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The design of cooling systems is very important in injection molding. An ideal cooling system can cool and solidify plastic parts can avoid unwanted deformation occurred during mold ejection. Since cooling time occupies 70-80% of the entire molding cycle, a well-designed cooling system will shorten the molding time and enhance production. On the contrary, improper cooling procedure and cooling design become one of the main reasons causing warpage, non-uniform shrinkage and distortion.
Plastic parts are cooled down by cooling channel running through the mold. The heat of polymer is transferred to the cooling lines, and is removed through heat convection of the coolants. As the process proceeds, the heat from polymer melt is continuously being transferred until the whole product is fully-ejected. After that, the ejected product continues to reduce its temperature to room temperature by dissipating its thermal energy into air, shown as below.
The schematics of the cooling
From the processing point of view, molten polymer fills the cavities during the filling process. This filling process arises the mold temperature because the heat transferred to the mold wall via heat conduction from the hot melt. A typical mold temperature variation cycle with steady coolant action is illustrated in the figure below. That is for conventional mold cooling, steady coolant is flowing through cooling pipe to deliver the heat out from hot melt. The mold temperature shows dynamic feature where the mold temperature is not a constant and is changed a long time domain. At the beginning, the temperature increases significantly for the first few shots until it reaches steady state of which the cyclic behavior of mold temperature does not deviate much from the average. In most cases, this deviation is less than 5oC. Hence, conventionally, it is reasonable to use the cycle-average temperature as the mold temperature along the time. However, for an unconventional complex mold temperature control system, the cycle-average approach is no longer suitable, for example, heating rod in the mold system, or Variotherm process. The temperature variation is very significant even at steady state. The deviation can easily go up to100oC or even more. Therefore, the cycle-average mold temperature approach can describe the mold behavior properly and transient cool is suggested.
Typical mold temperature variation cycle
In addition, the cycle time of injection molding comprises mold closing time, filling time, packing time, cooling time and ejection time. Cooling time occupies the biggest portion in the cycle and usually impacts the whole cycle and productivity directly. To avoid the undesired deformation, the temperature of plastic parts at the ejection stage should be cooled to a temperature lower than material’s deflection temperature. Otherwise, the relaxation of residual stress or warpage caused by ejection forces can be a serious problem. Since the cooling process, either for cycle-average approach or full transient cool, is very complicated, using Cooling Analysis of Moldex3D to simulate this process can help understand and solve related problems.