The opening of glue (bubbling) or weak glue (insufficient bonding strength) in each layer of plywood is a more common defect in the production process of plywood, and also an important factor affecting the quality of plywood. Therefore, managers at all levels and operators in related positions must understand and master the causes and prevention measures of unglued in order to apply them to production practice,Find problems and solve problems to ensure product quality.
There are many factors that affect plywood opening (bubbling) or weak glue. Whether it is raw materials, adhesive species, adhesive liquid index, operation method, process parameters, production time control and other factors are the influencing factors.The main influencing factors and preventive measures are described below.
1, Wood Moisture Content:
The amount of wood moisture content directly affects the strength of plywood. If the moisture content of the wood is more than 20%, it has a great impact on the strength of the plywood until it causes bubbling or weak glue. Therefore, when producing plywood, the moisture content should be controlled below 14%. The same is true for the veneer, the moisture content can not be too high. In particular, the water content of birch substrate veneer for the production of phenol glue (low solid content) should be controlled below 8%.
2, Hot Pressing Temperature And Time:
The low temperature of the pressing plate or the short hot pressing time is the main reason for the opening and weak glue.
(1) The hot pressing temperature of urea-formaldehyde glue is not less than 105 ° C, and the time is 1-1.2 minutes / ㎜.
(2) The hot pressing temperature of triamine glue is not less than 115 ° C, and the time is 1.2-1.5 minutes / ㎜.
(3) The hot pressing temperature of phenolic glue is not less than 120℃, and the time is 1.2-1.5 minutes / ㎜.
Hot pressing temperature is too high is an important reason for bubbling. Therefore, in the plywood production process, when meeting the glue pressure temperature, try to limit the pressure plate temperature is too high.
3, The Amount Of Glue:
The amount of glue also affects the bonding strength and an important factor causing product quality defects, generally controlled in the core plate glue 320 ~ 340g / ㎡. Veneer surface control at 290 ~ 320g / ㎡. If there is a certain temperature (not normal temperature) for the core plate, the amount of glue should be increased appropriately to avoid dry glue. For phenolic glue with low solid content, one is to increase the amount of flour (about 35%), and the other is to reduce the amount of glue when meeting the bonding strength.
4, Aging Time:
The length of aging time of the slab after gluing, not only affects the pre-pressing effect, but also because the glue has a certain temperature in the aging process, coupled with wind and other factors affecting the time is too long, may cause dry glue phenomenon. Therefore, the process time must be controlled in the production process. To avoid too long a time, generally without curing agent should be controlled between 20 to 24 hours on the hot pressing. The slab with curing agent should be controlled within 12 hours of hot pressing. For the parts that are easy to air dry, you can spray some water appropriately. For phenolic glue with low solid content, it is necessary to increase the aging time to make the water evenly distributed to avoid bubbling.
5, Loading Time:
The hot press operator should try to collect as short a time as possible when loading the plate. Too long time is easy to cause the adhesive layer of the laminated plate to dry glue. Generally, the board is controlled within 30 to 40 seconds, and the core board does not exceed 1 minute.
6, The Thickness Of The Board Or The Board:
The thickness of the board or the board is relatively large, which is also a reason for weak glue or opening glue, and the thickness of the board or the board should be controlled within a certain range.
7, The Plate Burr Groove Mark:
The plate burr groove mark is too large is also a cause of weak glue. Such boards should be removed without use.
8, Other Reasons:
(1) The flour contains more talc;
(2) Insufficient hot pressing pressure;
(3) The press plate is not correct;
(4) The middle plate stack is too large from the core;
(5) The core board is sanded or seriously unsanded.
The above reasons will be encountered in the production process, if there is a problem to understand the situation on the site, generally adopt the gradual elimination method to analyze the cause, finally determine the influencing factors, correct in time, and solve the problem.
