A Review of Different Sawing and Drying Techniques Used in Processing Small-diameter Logs
Aznor Marlina Abdullah,
Chee Yee Seng
The importance of small diameter logs in the sawmilling sector in the South East Asian region cannot be underplayed, as plantation-grown fast growing tree species is the main source of wood raw material for the rapidly expanding wood products industry in the region. The aim of this study was therefore to highlight the different sawing and drying techniques used in the processing of small-diameter logs from the fast growing forest plantations. A significant difference in processing yields and also the costs of the different techniques are obvious and the industry is poised to take advantage of the benefits provided by the saw-dry-rip (SDR) method of processing small-diameter logs. As opposed to the conventional live-sawing method widely used in hardwood sawing in the South East Asian region, the saw-dry-rip (SDR) method offers a more viable option to process the small-diameter logs, with significant reductions in rejects, while improving the overall yield. The overall economic benefits gained through the adoption of this technique warrants serious consideration by the sawmilling sector throughout the region.
Received: November 01, 2012;
Accepted: January 28, 2013;
Published: April 22, 2013
The conversion of saw logs into sawn-timber requires many manufacturing steps
from the moment it arrive at the saw-mill. For fast growing forest plantation
logs, such as Acacia mangium (Krishna et al.,
1998), Gmelina arborea, Albizia falcataria and rubberwood
(Hevea brasiliensis) (Teoh et al., 2011),
the processing has to be carried out as soon as possible. This is to prevent
insect and fungal attacks of the logs, as long-term storage of these logs may
not be economical (Table 1). In Malaysia and many part of
South East Asia, the application of portable sawmills that work on plantation
sites are common (Teoh et al., 2011), which
enable the logs to be sawn on-site immediately after being felled. Under such
circumstances, the challenge is to achieve high recoveries during the sawing
and drying processes. Ratnasingam (2000) reported that
the processing of rubberwood and other plantation logs contributed to the high
percentage of wastage, which has not improved over the past decade. The biggest
challenge is attributed to the small diameter of these logs, which makes its
handling and sawing difficult, compared to large diameter natural forest saw
logs. Kollert and Zana (1994) stated that the commonly
harvested sizes of plantation forest saw logs rarely exceed 50 mm in thickness
and 1800 mm in length. According to Sim (1989) and
Ratnasingam and Scholz (2009), the average recovery in
the saw-milling of plantation forest saw-logs was reported at 54%, while sawmills
processing natural forest saw-logs recorded recovery rates of up to 69%. The
best yield reported for rubberwood sawmilling is 35%, although a 25% yield in
rubberwood sawmilling is considered the industrial norm (Ratnasingam
and Scholz, 2009). The low density of fast growing plantation logs coupled
with a porous structure makes it easy to warp, bow and spring during sawing,
which will lead to lower yield. Yongdong et al. (2007)
claimed that the main reason for the low recovery in rubberwood sawmilling is
the irrational sawing techniques adopted in many sawmills in China and Asian
|| Factors affecting sawmilling yield
Therefore, the aim of this review study was to provide an insight into the
factors that governs yield in small-diameter logs processing and also highlights
the sawing techniques used to improve quality and yield.
FACTORS AFFECTING THE SAWMILLING YIELD
Sawing recovery refers to the amount of output (sawn timber) in percentage,
which results from the sawing of a certain volume of input (log). Sawmilling
recovery rates is very variable and differences have been observed within and
among the countries. Factors such as log sizes, dominant wood species processed
and state of technology/processing equipment and accepted quality standards
have significant influences on the sawing yield (Table 2).
Log dimension and quality: It has been shown in many researches that
the log dimension has a strong effect on recovery rate. The bigger the diameter
size, the higher is the recovery rate (Yang et al.,
2007). Log sizes of the same class or group also increased the recovery
with an increase in diameter. This was due to the sawing of wider and thicker
boards with lesser sawdust being produced. The reducing taper in large diameter
logs have also been reported to increase recovery (Yang
et al., 2007). The reason for this is that the taper is considered
a problem to the sawyer to remove rectangular solids (lumber) from a truncated
log. The more tapered the log is the shorter is the rectangular solid that can
be removed from the outside of this log (Steele, 1984).
Inevitably, with small-diameter logs, it is often the practice to cut the logs
into shorter lengths of not more than 2.0 m to reduce the tapering effect. As
expected, log quality is also a strong factor influencing sawmilling yield.
Studies by Ratnasingam and Yeo (2010) found that the
quality of logs can significantly affect sawmilling yield of rub berwood, especially
in logs with diameters below 30 cm. In this context, the quality of logs must
also be taken into consideration when determining sawmilling yield.
Size of sawn-timber produced: Wood recovery is also affected by the
size of sawn-timber produced. The greater the size of lumber produced, the higher
is the recovery obtained. For example, an average recovery rate, of as high
as 62% was obtained from cutting block-board sizes, meanwhile only 46% of recovery
was obtained from cutting furniture sizes (Lopez et
al., 1980). The latent reason behind this result is the fact that the
long cutting lines need to be done in order to produce smaller sawn-timbers,
which in turn increases the amount of sawdust produced.
Types of saws used: Traditionally, saw-millers did not realize that
thin and wide kerf can affect their mill recoveries. In fact, in most developing
countries such practices are still common in many small-scale sawn-timber processing
facilities, where the concern is on how to improve recovery based on the materials
input rather than the production system (Hallock, 1962).
Choosing of wide or thin kerf is closely related to the type of sawing equipments.
Circular saw is self supporting, thus it requires a thicker blade. A thicker
blade will produce a wider kerf, which lead to the low recovery. The study of
kerf was initially started by Hallock (1962). The result
from this study indicated that if the kerf width was reduced from 9.5 to 7.0
mm, the yield was increased on average of 7%. This is due to the reduction of
kerf that allowed the possibility to produce larger sizes of sawn-timber (Steele,
1984; Yang et al., 2007). The same scenario
occurs during manufacturing of boards which requires several rip-cuts, which
results in considerably higher waste. In other developing countries, wood-mizer
sawmills are becoming increasingly popular. A key feature of the wood-mizer
sawmills is the application of thin kerf narrow band-saw blades, which ensures
less valuable timber was lost as sawdust (Kutty, 2010).
It was reported by Ratnasingam and Scholz (2009) that
a sawmill which had used a vertical broad band saw for cutting rubberwood gave
a sawn-timber recovery of 41%, while sawmills with the wood-mizer
(horizontal narrow band saw) recorded recoveries of 75%.
Acceptable sawn timber quality: The quality of the sawn timber produced
also affects the overall sawmilling yield. With small-diameter plantation forest
logs, the challenge is often to prescribe an acceptable quality standard to
the sawn timber produced, which in most instances, is traded on a willing seller-buyer
basis. Therefore, the adoption of an internationally acceptable quality standard
which stipulated marketable grades or grading classes to the sawn timber, would
contribute significantly towards boosting the sawmilling yield when processing
small-diameter plantation forest logs (Ratnasingam and Scholz,
Cutting pattern: Live sawing is the most economical sawing method,
as it does not require any turning of the logs, nor is skill-demanding in making
cutting decisions. Furthermore, it is the fastest conversion method with the
highest production rate (Todoroki and Ronnqvist, 2002).
However, since the cutting of logs does not consider pieces of timbers with
defects (Todoroki and Ronnqvist, 2002), it has the
lowest value recovery. This cutting method is suitable for less defective and
large diameter logs from the natural forests (How et al.,
2007). According to Ratnasingam and Scholz (2009),
the most commonly used sawing patterns in the sawmilling industries in Malaysia
are as shown in Fig. 1 and 2. Pattern A
requires the log to be cut into halves and then sawn around, or live sawn to
be converted into the intended sizes. There is a situation where the slab is
cut to make an edge on one side of the log bolt as in pattern B. Then, the log
bolt is turned 90° to put the flat face on the in-feed table
of the band-mill. Boards of desired thicknesses are then cut according to the
However, it has been observed that plantation forest logs tend to wrap immediately
after sawing. This might be the results of the cutting pattern used, which is
mostly cutting through the core of the saw log. Boards cut through the centre
of the logs contain the core, which has a lower density than the outer parts
of the log (Ratnasingam and Scholz , 2009). Hence, some
techniques have been proposed to avoid this problem. For instance, Watcharakuldilok
and Vitayaudom (2008) suggested not sawing through the trunk and allowing
the logs to un-leave at the end. Then, it is fastened with a piece of wire,
or a nail is driven at the end of log. Practically, this method should be thoroughly
considered when dealing with the high volume production which requires fast
production flow. Another technique is the balanced cutting by sawing the right
and the left of the log at a distance equal from the pith together, by using
a twin band-saw. This method can release the stress in the log equally on opposite
sides, which reduces twisting in the sawn-timber.
Despite such techniques, the practicality of such techniques in high volume
production sawmills remain questionable. In recent years however, the application
of the saw-dry-rip (SDR) technique for small-diameter logs from plantation forests
has been widely considered as an option in the South East Asian region.
A COMPARISON OF THE SAW-DRY-RIP (SDR) AND CONVENTIONAL SAWING TECHNIQUES
In the conventional live-sawing process, the logs are usually cut into the
desired widths and thicknesses, with some allowances for shrinkage and planning
before the drying process. On the other hand, the SDR process involves the live
sawing of logs into nominal thickness flitches, which are dried before being
ripped to the desired dimensions (Erickson et al.,
1986). The principal behind the growing application of the SDR process is
the restraining effect of the wide flitches on warp development, during drying
(Maeglin and Boone, 1983). The growth stresses in the
logs are relieved, when the flitches dry due to slippage of the wood cells relative
to one another, while the piece holds its shape (Larson et
|| Distribution of growth stresses in a log
|| SDR sawing pattern
|| Conventional live sawing pattern
This reduction in stresses, inevitably leads to better quality, while improving
the saw-milling yield.
||Distributions of stresses in conventional live sawing
Warping in the form of spring or crook after sawing is common in forest plantation
timber due to the release of growth stresses (Cassens and
Serrano, 2004). Growth stresses is formed in three directions, known as
longitudinal, radial and tangential (Yang and Waugh, 2001).
However, in the case of hardwoods, it is the longitudinal stress which is the
most important because it leads to warp (Maeglin, 1987).
Figure 3 depicts the distribution of growth stress in a log.
When the log is cut, the growth stresses is released.
The success of the SDR process in reducing the percentage of warp is driven
by the application of live sawing of green logs into nominal width flitches
(Fig. 4) compared to conventional live sawing (Fig.
5). In the SDR process the stresses are balanced and restrained due to the
larger size of the pieces and symmetry of stress within the pieces (Layton
et al., 1986). The fact that growth stresses are present in almost
all logs is inevitable, however, through the use of appropriate sawing techniques
it is possible to minimize the detrimental effects of the stresses, especially
with regards to the resultant quality of the sawn timber produced.
When compared to the other sawing techniques, the application of live sawing
would produce stressed sawn-timber pieces (Fig. 6). Whether,
it is quarter sawing or flat sawing, the sawn-timber ends up with combination
of tension (-), T and compressive (+) forces, C. The prevalence of these opposing
stresses causes the sawn-timber to warp immediately upon sawing (Larson
et al., 1983) (Fig. 7). Further, in the conventional
live-sawing of fast growing plantation forest logs, the interaction of juvenility
(Cassens and Serrano, 2004) and tension wood (Yang
and Waugh, 2001) with the inherent growth stresses exacerbates the warp
to become even more crucial.
|| Distributions of stresses in quarter sawing
||Comparison between conventional and high temperature drying
techniques for wood
Juvenile wood has an excessive longitudinal shrinkage, while tension wood shrink
and swell about 2.5 times more longitudinally. In sawn-timber pieces which contain
normal and tension wood, the resulting warp is attributed to the differential
shrinkage (Maeglin, 1987).
DIFFERENT WOOD DRYING TECHNIQUES
The wood drying process is usually plagued with many drying defects, which
affects its productivity. A study by Ratnasingam et al.
(2010) found that the yield losses in small-diameter plantation forest logs
due can average 10% in the kiln dried sawn-timber, even with the application
of stress relieved treatment. This is significantly higher than the current
industrial allowance of 4% and hence, there is a growing need to find methods
of wood drying that can practically increase sawn-timber yield after drying
process. Nevertheless, this is a challenge because different wood species have
different optimum drying processes. For instance, drying of Yellow Poplar and
Basswood using High Temperature (HT) drying resulted in improved quality compared
to conventional drying. On the other hand, High Temperature (HT) drying was
not suitable for Aspen and Willow due to a higher incidence of collapse and
honeycomb in the dried sawn-timber (Boone, 1984). Ratnasingam
et al. (2011) have found that the SDR process in combination of high
temperature drying had produced the least amount of warp defects during drying
process of fast growing plantation forest wood species, such as rubberwood,
Acacia mangium and Gmelina arborea. The application of high temperature
(HT) drying to live-sawn flitches, allowed the stresses in the full width of
the boards to relax in a symmetrical manner so that sawn-timber can be ripped
subsequently from the dried cants and the flitches remains relatively straight
(Ishiguri et al., 2005). In high temperature
drying, the dry-bulb temperature which reaches 120°C in most instances,
allows for stress relieve in the wood through plasticization (Ratnasingam
and Scholz, 2012). In this context, it appears that the application of high
temperature drying may be a better option for fast growing plantation forest
wood species compared to the conventional drying processes. Table
3 provides a comparison of the conventional drying and high temperature
drying techniques for wood, based on a report published by Ratnasingam
et al. (2011). Despite the obvious benefits of the high temperature
drying techniques for wood materials, caution must be exercised especially when
dealing non-refractory or non-porous wood species, which will be severely affected
when exposed to high temperature regimes. Hence, the application of high temperature
drying technique is more suitable for porous wood species or fast-growing plantation
wood species (Ratnasingam et al., 2011), which
is less susceptible to drying defects.
DRYING STRESSES IN DIFFERENT DRYING TECHNIQUES
The drying of wood normally occurs unevenly. The outer part of the wood tend
to lose moisture faster which causes the wood to shrink, when it reach at Fibre
Saturation Point (FSP). However, the outer part of wood tends to shrink, although
the core wood is still green. Temperature is the key factor in the drying process.
Tenorio et al. (2012) mentioned that factors
of wood species, extractives content, tree age, longitudinal position of the
wood in the standing tree, initial moisture content, drying schedule and the
presence of sapwood and heartwood also affects the drying process. A study on
Acacia mangium wood showed that the drying stresses from kiln drying resulted
in colour changes, shrinkage, warp, split and check due to factors of climate,
grain pattern, initial and final moisture content as well as the drying schedule
used (Table 3).
Based from the Table 3, conventional drying specifies lower
temperature than high temperature drying. According to Chen
et al. (1997) and Nijdam et al. (2000),
high temperature drying reduces the energy consumption and drying times but,
nevertheless results in higher timber degradation.
|| Process economics of different sawing techniques for small-diameter
Surface checks may be found due to local drying stress when it reaches the
rupture strength which exceeds the allowable ultimate limit (Chen
et al., 1997). Thus, the mechanical properties of wood influence
the susceptibility of the wood to suffer from drying checks (Oltean
et al., 2007). Besides that, honeycomb and collapse may take place
due to drying stress with excessively high temperature (Oltean
et al., 2007). Moreover, stress due to drying at high temperature
consequently affects the mechanical properties of wood. Poncsak
et al. (2006) showed that the mechanical strength of birch wood decreases,
although improvements in dimension stability and resistance to decay were observed.
It has been suggested that the stresses during the high temperature drying contributes
to the modification of woodcompounds, where the hemicelluloses component is
normally modified in advance, followed by the depolymerisation of the cellulose
component in wood (Poncsak et al., 2006; Oltean
et al., 2007).
One fact that is often overlooked is the influence of stresses on sawing variation.
Ratnasingam and Scholz (2009) found that highly stressed
logs are more prone to warp, which in turn increases the sawing variation in
the sawn timber produced. It has been shown in rubberwood (Hevea brasiliensis)
that sawing variation of up to 9% can be accounted for by the stresses in logs,
which upon sawing is released and manifests as warped sawn timber.
ECONOMICS OF DIFFERENT SAWING TECHNIQUES FOR SMALL-DIAMETER LOGS
The sawmilling industry is very cost sensitive due to increasing production
cost and decreasing selling prices, especially in the plantation wood resources
market (Balsiger et al., 2000; Ratnasingam
and Booth, 2012). In several studies by Ratnasingam
et al. (2011) and Ratnasingam and Scholz (2012),
the economics of the using the saw-dry-rip method for sawing rubberwood logs
has been reported in detail. Table 4 provides a comparison
of the process economics for the three most common sawing techniques used for
small-diameter logs used in the South East Asian region.
It is apparent that the sawmilling of small-diameter logs from fast-growing
plantation is becoming increasingly important in many South East Asian countries,
which are dependent on such resources to fuel their growing demand from value-added
wood products manufacturing. Hence, sawmilling yield is becoming a crucial issue
in governing cost competitiveness, especially in the raw materials market. The
conventional live-sawing technique, which is more suited for the large diameter
natural forest logs, appears to be less suitable to handle the small-diameter
logs from the fast-growing plantations. In this context, the advent of the saw-dry-rip
(SDR) technique together with high temperature drying appears to be a promising
alternative to boost the yield from the sawmilling of such resources. This technique
has a positive overall benefit towards increasing the cost and quality competitiveness
of the sawmilling sector, especially when dealing with small-diameter fast-growing
1: Balsiger, J., J. Bahdon and A. Whiteman, 2000. Asia-Pacific Forestry Sector Outlook Study: The Utilization, Processing and Demand for Rubberwood as a Source of Wood Supply. FAO, Bangkok,.
2: Boone, R.S., 1984. High temperature kiln-drying of 4/4 lumber from 12 hardwood species. Forest Prod. J., 34: 10-18.
Direct Link |
3: Cassens, D.L. and J.R. Serrano, 2004. Growth stress in hardwood timber. Proceedings of the 14th Conference on Central Hardwood Forest, March 16-19, 2004, Madison, WI., USA., pp: 16-19.
4: Chen, G., R.B. Keey and J.C.F. Walker, 1997. The drying stress and check development on high-temperature kiln seasoning of sapwood Pinus radiata boards. Holz als Roh- und Werkstoff, 55: 59-64.
5: Erickson, R.W., H.D. Petersen, T.D. Larson and R. Maeglin, 1986. Producing studs from paper birch by saw-dry-rip. United States Department of Agriculture, Forest Service, Forest Products Laboratory, Madison, WI., USA.
6: Hallock, H., 1962. A mathematical analysis of the effect of kerf width on lumber yield from small logs. Report No. 2254, US Department of Agriculture, Forest Products Lab, Madison, WI., USA.
7: How, S.S., H.S. Sik and I. Ahmad, 2007. Review on six types of log cutting methods in various application: Part 1. Timber Technology Bulletin No. 45, Forest Research Institute Malaysia, Kepong.
8: Ishiguri, F., A. Mitani, K. Iizuka, S. Yokota and N. Yoshizawa, 2005. Effects of saw-dry-rip process on warp reduction in Japanese cedar 2 by 4's. Bull. Utsunomiya Univ. Forests, 41: 95-99.
Direct Link |
9: Kutty, J., 2010. Rubberwood processing company in India expands on thin kerf bandsaw. Wood Mizer Planet. http://www.woodmizerplanet.com/index.pl?act=PRODUCT&id203.
10: Kollert, W. and A.U. Zana, 1994. Rubberwood from agricultural plantations: A market analysis for Peninsular Malaysia. Planter, 70: 435-452.
Direct Link |
11: Larson, T.D., R.W. Erickson and H.D. Peterson, 1983. Saw-dry-rip processing: Taking the crook out of the stud game. United States Department of Agriculture, Forest Product Laboratory, Report No 1884, Madison, WI., USA., pp: 148-167.
12: Larson, T.D., R.W. Erickson and R.S. Boone, 1986. Comparison of drying methods for paper birch SDR flitches and studs. Research Paper FPL 465, United States Department of Agriculture, Forest Product Laboratory, Madison, WI., USA.
13: Layton, T.F., W.R. Smith and R.R. Maeglin, 1986. An evaluation of the saw, dry and rip process to convert red alder into studs. Wood Sci. Technol., 20: 185-200.
Direct Link |
14: Lopez, D.T., S. Mohd Arshad and A.G. Tan, 1980. Rubberwood-A study of recovery in production mills. Malaysian Forester, 43: 74-80.
Direct Link |
15: Maeglin, R.R., 1987. Applying the latest research to hardwood problems. Proceedings of the 15th Annual Hardwood symposium of the Hardwood Research Council, May 10-12, 1987, Memphis, TN., pp: 100-108.
16: Maeglin, R.R. and R.S. Boone, 1983. Manufacture of quality yellow poplar studs using the Saw-Dry-Rip (SDR) concept. For. Prod. J., 33: 10-18.
17: Nijdam, J.J., T.A.G. Langrish and R.B. Keey, 2000. A high-temperature drying model for softwood timber. Chem. Eng. Sci., 55: 3585-3598.
18: Oltean, L., A. Teischinger and C. Hansmann, 2007. Influence of temperature on cracking and mechanical properties of wood during drying: A review. BioResources, 2: 798-811.
Direct Link |
19: Krishna, P.B., A. Razak and M. Ali, 1998. Forest plantation potential in Malaysia. Paperasia, 14: 20-23.
20: Poncsak, S., D. Kocaefe, M. Bouazara and A. Pichette, 2006. Effect of high temperature treatment on the mechanical properties of birch (Betula papyrifera). Wood Sci. Technol., 40: 647-663.
21: Ratnasingam, J., 2000. Rubberwood supply in Malaysia. Asian Timber, 19: 16-19.
22: Ratnasingam, J. and F. Scholz, 2009. Rubberwood: An Industrial Perspective. WRI Publications, UK., ISBN: 978-983-44248-2-4 Pages: 74.
23: Ratnasingam, J., R. Grohmann and F. Scholz, 2010. Drying quality of rubberwood: An industrial perspective. Eur. J. Wood Prod., 68: 115-116.
24: Ratnasingam, J. and Y.C. Yeo, 2010. Yield improvements in rubberwood processing. IFRG Report No. 7, Singapore, pp: 41.
25: Ratnasingam, J., T.P. Ma, C.Y. Yoon and S.R. Farrokhpayam, 2011. An evaluation of the saw, dry and rip process for the conversion of rubberwood. J. Applied Sci., 11: 2657-2661.
CrossRef | Direct Link |
26: Ratnasingam, J. and F. Scholz, 2012. Yield improvement in Rubberwood sawmilling through the saw, dry and rip (SDR) technique. Eur. J. Wood Wood Prod., 70: 525-526.
27: Ratnasingam, J. and H.E. Booth, 2012. Techniques for processing small-diameter logs. IFRG Report No. 23, Singapore, pp: 73.
28: Sim, H.C., 1989. Yields of rubberwood sawn timber. J. Trop. Forest Sci., 2: 48-55.
Direct Link |
29: Steele, P., 1984. Factors determining lumber recovery in sawmilling. General Technical Report FPL-39, United States Department of Agriculture, Forest Product Laboratory, Madison, WI., USA., pp: 1-8.
30: Tenorio, C., R. Moya and H.J. Quesada-Pineda, 2012. Kiln drying of Acacia mangium wood: Colour, shrinkage, warp, split and check in dried lumber. J. Trop. Forest Sci., 24: 125-139.
Direct Link |
31: Teoh, Y.P., M.M. Don and S. Ujang, 2011. Assessment of the properties, utilization, and preservation of rubberwood (Hevea brasiliensis): A case study in Malaysia. J. Wood Sci., 57: 255-266.
32: Todoroki, C. and M. Ronnqvist, 2002. Dynamic control of timber production at a sawmill with log sawing optimization. Scand. J. Forest Res., 17: 79-89.
33: Watcharakuldilok, S. and S. Vitayaudom, 2008. The utilization of rubberwood in Thailand. Proceeding of the ITTO/CFC International Workshop on Rubberwood, December 8-10, 2008, Haikou, China -.
34: Yang, I., S.Y. Lee, R.W. Joo and Y.C. Youn, 2007. Factors affecting lumber conversion rate of sawmill industry in South Korea. J. Korean Foreest Soc., 96: 197-202.
35: Yang, J.L. and G. Waugh, 2001. Growth stress, its measurement and effects. Aust. Forestry, 64: 127-135.
Direct Link |
36: Yongdong, Z., J. Mingling and R.G. Xiaoling, 2007. Rubberwood processing manual: Demostration of rubberwood processing technology and promotion of sustainable development in China and other Asian Countries. Research Institute of Wood Technology, Chinese Academy of Forestry Beijing, Beijing, China.