Rapid prototyping (RP) is a widely used process in the industry to shorten development time.Another advantage of this technology is the ability to create conformal cooling systems, thus not only cooling time and cycle time can be shortened, but also shrinkage, thus warpage can be decreased. The main disadvantage of Rapid prototyping materials is their low thermal conductivity, which strongly influences cooling properties and warpage.The research based on a special developed injection mold for novel rapid prototyping based mold inserts with cooling systems. A method has been introduced to determine the most important thermal parameters for injection molding simulations using rapid tools. Those parameters, which can be measured such as the specific heat and thermal conductivity of the mold materials, are directly implemented into the software. The heat transfer coefficient between the polymer melt and the rapid tool insert surface cannot be measured in a reasonable way, thus simulation software was used to determine that based on indirect calculation 2 derived from real measurements. In the paper, the method was proved with Fused Deposition Modeling (FDM) and Polyjet mold inserts.

Abstract:The most time-consuming phase of the injection molding cycle is cooling. Cooling efficiency can be enhanced with the application of conformal cooling systems or high thermal conductivity copper molds. The conformal cooling channels are placed along the geometry of the injection-molded product, and thus they can extract more heat and heat removal is more uniform than in the case of conventional cooling systems. In the case of copper mold inserts, cooling channels are made by drilling and heat removal is facilitated by the high thermal conductivity coefficient of copper, which is several times that of steel. Designing optimal cooling systems is a complex process; a proper design requires injection molding simulations, but the accuracy of calculations depends on how precise the input parameters and boundary conditions are. In this study, three cooling circuit designs and three mold materials (Ampcoloy 940, 1.2311 (P20) steel, and MS1 steel) were used and compared using numerical methods. The effect of different mold designs and materials on cooling efficiency were examined using calculated and measured results. The simulation model was adjusted to the measurement results by considering the joint gap between the mold inserts.

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A common calculation error of thermal processes in injection molding simulation programs is caused by the inadequate description of the heat transfer between the polymer melt and the wall of the mold. No correct description of the effect of pressure on the heat transfer coefficient is available for the numerical calculations of injection molding yet. During the injection molding cycle, the pressure of the melt can vary from atmospheric pressure to thousands of bars. When such a high pressure is applied, the heat transfer coefficient can change several orders of magnitude. To calculate heat removal accurately, we developed a novel measuring method and based on the measurement data, we created a model that describes the heat transfer coefficient as a function of pressure and temperature. Calculated maximal heat transfer coefficients vary between 250 and 800 W/(mK) at the pressure of 500 bar. The pressure dependence of the heat transfer coefficient can be described with a sigmoid function. The heat transfer coefficient increases as the temperature difference between mold and melt increases and as pressure increases. We determined measurement error and found it to be less than 5%.

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