Multilayer birch plywood for floor substrates
In the production process, besides guaranteeing the bonding performance and formaldehyde emission meet the requirements, how to reduce warping deformation of multi-layer birch plywood used as floor substrate is also a problem to be solved.
1. The Generation and Solution of Warpage Deformation
In the production process of multi-plywood, warping deformation of plywood is a common problem, which is caused by many factors. The specific analysis is as follows.
(1) Water content factor of veneer: Because the moisture content of veneer is not uniform and different, the moisture content distribution inside the plywood is not uniform after hot pressing. Under room temperature and atmospheric pressure, the plywood should absorb moisture and regain moisture, resulting in uneven stress distribution inside the plywood and warping deformation. The countermeasure is that the newly dried veneer can not be hot-pressed immediately. It should be stored in the veneer warehouse for several days, and the moisture content of each veneer should be balanced so as to make the moisture content of each veneer as uniform as possible.
(2) Grouping factors: When grouping, attention should be paid to the tightness of veneer in symmetrical grouping, tightness facing outward. Otherwise, due to the asymmetry of the elastic surface after hot pressing, the stress distribution in the plate is not uniform, and M is warped. In order to reduce the cost and the number of coating layers, the veneer is often thicker, which increases the inhomogeneity of the veneer. Therefore, proper roughening and softening of the tight surface should be carried out to reduce the internal stress of the plywood.
(3) Added a suitable amount of filler when mixing: Phenolic resin adhesives increase the internal stress of plywood due to the shrinkage of the adhesive layer during curing, which not only reduces the bonding strength, but also easily causes the warping of plywood.
2. Production process
The moisture content of veneer should be strictly controlled (below 6%). Because phenolic resin is different from urea-formaldehyde resin and its solid content is relatively low (usually between 40% and 48%), controlling the moisture content of veneer can reduce blistering loss.
Settlement is particularly important.
(2) Controlling the thickness deviation of veneer. The thickness deviation of veneer should be less than (+0.2mm). Excessive deviation can easily lead to poor gluing. Therefore, the thickness deviation of veneer should be strictly controlled during rotary cutting.
Control the amount of glue. For birch plywood, veneer thickness is small
When the thickness of veneer is 1.5-3.5 ram, the amount of double-sided glue should be controlled between 240-280 g/m and 300-320 g/m. On the one hand, the production cost is increased, resulting in waste, on the other hand, the bubble defect is increased, and the qualified rate of finished products is reduced.
(4) The hot pressing temperature, time and pressure should be matched reasonably. The production practice for many years shows that it is reasonable to adopt the following hot pressing process parameters to produce multi-layer birch plywood: pressure 1.5 MPa, temperature 135~C and time 1.5 mln/mm. The qualified rate of multi-layer birch plywood produced by this process parameter is 98%.
The bubbling rate is less than 1%.
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