Influencing Factors of Plasticizing Ability of Injection Molding Machine

Abstract:

Injection molding is one of the most commonly used and most effective methods for processing plastic products, and it is widely used in mobile phone appliances, the automobile industry, aerospace, and other fields. Injection molding machine integrates mechanical, hydraulic, and control technologies, and can repeatedly process plastic parts with precise dimensions and complex shapes. It is also one of the most commonly used plastic molding devices, with high molding efficiency, strong adaptability, and less post-processing.

 

Over the years, the development of injection molding machines has been very rapid. The processing of products such as light guide plates has higher and higher requirements on the precision, performance, and production efficiency of injection molding machines, which requires continuous improvement and perfection of injection molding machines. In recent years, many domestic and foreign scholars have conducted a lot of research on precision injection molding, and have explored new injection methods, such as injection compression molding, high mold temperature injection molding, etc.

 

Main raw materials and equipment: 

The raw materials used in the experiment include: high-density polyethylene (PE-HD), melt flow rate 4g/10min; polypropylene (PP), melt flow rate 3, 11, 27 for 10min; acrylonitrile Butadiene-styrene copolymer (CABS), melt flow rate 23g/10min; use an oscilloscope as timing tool (192B/C, American FLUKE company), use the electronic scale as weighing tool (FM-048, Shanghai Yousheng Weighing Apparatus Co., Ltd.); the speed is tested by a speed measuring electric eye (PM08-02N, Taiwan Yangming FOTEK Company).

 

Test method: 

Heat the barrel to the set temperature (up to 2300C) and keep it warm for 20 minutes, then carry out the storage and injection actions, and start the test after observing that the storage time is basically stable (estimated more than 10 times); the nozzle aperture is the standard aperture of the injection molding machine, At 20% standard injection rate, inject 2.5 times the stroke of the screw diameter (D) and the injection volume of the whole stroke, record the storage time and the plastic quality after melting and solidification; when the machine is in a stable injection state, measure it in 6 times Get 6 data, take the average value to get a plasticizing capacity value (the ratio of the total injection volume to the pre-plastic time). Calculate the corresponding density according to the stroke and plastic quality; video the whole process of the test, calculate the relative density and time according to the screw displacement displayed in the video afterward to determine the instantaneous plasticizing capacity, record it in the chart, and use the software simulation curve function to make a corresponding chart.

 

The effect of screw structure on plasticizing ability: 

There are many types of screws for injection molding machines, and the internal parameters of different screws, such as compression ratio, pitch, segment, etc., are different according to different applications. This test did not study the internal parameters of the screw. Instead, the comparison is made on the basis of Haitian’s existing conventional screws, and the plasticizing capabilities of different types of screws under certain conditions are obtained. The D is 140mm and the aspect ratio is 24. Using PE-HD as the test material, the test conditions are screw speed 100r/min, screw displacement 680mm, back pressure 0.5MPa, and maximum molding temperature 230°C. The plasticizing ability of different screw structures under the same conditions is different. The instantaneous plasticizing ability of the separating screw is the highest among many screws, which is related to the structure and internal parameters of the screw, because the separating screw can effectively separate the solid-liquid phase, thereby producing the largest conveying capacity. In addition, the separating screw increases the thrust of the screw on the raw material under the same conditions, so that the conveying speed of the raw material is accelerated, while the single-edge screw has a larger groove depth and meal compression ratio than the double-edge screw, resulting in its instantaneous plasticizing ability slightly larger.

 

Effect of process parameters on plasticizing ability: 

The screw speed affects the transportation of plastic in the screw, the thermal history of plasticization, and shear efficiency, so it is an important parameter that affects the plasticizing ability, plasticizing quality, and molding cycle of the injection molding machine. Most of the heat energy required for plastic melting comes from the heat generated by the screw rotating and shearing. The higher the rotation speed, the higher the shear heat provided, and the faster the corresponding plasticizing speed. The test material is PE-HD, the test screw is a separate screw, D is 80mm; the test conditions are screw speed 108, 145, 156r/min, screw displacement 450~, back pressure 1.0MPa, and maximum molding temperature 230℃. According to the maximum line speed value of PE-HD material. .8m/s, it can be obtained that D is 80~, the highest reasonable speed for processing PE-HD can reach about 190r/min. When the rotational speed reaches 156r/min, the maximum instantaneous plasticizing capacity reaches 75 g/s, while when the rotational speed is as low as 108r/min, the maximum instantaneous plasticizing capacity is only 52g/s.

From the speed ratio (156/108=1.444) and the plasticizing capacity ratio (75/52=1.442), it can be seen that at a reasonable speed, the plasticizing capacity is basically linearly related to the screw speed. During the melting process of the plastic, the molten material continuously moves to the front end of the barrel (metering chamber), and accumulates more and more, gradually forming a pressure to push the screw back. In order to prevent the screw from retreating too fast and ensure the uniform compaction of the melt, it is necessary to provide pressure in the opposite direction to the screw. The pressure that prevents the screw from retreating in the opposite direction is called back pressure, and back pressure is also called plasticizing pressure. The test raw materials are PE-HD, PP (melt flow rate 11g/10min), ABS, the test screw is a separate screw, D is 80mm; the test conditions are screw speed 145r/min, screw displacement 450mm, back pressure 0.5, 1.0MP a, The highest molding temperature is 230°C. About 10% of the heat energy in the plasticizing process of the injection molding machine comes from the shear heat of the screw, and about 30% of the heat energy comes from the heat energy provided by the barrel heating ring.

Barrel heating has two main functions in the plasticizing process, melting cold material and assisting in plasticizing. Molding temperature is an important technical parameter in the production process, which is directly related to the plasticizing quality and plasticizing ability. The test material is PP, the test screw is a separate screw, and D is 150mm; the test conditions are screw speed 110r/min, screw displacement 680mm, back pressure 0.5MPa, and the highest molding temperature 210, 230, 250°C. The molding temperature of PP material is wide, the temperature is increased by 20°C, and the maximum plasticizing capacity is increased from about 230g/s to about 250g/s, which is about 8% higher. At the same time, the lifting amount at different displacement points is basically the same, so Under certain conditions, the effect of temperature change on plasticizing capacity should be basically the same. The most commonly used drive units for injection molding machine screws are hydraulic motors and electric motors. The power for screw rotation comes from the drive unit, which has a direct relationship with the screw speed and screw torque. In the early stage, there were few studies on the plasticizing ability of the drive unit. Due to the in-depth research on this test, the author found that the driving unit also has a certain influence on plasticizing ability. The test material is PE-HD, the test screw is a separate screw, and D is 110mm; the test conditions are screw speed 108r/min, back pressure 0.5MPa, and the highest molding temperature 230C. 

 

Effects of different raw materials:

The test raw materials are PE-HD, PP (melt flow rate 11g/10min), ABS; the test screw is a separate screw, and D is 80mm; 450~, back pressure 0.5MPa, maximum molding temperature 2300C. The maximum screw displacement point is 450mm, and it can be obtained that the PE-HD material is about 80g/s, the ABS material is about 68g/s, and the PP material is about 64g/s. The melting density of these raw materials: PE-HD is. .73g/cm3, ABS is 0.88g/cm3, and PP is 0.73g/cm3. From the melt density value of raw materials and the plasticizing ability value of raw materials, the plasticizing ability cannot be calculated simply by density conversion. 

 

The influence of different melt flow rates of the same raw material:

The test raw material is PP, and the melt flow rate is 3, 11, and 27g/10min respectively; the test screw is a separate screw, and D is 100~; the test conditions are screw speed 110r/min, screw displacement 470~, back pressure 0.5MPa, The highest molding temperature is 230°C. The same raw materials with different melt flow rates have different instantaneous plasticizing abilities under the same conditions. Under the condition of reasonable rotating speed for the same raw material (for PP material, the maximum allowable rotating speed of the screw is 100 to 150r/min), the higher the melt flow rate, the greater the plasticizing ability.

 

Conclusion: 

The screw structure has a great influence on the plasticizing ability. The separation screw can effectively separate the solid-liquid phase, and the instantaneous plasticizing ability is the best; with the increase of the screw speed and molding temperature, the plasticizing ability increases; as the screw back pressure, As the displacement of the screw increases, the plasticizing capacity decreases; different driving modes have different degrees of achieving the maximum instantaneous plasticizing capacity. Generally speaking, the effect of the electric driving mode is better than that of the hydraulic driving mode.

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