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The benefits of continuous pressing.

THE BENEFITS OF CONTINUOUS PRESSING

Wood supplies are becoming scarce and other manufacturing costs such as electricity and fuel are rising. These are the facts in the manufacturing of board in North America today. To see into the future, look across the Atlantic and see how the Europeans are responding to similar challenges, where costs are much higher.

The question to be asked is whether to continue to work with technologies that have served the industry for over the past 30 years or is there something new, more efficient, more profitable? The Europeans are embracing the continuous press technology. It is the author's contention that this technology is not only the future, but economically viable today in the North American market.

The board industry

In North America, particleboard was introduced as a "cheap" substitute to replace plywood in the building industry. Later, the product found its way into the furniture and cabinet businesses. The manufacturing process technologies were imported from Germany and Switzerland and modified to accommodate the fiber quality conditions of the waste woods generated by North American sawmill and plywood industries. Electric energy costs were only a fraction of the costs found in other parts of the world. Natural gas and oil were so cheap and convenient to use that it was more cost-efficient to burn wood waste in teepee burners than it was to use it for process fuel or for the co-generation of electricity. Petro-based adhesives were acceptably priced for this so-called "cheap" substitute.

That was yesterday. Today the North American business environment has changed dramatically. The oil embargo in 1974; the increasing regulation of environmental factors; and new companion materials such as melamine resin-impregnated papers, foils, acrylic and other liquid finishes, which can be used directly on boards; changed the perception of the reconstituted product in the market place. Today, it is more widely recognized as a material which can be engineered, or customized with sophistication, towards its end use.

Equally significant is the fact that there are no longer low-cost raw materials available to make the product. The thermal values of wood waste, even if round wood must be purchased to supplement process waste for fuel, now determines wood cost.

The sawmills and plywood plants which previously generated such huge quantities of residues are improving their own primary processes to continually achieve much higher wood utilization. The waste that is still produced has such a low fiber quality that it burdens board manufacturing costs with high board densities, hence, high raw materials and shipping costs. Even if the market is willing to accept these cost penalties now, it is questionable how long it can continue to do so in light of other materials which can perform the same functions as boards made from wood waste. Even though raw material is inexpensive relative to other basic materials, it is important to continually improve on the costs and efficiencies to maintain that difference and, thus, the industry's place in the market.

The European board industry started from different circumstances. In western Europe, forest resources, generally, are sparse. Traditional homes are made from masonry, concrete and stone. Thus, there is no significant demand for wood composite building panels. A huge demand for furniture panels came as a result of the destruction of homes and furniture during World War II.

The particleboard industry began to grow very rapidly in countries such as Germany, Italy, France, England, etc. Wood, as whole log or pulp chips, was the primary raw material employed. However, since the 1960s this has changed dramatically. Today, many of these plants are using an increasing percentage of waste wood with extremely poor fiber quality and high prices, prices which can be in excess of 120 percent of what is paid in North America.

Hence, while North American and European areas started from different forms of raw material, fuel, and energy, they appear to be converging rapidly to a common point. European manufacturers have become more conscious of operational efficiencies, which is reflected in the equipment they purchase. Because of the high cost for materials, North America has become more aware of the cost penalties from excessive board densities, thickness tolerances and sanding. The point of convergence for the two areas is process efficiency.

Process efficiency

Process efficiency has been one of the major driving forces behind the success of the continuous press in Europe. Wood costs in Europe are 120 percent and energy costs are 40 percent higher than in North America. The obvious objective has been to make better board with less material.

What is meant by better board? It could be, for example, board with a lower density, but very fine, tight surfaces suitable for overlying with thin film or paper foils. Core density of the board might become a secondary consideration since face-mounted hardware is now readily available in North America. If, however, the board were intended for exposed edge machining, a more homogeneous density could be engineered into the product. Most industrial-grade boards made today have a much higher density due in large part to the inconsistent board thicknesses which come out of a multi-opening cycle press. The average physical properties of the press load are adjusted high enough so that the properties of the poorest board meets the established standards. Such board inconsistencies do not occur in a continuous press because the press can respond to every foot of board as the production parameters change.

With the traditional cycle press, substantial process gains have been made to accommodate changing raw material supplies in North America. Computerized controls have made it possible to apply the knowledge of how to refine wood fiber, accurately distribute adhesives and press panels into a product with more predictable and confined parameters. It has also been found that with traditional systems, each successive step toward improvement in manufacturing costs or product flexibility has become progressively more expensive. An increasing number of people in the board industry believe the next step must be in making major changes in the manner in which board is pressed.

Continuous pressing

There is substantial evidence now to support the opinion that continuous presses are not only economically viable, but offer process control capabilities never available before.

An example of the continuous press, the ContiRoll, is a flat-bed type. A continuous mat ribbon is pressed and cured into board as it passes through the press between two steel belts. A unique feature of the ContiRoll is the fact that the press is built within a sturdy girder system that absorbs all the dynamic stresses, thus eliminating the need for expensive pits and foundations.

Heat and pressure are transmitted from the stationary press platens to the steel belts through a bed of precisely calibrated solid steel rods. These steel rods are called a "bed of rods." The length of the rods exceeds the width of the press platens and steel belt, have extremely low frictional forces and allow the pressing of a mat substantially narrower than the steel belt.

New dimensions in the marriage of technology and the manufacturing system are possible due to the zonal strategy of applied pressure/temperature in the press allowing each zone to be independently adjusted in heat and pressure from the adjacent one. Different values for temperature and pressure are applied over the length of the press. This flexibility permits modification of some of the board properties while the board is being pressed. This is not possible with a cycle press.

ContiRoll's hydraulic cylinders are arranged into three basic zones: high pressure zone, intermediate high to low pressure zone and low pressure zone. The high pressure zone has all cylinders for each frame operating at a selected pressure to give the desired closure rate. The intermediate zone is set up so that the interior cylinders are independently adjustable from either set of outer cylinders. The intermediate zone has two segments. In the first segment, the outside cylinders are adjusted independently from the inside on paired frames. The second segment allows each outside pair of cylinders to be adjusted independently from the inside cylinders. This feature allows the application of differential pressure which automatically adjusts to the mat-changing parameters. This gives precise control of the board thickness out of the press. The last zone has all the cylinders on the frame adjusted together and this location is where the board is decompressed.

There are four possible scenarios for the entry section of the ContiRoll. The infeed section of the ContiRoll is adjustable by distance. This, in conjunction with the variable pressures settings of the frames, allows for the adjustment of the closure rate of the press to give an engineered density profile to the board.

Most essential to the success of the system are the controls to apply the logic of the process technology. The ability of such state-of-the-art controls to sense and meticulously self-adjust to the normal variation encountered in board manufacturing manifests itself in an extremely consistent out-of-press thickness. Even though continuous pressing is still in its early stages of development, the sophisticated hardware/software, plus their implementation, exists now. Because the hardware/software, and their implementation is available, the user's learning curve is shortened. The operation can be optimized sooner.

Continuous presses, particularly in North America, are viewed as only having the ability to produce thin board. This is a myth that needs to be dispelled. This perception comes from the time when the only continuous press available was the calender, or drum type, Mende press. It was made specifically for producing thin boards.

The penalties for producing thinner boards in a multi-opening press are four-fold:

1. Heavy-precure

2. Excessive sanding relative to board thickness.

3. Board warpage

4. Production loss due to excessive dead time

All of these problems are eliminated with the continuous press. Dead time is the sum of the time needed for loading, unloading, opening, and closing the press. It is, however, a critical factor in the manufacture of thin boards in a cycle press because it limits production.

Dead time becomes a greater percentage of the available press time as the press cycle becomes shorter to accommodate pressing thinner boards in a multi-opening press. A continuous press has no dead time. Two presses, making 1-inch board at the maximum, had their capacities plotted for each board thickness from 1 inch to 0.250 inch. The difference in production between the multi-opening and continuous press lines are the affect of "dead time." This increased profitability illustrates a major reason for a continuous press to run thin board.

Europe is producing a full range of board thicknesses on these continuous presses. Eighty-five percent of the board produced exists within the thickness range of 3/8 inch to 1 inch. Extremes of thickness such as 1/10 inch to 1 1/2 inch are also manufactured. In looking at the North American market, when either particleboard or MDF are considered, the shipment records of the National Particleboard Assn. show that 100 percent of the thicknesses shipped in North America over the last six years fall within the range of that produced by the ContiRoll press. For North America, 92 percent of the particleboard shipped and 80 percent of the MDF shipped, fall within the 3/8-inch to 1-inch board thickness range found in Europe. This point is raised to demonstrate that while the continuous press is unquestionably the best machine on which to make thin board, it has proven itself as a truly universal press with the capability to produce an extremely wide range of thicknesses from 1/10 inch to 1 1/2 inch.

Attributes of continuous pressing

The following describes some of the attributes found with a continuous press:

1. Using the continuous press means that the time from blending to pressing is the same for every inch of the board. This provides for the opportunity to optimize the production variables such as resin and moisture, since they do not have to be averaged because of mat stand time, as is found in a multi-opening press.

2. Closure rates can be varied and thus, a unique density profile can be engineered for the board.

3. The ContiRoll continuous press system forms its mats on a fabric belt. The mat only contacts the hot steel belt when being hot pressed. This gives the board a smooth hard surface on both sides which are ideal conditions for a balanced construction that prevents warpage.

4. The continuous press can detect and respond to the changing parameters of the mat such as density, moisture content, etc. This gives a very consistent product out the end of the press.

5. The technologically optimum conditions of the ContiRoll allow for board to be produced with equivalent physical properties, but at a substantially 3 percent lower density than board made in multi-opening press systems.

6. With a continuous press there is minimum side trim for the product.

7. The continuous press produces a continuous ribbon of board. This means that the ability exists to cut the exact length of board desired from this ribbon without producing any end trim. By totally eliminating end trim of the total board produced, 1 to 3 percent is saved, and for that matter, more can be saved since dropends associated with cut-to-size and fixed platens lengths can be eliminated.

8. Another point related to any process is mat reject. When metal is detected in the mat, it can be rejected. The continuous press is able to reject a relatively small portion of the mat since it does not have to reject a whole platen length such as is the case with multi-opening press lines. This means the less recycled material to put back into the board, resulting in a smoother running operation, a higher quality product and less time lost.

9. The combination of being able to run significant width and length adjustments with no additional waste gives the continuous press tremendous flexibility to economically respond to market demands.

The future

The particleboard plants in North America are growing old. Seventy-one percent are more than 15 years old which represents 77 percent of the industry's production capacity. Forty-four percent of the MDF plants are 15 years or older. Though many plants have been modified periodically and upgraded to meet changing times, these "fixes" are mostly of the expensive band-aid type. Much of the rest of the world recognizes the need for fundamental changes in the way board is pressed and is doing something about it.

The author is sales engineer for Siempelkamp's U.S. office based in Marietta, Ga. This article was excerpted from his presentation at the 24th International Particle-board/Composite Materials Symposium held at Washington State University last April. A hard-bound copy of the full proceedings is available for $65 by contacting: Proceedings, Conferences and Institutes, Van Doren 208, Washington State University, Pullman, WA 99164-5222.

PHOTO : Continuous presses, like the ContiRoll from Siempelkamp, can produce a full range of board thicknesses, from 1/10 inch to 1 1/2 inch.
COPYRIGHT 1991 Vance Publishing Corp.
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 1991, Gale Group. All rights reserved. Gale Group is a Thomson Corporation Company.

Article Details
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Author:Mayner, Jon A.
Publication:Wood & Wood Products
Date:Apr 1, 1991
Words:2499
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