Analysis of the design of plastic extrusion die
1. Among the plastic products sold on the market today, the types of extruded products are increasing. Plastic pipes, plates, thin films and special-shaped materials have been playing a very important role in the application.
Extrusion forming is a continuous forming method for cooling and curing after heating and melting powder or granular raw materials. Therefore, the extrusion forming device must have three functions: melting, forming and curing.
The melting function is borne by the extruder. The forming function is assumed by the extrusion die and the plastic mold, and the curing function is borne by the plastic mould and the cooling mold. As a complete extruder, an auxiliary device, such as a tractor, a cutting machine and a winding machine, is required in order to finish the extrusion of the extruder.
The appearance and quality of the extrudate depends on the quality of the mold to a great extent. In addition, the uniformity of the molten material in the extruder and the mixing process in the mold will also affect the quality of the extrudate. For a variety of extrusion parts with different section shapes, they usually have the same shape section along the extrusion direction. However, in some special cases, the special section or special operation method can be used to change the section shape at the back part of the mold. Such as: embossing or blow molding.
In recent years, in addition to the single material extrusion forming, there are many kinds of plastics in the mold or die outside the mold as one of the co extrusion forming process. The co extrusion process with other materials such as aluminum, iron, wood and other materials has been developed to make the structure of the extrusion die more and more complex.
2, the mold design occupies an important position in the extrusion forming device. The pressure is continuously supplied to the mold from the screw ~3CMPa, and the extrusion material is formed by the mold to form the desired section shape. This process is a circular section melt material delivered from a cylindrical material barrel. Through the coupling, it enters into a circular or rectangular entry mold. After changing the shape in the mold, the extruder is extruded from the slot of the die terminal. For this reason, from the round or rectangular inlet to the melt flow path between the outlet part of the exit part close to the shape section, a streamline design is needed to ensure the smooth process and no stagnation location.
In addition, because the plastic melts used are non Newtonian fluids, they have elastic properties as well as viscous properties. Therefore, the effect of elasticity must be fully considered for the characteristics of melt flow.
Therefore, the following aspects must be taken into consideration when designing the extrusion die.
2.1, the rheological properties of molten materials in the mold are investigated for the melt flow characteristics in the mold, so as to get the extrudates that meet the quality requirements at the required extrusion volume, so we must design the melt flow path in the mold. Especially for the mold terminal gap, under the reasonable pressure distribution, we can get the high quality extruder with the same shape and size and the same shape as the mold gap through the required extrusion volume. The characteristics of non Newtonian fluid displayed by various kinds of molten materials are different from the viscous and elastic properties.
2.2 the shape of the die structure is different from the shape of the entrance and outlet. Therefore, the flow path section shape of the flow path changes during the flow of the melt to the outlet of the extrusion die. When designing the shape change of the section, we must consider the rheological characteristics of the plastic, and ensure its smooth and no stop flow, that is, according to the streamline shape. As for the pressure of the entry of the melt into the entrance of the gap, in order to stabilize the extrusion process, it should be able to make all the places uniform.
There are four parts in the structure of the extrusion die.
A portion of an adjustment part, a buffered part of a shaped part.
For convenience of analysis and discussion, now the extrusion die for extruded plate is introduced, so the round or rectangle section melt into the extrusion die should be expanded into flat extrusion. According to the vertical direction of the central axis, the distribution part of the melt is allocated according to the vertical distribution of the central axis. When the material expands to the two sides, the pressure also decreases, and the pressure of all parts is also different. That is, the pressure is inversely proportional to the distance from the entrance. Therefore, the next step must be adjusted to the pressure. The axial flow of melt under different pressure should be adjusted according to the principle of high flow resistance, low pressure and low resistance. But even if the pressure is the same, the shear state of the flow under the above conditions is different. The pressure at the outlet of the part is the same, but the shear experience of the melt is different. If the melt is directly fed into the shaped part, the pressure is the same everywhere, but the residual shear stress is different. So the expansion state of the extruded object from the mold gap is different, so that the shape of the extrusion part and the gap shape of the die are different.
It can be seen that the molten material entering the shaped part should not only be the same pressure, but also the stress state. Although the pressure state can be calibrated through the shape of the shaped part of the entrance, the practice is very difficult and unstable in the production application.
To this end, a buffer part is set before the form part to cushion the molten material with different shear records and become the same molten material as the stress state. If the shear in the distribution section and the adjustment part is the same in the section, it does not have to be buffered.
If the pressure is the same at the entrance of the shaped part, and the stress state is the same, the flow velocity at the exit of the die is the same by the shape of the shaped part.
2.3, the temperature adjustment usually needs to install the belt heater on the extrusion die. Therefore, the mold temperature should be adjusted to keep the mold temperature at the set temperature of 1 degrees C. This method is simple to consider the dependence of the plastic viscosity and temperature, and the appropriate temperature adjustment method can be used.
2.4, the flow status in different shape sections can be used to show the flow state in relation to flow velocity, pressure and flow volume for the flow path of the basic section. In order to find out such a relationship, the following assumptions are required: