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In traditional injection molding, the cavity surface did not need heating. Hence, when the melt filled the mold cavity, the cavity surface was heated. After the filling process was finished, the melt was cooled down by cold water in the cooling system. In such a process, the quality of the product is not high, and the quality of the surface often has many problems which need to be solved.
With the application of hot water vapor into injection molding, the cavity temperature in the filling period can be increased to a higher temperature, which helps the melt flows easily resulting in better packing. On the other hand, by using a low water temperature for the cooling period, the total cycle time is almost the same with traditional
Fig. 1. General process of steam heating.
Table 1
The thermal property of cooling water and the steam [15].
dimension of the coolant tank. Thermal properties of water are listed in Table 2.
Fluid Temperature (°C)
Density (kg/m3)
Thermal conductivity (W/m K)
Dynamic viscosity (kg/ms)
When fluid is steam or air, the heat transfer coefficient h can be calculated according to the following expression [12]:
Water 20 998.1 0.5984 1.00E-03
Steam 150 0.516 0.028 1.40E-05
"k3 ρ ðρ −ρ Þgr#
injection molding. Meanwhile, the melt flow ability can increase and
h = 0:555x
w w s
μDΔt
melt viscosity can be maintained at a lower value. In addition, the surface brightness and harness also improve.
3. Theoretical model
The government equation used to describe the heat transfer between fluid (steam, air and water) and mold is [15]:
q = hΔt
where: q is heat flux from fluid transfer to mold at the channel wall. Δt is the difference temperature between channel wall and fluid in each process. h is the heat transfer coefficient between the mold and fluid flow inside the channel system.
When fluid is water, the heat transfer coefficient is calculated with the following correlation advised by Sleicher and Rouse [14]:
8 . . λ
> h = 5 + 0:015ReaPrb
> D
>< 0:24
a = 0:88−> 4 + Pr
>: b = 0:333 + 0:5e−0:6Pr
where λ is thermal conductivity of water, D is the diameter of the channel, the Reynolds number and the Prandl number are defined by:
where k is the thermal conductivity of steam, g is the acceleration of gravity, r is the latent heat liquefaction, ρw and ρs are density of water and steam, and Δt is the difference temperature between steam and wall channel. Thermal property of steam is listed in Table 1.
4. Simulation and experiment work
The mold temperature control process with steam heating consists of a steam system, a coolant system, a valve exchange unit, a control and monitor unit, and an injection molding machine. All system structures are shown in Fig. 2. For the connection between these units, the continuous lines present for the pipeline of steam, air and cool water. The steam source is made by a boiler to support enough high temperature steam for the heating process. The steam unit can reach to 10 kg/cm3 steam pressure with the chiller tank at 230 L and the max power is 20 kW. For the coolant system, a mold temperature control was used to provide a low water temperature to cool the mold. Because of the lower water temperature, the faster cooling, the 20 °C water temperature was used for all cooling process in this research. In addition, the velocity of the cooling water must be high enough to maintain the turbulent flow for cooling the mold. To improve the heating efficiency after the cooling step is finished; the air is forced into the channel system for cleaning all water and preparing a good contact of steam on the walls of the channel. On the other hand, there is a saving of energy because the fluids are recovered. A valve exchange unit can alternate the steam, air and cooling water flow in to