Effectiveness of Asymmetrical Veneering with Hardwood Species of Varying Shrinkage and Porosity

The aim of the study was to check deformations of asymmetrically veneered panels using glue with elastic bonding. In order to achieve the aim, a special stand was designed to analyse the geometry of furniture elements. It is known that boards became convex after veneering with rigid glue. However, the level of deformations for varying parameters of veneering was not equal. As the result of veneering with elastic glue, the boards preserved their primary shape. In any case, the level of deformation depended on wood veneer species. The deformation of the boards was not infl uenced by their shrinkage. It is supposed that the main infl uence is the difference in porosity.


INTRODUCTION 1. UVOD
Veneering is one of the most important processes in today's furniture industry.The only fully recognised, tried and tested method is symmetrical veneering.In industry, the process of asymmetrical veneering was applied a long time ago (Hayward, 1949), but now is only used occasionally.On the one hand, it leads to cheaper products and production (Ostrowski and Roszkowski, 2009), but on the other hand, it causes warp of furniture elements.An analysis of deformation is espe-DRVNA INDUSTRIJA 65 (2) 139-142 (2014) cially important when composite materials are taken into consideration (Negri et al., 2009).Asymmetrically veneered wood-based panels, used as furniture elements, are composites consisting of materials of different physical and chemical properties glued together.Therefore, the glue is the key to fi nding a solution to avoid deformations in the composites.
Industrial asymmetrical veneering is currently done intuitively.Obviously, it would be better if this method was supported with the results of studies.The purpose of the initial experiments was to fi nd an adhesive that would bond veneer and board without causing signifi cant geometric deformations in asymmetrical veneering.In order to achieve it, samples were veneered only on one side using 5 industrial and 1 experimental glue (Olenska et al., 2010;Olenska et al., 2011 a,b).The present research showed that samples, asymmetrically veneered using rigid glue bonds, had a general tendency to become deformed.However, the properties of the glue are the most important factor affecting the level of geometric deformations of the board.The main conclusion from the preliminary studies is that it is possible to avoid casting asymmetrically veneered panels by using an elastic bonding adhesive.Therefore, the following research was performed with this type of glue.Primary research showed the importance of shrinkage on asymmetrical veneering (Olenska et al., 2011c), and therefore extended research was continued.
The aim of our studies was to analyse the infl uence of species of different shrinkage on the geometric stability of furniture elements in asymmetrical veneering.Boards were veneered on both large sides or on only one large side using an elastic bonding adhesive.

MATERIJAL I METODE
Measurements were done on a specially designed stand.The stand is composed of elements as shown in Figure 1.
The main idea of the stand is to defi ne a reference virtual plane (1).This plane is created with four pegs.
Three of them have a constant height of 30 mm (3,4,5).The fourth is adjustable (6) in order to avoid defl ection of the sample, which is attached to the pegs.The force is axially positioned with pegs.During measurements, panels are always placed equally in position to each peg.The position of the panels is secured with three fi xed blocks (7,8,9).The visible surface of the strip (10) forms the reference surface.The strip is placed with three blocks (8,9,11).The distance between the reference virtual plane and reference surface of the strip is constant.This is where the measured board is placed.The geometry is measured in fi ve areas (2), fi ve times in each area (3 mm gap) with a depth gauge.Its precision is 0.01 mm, and measurement error is 0.02 mm.
Four samples of the dimensions of 900 x 450 mm and 18 mm thick were veneered in the same way.After veneering, boards were conditioned at 20 ±2 ºC and 65 ±5 % relative humidity for seven days.All the boards were measured in three states, as follows: unveneered board, unvarnished asymmetrically veneered board and varnished asymmetrically veneered board.The results of these studies show that the examined adhesive provides stability of shape of the samples after veneering.Another advantage of the elastic glue bond line is small geometric deformation.At the critical point, deformation reached 0.37 mm.

ZAKLJUČAK
Samples veneered with elastic glue joint preserve their shape, or their defl ection is small.Therefore, obviously the properties of glue joint contribute much to the level of geometric deformations of the board.
Studies show that shrinkage in hardwood species does not have an infl uence on deformations in the ve-neer (Fagus sylvatica L.) is on one side.Figure 2 shows examples of the results obtained for the board veneered with African ebony (Diospyros spp.Hiern) on the other side -small difference of shrinkage values.Figure 3 shows examples of the results for the board veneered with Macassar ebony (Diospyros celebica Bakh.) on the other side -signifi cant difference of shrinkage values.
The results of these studies show that the chosen glue provides shape stability after the process of bothside asymmetrical veneering, even if the shrinkage difference was high, and the values of deformations were low.At the critical point of deformation, boards 1 and 2 reached 0.51 mm and 0.61 mm, respectively.
Larger geometric deformations of boards 1 and 2, even for small difference of shrinkage values, can be explained by differences in the porosity of veneers.In boards 1 and 2, the porosity difference between African ebony and beech was 35 %.In boards 1 and 2, the porosity difference between Macassar ebony and beech was only 13 %.Thus, it seems that porosity, apart from shrinkage, is one of the main factors determining stability of asymmetrical veneered panels.neered board.The porosity of species used was the main factor affecting the deformations of asymmetrically veneered boards.
A high difference in porosity values causes higher geometric deformations of asymmetrically veneered boards.This can be solved by using optimised technology of gluing and varnishing.

Figure 1 1 .
Figures 2 and 3 show as follows: ■ -difference between unveneered board and board veneered on both sides, unvarnished, • -difference between unveneered board and board veneered on both sides, varnished.Figures 2 and 3 show the results of the studies performed on board veneered on both sides.Beech ve-

Figure 4
Figure4shows the results of the studies made on one-side veneered board using beech veneer -large difference of shrinkage values.The following variables were used: ■ -difference between unveneered board and one-side veneered board, unvarnished, • -difference between unveneered board and one-side veneered board, varnished.The results of these studies show that the examined adhesive provides stability of shape of the samples after veneering.Another advantage of the elastic glue bond line is small geometric deformation.At the critical point, deformation reached 0.37 mm.