The formation characteristics of the vertical density profile of MDF are presented herein. Results of laboratory studies indicate that the vertical density profile of MDF is formed from a combination of actions that occur both during compaction and also after the press has reached final position. The methodology, which was used to describe the formation of the density profile for OSB that used two periods and five stages, can also be used to describe the density profile formation of MDF mats. There was a clearer surface layer consolidation stage for MDF mats when compared to OSB mats. At least 58% of the area in an MDF mat was in "spring status" when the press reached 120% of final panel thickness. The observed stress-strain response of MDF mats in hot-pressing with the one-step closing schedule was similar to OSB pressing. This was characterized by a long stress plateau followed by a rapid increase in stress, followed by an immediate reduction in stress after the press reached final position. Compared to OSB hot-pressing, the same pressing schedules (traditional or step-closure) did not result in similar MDF density profiles. It appears that high-density surface layers are easier to create in MDF than in OSB.

Industrially manufactured oriented strandboard (OSB) furnish was characterized by scanning images of a large number of individual strands and analyzing their shape properties. A Visual Basic macro in combination with commercially available image synthesis software was utilized to carry out the task of this analysis. The scanned images of these strands were used in a model that simulates the formation of layers within an OSB mat. These layers were simulated using three different orientation scenarios: random orientation. 100% strand alignment, and strand alignment based on industrial parameters. Information provided by the model includes the number and geometrical details of voids and strand overlap. Total void area for a mat is shown to be independant of strand orientation and the aspect ratio of strands. Interestingly, noticeable differences in the number, size, orientation, and shape factor of the individual voids, which make up the total area, were shown between various mat configurations.

The hot press is a key piece of equipment in composite panel production. The press influences processing efficiency of the whole production line and the performance of products. The objective of this study was to nondestructively monitor the bonding development of particleboard during hot-pressing using a dielectric system.With all other variables held constant, laboratory particleboard pressed with 20-s closure time reached the initial conductivity peak and the conductance valley the earliest and had the highest value at the initial conductivity peak and the conductance valley versus other closing times studied. There were strong relationships between the impedance signal and panel strength developments. There were significant effects of panel thickness on the characteristics of impedance curve.

A nondestructive optical technique was developed to determine thickness swell of discrete layers within intact samples of wood composites. Layer thickness swell of commercial medium density fiberboard (MDF) and oriented strandboard (OSB) are presented. Layer swell within the sample is important in understanding the swell phenomena of wood composites. Results from standard specimens show edge layer thickness swell after 2-,8-, and 24-hour water soak and include measures of precision and variation for this technique. The relative difference for overall thickness swelling compared between the optical technique and the traditional method decreased as water exposure time increased and was less than 3.44 percent after 24-hour water exposure. The contributions of high-density surface layers to overall MDF thickness swell were 95.76, 75.50, and 61.77 percent after 2-, 8-, and 24-hour water exposure, respectively. The contributions of high density surface layers to overall OSB thickness swell were 74.36, 64.39, and 57.30 percent after 2-, 8-, and 24-hour water exposure, respectively. Thickness swell for both products was dominated by the high density surface layers throughout the 24-hour soak cycle. However, dense surface layers contributed more to the overall swell measurement during the early period of the soak cycle compared to the swell measurement at the completion of the 24-hour soak cycle. The relative contribution of the core layers to overall thickness swell increased with length of exposure period. This optical technique is recommended for the. measurement and study of in-situ layer swell properties for all wood composite panel materials.

Dimensional instability of wood composite panels remains a key area of interest for many manufacturers. Cost-effective measures to minimize moisture-induced swelling of these hygroscopic products, while still retaining acceptable levels of mechanical strength, would provide obvious advantages within the highly competitive marketplace. A series of laboratory flakeboard panels were manufactured with the inclusion of discrete layers of furnish that had been treated with acetic anhydride. Acetylation is a treatment known to reduce the swelling and water absorption behavior of wood. Using treated material in discrete layers of the mat targets those layers of known significant thickness swell in a manner that may economically improve overall dimensional stability. A previously developed technique to determine the swelling of discrete layers within a panel was used to identify the contribution of a treated layer relative to the overall thickness swell for the whole panel. Results conclude that the thickness swell of oriented strandboard can be improved significantly by including discrete layers of acetylated strands when these layers are positioned appropriately in the mat. Acceptable internal bond strength was achieved for treated panels. Also, a semi-empirical model was used to verify the effective coverage area of the strands forming the discrete layers. The total uncovered area and the proportion and number of overlapping strands are included in the data output by the model.

The hot-press is undoubtedly a key piece of equipment in composite panel production. Process control of the pressing procedure can be divided into preprocess, in-process, and postprocess measurement and control stages. This paper presents an in-process measurement and control technique during the pressing cycle and discusses application possibilities based on the study. There are different curing conditions between the core layer and the surface layers of a panel, resulting in the core layer always having higher in-press expansion than the surface layers. This in-press expansion can be monitored by measuring in-situ density changes as the press platens open. It is proposed that when a predetermined value of controlled expansion is reached, the press process controller can immediately stop normal press opening and prolong the pressing cycle to ensure sufficient resin cure and prevent substandard production. Results showed that both the prolonging and secondary closing procedures could improve panel performance and prevent substandard production. In-process measurement and control of hot-pressing is made possible by monitoring internal density changes during press opening.

The process of bonding wood is a complex interaction of physical and chemical variables. The bonding of wood elements in a hot-pressed composite panel is a recognized complex phenomenon that includes polymerization of the resin and chemical reaction of the resin with the wood substrate. The bonding phenomena in hot-pressed composites occur under varying environmental conditions that include dynamic temperature, relative humidity, moisture content, pressing pressure, and steam pressure. During pressing, the wood furnish elements are not in a steady state of contact, due to significant consolidation changes that occur throughout the mat during the entire press cycle, even after the press reaches final position. Our previous research showed that the vertical density profile of wood composites is formed from a combination of actions that occurs both during consolidation and also after the press has reached final position. Recognition of the consolidation changes during pressing implies that there is not a steady state for resin bonding during pressing. This work reports on the nature of the unsteady contacting phase that exists during hot pressing and explains some phenomena relating to manufacturing processes of wood composites and end-product panel properties. The effects of the unsteady contacting phase on internal bond strength of medium density fiberboard are presented. An in-press impedance measurement was used to correlate the bond cure development with the unsteady contacting state of a particle-board mat during hot pressing.

The purpose of this study was to evaluate compression and swelling characteristics of individual furnish elements sampled through the thickness of lab panels pressed without resin. Commercial southern pine OSB furnish was used to press resinless mats so individual flakes could be removed from the panel after pressing and evaluated for compression behavior. 19 flake sets, each set consisting of 15 southern pine flakes with 0.65% wax, were marked and measured for thickness and mass. One set of marked flakes was randomly distributed in one layer of a mat which consisted of 19 total layers; each of the 19 layers had 15 marked flakes randomly distributed in the layer. After hot pressing each marked flake was removed from the mat. After achieving equilibrium at 35%, 65% and 98% relative humidity, each flake was again remeasured for thickness and mass. Experimental results include flake compaction ratio and its distribution through the mat thickness, flake thickness swelling under different RH environments, compaction ratio-thickness swelling relationship as well as individual flake compaction ratio and thickness swelling variations. Comparison is made to adsorption/desorption behavior of pressed flakes. Flakes from surface layers exhibited compression of 25 to 37%, about double that of flakes in core layers. As expected, flakes from surface layers showed much greater thickness swell than core flakes and the response was accentuated with higher EMC conditions.

The vertical density profile (VDP) is an important panel property that describes the change in density through the panel thickness. The panel industry has effectively used press closure rate to manipulate the VDP to alter product performance. Using a slow press closure rate to manipulate the density profile has some obvious limitations related to resin precure, density profile symmetry, and total press cycle time. Traditional press schedules consist of a single closing step until the press reaches final board thickness or position. In this research, OSB mats were pressed using both two- and three-step closing schedules. Mats were pressed to 110, 105, or 95 percent of target panel thickness and held at that position for times ranging from 20 to 120 seconds before closing to final thickness. For the three-step schedules, the mat was compressed to an additional intermediate position before final closure. The step closure schedules significantly changed the traditional shape of the vertical density profile of the laboratory-made OSB panels. The step schedules resulted in multiple surface densification peaks of the VDP rather than the traditional two density peaks of the VDP associated with conventional pressing. Step pressing schedules are one processing method that can be used to alter the densification process in the panel and subsequently influence panel physical properties. Results are unique in that both in-situ density profiles and density profiles measured after pressing are shown for different step-closure conditions and a comparison is made to traditional closing schedules. The physical properties for panels produced from each step-closure schedule are also shown. The step-closure procedure is one method that can be used to balance the structure of the panel density within the mat and in the resulting panel.

To achieve a more fundamental understanding of material behavior during the pressing process, a radiation-based system for measuring density of wood composite mats during consolidation is used to build in-situ cross-sectional density distributions of flakeboard mars with pressing time. The fundamentals of densification within flakeboard mars during hot and cold pressing are discussed in this paper. The pressing schedules included theoretical laboratory pressing schedules and schedules simulating industrial pressing. All tests were conducted at either ambient or 204 degreesC temperature. The results include stress relaxation of flakeboard mats during cold and hot pressing, stress-strain behavior, insitu density-strain behavior, and in-situ cross-sectional density distributions of flakeboard mats with Dressing time. Results of laboratory studies indicate that the stress relaxation during hot pressing after the press reached final position was much quicker than during cold pressing. The observed stress strain responses of flakeboard mats in hot pressing and cold pressing were similar, characterized by a long stress plateau followed by a rapid increase in stress and an immediate fall-down after the press reached final position. The process to simulate the industry operation resulted in another stress plateau. The stress-strain responses of flakeboard mats were characterized by a long stress plateau followed by a rapid increase in stress, and an additional high stress plateau followed by an immediate fall-down after the press reached final position. There was no clear indication that the maximum gas pressure attained is affected by press closing time.

We have designed a radiation-based system for measuring density of wood composite mats during consolidation. The system is installed on a laboratory hot-press and has been used to study consolidation of medium density fiberboard (MDF) and oriented strandboard (OSB) mats. Measuring density of the wood mat during consolidation is a key parameter for understanding subsequent product performance. The in-situ measuring system provides for density measurement at three horizontal planes in the wood mat, at positions of 25%, 50%, and 75% of the mat thickness at any time during the press cycle. The system incorporates three cesium 137 sources and electronic detection equipment, collimated to move in concert with the up-acting press platen. Radiation count data taken through the mat during pressing are converted to density after pressing. Press position and time are simultaneously recorded with the count data. Moisture migration during hot-pressing resulted in significant density changes as measured by the in-press radiation-based system. Clearly established in all laboratory pressing studies is the indication that the vertical density profile of wood composite panels is formed from a combination of actions that occur both during consolidation and also after the press has reached final position; measurements recorded in the press show that mat densification continues after the press has reached final position. A description of the radiation system and data from elementary pressing examples are presented, along with experimental results of the effects of moisture migration in the mat on measured density during pressing.

The vertical density profile or density distribution through the panel thickness has been identified as one of the important panel characteristics that correlates well with strength and physical properties of wood-based composite panels. We have studied the fundamentals of oriented strandboard (OSB) vertical density profile formation during hot-pressing. Experimental results are from the in-situ density measuring system installed on our laboratory hot-press. Results indicate that the vertical density profile of OSB is formed from a combination of actions that occur both during consolidation and also after the press has reached final position (i.e., thickness). We propose a methodology to describe the formation of the density profile into two periods and five stages. The consolidation period is the time of consolidation until the press reaches final position and contains two stages. The adjusting period is the time after the press has reached final position and continues until the culmination of the cycle. The adjusting period contains three stages. The resulting density profile is influenced by both periods and all five stages. The vertical density profile results from the combined effects of many process variables, but basically occurs from the effects of furnish moisture conditions, mat structure and the pressing environment. During pressing the mat is always in an unsteady state, and internal mat temperature, moisture content distribution, vapor pressure, layer density, and compaction stress are all related to the pressing operation. The unsteady state of the mat during the early stages of pressing may result in poor bonding strength development throughout the mat.

Southern pine and aspen are widely utilized species for oriented strandboard (OSB) production in North America. In general, aspen with a relatively low specific gravity and uniform cell structure arrangement is considered more suitable for composite panel manufacture than is southern pine. Aspen OSB typically is lower in density and has better dimensional stability when compared to pine OSB. The purpose of this study was to investigate the effects of species use and distribution within OSB panels on the formation of the vertical density profile, resulting layer thickness swelling, and end-product layer characteristics. Five species configurations were used in this study (by weight): all pine, all aspen, 50/50 percent mixture, 25 percent aspen faces/50 percent pine core, and 25 percent pine faces/50 percent aspen core. Results show that the shape of the vertical density profile was considerably affected by species acid species distribution. Resulting shapes of the vertical density profile were described as steep or gradual, referring to the densification of the face layers relative to the core. A steep profile has high density surface layers relative to the core and a gradual profile exhibits less difference between the face and core layers. The face density region of the density profile was further described as either a narrow density peak or a wide density peak, referring to the degree of density change within the face itself. The all-aspen panel had a steep density profile with a very narrow density peak. The 25 percent pine faces/50 percent aspen core panel had a more gradual density profile with a wide density peak. There was a strong relationship between layer density and layer thickness swelling. Physical and strength properties for all species combinations of panel types are reported.

Medium density fiberboard (MDF) experienced unprecedented capacity expansion in the mid-1990s. This expansion led to market competition never before seen in the MDF industry. Market competition and economic scarcity of raw material have led many MDF companies to redefine the method of MDF manufacture. Many companies have adopted the low-risk philosophy of continuous improvement as a strategy for long-term survival. Statistical process control (SPC) is the core principal of continuous improvement. This paper outlines continuous improvement strategies for MDF manufacture using SPC. Many SPC initiatives in MDF manufacture are unsuccessful because they lack focus on essential process variables that are linked to important product, attributes. Once these process variables for MDF manufacture are identified, additional obstacles for implementing SPC still exist. Most MDF is manufactured in continuous processes. An analysis of moisture data from one MDF process revealed autocorrelated data. Shewhart control charts of this data led to false signals of statistical control, i.e., 17 out of 20 out-of-control signals were false. A method for adjusting Shewhart control charts using the lag-1 autocorrelation coefficient is presented. Additional analysis of six product attributes for the same MDF manufacturing facility revealed that some attributes were highly correlated. Separate univariate Shewhart control charts of these attributes led to lack of detection of all special cause variation. A multivariate control chart of these six attributes using Hotelling's T-2 statistic was more accurate for defining statistical control.