To begin, the substrate, particleboard, is made up of a blend of sawdust, shavings, and wood chips. Wood chips, saw dust, and shavings are usually purchased from outside sources as chips, but some manufacturers chip round logs on site to supply their specific requirements. The ingredients are fed into a large hopper, where they are blown into a large filter where contaminates such as rocks, glass, dirt etc. are filtered out of the wood chips. From there, the wood chips are fed into a series of turbine style grinders which are designed to break the fiber down into the desired sizes, which usually include fines for the outer layer and larger chips, or fiber for the center of the board.
The ingredients then move into large dryers where the moisture is removed down to a 4% moisture content to ensure adhesion. The adhesive is then added to the ingredients typically applying a formaldehyde-based product. The mixture does not bond at this point as heat is required to activate the adhesive. The wood chips are thoroughly mixed with the adhesive as all particles must be coated to ensure proper bonding.
The chemically coated mixture is then poured onto a mat in layers. The first layer laid down is made up of fines, which will make up the surface of the board that will ensure a suitable and smooth surface, in so that the finished product might be painted or overlaid upon. Next the larger and more course chips, or fiber, are overlaid on top of the first layer fines. The larger chips ensure that the substrate will have good screw holding properties. Finally another layer of fines are overlaid over the course chips forming a sandwich of two layers of fines, with larger and more course chips in the center. This process is completed in a centennial flow creating a fluffy mat of 8 to 12 inches thick depending on the thickness of substrate being manufactured. Overhead magnets are situated over the mat that will collect any metal fragments that are still in the mat that somehow got through the filtering process.
The mat is then run through what is known as a pre press, which will press the mat down into a more workable thickness of approximately 2 to 4 inches via a large pressure roller. If the manufacturer is using a continuous press, the board is then fed into a continuous press for the final pressing. As the name suggests, it means exactly what it says. A rotating metal cylinder is hydraulically brought down onto the mat that both heats the mixture, activating the glue as well as pressing the mat into the desired thickness. The mat is pressed as it moves through the press where it will be cut to length, usually into 18′ or 24′ lengths, where it will be stored to cool and cure before further processing.
In a standard press-opening format, the procedure is very similar to that of the continuous press. In this process the 8 to 12 inch thick mat is cut to length usually anywhere from 48 to 128 feet depending on the press size capability. Once the mat is cut to length it is shuttled into the pre press and then very quickly shuttled into the press bed, in so that pre cure is prevented. The mat is then pressed to the desired thickness which is a process that will take anywhere from one minute to a minute and a half. The main difference in the standard pressing format from the continuous process is that the mat is cut to length and then pressed in set sizes and stages.
Once the board is pressed in either of the above said procedures, the board is then delivered into a giant cooling wheel that looks very much like that of a paddle wheel. The board is then cooled for approximately 45 minutes. The huge sheets are then stacked in giant lifts for a period of 2 to 4 days for further cooling and curing. Once cooled the sheets are then cut into usable sizes such as 4 x 16, 5 x 18, 5 x 16, etc. or even to exact final use sizes such as 4 x 8.
The cut sheets must now be sanded to remove the hard sheen caused by the press. The sanding can vary anywhere from a 100 grit, to 120 grit, to 140 grit, depending on what application the board is being manufactured for. The board is now ready for laminating.