Correct selection and calculation of the mold temperature machine When selecting the mold temperature controller, the following points are the main considerations;
1. Pump size and capacity.
2. The size of the internal throat.
3. Heating capacity.
4. Cooling capacity.
5. Control form.
A, the size of the pump
From the known heat dissipation per cycle, we can easily calculate the required volumetric flow rate of the coolant, and then get the correct cooling capacity required. The manufacturer of the DC temperature generator of the mold temperature controller mostly provides low calculation. Pump flow rate formula. Table 4.1 is useful when selecting a pump, which accurately lists the heat dissipation capabilities of different plastics.
The following rules govern the pump's need to provide a low flow rate of zui:
If the temperature difference across the surface of the cavity is 5 ° C,
0.75gal/min/kW @5°C temperature difference or
3.4151/min/kW @5°C temperature difference
If the temperature difference across the surface of the cavity is 1 ° C, the required low flow rate of zui needs to be multiplied by a factor of five, which is 3.75 gal/min/kW or 17.031/min/kW. In order to obtain the stability of product quality, many injection molding companies should control the temperature difference of the cavity surface to 1-2 °C, but in fact many of the injection molding manufacturers may not know the importance of this temperature difference or think that the temperature difference is good. The range is 5-8 °C.
To calculate the volumetric flow rate required for the coolant, the following procedure should be used:
1. First calculate the heat to be removed from the city where a plastic/mold combination is planted:
Taking the aforementioned PC cup mold as an example, the actual heat that needs to be dissipated is:
A module gross weight (g) / cooling time (s) = 208 / 12 = 17.33g / s
The heat dissipation rate of PC is =368J/g or 368kJ/kg.
Therefore, the heat that needs to be dissipated per cycle = 368 × 17.33 / 1,000 = 6.377 kW
2. Calculate the volumetric flow rate required for cooling:
According to the above rule of thumb, if the temperature difference of the cavity surface is 5 ° C, the flow rate = 6.377 × 0.75 = 4.78 gal / min or = 6.377 × 3.41 = 21.751 / min. If the temperature difference of the cavity is 1 ° C, the flow rate = 4.78 × 5=23.9gal/min or =21.75×5=108.731/min
3. Pump flow rate regulations
In order to get a good heat dissipation effect, the flow rate capability of the pump should be 10% less than the calculated result, so a 27 gal/min or 120/min pump is required.
4. Pump pressure regulations;
Generally, the operating temperature of the mold temperature controller is 2-5 bar (29-72.5 psi). Since the volume flow rate of the coolant is affected under the condition of insufficient pressure (the pressure of the flow causes the pressure loss), the higher the pressure of the pump, the flow rate. The more stable.
For very small molds for cooling pipes (for example, pipe diameters of 6mm/0.236in), the pump pressure needs 10bar (145psi) to provide sufficient heat dissipation (ie, coolant speed).
In general, the higher the volumetric liquid velocity requirement of the coolant, the smaller the diameter of the pipe, and the greater the pump output pressure required. Therefore, the pressure of the mold temperature controller should be more than 3bar (43.5psi).
B, heating capacity
Figure 4.8 is a typical heating calculation table that provides the amount of heating required for the mold weight. Figure 4.8 is calculated using:
1. The vertical axis represents the weight of the mold.
2. The horizontal axis represents the amount of heat that the mold warms up to the desired temperature, in kW/hr.
The temperature ramps of 3.37 ° C - 121 ° C provide the relationship between the mold weight and the heat capacity of the mold temperature controller at the corresponding temperature.
For example, we can find out from the map:
1. The heating capacity required to raise the weight of the 500 kg mold to 50 ° C is 3.3 kW / hr.
2. The heat capacity required to raise the temperature of the 700 kg mold to 65 ° C is 6.5 kW / hr.
In general, the stronger the heating capacity, the less the required heating time (the heating capacity is doubled and the heating time is reduced). Figure 4.8 provides a useful piece of information for the injection molding manufacturer to immediately find out the heating requirements of any mold to obtain the heat capacity of the correct mold temperature controller. Often because the ability of the mold temperature controller is too low, the mold can not reach the good temperature state. To know the actual performance of the mold temperature controller, we can compare its actual and calculated mold heating time.
Freezing capacity
The design of the freezing circuit of the mold temperature controller and the precise control of the component temperature on the mold temperature are important. When the temperature of the mold or heating liquid rises to the set value, the mold temperature controller must be able to quickly and effectively avoid the temperature from rising. The method is to introduce another lower temperature liquid, and the control of the introduction is controlled by the solenoid valve. . Therefore, the elimination and stability of the temperature override depends on the size of the solenoid valve.
The aperture of the cooling solenoid valve can be calculated using the following formula:
Freezing capacity (gal/min) = kW × 3.16 / Δt
Here Δt=the difference between the production temperature and the chilled water temperature set by the mold temperature controller:
kW = the heat that the mold needs to remove
The following table lists the volumetric flow rates available for different solenoid valve bore sizes:
Solenoid valve aperture volume flow rate
In mm gal/min 1/min
0.25 6.35 0.7 3.18
0.375 9.53 1.2 5.45
0.500 12.70 3.3 14.98
0.750 19.65 5.4 24.52
1.000 25.40 10.0 45.40
1.250 31.75 13.0 59.02
1.500 38.10 20.0 90.80
After calculating the freezing capacity, you can find the corresponding solenoid valve from the above table, as shown in the following example:
The heat that the PC cup mold needs to drain is 6.377 kW.
The set temperature for production is 90 ° C
The temperature of the chilled water is 18 ° C
△T=90-18=72°C
Therefore, the freezing capacity = 6.377 × 316 / 72 = 0.28 gal / min or 1.271 / min
It can be seen from the above table that the solenoid valve with a hole diameter of 6.35 mm / 0.250 in can provide sufficient volume flow rate, and is suitable for the precise requirement of the mold temperature control range of ± 1 ° C. The pressure drop of the solenoid valve affects the flow rate. The flow rate values ​​in the above table are based on a pressure drop of 1 bar (14.5 psi). Therefore, the higher the pressure drop, the faster the flow rate of chilled water. The typical pressure drop for a solenoid valve is 2 bar (29 psi).
C, liquid mold temperature heating control system
The main purpose of any one of the mold temperature controllers is to control the mold temperature within the range of (±2 °F). Therefore, the temperature rise control of the liquid running between the mold lines must be precise, otherwise the purpose of mold temperature control cannot be achieved.
The control method of some mold temperature controllers is in the on/off form, and its working principle is to compare the actual and set temperatures. If the actual temperature is much lower than the set temperature, the electric heat is fully opened. When the actual temperature reaches the set value, the electric heat is turned off, and the actual positive and negative temperature deviation is generated due to the on/off control. This temperature change not only directly affects the temperature of the liquid, but also indirectly gives the mold a large excessive lift, not to mention that the Zui will definitely reflect the quality of the finished product.
Therefore, we should use a heating control system in the form of PID (proportional, integral, derivative), which can ensure that the temperature control of the mold is maintained within ±1 °C (±2 °F).
Choose the appropriate mold temperature controller
Indiscriminate purchase of mold temperature control products DC high-voltage generator can bring 20% ​​profit loss at any time, so we must consider the production needs in detail when purchasing, and strictly verify the capabilities of the mold temperature controller, in order to make a decision. Unfortunately, people often overlook the extremely important part of this injection molding technology, and often wake up when there is a problem with productivity and quality.
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