Now the weather is hot, the temperature is rising , the time of the glue curing is shortened, especially when the wind is big, it is easier to solidify and curing .
So, this season, the factory need a good flexible management for their daily plywood production , in a timely manner to do some adjustments, shorten the conditioning time, at the same time, add some melamine powder, slow down the speed of the glue curing, while increasing the adhesion performance.
After the veneer core lay-up before hot pressing or cold pressing , if not timely to make the cold pressing or hot pressing, Spraying the water on the four sides to make the edges wet , aging is not easy.
If not well managed ,should be more risk of core delamination caused by the dry glue and curing .
If the factory has good equipment and management , can let the workers try to slow down the speed of the glue mixing and spraying . Try not to make too much glued veneer sent to the veneer core lay-up production area, prevent veneer from drying fast.
The glue ,is like the bones of the human body and the blood or tendons, linked with each layer of the core veneer .Without glue ,you can not make the plywood .Different glue and different glue machinery can make different quality levels of plywood .See below ,this is the traditional Glue production used for plywood .
Application of adhesive to layons can be achieved by any one of the following methods.
- Roller coaters
Roller coaters as their name suggest applies adhesive to the layons by means of rollers. Both sides of the layon are coated. The rollers are often corrugated to control the spread of the adhesive.
- Curtain coaters
These apply adhesive in a thin film or curtain of adhesive to one side of the veneer as it passes through the coater.
- spray coaters
These use air sprays to atomise the adhesive and apply a thin film of the adhesive on to one side of the veneer.Just test the glue coat how much weight ,can not accept thin glue or thick glue ,just suitable for the veneer gluing and also guarantee the quality .
- Liquid and foam extruders
These apply beads or rods of adhesive to the veneer surface which coalesce to form a complete covering of adhesive during the pressing process.It is hoped to develop new additives substituting for wheat flour used in UF-glue, which needs to investigate the interaction of flour with UF-glue as reference. Also need to mix the red or blue ,yellow color pigment to make different glue color .Why make different color ?Just make sure all the veneer are filled and sprayed with glue without bank area .As the poplar veneer are white color ,that is why the glue should be other types of colors .
The aim of all the coaters above is to deposit a regulated even cover of adhesive on to the veneer surface. Different wood species and grades of veneer require different levels of adhesive. Generally dense wood species and smooth cut veneers requires less adhesive than porous wood or rough veneers . Glue spread on the veneer surface is usually expressed in terms of grams of adhesive per square meter of veneer.
Preparation of Veneers for Gluing
Depending on the type and quantity of plywood to be produced, the steps to be taken in preparing the veneer stock for gluing and pressing, may range from direct transfer of the dry veneer. To the glue spreader, and then a series of operations (redrying, grading and matching, dry clipping, jointing taping and splicing, inspecting and repairing, conditioning) as required in the manufacture of high quality furniture plywood.
In general however most veneers wether rotary cut or sliced, are cut to size and squared by a dry clipper, edge jointed, taped or spliced and graded into cores, cross bands, or face veneers prior to being assembled for gluing and pressing.
Sliced veneers are processed, basically the same as rotary cutting. However, difficulties are aggravated by usually greater working lengths. Also more waste and loss are more serious when working with expansive material and large sized veneers .
Glue and Gluing Procedures
Synthetic resign glues replaced protein and starch glues for plywood manufacture, with urea formaldehyde predominating because of price, properties and ease of application. Australian mills once used tannin formaldehyde resigns or casein and soybean glues.
- Urea formaldehyde glues are extensively used for interior and intermediate grade bonding, which covers the majority of hardwood plywood produced. Formulas differ somewhat and can be adapted to meet specific conditions.
- Polyvinyl acetate glues and also polyvinyl acetate/urea formaldehyde glues are used for edge jointing and veneering. Polyvinyl acetate glues resistant to boiling water are available.
- Melamine formaldehyde glues are not extensively used for plywood gluing but are used where a high grade bond is required, and where black phenolic glues cannot be tolerated. They are used to fortified urea-formaldehyde glues to increase the weathering resistance of the bond. The largest application of the melamine formaldehyde resin is in the production of decorative overlays.
- Phenol-formaldehyde glues are standard for exterior bonds. With a glue properly formulated for exterior use and suitably employed, no exposure conditions or laboratory testes are known which will degrade hot pressed phenolic glue bonds without destroying the adjacent wood layers. Phenolic glues are also used for impregnating veneers and paper overlays for plywood.
- Resorcinol-formaldehyde and phenol/resorcinol-formaldehyde glues are similar to phenolic glues in quality of performance but being more reactive, can be cured at room temperature. They are more expensive than phenolic glues, and therefore limited in use to special applications.
Fillers may be used to assist in reducing penetration of the glues into the wood, increase its viscosity at the critical time before setting, and tend to fill small cavities between veneers, thus preventing starved joints. Fillers are usually finely ground inert minerals such as clay or wood/grain flour finer than 180 mesh.
Cereal flours are used as extenders. These reduce the resign content of glue to the minimum necessary for the required bond but generally require a heavier spread of glue mixture.
Mixing and Spreading
Glues are cheapest if bought in liquid form in bulk and handled by pump to storage and factory. They can be purchased in containers in syrup or dry form, if stored in a cool dry place they will keep for several months even in the tropics.
Glue mixers are much the same for all kinds of glue, and usually consist of a mechanically driven paddle device for stirring the mixture in a container. It is general practice to mix the ingredients on a weight basis. A glue with a short working life must be prepared in small batches. When mixing several types of glue, special care should be taken when cleaning between mixers, since even traces of one type may seriously affect the propertises of another.
Glue spreading machines usually have power driven rollers, ether of steel or rubber, with groove patterns consistent with the type of glue being applied. The amount of glue spread is controlled by one or more doctor rolls and varies with the type of glue, the species and quality of the veneer, and other factors.
It is essential to the successful production of plywood that there should be a strong glue bond between plies. Not all woods behave equally well in this connection. A strong bond is difficult to obtain with some of the heavier timbers and here a strong bond is needed most. As the greater stiffness of the denser wood causes increased strain on the bond with shrinkage and swelling tendencies in the plywood, as a result of changing atmospheric conditions. Where gluing to the surface of plywood is necessary, as in the case of overlays, the face veneer should exhibit good gluing properties.
Some species absorb glue much more readily than others and glue viscosity has to be adjusted to meet the required of the wood being used. This can cause problem when absorbent and non-absorbent species are used in combination . Some woods are inherently bad for gluing and finishing standpoints, because of natural constituents in the wood for example, oil or waxy materials may bleed to the surface during drying or storage and present a poor gluing surface, or result in uneven staining or painting characteristics. Steaming of logs prior to veneer manufacture, control of drying conditions, and their use as soon as possible after the production of the veneer can help in some cases.
let’s also talk about the glue types here used for plywood and other wood panels ,not just the production process .
Urea and formaldehyde reaction to obtain the polymer. Urea formaldehyde resin. English abbreviation UF. When forming, the product is crosslinked and the product is a thermosetting resin. After curing of urea formaldehyde resin color than the phenol formaldehyde resin shallow, translucent, resistance, weak acid and weak alkali, good insulation performance, excellent wear resistance, and the price is cheap, but in case of strong acid, alkali decomposition, weather resistance, poor.
Urea formaldehyde resin has low cost, light color, high hardness, oil resistance, mildew resistance, good insulation and temperature resistance, but the weather resistance and water resistance is poor. It is one of the earlier thermosetting resins.
Urea formaldehyde resin is the main adhesive of wood industry at home and abroad. It has been widely used because of its high bonding strength, fast curing, good operability, low production cost, rich raw material and easy to get.
There are a variety of glue can be used for plywood. As for the specific use of what kind of glue, to look at the quality requirements of plywood.
(a) according to the composition of the glue, for the construction of plywood with the following kinds of glue:
Urea formaldehyde adhesive: moisture proof glue / for indoor decoration, furniture, can not be used for outdoor.
Melamine glue: moisture, and has a certain water boiling, weather resistance characteristics / can be used for outdoor or indoor, but can not often be immersed in water.
Phenolic glue: moisture proof, excellent water boiling resistance and climate resistance characteristics can be used for outdoor or indoor / outdoor, can withstand exposed to wind and rain.
(two) according to the environmental protection grade, the plastic used for plywood can be divided into E0 grade, E1 glue, E2 glue (and even E2 grade can not reach the unqualified plastic). E0 level is the highest level of environmental protection. E1 grade plywood can be used directly in the interior of the.
High quality urea formaldehyde / melamine / phenol formaldehyde adhesive can reach E0 grade, E1 grade, which can be directly used for indoor.
(three) environmental protection proposals:
1, if to indoor generally used for the production of wood products, you can use E1 class (even E0 class) urea formaldehyde glue; if further want to let the wood products have stronger waterproof property, you can use the E1 (even E0 grade melamine glue.
2, if you want to make outdoor use of wood products, you can use E1 grade (or even E0 grade) phenolic adhesive.