise with sharply defined contours. This has a positive effect on pressing in the folding process. Modern swirl applicators also offer the possibility of precisely adjusting the width of the application. In places where less material is required, the applied volume can thus be systematically reduced while still maintaining a consistently high application quality. This allows perfect adjustment of the bonding seam to the geometric shape of the flange. Material leaks and reworking can thus be avoided. At the same time, it reduces material consumption.


Summary

Hem flange bonding are one of the most challenging tasks in body-in-white manufacturing. The material distribution in the hem has major impacts on the long-term quality of the adhesive bond. Defects can cause corrosion and lead to high reworking costs. A swirl application can counteract this and increase the process reliability. 

 There are, however, diverse factors that influence the quality and productivity of hem flange bonding, and they require comprehensive joining and process expertise. In its global Innovation Centers, Atlas Copco works closely together with customers and material suppliers – from application development through material testing to process optimization and quality assurance. 


Bead and swirl applicator in one

Modern vehicle designs require increasingly flexible application solutions. The applicator E-Swirl 2 AdX BIW of Atlas Copco’s product line SCA masters traditional bead applications as well as swirl applications with equal precision. Due to a flexibly adjustable application width and accurate material distribution, the E-Swirl 2 AdX BIW is particularly suitable for hem flange bonding.

For more information on the E-Swirl 2 AdX BIW watch our video:

To learn even more about hem flange bonding, please refer to our Pocket Guide: