Focus on optimal bondline quality

With the advent of new methods for narrow-surface lamination making use of laser or plasma technology, there is now again a strong focus on bondline quality and on the “zero bondline” in particular. But bondline appearance is just as important in conventional edgebanding with hotmelt adhesives. Recent developments in both machinery and adhesives aim to optimize it further.

As the joining means in conventional edgebanding, the hotmelt is decisive for the quality of gluing and for the achievement of an extra-thin or even invisible bondline. This is where the processer himself can exercise influence by choosing the right adhesive and applying it correctly. Processers for whom the investment in new laser and plasma techniques is out of the question can benefit not only from longstanding experience with the hotmelt method but also from innovative and further-developed products.

Adhesive types and application weights
Different adhesives are available to suit different substrates and purposes. EVA edgebanding hotmelts, which have been undergoing consistent further development since their introduction almost 50 years ago, are processed with application weights of 200 to 350 g/m². With a density of 1.2 g/m³, this yields a bondline thickness of only 0.2 to 0.3 mm. The application weight depends, first, on the percentage of filler in the adhesive. In machines with the right settings, unfilled hotmelts can thus achieve considerably lower application weights than hotmelts with a high filler content. Also important are the type of substrate and factors like correct workpiece preparation and finishing. An EVA hotmelt introduced by Henkel in 2010 can be processed at much lower temperatures than conventional products, thus cutting energy costs and minimizing vapor generation. Thanks to the low application volume, this adhesive helps to achieve the desired zero bondline appearance.

For particularly high-grade and highly stressed bonds, PU edgebanding hotmelts are employed, and low application weights can be achieved with these as well. Special developments like granulated PUR hotmelts, which can be processed with conventional melters and applicators, and Micro-Emission PUR adhesives are now contributing to the steadily growing importance of these adhesives in edgebanding.

Application weights that are even lower than those of unfilled EVA hotmelts and hence markedly improve bondline appearance have now been made possible by edgebanding hotmelt adhesives based on new thermoplastic polymers. These were launched by Henkel for industrial use under the Technomelt Supreme brand in 2011. Another advantage during processing is their high thermal stability and the associated higher heat resistance of the bond.

In terms of bondline appearance, the adhesive’s color is also significant. To render the bondline invisible as such, it would be ideal to produce the adhesive in the same color as the substrates being bonded, i.e. the panel surface and the edge band. However, this is not feasible in practice with conventional methods. Adhesives are usually supplied in their unpigmented form and also frequently in white, brown and black varieties.

Tried-and-tested selection criteria
In order to select the right adhesive, a number of further important factors also have to be considered. The available machine, the materials being bonded and the application conditions have to be mutually adapted. The following five questions have proven useful in this connection:

1. Machine. Which machine is to be used and what effect does it have on bondline appearance? Edgebanding machines come in many different designs, ranging from stand-alone single-sided solutions through to interlinked high-volume four-sided banding lines. Choosing the right adhesive depends essentially on the following main factors: a) Is the line a through-feed system or does it contain machining stations? b) What is the shape of the narrow surface – is it a straight edge or will soft-forming be required? c) Is this a special case involving a supporting edge? This two-stage edgebanding process was developed in order to be able to process frameless lightweight panels. The supporting edges have shoulders that are glued into the panels using low-viscosity, fast-setting assembly adhesives with a short open time. Conventional edge bands are then glued onto the supporting edges.
d) How will the adhesive be applied – by roller or nozzle? The adhesive can be applied either to the band or to the panel using a roller (passive method, where the substrate picks up the adhesive as it passes the roller) or a nozzle (active method). The roller method has the advantage that there is no need to adjust any settings when the panel thickness changes. The nozzle method, on the other hand, makes it possible to apply the adhesive more evenly and tends to reduce the amount of adhesive required. With the roller application technique, the right roller coding is important to achieve the low application weights enabled by the new adhesives. Minor investments made at the roller level pay back within a very short time. e) Finally, the feed speed at which the edgebander is to operate is also an important criterion for getting the most out of the adhesive selected.

2. Ambient conditions in the edgebanding shop: Edgebanding adhesives are processed within certain temperature ranges that can vary depending on their chemistries. The specific processing temperatures and the required volume of adhesive in each case affect the adhesive’s open time, i.e. the period within which it can achieve its optimal processing window and which ultimately determines its optimal final bond strength. Other temperature factors, such as excessively cold panels in winter or drafts, can have a negative effect on bonding.

3. Substrates to be bonded: The two materials involved – the panel material and edge band – have to be taken into account when choosing the adhesive. The panel in particular has a big effect on adhesive application weights. This is where panel thickness and density profile play a major role.

4. End use and the resulting requirements: Both the end use and the downstream processing steps influence the choice of the most appropriate adhesive. The place where the furniture item is to be used must be known beforehand. Kitchen and bathroom units, for example, have to meet higher requirements regarding resistance to heat and water vapor than items intended for use in living rooms. Temperature and humidity also have to be considered when furniture items are to be used in regions with warmer climates or if they are to be exported and will be exposed to fluctuating conditions during transport. During the manufacturing process itself, the adhesive bondline may be subjected to thermal loading as well, for example when the workpiece has been painted and is then exposed to high temperatures in drying tunnels.

5. Cost. The actual cost per meter of edge depends on the selected adhesive and the required application volume. Judging adhesives purely on their purchase price is therefore not realistic. The new thermoplastic edgebanding hotmelts now being introduced to the market additionally score with their high thermal stability. This has a positive effect on costs because it significantly reduces the cleaning and maintenance effort. The volume of adhesive applied also influences the overall cost. If too little adhesive is applied, this increases the risk of inadequate gluing and hence the scrap rate. If too much is applied, the excess adhesive might not be properly removed from the panel surface in the post-processing zone. In the past, many processors found it difficult to measure and precisely adjust the adhesive application weights. Henkel has now developed an easy-to-use system (ProMeter) which helps users to determine the exact application weights. Its greatest advantage is that the results are instantly available, helping users to determine the optimal machine settings. Special systems record all of these data so that they can be retrieved at any time for future jobs.

Correct preparation and post-processing
Once the processer has chosen the adhesive that best fits the manufacturing conditions and customer specifications or end use, it then has to be applied with the appropriate pre- and post-processing so that it will deliver optimal performance. Special attention must be paid to the following points: Adhesive application is preceded by workpiece preparation in order to achieve good formatting, the precondition for a neat bondline. Once the right adhesive has been found for the substrates and feed rate, the appropriate application rate has to be set. This has an effect on the thickness of the bondline as well as on the adhesive’s setting time. The faster the adhesive hardens, the easier mechanical trimming will be. To achieve an attractive bondline appearance, correct adjustment of the pressing zone is important as well. After the adhesive has been applied, the use of trimming tools, such as scrapers, also has a large bearing on joint appearance. If too much material is removed, the substrates may be damaged, and this can impair joint appearance.

Reaping the benefits of new adhesives
The fact that appearance is now being focused on alongside technical performance when it comes to judging edgebanding quality is demonstrated by the high importance that end users attach to it. It therefore makes sense to reduce the volume of adhesive as far as possible by considering all the relevant factors when selecting the adhesive and devoting great attention to correct preparation and post-processing. This way, visually appealing joints can be achieved without great technical effort. New hotmelt adhesives and control methods offer processers extra scope for boosting quality still further.


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