【正文】 ample ponent 5 2 The current practical approach As shown in , the current approach consists of three phases: product design, mold design and molding process parameter setting. A main objective in the product design is to establish the physical dimensions of the part such as its thickness, width and length. The phases of molded sign and molding subsequently treat the established physical dimensions as given inputs to calculate the required details for mold making and molding operations. When applying the current practical approach for tackling the given example, the key variables are handled by the three phases as follows: Product design * Establish the minimum thickness (height) HP, and then calculate the material cost. HP is then treated as a predetermined input for the calculation of the costs of mold design and molding operations. HP Mold design * Calculate the cooling time for the determined minimum thickness HP in order to obtain the number of mold cavities required. The mold making cost is then the sum of the costs to machine the: – Depth of cutting (thickness) HP – Number of cavities – Runner diameter DR – Gate thickness HG Molding process * Determine the injection pressure Pin, and then the cost of power consumption ? Determine the cooling time t co, and then the cost of machine operations. The overall molding cost is the sum of the power consumption cost and machine operating cost. The total manufacturing cost is the sum of the costs of plastic material, mold making and molding operations. Note that, in accordance with typical industry practice, all of the following calculations are in terms of unit costs. design This is the first manufacturing phase of the current practical approach. The design minimizes the thickness HP of the plastic ponent to meet the creep loading deflection constraint , Y (),and to minimize plastic material usage cost Cm. Minimizing HP requires [21]: 6 Figure 3 plots changes in HP through and graphs show that the smallest thickness that meets the maximum creep deflection constraint is 0 .75mm,with a plastic material cost of $units. This thickness will be treated as a given input for the subsequent molded sign and molding process analysis phases. design Determination of cooling time The desired mold temperature is 25 C. The determined thickness is . Figure 4 shows the cooling channels layout following standard industry practices. The cooling channel diameter is chosen to be 3mm for this example. From [22], the cooling time t co: And the location factor, , and substituting HP = and the given values of the cooling channel design parameters, the cooling time () is obtained. The cycle time t cycle, given by E q. 5, is proportional to the molding machine operating costs, and consists of injection time (t in), ejection time (t e j), dry cycle time (t d c), and cooling time (t c o). Determination of the number of mold cavities In general, the cost of mold making depends on the amount of machining work to form the required number of cores/cavities, runners, and gates. The given example calls for a twoplate mold 7 . Deflection and plastic materials costs versus part thickness . Cooling channel layout that does not require undercut machining. Therefore, the ma chining work for cutting the runners and gates is proportional to the work involved in forming the cores/cavities and need not be considered. In the example, mold making cost Cmm is governed by (n, HP). Generally, the minimum number of cavities, Nmin, is chosen to allow for delivery of the batch of plastic parts on time圖 3 。 Department of Manufacturing Engineering amp。 This approach treats the ponent dimensions established in the product design phase as the given input, and uses optimization techniques to reduce the manufacturing cost of mold design and molding for producing the ponent. In most cases, the current approach provides the correct solution for minimizing the manufacturing cost. However, when the approach is applied to a thin ponent typically when miniaturizing products,it has problems finding the true minimum manufacturing cost. This paper analyses the shortings of the current approach for handling thin plastic ponents and proposes a method to overe them. A worked example is used to illustrate the problems and pare the differences when using the current approach and the new method proposed in the paper. Keywords Miniaturization of plastic parts Minimization of manufacturing Plastic part design and manufacturing cost . Nomenclature The thickness of gate The thickness of the rectangular channel Latent heat offusion ofPP=130kJ/kg The length of gate=– The length of circular channel The length of rectangular channel the consistency index 1/Poisson ratio of PP= Plasticmaterial constant, The volume flow rate The volume flow rateinside the rectangular channel The volume flow rateinside the circularchannel The radius of the circularchannel Distance of piston movement Loading time=31536000s Time for making single cavity mold insert=15h Dry cycle time= Ejection time= Injection time= 2 Demolding temperature of PP=70 C Melt temperature ofPP=190 C The width of gate The width of the rectangular channel the viscosity Strain of materials Stress of materials Thermal conductivity of steel=45W/mK Shear stress of plastic material Shear rate of plastic material Pressure drop of sprue Pressure drop of secondary runner Pressure drop of tertiary runner Pressure drop of gate Pressure drop of cavity Pressure drop of circular channel Pressure drop of rectangular channel () 1 Minimizing manufacturing costs for thin injection molded plastic ponents Abstract： Minimizing the cost of manufacturing a plastic ponent i