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Highlights:
Modern sawmills had 7 % higher yields compared to band sawmills.
Diameter and taper of logs had no effect on the sawing coefficient.
The product distribution according to thickness, width and length was not different between types of industries.
Equipment with modern technology allow efficient utilization of raw materials.
Introduction
Long before techniques and tools for cutting wood were developed, humans collected food, medicines and other resources that met their basic needs in forests (Sheppard et al., 2020; Tymendorf & Trzciński, 2020), ecosystems where natural resources with economic, cultural and ecological values are still extracted (Cardenas et al., 2018). For this reason, the forestry sector is often economically important in countries gifted with forests (Lundmark et al., 2021).
The sawmill industry is one of the most important economic activities in several regions (Lundmark et al., 2021; Marchesan et al., 2014; Makkonen, 2018; Ortiz et al., 2016). In the north of Mexico, the sawmill process is the industrial labor of greatest relevance in social organizations for processing roundwood (Zavala & Hernández, 2000). Therefore, sawmill practices are the main forestry industrial activity in the state of Chihuahua because it is the most accessible methodology to generate wood products in ejidos, communities and private microenterprises.
Rascón-Solano et al. (2020, 2022) mentioned that the state of Chihuahua has undergone changes in timber processing technologies over the last decade. The industrial technological transition is mainly due to the need to be more competitive and efficient in the processes (Herrera-Medina & Leal-Pulido, 2012).
Currently, some activities are challenged by low-value wood products (Townsend et al., 2019), high wood extraction costs or limitations in operational productivity (Holzfeind et al., 2021) and changes in the utilized and installed capacity of forest industries. Melo et al. (2012) and Rascón-Solano et al. (2022) identified that the timber activity shows problems due to the age of the equipment, little knowledge in the efficient use of the dimensional and quality characteristics of the raw material, and changes in production and consumption patterns (Lauri et al., 2021).
Small and medium-sized enterprises in the sawnwood producing sector play a crucial role in improving the productivity and competitiveness of a region (Lähtinen et al., 2016). For this, the evaluation of efficiency in firms sharing similar production processes is necessary in order to obtain information on relative success based on the choice of optimal sets of technologies (Hetemäki & Hurmekoski, 2016). The objective of this research was to statistically contrast the volumetric yield and percentage distribution of sawn products by taking two industrial technologies as a comparison factor in ejido sawmills in the state of Chihuahua.
Materials and Methods
Study area site
This research was carried out in the physiographic province of the Sierra Madre Occidental in the municipalities of Balleza, Guachochi and Urique in the state of Chihuahua. Data collection was carried out in the first quarter of the year 2021 in band sawmills in the ejidos Cieneguita de la Barranca and Corareachi, as well as in horizontal thin-cut sawmills in the ejidos Aboreachi, Tecorichi and San Carlos.
Sawmill technical description
The sawmills at the Aboreachi, Tecorichi, and San Carlos ejidos use Wood-Mizer® model MW3500 horizontal-cutting equipment with 635 mm diameter flywheels; the band is made of fine-cut steel, has locking teeth and a – in. tooth pitch; the band is 35 mm wide and 1.07 mm thick. The equipment has a processing platform with leveling rollers, traction toothed rollers, locking jacks and bidirectional chain to rotate the logs. This study considered this type of industry as a modern sawmill. Due to their recent installation, it was necessary to train personnel in the use of raw material, the use of consoles for programming cutting diagrams, and the preparation of raw material for the multiple saws or resawing machines.
The sawmills in the Cieneguita de la Barranca and Corareachi ejidos were installed in the 1970s by Productos Forestales de la Tarahumara (PROFORTARAH). The sawmills have a vertical band saw with 1 473 mm diameter flywheels, 203.2 mm wide cutting bands, 17 gauge (1.37 mm), 44.45 mm tooth pitch, 15.88 mm throat depth, and 30° cutting angle. Industries with these characteristics were considered traditional sawmills for this research. Due to their age, the operators' skills have been based mainly on obtaining knowledge and experience in raw material utilization and equipment handling, based on empirical perceptions.
Sawing yield
The largest and shortest length and diameter (with and without bark) were measured on a sample of at least 90 logs per sawmill. Total volume with bark, timber volume, bark volume and log taper were determined from this information. Log volume (V) was determined with the Smalian formula (Husch et al., 2003): V = [(S0 + S1) / 2] * L; where, S0 = larger diameter area, S1 = smaller diameter area and L = length.
The percentage yield (Y) of sawn wood according to nominal thickness, width and length was determined with the equation R = (Va / Vr) * 100 (Quirós et al., 2005); where, Va = volume of boards according to nominal thickness, width and length (m3) and Vr = volume of roundwood without bark (m3 r).
Timber size
The most popular timber sizes are: 22.23, 31.75 and 38.10 mm (7/8 in, 5/4 in and 6/4 in, respectively) plus reinforcement; for nominal thicknesses of 22.23 mm, the reinforcement used is 3.17 mm (1/8 in) and, from 31.75 mm onwards, the reinforcement is 6.36 mm (1/4 in). Also, 88.90 x 88.90 mm (3 ½ in x 3 ½ in) poles are produced. Wood widths range from 101.60 to 304.80 mm (4 in to 12 in), plus 12.70 mm (1/2 in) reinforcement. Piece lengths range from 1 219.20 to 4 876.80 mm (4 ft to 16 ft), plus 50.80 mm (2 in) reinforcement. The volume of each piece (V, m3) was estimated with the expression V = t * w * l; where, t = thickness (m), w = width (m) and l = length (m).
Statistical Analysis
The smallest diameter without bark and taper category of logs in the five sawmills were evaluated by Duncan's multiple comparisons test (P = 0.05). Statistical differences in log to timber conversion ratio and volumetric yield of thickness, width and length between industry types were identified by Student's t-test (P = 0.05). The statistical package IBM-SPSS version 25 (IBM Corp., 2017) was used for the development of information analysis and determination of results.
Results
Description of raw material
The categories of smallest diameter without bark among sawmills show significant differences (P = 0.0000) in a sample of 486 pine logs entering the sawmilling process. The sawmill owned by the Tecorichi ejido included logs with smaller diameters (28.34 cm) in the sawmilling process; the rest of the ejidos had logs with larger diameters very similar to each other (Cieneguita de la Barranca [31.25 cm], Corareachi [31.97 cm], San Carlos [33.81 cm] and Aboreachi [36.20 cm]). Table 1 shows the estimated values of raw material for each of the industries evaluated.
Table 1 Characteristics of raw material in five sawmills in the state of Chihuahua, Mexico.
| Sawmill | Logs (n) | Sampling error (%) | Minimum diameter with bark (cm) | Taper (cm∙m-1) | Total volume (m3r) |
|---|---|---|---|---|---|
| Aboreachi | 102 | 9.70 | 36.20 ± 6.23 | 1.26 ± 0.69 | 53.90 |
| Tecorichi | 95 | 10.06 | 28.34 ± 6.85 | 1.28 ± 0.60 | 45.79 |
| San Carlos | 91 | 10.27 | 33.81 ± 9.71 | 0.79 ± 0.55 | 56.50 |
| Corareachi | 94 | 10.11 | 31.97 ± 7.32 | 0.90 ± 0.63 | 51.06 |
| Cieneguita | 104 | 9.61 | 31.25 ± 6.19 | 1.38 ± 0.80 | 59.27 |
Mean values ± standard deviation.
Regarding integrated raw material, Table 2 indicates significant differences (P = 0.0001) in mean log diameter among sawmills; however, between type of technology involved in the process, the differences were not significant (P = 0.0842).
Table 2 Comparison of categories of smallest diameter without bark in logs from five sawmills with different industrial technology in Chihuahua.
| Sawmill | Treatment | Logs (n) | Minimum diameter without bark (cm) | Medians | P-value |
|---|---|---|---|---|---|
| Aboreachi | 1 | 102 | 31.32 ± 5.49 a | 30 | 0.0001 |
| Corareachi | 3 | 94 | 32.13 ± 7.46 abd | 30 | |
| Cieneguita | 2 | 104 | 31.73 ± 5.98 ad | 30 | |
| Tecorichi | 5 | 96 | 28.13 ± 6.85 c | 25 | |
| San Carlos | 4 | 91 | 33.79 ± 9.81 b | 35 | |
| Moderna/Modern | 1 | 289 | 31.04 ± 7.84 a | 30 | 0.0842 |
| Tradicional/Traditional | 2 | 198 | 31.92 ± 6.71 a | 30 |
Mean values ± standard deviation with a common letter are not significantly different among sawmills (Duncan's multiple comparison P > 0.05) and between industry types (Student's t P > 0.05).
Table 3 shows that sawmills in ejidos San Carlos and Corareachi had logs with lower average taper, 0.80 and 0.91 cm∙m-1, respectively. This results in significant differences compared to the other sawmills (P = 0.0000); however, the log taper has similar values in the comparison between industry types (P = 0.6620).
Table 3 Comparison of taper categories in logs from five sawmills with different industrial technology in Chihuahua.
| Sawmill | Treatment | Logs (n) | Taper (cm∙m-1) | Medians | P-value |
|---|---|---|---|---|---|
| Aboreachi | 1 | 102 | 1.25 ± 0.64 a | 1 | 0.0000 |
| Tecorichi | 2 | 96 | 1.29 ± 0.61 ab | 1 | |
| Cieneguita | 5 | 104 | 1.38 ± 0.83 ab | 1 | |
| San Carlos | 3 | 91 | 0.80 ± 0.55 c | 1 | |
| Corareachi | 4 | 94 | 0.91 ± 0.65 c | 1 | |
| Moderna/Modern | 1 | 289 | 1.12 ± 0.64 a | 1 | 0.6620 |
| Tradicional/Traditional | 2 | 198 | 1.16 ± 0.79 a | 1 |
Mean values ± standard deviation with a common letter are not significantly different among sawmills (Duncan's multiple comparison P > 0.05) and between industry types (Student's t P > 0.05).
Sawing coefficient
The modern sawmills considered in this research processed 156.19 m3r (cubic meter in logs) with bark or 138.00 m3r without bark that generated 83.72 m3 of saw timber. The average sawing coefficient with bark corresponds to 53.63 %, and without bark, the yield increases to 60.93 (influence of bark 7.55 %); therefore, considering these yields and that 1 m3r = 424 BF, it is possible to achieve 227.27 and 257.23 board feet (BF), respectively, when processing 1 m3r of pine.
On the other hand, traditional sawmills generated 51.60 m3 of sawn timber, resulting from processing 110.33 m3r with bark or 97.09 m3r without bark. The average sawing coefficient without bark was 53.09 %; according to the influence of bark (6.36 %), a yield with bark of 46.73 % was determined.
The Student's t test showed highly significant differences when comparing transformation coefficients with bark (P = 0.0006) and without bark (P = 0.0074) between both types of industry. Finally, the estimated bark percentage had no significant differences (P = 0.4415).
Yield per nominal thickness
Figure 1a shows that sawing production in modern industries is concentrated in the nominal thickness of 7/8 in (22.23 mm) with a mean and standard deviation of 57.05 ± 25.45 % of the total volume sawn, followed by 5/4 in (31.75 mm) with 21.39 ± 18.63 %. The 6/4 in (38.1 mm) size represents a production of 9.67 ± 4.18 % and 3 ½ in (88.90 mm) poles occupy 11.89 % ± 6.01 % of the sawn volume. On the other hand, the traditional industries mainly produce nominal thickness of 7/8 in with an average of 39.33 ± 5.38 %, followed by 6/4 in which represents 23.59 ± 26.30 %. The 5/4 in size represents 15.15 ± 16.89 % and the 3 ½ in logs correspond to 21.93 ± 14.78 % of the total volume sawn (Figure 1b).
Figure 1 Mean volumetric frequency in the production of nominal thickness, proportion (yield) of products and standard deviation (SD) in modern sawmills (a) and traditional sawmills (b). Student's t-tests show no significant differences in the yield of nominal thickness of 7/8 in (P = 0.424), 5/4 in (P = 0.731), 6/4 in (P = 0.594) and 3 ½ in (P = 0.345) between sawmill types.
Yield per nominal width
According to Figure 2a, in modern industries, the production per nominal width of 4 in (101.60 mm), 6 in (152.40 mm) and 8 in (203.20 mm) dominates with a volumetric yield of 83.83 % altogether, while the nominal widths of 10 in (254.00 mm) and 12 in (304.80 mm) only represent 16.17 % as a whole. Similarly, on average, the traditional industries produce mainly the nominal widths of 4 in, 6 in and 8 in with a yield of 91.53 % (Figure 2b) and the nominal widths of 10 in and 12 in generated an overall yield of 8.47 %.
Figure 2 Mean volumetric frequency in nominal width production, proportion (yield) of products and standard deviation (SD) in modern sawmills (a) and traditional sawmills (b). Student's t-test indicates no significant differences in yield per nominal widths of 4 in (P = 0.484), 6 in (P = 0.477, 8 in (P = 0.237) and 10 in (P = 0.344) between sawmill types; 12 in timber had a P value = 0.057.
Yield per nominal length
For the distribution of volumetric yield per nominal length, modern sawmills had an average yield of 77.08 ± 2.32 % for 16 ft (4 876.80 mm); dimensions from 4 to 14 ft (1 219.20 to 4 267.2 mm) accounted for only 22.92 % of the total yield (Figure 3a). The estimated average production by traditional sawmills (Figure 3b) has similar values; the 16 ft length represents 73.63 ± 4.03 % of the total yield and the rest of the dimensions add up to 26.37 %.
Figure 3 Mean volumetric frequency in nominal length production, proportion (yield) of products and standard deviation (SD) in modern sawmills (a) and traditional sawmills (b). Student's t-test indicates significant differences in yield for nominal lengths of 4 ft (P = 0.038) and 6 ft (P = 0.045), but not for lengths of 8 to 16 ft (P > 0.05) among sawmills.
Discussion
Deficiencies in the sawing process and production efficiency are determined by evaluating the sawing coefficient and volumetric yield (Borz et al., 2021; Stragliotto et al., 2019), which refers to the relationship between the volume of roundwood and the resulting volume of products (Rascón-Solano et al., 2022).
When analyzing the dimensional characteristics of the raw material, Zavala and Hernández (2000) found no increase in the sawing yield coefficient according to the diameter and quality of the logs. Nájera Luna et al. (2012) conclude that sawtimber yield is not affected by diameter, but it is affected by log length and taper in two private sawmills in El Salto, Durango, Mexico. Grigsby et al. (2015) reported that dimensional characteristics and high log quality result in better economic benefits. Murara et al. (2005) evaluated two sawmilling systems and found that yield is affected by the sawmilling system according to the diameter of the log; in the traditional system they found no increase in yield with increasing diameter, but yield improved with an optimized system as the diameter of the logs increased. Meanwhile, Vaughan et al. (2018) and Fekiač et al. (2021) indicate that the continuous deterioration of raw material in the field can affect the potential yield and quality of forest products; also, Monserud et al. (2004) mention that twisted or curved logs have potentially lower yields.
Some studies agree with that described in this study. When considering a P value = 0.0842 in the log diameter contrast, the results indicate that this is not a factor that affects the sawtimber yield coefficient; furthermore, taper had no influence on the sawing yield between the type of technology used, with a P value = 0.6620 in the contrast performed. These results suggest that the sawing coefficient is mainly influenced by the technological capacity of the sawing equipment. Modern sawmills are designed primarily to program the cutting diagrams and enable resaws that generate timber of greater thicknesses (5/4 in and 6/4 in). To achieve this, it is necessary for the operator to have constant training, which results in a better knowledge of raw materials, processes and products. Band sawmills lack the technology to program the cuts; also, due to the traditional use of this old equipment, operators have transmitted their knowledge based on empirical perceptions, related to the dimensional and physical characteristics of the raw material, but without receiving formal training.
Concerning the yield per band sawmill (traditional in this study), Gatto et al. (2005) evaluated micro and small sawmills with low industrialization in Brazil and found that many technological aspects are unknown or neglected, which causes poor management of raw material, low yield and low quality of the final products. Ortiz et al. (2016) carried out a comprehensive study in a traditional sawmill in Oaxaca and calculated a yield coefficient of 44.18 % when considering volume with bark and 48.27 % without bark; they also indicated a direct increase in the sawing coefficient according to the diameter of the logs and higher yield at lower taper. The results of this analysis indicate that it is possible to produce up to 257.23 BF, when the estimation is based on processing 1 m3r without bark in modern sawmills, and 227.27 pts when processing 1 m3r without bark in traditional sawmills. On the other hand, it was found that the volume of bark has no influence on the sawing yield of the two variants of industries analyzed; the bark influences 7.55 % on the yield in modern sawmills and 6.36 % in traditional sawmills, representing no statistically significant differences (P > 0.05). This information leads to the conclusion that the characteristics of the raw material have no effect on sawmill productivity.
Regarding thin-cutting sawmills (modern in this study), Kehinde et al. (2010) evaluated 170 small (portable sawmills) and medium-scale sawmills (similar to those in this study) in Nigeria and found a log conversion rate of approximately 58.00 %; however, the results indicate that medium-scale sawmills are more efficient than small-scale sawmills. In Finland, Hyytiäinen et al. (2011) estimated a sawing coefficient of 61 to 66 % in the production of private medium-scale sawmills, which represents heterogeneous productivity; moreover, they indicate that it is possible to increase production by integrating the use of current processing technology. Borz et al. (2021) evaluated the sawing coefficient of Norway spruce (Picea abiesLam. [Limk.]) and silver fir (Abies albaMill.) in a small-scale sawmill in Romania and considered the processing of 26 m3r, resulting in a sawnwood volume of about 18 m3. This represented a sawing yield that varied widely between 38.80 and 95.00 % per log with an average of 69.00 %. In this study, the modern sawmills achieved an average yield of 60.93 %, a value that is in a medium range when considering the studies used to compare this result.
The research previously described is consistent with that described in this study, except for the study developed by Borz et al. (2021), whose equipment exceeds productivity of the modern industries evaluated by approximately 8.0 %. According to these studies, raw material, operators' knowledge, sawing equipment capacity and technology used are variables affecting productivity and efficiency of the industries.
In this study, the differences in volumetric production by each sawmill are mainly attributed to the demands of the market where they market their products; however, volumetric difference had no effect on the percentage contrasts of production between the type of industrial technology used, since the standard deviation of the thicknesses generated allowed improving the contrast adjustment in the statistical estimations made. In the case of the yield per nominal thickness, regarding the conversion coefficient (60.93 %), modern industries concentrate their production in the 7/8 in (22.23 mm) thickness with an average of 34.76 %, followed by 5/4 in (31.75 mm) with 13.00 % and the 6/4 in (38.1 mm) size represents 5.89 %. Similarly, traditional industries with a sawing coefficient of 53.09 % produce, mainly, nominal thickness of 7/8 in with an average of 20.88 %, followed by 6/4 in with 12.52 % and 5/4 in with 8.04 % of the total. In this regard, the percentage distribution of products is similar to the results found by Nájera Luna et al. (2011), who observed that sawnwood production in the El Salto region is concentrated in the nominal thickness of 7/8 in with 22.40 % of the total volume sawn, followed by 5/4 in with 13.90 % and 6/4 in with 7.50 %; these values vary according to a sawing coefficient of 57.50 %.
Regarding width yield, Ortiz et al. (2016) found that the 12 in (304.80 mm) width excels with 54.18 %, while the nominal widths of 6 in (152.40 mm), 8 in (203.20 mm) and 10 in (254.00 mm) account for 40.30 %, and the 4 in (101.6 mm) only 5.50 %. The report of Ortiz et al. (2016) differs from the results, as the production in this study was mainly concentrated on the lower dimension widths. In modern industries, in the volumetric yield per nominal width, the production of 4 in, 6 in and 8 in stood out with 83.83 % and the nominal widths of 10 in and 12 in only represented 16.17 % as a whole. Similarly, traditional industries mainly produce nominal widths of 4 in, 6 in and 8 in with a production of 91.53 %. The distribution of these products is mainly related to the average log diameter (≈30 cm) and market requirements. However, in this study, calculations indicate that the yield of 12 in (304. 8 mm) timber ranges from 3.89 ± 1.46 % to 0.59 ± 0.29 %; in such a context, there were evident percentage differences in the production of this nominal width. Therefore, it is suggested that the calculated statistical difference may be affected by the low proportion of this product.
Regarding the distribution of volumetric yield by nominal length, Nájera Luna et al. (2011) found that the length of 16 ft (4.87 m) had the highest value with 27.20 % of the total volume sawn, regarding a yield of 57.50 %. In this study, it was determined that, in 16 ft timber, modern sawmills had an average yield of 46.96 % regarding a yield of 60.93 %, while traditional sawmills (yield 53.09 %) had 39.09 % of the total yield. This is because most of the raw material sourced in the north of Mexico corresponds to 16 ft in length plus reinforcement.
Conclusions
The sawing coefficient varies significantly when comparing the two sawmilling technologies used. Traditional and modern sawmills have adequate yields (53 to 60 %); however, the use of equipment with modern technologies allows efficient use of the available raw material. In this study, log diameter and taper had no effect on the sawtimber yield coefficient between types of technology used, suggesting that the sawing yield is mainly influenced by the technological capacity of the sawing equipment.
