Thermal Mold Design

The selection of the most effective demolding principle is dependent on the order and is always a fundamental decision. Depending on this choice is whether the parts are damaged during demolding or remain usable. The principal ejector types are  

These ejectors, or combinations, are advantageous because the injection molding machines are designed for them speciflcally, and the cycle times are kept short. Gate marks and deformations of thin-walled parts have to be taken into consideration with these solutions. 

In contrast, one could also use the following demolding types  

These demolding types require a simple mold structure and are often used in silicone mold making. Their disadvantages are the longer cycle times. 

In addition to these demolding types, techniques are used like ejecting with air ejectors by which the parts are blown out of the mold. Mostly, these valves are designed as mushroom ejectors. This technique is effective but complex. Cylindrical ejectors are usually less suitable due to the tight tolerances required. 

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In practice, the calculated thermal mold design is done in two ways ... 

Short shot can be avoided by proper mold design and control of polymer melt conditions—namely, temperature and injection pressure. This relationship is shown in Figure 7.76. Within the area bounded by the four curves, the specific polymer is moldable in the specific cavity. If the pressure and/or temperature are too low, short shot will result. If the temperature is too high, thermal degradation of the polymer can occur. If the temperature is too low, the polymer will not be molten. If the pressure is too high or the polymer is too fluid, the melt can flow into the gaps of the mold, creating thin webs of polymer attached to the molded article in an undesirable part... 

The dimensioning of the mold (phase II of the mold design) essentially includes the rheological, mechanical, and thermal mold layout. These aspects of the layout can be further divided into several steps, each with its own special emphasis ... 

CAD and CAE programs are available that can aid in mold design and in setting up the complete process (1, 2, 4, 9, 29, 99). These programs are concerned with melt flow to part solidification, and the meeting of performance requirements. Many different factors are incorporated, including heat transfer, thermal conductivity, thermal expansion, coefficients of friction, machine and mold operating setup, and so on. 

Park SJ, Kwon TH (1998) Thermal and design Sensitivity analysis cooling system of injection mold. Part 1 thermal analysis. J Manufact Sci Eng 120 287-295 Patankar SV (1980) Numerical heat transfer and fluid flow. McGraw-Hill, New York Pearson JRA (1966) Mechanical Principles of Polymer Melt Processing. Pergamon Press, Oxford Pearson JRA, Petrie CIS (1968) On the melt flow instability of extruded polymers. In Wetton RE, Whorlow RW (eds) Polymer systems deformation and Flow. Macmillan, London, pp 163-187... 

As the composite molds have much lower thermal conductivity than metallic molds, mold designers should consider having more and closer heating channels to the cavity in composite molds than in metallic molds if necessary. 

The die must be designed to control the pressure drop rate, which controls the uniformity of cell size. The desired pressure drop rate is greater than 1 GP/s. It also removes the thermal energy from the molded part. The die also creates 3-D shapes in the case of injection molding or the profile in the case of extrusion. It can be seen that the die design is as important as the extruder or the injection-molding design. 

Su F-Y and Wu X-Q (2010) Experimental study on silicone rubber mold design on thermal expansion RTM, J Reinf Plast Compos 29 2698-2704,... 

Once a geometric model of data base is created it may be shared for many of the product development steps described earlier. The data base may be copied and scaled for shrink factors to enable a mold designer to do preliminary mold design, or an engineer can use the same data base to perform structural analysis. Numerical control tool paths may be developed concurrently with the final mold design. Such modeling tools as finite element analysis, mold fill, and mold thermal... 

These elements must be considered in combination in order to understand the cooling performance of the mold. Improper mold design may result in a high thermal resistance that will lead to the quality and productivity problems previously mentioned. 

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