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What are the process factors affecting plastic extrusion of vertical servo injection molding machines?

Dec 05, 2019

(1) Temperature. Temperature has a significant effect on the extrusion of the plastic and the performance of the parison of the vertical servo injection molding machine. Increasing the heating temperature of the vertical servo injection molding machine will have the following effects: it can reduce the melt viscosity, improve the melt flowability, and reduce the power consumption of the vertical injection molding machine; it can appropriately increase the screw speed without affecting the mixing and plasticity of the material Effect; help to improve the strength and brightness of the final product; help to improve the transparency of the final product. However, if the melt temperature is too high, the extruded parison is prone to sag under its own weight, causing uneven longitudinal wall thickness of the parison; it will extend the parison inflation cooling time; it will expand the shrinkage of the final product; it will cause pvc Isothermal-sensitive plastics degrade, which significantly reduces the strength of engineering plastics such as Pc. Therefore, the heating temperature of the extruder should be set in accordance with the principle that under the premise that it can extrude a smooth and uniform parison without overloading the transmission system, in order to ensure a high melting of the parison Body strength should be as low as possible.


When setting the heating temperature of the vertical servo injection molding machine, the temperature of the feeding section should be relatively low, which prevents the material from blocking at the feeding port and affects the transportation of the material; the higher temperature of the compression section is conducive to the mixing and plasticization of the material; the extrusion section The temperature can be lower than the compression section and higher than the feed section, which is conducive to the stable and uniform feeding of the machine head.


The heating device of servo injection molding machine generally adopts electric heater and air cooler at the same time to adjust the heating temperature of vertical servo injection molding machine. In order to control the deviation of the heating temperature of the extruder, a proportional (P) integral (I) differential (D) temperature controller is often used. Their functions are:


① Proportional effect. There is a proportional relationship between the heating current and the temperature deviation. The smaller the heating current, the smaller the temperature deviation.

② Integral effect. The heating current is proportional to the integral of the temperature deviation over time. In this way, even if the deviation is very small, after a certain period of time, the static deviation can be eliminated, and the static accuracy of the temperature control system can be improved.

③ Differential action. The heating current is proportional to the temperature deviation versus time. Then, the faster the temperature deviation appears, the larger the corresponding change in heating current is, which can improve the ability of the heating system to suddenly interfere with the outside world.

Because of these effects, the PID temperature controller can control the temperature deviation of the extruder within the range of ± 1 ° C, and the power consumption is reduced by about half accordingly.


(2) Melt pressure. The melt entering the nose should be evenly pressurized. Properly increasing the pressure of the melt in the vertical injection molding machine can make the materials evenly mixed and stabilize the performance of the parison and the final product. For high-molecular-weight polyethylene, blends of high-density polyethylene and low-density polyethylene, and polymers colored with color masterbatches, sufficient melt pressure can make the parison have a good appearance and reduce "crystal points" and cloudiness Shape pattern. However, due to debris blocking the screen of the machine head, the melt pressure is too high, which will increase the load of the vertical injection molding machine and damage the machine. The screen should be replaced and cleaned in time to maintain a stable pressure for the melt entering the machine head. Adopting a resin with a low melt flow rate, reducing the heating temperature of the extruder, and increasing the speed of the extruder screw can increase the melt pressure of the extruder.

In order to control the melt extrusion pressure, a melt pressure measuring instrument can be installed in the discharge section of the vertical servo injection molding machine. Commonly used instruments for measuring melt pressure are mechanical pressure gauges, hydraulic (silicone oil, silicone grease, mercury) pressure gauges, pneumatic pressure gauges and electrical pressure gauges.


(3) Melt conveying speed. The larger the melt transfer speed, the larger the extrusion capacity of the vertical servo injection molding machine. Increasing the conveying speed of the melt can improve the self-weight sagging of the parison and increase the wall thickness of the parison. The melt conveying speed increases with the increase of the screw diameter and is less affected by the head pressure. Increasing the speed and heating temperature of the extruder screw can increase the melt transfer speed accordingly. However, when the screw rotation speed is increased to a certain value, melt fracture is liable to occur. In the plastic injection experience of vertical injection molding machines, the three factors mentioned above are interdependent. At the same time, the volatility of these three factors is required to be small. If there are large fluctuations in temperature, pressure, and melt conveying speed, not only the uniformity and repeatability of the wall thickness of the parison will deteriorate, but also the mechanical properties and dimensional stability of the final product will have a large difference Down.