The sanding process is an important value-addition task in the wooden furniture
manufacturing industry. It is regarded an orthogonal cutting process with a
negative rake angle, which implies that wood removal is achieved by crushing
and scraping actions (Ratnasingam et al., 2004;
Welling et al., 2009). Inevitably, the sanding
process results in the production of wood dust that is injurious to the health
of the workers (Whitehead, 1982; Black
and Dilworth, 2007; Wutjaree et al., 2009).
When the health of the workers suffers there is a corresponding loss in labor
productivity, which will impair the business performance (Bemer
et al., 2000; Schlünssen et al.,
2008; Hursthouse et al., 2004). Dust from
wood sanding processes is of particular interest in Malaysia because many different
hardwood species, ranging from the light hardwood (<600 kg m-3
in density) to heavy hardwoods (>800 kg m-3 in density) are used
in the manufacture of wooden furniture (Graham and Ratnasingam,
2007). Despite its industrial implications, studies on dust emission during
the sanding processes of Malaysian hardwoods in the wood products and furniture
industry are very limited (Graham and Ratnasingam, 2007).
A number of parameters have been found to affect the production of dust during
the wood sanding process, such as the type of wood, density and hardness and
also the sanding grit of the abrasives used in the process (Thorpe
and Brown, 1995; Vinzents et al., 2001;
Harper and Muller, 2002). By monitoring the quantity of dust produced as
these parameters are varied it is possible to examine the dust-generating characteristics
of Malaysian hardwoods during the sanding process. Therefore, this study aims
to evaluate the dust-generation characteristics of selected Malaysian hardwoods
and provide benchmark values on the amount of dust generated during the sanding
operation in the Malaysian wooden furniture industry.
MATERIALS AND METHODS
Twelve species of hardwoods, which represented the commercially important hardwoods
used in the Malaysian wooden furniture manufacturing industry, were selected
based on its density and hardness for this study (Table 1).
The materials were obtained from a local sawmill. The materials in the form
of planks of 25x50x50 mm in dimension were conditioned to a final moisture content
of 12±2%, before experimentation. A total of 5 planks of each species
were used in this study.
||Malaysian hardwood species used in the study
|Density and hardness measurements taken from the sample boards.
Hardness is defined as the force, in kg wt., applied to an 8 mm diameter
sphere to produce a 2 mm depression in the wood sample
The density of the wood samples was determined gravimetrically
by taking the weight and volume of a specimen (20x20x25 mm) from each wood species
at the final moisture content, while the hardness was determined by enumerating
the force applied by 8 mm steel ball to result in a 2 mm depression on the surface
(Desch and Dinwoodie, 1996). The density and hardness values of the wood species of the samples obtained
in this study were within the 5% difference of the average published values
for the respective wood species.
The sanding experiment was carried out using a 3 M pneumatically-operated orbital sander, which had an orbit diameter of 80 mm and rotating at 10,000 revolutions per min. Aluminum oxide abrasive of the sanding grit size 150 was used in this study. The study backed abrasive was attached to the orbital sander using the velour hook-and-loop system.
The orbital sander was fixed in position by a specially made bracket, while
the wood plank was attached on to a reciprocating platform that moved at a speed
of 15 mm sec-1. A force equivalent to 5 kg was applied on to the
sander, to ensure consistent sanding operation (Fig. 1). This
experimental configuration represented the typical sanding operation in the
wood products manufacturing industry, as reported by Graham
and Ratnasingam (2007).
The sanding experiments were carried out in a large dust tunnel at the experimental facility of Petaling Corporation in Sungei Buloh in Malaysia from March to May 2009, to ensure that the experiments were carried out in a moving air stream. The air velocity in the tunnel was maintained at 25 cm sec-1, which was typical of industrial conditions. The experimental equipment was placed within a working section of approximately 3.0 by 3.0 m and all dust measuring instrumentations was placed several meters downstream. For all the hardwood species tested, sanding was carried out for a period of 30 min.
The quantity of dust removed from the hardwood planks during the 30 min of
sanding was determined by weighing the planks before and after the operation.
||Sanding experiment configuration
The dust concentration in the tunnel was measured using two gravimetric isokinetic
samplers placed two meters apart long the length of the air tunnel. The concentration was taken as an average of the two samplers. The size distribution
of the dust was measured using an aerodynamic particle sizer (APS 3300). The
experimental techniques and the dust particle evaluation method used in this
study, together with the statistical analysis undertaken were as reported in
the previous studies by Lehmann and Fröhlich (1988),
Thorpe and Brown (1995) and Graham
and Ratnasingam (2007).
In order to evaluate the dust concentration and its effects on the labor productivity in the value-added wood products industry, dust-concentration measurements using the portable gravimetric isokinetic samplers placed 2.0 m from the various sanding machines, at 25 wooden furniture manufacturing enterprises in the Klang Valley in Malaysia. The average measurements at each sanding stations at the mills, were taken over a period of 30 min. Further, the workforce records at these mills were examined and the relevant labor productivity data calculated. All the mills consented to participate in this study prior to the implementation of the field survey, which was carried out from January to May 2009 around the Klang Valley, Malaysia.
RESULTS AND DISCUSSION
Effect of wood density and hardness on dust concentration: The results
from the study revealed that there is a good correlation between the mass of
wood removed during the abrasive sanding process and the concentration of airborne
dust produced, irrespective of the sanding grit and the type of hardwood specie.
Generally, the amount of wood removed and airborne dust produced both varied
inversely with density and hardness as shown in Fig. 2.
||Effect of wood density on dust concentration
the correlation between wood density and hardness was very good, with a coefficient
correlation of 0.981, it may be implied that the correlation of other parameters
with any of these intrinsic characteristics of the hardwoods will results in
similar results, as suggested by Thorpe and Brown (1995) and Desch and Dinwoodie (1996). Consequently, any change in wood density or hardness will results in a comparable
rate of wood removal and rate of airborne dust production. Martin
and Zalk (1997) reported that the crushing effect of the hardwood cells
is more pronounced in low density materials compared to high density hardwood
species. In fact, abrasive sanding is more efficient on materials of lower density
and hardness, as the abrasive grains are able to penetrate deeper into the material
during the sanding process, thus resulting is a higher mass of material removed,
when all other conditions are kept constant (Ratnasingam et
Effect of wood species on dust particle diameter: The study revealed
that the geometric mean particle size of the dust produced during the sanding
process was significantly influenced by the wood density and hardness. Softer
and low density wood species produced coarser dusts, while harder and high density
wood species resulted in finer dust particles (Fig. 3). This
observation is attributed to the fact that during the sanding process, the abrasive
grits penetrated deeper into the softer wood species to remove a larger amount
of wood material, resulting in coarser dusts. On the other hand, harder wood
species restrained the penetration of the abrasive grits, thus resulting is
smaller amount of wood removed, which in turn manifested as finer dust particles
(Ratnasingam et al., 2004). Nevertheless, denser
hardwoods give rise to airborne dust at a lower rate compared to the less dense
hardwoods, but the total amount of airborne dust produced is a function of the
total amount of wood removed during the sanding process, attributed to the mechanics
of the abrasive sanding process (Ratnasingam et al.,
||Effect of wood density on mean airborne-dust particle diameter
This result suggest that contrary to common belief, the amount of airborne
dust produced during the abrasive sanding process is not influenced by the density
of the wood and hence, dust emission during the process can be reduced by minimizing
the amount of abrasive sanding carried out.
Sanding dust-emission profile in the malaysian wooden furniture industry:
From the field survey conducted, the dust concentrations at the various sanding
stations in the furniture manufacturing mills were markedly different, as shown
in Fig. 4. It is obvious that this difference is closely related
to the amount of wood removed at the sanding station. Increasing amounts of
wood removed would lead to greater amount of dust concentration and it appears
that the wide-belt sanding machine has the highest level of dust concentration.
Further, all the sanding sections recorded dust exposure levels in excess of
the standard value of 5 mg m-3 (Whitehead, 1982),
implying that the use of dust protection gears or dust mask by workers is a
necessity in order to ensure their health and safety.
Dust emission and labor productivity in the wooden furniture industry:
The labor records examined show that labor productivity among the workers in
the sanding section at the 25 furniture manufacturing mills was consistently
lower than the workers from the other section. On the average, non-productive
time of an average of 27% were recorded among the workers due to respiratory
related illness. As a result, workers turnover was also the highest among
workers in the sanding section. On the other hand, productive-time loss was
much lower among workers from the other sections in the furniture manufacturing
mills, averaging 8% in the 25 mills surveyed (Fig. 5).
||Dust concentration at different sanding stations
||Average productive-time losses in furniture manufacturing
findings of this study corresponds with the previous report by Whitehead
(1982) and Graham and Ratnasingam (2007), who found
that labor productivity was significantly effected by dust emission in the value-added
wood products industry.
Industrial implications: The results from this study reveal the abrasive
sanding characteristics of selected Malaysian hardwoods and it is apparent that
the amount of airborne dust produced is a function of the amount of wood removed
during the abrasive sanding process, rather than the density of the wood, as
previously thought. Hence, the hardwoods used in this study resulted in similar
amount of airborne dust, but only differed in the rate at which the dust was
produced. The results of this study also confirm the observation that the sanding
operation in the furniture manufacturing industry generates a high concentration
of airborne dust. Although, the dust concentration is related to the amount
of wood removed during the sanding operation and indirectly, to the wood species
used, the airborne dust concentration levels recorded at the mills which exceeds
the standard 5 mg m-3, suggest that dust-emission control and dust
protection gears for the workers are essential requirements that must be enforced
in the furniture manufacturing mills in order to ensure the health and safety
of the workers. Further, the study also suggests that the sanding operation
should be optimized to ensure minimal wood removal, which in turn, will reduce
the dust emission. Such an effort will help create a more hygienic and comfortable
working environment in the furniture manufacturing mills. It is also recommended
that a further study of industrial air-borne dust concentration levels be carried
out in the wooden furniture-manufacturing factories using different wood species,
to ascertain wood species influences on the overall dust concentration in the
Dust emission during the sanding process of hardwoods is influenced primarily by the amount of wood removed during the sanding operation and hence, dust emission control can be achieved by minimizing the amount of wood removed during the sanding operation. The study also reveals that airborne dust concentration in the sanding operation exceeds the allowed standard and hence, control of dust emission and the use of dust protection gears by the workers, must be enforced in the furniture manufacturing industry to ensure workers health and safety.
The authors wish to express their appreciation to Petaling Corporation Sdn. Bhd. in Sungei Buloh, Selangor, Malaysia, for the support with the experimental facilities and equipments during the study.