Sawmill Process

Sawmills can vary greatly in size. The smallest are either stationary or portable units consisting of a circular saw headrig, a simple log carriage and a two-saw edger (see descriptions below) powered by a diesel or gasoline engine and operated by as few as one or two workers. The largest mills are permanent structures, have much more elaborate and specialized equipment, and can employ over 1,000 workers. Depending on the size of the mill and the climate of the region, operations may be performed outdoors or indoors. While the type and size of logs determine to a large degree what types of equipment are needed, the equipment in sawmills can also vary considerably based on the age and size of the mill as well as the type and quality of boards produced. Below is a description of some of the processes conducted in a typical sawmill.

After transport to a sawmill, logs are stored on land, in water bodies adjacent to the mill or in ponds constructed for storage purposes (see figure 1 and figure 2). The logs are sorted according to quality, species or other characteristics. Fungicides and insecticides may be used in land-based log storage areas if the logs will be stored for a long time until further processing. A cut-off saw is used to even up the ends of the logs either before or after debarking and prior to further processing in the sawmill. The removal of bark from a log may be accomplished by a number of methods. Mechanical methods include peripheral milling by rotating logs against knives; ring debarking, in which tool points are pressed against the log; wood-to-wood abrasion, which pounds the logs against themselves in a rotating drum; and using chains to tear away the bark. Bark may also be removed hydraulically by using high-pressure water jets. After debarking and between all operations within the sawmill, logs and boards are moved from one operation to the next using a system of conveyors, belts and rollers. In large sawmills these systems can become quite complex (see figure 3).

Figure 1. Chip loading with water storage of logs in background

Source: Canadian Forest Products Ltd.

Figure 2. Longs entering a sawmill; storage and kilns in background

Source: Canadian Forest Products Ltd.

Figure 3. Mill interior; conveyor belts and rollers transport wood

British Columbia Ministry of Forests

The first phase of sawmilling, sometimes referred to as primary breakdown, is performed at a headrig. The headrig is a large, stationary circular saw or band-saw used to cut the log longitudinally. The log is transported back and forth through the headrig using a travelling carriage which can rotate the log for the optimum cut. Multiple band-saw headrigs may also be used, especially for smaller logs. The products of the headrig are a cant (the square centre of the log), a series of slabs (the rounded outer edges of the log) and, in some cases, large boards. Lasers and x rays are becoming common in sawmills for use as viewing and cutting guides in order to optimize wood use and the size and types of boards produced.

In secondary breakdown, the cant and large boards or slabs are further processed into functional lumber sizes. Multiple parallel saw blades are usually used for these operations - for example, quad saws with four linked circular saws, or gang saws which may be of the sash or circular saw type. Boards are cut to the proper width using edgers, consisting of at least two parallel saws, and to the proper length using trim saws. Edging and trimming are usually performed using circular saws, though edgers sometimes are band-saws. Manual chain-saws are usually available in sawmills for freeing lumber caught in the system because it is bent or flared. In modern sawmills, each operation (i.e., headrig, edger) will generally have a single operator, often stationed within an enclosed booth. In addition, workers may be stationed between operations in later stages of secondary breakdown in order to manually ensure that the boards are properly positioned for subsequent operations.

After processing in the sawmill, the boards are graded, sorted according to dimensions and quality, then stacked by hand or machine (see figure 4). When lumber is manually handled, this area is referred to as a “green chain”. Automated sorting bins have been installed in many modern mills to replace labour-intensive manual sorting. In order to increase airflow to assist in drying, small pieces of wood may be placed between the boards as they are being stacked.

Figure 4. Fork-lift with load

Canadian Forest Productions Ltd.

Construction grades of lumber may be seasoned in the open air outdoors or dried in kilns, depending on local weather conditions and the wetness of the green lumber; but finishing grades are more commonly kiln dried. There are many kinds of kilns. Compartment kilns and high-temperature kilns are serial kilns. In continuous kilns, stacked bundles can move through the kiln in a perpendicular or parallel position, and the direction of air movement can be perpendicular or parallel to the boards. Asbestos has been used as an insulating material for steam pipes in kilns.

Prior to storage of green lumber, especially in wet or humid locales, fungicides may be applied to prevent growth of fungi which stain wood blue or black (sapstain). Fungicides may be applied in the production line (usually by spraying) or after bundling lumber (usually in dip tanks). The sodium salt of pentachlorophenol was introduced in the 1940s for the control of sapstain, and was replaced in the 1960s by the more water soluble tetrachlorophenate. Chlorophenate use has largely been discontinued because of concern regarding health effects and contamination with polychlorinated dibenzo-p-dioxins. Substitutes include didecyldimethyl ammonium chloride, 3-iodo-2-propynyl butyl carbamate, azaconazole, borax and 2-(thiocyanomethylthio)benzthiazole, most of which have been little studied among user workforces. Often lumber, especially that which has been kiln dried, does not need to be treated. In addition, wood of some tree species, such as Western red cedar, is not susceptible to sapstain fungi.

Either before or after drying, the wood is marketable as green or rough lumber; however, the lumber must be further processed for most industrial uses. Lumber is cut to final size and surfaced in a planing mill. Planers are used to reduce the wood to standard marketable sizes and to smooth the surface. The planer head is a series of cutting blades mounted on a cylinder which revolves at high speed. The operation is generally power fed and performed parallel to the wood grain. Often planing is performed simultaneously on two sides of the board. Planers which operate on four sides are called matchers. Moulders are sometimes used to round the edges of the wood.

After final processing, the wood must be sorted, stacked and bundled in preparation for shipping. Increasingly, these operations are being automated. In some specialized mills, wood may be further treated with chemical agents used as wood preservatives or fire retardants, or for protection of the surface from mechanical wear or weathering. For example, railroad ties, pilings, fence posts, telephone poles or other wood expected to be in contact with soil or water may be pressure treated with chromated or ammoniacal copper arsenate, pentachlorophenol or creosote in petroleum oil. Stains and colourants may also be used for marketability, and paints may be used to seal the ends of boards or to add company marks.

Large amounts of dust and debris are generated by saws and other wood-processing operations in sawmills. In many sawmills the slabs and other large pieces of wood are chipped. Chippers are generally large rotating discs with straight blades imbedded in the face, and slots for the chips to pass through. The chips are produced when logs or mill wastes are introduced to the blades using inclined gravity feed, horizontal self-feed or controlled power feeding. Generally the cutting action of the chipper is perpendicular to the blades. Different designs are used for whole logs than for slabs, edgings and other pieces of waste wood. It is common for a chipper to be integrated into the headrig to chip unusable slabs. Separate chippers to handle waste from the rest of the mill are also used. Wood chips and sawdust may be sold for pulp, reconstituted board manufacturing, landscaping, fuel or other uses. Bark, wood chips, sawdust and other material may also be burned either as fuel or as waste.

Large, modern sawmills will typically have a sizeable maintenance staff which includes clean-up workers, millwrights (industrial mechanics), carpenters, electricians and other skilled workers. Waste material may collect on machinery, conveyors and floors if sawmill operations are not equipped with local exhaust ventilation or the equipment is not operating properly. Clean-up operations are often performed using compressed air to remove wood dust and dirt from machinery, floors and other surfaces. Saws must be regularly inspected for broken teeth, cracks or other defects, and must be properly balanced to prevent vibration. This is done by a trade that is unique to the wood industries - saw filers, who are responsible for the re-toothing, sharpening and other maintenance of circular saws and band-saws.

Sawmill Health and Safety Hazards

Table 1 indicates the major types of occupational health and safety hazards found in the major process areas of a typical sawmill. There are many serious safety hazards within sawmills. Machine guarding is necessary at the point of operation for saws and other cutting devices as well as for gears, belts, chains, sprockets and nip points on conveyors, belts and rollers. Anti-kickback devices are necessary on many operations, such as circular saws, to prevent jammed lumber from being ejected from machines. Guard rails are necessary on walkways adjacent to operations or crossing over conveyors and other production lines. Proper housekeeping is necessary to prevent dangerous accumulation of wood dust and debris, which could result in falls as well as presenting a fire and explosion hazard. Many areas which require clean-up and routine maintenance are located in hazardous areas which would normally be inaccessible during times when the sawmill is in operation. Proper adherence to machinery lock-out procedures is extremely important during maintenance, repair and clean-up operations. Mobile equipment should be equipped with audible warning signals and lights. Traffic lanes and pedestrian walkways should be clearly marked. Reflective vests are also necessary to increase the visibility of pedestrians.

Table 1. Occupational health and safety hazards by lumber industry process area

* Signifies high degree of hazard.

Sorting, grading and some other operations may involve the manual handling of boards and other heavy pieces of wood. Ergonomic design of the conveyors and receiving bins, and proper material-handling techniques should be used to help prevent back and upper extremity injuries. Gloves are necessary to prevent splinters, puncture wounds and contact with preservatives. Panels of safety glass or similar material should be placed between operators and points of operation because of the risk of eye and other injuries from wood dust, chips and other debris ejected from saws. Laser beams are also potential ocular hazards, and areas using Class II, III or IV lasers should be identified and warning signs posted. Safety glasses, hardhats and steel-toed boots are standard personal protective gear that should be worn during most sawmill operations.

Noise is a hazard in most areas of sawmills from debarking, sawing, edging, trimming, planing and chipping operations, as well as from logs striking each other on conveyors, rollers and drop-sorters. Feasible engineering controls to reduce noise levels include sound-proof booths for operators, enclosure of cutting machines with sound-absorbent material at the in- and out-feeds, and construction of sound barriers of acoustical materials. Other engineering controls are also possible. For example, idle running noise from circular saws may be reduced by purchasing saws with a suitable tooth shape or adjusting the speed of rotation. The installation of absorbing material on walls and ceilings may aid in reducing reflected noise throughout the mill, though source control would be necessary where noise exposure is direct.

Workers in almost all areas of the sawmill have the potential for exposure to particulate matter. Debarking operations involve little or no exposure to wood dust, since the goal is to leave the wood intact, but exposure to airborne soil, bark and biological agents, such as bacteria and fungi, is possible. Workers in almost all sawing, chipping and planing areas have the potential for exposure to wood dust. The heat generated by these operations may cause exposure to the volatile elements of the wood, such as monoterpenes, aldehydes, ketones and others, which will vary by tree species and temperature. Some of the highest wood dust exposures may occur among workers using compressed air for clean-up. Workers near kiln drying operations are likely to be exposed to wood volatiles. In addition, there is a potential for exposure to pathogenic fungi and bacteria, which grow at temperatures below 70°C. Exposure to bacteria and fungi is also possible during the handling of wood chips and waste, and the transport of logs in the yard.

Feasible engineering controls, such as local exhaust ventilation, exist to control the levels of airborne contaminants, and it may be possible to combine noise- and dust-control measures. For example, enclosed booths may reduce both noise and dust exposures (as well as preventing eye and other injuries). However, booths provide protection only to the operator, and controlling exposures at the source through enclosure of operations is preferable. Enclosure of planing operations has become increasingly common and has had the effect of reducing exposure to both noise and dust among persons who do not have to enter the enclosed areas. Vacuum and wet clean-up methods have been used in some mills, usually by clean-up contractors, but are not in general use. Exposure to fungi and bacteria may be controlled by reducing or increasing kiln temperatures and taking other steps to eliminate the conditions which promote the growth of these micro-organisms.

Other potentially hazardous exposures exist within sawmills. Exposure to cold and hot temperature extremes is possible near points where materials enter or leave the building, and heat is also a potential hazard in kiln areas. High humidity may be a problem when sawing wet logs. Exposure to fungicides is primarily via the dermal route and may occur if the boards are handled while still wet during grading, sorting and other operations. Appropriate gloves and aprons are necessary when handling boards that are wet with fungicides. Local exhaust ventilation with spray curtains and mist eliminators should be used in spraying operations. Exposure to carbon monoxide and other combustion products is possible from mobile equipment used to move logs and lumber within storage areas and to load semi-trailers or railroad cars. Saw filers may be exposed to hazardous levels of metal fumes including cobalt, chromium and lead from grinding, welding and soldering operations. Local exhaust ventilation as well as machine guarding are necessary.

Veneer and Plywood Mill Processes

The term plywood is used for panels consisting of three or more veneers which have been glued together. The term is also used to refer to panels with a core of solid wood strips or particleboard with top and bottom veneer surfaces. Plywood can be made from a variety of trees, including both conifers and non-conifers.

Veneers are usually created directly from debarked whole logs using rotary peeling. A rotary peeler is a lathe-like machine used to cut veneers, thin sheets of wood, from whole logs using a shearing action. The log is rotated against a pressure bar as it hits a cutting knife to produce a thin sheet between 0.25 and 5 mm in thickness. The logs used in this process may be soaked in hot water or steamed to soften them prior to peeling. The edges of the sheet are usually trimmed by knives attached to the pressure bar. Decorative veneers may be created by slicing a cant (the square centre of the log) using a pressure arm and blade in a manner similar to peeling. After either peeling or slicing, the veneers are collected on long, flat trays or rolled onto reels. The veneer is clipped into functional lengths using a guillotine-like machine, and dried using artificial heating or natural ventilation. The dried panels are inspected and, if necessary, patched using small pieces or strips of wood and formaldehyde-based resins. If the dried veneers are smaller than a standard-size panel, they may be spliced together. This is done by applying a liquid formaldehyde-based adhesive to the edges, pressing the edges together, and applying heat to cure the resin.

To produce the panels, veneers are roller- or spray-coated with formaldehyde-based resins, then placed between two unglued veneers with their grains in the perpendicular direction. The veneers are transferred to a hot press, where they are subjected to both pressure and heat to cure the resin. Phenol-resin adhesives are widely used to produce softwood plywood for severe service conditions, such as for construction and boat building. Urea-resin adhesives are used extensively in producing hardwood plywood for furniture and interior panelling; these can be fortified with melamine resin to increase their strength. The plywood industry has used formaldehyde-based glues in assembling of plywood for over 30 years. Prior to the introduction of formaldehyde-based resins in the 1940s, soybean and blood-albumen adhesives were used, and cold pressing of panels was common. These methods may still be used, but are increasingly rare.

The panels are cut to the proper dimensions using circular saws and are surfaced using large drum or belt sanders. Additional machining may also be performed in order to give the plywood special characteristics. In some cases, pesticides such as chlorophenols, lindane, aldrin, heptachlor, chloronaphthalenes and tributyltin oxide may be added to glues or used to treat the surface of panels. Other surface treatments may include the application of light petroleum oils (for concrete-form panels), paints, stains, lacquers and varnishes. These surface treatments may be performed at separate locations. Veneers and panels are often transported between operations using mobile equipment.

Veneer and Plywood Mill Hazards

Table 1 indicates the major types of occupational health and safety hazards found in the major process areas of a typical plywood mill. Many of the safety hazards in plywood mills are similar to those in sawmills, and the control measures are also similar. This section deals with only those issues which differ from sawmill operations.

Both dermal and respiratory exposure to formaldehyde and other components of glues, resins and adhesives is possible among workers in glue preparation, splicing, patching, sanding and hot pressing operations, and among workers nearby. Urea-based resins more readily release formaldehyde during curing than phenol-based ones; however, improvements in resin formulation have reduced exposures. Proper local exhaust ventilation and the use of appropriate gloves and other protective equipment are necessary to reduce respiratory and dermal exposure to formaldehyde and other resin components.

The wood used to produce veneers is wet, and the peeling and clipping operations do not generally produce much dust. The highest wood dust exposures during the production of plywood occur during the sanding, machining and sawing necessary to finish the plywood. Sanding, in particular, can produce large amounts of fine dust because as much as 10 to 15% of the board may be removed during surfacing. These processes should be enclosed and have local exhaust ventilation; hand sanders should have integral exhaust to a vacuum bag. If local exhaust is not present or it is not functioning properly, significant exposure to wood dust may occur. Vacuum and wet clean-up methods are more commonly found in plywood mills because the fine size of the dust makes other methods less effective. Unless noise control measures are in place, noise levels from sanding, sawing and machining operations are likely to exceed 90 dBA.

When veneers are dried, a number of chemical constituents of the wood may be released, including monoterpenes, resin acids, aldehydes and ketones. The types and amounts of chemical released depend on the species of tree and veneer dryer temperature. Proper exhaust ventilation and the prompt repair of veneer dryer leaks are necessary. Exposure to engine exhaust from fork-lifts may occur throughout plywood mills, and mobile equipment also presents a safety hazard. Pesticides mixed in glues are only slightly volatile and should not be detectable in workroom air, with the exception of chloronaphthalenes, which evaporate substantially. Exposure to pesticides may occur through the skin.

Other Manufactured Board Industries

This group of industries, including the manufacture of particleboard, waferboard, strandboard, insulation board, fibreboard and hardboard, produces boards consisting of wood elements of varying sizes, ranging from large flakes or wafers to fibres, held together by resinous glues or, in the case of wet process fibreboard, “natural” bonding between fibres. In the simplest sense, boards are created using a two-step process. The first step is the generation of the elements either directly from whole logs or as a waste by-product of other wood industries, such as sawmills. The second step is their recombination into sheet or panel form using chemical adhesives.

Particleboard, flakeboard, strandboard and waferboard are made from chips of wood of varying sizes and shapes using similar processes. Particleboard and flakeboard are made from small wood elements and are often used to make wood-veneered or plastic-laminated panels for the manufacture of furniture, cabinets and other wood products. Most elements may be made directly from wood waste. Waferboard and strandboard are made from very large particles - wood shavings and strands, respectively - and are primarily used for structural applications. The elements are generally made directly from logs using a machine containing a series of rotating knives which peel thin wafers. The design can be similar to a chipper, except the wood must be fed to the flaker with the grain oriented parallel to the knives. Peripheral milling designs can also be used. Water-saturated wood works best for these processes and, because the wood must be oriented, short logs are often used.

Before making sheets or panels, the elements must be sorted by size and grade, and then dried using artificial means, to a closely controlled moisture content. The dried elements are mixed with an adhesive and laid out in mats. Both phenol-formaldehyde and urea-formaldehyde resins are used. As is the case with plywood, phenolic resins are likely to be used for panels destined for applications requiring durability under adverse conditions, while the urea-formaldehyde resins are used for less demanding, interior applications. Melamine formaldehyde resins may also be used to increase durability, but rarely are because they are more expensive. In recent decades a new industry has emerged to produce reconstituted lumber for various structural uses as beams, supports and other weight-bearing elements. While the manufacturing processes used may be similar to particleboard, isocyanate-based resins are used because of the added strength needed.

The mats are divided into panel-sized sections, generally using an automated compressed air source or a straight blade. This operation is done in an enclosure so that the excess mat material can be recycled. The panels are formed into sheets by curing the thermosetting resin using a hot press in a manner similar to plywood. Afterwards the panels are cooled and trimmed to size. If necessary, sanders may be used to finish the surface. For example, reconstituted boards which are to be covered with a wood veneer or plastic laminate must be sanded to produce a relatively smooth, even surface. While drum sanders were used early in the industry, wide belt sanders are now generally used. Surface coatings may also be applied.

Fibreboards (including insulation board, medium-density fibreboard (MDF) and hardboard) are panels consisting of bonded wood fibres. Their production varies somewhat from particle- and other manufactured boards (see figure 5). To create the fibres, short logs or wood chips are reduced (pulped) in a manner similar to that used for producing pulp for the paper industry (see the chapter Paper and pulp industry). In general, a mechanical pulping process is used in which chips are soaked in hot water and then mechanically ground. Fibreboards can vary greatly in density, from low-density insulation boards to hardboards, and can be made from either conifers or non-conifers. Non-conifers generally make better hardboards, while conifers make better insulation boards. The processes involved in pulping have a minor chemical effect on the ground wood, removing a small amount of the lignin and extractive materials.

Figure 5. Classification of manufactured boards by particle size, density and process type

Two different processes, wet and dry, may be used to bond the fibres and create the panels. Hardboard (high density fibreboard) and MDF can be produced by “wet” or “dry” processes, while insulation board (low density fibreboard) can be produced only by the wet process. The wet process was developed first, and extends from paper production, while the dry process was developed later and stems from particleboard techniques. In the wet process, a slurry of pulp and water is distributed on a screen to form a mat. Afterwards, the mat is pressed, dried, cut and surfaced. The boards created by wet processes are held together by adhesive-like wood components and the formation of hydrogen bonds. The dry process is similar, except that the fibres are distributed on the mat after addition of a binder (either a thermosetting resin, thermoplastic resin or a drying oil) to form a bond between the fibres. Generally, either phenol-formaldehyde or urea-formaldehyde resins are used during the manufacture of dry-process fibreboard. A number of other chemicals may be used as additives, including inorganic salts as fire retardants and fungicides as preservatives.

In general, the health and safety hazards in the particleboard and related manufactured board industries are quite similar to those in the plywood industry, with the exception of pulping operations for fibreboard production (see table 1). Exposure to wood dust is possible during the processing to create the elements and may vary greatly depending on the moisture content of the wood and the nature of the processes. The highest wood dust exposures would be expected during the cutting and finishing of panels, especially during sanding operations if engineering controls are not in place or not functioning properly. Most sanders are enclosed systems, and large capacity air systems are needed to remove the dust generated. Exposure to wood dust, as well as fungi and bacteria, is also possible during the chipping and grinding of dried wood and among workers involved in the transport of chips from storage to processing areas. Very high noise exposures are possible near all sanding, chipping, grinding and related wood-processing operations. Exposure to formaldehyde and other resin constituents is possible during the mixing of glues, laying of the mat and the hot pressing operations. The control measures for limiting exposure to safety hazards, wood dust, noise and formaldehyde in the manufactured board industries are similar to those for the plywood and sawmill industries.

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