Delamination currently accounts for approximately 85% of customer complaints about plywood as a sub-flooring product. It has become an urgent issue to many of our plywood members. It is estimated that by merely reducing 1% delamination in a 250 million ft2 (3/8 –in basis) plywood mill, the potential annual savings will be approximately $650,000. To help reduce plywood delamination, the key objective of this project was to develop a generic best practice checklist as a guide for manufacturing plywood.
A generic best practice checklist for manufacturing plywood was compiled with a focus on the following four key checkpoints: veneer peeling, veneer drying, panel gluing/lay-up and hot pressing. Key process variables at each checkpoint were determined as follows: peeling related veneer surface roughness and thickness variation, drying related veneer moisture content (MC) variation and surface inactivation, veneer temperature, glue coverage and dryout, and pressing time and pressure. Some technical issues were proposed to revisit as a strategy to reduce panel delamination. Among them include optimal lathe bar gap and pitch profiles, and proper knife sharpening for peeling, reduction of veneer overdry during drying, real-time adjustment of glue spread for adequate glue coverage, and use of optimum pressing time/pressure for adequate level of panel compression and glue curing. The resulting generic checklist can be modified for individual mill use.
Through literature review, pilot plant tests, and mill trials, the main causes of panel delamination were identified as: 1) glue dryout from long assembly time and high veneer temperature; 2) low panel compression, light glue spread or glue skips due to rough veneer; 3) little glue transfer due to veneer surface inactivation; 4) inadequate glue cure due to heavy glue spread, overwet veneer, sap wet spots, and short pressing time; and 5) combined effects of the above. It was found that veneer surface roughness had a significant effect on plywood gluebond quality, and excessive roughness and combined effect of veneer roughness, overdry, and glue dryout, were key causes of the low percentage wood failure. A statistical model was also developed from mill trials to predict the percentage wood failure in terms of veneer temperature, open assembly time and glue spread. The model helps establish an operating window for each key variable and adjust the gluing/layup process to reduce glue dryout. Furthermore, a practical method was developed to determine the optimum pressing parameters to achieve target gluebond quality while minimizing plywood thickness loss.
In this work, the properties of aspen veneer from two mills (A and B) were compared. The comparisons between the incised veneer and non-incised veneer for mill A were made in terms of veneer thickness, ultrasonic propagation time (UPT), density and MOE. The aspen veneer was further characterized for LVL/plywood products by tailoring veneer grades to the requirements of final veneer products. In addition, MOE-based veneer stress grading and UPT-based veneer stress grading were compared for the aspen veneer. The advantages of MOE-based veneer stress grading over UPT-based veneer stress grading were identified in terms of veneer grade MOE and volume breakdown. The main results are summarized as follows:
1) Aspen veneer properties change from mill to mill. The differences in aspen veneer density and MOE between mill A and B are significant with mill A producing denser and stronger aspen veneer.
2) For aspen veneer in the mill A, the distribution shapes of veneer thickness, UPT, density and MOE between the non-incised and incised veneer are quite similar. Although the differences in veneer thickness, UPT and density between the non-incised veneer and incised veneer are identified as significant, the difference in veneer MOE is not significant due to the effect of both veneer UPT and density. The incised veneer has a slightly higher variation in thickness and is also slightly thicker compared to the non-incised veneer. This could due to the change of lathe settings or the property variation of aspen species as indicated with the veneer density variation.
3) Of the aspen veneer from mill A, using the optimum UPT thresholds, about 27.5 ~ 30.9% can be extracted through veneer stress grading to make 2.0 million psi LVL; about 43.4 ~ 59.9% can be sorted out for 1.8 million psi LVL; and the remaining 12.6 ~ 25.7% can be used for 1.5 million psi LVL or for plywood. It was also found that the incised aspen veneer generates 3.4% less of top stress grade G1 but 16.5% more of stress grade G2 compared to the non-incised aspen veneer if performing the optimum UPT-based stress grading.
4) The MOE-based veneer stress grading not only results in a smaller variation in MOE of each grade, but also higher volume percentages of stress grades G1 and G2 compared to the UPT-based veneer stress grading. This smaller variation in MOE of each stress grade will be very beneficial to the industry and structural applications since higher design stress can be assigned for the wood structural components. Also the higher percentages of stress grades G1 and G2 with the MOE-based veneer stress grading has significant economical implications and should be recognized by the industry.
5) To maximize mill profits, veneer sheets need to be periodically sampled and analyzed using the VGrader software. The optimum grading thresholds for the specific veneer can be established for on-line veneer stress grading based on the current market and requirements of final veneer products, providing a real solution to characterize and make best use of the specific veneer for LVL/plywood products.
A series of plywood and laminated veneer lumber (LVL) panels were prepared using veneers with higher than normal moisture contents in face and back layers. The purpose of the work was to evaluate the effects of self-generated steam on the pressing times and panel warpage. Panels made with 6% and 10% m.c. faces and backs were compared with control panels made with all dry veneer. Thirteen- ply 40 mm (1 5/8 inch) thick panels were evaluated for press times and thin 9.5 mm (3/8 inch) panels were evaluated for cupping and bowing. Normal plywood press temperatures and adhesives were used. All panels were made with incised 3.2 mm (1/8 inch) SPF veneers. The project demonstrated that substantially shorter press times and more dimensionally stable panels can potentially be made using higher moisture content outside veneers.
A series of plywood and laminated veneer lumber (LVL) panels were prepared using incised veneers in the second phase of this two year project. The primary purpose of the work was to evaluate the effects of steam injection on the pressing times. A secondary objective was to expand the study of warpage in three-ply and four-ply plywood which was begun in phase one. Thirteen-ply 40 mm (1 5/8 inch) thick panels were evaluated for press times and thin 9.5 mm (3/8 inch) and 12.5 mm (1/2 inch) panels were evaluated for cupping and bowing. Press temperatures of 150 degrees C, 175 degrees C and 204 degrees C were used with a commercial adhesive mix for the LVL study while normal plywood pressing conditions were used for the plywood. For the plywood warpage study, the effect of lathe check orientation and species mix were evaluated. The lathe check orientation had little effect while the surface veneer species had a pronounced effect on the warpage in the plywood. Steam used for injection was heated to 260 degrees C at 450 KPa (65 psi) with a super-heater. All panels were made with incised 3.2 mm (1/8 inch) SPF veneers. The project demonstrated that steam injection can shorten press times by fifty percent if incised veneers are used.
This report investigated the effect of veneer incising (incisor teeth patterns) on conventional hot-pressing time and compression behavior of spruce LVL/plywood. The 4 factors taken into account were panel type (LVL or plywood), number of panel layers, veneer moisture content and veneer type. Three veneer types were considered: non-incised veneer, lightly-incised (lathe-incised) veneer and heavily-incised veneer (similar to veneer incising in front of dryer). The three veneer moisture levels considered were 0%, 3% and 6%. The number of panel layers used was 5, 9 and 13. A mixed-level experimental design was employed. Using a statistical software program, JMP, the main factors affecting the LVL/plywood hot-pressing time were identified.
Veneer incising at the peeling lathe, a new technology developed at Forintek, has been increasingly applied in Canadian softwood plywood mills. Significant benefits include reducing veneer curl-up and spin-outs and increasing veneer recovery. However, a comprehensive study of the effect of veneer incising on the conventional hot-pressing process has not been undertaken so far. As part of the work on optimization of LVL/plywood hot pressing process, this report investigated the effect of veneer incising on the strength properties of Douglas-fir LVL/Plywood products. Lightly-incised Douglas-fir veneer peeled with a Forintek mini-lathe was used in comparison to non-incised Douglas-fir veneer. The three veneer moisture levels considered were 0%, 3% and 6%. The number of plies for plywood and LVL panels were 5 and 13, respectively. A mixed-level experimental design was employed. Using a statistical software program, JMP, the importance of factors affecting Douglas-fir LVL/plywood hot-pressing and strength properties were identified. Also the t-test was used to test the significance of the difference in panel mean strength. The results showed that:
1. For Douglas-fir species, the lightly-incised veneer (similar to that now used at mills) does not significantly affect the conventional hot pressing times for 1) 5-ply plywood panels if the target core temperature is 115 0C and 2) 13-ply LVL if the target core temperature is 105 0C, compared to non-incised veneer. The hot-pressing time increases with increase in veneer moisture content ranging from 0 to 6% for 5-ply plywood.
2. For Douglas-fir veneer, no occurrences of blows were observed after unloading the press even with 5-ply plywood panels at 6% veneer moisture content. Under the same conditions, the compression ratios of 5-ply plywood and 13-ply LVL using the lightly-incised veneer are slightly larger compared to non-incised veneer. Also higher moisture veneer results in higher panel compression ratio.
3. For 5-ply Douglas-fir plywood, there are no significant differences in plywood bending MOE and shear strength (lap-shear) between non-incised and lightly-incised veneer at either 0% or 3% veneer moisture content. However, at 6% veneer moisture content, the differences in mean plywood bending MOE and shear strength between non-incised veneer and lightly-incised veneer were identified significant; the mean bending MOE of 5-ply plywood using the lightly-incised veneer is about 10% higher compared to the non-incised veneer; on the contrary, the shear strength (lap-shear) of 5-ply plywood using the lightly-incised veneer is about 20% lower compared to the non-incised veneer. On a statistical basis, there is no significant difference in plywood mean bending MOR between non-incised veneer and lightly-incised veneer at each of the three moisture content levels.
4. For 13-ply Douglas-fir LVL, there is no statistical difference in edgewise bending MOE and MOR between the incised veneer and non-incised veneer at 3% veneer moisture content. Based on the limited number of replicates, the block shear strength through-the-thickness using the lightly-incised veneer is found to be approximately 10% higher compared to the non-incised veneer and this difference is found to be significant on a statistical basis. But the difference in block shear strengths parallel to grain between the incised veneer and non-incised veneer is not significantly different.
Steam press drying of green SPF veneers was evaluated on a 4 x 8 foot prototype steam press set up at a local plywood mill. A used industrial press has been modified into a single opening oil heated hot press with grooves machined into the upper platen. Standard green nominal 4 x 8 foot 3.2 mm (1/8 inch) thick mill veneer was dried with super-heated steam injected through 6 mm (1/4 inch) holes in the bottom platen. All veneers were incised with Forintek's patented veneer incisor. The project demonstrated that steam injection can dry green veneers three to four times faster than conventional veneer driers. From the results of this project, a study on the press drying of redry veneer was initiated and showed good potential for industrial implementation.