United States

Department of

Agriculture

Forest Service

Forest

Products

Laboratory

Research

Paper

FPL 456

Steam-Injection

Pressing of

Isocyanate-Bonded

Aspen Flakeboards

Latitudes and Limitations

Robert L. Geimer
Abstract

Injection of steam into a flakeboard mat during pressing

significantly reduces press times. One-half-inch-thick 0.640

specific gravity (SG) isocyanate-bonded aspen flakeboard

can be cured in 60 seconds by injecting 99 Btu’s of steam

energy per pound (Btu/lb) of board. Press time may be

reduced to 40 seconds when energy consumption is

increased to 177 Btu/lb of board. Two-inch-thick 0.640 SG

board can be pressed in 201 seconds with 185 Btu/lb.

Blister formation in high-density thin boards and temperature

variation in low-density thick boards were compensated for

with press cycles individually suited for each combination of

board thickness and SG.

Keywords: Steam, steam injection, heat, heat transfer,

isocyanate, aspen, flakeboard, ring flake, disk flake, energy,

press schedule, press time.

July 1985

Geimer, Robert L. Steam-injection pressing of isocyanate-bonded aspen

flakeboards: Latitudes and limitations. Res. Pap. FPL 456. Madison, WI:

U.S. Department of Agriculture, Forest Service, Forest Products

Laboratory; 1985. 16 p.

A limited number of free copies of this publication are available to the

public from the Forest Products Laboratory, One Gifford Pinchot Drive,

Madison, WI 53705. Laboratory publications are sent to over 1,000

libraries in the United States and elsewhere.

The Laboratory is maintained in cooperation with the University of

Wisconsin.
Steam-Injection

Pressing of

Isocyanate-Bonded

Aspen Flakeboards

Latitudes and Limitations

Robert L. Geimer, Technologist

Forest Products Laboratory, Madison, WI

Introduction

Particleboard and flakeboard manufacturers operating in a

competitive economy constantly strive to increase production

rates without increasing costs or sacrificing quality. In the

past 30 years we have seen substantial improvements in the

manufacturing process; however, motivation for improved

production remains. In modern plants, the production

bottleneck is the hot press. Specifically, the press cycle is

limited by the time it takes to heat the center of a board to a

temperature that activates the thermosetting resin. This time

increases in a nonlinear fashion with board thickness. The

45- to 90-second warmup period characteristic of a

1/2-inch-thick board may be as long as 45 minutes for a

2-inch-thick board. Substantial reductions in thickboard press

times would create new, economically viable markets for

what is now considered a low profit item.

A process developed at the Forest Products Laboratory

shortens press times by injecting saturated steam directly

into the mat (Geimer 1983) (fig. 1). The process reduces

press times for 1/2-inch-thick flakeboard by 60 percent, from

4 minutes to 1-1/2 minutes, without degrading internal bond

(IB) or flexure properties (Geimer 1982). Press-time

reductions of 90 percent are possible when using this

steam-injection pressing system with thicker boards.

In past work, we developed the principles of steam-injection

pressing using Douglas-fir ring flakes and phenolic resin.

Research was mainly restricted to the fabrication of

l/e-inch-thick boards having a specific gravity (SG) of 0.640.

The work described here was conducted using an aspen

furnish and an isocyanate binder and extended the

application of steam-injection pressing to other board

thicknesses and SG’s. Of particular interest was the

manipulation of pressing variables (i.e. steam flow, steam

time, closing rate, etc.) to enhance the relationship between

energy consumption and total press times.

Figure 1.—Saturated steam is injected into mat

through top and bottom perforated platens during

press closure. (ML85 5071)
Experimental Procedures

Board Fabrication

Flakeboards were constructed to three thicknesses and two

levels of SG.

Thickness SG

1/2 inch 0.640-0.801

1-1/4 inches 0.560-0.721

2 inches 0.480-0.640

The assignment of SG levels was based on current product

trends wherein SG decreases with increasing board

thickness. For each board thickness and SG level, a series

of from five to nine individual flakeboards were made at

varying press and steam times. No particular set of

conditions were replicated for the purpose of establishing

statistically significant board properties.

The furnish consisted of a 50:50 mixture of 0.020-inch-thick

by e-inch-long by random-width aspen ring flakes and

0.020-inch-thick by 2-1/2-inch-long by random-width aspen

disk flakes. With the exception of a few 1/2-inch-thick 0.801

SG boards made with 7 percent isocyanate, all boards were

constructed using 3 percent isocyanate resin based on

ovendry (OD) wood. Moisture content of the mat prior to

pressing was below 4 percent (OD wood basis).

The board dimensions of 25 by 29 inches provided a

1-1/2-inch margin beyond the periphery of the steam platen

perforations to act as a steam seal. The steam-injection

platens, perforated with 3/32-inch-diameter holes on a

1/2- by 2-inch spacing, were attached to the regular

oil-heated platens which in turn were maintained with few

exceptions at a temperature of 375 °F. The steam-injection

system is detailed in a previous report (Geimer 1982).

Press Control and Data Collection

The press was programmed through a computer, which in

conjunction with the press’ electronic system, provided

excellent control of platen position and board pressure

(Geimer et al. 1982). The computer also performed auxiliary

functions of steam valve actuation and data collection.

Variables measured included board temperature, hydraulic

pressure, press platen position, steam flow, and steam

platen manifold pressure and temperature. Board

temperature was initially measured only at the mid-thickness

‘of the board’s center location. Measurements were later

expanded to include edge and corner positions. During the

critical portion of the press cycle beginning some 5-10

seconds prior to steam injection and lasting for a total of

45-60 seconds, information was recorded every 0.5 second.

Thereafter, the recording time was lengthened to between

1 to 5 seconds. Within 5 minutes after press opening, data

were available in the form of both a hard copy readout and

graphs of selected variables versus time. Press schedules

were altered after each pressing to provide what was

believed would be the most useful information in regard to

press times, steam-injection schedule, total energy

consumption, and visual board quality.

Testing

Cutting diagrams for test specimens are shown in figures 2

and 3. The samples were tested for internal bond (IB) and

bending modulus of elasticity (MOE) and modulus of rupture

(MOR) according to procedures outlined in ASTM D

1037-72A (American Society for Testing and Materials

1976). Linear expansion (LE), thickness swelling (TS), and

water adsorption (WA) were measured after each

successive exposure to ovendry (OD), 65 percent relative

humidity, vacuum pressure soak (VPS), and a second

ovendrying condition. In addition, density gradients were

determined with a gamma ray density measuring device’

(Laufenberg 1984). Corner or edge delaminations prevented

complete testing of some boards.

1

Laufenberg, T. L. Using gamma radiation to measure density gradients

in reconstituted wood products. Manuscript in preparation. U.S.

Department of Agriculture, Forest Service, Forest Products Laboratory,

Madison, WI; 1985.

2
Figure 2.—Cutup diagram for 1/2-inch-thick

boards. Sample size: Internal bond (IB) 2

by 2 inches: modulus of elasticity (MOE)

and modulus of rupture (MOR) 3 by

13 inches: thickness swell (TS), linear

expansion (LE), water absorption (WA) 3

by 12 inches; and density gradient 2 by

10 inches. (ML85 5073)

Figure 3.—Cutup diagram for 1-1/4- and

2-inch-thick boards. Sample size: same as

figure 3 except modulus of elasticity (MOE)

and modulus of rupture (MOR) 3 by

26 inches. (ML83 5072)

Discussion

I chose aspen for this study because of its present

widespread use in structural flakeboards. Boards from this

low-density species generally are manufactured to a

relatively high compaction ratio (ratio of board density to

species density) and are prone to moisture release problems

in the form of blows or blisters. The extent of this problem is

of major interest in the investigation of steam-injection

pressing, where additional moisture is added during the

press cycle. Disk flakes were mixed with the ring-flake

furnish to intensify the problem of moisture release. Although

relatively new to the particleboard industry, isocyanate resin

was chosen for this study because exploratory work showed

the resin’s characteristic of fast cure at low temperatures to

be well suited to steam-injection pressing.

Pressing variables and board properties are shown in

tables 1-6. The board number indicates the month, day, and

sequence of board manufacture. To make comparisons

easier, the board variations are arranged in descending

order of (1) press time and (2) energy consumption rather

than chronologically. Press schedules shown here do not

necessarily represent minimum or optimized conditions but

rather were chosen to ascertain the latitudes and limitations

of the steam-injection system. Total press times were kept

relatively short to emphasize the effect of the other

variables.

During the course of the experiment, it became apparent

that a set of press conditions that worked well with one

combination of board thickness and SG was not necessarily

correct for another board type. Although considerable

changes were made in press schedules, they were all

intended to meet the basic criteria of steam-injection

pressing, i.e., introduction of steam prior to the mat reaching

a SG of between 0.416 and 0.448 and development of

212 °F temperature in the mats centerline prior to the

partially compressed mat reaching a SG of 0.577

(Geimer 1983). Failure to obtain these conditions in a few

cases proved disastrous to bond formation.

With the exception of steam flow rates and steam energy

consumption, all pressing data shown were taken from

actual computer-monitored information. Steam flow rates are

target values based on valve settings and flow

measurements made with an open press. Resistance to

steam flow because of mat compaction actually, causes a

gradual reduction of steam flow during the press cycle.

Steam energy consumed by boards made in the early

stages of the experiment was calculated using target steam

flow rates and the programmed time schedule. Energy

values for boards numbered higher than 5/4 were calculated

using computer-measured flow rates. Energy values are

expressed as Btu’s per pound (Btu/lb).of OD board and are

referred to in the text as Btu’s.

Values in the column entitled “entry specific gravity” give the

actual SG of the mat at the time steam was first introduced.

In most cases, steam introduction was programmed to start

at a specific mat density, determined by the computer

monitored press position. Another method, which indirectly

reflects mat density and proved to be repetitive, initiated

steam flow when a selected hydraulic pressure was

reached.

3
Results

Equipment modifications made during the course of the

experiment enabled steam flow to be changed between two

rates. This ability to change flow rates permitted a finer

degree of optimization of press time and energy

consumption. With thick boards, high flow rates are often

needed to achieve fast temperature rise in the board. In

some cases, once the maximum temperature has been

reached in all sections of the board, temperature can be

maintained with reduced steam flow. Conversely, with

thinner boards it sometimes is advantageous from an energy

standpoint to introduce steam at low flow rates and increase

flow when the mat SG becomes higher. In this way, higher

temperatures can be reached.

Press closing rates are given in distance rate change (in./s)

and average SG rate change (ΔSG/s). these figures are

taken from the monitored data and reflect any deviation from

the programmed events caused by either controller

characteristics or dynamics of the system. Note that press

closure rates as measured by changing distance do not

follow the same pattern as those calculated from changing

SG. The difference reflects the nonlinear relationship

between mat SG and mat thickness (fig. 4). The rate of

change in SG may be more meaningful than distance rate

change in explaining the effects of press closing rates on

board characteristics. Consideration should be given to this

method of monitoring and controlling closure in the

development of modern press schedules.

Total press time as referred to herein is the elapsed time

between initial introduction of steam and final press opening.

In most cases, total press time included a lo-second

decompress period. When pressing 1/2-inch-thick 0.640 SG

board, approximately 10 seconds were needed to close from

a position where both platens touched the mat to the point

of steam injection. This period increased to approximately

20 seconds for a 2-inch-thick board.

Figure 4.—Specific gravity (SG) is a nonlinear

function of mat thickness. SG = K/t where

t = thickness and K = weight/area. Relation is shown

for a 1/2-in-thick 0.640 SG board. K = 0.032.

(ML85 5074)

For those unaccustomed to steam-injection pressing

techniques, the difference of a few seconds may seem

trivial. The response of variables to changes in the press

cycle, however, is often rapid and decisive. Data given

herein, although not showing all the changes occurring

within the boards, will provide an understanding of the

parameters of steam-injection pressing. Close scrutiny of

tables 1-6 will indicate the sensitivity of the system.

1/2-Inch-Thick 0.640 SG Boards

(Table 1)

No problems were encountered in manufacturing the

1/2-inch-thick 0.640 SG boards except when press time was

reduced to 22 seconds or less. A press schedule that

worked well with this group of boards called for an early

(0.260 SG) introduction of steam at a medium flow rate

(540 lb/h). Later, when the mat had been compressed

considerably (0.500 SG) and higher temperatures could be

reached, the steam flow was increased (board 5/24a). In

several cases, even more energy was saved by interrupting

the steam flow for a few seconds until the mat was

compressed (5/25 and 5/24d). This interruption of steam

flow works well with boards having a pressed SG less than

0.640. Problems with steam reentry are likely to be

encountered in boards whose final SG is above 0.640.

Temperature in the 1/2-inch-thick 0.640 SG boards was

monitored in all three positions: center, edge, and corner. In

most cases there is little difference between maximum

temperatures reached at the different locations. The

temperatures are well above the activation point,

approximately 220 °F, for the rapid curing of an isocyanate

resin. Data given under the table heading “temperature

delay” show the delay in seconds from the time steam was

introduced to the point where the temperature in the center

of the board at midthickness begins to rise. This time is

approximately 1 second for 1/2-inch-thick board but may be

as long as 8 seconds in a 2-inch-thick board. As a result of

the fast temperature transfer, all 1/2-inch-thick boards had a

uniform density profile across their thickness. Maximum

temperatures are reached during the period of steam flow or

shortly thereafter and decline rapidly to a level between 225

and 250 °F when the steam is shut off. Temperatures

stabilize at this level for the remainder of the press cycle.

A blister formed on the surface of the board (5/24b) made in

22 seconds using steam energy of 158 Btu/lb of OD board

(Btu’s). Board 5/24e, made in 22 seconds using 101 Btu’s,

did not blister but had more out-of-press springback and

showed a decline in board properties. The board (5/25a)

made in 13.5 seconds had a slight delamination along one

edge, which proved to be characteristic of short press cycles

or poor temperature distribution and indicated insufficient

resin cure. Data show that l/Pinch-thick 0.640 SG boards

can be made in 60 seconds using 99 Btu’s, or in 40 seconds

using 177 Btu’s steam energy. Bending properties of the

sanded and unsanded specimens were statistically identical.

Modulus of elasticity and MOR values given in tables 1-6

are an average of all bending specimens.

4
1/2-Inch-Thick 0.801 SG Boards

(Table 2)

Moisture release was the main problem encountered when

making these higher density boards. At a SG of 0.80

(compaction ratio of 2.1:1) blisters were prone to form on the

surface of the boards. Blisters result because of excessive

steam pressure disrupting the bond, a localized high

moisture spot preventing resin cure, or a combination of

both. Reduction of backup platen temperature did not help.

In an attempt to reduce the total steaming time, steam

injection of board 5/25d was delayed until a mat SG of 0.395

was reached. Because of poor steam penetration and

consequently low temperatures, this board contained

delaminations in addition to some blisters.

Major changes were necessary to prevent blisters. We

(1) increased resin content from 3 to 7 percent, (2) modified

the press schedule to introduce steam at a lower mat

pressure and reduce final closing pressures, and

(3) increased press times and steam energy consumptions.

A board, free of blisters and having good physical

properties, was made in 172 seconds using 373 Btu’s.

Successive trials showed that press times and energy levels

could be reduced. The minimum time used to press a

blister-free board was 152 seconds. One board (5/26e) was

made with an even shorter press time but an excess of

steam. Blisters developed, indicating that a longer time at

maximum temperature could not compensate for reduced

press time and excessive moisture. Therefore, when

pressing high-density boards, a balance between maximum

temperature duration and total press time must be

maintained. No attempt was made to produce boards at the

new press cycles with reduced resin content.

1-1/4-Inch-Thick 0.56 SG Boards

(Table 3)

Edge and corner delaminations in the 1-1/4-inch-thick 0.56

SG boards were common. The problem proved to be related

to poor temperature distribution which may be attributed to a

progressive pressure differential occurring from the center of

the board to its edges. The pressure drop is greater in thick,

low-density boards where steam can escape easily.

With the exception of board 5/23, temperature was

monitored only at the center position. Thermocouples placed

in board 5/23 showed a large temperature difference

between center, edge, and corner positions. Maximum

temperature decreased from 318 °F in the center to 245 °F

at the corners. Although this temperature differential is quite

large, a delamination-free board was pressed in

140 seconds using 155 Btu’s steam energy. It is assumed

that the temperature differential was even greater in those

boards where delamination occurred. Because of the design

of the platen steam passages, the temperature differential

was not symmetric, and delaminations occurred

predominantly along the same edge of each board. In those

cases where delamination was not severe, board properties

were tested. Internal bond data showed that the resin cure

was adequate in those portions of the boards receiving

ample heat.

Most of the good boards, i.e., those containing no

delaminations, were made using a high flow rate of steam

(720 lb/h), which continued for some time following closure

to final thickness. This permitted the temperature to rise to

curing levels in the board extremities. The effect of steam

flow rate can be seen by comparing results for board 4/14d,

steamed at 260 lb/h for 12 seconds, to board 5/23, steamed

at 720 lb/h for the same period. The low energy input of

55 Btu’s obtained with the lower flow rate was insufficient to

make a good board. Even when total energy input was

approximately equal, higher flow rates produced better

results (boards 4/15 and 4/18b). The minimum press time

attempted with the 1-1/4-inch-thick 0.56 SG boards was

60 seconds. A board having no delaminations was made in

this time using 370 Btu’s steam energy.

As mentioned previously, press conditions were deliberately

chosen to emphasize the effect of variables. This often

resulted in a very fine line between success and failure in

board production. Such was the case for boards 4/13 and

4/13a. The press cycle resulting in a board with good

properties (4/13) was slightly altered for board 4/13a. A

4-second decrease in closing time and an b-second

reduction of steam precluded resin cure and caused the

complete delamination of board 4/13a.

The press schedule including monitored rates of closure are

shown for both boards in table 3. Steam flow was adjusted

to 540 lb/h. Pressure, temperature, and position variables

are plotted against time in figures 5 and 6. For the purpose

of this description, zero time is designated as the moment

when manifold pressure and temperatures suddenly rise

indicating the entry of steam. At this point the SG of both

boards was recorded as 0.358. The difference between this

value and the programmed entry of 0.352 implies an error of

0.033 inch or 0.13 second, well within the accuracy limits of

the control equipment and recording devices. At zero time,

the pressure on board 4/13 was 265 pounds per square inch

(Ib/in.2

) and the pressure on 4/13a was slightly higher,

285 Ib/in.2.

5
Figure 5.—Proper selection of variables: 0.093 in./s closing speed and 16 seconds of steam

permit a 1-1/4-inch-thick 0.560 SG board (4/13) to be cured in 140 seconds press time.

(ML85 5078)

Figure 6.—Board delamination occurs when centerline temperature rise is delayed by fast

closure (0.192 in./s) and a reduced (8 s) steam time— 1-1/4-inch-thick 0.560 SG board

(4/13a). (ML85 5077)

6
The fate of the boards was decided in the next 8 seconds.

The pressure in the board made at the slower press closing

rate (4/13) was arrested for 2 seconds, indicating that steam

was penetrating and plasticizing the material. At the end of

6 seconds, the board had been compressed to a SG of

0.550 and board pressure peaked at 350 lb/in.2

. Centerline

temperature began to rise after 7.5 seconds and passed the

212 °F mark 9 seconds after initial steam entry. This point

(212 °F centerline temperature) occurred 0.5 second after

the press had reached its minimum opening and the board’s

SG was 0.575, very close to the upper limit of 0.577 defined

as critical in the basics of steam pressing. Centerline

temperature climbed to a maximum of 315 °F by the end of

the 16-second steaming period. Sixty seconds into the press

cycle the centerline temperature had dropped to, and

stabilized at, 240 °F. Board pressure at this time was only

35 Ib/in.2

. The induced heat was sufficient to accelerate the

resin cure, and a board free of delamination was produced

in 140 seconds.

In contrast, steam failed to penetrate to the center of board

4/13a. The faster closing rate, 0.193 versus 0.093 in./s,

permitted only a slight alteration in the rate of pressure

increase during closure, and a maximum board pressure of

570 Ib/in.2

 was reached 4.5 seconds after steam

introduction. This point of maximum pressure occurred

0.5 second prior to the board reaching a maximum SG of

0.575. The combination of the fast close and a short,

8-second steam period precluded an early rise in centerline

temperature. Sixty seconds elapsed before temperatures

finally surpassed 212 °F. The 80 seconds remaining in the

press cycle were insufficient to allow the resin to cure. The

board completely delaminated.

Steam injection not only provides a stabilized resin-curing

temperature (approximately 225 °F) sooner than

conventional pressing but also accelerates the process by

exposing the board to elevated temperatures for a short

period of time. Contrast the resin-curing, time/temperature

relations occurring in board 4/13a with those that took place

in the 1/2-inch 0.640 SG board 5/24d (table 1 and fig. 7).

Low levels of steam energy, 80 and 99 Btu’s, were used to

manufacture both boards. Centerline temperatures stabilized

between 230 and 235 °F in each case. However, only

55 seconds above a temperature of 212 °F were sufficient to

complete the resin cure in board 5/24d after a very short

exposure to a temperature of 300 °F had accelerated the

process.

1-1/4-Inch-Thick 0.721 SG Boards

(Table 4)

The heat transfer problem caused by easy escape of steam

through the edge of a low-density thick board is replaced by

an entry problem when the target board density is increased.

An increase in mass demands that more time be allowed at

lower mat densities to obtain adequate steam penetration.

Timing is critical in achieving optimum heat transfer.

A press closing schedule that worked well with the

1-1/4-inch-thick 0.721 SG board type incorporated closure

rates of approximately 0.12 in./s during initial steam entry.

After centerline temperature had risen above 212 °F, closure

speed was increased to compress the mat rapidly and

achieve maximum temperature with a minimum use of

steam. When steam injection was postponed beyond a SG

of 0.260 as in boards 4/18c and 5/23a, the delay in

centerline temperature rise resulted in severe board

delamination. Press programs in both of these cases

incorporated low or medium steam flow rates. Edge and

corner delaminations in boards 4/21c and 4/21d were also

attributed to low steam flow rates and the accompanying

temperature differentials.

Resin cure as discussed earlier is a function of time as well

as temperature. The corner delamination in board 5/23c is

attributed to a very slow rise in temperature at this position.

While it took only 5.5 and 9.0 seconds, respectively, to

reach 220 °F in the center and edge positions, 66 seconds

elapsed before the temperature passed 220 °F in the corner

(fig. 8).

In contrast, all monitored positions-center, edge, and

corner-of a similarly pressed but fully cured board (5/23b)

passed the 220 °F mark within 15 seconds. Except for an

increase from 540 to 720 lb/h steam flow rate during the first

4 seconds of pressing board 5/23b, the press closing

schedules for both boards were identical. Results from this

portion of the study indicate higher steam flow rates permit

shorter press cycles by reducing temperature variations

within the board. When cure temperatures have been

reached in all sections of the board, they can be maintained

by a reduced flow. A 1-1/4-inch 0.721 SG board having high

bending properties and adequate IB was made in

71 seconds using 167 Btu’s steam energy.

2-Inch-Thick 0.480 SG Boards (Table 5)

No problems were encountered in pressing 2-inch-thick

0.480 SG boards. Press schedules were compromised

between steam flow rates, entry SG, and press closure rate

to provide adequate steam penetration while preventing

excessive resin precure or steam energy consumption.

Moderate steam flow rates of 540 lb/h were used during the

initial period of steaming. Because of the mass and

thickness of the mat, the delay in center temperature rise is

relatively long, 4-8 seconds. Corner position temperature

rise is usually delayed an additional 2-5 seconds. 

7
Figure 7.—A brief exposure to maximum temperatures of 325 °F permits resin cure at 235 °F

in 60 seconds press time, 1/2-inch-thick 0.640 SG board (5/24d). (ML85 5076)

Figure 8.—Delay in corner temperature rise results in delamination of this portion of the

board, 1-1/4-inch-thick 0.721 SG board (5/23c). (ML85 5075)

8
Summary

A high steam flow rate (720 lb/h) was used in some portion

of each press cycle to reduce within-board temperature

differentials. Maximum temperatures at the corner position of

the various boards ranged from 227 to 270 °F. A board

made in 150 seconds with 111 Btu’s steam energy had

acceptable board properties. Press time was shortened to

55 seconds when steam energy was increased to 204 Btu’s.

Density profiles were uniform in all boards except 5/19b and

5/20. Press cycles specified a late steam entry and a shorter

steam time for both of these boards. Density levels varied

from a plus 30 to a minus 20 percent of average.

2-Inch-Thick 0.64 SG Boards (Table 6)

High flow rates were again used in manufacturing the

2-inch-thick 0.64 SG boards. A low steam flow rate in the

initial period of steaming was disastrous and caused

complete delamination of board 5/2Oc. Press times were

successfully reduced from 320 to 201 seconds in boards

5/3a, 5/3b, and 5/20b using steaming schedules that

provided between 185 and 200 Btu’s energy. Change to a

faster closing rate and overpressing slightly to compensate

for springback resulted in lower corner temperatures and

caused corner delamination in board 5/3c.

Two inches is about the maximum board thickness that can

be pressed with the described steam-injection equipment.

Steam passing through the face layers of a thicker board

escapes through the edges before it can reach the core.

increasing steam-line pressure would increase the practical

thickness range for steam-injection pressing in addition to

alleviating some of the temperature differential problems in

thinner boards. Means to prevent large temperature

differentials from occurring throughout the board should be

considered in any scale-up to commercial size panels.

A pressing method utilizing saturated steam injected directly

into the mat during the closing period can be used to

accelerate the cure of 2-inch-thick isocyanate-bonded aspen

flakeboards. Pressing schedules and resultant board

properties given here show the latitudes and limitations of

the system for boards of various thicknesses and SG’s. A

2-INCH-THICK 0.640 SG BOARD WAS MADE IN

201 SECONDS using added steam energy of 185 Btu/lb of

OD board (Btu’s). One-half-inch-thick 0.640 SG boards can

be made in 60 seconds using 99 Btu’s. In some cases

increasing the amount of steam energy permitted press

times to be reduced. A 1/2-inch-thick 0.640 SG board was

made in 40 seconds using 177 Btu’s. In other cases,

especially involving high SG boards (0.800) which are prone

to blister, a balance between press time and energy input

must be maintained.

Corner and edge delaminations were traced to temperature

differentials occurring at these extremities and are believed

to be caused by a pressure drop between the center and

edge of the boards. This problem, which is more severe in

thicker, low-density boards, can be reduced by the use of

high steam flow rates.

Steam flow rates can be changed during the press cycle to

optimize press time and energy consumption. Each board

thickness and SG combination require a different press

schedule. Whereas increasing the flow rates during

steaming time is advantageous to thin boards, the reverse is

true for some SG levels of thick boards.

The upper thickness limit of steam-injection pressing using

the described equipment is about 2 inches. Steam passing

through the face layers of a thicker board escapes through

the edges prior to reaching the centerline. The basics of

steam-injection pressing using saturated steam in an

unsealed system were found to apply to all ranges of board

thicknesses and SG: steam must be introduced prior to the

mat reaching a SG level of 0.448, and a centerline

temperature of 212 °F must be reached before the mat is

compressed to a SG of 0.577.

9
Literature Cited

American Society for Testing and Materials. Standard

methods of evaluating the properties of wood-base fiber and

particle panel materials. ASTM D 1037-72A. Philadelphia,

PA: ASTM; 1976.

Geimer, R. Method of pressing reconstituted lignocellulosic

materials. U.S. Patent 4,393,019. 1983 July 12.

Geimer, R. Steam injection pressing. In: Proceedings, 16th

international particleboard symposium; 1982 March

30-April 1; Pullman, WA. Pullman, WA: Washington State

University; 1982: 115-134.

Geimer, R.; Stevens, G.; Kinney, R. Automation of a

laboratory particleboard press. Forest Products Journal.

32(4): 34-36; 1982.

10
Table 1.—Pressing schedule and Physical properties of 1/2-inch-thick 0.640 specific gravity steam-lnjected boards

Pressing variables Board properties

Board Total

number press Steam

time energya

Steam flow Maximum temperature Pressure Moisture Bending Linear expansiong 

contentf

Specific

gravity Entry Tempera- Modulus Modulus

specific Time Rate at end Closure ratec ture Center Edge Corner Steam Maximum In Out Thick- Specific of of Internal 65 percent Vacuu

gravityb period of entrye ness gravity elastlcity rupture bond relative pressu

delayd humidity soak 

period

a

Steam energy in Btu/lb of ovendry board.

b

Mat specific gravity (SG) at the time of initial steam flow.

c

ΔSG/s = average change in SG per second for the period. d

Time between steam entry and temperature rise in the center of the board.

e

Mat pressure at the time of initial steam flow.

f

Moisture content of mat going in and coming out of the press.

g

Values based on preliminary OD conditions.

11
Table 2.—Pressing variables and physical properties of 1/2-inch-thick 0.801 specific gravity steam-injected boards

Pressing variables Board properties

a

Steam energy in Btu/lb of ovendry board.

b

Mat specific gravity (SG) at the time of initial steam flow.

c

ΔSG/s = average change in SG per second for the period. d

Time between steam entry and temperature rise in the center of the board.

e

Mat pressure at the time of initial steam flow.

f

Moisture content of mat going in and coming out of the press.

g

Values based on preliminary OD conditions.

12
Table 3.—Pressing variables and physical properties of 1-1/4-inch-thick 0.560 specific gravity steam-injected boards

a

Steam energy in Btu/lb of ovendry board.

b

Mat specific gravity (SG) at the time of initial steam flow.

c ΔSG/s = average change in SG per second for the period. d

Time between steam entry and temperature rise in the center of the board.

e

Mat pressure at the time of initial steam flow.

f

Moisture content of mat going in and coming out of the press.

g

Values based on preliminary OD conditions.

13
Table 4.—Pressing variables and physical properties of 1-1/4-inch-thick 0.721 specific gravity steam-injected boards

Pressing variables Board properties

a

Steam energy in Btu/lb of ovendry board.

b

Mat specific gravity (SG) at the time of initial steam flow.

c

ΔSG/s = average change in SG per second for the period. d

Time between steam entry and temperature rise in the center of the board.

e

Mat pressure at the time of initial steam flow.

f

Moisture content of mat going in and coming out of the press.

g

Values based on preliminary OD conditions.

14
Table 5.—Pressing variables and physical properties of 2-inch-thick 0.480 specific gravity steam-injected boards

Pressing variables Board properties

a

Steam energy in Btu/lb of ovendry board.

b

Mat specific gravity (SG) at the time of initial steam flow.

c ΔSG/s = average change in SG per second for the period. d

Time between steam entry and temperature rise in the center of the board.

e

Mat pressure at the time of initial steam flow.

f

Moisture content of mat going in and coming out of the press.

g

Values based on preliminary OD conditions.

15
The Forest Products

Laboratory (USDA Forest

Service) has served as the

national center for wood

utilization research since

1910. The Laboratory, on the

University of WisconsinMadison campus, has

achieved worldwide

recognition for its

contribution to the knowledge,

and better use of wood.

Early research at the

Laboratory helped establish

U.S. industries that produce

pulp and paper, lumber,

structural beams, plywood,

particleboard and wood

furniture, and other wood

products. Studies now in

progress provide a basis for

more effective management

and use of our timber

resource by answering critical

questions on its basic

characteristics and on its

conversion for use in a variety

of consumer applications.

Unanswered questions remain

and new ones will arise

because of changes in the

timber resource and

increased use of wood

products. As we approach the

21st Century, scientists at the

Forest Products Laboratory

will continue to meet the

challenge posed by these

questions.