Best Practice Checklist for Plywood Manufacturing - A Strategy to … · FPInnovations – Forintek...

FPInnovations – Forintek Division

Western Region 2665 East Mall

Vancouver, British Columbia V6T 1W5

General Revenue Report Project No. 5727

Final Report 2007/08

Best Practice Checklist for Plywood Manufacturing - A Strategy to Reduce Delamination

by

Brad Jianhe Wang Research Scientist Composites Group

Chunping Dai Group Leader

Composites Group

August 2008

FPInnovations - Forintek would like to thank its industry members, Natural Resources Canada (Canadian Forest Service); the Provinces of British Columbia, Alberta, Saskatchewan, Manitoba, Ontario, Quebec, Nova Scotia, New Brunswick, as well as Newfoundland and Labrador and the

Government of Yukon for their guidance and financial support for this research

Notice This report is an internal FPInnovations – Forintek Division (Forintek) document, for release only to Forintek members and supporters. This distribution does not constitute publication. The report is not to be copied for, or circulated to, persons or parties other than Forintek members and supporters, except with the prior permission of Forintek. Also, this report is not to be cited, in whole or in part, unless prior permission is secured from Forintek. Neither Forintek, nor its members, nor any other persons acting on its behalf, make any warranty, express or implied, or assume any legal responsibility or liability for the completeness of any information, apparatus, product or process disclosed, or represent that the use of the disclosed information would not infringe upon privately owned rights. Any reference in this report to any specific commercial product, process or service by trade name, trademark, manufacturer or otherwise does not constitute or imply its endorsement by Forintek or any of its members. ©2008 FPInnovations – Forintek Division. All Rights reserved. No part of this published Work may be reproduced, published, stored in a retrieval system or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording or otherwise, whether or not in translated form, without the prior written permission of Forintek, except that members of Forintek in good standing shall be permitted to reproduce all or part of this Work for their own use but not for resale, rental or otherwise for profit, and only if Forintek is identified in a prominent location as the source of the publication or portion thereof, and only so long as such members remain in good standing

Best Practice Checklist for Plywood Manufacturing – A Strategy to Reduce Delamination

iii

Abstract 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.


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Acknowledgements FPInnovations - Forintek Division would like to thank its industry members, Natural Resources Canada (Canadian Forest Service), British Columbia, Alberta, Saskatchewan, Manitoba, Ontario, Quebec, Nova Scotia, New Brunswick, Newfoundland and Labrador, and the Yukon Territory, for their guidance and financial support for this research.

The authors would like to thank following project liaisons for their technical guidance and useful input during the course of this project:

• James McPherson, Longlac Wood Industries; • Steve Wharton, Tolko Industries Ltd.; • Dale Black, CertiWood Technical Centre; • Dan Gouthro, Hexion Specialty Chemical; • Tim Quebec and Greg Crawford, Canadian Forest Products Ltd.


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Table of Contents Abstract......................................................................................................................................................................... iii Acknowledgements ....................................................................................................................................................... iv Table of Contents...........................................................................................................................................................v List of Tables................................................................................................................................................................. vi List of Figures ............................................................................................................................................................... vi 1 Introduction .............................................................................................................................................................1 2 Key Objectives ........................................................................................................................................................2 3 Staff ........................................................................................................................................................................2 4 Development of a Best Practice Checklist ..............................................................................................................2

4.1 Main Causes of Plywood Delamination........................................................................................................2 4.2 Key Checkpoints and Variables ...................................................................................................................4

4.2.1 Veneer surface roughness.............................................................................................................5 4.2.2 Dry veneer thickness .....................................................................................................................6 4.2.3 Ambient and dry veneer temperatures...........................................................................................7 4.2.4 Dry veneer MC...............................................................................................................................8 4.2.5 Glue spread level ...........................................................................................................................8 4.2.6 Open and close assembly times ....................................................................................................9 4.2.7 Variation of percentage wood failure within a full-size panel .........................................................9

4.3 A Generic Best Practice Checklist..............................................................................................................11 4.3.1 Improving veneer peeling.............................................................................................................11 4.3.2 Optimizing veneer sorting and drying...........................................................................................12 4.3.3 Improving glue coverage and reducing glue dryout .....................................................................14 4.3.4 Optimizing pressing parameters ..................................................................................................16

5 Summary and Conclusions ...................................................................................................................................18 6 Recommendations ................................................................................................................................................19 7 References ...........................................................................................................................................................19 Appendix I: Effect of veneer surface roughness on plywood gluebond quality ...........................................................24 Appendix II: Establishing operating windows to reduce glue dryout ...........................................................................27 Appendix III: Optimizing plywood pressing parameters ..............................................................................................31


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List of Tables Table 1: A generic checklist for veneer peeling............................................................................................................12 Table 2: A generic checklist for veneer drying .............................................................................................................14 Table 3: A generic checklist for gluing and lay-up........................................................................................................16 Table 4: A generic checklist for panel hot pressing......................................................................................................18 Table 5: Veneer thickness measurement for three roughness groups.........................................................................25 Table 6: Panel compression ratio and gluebond quality of 3-ply plywood....................................................................26 Table 7: An experiment design involving three key variables .....................................................................................28 Table 8: The gluebond test results from the mill trial...................................................................................................29 Table 9: The layout of pilot plant plywood manufacturing ............................................................................................32 Table 10: The gluebond quality and density of 5-ply coastal Douglas-fir plywood panels ...........................................33

List of Figures Figure 1: Causes of plywood low percentage wood failure......................................................................................3 Figure 2: The key process variables pertaining to panel delamination ....................................................................5 Figure 3: Variation of veneer surface roughness from laboratory measurements ...................................................5 Figure 4: Variation of veneer surface roughness as seen in photos taken in a plywood mill ...................................6 Figure 5: Variation of dry veneer thickness from a mill trial .....................................................................................6 Figure 6: Variation of dry veneer temperature in terms of species and loads from a mill trial..................................7 Figure 7: Variation of ambient and dry veneer temperatures during a day from a mill trial......................................7 Figure 8: Variation of dry veneer MC in terms of species and loads from a mill trial ...............................................8 Figure 9: Variation of glue spread level from a mill trial ...........................................................................................8 Figure 10: Variation of open and closed assembly times for 6-ply spruce plywood from a mill trial ..........................9 Figure 11: Lowered percentage wood failure due to excessive localized veneer roughness from a mill trial ..........10 Figure 12: Lowered percentage wood failure due to veneer overdry and glue dryout from a mill trial .....................10 Figure 13: Lowered percentage wood failure due to veneer roughness and glue dryout from a mill trial ................10 Figure 14: Programming lathe, pitch and gap profiles for optimum peeling.............................................................11 Figure 15: The effect of veneer drying on percentage wood failure.........................................................................13 Figure 16: Upgrading conventional glue spreader to a foam or curtain coating system ..........................................15 Figure 17: Ensuring a full glue coverage during gluing and lay-up ..........................................................................15 Figure 18: The variation of percentage wood failure in terms of pressing time........................................................17 Figure 19: Adjustment of glue spread for target percentage wood failure ...............................................................30 Figure 20: Variation of plywood percentage wood failure versus pressing pressure ...............................................33 Figure 21: Relationship between plywood percentage wood failure and pressing pressure....................................34 Figure 22: Variation of plywood percentage wood failure versus panel CR.............................................................34


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1 Introduction The plywood manufacturing process includes log conditioning, veneer peeling, clipping and sorting, drying, composing, grading, gluing and layup and hot pressing (Sellers 1985; Demas 1992; Baldwin 1995). The quality of plywood products starts right at the beginning of the process. Delamination has been one of the biggest concerns in the plywood industry (Perkins 1950; Finley 1964; Koch 1965; Koch 1967; Koch 1970; Sellers 1985; DeVallance et al 2004; DeVallance et al 2007), and becomes the most urgent issue to many of our plywood members. It currently accounts for approximately 85% of customer complaints about plywood as a sub-flooring product.

Previous studies have shown that veneer surface roughness has a significant effect on plywood gluebond quality as measured by shear strength and percentage wood failure (Yavorsky et al. 1955; Neese 1997; Neese et al. 2004; Wang et al 2006a and b). Percentage wood failure can be increased by reducing veneer roughness, and shear strength can be increased by reducing the number of lathe checks per unit width. Lathe checks in the loose side of veneer may be the initial zones of weakness but they seemed to favor wettability since the contact angle decreases more rapidly on loose sides than on tight sides (Kaneda et al. 1968; Vazquez et al 2003). Lathe checks might interact with veneer roughness in determining gluebond quality (DeVallance 2007).

The moisture variation of dry veneer is one of the main factors contributing to delamination (Troughton 2001; Dai et al. 2003; Wang et al. 2007). Improper green veneer moisture sorting and aggressive drying schedule appear to cause both overdried and underdried veneer (Wang et al. 2004; Wang and Dai 2005; Wang 2006). Glue dry-out is another key contributor to panel delamination, which is mainly affected by veneer temperature, assembly time, and glue spread level and coverage (Feihl 1982; Sellers 1985; Troughton 2001). Although many research studies have been devoted to clarify their individual effect on gluebond quality, their combined effects are not fully known (Faust and Rice 1986a and 1986b; Faust and Rice 1987; Byrne and Peters. 1994; Zavala and Humphery 1996; DeVallance 2007). Hot pressing is the key step in the plywood manufacturing process, the drive on increasing productivity and recovery may have contributed to panel delamination (Wellons et al. 1983; Wang 2003; Wang and Yu 2003; Wang et al. 2006). There are numerous key process variables in the plywood manufacturing. Significant variations in these variables may exist in today’s fast production. Over the past several years, FPInnovations-Forintek Division has investigated the full process of plywood manufacture (Dai et al. 1998; Dai and Wang 1998; Dai and Wang 1999; Dai and Wang 2001; Wang and Dai 2001; Dai and Wang 2003; Wang 2003; Wang and Yu 2003; Wang et al. 2004; Wang and Dai 2005; Dai and Wang 2006; Wang and Dai 2006a and b; Wang et al. 2006; Wang et al. 2006 a, b and c; Wang and Dai 2007). The key results include best practice and software programs for each of the manufacturing phases (Dai and Wang 1998; Dai et al. 2002; Dai and Wang 2006; Wang and Dai 2006a). But so far, a generic best practice checklist that member mills can use for the purpose of identifying any gaps in their quality control system has not been available. For this project, previous work and literature were reviewed. The main causes of panel delamination were ascertained. The four key checkpoints for reducing panel delamination were determined in the manufacturing process. The key process variables at each checkpoint were identified. Through mill visits and field studies, the variations of these key variables were monitored to determine their working ranges that minimize delamination. A generic best practice checklist was devised, which can be tailored to individual mills.


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2 Key Objectives The key objectives were to develop a generic best practice checklist to reduce plywood delamination, and determine the effects of key operation variables on plywood gluebond quality.

3 Staff Brad Jianhe Wang Project Leader, Composites Research Scientist, Composites Group Chunping Dai Group Leader, Composites Group Gordon Chow Industry Advisor, Composites Group Heng Xu Composites Research Scientist, Composites Group Steve Wharton Quality Control Supervisor, Armstrong Division, Tolko Industries Ltd. Peter Ens Senior Technologist, Composites Group Axel Andersen Senior Technologist, Composites Group Don Moffat Technologist, Composites Group

4 Development of a Best Practice Checklist 4.1 Main Causes of Plywood Delamination Plywood gluebond quality is a key criterion to assuring the panel structural integrity and guarding against service delamination. Currently, there are several standards available to determine plywood gluebond quality (NIST 1996; European Standard EN314 1993; JAS SE-3 2000; CSA O151 2004). The European Standard primarily measures the shear strength of plywood specimens as the quality indicator while Canadian standard primarily targets on percentage wood failure (CSA O151 2004). Plywood delmination is closely associated with low percentage wood failure (Chow and Warren 1972; DeVallance 2007). Based on the historical laboratory plywood gluebond (shear) test data, veneer surface roughness was found to be the key factor to the reduced percentage wood failure, followed by knotty surface, missing glue due to thick and thin veneer, surface inactivation (hardening), inadequate glue cure and others such as gaps and strings. Figure 1 shows the fraction of the causes of low percentage wood failure. Note particularly that glue dryout was normally excluded from as a variable in the manufacture of these laboratory plywood panels. In the mill conditions, glue dryout was normally the no.1 cause due to long assembly time and high veneer temperature. In most cases, the low percentage wood failure was caused by the combined effect of veneer roughness, temperature and assembly time.


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0%

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Inactivation Inadequatecuring

Other (gapetc.)

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of lo

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Figure 1: Causes of plywood low percentage wood failure While it is generally agreed that veneer roughness leads to lower gluebond quality (George and Miller 1970; Faust and Rice 1986a; Faust and Rice 1986b; Faust and Rice 1987; Neese 1997; Neese et al. 2004), current industrial practices such as the utilization of small and low quality logs, high speed peeling, small core drop size, and sorting out higher-grade veneer to make laminated veneer lumber (LVL) all contribute to rougher veneer being used to produce plywood (JAS SIS 1993; Wang and Dai 2001; Wang and Dai 2005; Wang and Dai 2006b). Processing variables such as log conditioning time, conditioning temperature, knife sharpness, knife angle, lathe settings, peeling speed, and final veneer thickness may also affect veneer roughness (Lutz 1956; Lutz 1964; Leney 1960; Knospe 1964; Feihl 1971; Palka and Holmes 1973; Resch and Parker 1979; Faust 1987). Variation in wood density can cause vibration forces during peeling, resulting in rough veneer being produced (Feihl and Godin 1970). Anatomical factors that influence mechanical processing include tree growth rate, annual ring symmetry, and size and frequency of knots. Rough veneer contributes to poor quality glue bonds because of peaks and valleys on the surface that result in only the peaks making intimate contact with each other during pressing (Marian et al. 1958; Stumbo 1960; Wang et al. 2006a and b). Roughness was also found to affect the bond strength perpendicular to the glueline of waferboard (Pfaff 1989). In plywood manufacturing, the rough veneer was generally coupled light glue spread or glue skips under the same platen pressure. Veneer surface inactivation would cause insufficient glue transfer during pressing. Inadequate glue cure was mainly due to heavy glue spread, overwet veneer, sap wet spots, and short pressing time (Chow and Hancock 1972; Sellers 1985).


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4.2 Key Checkpoints and Variables Based on the review of literature, previous work performed by FPInnovations - Forintek and CertiWood and feedback from the mill visits, the following four key quality checkpoints for reducing panel delamination were determined: veneer peeling, veneer drying, panel gluing/lay-up and hot pressing. For veneer peeling, the focus should be placed on how to improve veneer quality. For veneer drying, the priority should be put on how to increase veneer final moisture content (MC) uniformity and reduce veneer overdry. For panel gluing and lay-up, improving glue coverage and reducing glue dryout are the two key factors. Reducing open assembly time and veneer temperature helps reduce glue dryout. As for panel pressing, pressing schedules (or recipes) are the key to the quality of final plywood products, which should be optimized based on species and panel construction to achieve target gluebond quality while minimizing panel thickness loss. Figure 2 shows the key variables at each of the four checkpoints. In the process of veneer peeling, veneer surface roughness and thickness variation are the two main variables. Currently in the mill, the evaluation of veneer surface roughness is mainly based on visual inspection. During peeling, veneer ribbon generally becomes rougher from sapwood to heartwood. Some mills purposely scrap very rough veneer close to the core for chipping. Although this may cause a reduction of material recovery, the required level of panel compression ratio (CR) and the rate of panel delamination can be reduced. The net effect could be positive. For veneer drying, three key variables are identified as veneer MC variation, surface inactivation and veneer temperature. Due to the inaccurate sorting of green veneer MC and aggressive drying schedules, variation of dry veneer MC (peak or average), either between sheets or within-sheet, is a big issue. In current practice, both overdried and underdried veneers co-exist and significantly reduce gluebond quality. Veneer overdrying is generally associated with surface inactivation, which has a direct connection with green veneer sorting and drying settings such as temperature and humidity. The veneer sheets containing both sapwood and heartwood are more difficult to dry to a uniform MC. As a result, improving green veneer sorting and optimizing drying temperature and humidity are essential to reduce surface inactivation. To achieve good gluebond quality, the target veneer MC should be higher than 2% (Sellers 1985). The best drying strategy should balance drying productivity and dry veneer quality. Compared with other zones of the dryer, the set temperature at the last zone should be lowered to save energy and improve veneer quality, which can also help reduce veneer temperature after drying. For gluing and lay-up, three key variables are glue spread level, glue coverage and open assembly time. Statistical quality control methods can be used to monitor in-situ variation of these variables (Montgomery 2005). Glue spread level should be adjusted based on veneer quality, ambient temperature, veneer temperature and assembly time. Glue coverage should be monitored to minimize glue skips and ensure a full glue coverage. The open assembly time is the time from the gluing of the first sheet to the full pressure of the load from the cold press, which has a substantial effect on glue dryout. The stoppage of the lay-up line should be avoided as much as possible to reduce glue dryout. For panel pressing, pressing pressure, pressing temperature and time are the three key variables. Pressing temperature of the platen should reach the target and be as uniform as possible. Optimized pressing pressure and time help overcome veneer surface roughness for adequate bonding contacts without sacrificing the productivity and material recovery.


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No. of samples (63.5 x 63.5 -mm)

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Rq at tight side

Equivalent Rq

Peeling-inducedveneer surface roughnessthickness variation

Drying-inducedveneer MC variation surface inactivation veneer temperature

Drying-inducedveneer MC variation surface inactivation veneer temperature

Gluing-inducedglue spread levelglue coverageopen assembly time

Gluing-inducedglue spread levelglue coverageopen assembly time

Pressing-inducedpressing pressure pressing temperaturepressing time

Figure 2: The key process variables pertaining to panel delamination Through laboratory tests and mill studies, the typical variation of the above key variables was shown as follows:

4.2.1 Veneer surface roughness

Veneer surface roughness can be measured by either laser/vision (non-contact) or stylus (contact) methods (Stumbo 1963; Funck et al. 1992; ASME 2003). Using a conventional stylus profilometer (Mitutoyo 2004), the surface roughness (root mean square or RMS roughness Rq) of 24 representative dry aspen veneer pieces (2.5 x 2.5 -in or 63.5 x 63.5 –mm) was measured at both tight side and loose side. As shown in Figure 3, quantitatively, there was a significant variation in Rq among those 24 pieces ranging from 15 to 105 μm. In general, the loose side was rougher than the tight side but with some exceptions.

Figure 3: Variation of veneer surface roughness from laboratory measurements


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Whitewood

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Composed core (random sheets) Half sheets Figure 4: Variation of veneer surface roughness as seen in photos taken in a plywood mill Qualitatively, as shown in Figure 4, based on the glue coverage, both within sheet and between sheets veneer surface roughness variations existed. Rough peel was the primary cause of severe surface glue skips. Composed core with random sheets was generally worse than that with half sheets since random sheets might introduce a significant veneer thickness variation.

4.2.2 Dry veneer thickness

As shown in Figure 5, a significant dry veneer thickness variation existed for the two common softwood species: Douglas-fir and whitewood mix comprising spruce, lodgepole pine and alpine fir (SPF). Douglas-fir veneer had a mean of 3.17 mm with a standard deviation of 0.180 mm while whitewood veneer had a mean of 3.18 mm with a standard deviation of 0.189 mm. In general, mills peeled Douglas-fir veneer slightly thinner than whitewood veneer. Figure 5: Variation of dry veneer thickness from a mill trial


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4.2.3 Ambient and dry veneer temperatures

Figure 6 shows the variation of dry veneer temperature for two common softwood types, Douglas-fir and whitewood mix (SPF). Comparatively, whitewood veneer had a higher temperature than Douglas-fir veneer. As well, the temperature of Douglas-fir veneer varied from load to load, probably due to the variation of veneer MC. Figure 6: Variation of dry veneer temperature in terms of species and loads from a mill trial Figure 7 shows the variation of ambient temperature and dry veneer temperature during a typical operation day from morning through afternoon. It was noticed that both ambient temperature and veneer temperature increased from morning to afternoon.

Figure 7: Variation of ambient and dry veneer temperatures during a day from a mill trial


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Figure 8 shows the variation of dry veneer MC from species to species and load to load. Note that a large portion of veneer sheets had a MC below 2%, indicating a potential problem of veneer overdry (Sellers 1985). Veneer overdry will cause a serious problem of glue bonding. The results indicated that the drying schedule in the mill needs to be adjusted. Figure 8: Variation of dry veneer MC in terms of species and loads from a mill trial

4.2.5 Glue spread level

Figure 9 shows the variation of glue spread level from a mill based on camera detection (Groves 2007). For the 1000 sheets monitored, there was a variation of glue spread from 28 to 34 lb/1000 ft2 per single glueline when a roller spreader was used.

Figure 9: Variation of glue spread level from a mill trial


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4.2.6 Open and close assembly times

Figure 10 shows the variation of open assembly time (OAT) and closed assembly time (CAT) in the mill for 6-ply select-grade spruce plywood. In general, OAT had a significant effect on glue dryout. For this particular panel type, the OAT varied from 5 to 10 min while CAT varied from 5 to 16 min. The variation of OAT was mainly caused by the stoppage of the lay-up line, debris on veneer sheets and veneer quality. In general, the select grade panels had shorter OAT than sheathing grade panels. The variation of CAT was mainly caused by panel unloading in the cold press and loading/unloading in the hot press. Figure 10: Variation of open and closed assembly times for 6-ply spruce plywood from a mill trial

4.2.7 Variation of percentage wood failure within a full-size panel

Due to the variation of localized veneer roughness, the percentage wood failure inevitably fluctuates within the panel. The current plywood standard only requires ten shear specimens cut from one end of the panel (CSA O151 2004), which may not accurately reflect the true panel gluebond quality. Through the mill trial, it was found that the cause of low percentage wood failure was rather complex. The three primary causes were attributed to: 1) excessive localized veneer roughness; 2) combined effect of veneer overdry and glue dryout; and 3) combined effect of veneer roughness and glue dryout. Figure 11 shows the variation of the percentage wood failure within a 4 x 8 –ft 5-ply sheathing grade Douglas-fir plywood. The lowered percentage wood failure was caused by excessive localized veneer roughness. Figure 12 shows the variation of the percentage wood failure within a 4 x 8 –ft 6-ply select grade white spruce plywood. The lowered percentage wood failure was mainly caused by combined effects of veneer overdry and glue dryout. Figure 13 shows the variation of the percentage wood failure within a 4 x 8 –ft 6-ply select grade white spruce plywood. The lowered percentage wood failure was mainly caused by combined effects of veneer roughness and glue dryout.


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48"

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6-ply 3/4” Spruce plywood (select)Test conditions: OAT = 7’40”, CAT = 12’30”

36.7%

64.5%

54.0% 37.8%

35.5%

45.7%Severedryout

48"

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75.0% 52.2%

69.3%

75.0% 67.0%

67.7%Roughness& dryout

6-ply 3/4” Spruce plywood (select): Test conditions: OAT = 6’33”, CAT = 14’34”

Figure 11: Lowered percentage wood failure due to excessive localized veneer roughness from a mill trial Figure 12: Lowered percentage wood failure due to veneer overdry and glue dryout from a mill trial Figure 13: Lowered percentage wood failure due to veneer roughness and glue dryout from a mill trial

5-ply 5/8” Douglas-fir plywood (sheathing)Test conditions: OAT = 7’46”, CAT = 6’39”

48"

96"

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42.1%

83.0%

76.0% 56.9%

89.5%

69.5%


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Positioning knife, bar and block

Programming pitch & gap profiles

4.3 A Generic Best Practice Checklist As a strategy to reduce plywood delamination, a generic best practice checklist was designed for the four key checkpoints: veneer peeling, veneer drying, panel gluing/layup and hot pressing. To help explicitly identify quality control gaps, the checklist was devised in such a way that the answers to all the questions are “Yes”. Otherwise, corresponding actions need to be taken.

4.3.1 Improving veneer peeling

The key to veneer peeling is to reduce veneer surface roughness and thickness variation. The effect of veneer surface roughness on plywood gluebond quality was studied in the laboratory tests. The results, as shown in Appendix 1, demonstrated that the rough veneer tends to result in a low percentage wood failure under the same manufacturing conditions. To improve veneer peeling, log conditioning recipes, such as heating temperature and time, should be species-dependent. Both species sorting and log diameter sorting should help improve green veneer quality. As shown in Figure 14, the positioning of bar, knife and block gap should be checked with an appropriate pressure, and optimized pitch and gap profiles should be used in terms of species. The pressure of the backup roll should also be checked to make sure it is working properly as specified. The green veneer thickness at the three locations: left, center and right, should be regularly monitored or measured to diagnose if an abnormal peeling results.

Figure 14: Programming lathe, pitch and gap profiles for optimum peeling As shown in Table 1, eight questions related to veneer peeling should be checked. The checklist questions from no. 1 to no. 2 deal with conditioning temperature and time for achieving optimum heating. Log diameter sorting should also help achieve target temperature for peeling. The question no. 3 concerns the optimum lathe settings for specific species. The lathe settings used by operators should be validated from green veneer quality check in terms of thickness, roughness and lathe checks. The question no. 4 concerns the lathe maintenance. The performance of backup rolls can be identified by the checklist question no. 5. If the pressure from the backup rolls is too high, hourglass-like core will result. In contrast, if the pressure from the backup rolls is too low, cigar-like core will result. The question no. 6 is knife related. The knife test should be performed according to prescribed frequency. Proper knife sharpening and timely change are important to the peel of smooth veneer. Log quality, diameter and MC have a significant effect on


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veneer quality. The smoothness and flatness of veneer ribbons should be checked, as indicated by the question no. 7. In general, at the end of each peel, veneer ribbon tends to be more curly and rougher due to large curvature and low MC. As stated in the question no. 8, if the ribbon is too rough, the sheets should be pulled out for chipping. In this way, the required compression level during plywood hot pressing can be reduced to increase material recovery. Table 1: A generic checklist for veneer peeling

4.3.2 Optimizing veneer sorting and drying

To perform uniform drying, improved green veneer sorting is essential. Increased number of sorts and reduced MC variation for each sort could help reduce veneer overdry and redry. During drying, optimized temperature and humidity settings should be used to balance productivity and veneer quality. To reduce dry veneer brittleness and extend its shelf “life”, use of mist spray and lower temperature at the last drying stage are recommended. Hot stacking is favoured rather than overdrying. Effective veneer cooling also helps subsequent gluing. As shown in Figure 15, to achieve target glue bond quality, plywood panels made from mill dried veneers required significantly longer pressing time than those made from laboratory dried veneers. The reason was that the laboratory dried veneer adopted a lower drying temperature than the mill dried veneer. In the mill, at a higher drying temperature (over 175°C), a significant portion of veneer sheets could be overdried (Figure 8), leading to little glue transfer during gluing and pressing.


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0

10

20

30

40

50

60

70

80

90

100

2 3 4 5 6 7 8

Pressing time of 5-ply plywood (min)

Woo

d fa

ilure

(%)

Veneer dried by mill A

Veneer dried by mill B

Veneer dried in the lab

Figure 15: The effect of veneer drying on percentage wood failure It was reported that green veneer treated with chemical (borax etc.) spray can help protect bonding sites during drying (Chow 1975). If applicable, light sanding of dry veneer could help reduce veneer surface roughness and thickness variation, and alleviate the effect of veneer overdry. In addition, compressing dry veneer using a hot roller could improve veneer surface smoothness for better gluing. As shown in Table 2, ten questions related to veneer sorting and drying should be checked. As far as green veneer sorting is concerned, two checklist questions (no.1 and no.2) are enlisted. Currently, a trial and error method is used to determine the boundaries of green veneer MC sorting. In general, there exists a significant overlapping in MC between sap and light-sap sorts using the conventional RF sensors (Wang and Dai 2005). Improved sorting appears to be necessary. In the mill, the drying recipes or schedules (questions no.3 and no.4) could vary from operator to operator. To increase the drying productivity, the drying temperature used in the mill is generally too high, leading to an increase in veneer overdry and energy consumption (Christiansen 1990). The checklist question no. 5 addresses redry and rotation (stacking) rate, which also influence actual drying productivity and effectiveness of the drying recipes. Currently, the target rate of redry plus stacking is generally set at approximately 20% for most softwood species to maximize drying productivity. The checklist question no. 6 deals with peak veneer MC and average veneer MC after drying. Due to the significant within-sort MC variation, veneer overdrying may result with a large portion of dry veneer having a peak MC below 2% (Sellers 1985). So far, the optimum drying temperature and humidity settings have not been fully established for common veneer species. Further work is needed in this area. The checklist question no. 7 relates to veneer temperature after drying, which should be effectively reduced through cooling to help reduce glue dryout. The checklist questions no. 8 and no. 9 concern dry veneer quality, which should be controlled to meet the mill specification. Note that the veneer after drying would appear rougher. The last checklist question no. 10 is in connection with dry veneer MC. Manual check of dry veneer MC helps calibrate the on-line dry veneer MC meter. It also helps ensure that the rotation veneer after stacking meets the target MC required for gluing.


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Table 2: A generic checklist for veneer drying

4.3.3 Improving glue coverage and reducing glue dryout

Currently, a conventional contact-type glue application system represented by a roller glue spreader has a drawback being sensitive to veneer surface roughness and thickness variation. As shown in Figure 16, the glue coverage from either foam coating or curtain coating is more uniform with less glue skips than that from spreader coating. To reduce the effect of veneer surface roughness and improve the glue coverage, non-contact type of glue application systems, such as foam coating or curtain coating, should be adopted. To ensure a full glue coverage, composed core should use the same species (thickness) with a preference giving to half sheets. Some very rough veneer sheets (random or full width) can be visually identified. As mentioned earlier, they should be segregated out for chipping (Figure 17). During gluing, the debris on veneer sheets should be removed since they will affect the glue coverage and subsequent pressing.


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Foam coating

Curtaincoating

Figure 16: Upgrading conventional glue spreader to a foam or curtain coating system

Figure 17: Ensuring a full glue coverage during gluing and lay-up As shown in Table 3, ten questions related to gluing and lay-up should be checked. Among them, no.1 has long been overlooked. Slow oxidation takes place during the storage time (or aging time). In general, the longer the aging time of dry veneer, the lower the plywood gluebond quality. The difference in contact angle (or wet ability) was significant between that measured immediately after drying and that after 30 days from drying (Furuta et al 2004). The veneer loads should be used when they are “fresh”. The questions from no. 2 to no. 5 are related to glue dryout, which is affected by veneer temperature, bondability of plywood glue mix, glue spread level and open assembly time, and so on. The questions from no. 6 to no. 8 deal with the actual glue spread level and its uniformity, which can be measured with sample sheets and monitored by cameras (Groves 2007).


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No Quality control check Yes NoIf “no”, explain

1

2

3

4

5

6

7

8

9

10

Is the duration of veneer loads after drying within the specified time (two weeks for example) ?

Has the spread level been adjusted in terms of veneer temperature, ambient temperature, open assembly time and panel construction ?

Is the spread level consistent from the left to right for the spreader, or foam coater, or curtain coater ?

Is the glue being mixed according to the resin supplier's specification,and is its viscosity regularly checked and details recorded ?

Have the temperature and MC of loads been checked before gluing ? Have the temperature and MC of loads been checked before gluing ? Is the veneer temperature at lay-up stations within the target range ?Is the veneer temperature at lay-up stations within the target range ?

Is the open assembly time within the target range in this season ?Has the spread level been checked according to the set frequency ?Has the spread level been checked according to the set frequency ?

Is the glue coverage uniform with minimum glue skips ?Is the glue coverage uniform with minimum glue skips ?

Are the pre-pressing pressure and time set as specified ?Are the pre-pressing pressure and time set as specified ?

To help reduce the glue dryout, a systematic mill study was conducted involving the three key variables: glue spread level, veneer temperature and open assembly time. Based on the gluebond test results, a statistical model was established to 1) examine the interactions of the three variables for achieving a target percentage wood failure; and 2) deal with “what-if” scenario to reduce glue dryout. As a result, the working ranges of the three key variables can be established. The details of the methodology are shown in Appendix II. The question no. 9 pertains to the real-time adjustment of glue spread level, which should be a function of target percentage wood failure, veneer temperature, open assembly time, and ambient temperature (operation season dependent). Panel construction is also a potential factor. The question no. 10 deals with pre-pressing which was aimed at creating an initial tack between veneers while not squeezing out the glue. Table 3: A generic checklist for gluing and lay-up

4.3.4 Optimizing pressing parameters

The key to panel pressing is to apply pressure and heat to consolidate panels to the target thickness and cure the glue. To achieve target gluebond quality, veneer surface roughness needs to be overcome for intimate bonding contact. Use of species-dependent pressing schedules is essential to ensure adequate panel compression ratio (CR) and glue curing. With adequate pressing time and/or pressure, the effect of veneer surface roughness and overdry can be partially overcome to achieve target gluebond quality, and the variation of percentage wood failure can be significantly reduced (Figure 18). As a result, the issue of panel delamination can be alleviated.


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0

10

20

30

40

50

60

70

80

90

100

6.5 7.0 7.5 8.0

Pressing time of 8-ply plywood (min)

Woo

d fa

ilure

(%)

Mean Std. Dev.

Figure 18: The variation of percentage wood failure in terms of pressing time As shown in Table 4, eleven questions should be checked at this checkpoint. The question no. 1 concerns the loading/unloading efficiency of cold pressing and hot pressing. The question no. 2 deals with the maintenance and heat uniformity of the hot press, which are important to the manufacturing of quality products. The questions no. 3 and no. 4 concern the hot pressing recipes to ensure that an adequate bonding contact is achieved and the innermost glueline is fully cured. The pressing pressure should be adjusted in terms of species, veneer MC and panel thickness and construction. With the monitoring of the innermost glueline temperature, manipulating panel pressing pressure can lead to improved plywood gluebond quality while minimizing panel thickness loss. The actual panel compression ratio (CR) should be checked for each species and panel construction. As shown in Appendix III, to optimize plywood hot pressing parameters, a practical method was developed for use in the mill. The method resulted in an optimum pressing pressure in terms of given variation of veneer surface roughness and MC, which leads to increased material recovery and reduced variation of plywood gluebond quality. The questions no. 5, no. 6, and no. 7 tackle the gluebond quality check and effectiveness of the quality control tests. The questions no. 8, no. 9, no. 10 and no. 11 link the glue bond quality results and required actions.


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Table 4: A generic checklist for panel hot pressing

5 Summary and Conclusions Based on this work, the main causes of panel delamination were methodically identified as 1) glue dryout from long assembly time and high veneer temperature; 2) light glue spread (or glue skips) and low panel compression due to rough veneer; 3) poor 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) the combined effect of any of the above four mentioned causes. The potential root causes of panel delamination were found to be evident from the very beginning of the production process. Consequently, four key checkpoints were identified: veneer peeling, veneer drying, panel gluing/lay-up, and hot pressing. Furthermore, key process variables at each checkpoint were determined as follows: peeling-related veneer surface roughness and thickness variation, drying-related veneer MC variation and surface inactivation, veneer temperature, glue coverage and dryout, and pressing time and pressure. A generic checklist was compiled to help establish best practices for manufacturing plywood with a focus on the above four key checkpoints. The checklist can be modified for individual mill use. The strategies to reduce panel delamination were proposed and discussed. Based upon the associated mill trials, the variation of the above mentioned key variables was ascertained. It was found that excessive localized roughness and the combined effect of the following factors, namely, veneer overdry, glue dryout and roughness, were the key causes of a low percentage wood failure. A statistical model was also developed to predict the percentage wood failure in terms of veneer temperature, open assembly time and


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glue spread, which helps establish an operating window for each key variable, and adjust the gluing/layup process to reduce glue dryout. A practical method was further developed to determine the optimum pressing parameters to achieve target gluebond quality while minimizing panel thickness loss.

6 Recommendations It is estimated that a 1% reduction in delamination for a 250 million ft2 (3/8 - in basis) plywood mill, represents a potential annual savings of approximately $650,000 to the company (RISI – Variable Costs – BC – 2006). To help reduce plywood delamination, the generic checklist established should be modified for individual mill use as a guide for manufacturing plywood. It is recommended that borax in a low concentration be applied to green veneer before drying to reduce surface inactivation, and the optimum pressing schedules be established in terms of species, panel type and thickness, and veneer MC.

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Appendix I Effect of veneer surface roughness on plywood gluebond quality


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Experimental To investigate the effect of veneer surface roughness on plywood gluebond quality, 1/8-in dry western hemlock veneer sheets were acquired from one BC plywood mill and cut into 12 x 12 –in size and visually segregated into 3 roughness classes: 1- smooth veneer; 2- medium rough and 3- rough. Veneer moisture content (MC) was 3 -5 %. The thickness was measured from 9 points of each sheet. Then they were pressed into 3-ply plywood panels following 3 different lay-up constructions, namely, class 1 -class 1 -class 1; class 1- class 2- class 1 and class 1 - class 3 - class 1. Note that for each lay-up, smooth veneer (class 1) was always used as face and back with the tight side facing outwards. Three replications were used for each construction. The lay-up thickness of each panel was calculated. A commercial plywood phenol formaldehyde (PF) glue mix was used with a solids content of 45%. The glue spread level was 32 lb/1000 ft2 per single glueline. Total assembly time was set at 10 min to avoid glue dryout. A small laboratory press (15 x 15 –in) was used. The hot pressing parameters were 150°C for pressing temperature and 180 psi for pressing pressure. The pressing time was 4 min with which the full glue cure can be ensured. After pressing, the panels were stacked for 48 h. Then 9-point panel thickness was measured to calculate panel compression ratio (CR). Subsequently, sixteen (16) shear specimens (half of the shear specimens, lathe checks pulled open; other half, lathe checks pulled close) were cut from each panel. The panel gluebond quality was determined after vacuum pressure treatment in terms of shear strength and percentage wood failure (CSA O151 2004). In addition, 8 specimens (1 x 5 -in) were cut from each panel to perform 6-cycle delamination tests in accordance with the CAN/CSA –O325 standard (CSA O325.1 1988). To meet target gluebond quality requirements, following three criteria must be met:

1. the average percentage wood failure: equal or greater than 80%; 2. at least 90% of panels represented by shear specimens having wood failure greater than 60%; and 3. at least 95% of panels represented by shear specimens having wood failure greater than 30%.

Results Table 5 summarizes the results of veneer thickness in terms of 3 roughness classes. Compared with smooth veneer, rough veneer was thicker, had higher thickness variation and generally associated with deeper lathe checks. Table 5: Veneer thickness measurement for three roughness groups

Roughness class Veneer thickness

(mm) 1 - Smooth 2 - Medium rough 3 - Rough

Average 3.26 3.34 3.41 Maximum 3.81 3.94 5.03 Minimum 2.93 2.88 2.96 Std. Dev. 0.15 0.24 0.35

Table 6 summarizes the panel thickness, panel compression ratio (CR) and gluebond quality for 3 panel lay-up constructions. Using the same pressing parameters, the resulting panel thickness seemed to be similar but the panels with rough veneer had a larger CR, which means a larger panel thickness loss (Wellons et al. 1983). The average percentage wood failure decreased and its variation increased with increasing veneer roughness. Note that the panel lay-up construction class 1-3-1 (smooth-rough-smooth)


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failed to meet the CSA standard 80% wood failure requirements. Three panels with the lay-up construction 1-3-1 did not survive the first cycle of delamination test in accordance with CAN/CSA-O325.0-88 either. This demonstrated that under the same pressing conditions, placing rough veneer as the crossband (core) yielded a low percentage wood failure. As a result, rough veneer could be one of the major causes of field panel delamination. Table 6: Panel compression ratio and gluebond quality of 3-ply plywood

Panel gluebond quality Lay-up thickness

Panel thickness

Panel CR Shear strength (psi) Wood failure (%)

3-ply plywood lay-up

(mm) (mm) (%) Average Std. Dev. Average Std. Dev. Roughness class

1-1-1 9.65 8.82 8.6 152.8 45.1 89.7 12.6

Roughness class 1-2-1 9.93 8.75 11.9 152.7 34.7 84.2 14.3

Roughness class 1-3-1 10.12 8.79 13.2 131.8 40.4 73.2 16.0


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Appendix II Establishing operating windows to reduce glue dryout


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Mill Trials Based on the feedback from mills, glue dryout is the no.1 contributor to the panel delamination. To reduce glue dryout, mill trials were conducted to 1) monitor the variation of key process variables; 2) examine the variation of plywood gluebond quality as measured by shear strength and percentage wood failure; and 3) determine how the key gluing and lay-up variables such as veneer temperature, glue spread level and assembly time affect glue dryout and panel gluebond quality. Based on real-time monitoring of these key variables, their variations were determined. As shown in Table 7, an experiment layout with a response surface method was devised to investigate the effect of the three key variables: open assembly time (X1), glue spread level (X2) and veneer temperature (X3), on plywood gluebond quality and define the operating window for reducing plywood delamination. This method has been widely used in wood industry and other fields to optimize the process variables (Warren and Hailey 1980; Khuri and Cornell 1996; JMP 2000; Jimenez et al. 2000; Rosli et al. 2003). In the trial, the following process variables were not differentiated: veneer MC, surface roughness and inactivation. Western white spruce, western hemlock and interior Douglas-fir were used in the panel manufacturing. Five -ply 5/8 -in plywood panels were manufactured with 285°F (140.5°C) pressing temperature, 190 psi platen pressure and 255 s pressing time. Thirty panels (one load) were made for each combination. Two panels were sampled from each load. Thus a total of thirty 4 x 8 –ft plywood panels were sampled and shipped to FPInnovations –Forintek Division for gluebond quality tests. From each panel, five locations were labelled for cutting shear specimens. Ten specimens each were cut from each location to measure shear strength and percentage wood failure in accordance with CSA O151-04 standard (CSA O151 2004). Table 7: An experiment design involving three key variables

Record Open assembly time (OAT, X1)

Glue spread level (double, X2)

Veneer temperature (X3) Exp.

no. (min-sec) (lb/1000 ft2) (°F)

Ambient temperature

(°F) Species Mill no.

1 4’30” -1* 58 -1* 85-100 0* 70 Hemlock 1

2 4’30” -1 66 1 85-100 0 67 SPF (pine) 7

3 13’30” 1 58 -1 85-100 0 70 SPF (pine) 2

4 13’30” 1 66 1 85-100 0 70 SPF (pine) 6

5 9’ 0 58 -1 70-85 -1 67 Hemlock 12

6 9’ 0 58 -1 100-115 1 72 SPF (pine) 8

7 9’ 0 66 1 70-85 -1 67 Hemlock 15

8 9’ 0 66 1 100-115 1 72 SPF (pine) 11

9 4’30” -1 62 0 70-85 -1 67 Hemlock 13

10 13’30” 1 62 0 70-85 -1 74 Douglas-fir 14

11 4’30” -1 62 0 100-115 1 72 SPF (pine) 9

12 13’30” 1 62 0 100-115 1 63 Douglas-fir 10

13 9’ 0 62 0 85-100 0 70.5 Hemlock 3

14 9’ 0 62 0 85-100 0 67 SPF (pine) 4

15 9’ 0 62 0 85-100 0 67 SPF (pine) 5


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• WF (%) = 1187 – 61.0 X1 – 47.1 X2 + 12.2 X3

+ 1.045 X1 X2 – 0.094 X1 X3 – 0.196 X2 X3

+ 0.236 X12 + 0.467 X2

2 + 0.002 X32

Results Table 8 shows the gluebond test results. Note that the true values of the three variables were recorded, which were close to the design values. Based on the visual inspection of shear specimens, veneer roughness was found to affect the percentage wood failure. To examine the effect of veneer surface roughness on gluebond quality, the results were summarized in two categories: with roughness and exclusion of roughness. It was found that on average, the roughness reduced the percentage wood failure by about 3.2% from 68.1% to 64.9% when rough specimens were taken into account. Table 8: The gluebond test results from the mill trial

With roughness Excluding roughness Open assembly time (X1)

Glue spread level (X2)

Veneer temperature

(X3) Shear

strength Wood failure

Shear strength

Wood failure Exp. No.

(min) (lb/1000 ft2 double glueline) (°F) (psi) (%) (psi) (%)

1 4.75 58 90 148 68.6 148.4 78.6

2 4.5 66 99 142.6 67.7 129 69.9

3 13.5 58 107 102.9 34.6 105.2 35

4 13.5 66 106.5 133.8 81.6 127 80.4

5 9.25 58 73.5 118.9 48.8 121.2 55.2

6 9 58 117.5 127.9 78.6 127.9 87.5

7 9 66 77.5 121.7 77.1 123.5 87.7

8 9 66 125 117 42.3 120 43.2

9 5 62 73.5 146.6 76.8 147.5 73.8

10 14.75 62 77.5 104.3 77.7 102.4 84.6

11 4.5 62 133 143 73.6 124.2 70.8

12 13.5 62 82.5 146.8 76.4 140.3 75

13 9 62 93 134.8 72.1 130.8 78.7

14 9 62 99.5 121.4 44.1 120.2 45.7

15 9 62 97 128.5 53.8 129.6 55.5

Average 64.9 68.1 Based on the results, a quadratic prediction model based on response surface method (RSM) was established with a R2 of 0.74:


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Veneer temperature = 85 oF

0

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80

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100

57 58 59 60 61 62 63 64 65 66 67

Glue spread level (double glueline, lb/1000ft2)

Woo

d fa

ilure

(%)

Assembly time = 4 min



Target

Where X1 is open assembly time, X2 is glue spread level and X3 is veneer temperature. Note that two interactions X1* X2 and X2 *X3 are significant. Based on this prediction model, to minimize glue dryout and achieve target gluebond quality, an operating window can be established for the three key variables. As shown in Figure 19, the glue spread level should be adjusted in terms of assembly time and veneer temperature. In addition, ambient temperatures should also be taken into account since they are significantly different between summer and winter times. Figure 19: Adjustment of glue spread for target percentage wood failure


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Appendix III Optimizing plywood pressing parameters


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Experimental The effect of pressing pressure on plywood gluebond quality and compression ratio (CR) was investigated with 1/8- in dry coastal Douglas-fir veneer, which was randomly acquired from a BC plywood mill. Seventy-five sheets of 24 x 24 -in coastal Douglas-fir veneer were cut for manufacturing 5-ply plywood. The average veneer MC was 3%. As shown in Table 9, five levels of pressing pressure were used from 150 to 250 psi with an interval of 25 psi. Three replicates were used. Thus, fifteen 24 x 24 –in plywood panels were manufactured. Table 9: The layout of pilot plant plywood manufacturing

Pressing pressure (psi)

150 175 200 225 250 5-ply plywood

1 2 3 1 2 3 1 2 3 1 2 3 1 2 3 Lay-up thickness Panel thickness Panel CR (%)

Wood failure (%)

A commercial plywood PF glue was used with a solids content of 45%. The glue spread was 32 lb/1000 ft2 per single glueline. A laboratory 35 x 35 –in press was used to manufacture panels. Pressing temperature was set at 155°C, and the target innermost glueline temperature was 110 °C. Before making panels, the 9-point thickness of each veneer sheet was measured. For each panel lay-up, the total thickness was calculated. After pressing, the panels were hot stacked for 48 h. Subsequently, they were trimmed to measure 9-point thickness for calculating panel compression ratio (CR). Furthermore, 20 shear specimens were cut from each panel. After vacuum pressure treatment, the gluebond quality of each specimen was determined in terms of shear strength and percentage wood failure in accordance with CSA O151-04 standard (CSA O151 2004). Results Table 10 summarizes the results of 15 plywood panels in terms of panel density, compression ratio (CR) and gluebond quality. Note that both panel CR and density increased linearly with increasing pressing pressure. As well, both panel average shear strength and its variation seemed to increase with increasing pressing pressure, but the correlation was weak. Furthermore, the correlation between shear strength and percentage wood failure was weak. Figure 20 shows how plywood percentage wood failure changes with pressing pressure. For a pressing pressure of 150 psi, all three panels failed to reach the target 80% wood failure. For a pressing pressure of 175 psi, one of three panels failed to meet the 80% requirements. For a pressing pressure equal to or greater than 200 psi, all panels met the standard requirements. The results indicated that to achieve target gluebond quality while reducing panel thickness loss, the optimum pressing pressure should be 200 psi for pressing 5-ply coastal Douglas-fir plywood.


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0

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150 150 150 175 175 175 200 200 200 225 225 225 250 250 250


Woo

d fa

ilure

(%)

Table 10: The gluebond quality and density of 5-ply coastal Douglas-fir plywood panels

Panel gluebond quality Pressing pressure

Compression ratio (CR) Wood failure Shear strength

Panel density

Average Std. Dev. Average Std. Dev. Panel no.

(psi) (%) (%) (psi)

(g/cm3)

1 150 5.9 49.7 25.4 166.2 36.9 0.566 2 150 4.1 61.2 23.2 157.2 55.0 0.583 3 150 6.1 70.7 17.7 192.2 40.1 0.575 4 175 6.8 81.2 18.1 215.9 54.6 0.593 5 175 5.8 70.7 24.4 183.4 52.0 0.578 6 175 6.9 92.8 5.5 219.4 48.3 0.601 7 200 7.3 93.3 5.9 178.3 32.6 0.602 8 200 5.5 87.8 10.3 193.4 56.5 0.620 9 200 5.7 90.6 4.3 222.1 47.6 0.609 10 225 9.0 86.4 13.1 208.1 66.0 0.595 11 225 9.0 81.2 8.7 206.2 78.0 0.631 12 225 10.1 86.6 10.0 186.5 38.8 0.602 13 250 9.0 86.8 12.1 201.1 60.2 0.656 14 250 8.1 89.0 8.5 240.3 67.1 0.607 15 250 8.9 92.7 6.0 187.7 56.7 0.642

Figure 20: Variation of plywood percentage wood failure versus pressing pressure


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4.1 5.5 5.7 5.8 5.9 6.1 6.8 6.9 7.3 8.1 8.9 9.0 9.0 9.0 10.1

Plywood CR (%)

Woo

d fa

ilure

(%)

y = -0.0054x2 + 2.41x -177.5R2 = 0.69

y = 0.0025x2 - 1.12x + 134.3R2 = 0.56

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125 150 175 200 225 250 275


Woo

d fa

ilure

(%)

Average

Standard deviation

Figure 21 shows the relationship between panel percentage wood failure and pressing pressure. It seemed that average percentage wood failure increased with pressing pressure while its variation decreased from 150 to 200 psi, but both levelled off at 225 psi. Again, the results demonstrated that the optimum pressing pressure for this mill peeled coastal Douglas-fir veneer should be approximately 200 psi. Figure 21: Relationship between plywood percentage wood failure and pressing pressure Figure 22 shows how plywood percentage wood failure changes with the panel compression ratio (CR). The variation of panel percentage wood failure decreased with increasing panel CR. For this mill peeled coastal Douglas-fir veneer with given variation of surface roughness, the resulting panel CR ranged from 5.5 to 7.3% when the optimum 200 psi pressing pressure was used. Figure 22: Variation of plywood percentage wood failure versus panel CR

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