1. Core stacking, off-seam
Core stacking refers to the phenomenon that two adjacent core veneers (or two parts of a cracked veneer) overlap each other in the same layer of plywood.
Off-joint, also known as off-core, refers to the phenomenon of separation between two adjacent cores in the same layer of plywood or between two spliced cores.
Overlapping core, off-seam in the plywood production will often encounter, should be analyzed and controlled from the following aspects:
(1) core grouping, core board overlap, misalignment, or reserved gap is too large or too small. These problems caused by the core grouping need to strengthen the training and responsibility of the core grouping operator, improve the technical level of the operator grouping, to ensure that the combination between the veneer and the veneer is just right when the core grouping.
(2) The core board moves or is misaligned when loading the board or moving the slab. In this case, it is necessary to put the coated core board for a period of time before forming the blank and to carry out pre-pressing after forming the blank to make the board bonded together initially and to keep the board as smooth as possible when moving the blank.
(3) Core board warping and deformation are serious or the edge of the core board is not straight enough. In this case, the warped core board should be softened and leveled before forming the core board, and the second flush edge should be done to make the surface flat and the edge straight.
2. Low gluing strength or glue layer debonding
Gluing strength refers to the solidity of the bonding between the veneers in plywood. Gluing strength is one of the important indicators to measure the quality of plywood.
The quality problem of low bonding strength or glue layer debonding encountered in the production process should be analyzed and controlled from the following aspects.
(1) There are quality problems with the adhesive used, such as poor quality, deterioration, or too low a concentration of adhesive. In the production process should always check the quality of the adhesive, poor quality or deterioration of the adhesive should not be used; if concentration is too low can consider adding flour for thickening adjustment.
(2) The quality of glue coating does not meet the process requirements, such as too little glue coating or uneven glue coating. In the production process, we need to choose the right amount of glue according to the different species of veneer and different thicknesses, and pay attention to the glue application should be appropriate and even.
(3) The moisture content of the veneer is too high or too low. The moisture content of the core board should be checked before gluing, and after drying, the moisture content of the core board should be controlled within the requirements of the production process.
(4) The aging time of the coated veneer does not meet the requirements, such as the aging time is too long, and the glue is already dry when the group is hot pressed; the aging time is too short, and the group is hot pressed without forming a continuous glue film. In order to make the glue effectively penetrate into the board, and increase the viscosity, hot pressing is conducive to the formation of a continuous layer of glue and improves the bonding strength, the group should be aged for 15-30 min before being sent to the next process, and the specific aging time after gluing should depend on the weather conditions.
(5) The veneer has quality problems, such as the veneer burr grooves are too deep, veneer rotary cutting quality is poor, etc. The quality of veneer is an important factor affecting the strength of the glue, so the veneer sorting and finishing process should be strictly controlled to improve the quality of veneer cutting to ensure that the quality of veneer meets the requirements of the production process.
(6) Hot pressing process parameters are unreasonable, such as insufficient hot-pressing pressure, too low hot pressing temperature or short hot pressing time. The hot pressing process affects the quality of plywood, so it is necessary to choose suitable hot pressing process parameters according to different products, and appropriately increase the hot pressing temperature and pressure or extend the hot pressing time under the premise of meeting the production requirements.
(7) There are problems with the hot pressing equipment, such as pressure increase does not go up, temperature transfer is not in place, etc. To frequently test whether the work of hot pressing equipment is intact, any problems should be repaired in time to ensure that the working condition of the hot pressing machine is intact.
3. bubble and local open glue
Blistering is the bulging caused by the gas on the surface of the product. Local open glue is a part of the board is not bonded firmly and cracked. The bulging and local open glue problem should be analyzed and controlled from the following aspects.
(1) Veneer moisture content is too high or uneven. Veneer moisture content largely affects the quality of plywood, too high, or too low are not conducive to plywood production, therefore, the moisture content of the veneer after drying should be strictly controlled in the range of 8% to 14%, the moisture content is too high, it is easy to open glue bubbles.
(2) When gluing, there are local blank spots, or the veneer is covered with debris or is sticky and dirty. In order to ensure the quality of glue, the amount of glue should be evenly applied, and the parts not coated with glue should be manually brushed, no blank spot is allowed, and the place where glue is piled up should be cleared of excess glue. The veneer entrapped debris and sticky dirt must be cleaned up.
(3) The hot pressing process is unreasonable, such as insufficient hot pressing time, the local hot pressing temperature is too low, resulting in an insufficient degree of resin condensation. The hot pressing process in the whole plywood production process always needs to be strictly controlled and must be selected according to different products to meet the production needs of the hot pressing process parameters.
(4) Hot pressing is completed and the speed of lowering is too fast. The plywood production process should control the speed of hot pressing pressure drop from the equilibrium pressure to zero, this stage of the pressure drop speed to slow so that the hot pressing temperature is properly reduced, the pressure drop is too fast in the board water vapor discharge speed is too fast, easy to produce bulging.
4. permeable glue
Permeable glue is the defect that the adhesive penetrates into the surface of the plywood through the surface plate and causes plate surface pollution. The problem of glue permeability should be analyzed and controlled from the following aspects.
(1) check the quality of the surface plate, such as if the surface plate gap is too large, and the surface plate is too thin. In the production of plywood, the surface board should not be too thin, and should ensure that it has a certain covering ability; if the surface board crack is too large, it needs to be repaired.
(2) Adhesive viscosity should be appropriate.
(3) The hot pressing process is unreasonable, such as the pressure of hot pressing is too large. In production, if the pressure is too large, the adhesive will be extruded through to the surface of the board, and thus the production should choose the right pressure to make the glue flow reasonably between the slabs.
5. warpage
Warpage is a concave phenomenon formed by the two ends of plywood warping or arching in the middle. Plywood warpage is caused by the high internal stress of plywood. The warpage problem should be analyzed and controlled from the following aspects.
(1) plywood structure is not symmetrical. The production process should follow the principle of an odd number of layers, species with reasonable, from the middle layer of the two symmetrical layers of veneer species, thickness, moisture content should be consistent.
(2) Veneer moisture content is not uniform. When drying veneer, it should be dried naturally first, and sent to the dryer for drying after drying to a certain degree, in order to control the veneer will not be twisted and deformed in the drying process. Monitor the temperature of each point in the dryer at any time during the drying process to ensure that the moisture content of each part of the veneer after drying is uniform, so as to improve the drying quality.
(3) The setting of the hot pressing process should be reasonable. During normal production, the temperature of the upper and lower platens of the hot press should be close to each other to ensure that the upper and lower platens are heated consistently.
6. Surface indentation
Indentation refers to the local dent on the surface of plywood caused by external factors. The surface indentation problem should be analyzed and controlled from the following aspects.
(1) The surface of the hot pressing pad itself is uneven, and the surface of the hot pressing pad is stuck with glue blocks or debris. Before the hot pressing starts, the hot pressing mat board should be checked and cleaned up in time; if the surface of the hot pressing mat board is uneven, the hot pressing mat board should be replaced.
(2) There are glue blocks or debris on the surface after the surface is closed. After finishing the surface of the plain slab and hot pressing the board should check whether there are glue blocks or debris on the surface of the board, and clean it up in time.
7. Corner loss or open glue
Corner loss refers to the phenomenon of missing or damaged product corners or edges caused by the improper mechanical or human operation. Corner gluing is the product corners are not bonded firmly and are cracked. This problem should be analyzed and controlled from the following aspects.
(1) The quality of the core board, such as the core board length is not enough, and the core board corners are missing. The length of the purchased core board should be strictly controlled to ensure that the length of the core board meets the requirements of the production process; for the core board with defective corners, it should be handled in time when the veneer is cut.
(2) Grouping operation is carried out with a right-angle reference edge according to the principle of “one side and one end flush”.
(3) There are problems in hot press loading, such as the upper and lower slab corners are not aligned when the slab is loaded into the hot press, and the slab is not evenly loaded and pressed. Strengthen production management, improve operator skills and responsibility, and keep the position of the slab in each layer of the press as flush as possible (i.e. top and bottom, left and right alignment) to ensure that the plywood is evenly pressed.
(4) hot pressing equipment problems, such as hot pressing plate edge wear caused by insufficient pressure and poor heat transfer to the edges caused by low corner temperature. To frequently check the hot pressing pad, found that the corners wear should be replaced in a timely manner; often check whether the heat-conducting oil pathway of the hot pressing plate is smooth and if the corner temperature of the hot pressing plate is found to be low, to unblock the heat-conducting oil pathway.
5) Glue coatings quality problems, such as lack of glue at the edges and corners, the amount of glue coating is small, and dries up prematurely.
8. Formaldehyde release exceeds the standard
Formaldehyde release refers to the use of urea-formaldehyde resin and other formaldehyde series of adhesives pressed into the artificial board, in the manufacturing, stacking, and use the process to the outside world constantly emit formaldehyde gas phenomenon, the problem should be analyzed and controlled from the following aspects.
(1) the formaldehyde content of the adhesive used exceeds the standard. Manufacturers each purchase a batch of adhesives are required to test, once found that the formaldehyde content exceeds the standard adhesive resolutely not used.
(2) the amount of glue is too large, and the amount of glue is not uniform.
Eden Chang Whatsapp:+8617712963728 Email: eden@jldwood.com
Plywood/MDF/BlockBoard/LVL
