Among the plastic products sold on the market today, the number of extruded articles is increasing. Plastic pipes, plates, films and profiled materials have been in the application of a very important position. In the case of
Extrusion forming is a continuous molding method in which a powdery or granular raw material is heated and melted and then subjected to a cooling method by extrusion pressing and then subjected to cooling and solidification. So the extrusion molding device must have the three functions of melting, shaping and curing. In the case of
The melting function is borne by the extruder. The shaping function is borne by the extrusion mold and the shaping mold, and the curing function is borne by the shaping mold and the cooling mold. As a complete extrusion device, in order to organize the extrusion of the need to set the traction machine, cutting machine and crimping machine and other auxiliary devices. In the case of
Extruded appearance and quality of good or bad, to a large extent depends on the quality of the mold. In addition, the uniformity of the melt delivered to the mold in the extruder and the mixing process of the melt in the mold also have an effect on the quality of the extrudate. For a variety of cross-sectional shapes of the extrudate, generally along the extrusion direction of the same shape of the cross-section, but in some special circumstances, the rear of the mold set a special device or a special method of operation can change its cross-sectional shape. Such as embossing or blow molding
In recent years, in addition to a single material extrusion molding, there are a variety of plastic in the mold or die as one of the extrusion molding of the co-extrusion method. Also developed with aluminum, iron, wood and other materials co-extrusion process, so that the structure of the extrusion die is also increasingly complex.
Second, mold design
A mold which occupies an important position in the extrusion molding apparatus is supplied with a melt continuously from the pressure of the screw to 3CMPa and the melt is formed into a desired cross-sectional shape by the die. The process is a circular cross section melt fed from the cylindrical barrel through a coupler into a mold having a circular or rectangular inlet which, after changing the shape in the mold, squeezes the desired cross-sectional shape from the gap of the mold end Out of things. For this reason, from the circular or rectangular inlet to the melt flow path in the mold between the openings near the exit portion of the cross-sectional shape of the molded article, a streamlined design is required to ensure that the change process is smooth and non-retentive. In the case of
In addition, since the plastic melt used is a non-Newtonian fluid, there is also an elastic property while having a viscous property, so that the characteristics of the melt flow must take full account of the effect of elasticity.
Therefore, in the design of extrusion die must focus on the following aspects. In the case of
1, the rheological properties of the melt within the mold on the mold melt flow characteristics of the inspection is to facilitate the required amount of extrusion to meet the quality requirements of the extrudate, which must mold the melt flow path design The In particular, the gap of the mold end, under a reasonable pressure distribution, through the required amount of extrusion, in order to obtain the shape and size of the mold gap shape of the same high-quality extrudate. The non-Newtonian fluid properties exhibited by the various ferrules are different from those of the viscous and elastic properties. In the case of
2, the mold structure of the extrusion die at the entrance and exit of the different shapes. So that the flow of the flow path into the extruder inlet is changed during the flow of the melt into the outlet. In the design of such changes in the shape of the cross-section, must be used in considering the rheological properties of plastic used to ensure its smooth and non-stranded flow, that is, according to the streamlined design. As for the melt into the gap at the entrance of the pressure, in order to make the extrusion process is stable, should be able to make uniform everywhere. In the case of
The structure of the extrusion die consists of four parts: the melt distribution part of a adjustment part of a buffer part of a part of the shape to facilitate the analysis and discussion, is now extruded sheet extrusion die introduced, should be into the extrusion die round Or rectangular cross-section of the melt into a flat extrudate. The melt distribution portion is provided in accordance with the distribution of the melt to the vertical direction of the center axis, and the pressure is also decreased when the melt is expanded to both sides, and the pressure of each portion is also different. That is, the pressure is inversely proportional to the distance from the entrance.
Therefore, the next step must be to adjust the pressure. In the axial flow generated by the melt under different pressures, the adjustment part is set in order to realize the vertical plane with the central axis in accordance with the principle that the flow resistance is large and the low resistance part is encountered The pressure within the equal. But even if the pressure values are the same, the melt in the case of the flow of the shear state is not the same. The pressure at the outlet is the same, but the shear is subjected to different shear.
If the melt of this state is fed directly into the forming portion, the pressure is the same but the residual shear stress is not the same, so that the expansion state of the extruded object from the mold gap is different, so that the extruder shape And the mold gap shape differences. In the case of
It can be seen that the melt into the forming part not only requires the same pressure, but also the stress state should be the same. Although the pressure state can be different through the shape of the part of the entrance to the shape of the calibration, but in fact this practice is very difficult, unstable in the production applications. In the case of
To this end, a buffer portion is provided before the forming portion, and the melt having the different shear history is buffered to be the same as the stress state. If the cut in the distribution and adjustment parts are the same in the section, there is no need for buffering. In the case of
If the pressure of the melt at the inlet of the forming portion is the same as that of the inlet, the flow rate at the exit of the die gap is the same as the shape of the forming portion. In the case of
3, the temperature adjustment In the extrusion die are generally installed with a belt heater, therefore, the need to adjust the mold temperature, the mold temperature is maintained at the set temperature range of 1 ° C. This method is simple to consider the plastic viscosity and temperature dependence, the use of appropriate adjustment of the temperature can be. In the case of
4, the flow of different shapes in the cross-section of the basic cross-sectional shape of the melt flow path, you can use the flow rate, pressure and flow to show the relationship between the flow state. To find this relationship, the following assumptions are made: the melt is a non-compressible material;
The melt satisfies the power function: ie, or t T – shear stress, kn – coefficient, n, n flow index; flow wall without melt sliding, ie wall velocity v = 0. Similarly, , It can be extended to the quantitative analysis and calculation of melt in a variety of cross-section flow path flow state, to ensure the rationality of the design of extrusion die structure.