RU2708323C1 - Method for production of particle board - Google Patents

Abstract

FIELD: woodworking industry.SUBSTANCE: invention relates to woodworking, particularly to production of particle boards. Performing formation of chip mat, hot pressing of plates, measurement of direction and size of plates warpage at least 5 minutes after unloading of plate from press. Surface of the chip mat, towards which the middle of the plate is concaved, is irrigated with water, the amount of which depends on the warpage size, thickness and density of the plate and the time of measuring of its warpage.EFFECT: reduced warpage of particle boards.1 cl, 1 dwg, 1 tbl

Description

The invention relates to the woodworking industry and can be used in the manufacture of chipboards.

Known methods for the manufacture of chipboard, implemented in lines for the manufacture of chipboard, including the formation of chipboard, hot pressing of the boards, measuring the magnitude and direction of warp of the finished boards and irrigation with water on the upper or lower surface of the chipboard during its formation [Patent 2453428 RF, IPC B27N 3/02, B27N 3/18, Line for the manufacture of chipboards. - publ. 06/20/2012. Bull. No. 17], [RF Patent 2508193, IPC 27 N3 / 02. Line for the manufacture of chipboards. - publ. 02/27/2014. Bull. No. 6].

In the known methods there is no information about what surface of the carpet should be irrigated and about the amount of water needed for irrigation in order to eliminate the distortion of a certain size in the finished plate.

A known method of manufacturing chipboards, including the formation of a particleboard carpet, hot pressing of the plates, measuring the magnitude and direction of warp of the plates and creating a certain temperature difference between the pressing surfaces in the course of subsequent pressing cycles [A.S. 1653961 USSR, IPC B27N 3/02, Method for the manufacture of chipboards. - publ. 06/07/1991, Bull. No. 21].

For the implementation of this method requires special precision regulators of the coolant of the press and the special design of the channels of the pressing plates.

The closest is a method of manufacturing chipboards, including the formation of a chipboard, hot pressing of the boards, measuring the size and direction of warp of the boards at least 5 minutes after unloading the boards from the press and creating a certain moisture difference between the outer layers of the chipboard during its subsequent formation [ A.S. 1653960 USSR, IPC B27N 3/02, Method for the manufacture of chipboards. - publ. 06/07/1991, Bull. No. 21].

The known method involves the creation of different humidity of the outer layers of the carpet, which can be implemented at the stage of grinding the chips. Resin is produced earlier than the formation of the carpet, so the known method is inertial, which affects the quality of chipboards. In addition, the implementation of this method is impossible in the manufacture of single-layer plates, and in the manufacture of multilayer plates, the number of mixers increases, because different humidity in the outer layers of the carpet is set by different mixers.

The invention solves the problem of improving the quality of particle boards.

The technical result is to improve the quality of chipboards, by reducing or completely eliminating their warp.

The objective is achieved by the fact that in the method of manufacturing chipboards, including the formation of chipboard carpet, hot pressing of the boards, measuring the size and direction of warping of the boards at least 5 minutes after unloading the board from the press, according to the invention, the surface of the carpet, to the side of which the middle is curved plates, irrigated with water, the amount of which is determined from the expression:

where Δƒ is the warp of the plate, mm;

t is the period between unloading the plate from the press and the moment of measuring its warp, min .;

L is the length of the plate, m;

b - plate thickness, mm;

ρ is the density of the plate, g / cm ³ .

The inventive method for the manufacture of particle boards is thus distinguished by irrigation with water of the surface of the chip carpet, towards which the middle of the plate is curved, the amount of which depends on the amount of deflection of the plate, its format, thickness, density and time of measurement of warp.

In FIG. 1 shows the amount of water W (g / m ² ) applied to one of the surfaces of the chip carpet to eliminate distortion (the deflection arrows at the diagonal intersection) Δƒ (mm) of the finished plate with a density of ρ (g / cm ³ ) 16 mm thick, format 1 m ² . Deflection was measured 20 minutes after unloading the plate from the press.

The dependence of the distortion of the plate (the arrow of the deflection at the intersection of the diagonals) on the moisture imbalance of the upper and lower surfaces of the initial chip package was established experimentally. [Plotnikov S.M. The study of the warp of chipboards with an asymmetric structure. - Izv. Universities, Forest Journal. - 1989. - No. 1, p. 51, table 3]. The warp Δƒ (mm) measured 20 minutes after unloading from the press a plate with a format of 0.4 * 0.4 m ² and a thickness of 16 mm takes on the value

where ΔW is the moisture difference between the upper and lower surfaces of the chip package,%;

ρ is the density of the plate, g / cm ³ .

It was also established that the warping of the plate occurs exponentially with a constant warping time T _k = 10.5 min., Regardless of the thickness and format of the plates [see Plotnikov S.M. Improving the formation and pressing of chipboards: abstract dis. for a job. scientist step. Doct. tech. Sciences (05.21.05) / SibGTU. - Krasnoyarsk, 2009. - 38 p., P. 23].

From expression (1), we can obtain an inverse relationship, from which it follows that in order to eliminate the distortion Δƒ (mm) of a plate with a format of 0.4 * 0.4 m ² and a thickness of 16 mm, measured 20 minutes after unloading the plate from the press, it is necessary to create moisture difference ΔW (%) of the outer layers

where Δƒ is the warp of the plate, mm.

The correction for the time of measurement of warp is:

where t is the time between unloading the plate from the press and the moment of measuring its deflection boom, min .;

T _to - warping time constant, min.

Here, the coefficient 0.851 reflects the fact that expressions (1) and (2) are valid for the period corresponding to 20 minutes after unloading the plate from the press. Substitution in (3) t = 20 min. and T _k = 10.5 min. we get K _t = 1. In order for expressions (1) and (2) to have acceptable certainty, the time t must be at least 5 minutes. The control action introduced to eliminate the distortion Δƒ, the value of which was measured 5 minutes after unloading the plate from the press, according to formula (3), should be increased 2.25 times (K _t = 2.25). If the warp of the plate was measured after an infinitely long time (t → ∞), then the control action should be multiplied by 0.851 (K _t = 0.851). Thus, the coefficient K _t shows the degree of reduction of the impact introduced into the technological mode of manufacturing the plate as the time elapsed from unloading the plate from the press to measuring warp.

Experimental studies have shown that the quantity Δƒ obeys the similarity condition, i.e. for a square plate with a side length of L (m), warp differs in L / 0.4 from the warp of plates for which equation (1) is obtained, i.e. for plates with a format of 0.4 * 0.4 m ² . Therefore, the correction for the format of the plate is:

For a plate with a format of 0.4 * 0.4 m ^{2, the} coefficient K _L = 1. To eliminate distortion of a slab with a format of 1 * 1 m ² , ceteris paribus, it is required to introduce a control action that compensates for the warp of such a slab, 2.5 times weaker than for a slab with a format of 0.4 * 0.4 m ² , i.e. K _L = 0.25. For a non-square plate, instead of L, substitute the arithmetic mean between the length and width of the plate.

The relationship between the warp (deflection arrow) of a particle board Δƒ and its thickness b was established in [Cai, Z., Dickens J.R .. 2003. Wood Composite Warping: Modeling and Simulation // Wood and Fiber Science. - 2004. - 42. - P. 174-185, p. 180]:

Δƒ * b = const.

Studies were conducted on samples with a thickness of 2.54 to 25.4 mm. Expression (1) was obtained for a plate with a thickness of b = 16 mm, therefore, taking into account the constancy of the product Δƒ * b, the correction for the thickness of the plate is:

For a plate with a thickness of 16 mm, the coefficient K _b = 1. All other things being equal, the warp of a slab 32 mm thick will be half that of a slab of 16 mm thick; therefore, the control action compensating for the warp of a thicker slab should be twice as strong.

Subject to corrections for the measurement time, format and thickness of the plate, expression (2) has the form:

т.е. коэффициент перехода от % к граммам составляет 1,2. Выражение (6) получено для плиты форматом 0,4*0,4 м². Для площади 1 м² следует уменьшить воздействие в 1/04²=6,25 раз. Поэтому, умножив коэффициент 12,21 в формуле (6) на 1,2 и разделив на 6,25, от относительной влажности перейдем к абсолютной влажности (г/м²) одной из поверхностей ковра при условии, что противоположная поверхность не орошается:It was experimentally established that 100 percent moisture content of the surface of the particleboard carpet, taking into account its surface roughness, corresponds to 120 g of water per 1 m ^{2 of} surface. For example, to create the required moisture difference ΔW = 50%, one of the carpet surfaces should be irrigated on

those. the conversion rate from% to grams is 1.2. Expression (6) was obtained for a slab with a format of 0.4 * 0.4 m ² . For an area of 1 m ² , the impact should be reduced by 1/04 ² = 6.25 times. Therefore, by multiplying the coefficient 12,21 in the formula (6) by 1.2 and dividing by 6.25, the relative humidity pass to the absolute humidity (g / m ²⁾ of one of the carpet surface, provided that the opposite surface is not irrigated:

The formula for the amount of water needed to irrigate the surface of the original particleboard carpet to eliminate the warp of the finished slab, depending on its format, thickness, density, time of measurement of warp and the amount of deflection obtained by the author for the first time.

An example implementation of the method.

From a mixture of chips and a binder, for example, urea-formaldehyde resin, a chip carpet is formed and it is hot pressed. Not less than 5 minutes after unloading the slab from the press, measure the magnitude and direction of warp of the resulting chipboard. In the presence of warping of the finished plate, the surface of the newly formed chip carpet, to the side of which the middle of the plate is concave, is irrigated with water, the amount of which is determined from expression (7). For example, when bending 2 mm with a middle downward slab 16 mm thick, 1 * 1 m ² format and a density of 0.7 g / cm ^{3, the} surface of the forming conveyor is irrigated in front of the first forming machine, according to expression (7), 135 g of water (a layer of water with a thickness of 0.135 mm). When the same plate is bent, the surface of the chip carpet emerging from the last forming machine is irrigated with the same amount of water. (For the manufacture of a single-layer plate, one forming machine is used).

As a result of the asymmetric humidity of the carpet surfaces, moisture and heat from the surface are transferred into the carpet (package) at different speeds, the binder in the upper and lower layers of the carpet (package) is cured at the same time. The chips of the wetter layer swell longer, expanding in volume, and internal mechanical stresses arise over the thickness of the resulting plate, which begin to balance by warping the plate after opening the press. The middle of the plate bends towards a wetter surface, and the warping that took place in the previously formed plates is compensated.

To confirm the operability of the proposed method, an experiment was conducted in laboratory conditions.

Four batches of single-layer boards from pine shavings (10 boards in each batch) 16 mm thick, format 0.4 * 0.4 m ^{2 with a} density of 0.7 g / cm ³ were made on an electrically heated press. The humidity of the initial carpet (package) was 12%, the average pressing temperature was 170 ° C. The temperature values of the upper and lower pressing surfaces T _in and T _n , the amount of water applied to the upper and lower surfaces of the particleboard carpet (package) W _in or W _n , as well as the warp values of the finished plates (average of 10 plates) are presented in table 1.

The amount of water applied to the surface of the chip carpet (package) to eliminate the warp of the slab Δƒ = 1.9 mm was determined by the formula (7)

moreover, in the manufacture of batch plates, the upper surface of the carpet (bag) was irrigated, and in the manufacture of batch plates G, the surface of the tape (pallet) on which the carpet (bag) was formed, i.e. moistened the bottom surface of the carpet (package). After unloading the plates from the press, the plates were kept in a horizontal position for 20 minutes, after which the warping was measured. For plates of batch A, the warp was 0.1 mm (commensurate with the measurement error), for plates of batch B - 1.9 mm, for plates of batch B - 3.9 mm and for plates of batch G - 0.2 mm (average 10 plates).

- (numerator for the upper surface, denominator for the lower). Here the deflection with the “+” sign means the bend of the middle of the plate up, “-” - down.

In the plates of party B, warping was artificially caused by the temperature asymmetry of the pressing surfaces. In the slabs of party B, warp was immediately caused by two reasons: temperature asymmetry during pressing and asymmetry of humidity of the surfaces of the initial chip carpet. In the plates of party D, the warp caused by temperature asymmetry during pressing was compensated by the “reverse” warp caused by moisture asymmetry of the upper and lower surfaces of the original chip carpet.

The invention allows to eliminate such a defect as warping of the manufactured plates, regardless of the technological reasons that caused warpage, which makes the production process less critical to errors. In addition, the raw materials used to eliminate distortion are saved (grinding and plate calibration).

Claims (7)

A method of manufacturing chipboards, including the formation of a chipboard carpet, hot pressing of the boards, measuring the size and direction of warping of the boards at least 5 minutes after unloading the board from the press, characterized in that the surface of the carpet, to the side of which the middle of the board is curved, is irrigated with water, the amount of which is determined from the expression:

where Δƒ is the warp of the plate, mm; t is the period between the unloading of the plate from the press and the moment of measuring its warp, min; L is the length of the plate, m; b - plate thickness, mm; ρ is the density of the plate, g / cm ³ .

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