overview
With the rapid development of plastic industry, the raw material supply in the process of material extrusion simply rely on manual weighing, mixed feeding this way is difficult to ensure its work quality and efficiency, especially with the rapid development of multi-layer co-extrusion technology, The raw material supply of plastic extrusion process has gradually evolved into an automatic system engineering integrating raw material drying, storage, transportation, weighing, mixing and extrusion control. On the other hand, today's plastic processing industry is facing increasingly serious competition and continuous market changes, how to ensure the quality of products and competitive position in the market, a high degree of flexibility in production is very and absolutely necessary, and reduce the cost of products is becoming more and more important. Weighing measurement control technology is a new technology which has been developed rapidly to meet the increasing demand of people in the plastic extrusion industry, and has attracted wide attention.
In the process of plastic extrusion molding, the raw material supply of extruder is usually divided into three steps: the first is to weigh each component required by the extruder, the second is to mix the components required by each extruder, and the last is to supply the evenly mixed material to the extruder hopper through the screw plasticizing and melting extrusion. The conventional practice is to carry out off-line weight, that is, how many packages of main materials (25kg) and how many packages of auxiliary materials, and then artificial mixing into the extruder hopper. Such a practice often produces a series of problems: such as inaccurate weighing; The mixture is not uniform, especially the dispersion of additives in multi-component ingredients is not sufficient; Sometimes the waste of raw materials and the increase of labor cost are caused by excessive artificial ingredients. Unable to provide real-time extruder production, lack of quality control and so on. Therefore, it is of great importance to use on-line continuous weighing of the mixture, which can effectively ensure that each component is measured and weighed according to the predetermined weight percentage, so as to achieve the purpose of uniform mixing.
On the other hand, the conventional extrusion control in the process of plastic extrusion requires the operator to set the speed of the extruder and manual adjustment, and this cannot guarantee the constant extrusion output, and over time, due to the fluctuation of temperature and pressure, the condition of the filter screen and other related process changes, extrusion output will often change periodically, Because the speed of screw and tractor is relatively constant, the thickness deviation of the product in the meter weight, that is, the longitudinal thickness of the product, seriously affects the quality of the product. The figure below shows the fluctuation of the meter weight of the product with the extrusion production.
In terms of extrusion control, namely meter weight control, it is usually divided into three ways: traction meter weight control mode, extruder meter weight control mode and traction extruder synchronous meter weight control mode, as shown in the figure below. 1) Traction meter weight control means that the meter weight of the product can be set in advance. In the automatic mode, the deviation between the set value and the actual value can be eliminated by adjusting the speed of the tractor; 2) The meter weight control mode of extruder means that the meter weight of the product can be set in advance. In the automatic mode, the deviation between the set value and the actual value can be eliminated by adjusting the speed of the extruder screw. If the extruder is fed by a metering device, the screw speed of the metering feeding device will be adjusted. 3) Extruder meter weight control and traction extruder synchronous meter weight control means that the output of the product can be pre-set, and can be adjusted respectively the new working point, the controller synchronously adjust the extruder screw speed and the speed of the tractor, so that the product meter weight is constant. It is mainly used in the production process of recycled materials.
Due to the use of volume measurement it exists:
1. Measurement accuracy must be adjusted depending on time; 2. The setting of equipment data shall be operated by professional personnel; 3. Formula and processing data need to be converted and corrected; 4. The uniformity of the volume and density of the raw material particles is required to be high. Therefore, it is rarely used in the process of plastic extrusion.
Due to the use of gravimetric method, it has the following advantages:
1. Stable and accurate control; 2, can use a variety of different raw materials; 3, automatic calibration function; 4, the setting is simple - only a very short time; 5. Measure by weight - not by volume; 6, eliminate fuzzy operation; 7. Fast ROI. The weightless metering method is suitable for extrusion production with high yield and provides auxiliary control for multiple mixers used in multi-layer co-extrusion equipment. The weight measurement method is suitable for all raw materials with weight measurement has: 1, extremely high accuracy; 2. Automatic correction; 3, directly set the percentage of each component; 4, set directly with the dial wheel; 5, the operation of the machine is simple.
Several typical weighing devices are described
