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Plywood Pressing

Plywood Pressing

Topics

Pre-pressing

Prior to pressing the veneers are assembled together to form piles consisting of unglued, and glued veneers laid alternatively, with their grains mutually perpendicular. The assembled plywood panels may then be grouped together and pressed in a pre-press.

The pre-press (see Pic 1) which is unheated operates at pressure slightly below that of the hot-press. Pre-pressing can takes up to 50 sheet at a time, with a pressing time of 3 to 4 minutes.(Baldwin 1981). Prior to the development of thermo-setting resigns (urea formaldehyde,phenolic formaldehyde), all plywood panels were simply cold pressed

Picture 1:- Pre-press


(Courtesy of Dr. P.D.Evans)

Hot Pressing

Hot pressing is carried out in a hydraulic press incorporating multiple heated platens between which each individual panel assembly is subjected to heat and pressure. Hot press pressure for softwood pine plywood usually range between 1242 to 1380k Pa. Platen temperatures of around 120 degree Celsius are used and pressing times for panels 3.5 and 22 mm are 2.75 and 9.5 minutes respectivelyBaldwin (1981). Very accurate control of pressing times, temperatures and pressure are necessary to ensure adequate adhesive bond development.


(Courtesy of Dr. P.D.Evans)

Cold Pressing

Cold pressing is carried out at room temperature with either mechanical (screw type) or hydraulic presses. The glues employed are low temperature curing synthetic resin adhesives such as urea-formaldehyde. Cold presses are usually single opening and a complete bundle of uncured panels held together by clamps between top and bottom retainer boards and I-beams, is pressed at one time. As pressure is applied, the retainer clamps are taken up until full pressure has been obtained. The press is then opened and the entire bundle or package held under pressure by the clamps is removed. After curing period ranging from 4 to 24 hours depending upon the glue used, the retainer clamps, retainer boards and I-beams are removed and the panels sent for further processing.(Baldwin 1981).

Assembly and Pressing Veneers into Plywood

The panels to be pressed are assembled directly behind the spreader, building up a bundle or package of uncured panels with aluminium caul plates interspersed alternatively with every one or two plywood panels. The caul plates assist in handling, protect the face veneers and help keep the panels flat. The time between the spreading of the glue and the application of full pressure in the press of the uncured panels is called the assembly time or period.

(Baldwin 1981).advises that close control of the assembly time is necessary in order to prevent defective glue joints occuring due to either;

Most synthetic resign adhesives are cured by hot pressing. Each platen of a multi platen press is heated with steam hot water or hot oil as the heating medium, and either one or two plywood assemblies are cured between aluminium cauls placed in each press opening. Ordinarily a complete press load is assembled at the glue spreader, and the load transferred by conveyer rolls to the press-loading station. For small presses up to 10 openings, loading may be by hand. In hot pressing it is absolutely necessary to load and close the press and to apply full pressure as quickly as possible, otherwise the glue lines, particularly those adjacent to the platens will dry or cure without adequate pressure and a defective (precured) joint will resultBaldwin (1981). Pressing times range from less than 4 minutes up to 10 to 15 minutes depending upon the thickness of the plywood assemblies, the type and rate of cure of the glue, and the percentage of cure required. In practice the press load is removed when glue joints are strong enough to permit handling.

Conditioning of Plywood

In hot pressing a considerable amount but not all of the water in the glue line escapes in the form of steam. Moreover, hot pressed panels are often dipped or sprayed with water immediately after removal from the press in order to restore the moisture in the face plies. Consequentially, a conditioning or redrying period is necessary and hot pressed plywood is usually hot stacked in solid piles for periods ranging from one or two hours to overnight. In this way the glue joints attain full cure and the moisture content of the panels is reduced and equalised through out the panels before further processing is carried out

In cold pressing, particularly all of the water from the glue line is taken up by the wood layers and redrying is mandatory. Usually the panels are piled with tiers separating the panels at regular intervals to permit air circulation throughout the pile. Conditioning rooms are often used where panels are dried from 8 to 24 hours at temperatures up to 49 degrees CelsiusBaldwin (1981)..