Views:4 Author:Site Editor Publish Time: 2020-07-09 Origin:Site
The cold press of wet fiber slabs is mainly for slab dehydration, and the roller dehydration method is commonly used. In order to prevent the sudden increase of applied pressure and cause the slab to rupture, four pairs of pre-pressing rollers were gradually pressurized, and then two pairs of rolls reached the maximum pressure to reduce the slab moisture content to 65% to 70%. A small number of flat cold presses that also use the upper pressure type have a drainage hole on the lower plate, and the wet slab formed by the long net and cut according to the specifications is placed in the flat cold press for dehydration under pressure, and the dehydration efficiency is higher. The water content of the billet can be reduced to 50% to 55%.
The cold pressing of dry fiber slabs is basically similar to the cold pressing of particle board slabs. The thickness of the slab after dry fiber forming reaches 30-40 times the thickness of the product, so cold pressing is necessary. The degree of compression of the fiber slab cold press is related to many factors such as fiber separation quality, water content, raw material type, sizing amount, cold pressure, molding method and process.
The cold pressed slab is 2/3 -3/4 less than the original thickness, or 8-10 times the thickness of the finished product. After cold pressing of the slab, there is rebound in the thickness direction. The compression rate and rebound rate can be calculated as follows:
Compression rate η=×100% rebound rate β=×100%
In the formula: h1 = slab forming thickness (mm);
h2 = stable thickness after slab rebound (mm);
h3=The minimum thickness (mm) of the slab during compression.
For wood raw materials, the compression rate is generally 60% to 75%, and the rebound rate is 15% to 30%. For bagasse as the raw material, the compression rate is 70% to 90% higher than the wood raw material. The width of the slab after cold press The direction should stretch about 5%.
The types of cold presses for dry fiber slabs are divided into periodic flat cold presses and continuous belt cold presses, and continuous cold presses are the most commonly used. The machine is divided into three parts: the first part is the guide part with an inclination angle, which slowly reduces the thickness of the slab with the smallest inclination angle; the second part is the main pressing part composed of the upper and lower pressing rollers, the pressing roller Gradually pressurized, the maximum linear pressure is 1.4 ~ 2.5kN/cm, supplied by the hydraulic cylinder; the third part is the pressure holding section, because the dry fiber rebounds large and is difficult to compress, so there must be a certain length of pressure holding section for the slab Holding pressure, the pressure in this section can be the same as the pressing section or lower than the linear pressure in the pressing section. The holding time is 6-7s. The opening and pressure of each pair of rollers can be adjusted. The length of the cold pressing time is almost Does not affect the final thickness of the slab, the general pressure and holding time is 10-30s.
The exit end of many pre-presses now has a small inclination angle, called the departure angle, the purpose is to gradually relieve the pressure to prevent the slab from rebounding too fast, and re-inhale the air, while avoiding the separation of the forming mesh belt and the slab Peel and bring up the surface fibers.
Cold-pressed belts used steel belts in the past, but they are mostly reinforced steel wire rubber belts or PTFE nylon belts.
In a continuous belt cold press, the slab moves under compression, so the operating speed of the cold press and the speed of the forming mesh belt must be strictly synchronized.
When the slab is cold-pressed, high-frequency preheating can be used at the same time to increase the slab compression rate and reduce springback. At the same time, the hot press cycle can also be shortened. The temperature is generally not more than 70°C to prevent the resin from curing in advance.