Through a large number of tests and analyses, we learned the shape and characteristics of back cracks in basswood and poplar veneers and the factors that affect their deformation. The test results show that the depth of the crack on the back of the single plate is 30% to 60%. The mechanism that causes the deformation of plywood is caused by uneven cracks on the back of the veneer due to differences in wood structure. The amount of deformation of the plywood can be reduced through cold pressing and shaping. Keywords: veneer; crack; plywood deformation 1. Test purpose Since the national plywood standard is formulated with use as the goal, the deformation index of 3 to 5 layers of plywood below 6mm is not tested, which makes it difficult to test the product in terms of standards. Constraints make secondary veneer decoration difficult. In order to solve this problem, in 1994, the Heilongjiang Academy of Forestry Science assigned the topic "Research on the Influence of Veneer Back Cracks on Plywood Deformation" to conduct a special study. The purpose was to study the impact of veneer back cracks on plywood deformation. The purpose is to understand the role and mechanism of veneer back cracks in the deformation of plywood through research, and to have a certain effect on improving the current production process of 3 to 5-layer plywood products to reduce the occurrence of deformed plywood. A camera was used to analyze the shape, location and reaction of veneer cracks to gluing and glue, etc., and two commonly used tree species, basswood and poplar, were selected; (2) Observe the veneer crack conditions when log sections have different diameters, so as to Mainly based on research; (3) Observe the impact of the log softening process on cracks; (4) In the normal rotary cutting process, the impact of rotary knife grinding, installation, etc. on the cracks on the back of the veneer; (5) Different veneer thicknesses and different drying The cracks in the veneer under the conditions; (6) The impact of artificial repair and natural defects of the veneer on its finished plywood; (7) Instruments and equipment, using a wood universal testing machine, a hot press for testing, an electric drying oven, and a high-frequency wood moisture tester Rate meters, calipers, tape measures, micrometers and other measuring tools, as well as universal microprojectors, cameras, etc. 2. Test method (1) Using the microscopic microscope in the Sino-Japanese technical cooperation project. 3 During the project establishment and research, researchers Liu Zhenguo and Fu Chaochen of our institute, Wang Jinlin of the Institute of Wood Industry, Chinese Academy of Forestry, and Northeast Forestry University I would like to express my sincere gratitude to Professor Shen Yaowen for his enthusiastic guidance. Submission date: 1996-06-16 Responsible editor: Pan Qiying 3 Test materials (1) Veneer (basswood, ) purchased from the state-owned Songjiang Glued Poplar Board Factory; (2) Urea-formaldehyde resin glue, milky white glue, chloroprene glue etc., purchased from Songjiang Plywood Factory or purchased from the market. Issue 1 Xu Lanying et al.: The influence of cracks on the back of veneer on the deformation of plywood 35 When applying glue, uneven absorption of glue occurs, that is, compared with normal materials 4 Experimental analysis 4. 1 Observe that the glue absorption performance of the material is poor, or it is called poor wettability. During the aging and expansion process of the veneer, it is easy to dry out, become thin, and lack glue. It is assumed that various local stresses will be generated during pre-pressing and hot pressing, causing the overall deformation of the plywood. When entering the finished product state, due to the rapid loss of a large amount of moisture, the moisture content of the plywood is further caused to be uneven. The stress caused by the loss of moisture eventually leads to the deformation of the plywood. Microscopic analysis has proven that the defect is a lack of glue. 4. 5 The impact of repairing defects on deformation has been analyzed and it is believed that the origin of the wood, log diameter grade and hydrothermal softening treatment will not have a major impact on the cracks in the veneer. As long as the operation process is strictly controlled, accidental cracking and deformation will not occur. But cracks on the back of the veneer always exist. 4. 2 Analysis of cracks on the back side of veneer sampling The moment the veneer leaves the tip of the rotary cutter of the rotary cutting machine, the veneer (or rotary cutting chips) contacts the surface of the rotary cutter, along the thickness direction of the veneer and along the back of the rotary cutter. In the direction of the extended line of the knife tip, it penetrates into the veneer to a certain depth and creates cracks along the length of the wood fiber. Generally, it is about 30% to 60% of the thickness of the veneer. For example, when the thickness of the veneer is 1mm, the back crack is about 0.30~0.60mm. Usually, the surface of a veneer with cracks is called the back side, and the cracks on the back side of the veneer come from this. Use the microscopic analysis method to measure the back cracks of the veneer on a universal microprojector, as shown in the table. Cracks Average number of main cracks Samples Single plate thickness Number of cracks Shape mm (strips/cm) (%) 46 46 49 50 1. 00 1. 40 7 8 5 6 62. 4 Defects caused by natural and processing , defects will also form after manual repair, including: patches, patch strips, laminates, paper tapes (perforations), seams, etc. After observation, it was found that there were also differences with the surrounding areas. Therefore, uneven expansion and stress imbalance also occurred when applying glue.

In this case, stress occurs after gluing only because foreign materials such as patches and veneers are not unscrewed from the same wood section or their moisture content and texture are inconsistent. 4. 6 Comprehensive analysis and verification 4. 6. 1 Analysis Tree species Basswood Poplar 3 1. The average number of cracks measured on a 4cm wide veneer; 2. The ratio of the total height of cracks to the thickness of the veneer is the average crack degree. 4.3 The influence of veneer processing on the cracks on the back of the veneer. Comparative drying observations of the dryer and drying oven show that drying has no effect on the lengthening of the crack depth on the back of the veneer. On the contrary, the existence of cracks on the back of the veneer promotes the completion of drying and reduces drying costs. 4. 4 The impact of veneer material defects on plywood deformation. Tests of plywood by plywood factories have shown that natural wood structural defects on veneers have a great impact on plywood deformation. Among them, defects in medium and small-sized logs appear on veneers. The frequency is very large. Microscopic analysis shows that the cracks behind the defect are shallow and few, and the material density is uneven. When there are defects in the back cracks of basswood and poplar veneer characteristics 3 70. 1 61. 0 1. 00 1. 40 64. 8 Oblique curves Oblique curves Oblique curves Oblique curves The test proves that the finished plywood will deform. The key lies in the hot pressing stage. During the hot pressing process, the thermosetting adhesive in the slab solidifies due to heat, bonding the veneers, but it cannot be completely solidified, especially when moisture is released between the 2nd and 3rd curves, the outer surface The escaping steam will probably be discharged from the edge of the veneer along the grain first, and the defective parts will show advanced or delayed exhaust. When the third section ends, its lagging part has not yet been exhausted. When the pressure is relieved, a core is formed from the defective parts of the plywood, resulting in several stress points. When shrinkage occurs due to gluing, deformation occurs under the action of stress. For many years, the factory's solution has been to stack the plywood while it is still hot after being unloaded from the hot press for about 3 to 21 hours to further promote the complete solidification of the incomplete glue. This is beneficial in terms of flatness, but only solves temporary deformation correction problems. When the stack is opened and placed alone, the deformation will reappear. The reason is that the pressure of the stack (hot stack) is not enough to restrain the stress of the plywood. Therefore, hot stacking still cannot fundamentally solve the deformation problem. 4. 6. 2 Verification The cracks on the back of the veneer can be identified as the main factor affecting the deformation of the plywood due to the differences and defects in the material structure of the veneer 36 Forestry Technology Volume 22. At the same time, cracks are prevalent on the back surface of the veneer in varying degrees and forms. Due to different causes of cracks, the deformation of 3 to 5-layer plywood products in the conventional plywood production process is inevitable. The cold pressing shaping process after hot pressing can correct the internal stress generated in the plywood. When the thermosetting resin adhesive in it has not been completely cured, that is, before an irreversible body structure has been formed, the plywood is kept under the constraints of the cold press for a period of time. , allowing the adhesive within the plywood to complete curing. When the plywood is unloaded from the cold press, it becomes a plywood product with minimal deformation, which improves the utilization rate. According to the follow-up inspection and confirmation of the plywood that has been finalized for 7 months, the above method can be used to successfully finalize the plywood, which has practical significance for guiding the practice of plywood production. The unevenness caused by it shows changes in the presence, absence, and depth. Due to uneven absorption and expansion of the rubber material, stress is formed during hot pressing, which appears after the pressure is released, and cannot be corrected under relatively small pressure (such as stacking). In order to verify it, a correction method under larger pressure of the cold press was designed to correct and verify. The method is: send the plywood that has just been unloaded from the hot press into the cold press platen room while it is still hot, with a pressure of about 0.3 to 0.4 MPa, and maintain it for 15 to 30 minutes to correct its shape or called shaping processing. Tests have proven that the effect is very good, indicating that the cracks on the back of the veneer play a further solidifying and shaping role in the post-processing of hot-pressing gluing.

5. Conclusion Microscopic analysis and various tests and verifications show that the names and symbols of single commonly used legal units of measurement are simplified. Name unit symbol International symbol m cm mm μm km m2 cm2 mm2 km2 hm2 m3 cm3 mm3 L ( I) mL ( ml) a h min s Hz kHz MHz m/ s km/ min km/ h m/ s2 Chinese symbols meters centimeters millimeters microns kilometers (km) meters 2 cm 2 mm 2 kilometers 2 (km 2) hectares meters 3 cm 3 mm 3 liter milliliter year hour minute second hertz kilohertz megahertz meter/second kilometer/minute kilometer/hour meter/second 2 Quantity name unit symbol international symbol t kg g mg Chinese symbol ton kilogram grams milligram length (including distance, height, ) Diameter, mass (weight), pressure, pressure (should), elastic modulus Pa, area force (gravity), energy, , work, thermal power amplitude difference, sound pressure level, power difference N J w? h w NJ kilowatt? hour watt decibel volume volume dB Time frequency velocity acceleration In scientific and technological literature (including other aspects), the following commonly used units can no longer be used: (1) All municipal units, such as inches, , , , , , , feet and feet, two kilograms per acre, k etc.; (2) centimeters, meters, , , , quintals, horsepower, etc.; (3) inches, feet, , , , miles, gallons, pounds, tons, etc.; (4) ppm shall not be used as the unit, for example 2ppm shall be Written as 2× -6 10