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Key words refer to the production technology and equipment of the joint material
1 finger joint production process and equipment
In the process of wood processing and wood product manufacturing, in order to obtain a square or square material of a certain width or thickness, two or more pieces of wood can be spliced ​​and glued in the width or thickness direction of the wood with various adhesives, but the wood is short. It can be lengthened in the length direction of the wood (end direction) but requires the joint to have strength close to the original wood. The method of mitering was invented to make the wood longer, but the miter wasted wood, which was difficult to manufacture and had low productivity. If the long bevel is folded into a fold of the same shape, the finger joint is formed. Finger-joining is one of the most commonly used and most cost-effective methods of wood lengthening in the world. In the late 1950s and early 60s of China, finger joint technology was also adopted, such as in the production of anchor rods and door and window production. In recent years, due to the decline of raw wood quality, the increase in demand for wood, and the production of some large components, the finger-joining technology has gradually received attention and adoption. The advantages of finger-joined wood are as follows: 1. The short material is a long material, and the curved material is a straight material. Small-diameter wood and short materials are processed into building materials, furniture, automobile materials, etc. by sawing, planing, finger-joining and so on.
2. Increase wood production and ease the contradiction between supply and demand of wood.
3. Finger-joined wood processing plants are easy to get on.
4. Conducive to the economic development of the forest area. The development of thinning wood refers to the production of wood, which not only increases income, but also provides employment opportunities. 5. The use of finger joint wood and wood-based panels can better exert the effect of comprehensive utilization.
1.1 Finger joint design
A comprehensive study of the effect of fingerboard geometry on tensile strength, the main conclusions are as follows: 1. Generally speaking, the tensile strength of the finger joint increases with the decrease of the slope of the finger, but the proportion of growth decreases with the slope. And decrease. When the slope is reduced from 1:12 to 1:16, the increase in tensile strength is very small.
2. When the slope and the top width of the finger are kept constant, the bonding strength increases as the finger distance increases, but the proportion of growth decreases.
3. There is a good correlation between the bonding strength and the effective bonding area. This indicates that in order to achieve high strength, the fingerprint must be of sufficient length and the slope is small enough to provide a sufficiently large effective bonding area.
4. If the first three conditions are satisfied, the finger width is the determining factor of the bonding strength, and the finger width is smaller and the strength is higher.
5. The stress in the resulting net section of the finger joint (the area of ​​the total section minus the width of the top of the finger), in the range of 1:10 to 1:16, does not depend to a large extent on the slope of the fingerprint. Depending on the miter area or the ratio of finger length to finger pitch (1/t), the maximum 1/t range is greater than about 4. This maximum stress is about 17% lower in spruce than the strength of the clear material (perhaps due to stress concentration at the top of the finger). Therefore, the strength of the finger joint depends on the area of ​​the net section and the strength of the miter in the net section.
6. The research data shows that the finger joint with smaller tip width can obtain more obvious high strength, and if the maximum strength is required, such as stretching or bending, the thinner top width which can be obtained in practice should be used as much as possible. The national standard GB11954-89 "finger-joined materials" specifies the size series of the fingernails. Among them, the category of "finger type" is used for finger joints for structures, and the type is used for finger joints for non-structural use.
1.2 Finger-joined manufacturing
1.2.1 Wood selection and requirements
1. Tree species All coniferous wood and broadleaf wood with a dry density of less than 0.75g/cm3 can be used for finger joint wood production. The same piece (root) finger joint wood should in principle use the same tree species for wood finger joints.
2. The material used for finger joint production shall have certain material requirements, such as the requirements for knots and other material defects. 3. Water content refers to the sawn timber used for the wood. The kiln should be dried. The upper limit of water content is 15% and the lower limit is 8%. In order to ensure that the moisture content of the two pieces of finger jointed together should be as close as possible, the difference in moisture content should not exceed 5%.
1.2.2 Open Finger Fingers Open the required fingertips at both ends of the block. There are three methods: cutting method, compression molding method and combination of cutting and compression molding.
1.2.3 Glue In order for the butt fingers to be firmly bonded together to transmit stress, an appropriate amount of adhesive must be applied evenly to the surface of the finger. The appropriate amount of glue is one of the factors to ensure the strength of the finger joint. The amount of glue applied is too much, which not only wastes the glue, increases the cost of the product, but also the glue that is squeezed out by the extruded glue, and the surface of the wood is accelerated. This will cause the adhesive to accumulate on the splicer and damage its operation. The amount of glue applied is insufficient, so that the surface of the fingerboard cannot form a uniform layer of glue of a certain thickness, so that the bond strength is lowered. Therefore, according to research and production practice, it is proved that the amount of glue applied is about 350g/m2, and the suitable amount of glue is determined according to the glue type, viscosity, and tree species.
1.2.4 splicing (finger joint)
1. Stitching method There are two methods of splicing (finger joint), one is dry finger joint, and the other is wet finger joint. The so-called dry method refers to the joint material that has been dried in the kiln before finger jointing, and the moisture content is in the range of 8 to 15%. The dry-fingering practice is the most widely used, and is currently widely used in countries around the world. Wet finger joint refers to the moisture content of the blank before fingering is much higher than the above range, even higher than the fiber saturation point. After the finger is opened, it is specially heated and then coated, so that the wet material can also be finger jointed.
2. The main parameter of the finger joint (end pressure) The end pressure refers to the pressure applied at the end of the finger joint along the length direction of the finger joint. The appropriate end pressure size is an important factor in ensuring the quality of the finger joint. Insufficient end pressure, the finger can not be completely compacted, leaving a large fingertip gap, so that the strength of the finger joint wood is greatly reduced; the end pressure is too large, causing the finger top to be excessively pressed into the finger valley, causing the finger to split and also The strength of the finger joint wood is greatly reduced. The appropriate end pressure is determined by the geometry of the finger, the hardness of the wood, and the anti-cracking properties of the wood. End pressure time: the dwell time after the pressure limit is reached. There is no limit in GB11954-89. It is recommended to select it within 10~30 seconds. According to the test and production practice, the end pressure time of 2 to 3 seconds can meet the requirements.
2. The splicing equipment can be divided into two types: intermittent splicing machine and continuous splicing machine according to the working form of splicing equipment. The splicing process of the intermittent splicing machine is intermittent, that is, the splicing of a pair or a set of finger back end cylinders is paused for a while, at this time, the feeding is completed, and then the second splicing is performed after the feeding is completed. The continuous splicing machine is a continuous movement of the workpiece during the splicing process. The advantage is that the length of the wood is not limited.
1.2.5 curing of the adhesive
1. Normal temperature curing usually refers to the curing of most adhesives at room temperature (21 ° C and above). Therefore, the finger jointed wood after the finger joint is only placed in the indoor temperature for the above-mentioned temperature for a certain period of time, and the adhesive is solidified, and further sawing and other mechanical processing can be performed. The length of curing time depends on the indoor temperature, the type and performance of the adhesive, and the size of the finger joint wood, which is generally in the range of 8 to 48 hours.
2. Heating and curing The heating method can greatly shorten the adhesion time of the adhesive. High frequency heating is one of the fastest ways to accelerate the curing of adhesives. The frequency range used is 1 to 27 MHz. Wood moisture content is an important factor when curing at high frequency. If the control is reasonable, the quality of high frequency bonding is as good as other methods. The problem of curing the adhesive in the finger-joined wood production in winter is a headache. In order to accelerate the curing, a curing room with enough steam heating tubes can be set up in the splicing workshop, or a maintenance room can be specially set up to make the temperature in the shed (room) Above 25 ° C. This eliminates the need to consume too much heat to solve the problem of curing adhesives produced in winter. What method is used to accelerate the curing of the adhesive, as described above, according to local conditions, according to the specific conditions of each plant.
1.3 Fingering efficiency and strength
1.3.1 Finger-joining efficiency The strength of the non-fingered wood without wood defects is taken as 100%, and the strength of the finger-joined material of the same material is compared with the former, and the percentage obtained is used as the finger-joining efficiency. Finger-joining efficiency varies with factors such as seed, wood quality, type of adhesive, geometry of the finger, and type of finger joint. Many studies have shown that the direction of the orientation of the finger and the direction of the static stress affect the efficiency of the finger joint. The horizontal finger joint (side thickness refers to the type A) has a static bending strength that is about 5% lower than that of the vertical finger joint (wide face refers to the type B). China's standard GB11954-89 "finger-joined materials" appendix F, G, H, I respectively listed different finger lengths, wood gas dry density, finger joint material bending strength, grain resistance, and grain resistance A table of coefficients for tensile strength and impact toughness (ie, finger joint efficiency). Chinese scholars use the finite element method to determine the stress uneven distribution coefficient between the finger trapezoidal notch and the transfer efficiency of the finger joint. The obtained data are very similar to the test results. They derived the formula for the efficiency of the finger-joining force, so that the finger joints with the required force transmission efficiency can be designed according to the requirements.
1.3.2 Strength China's standard GB11954-89 "finger-joined materials" for the finger joint (no substructure and non-structural use) strength requirements, its resistance strength σb and the grain compressive strength σc should not be lower than the limit of Appendix B value. In Appendix B, the effects of factors such as air dry density, rubber type, finger length, and direction of finger lining in the finger joint are considered, and the corresponding resistance strength and compressive pressure are listed. The limit of strength. National standards stipulate that there is generally no requirement for the tensile strength and impact toughness of the finger joints under normal use conditions; if there are special requirements, their tensile strength and impact toughness should not be lower than the limit of Appendix C.
Finger joint material, integrated material production technology and equipment (1)
Abstract This paper gives a detailed introduction and in-depth discussion on the production technology, technology and equipment of finger joints and laminated materials. It is pointed out that the production process is not complicated, the equipment investment is small, and the sales are good. The development of finger-joined materials and laminated timber can increase timber production, ease the contradiction between supply and demand of timber, and contribute to the economic development of forest areas, that is, economic benefits and social benefits.