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

Mar 18, 2020

(1) Temperature. Temperature has a significant effect on the extrusion of plastics and the performance of parisons for hybrid injection molding machines. Increasing the heating temperature of the vertical injection molding machine will have the following effects: it can reduce the viscosity of the melt, improve the melt flowability, reduce the power consumption of the vertical injection molding machine; can properly increase the screw speed without affecting the mixing and plasticizing of the material Effect; Conducive to improving the strength and brightness of the final product; Conducive to improving 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 Degradation of isothermally sensitive plastics significantly reduces the parison 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 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 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 the oil-electricity hybrid injection molding machine generally adopts an electric heater and an air cooler is installed at the same time to adjust the heating temperature of the vertical 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 heating current and 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 correspondingly reduced by about half.

(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 concentrates, sufficient melt pressure can make the parison have a good appearance and reduce "crystal points" and cloud Shape pattern. However, due to debris blocking the screen of the head, the melt pressure is too high, which will increase the load on the vertical injection molding machine and damage the machine. The screen should be replaced and cleaned in time to maintain a stable pressure of the melt entering the 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 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 injection molding machine. Increasing the transfer 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 rotation speed of the screw is increased to a certain value, melt fracture easily occurs. 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 the temperature, pressure and melt conveying speed fluctuate greatly, not only the wall thickness uniformity and repeatability of the parison will worsen, but also the mechanical properties and dimensional stability of the final product will be greatly different. Down.