Rheological design of the hottest rapid vulcanizat

2022-09-19
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Rheological design of rapid vulcanization system

authors: Jan gogolin, F. svaricek, V. Hartel (Universitat der BW Munchen), Peter Lipp (yellowbox engineering/lwb)

injection molded parts, EFE, that is, the development of two-stage injection molding process based on temperature control, is carried out within the framework of the cooperative project between LWB Steinl GmbH and the school of dynamic systems of the German Defense University (ubwm). Compared with the usual primary process, the system can provide more accurate conversion of thermal energy to mechanical energy. A large number of sprays from the machine completely solve the problem of vibration or direction change at low speed, and the temperature of the rubber sprayed from the nozzle is close to the temperature of the mold. Therefore, it becomes feasible to minimize the vulcanization time and vulcanization cycle time

in order to establish a controllable EFE component based on mathematical model and independently in an adjustable throttle valve, it is necessary to know the rheology and heat of the whole rubber flow. The first attempt to directly detect the temperature in the flow channel failed because the thermocouple in the rubber flow kept heating up. Therefore, in order to determine the temperature and pressure distribution of rubber under flow conditions, we simulated the injection process by computational fluid dynamics (CFD) analysis. The calculated temperature distribution is verified by a specially developed indirect temperature measurement method. This kind of sensor is also suitable for control

mechanical energy is converted into heat energy

the analysis of the whole injection molding and vulcanization process shows that it is very beneficial to increase the average temperature of the injected material in a certain step, about 20 ℃ to 50 ℃ in the initial stage of the injection molding cycle. Considering the thermal properties of rubber, this temperature increase requires a power input of at least 20 kW during the injection phase. Considering the law of conservation of energy, the maximum temperature it follows may also rise:

△ t is the volume average temperature difference, △ P is the pressure difference between the inlet and outlet. The distribution Q of heat conduction (cooling or heating separately) is inversely proportional to the flow rate V, which shows a higher flow rate (Fig. 6), which can be ignored. The only material characteristic parameter that leads to the rise of temperature is the specific heat capacity CP and density ρ。 For most rubber compounds, it causes a temperature rise of 4 to 5 ° C per 100bar

runner 3 The spindle speed can be divided into four levels 334; 673; 1470; Temperature distribution in 2961 RPM

the loss of mechanical energy of flowing rubber in the channel occurs between layers, which is based on the friction caused by its different moving speeds. In addition, because the viscous fluid has no slip phenomenon, the highest shear rate (the source of friction) occurs at the pipe wall. The middle zero shear rate of the runner is dominant. Therefore, the final temperature distribution of the channel section is uneven. In the case of heat insulation, the temperature generated by friction reaches the maximum on the pipe wall or on the cooling channel near the wall. The main part of the channel cross section is filled with cold rubber core (Figure 1)

Fig. 1 velocity distribution diagram (gray) and corresponding temperature distribution of the cold runner in viscous heating

the thermal diffusion coefficient of the rubber compound is too low to be balanced. In a relatively short injection phase, the side heat conduction leads to temperature difference. This brings about a well-known process problem, namely thermal isolation. When the temperature curve of the flowing rubber material is uneven, it will face a series of problems of branch of the channel system, which will produce diversion according to different temperatures. As a result, different flow channels even have different thermal materials geometrically (Fig. 2). Due to the temperature dependence of rubber viscosity, the cavity filler is uneven and the initial temperature of vulcanization stage is different

Figure 2 temperature curve of a typical cold runner system, resulting in uneven packing

in order to overcome this disadvantage of viscous heating, LWB Steinl developed an injection system that produces the highest temperature in the middle of the runner

in the first design, the injection molding device (EF) has a plug valve as a regulating restrictor, which is a special function of the ef-e system - this is a complete spray nozzle. This function allows the temperature rise to approach the scorching limit of the compound, and there is no risk of contamination from the previous scorched material for the next injection molding. The throttle valve component in the second design is a controllable aperture based on the wedge-shaped gap between the material inlet and the outer surface of the connecting rod of unit e (Fig. 3). It creates a rubber laminar flow with a high heat resistant layer into the middle of a fairly wide channel of the E-Cell

Figure 3 contains an ef-e injection molding system with adjustable flow valve and no nozzle

the flow numerical simulation software is used for the analysis of this system

cfd simulation

◆ three dimensional steady-state flow simulation

the flow calculation has been completed under the ANSYS CFX software platform. The numerical solution of the flow equation -navier Stokes equation - obtained in the first step is to discretize the geometric domain. In ef-e geometry, the numerical lattice is subject to large parameter changes in the transverse section. Especially at the throttle slot, the velocity and temperature gradient become large, which requires a better fine grid to cross the flow direction. The finite element simulation of flow between EF and machine nozzle requires about 4million so-called hexahedral elements. The boundary condition of the tube wall is zero velocity, the temperature is 80 ℃ and the heat transfer coefficient is 25000w/mk. The simulated rubber is an EPDM compound with a hardness of 65, with a flow index of m= 3.7, a density of 1019kg/m3, and a specific heat capacity of 1900j/(KGK)

the simulation results are shown in Figure 4 (temperature distribution), which calculates that the throttle valve position is 80% and the steady flow rate is 50 ml/s

Figure 4. Temperature distribution on the symmetry plane and wall

the results show that the temperature distribution reaches the maximum in the middle of the channel. But it is not in the absolute center, but a little towards the pipe wall. Remember that the modified thermal boundary conditions, especially for the top of the e-piston, are only an approximation of the real situation. The heat flow between the rubber flow and the surrounding metal is a transient phenomenon, which should be simulated in the form of conjugate heat transfer (CHT). Among them, the flow simulation and the temperature calculation of the metal body are carried out at the same time

◆ two dimensional transient conjugate heat transfer (CHT) simulation

in order to judge the thermal effect of surrounding metals in the thermal distribution of throttle valve, we conducted two-dimensional transient conjugate heat transfer (CHT) simulation. The total injection time is 10 seconds; The temperature of the metal part of the outer wall is set at 80 ℃, which is different from the previous three-dimensional simulation. This calculation is short-lived and involves both fluid (rubber) and solid (metal) domains. Figure 5 shows that after the injection time of 5 seconds, the dispersion of filler can be improved, the storage length of glass fiber can be increased, and the floating fiber can be improved. The temporary temperature distribution around the throttle valve slot is inserted into the two points of the thermocouple marked as S1 and S2 (see Figure 6). The initial temperature of the rubber compound is 80 ℃, and a very thin sheet along the throttle wall is heated to more than 180 ℃. The heat conducted to the metal body on the wall inhibits the rise of the nearby wall temperature and limits it to an acceptable value. The temperature distribution of the metal is probably around the narrowest position of the throttle slot

Figure 5. Temperature distribution of throttle valve after 5 seconds of injection molding time

Figure 6. Temperature distribution of thermocouples S1 and S2 close to throttle valve

simulation results show that for the function of throttle valve, the effect of external heat flow to surrounding metal is very important, so it is necessary to consider establishing a model-based control system

experimental test of E-Fe system

◆ spray experiment without mechanical nozzle

one of the series experiments, rubber is sprayed through the throttle hole, which is regarded as a thick rubber strand. In order to avoid superimposing shear effect on temperature, we removed the mechanical nozzle

this test ran seven different electronic piston positions, ranging from 0% (aperture fully open) to 100% (aperture fully closed), and six injection rates. The injection volume of each injection is still 800 ml. Record the transient data of 15 sensors. The resulting pressure flow diagram is shown in Figure 7

Figure 7. Injection pressure as a function of production capacity

one of the results shows the temperature measured by the thermometer inserted into half the length of the rubber. Figure 8 shows that different throttle valves rely on these temperatures as a function of injection pressure. Different colors mark the actual flow rate

figure 8. The temperature at the center of the middle length of the rubber strand, as a function of the injection pressure of different throttle valve apertures

the measuring points of each group themselves are incomplete straight lines, and their starting points are the same (the pressure is zero), but the slope is different. The point at the bottom marks that the injection speed is low, which lasts for about 120 seconds, and plays a role in cooling the pipe wall. At the point concentration above the center line, the spray can be seen, because formula (1) is approximately insulated. The measured temperature depends only on the effective pressure loss. The deviation of the measured points higher than 800bar led to a significant rise in the stranding temperature, which did not subside after 1 minute. The simplified point, the dotted line in the temperature pressure diagram, shows a proportional dependence on pressure loss, with a slope of 5 ℃ per 100bar

direct temperature measurement of flowing rubber is still an unsolved problem. First, the friction at the top of the thermocouple makes the temperature unreal. Second, the sensor must be stable. Its specific heat capacity and the temperature of the pipe wall will affect the transient temperature measurement

in order to verify the correctness of the flow simulation results, especially the prediction of abnormal hot spots in the rubber chain, we carried out a special experiment with rapid vulcanization rubber. To this end, the rubber began to spray. As described, the throttle valve was closed by 90%, but only for EF unit. The material in unit e contacts with the cold pipe wall at 80 ℃ and stays stationary for 10 minutes, then spray. In the cross section obtained by cutting the rubber strand, we found an abnormal vulcanized rubber core with similar shape and position. However, it is difficult to take out photos and other documents after long-term development, because there is little difference between vulcanized and unvulcanized rubber

spray experiment for closed mold

theory and experiment have proved that the eccentricity of rubber strand hot spot will affect the packing of multi cavity mold. We specially designed a mold with eight cavities with symmetrical stars (as shown in Figure 9) for inspection and verification

Figure 9. Injection mold with star runner system - results of short injection

the mold is equipped with a series of thermocouples, eliminating the possibility of uneven temperature field in the mold during the test. The mold uses a quantitative flow rate of 40 ml/s to fill the throttle valves at different parts, and the injection volume is about 25% and 100%. The filling weight of each cavity is determined and shown in a radar chart (Figure 10)

Figure 10. Radar chart of packing weight (cavity 5 - throttle valve side)

interestingly, the slight eccentricity of hot spots in the rubber strand indicates that it is not ahead of the corresponding cavity, as expected, but the packing is equivalent to 80% of the throttle valve. First, for very small orifice sizes (90% or less), centrifugation becomes obvious. At this time, as expected, there is a delay in cavity 1, which is due to the early immature vulcanization, and the corresponding cavity filler becomes slow and incomplete

summarize the rheological and thermal properties of

efe system, which are verified by the method of fluid mathematical simulation. At the same time, practical tests were carried out to prove

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