Quality problems and solutions in plywood production in China
In the production process of any kind of product, there will be some quality problems. There are many and complex factors that affect the product quality, including the quality of raw materials, the insufficient precision of processing equipment, or the unreasonable processing technology arrangement. Of course, the other important reason is human factors, such as the operator’s lack of technical proficiency and sense of responsibility Emotional fluctuation of operators…… Plywood production is no exception, and the above situations will also occur.
In the production process, the product quality fluctuation caused by the above reasons can only be found out through analysis, so that it can be solved in time to stabilize the product quality.
1、 Quality problems of veneer joint
As the number of high-quality logs suitable for making surface boards is decreasing, most plywood manufacturers adopt gluing process for surface boards to improve the yield of surface boards. The gluing quality of the surface plate is related to the later assembling operation and the quality of the plywood. The possible quality problems, causes and solutions in the gluing are as follows:
Quality problem: 1. The end of the assembled board is not complete
Cause: It is caused by improper splicing operation, and the end of the veneer on both sides is not aligned when feeding on the longitudinal belt glue splicing machine; Or the veneer is not rectangular when it is flush.
Solution: strengthen the sense of responsibility of operators and improve the level of operation technology; When veneer is flush, there should be a benchmark. It is not allowed to cut veneer into parallelogram.
Quality problem: 2. The seam is too tight or loose
Cause: Improper adjustment of the feeding squeeze roll of the sewing machine.
Solution: Adjust a pair of conical squeezing rollers under the worktable of the sewing machine to make them suitable.
Quality problem: 3. The adhesive tape is not firmly stuck
Causes: The adhesive tape is not fully wetted, and the temperature of the heating roller of the sewing machine is too high or too low; The upper rolling rod pressure is not enough.
Solution: fully wet the tape with water; Check the heating roller; Increase the pressing force of the upper rolling rod.
Quality problem: 4. The color or finish of the assembled veneer is inconsistent.
Cause: The matching worker did not select the materials properly when matching the boards, and failed to “match the original trees”; During the splicing operation, the front and back of the two boards are inconsistent, and the loose and tight sides are reversed.
Solution: improve the technical level of operators, conduct more technical training, and strengthen the sense of responsibility of operators.
2、 Defects, causes and solutions of billet assembling
Defect 1: core separation
Cause: The position between parts and veneers is not properly placed during blank assembly; The core plates are dislocated due to collision during slab moving.
Solution: The core board coated with glue shall be put in place for a while before being assembled; Pre pressing shall be carried out after assembling to make the slabs initially bonded together and not easy to shift; Improve the technical level of the billet assembly worker.
Defect 2: edge defect and slab edge irregularity
Cause: The length of the core plate is insufficient, and the principle of “one side and one end are flush” is not followed during the billet assembly operation; Surface plate edge defect.
Solution: Strictly control the length of the core plate and the breadth of the surface plate when purchasing; There shall be a right angle datum edge when assembling; Strengthen production management and improve the skills and sense of responsibility of operators. Try to use skilled workers.
3、 Quality problems and solutions of preloading
In the plywood production process, the quality problems in the prepressing process are mainly reflected in the unsatisfactory prepressing effect, the slabs after prepressing can not be formed, the effect of pre bonding can not be achieved, or the core separation phenomenon occurs after prepressing. The defects, causes and solutions are as follows:
Defect 1: The prepressing effect is not ideal, and each layer of veneer is not well bonded.
Cause: insufficient viscosity of prepressing glue; Improper control of preloading time; The thickness of the plate stamp is too large for one preloading.
Solution: The viscosity of the glue shall be controlled during the glue preparation. The urea formaldehyde glue shall be 800-1400cp, and the phenol formaldehyde glue shall be 1000-1800cp; During glue mixing, flour or polyvinyl alcohol can be added in an appropriate amount to increase the viscosity of glue, and the active period of glue mixing under different climatic conditions can be well controlled, so that the prepressing is in the period of resin transformation from phase A to phase B. The active period of prepressing glue should be controlled at about 2h. It is generally said that the glue should be slightly “drawn” when the veneer is lifted from the prepressed slab.
In order to ensure that the preloading pressure is evenly transmitted to the width of each slab, it is better that the height of the plate stamp for one preloading should not exceed 60cm.
Defect 2: core separation after preloading
Cause: The core plate moves wrongly when moving the slab into the pre press; The core plate used for assembling is seriously warped and deformed, and the edge is not cut straight.
Solution: In the production process, civilized production and operation shall be carried out; At the same time, it should be noted that the core boards of fast-growing poplar species should be leveled (softened) before use, and the edges should be aligned twice to make the surface flat and the edges straight.
4、 Plywood hot pressing quality problems, causes and solutions
Defects: 1. Low bonding strength or degumming of adhesive layer
Cause: the adhesive deteriorates or the concentration is too low, or the amount of glue is too little; The moisture content of veneer is too high or the veneer peeling quality is poor; The storage time of glued veneer after gluing is too long; The hot pressing temperature is low, or the hot pressing pressure is small, or the hot pressing time is short.
Solution: timely check the quality of adhesives, and do not use unqualified adhesives; Select the appropriate amount of glue according to different tree species and different thicknesses of veneer; Check whether the moisture content of the core plate is within the required range before gluing; The storage time of the glued boards depends on the climatic conditions; Select appropriate hot pressing process according to different products, and do not use the same hot pressing parameters.
Defects: 2. “Bubble” or partial opening
Cause: After hot pressing, the pressure drop speed is too fast; High moisture content of slab; The core plate is partially short of glue; The veneer surface is partially stained with grease.
Solution: strictly control the speed of pressure drop from equilibrium pressure to zero when the hot pressure is reduced; Control the amount of glue; Check the moisture content of the core plate, which shall not exceed 10%, otherwise it is easy to open the glue and bubble; Pine wood rich in resin shall be subject to hydrothermal treatment before rotary cutting to extract part of turpentine and turpentine.
