Tim Chou, Engineer at Product R&D Division of Moldex3D
In injection molding, the low-temperature plastic solid pellets are heated and transferred to high-temperature melt through the screw rotation and heater on the barrel. Melt temperature will affect the product quality. If the temperature is too high, material degradation and yellowing can occur. If the temperature is too low, the material fluidity will decrease and thus cause more flow resistance. The melt temperature cannot be directly controlled by the injection machine setting but can be controlled indirectly by the plastification process. There are three major factors in plastification that will affect the melt temperature, which are, the heater temperature, the screw speed, and the back pressure. How the plastification process affects the material temperature will be interpreted as follows.
The relationship between the screw rotation speed and melt temperature
The rotation of the screw will drive the plastic flow, and the shear heat generated in the process will increase the melt temperature. If the screw speed is too high, the material temperature will deviate too much from the heater temperature; if the speed is too low, it will increase the metering time and reduce production efficiency. As shown in Fig. 1, when the screw speed is higher, the temperature at the center is higher; while the two sides are still at the temperature of the heater, resulting in uneven melt temperature. When increasing the screw speed to reduce the metering time, it is necessary to be aware of the effect of the consequent temperature rise on the plastic.
Fig. 1 The relationship between the screw rotation speed and melt temperature
The relationship between the back pressure and melt temperature
During metering, the screw will transport the melt to the front end of the barrel, and the melt stored at the front end will cause the pressure to rise. When the front pressure is higher than the back pressure, the screw will be pulled back. If the back pressure is too high, the screw cannot easily retreat, and the contact time between the plastic and the screw increases, generating too much shear heat that causes the material temperature to rise. If the back pressure is too low, the screw will retreat too fast, resulting in inaccurate and insufficient melt metering that may contain air. Proper back pressure can make a proper time for the plastic to interact with the screw so that the melt temperature can be well controlled.
The relationship between the heater and melt temperature
Each heater generally has different settings, according to the temperature sensitivity of the material. Before the plastic melts, the heat majorly comes from the heater. At this time, the plastic temperature can be lower than the temperature of the heater. As the plastic melts down and enters the rear section of the screw, the shear heat gradually increases, and the melt temperature may be higher than the heater temperature.
Fig. 2 The relationship between the heater and melt temperature
During plastification, the phase change of the plastic and the space change of the screw rotation needs to be considered. The two factors make the simulation too complicated, so the model needs to be simplified. The process of plastic melting from solid particles can be divided into three parts : solid bed, melt pool and melt film, on which the phase change simulations are based. For the complex geometry of the screw, we can use the Unwound Method to straighten the spiral channel between the screw and the barrel into a two-dimensional  or three-dimensional [3,4] channel, and then assume that the screw is stationary, and the barrel rotates in the opposite direction. Then, the dynamic computation field can be transformed into a fixed space, greatly reducing the computation complexity. Taking Moldex3D as an example, we only need to fill in the dimensions of the screw in the window, and the software can automatically complete the model required for simulation. Each analysis with the simplified model takes about three minutes, and data can be quickly generated.
Fig. 3 The screw parameter settings
The results of the screw at each location can be displayed in Moldex3D. Fig. 4 shows the temperature distribution from the screw to the barrel surface under different cross-sections. Usually, we are mostly concerned about the final melt temperature. We can see that the highest temperature here is 2℃ higher than the heater. Users can check if the melt temperature is within the required range.
Fig. 4 The distribution of the melt temperature
Fig. 5 & 6 are respectively the average temperature and average pressure at different locations. The average temperature can be the basis for adjusting the temperature of every heater. The average pressure can be the reference for the screw rotation speed and back pressure settings.
Fig. 5 The distribution of the average melt temperature
Fig. 6 The distribution of the average melt temperature
Fig. 7 shows the solid plastic proportion at different locations. When the value is zero, it means the plastic has fully melted. We can check whether the plastic is completely melted under the current conditions from this figure.
Fig. 7 The distribution of the melted plastic proportion
The table below is the comparison between the simulation and experiment. The molding conditions are the screw speed of 150 RPM and the back pressure of 4.5 MPa. The maximum temperature rise represents the difference between the highest temperature of the melt and the heater. In general, the smaller the temperature difference the better, meaning that the plastic temperature is uniform and almost equal to the temperature of the heater. The plasticizing time is the time required for the screw to reach the specified metering position, which must be lower than and is better to be close to the cooling time. While the plastification process is performed inside the barrel, the previous molding shot is getting cooled inside the mold at the same time. If the plasticizing time is less than the cooling time, it means that the plastic stays in the barrel for a long time and may get degradation; if the plasticizing time is longer than the cooling time, it means that the metering cannot be completed before the mold open, which will delay the next cycle.
Table 1 Comparison between the simulation and experiment of HIPS60
Moldex3D’s plastification analysis provides a feature to evaluate the screw movements as well as the pressure and temperature variation inside the barrel. Simultaneously, we can observe the temperature rise and pressure drop caused by the different process settings and the geometric structures during material plastification. Thus, we can control the molding conditions more efficiently.
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