Plywood QC Inspection Checklist : Part 18 On-Site Management of Stabilizing Plywood Quality
China is a big country in plywood production, but not a powerful country, mainly reflected in the current level of equipment and technology. The quality of plywood products is related to the raw and auxiliary materials such as veneers and adhesives, and the strict implementation of relevant production process requirements in the production process. At present, the veneers used in the production of plywood in our country are seldom used for the whole veneer which is suitable for the production of machine assembly line. The veneers are mainly small veneers (commonly referred to as medium veneers) whose width and width are 1 270 mm x 630 mm and 960 mm x 480 mm. Therefore, in the actual production of plywood in our country, whether they are plywood of 3’x 6’or plywood of 4′. The plywood of’*8’specification is mainly made by manual splicing of multiple veneers. The mechanized production level of other processes is not high, and many processes are manual operation. Therefore, there inevitably exists the influence of human factors on the quality stability of plywood. In order to stabilize and improve the quality of plywood, besides strictly controlling the quality of veneers, adhesives and other raw and auxiliary materials, we should not neglect the on-site management of the following production processes, try to minimize the impact of human factors and implement the specific requirements of the process.
I. Gluing Procedure
In the production of plywood, the different amount of veneer glue has a great influence on the bonding strength of plywood: under certain hot-pressing conditions, when the amount of glue increases gradually from the low amount of glue, the bonding strength also increases, in a certain range, the amount of glue is proportional to the bonding strength. When the amount of glue is increased to a certain value, the proportion relationship begins to change, and the degree of increase of glue strength gradually slows down with the increase of the amount of glue, and finally tends to be stable until it decreases. When this positive proportion relationship disappears, it is the critical point, and the critical amount of glue applied at the critical point is called the critical amount of glue applied . The amount of glue coating is closely related to the surface burrs, grooves and cracks of veneers, so the critical amount of glue coating should be determined according to the actual situation of raw materials. In actual production, the process glue amount should be slightly higher than the critical glue amount of 200 g/m2. If the glue amount is small, the bonding strength will decrease. Excessive glue amount will not increase but decrease the bonding strength, but also increase the cost of plywood.
In production, we should control the glue amount steadily, so as to achieve the purpose of controlling the glue strength of plywood steadily. This requires the production managers of enterprises to strengthen the on-site management of the glue process. After the glue machine works normally every day, we should first check whether the glue amount meets the technological requirements, and then check the glue amount every 2-3 hours. Once it is found that the amount of glue coating is not within the scope of process requirements, it should stop gluing immediately, check and analyze from veneers, adhesives, gluing machines and personnel operation, and solve the problem of abnormal amount of glue coating before continuing production. By strengthening the on-site management in this area, problems can be solved in time, so as to avoid the quality problems of batch products caused by the problems of glue quantity.
2. Hot pressing process
Hot-pressing process is the most important process in plywood production. Hot-pressing time, temperature and pressure are three important hot-pressing parameters affecting plywood bonding quality. Every manufacturer attaches great importance to them. However, it is not enough to pay attention to the requirement that the hot-pressed plywood be inspected several times a day in order to ensure the hot-pressing effect. Many enterprises think that if the three hot-pressing parameters of the hot-press are well controlled, the quality of the plywood produced should be no problem. In actual production, although three hot pressing parameters have been strictly controlled in some enterprises, the accidents of plywood delamination or partial degumming which are claimed by customers still occur from time to time. The reason is that the on-site inspection of hot pressing effect has been neglected, thus the problems can not be found in time, resulting in serious quality problems of plywood in batches.
The on-site inspection of hot pressing effect refers to the inspection of quality defects of plywood by visual inspection, tool knocking and instrument measurement. Visual inspection has become the norm in most enterprises, without further elaboration, but the first batch of plywood produced by hot pressing can not only check the appearance quality, but also insist on measuring the thickness regularly with thickness gauge or caliper, and tapping the parts of the whole board with tools such as wooden hammer to determine whether there are serious defects such as delamination or partial degumming, but not in every production. Enterprises can do it. Check every 2 to 3 hours after the first examination. If defects are found in plywood, pressing should be stopped immediately, and production can continue until the quality defects are solved. Such meticulous on-site management is an effective and simple way to avoid mass production of substandard products.
If the thickness of plywood is found to be too thick, it is necessary to check what causes it, whether the compression ratio of plywood is too small due to the pressure of the hot press or the misuse of other thicker specifications of the single slab. When the phenomenon of separation is found, it is necessary to check whether the problem is caused by the time, temperature and pressure of hot pressing, the quality of adhesives, or the excessive moisture content of veneer, or the failure of the machine, which results in the inaccuracy of the instrument displaying three parameters of the hot press, or the obstruction of the heat transfer medium passage of one or several layers of the hot pressing plate, which results in the temperature of the hot pressing plate not reaching. To process requirements. When local degumming is found, it is necessary to check whether the patch is too thick or the moisture content of veneer is too high when the billet is assembled; check whether the heat transfer medium channel of the hot-pressing plate is not smooth, resulting in uneven temperature in different parts of the plate and so on. Through meticulous on-site management, problems can be found in time, problems can be solved in time, and large quantities of products can not be qualified, thus avoiding large economic losses for enterprises.
A plywood manufacturer once produced a batch of urea-formaldehyde plywood with specifications of 2 440 mm *1 220 mm *18 mm, which suffered customer complaints and claims for quality problems of some products. After a comprehensive inspection of the products, it was found that about 10% of the plywood had one or more layers of large area degumming, and the remaining 90% of the products had no quality problems. By checking the degummed plywood, the moisture content of the core is 13%-14%. The degummed part is in the middle layer of the plywood or several successive layers on the side of the plywood. Comprehensive analysis can eliminate two factors: quality problems of adhesives and high moisture content of veneers, which should be caused by incomplete curing of local adhesives in a small number of products. After a comprehensive inspection of the two hot pressing machines used for pressing plates, it was found that the two adjacent hot pressing plates of one hot pressing machine had the faults of unobstructed heat transfer medium channel, which resulted in the low temperature and inadequate solidification of adhesives in this part of the plywood, leading to the serious quality defect of degumming in this part of the plywood. If the field management of hot pressing effect is strengthened during production, problems will be found in time, so that quality accidents can be avoided and economic losses claimed by customers can be avoided.
3. Triming Procedure
Timing monitoring of the cutting process is to check the verticality of the finished plywood by measuring the diagonal line of the plywood or overlapping the positive and negative sides of the plywood after the production of the cutting machine every day. If it is found that the verticality is not up to standard, it is necessary to stop the machine and check the trimming saw and other equipment to eliminate the failure of producing the verticality not up to standard before allowing the production to continue. After that, the verticality of the finished plywood products should be checked every 2 to 3 hours. Every enterprise knows the importance of edge cutting process, but many of them do not repair the edge cutting equipment until the uneven side of the sheet bundle is found in the packaging of finished products. Few enterprises take regular inspection of the edge cutting equipment every day as normal. Through the timing monitoring of the cutting process, the faults in the production of cutting equipment can be found in time, and the maintenance can be arranged to effectively avoid mass production of products with unqualified verticality.
4. Appearance inspection
Whether the on-site supervision and management of billet forming, filling and table selection boards are in place is closely related to the appearance quality of products. In addition to affecting the internal structure of plywood, patches and lapping joints will also have a greater impact on the smoothness of the plywood surface. If the interface of each layer is in the same position and the interface of each layer is docked, the plywood will easily break in this position. If the joint is lapped without oblique grinding, the plywood surface will protrude in this position, which will lead to sand penetration and affect the appearance quality of the plywood surface.
In filling process, according to the requirements of plywood technology, the gap width between adjacent surface panels is more than 3 mm or 3 mm. It must be repaired with single slab with the same thickness as the surface panels and filled with putty. If the excessively wide gap is not repaired by single strip, but directly filled by putty but not by twice putty, because the putty shrinks, the gap fills the surface of micro-concave. After hot pressing, a dark mark will appear on the surface of plywood, which will affect the appearance of the plywood.
For the selection of the sheet, besides requiring the veneer surface to be free of defects such as dead joints, jackets, cracks and holes, if there is no shearing machine, the veneer with flat edges should be selected as the sheet as far as possible, so as to avoid large gaps between the sheets of the slab; in the process of forming the sheet, attention should be paid to the face of the sheet, because the flatness and smoothness of the surface of the veneer should be kept. It is much better than the back of the plywood. Only in this way can the smoothness of the plywood surface be improved and the appearance quality of the plywood surface be improved.
Because these processes are mainly operated manually at present, the human factors have a great influence, and workers tend to neglect these details in order to improve work efficiency, so enterprise managers must strengthen the on-site management of these processes in order to ensure the appearance quality of plywood.
Under the present conditions of plywood production, it is not difficult to produce plywood, but it is not easy for every production enterprise to produce high quality and stable plywood. In order to produce high quality plywood continuously and steadily, besides ensuring the quality of veneers, adhesives and other raw materials, the site management of the above processes should be strengthened and the process requirements should be implemented in place. If the details of field management of gluing, hot pressing and edge cutting processes are put into practice, problems existing in production are found as soon as possible and corrected in time, the unqualified products with large deviation of separation layer, degumming and verticality can be avoided in batch production, and the internal quality of products can be effectively maintained; if the processes of billet formation, filling and table selection are strengthened, the internal quality of products can be maintained. On-site supervision and management can effectively maintain the stability of appearance quality. In addition to the above mentioned site management, other processes should also pay attention to site management, in order to facilitate the stability and improvement of product quality. Only by continuously and steadily producing high-quality products, an enterprise will continue to develop healthily.
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