MDF QC Inspection Checklist : Part 6 Manufacturing Defects and Grading Rules
Common quality defects in MDF production, such as surface hair, watermarks, surface cracks and stratified bubbling, are analyzed. The prevention and cure methods are introduced.
In actual production, there are cracks, surface hair, concave and convex marks, watermarks and other defects. Based on the experience accumulated over the past few years, the common quality problems are analyzed as follows.
1 Surface hair
When continuous flat pressing is used to produce sheets less than 6 mm, it can be done without sanding or to reduce sanding allowance, which reduces consumption and improves the economic benefits of enterprises. However, there is surface hair phenomenon in the production of calender, which directly affects the later use of customers. The common hair phenomena are as follows:
1) Edge hair with soft edge (brittle edge) defect. Because the production speed of sheet metal is relatively fast, in order to prevent stacking at the inlet, the edge exhaust is usually set. In the process of pressing the slab at the inlet of the press, the exhaust gas is exhausted to both sides and some fibers are taken away, which makes the density of the edge of the sheet low and soft, brittle and brittle after cooling. The ways to solve this defect are as follows: firstly, increase the width of the flat edge of the slab so that the hair on the edge of the finished slab can be cut off during sawing; secondly, slightly adjust the proportion of the fibers, i.e. the dust in the fine fibers is more harmful to exhaust gas, so it is necessary to increase the production speed of the thin slab. In addition, some dust is emitted during the exhaust process at the entrance, which can easily adhere to the steel belt and cause rough edges.
2) irregular hairs on the surface of the plate. If there is a deposit in continuous production
In this defect, the possible reasons are the low moisture content of fibers and dirty steel strips. In the hot pressing process, the moisture content of the fibers mainly plays the role of heat transfer. Continuous flat press is relatively long. In the production of thin plate, if the temperature is high and the moisture content of the fibers is low, the water loss in the hot pressing process is very fast, which reduces the plasticity of the fibers and will cause the surface hair. Therefore, the moisture content of the fibers and the heating temperature of the press should be increased as much as possible in the production of thin plate. In production, if the surface of the strip appears foggy, it shows that the strip is dirty. After the strip is dirty, the surface of the strip will be brushed lightly. In serious cases, the surface of the strip will produce concave and convex marks. Therefore, it is necessary to keep the strip clean in the production of thin sheets.
3) The strip hairs with the same width parallel to the direction of the press outlet are produced on the surface of the plate. This kind of defect is usually after the plain plate press. Because the clamping force of the pressing wheel is too large during the deceleration process of the plain plate processing system, it is necessary to find the position and adjust the clamping force to eliminate hair.
2 Watermarks or traces
Generally, there are two kinds of watermarks: one is the climate impact, such as rain and snow weather caused by droplets in the system, the formation of watermarks will have a distinct protrusion, watermarks in places even after sanding will be more soft. This situation is generally due to the roof or skylight seals are not strict, so we should pay attention to the roof and skylight seals to prevent the formation of water droplets and dripping. The second is the condensate water caused by the temperature difference near the outlet of the press. When the hot air emitted from the press meets the chiller rack of the outlet line, condensate water will be produced and brought to the board, especially in the middle and night shift production, especially in the north. Therefore, it is necessary to increase the outlet heat preservation and indoor heat preservation to prevent the formation of condensate water.
3 plate surface crack
In the production of sheet metal, cracks on the surface of sheet metal occur mostly in hot-pressing and pre-pressing stages. Generally, there are several reasons as follows:
1) Press and pre-press speed matching is not appropriate, resulting in plate surface tension
Break. In general, there is a speed difference between the press and the pre-press and the paving line. When producing the same size plate, the transmission ratio is different under the conditions of high-speed and low-speed operation. Therefore, if the press accelerates, the speed ratio must be adjusted appropriately to match the speed of the pre-and post-process.
2) The pressure of the preloader is too large, and the slab is damaged in the preloading stage.
Generally, the height of the slab compressed by the pre-press is 6-8 times of the thickness of the plain plate after hot-pressing, and the pressure at this time should be the appropriate pressure of the pre-press.
3) The axis line of the upper and lower pressure rolls of the preloader shall be in the same vertical plane if
The offset displacement deviates the direction of roll pressure from the direction of slab movement, tearing slab causes slab cracks. Therefore, in the production process, attention should be paid to overhaul the preloader to ensure the correct installation of the upper and lower pressure rollers.
4) Press inlet pressure is set too high, which is easy to break in the process of inlet exhaust.
Cracks are formed in bad slabs, and even discount of entry slabs may occur in severe cases. For the Difenbach press used by our company, the pressure here is 2.5-3.0 MPa.
5) The moisture content of fibers is too high or there are many fine fibers. In the course of paving, the qualified fibers per unit area are less, the toughness of slab is poor, and it is easy to be cracked in high-speed transmission. Because of the high moisture content, the adhesion of slab to conveyor belt is large, so it is possible to crack the surface of slab in high-speed transmission. Therefore, the moisture content of fibers should not be too high, and the proportion of qualified fibers should be ensured.
4. Layered Bubble and Bubble of Products
There are three reasons for layered bubbling of products: first, the higher moisture content of fibers, the bubbling or direct stratification occurs when the steam pressure in the plate is greater than the bonding force during hot pressing; second, the lower moisture content of fibers and the faster speed of press; third, the bubbling stratification occurs when the glue in the core layer of slab does not reach the curing temperature; third, the steel strip is repaired (especially after a large area of excavation) because of the steel strip. Because of the thickness and stress, the poor heat transfer results in bubbling or bulging. According to the causes of stratified bubbling, the process regulations should be strictly observed, and the moisture content of fibers and the press speed should be controlled.
The reason for bulging is that fibers or other sundries from the roof fall on the slab and enter the press from the scraping head to the entrance of the press. Therefore, in the course of paving, it is necessary to prevent agglomerated fibers and foreign materials from falling into the slab. With the rapid development of medium density fiberboard production, its production enterprises are facing serious challenges. How to improve product quality to meet market requirements and deal with the relationship between product quality and production cost has become an important issue for entrepreneurs to deal with. Imported products of continuous press production line lead the market with high quality and multi-specifications. In the fierce market competition in the future, we should control every quality link, reduce quality loss, constantly improve product quality, provide customers with high-quality products, and increase benefits for enterprises.
But black paper or dark decorative paper does not need the opacity of urea-formaldehyde resin, and the use of urea-formaldehyde resin will also make the paper surface white, white stripes or whitening of the board surface easily appear in the process of pressing, so it is better to use pure melamine resin in the impregnation process of black or dark paper. In addition, for products with high surface quality requirements of wear resistance and corrosion resistance, such as impregnation of decorative paper for countertops and floors, pure melamine resin should be used.
Impregnated paper production technology involves a wide range of factors, from equipment adjustment to production process, and even production environmental hygiene will have a great impact on it. In actual production management, managers are required to have sufficient professional knowledge and practical management experience at the same time, and to exchange and cooperate with experts at home and abroad, so as to continuously improve the production technology of impregnated paper.