Application of High-temperature Water in Mold Temperature Controller

Under standard atmosphere, liquid water will turn into vapor when heated to 100℃. The boiling point of water is directly proportional with the air pressure. That is to say, the boiling point of water rises as air pressure increases, and vice versa. On the mountain of more than several kilometers, the air pressure is much lower than that at sea level, and the boiling point of water is low because of atmospheric depression. So the water will boil below 100℃ there.

STM is abbreviation of mold temperature controller or mold constant temperature machine which is widely used for maintaining constant temperature for molds in plastic molding industry. It has other names in different industries, such as oil heater and constant temperature machine. In a broad sense, its name is temperature controller which consists of heating and cooling.

STM can be divided into water type and oil type according to heating medium it utilizes. Within acceptable range of temperature, water type is preferred in view that water has favorable thermal conductivity and is also economic. The maximum heating temperature of current water type STM is higher than 100℃. It is generally recognized that under standard atmosphere, liquid water will turn into vapor when heated to 100℃. Well then, is it water vapor that circulates in the water type STM with controlling temperature above 100℃?

We can conclude a general rule from the above two phenomena that the boiling point of water is variable by altering air pressure in the barrel of STM.

Saturation pressure refers to air pressure at which the liquid water changes into gaseous form. For instance, the standard atmosphere is saturation pressure of water of 100℃. Liquid water will not vaporize if the given pressure is higher than its saturation pressure, and by this method, the water heated to 100℃is called high-temperature water. Under standard atmosphere, water temperature below 100℃is called low-temperature water, and both high and low-temperature water are called warm water. The following table shows saturation pressure of water varying with different temperature from which we can find out saturation temperature of water is 1.03323 kgf/cm² (also the standard temperature) at 100℃. The water boils at 70℃ on the Mt. Everest because the air pressure there is only 0.3116 kgf/cm².

The maximum operating temperature of Shini STM-W, STM-PW and STM-HPW is 120℃, 160℃ and 180℃ respectively, and the corresponding pressure are 4.0kgf/cm², 7.0 kgf/cm² and 12.0 kgf/cm².

What we learn from this table is that system pressure is higher than water saturation pressure at the corresponding temperature. Consequently, Shini STM that utilizes water as heating medium applies high-temperature water instead of water vapor to heat the mold.

Take 1m3 of both vapor and high-temperature water as example, the latter transfers heat of 28,152 kcal and it is 16 times more than that vapor takes which is only 1,732 kcal. To use high-temperature water will achieve better heat transfer efficiency.

Annotation:

1. Saturation Pressure: Molecules burst out of liquids that placed in the airtight container gather and move randomly within spaces on the liquid surface. Part of gas molecules collide the surface and back to liquid, however, some more molecules rise from the surface and fly into gas space. When the two strike a balance, specific volume of gas is unchanged; liquid and its vapor keep dynamic equilibrium; quantity of molecules doesn’t increase, and this is saturation. Vapor under this state is called saturated vapor, and its temperature and pressure are saturation temperature and saturation pressure.

2. Added Value of Saturation Pressure: value added to saturation pressure whenever temperature increases one degree.