This article discusses the process of laying and gluing veneer plywood. The article examines the effects of log diameter, geometry, and roughness on bonding quality and net recovery of marketable veneer. It also discusses the factors that affect the recovery of marketable veneer from log volume. It concludes with an overview of some of the available options. This article focuses on the most important factors that should be taken into account when reducing manufacturing costs in veneer plywood.
Process of laying and gluing veneer plywood
The assembly process begins with the gluing of veneer pieces to form a plywood sheet. This process is often done by hand or semi-automatically. For a three-ply plywood sheet, the back veneer is laid flat, run through a glue spreader, and then short pieces of core veneer are laid crossways on top of the glued back. The veneer sheet is then stacked on top of other sheets waiting to be pressed.
The process of laying and gluing veneer plywood is the most common form of manufactured wood panel. Veneers are thin slices of wood, typically less than one eighth-inch thick, and are glued together with the grain running at right angles. It is a lightweight, inexpensive alternative to other types of wood. The face veneer does not have to be prepared, but poorly made plywood may split or peel.
After the veneers are glued, the panels are assembled in hot presses. Glue types are dependent on the type of plywood being manufactured. Exterior-use plywood requires phenol-formaldehyde resin, while interior-use plywood is made from urea-formaldehyde. Once the veneers are glued, the panels are stacked and allowed to cool. Plywood can range in thickness from three millimetres for all-veneer panels to thirty millimeters for lumber-core plywood.
Proper veneer drying is the key to a successful product. The proper pre-sorting process is the foundation for a successful veneer drying process. By optimizing the drying process, manufacturers can reduce manufacturing costs and improve product quality. For example, it is possible to produce veneer plywood with less glue and a more consistent quality. The ideal end moisture content varies depending on the type of wood and the final product. Proper drying results come from optimizing process control and measurement.
Effects of pressure and time of pressing on bonding quality
The paper discusses the effects of pressure and time on the bonding quality of veneer plywood. The study was based on laminated veneer lumbers manufactured from sliced half-round I-214 hybrid poplar clone veneers using MUF adhesives. The thickness of the glue line was found to increase with increased press pressure while its specific gravity and GLT decreased with decreased pressure. The optimum press pressure was identified as 10 kg cm-2 and significant linear correlations were found between GLT and mechanical properties.
The three-layer samples of veneer plywood were made using a laboratory hydraulic press heated to 150 deg C. Pressure and time of pressing were varied, and veneer sheets were laid loosely or tightly. A hand roller spreader was used to apply adhesive to one side of each uneven ply. The results showed that the PF resin modified with ethyl malonate produced plywood with high mechanical properties and good bonding quality. The resulting plywood also reduced the pressing temperature by 20 degC.
The press temperature was high and the core layer rapidly heated to 100 degC. The steam in the core layer caused the veneer layer to densify, reducing the porosity and making it more difficult to pass steam. The high-pressing temperatures caused the surface veneer to become denser while the non-densified veneer took longer to reach pressing temperature. The higher the moisture content, the more moisture is able to pass through the wood.
If you use a vacuum press, be sure to carefully monitor the veneer moisture content before and after pressing. If the moisture content is too high, the veneer will curl up and may even split later. If the veneer is too wet, the moisture will cause excessive expansion in the wood cells, which could lead to cracking and delamination. The hot press temperature may also shorten the pressing time.
Effects of veneer roughness on bonding quality
Veneer roughness has important impact on veneer bonding quality. Roughness of veneers can reduce the bonding strength of plywood. We studied the effect of corona treatment and NaOH activation on the bonding quality. The corona treatment increased wettability of veneer surface and improved bonding quality. These findings have implications for improving the bonding strength of plywood. To improve bonding strength, higher adhesive amount could be used for veneers.
We also evaluated the effect of corona treatment on birch veneers. Corona treatment enhanced bonding quality without changing the statistical differences between corona-treated veneers and reference samples. The soaked corona samples in the group with pulled lathe checks had higher shear strength than the reference samples. In the comparison between the two groups, the corona-treated group had higher SD than the open group.
The surface roughness of veneers affects bonding quality and the suitability of plywood for end-use and environment. In particular, roughness plays an important role in penetration and uniform distribution of adhesive. For high-quality adhesion, close molecular contact is needed. Surface activity can be assessed using contact angle measurements. In addition to the surface roughness, the surface energy of veneers varies according to wood species, surface treatment, age, and drying process.
Densified veneers had improved bonding strength and wettability in our experiments. Corona treatment improved bonding strength between two layers of plywood but had no effect on spruce plywood’s properties. We found that corona treatment improved bonding between veneers and plywood at a reduced adhesive spread rate. Despite this effect, we did not observe a substantial improvement in bonding strength.
Effects of log dimensions and geometry on recovery of marketable veneer from log volume
This study investigated the impact of log dimensions and geometry on the recovery of marketable veneer from log volumes. The researchers then devised a metric to support log procurement decisions. This metric provides a maximum amount of marketable veneer that can be recovered per mill-delivered log in order to meet a target gross margin, defined as the market value of dry-graded veneer less the cost of the log.
The study showed that the cost per log increased with diameter, and the trade-off between large diameter logs and lower margins is often not advantageous for the veneer manufacturer. In this study, the results were consistent with other studies, which indicated that larger diameter logs may not provide the most profitable returns. Therefore, it is important to consider these trade-offs when selecting logs for processing.
In the United States, Wang and Dai reported that the average dry veneer recovery per log was 48.7 percent, with the proportion being higher when the log length is short. Short log lengths also reduced the overall volume loss due to sweep and taper. Other researchers have reported recovering 62% to 65% of marketable veneer per log volume in dry conditions for 1.2-m logs of rubber trees with diameters ranging from 15 to 18 cm.
The results of the study have implications for the veneer manufacturing industry. Depending on the log diameter and wood species, the recovery of marketable veneer from log volume varies. The USFS timber assessment states that a single cubic foot of log can produce 14.1 square feet of 3/8-inch nominal veneer. However, when the log diameter is too small, the yield may fall below this value.
Effects of PF resin on bonding quality
The main goal of this study was to determine the effects of PF resin on veneer plywood bonding quality. The PF resin is a polymer that forms vast networks upon cure. This increased molecular mass of the resin enables it to achieve a quicker gelation rate at 120 deg C. Increasing the molecular mass of the resin also decreases its activation energy. Therefore, the higher the molecular weight of the resin, the lower the activation energy needed to cure it.
The press temperature is a critical factor in improving the bonding properties of PF adhesives in veneer plywood. For exterior structural applications, plywood is often made from thermosetting PF adhesives. The PF adhesive is required to harden at temperatures greater than 100 degC inside the veneer stack. The heat generated by the hot plates should be transferred to the core layer as quickly as possible. If the heat transfer process is prolonged, the adhesive may prematurely harden and eventually fail to bond with the veneer plywood.
The effects of PF resin on veneer plywood bonding quality were determined using two different types of OPS veneer. Untreated OPS veneers are less acidic than those that have been phenolic-preg. Commercial PF resin, on the other hand, has a high pH and high WPG. The untreated veneers are more acidic than commercial PF resin. However, phenolic-treated OPS veneers are more resistant to acid and alkali and are less sensitive to alkali.
The effects of PF resin on the bonding quality of veneer plywood were studied to determine their effect on the wood strength. The results revealed that higher molecular weight of PF resin leads to higher wood failure. The phenolic resin also exhibits significant degradation at low temperatures, whereas high molecular weight resin results in higher failure rates. Thus, high-quality PF resin can improve the bonding quality of veneer plywood.