Forming and pre-pressing of plywood

Aging and drying of glued veneer

Aging of glued veneer

After the veneer has been glued, it is left for a period of time before being hot pressed, a process known as ageing time. The purpose of this is to concentrate the glue to avoid blistering during hot pressing. This allows the veneer to expand sufficiently after gluing to overcome defects such as cores leaving the seam. There are two ways of ageing: after gluing the veneer is left for a period of time before it is formed, called open ageing. After the veneer has been glued, it is placed in the press for a period of time called closed ageing.

This process is generally used for adhesives with a high moisture content, such as bean glues, non-dehydrated urea-formaldehyde resin glues and phenolic resin glues that do not dry after gluing in the centreboard.

Common glue spreader

The ageing time varies according to the temperature of the workshop and the type of glue. The ageing time for urea-formaldehyde resins is 60min at a workshop temperature of 15-25°C and 30min at a workshop temperature of 26°C or more.

After gluing the veneer if it is dense for a longer period of time in the stack up, if a high molar ratio of glue is used, there will be an increase in temperature in the slab, e.g. birch slabs can be as high as 50°C. The main reason for this is the accumulation of heat released by the wood after it has absorbed moisture. This accumulation of heat causes early curing of the glue, which reduces the viscosity and leads to excessive glue penetration in the veneer, thus reducing the glue adhesion. Attention should be drawn to this in production.

Drying of glued veneer

Glued veneer is generally not dried. Only some special plywood, surface thinning ordinary plywood or glued veneer moisture content is too high, the veneer for drying treatment. The drying of glued veneer should be done quickly before the moisture in the glue penetrates into the interior of the veneer, otherwise too much moisture penetrates into the veneer. The amount of glue on the board surface is reduced and the strength of the glue will be reduced.

Pneumatic glue spreader

The moisture content of the veneer, the amount of glue applied, the type of adhesive, the temperature of the medium in the drying equipment, the wind speed and the relative humidity are the conditions that determine the drying process of the veneer. The drying temperature is the main condition. The higher the drying temperature, the faster the drying speed. However, too high a temperature will cause some resin molecules to cure and even bubbles to appear on the surface. Therefore, the drying temperature should be appropriate, generally within the range of 80-90°C and up to 110°C. The quality requirements after drying are: moisture and volatile matter content of 6% to 12% (thin board can be 8% to 12%, thick board and multi-layer board can be 6% to 8%); the curing rate of the adhesive is less than 2% to 3%.

There are three types of equipment for veneer drying: simple drying chambers, flatbed presses and conveyor dryers.

Forming of plywood

Blanking is the combination of face, back and glued core boards into a slab. The thickness of the slab is determined by the thickness of the finished product, the compression rate of the slab during the pressing process and the size of the surface finish allowance.

The size of the compression rate varies depending on the species, moisture content and hot pressing conditions. Usually the compression rate is high when the material is soft, the moisture content is high, the pressure is high, the temperature is high and the pressing time is long. Conversely, the compression rate is small. The compression rate is closely related to the performance of the finished product. The compression rate of the slab is generally 5% to 10% for ordinary plywood. The plywood layers can be of equal thickness, or a thin surface and thick core structure. The latter makes better use of quality wood. The thickness of the veneer is currently 0.5 mm.

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After calculating the thickness of the slab, the thickness of each layer of veneer should be determined and then matched. In practice, this should also be adapted to the nominal thickness of the veneer manufactured by the rotary cutter. The thickness of the veneer should be within the standard deviation.

The quality requirements for each layer of veneer in plywood are different. The highest requirements are for the plywood, which is an important basis for differentiating between plywood grades. The quality of the backing board can be slightly lower. The quality requirements of each plywood panel and backing board should be in line with the national standards when forming the blank.

1.Hand assembled plywood

At present, the production of plywood in China is still dominated by hand-assembled blanks. The following points should be noted in the hand-assembled blanks:

The cores should be arranged “one side at a time” to establish a benchmark for the gluing and edge-planting of the board.

For closed-ageing methods, use zero-piece blanks to leave gaps according to the law of expansion of the veneer after water absorption. Avoid defects in the plywood such as stacked cores and off seams.

The tight side of the surface plate should face outwards.

The texture of adjacent layers of veneer should be perpendicular to each other.

Small narrow veneer should be placed in the middle to prevent misalignment and skewing in handling and loading of the board, resulting in inferior products.

The processing allowance of the core board should be adapted. The core board is slightly smaller than the surface board, to prevent the glue from being squeezed out when the glue is pressed and contaminated and corrosive hot press board.

2.Mechanised plywood forming

The labour spent on the “sizing, forming and hot pressing” section is equivalent to about 1/4 of the entire plywood production. Therefore, the mechanisation of the plywood production process is an important part of improving labour productivity and achieving continuous plywood production. The mechanization of the group must be achieved first of all by the whole sheet of the core board, so as to solve the difficulties caused by the narrow core board of different widths and the stacked cores away from the seams. At present, the whole sheet of core board is more successful way is to use three or four hot melt resin nylon line will core board glue into a whole sheet.

Pre-pressing of slab

Before the slab enters the hot press, it is placed in the cold press where it is pressurised and the veneer is bonded together by virtue of the adhesive. This process is called pre-pressing.

1.The significance of slab pre-pressing

The use of pre-pressing is beneficial to the improvement of product quality. This reduces the auxiliary time in the hot pressing process, shortens the hot pressing cycle and increases the productivity of the hot press. The ease of transport and loading of the slabs facilitates the use of machinery for loading and unloading the slabs, which reduces the labour intensity of the workers. Reducing the distance between the hot press plates and the pad transport equipment can save energy. The slabs can also be checked sheet by sheet after pre-pressing before they enter the loading rack. If defects are found, they are repaired.

In order to adapt to the requirements of the pre-pressing process, so that the adhesive has a certain initial viscosity, the adhesive should be modified. Phenolic resin adhesives are very viscous after ageing and can meet the requirements of pre-pressing. Urea-formaldehyde resin adhesives need to be modified by adding the right amount of filler, such as flour. Or add the right amount of polyvinyl alcohol for modification when synthesising.

Cold press machine

Pre-pressing of slab

Before the slab enters the hot press, it is placed in the cold press where it is pressurised and the veneer is bonded together by virtue of the adhesive. This process is called pre-pressing.

1.The significance of slab pre-pressing

The use of pre-pressing is beneficial to the improvement of product quality. This reduces the auxiliary time in the hot pressing process, shortens the hot pressing cycle and increases the productivity of the hot press. The ease of transport and loading of the slabs facilitates the use of machinery for loading and unloading the slabs, which reduces the labour intensity of the workers. Reducing the distance between the hot press plates and the pad transport equipment can save energy. The slabs can also be checked sheet by sheet after pre-pressing before they enter the loading rack. If defects are found, they are repaired.

In order to adapt to the requirements of the pre-pressing process, so that the adhesive has a certain initial viscosity, the adhesive should be modified. Phenolic resin adhesives are very viscous after ageing and can meet the requirements of pre-pressing. Urea-formaldehyde resin adhesives need to be modified by adding the right amount of filler, such as flour. Or add the right amount of polyvinyl alcohol for modification when synthesising.

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