外文翻译英文.pdf

ORIGINAL ARTICLE A scaffolding architecture for conformal cooling design in rapid plastic injection moulding K M Au S is the original solid box S is the negative offset solid from S The unionization of the discrete scaffolding elements generates the whole cooling channel conformally In this approach set Z denotes the set of integers Z3becomes the set of points whose coordinates are all integers in the three dimensional Euclidean space E3 and a set of discrete volume data is given as a finite subset of Z3 A primitive of scaffolding element in Z3are defined The union of the scaffold elements is based on the connectivity of the chain structure The chain structure is obtained by the vertex edge and face connection to generate the whole cooling channel We can define and locate the solid volume with the union of the scaffolding primitives Let C be a Euclidean cube within the subset Z3 Then we define the scaffolding elements of C as follows Vertices Va a 1 2 3 8 are labeled with i j k where i j k are the three plane indices and those planes have at least one point in common An edge Eb b 1 2 3 12 is drawn between two vertices if the vertices labels have two planes in common The Euclidean cube primitives within the subset Z3 develop the shape of the cooling system The whole structure is defined by the corresponding attributes of vertices and edges of the primitives The position of the cooling channel is tracked from the previous section of scaffolding curve approximation process The connectivity of the scaffolding element primitives is based on the Boolean operation Figure 24 shows the union of two consecutive scaffolding elements The interior surfaces of the scaffolding elements will form the cooling surface for the proposed model 4 3 Coolant flow through the scaffolding architecture The scaffold cooling system is designed with a complete coolant circulation which has an inlet an outlet and a pumping system The coolant inlet and outlet are connected directly to the mould halves Heat transfer during the injection moulding cycle includes heat exchange originated from polymeric melt to the mould material by conduction Fig 19 Flowchart of scaffold cooling surface approximation Fig 20 Surface offsetting of mouse model mould cavity Int J Adv Manuf Technol 2007 34 496 515503 Fig 21 Schematic diagrams of the injection mould half a Modeling of mould cavity surface b Cavity mould half inserted with scaffolding ele ments inserted for uniform cooling Fig 22 Graphical representa tion of a solid scaffolding element Fig 23 Assembly of scaffolding elements Fig 24 Union of two consecu tive scaffolding elements a Before merging b After merging 504Int J Adv Manuf Technol 2007 34 496 515 The heat is then conducted from the mould material to the coolant in the cooling passageway via the scaffold cooling passageway For the direction of coolant flow a single scaffolding element has six faces that provide one face as the inlet and five faces as the outlet pathways for the coolant flow Figure 21b is an example of a cavity mould half that is integrated with a scaffold cooling architecture When the coolant inlet and outlet are connected with a high pressure water pump and connector a complete coolant circuit is formed The assembly of numerous scaffolding element forms the conformal cooling surface which generates a multiple orientation passageway The coolant flows from the inlet with high fluid pressure and run into the scaffolding architecture passageway in the cavity mould half The coolant then brings the heat from the polymer and flows away via the outlet As the scaffolding architecture follows the shape of the mould cavity surface it increases the contact area of heat transfer from the polymeric melt and a near uniform cooling performance can be achieved 5 Results of scaffold cooling performance The advent of computer aided engineering CAE technol ogy for plastic injection moulding provides a large support to injection mould design Injection mould design simula tion modules allow precise determination of the effective ness of the mould cooling system at the desired mould temperature avoiding some mould defects and finding the desired injection moulding cycle time A variety of CAE simulations are performed for the proposed scaffold cooling system Section 5 1 deals with the cooling performance analysis Section 5 2 considers the mechanical properties of the scaffold cooling design method for loadings during the injection moulding cycle Section 5 3 discusses the thermal management of the proposed method Section 5 4 tests the effect of dimensional stability from shrinkage analysis The CAE results illustrate the feasibility of the proposed scaffold cooling approach as for rapid plastic injection mould 5 1 Cooling performance investigation of cooling channel by meltflow analysis Cooling performance analysis will find the temperature distribution in a plastic injection mould during the moulding process Heat transfer will be analyzed between the plastic the mould material and the coolant within the cooling system An optimal cooling performance for designing the cooling system can be identified Moreover shrinkage and thermal stress analysis are conducted In this research Moldflow Plastic Insight 3 1 23 is used to investigate the thermal effects of cooling channel design on the injection mould The set of analysis sequence in this study is cool and flow The parameters included injection mould pressure maximum temperature of part thermal stress cooling time and volumetric shrinkage The aims are to create uniform cooling along the circular cooling channel above and below the injection moulded part Figure 25a shows the mouse model for the meltflow analysis It consists of a thin shell with three buttons Figure 25b illustrates the modeling of the mould cavity and core mould halves with scaffolding architecture Figure 25c shows the opening and closing of the mould Figure 26 shows the meltflow analysis workflow by Moldflow Plastics Insight 3 1 The procedures can be grouped into the pre processing step the solver and the post processing step Tables 2 and 3 tabulate the specifica tions and cooling parameters for meltflow analysis Figure 27 compares geometric design of traditional and scaffold cool Fig 25 CAD models for CAE analysis a Mouse model b Cross section of mould cavity and core and c mould closing and opening Int J Adv Manuf Technol 2007 34 496 515505 ing channels The geometry of the scaffold cooling method follows closer to the mouse model surface Many injection mould parts have complex three dimen sional geometries Figure 28a reveals that the mould temperature distribution of scaffold cooling method is more uniform than the traditional cooling method Fig 28b The reason is that the scaffold cooling configuration closely matches the shape of the part being moulded Heat can transfer more evenly from the mould surface Figures 29 and 30 indicate that residual stress and volumetric shrink age accumulate near the corner of the mouse model under scaffold cooling Also mould defect occurrence in the traditional cooling method is higher than the scaffold cooling method In both cases the mould defects are built up on the surface and corner of the model This is because heat cannot be effectively transferred without any cooling channel insertion Uniform mould temperature distribution cannot be obtained in the case of restricted cooling in traditional cooling method 5 2 Mechanical properties investigation The mechanical performance of an injection mould is important as it directly affects the durability of the injection moulded part production The mechanical strength of the mould material has to withstand any force or load produced from damaging or dislocation during the mould opening and closing stages In injection mould production material selection depends on the experience of the mould engineers It is necessary to select a suitable mould material to prevent any chemical deterioration during the moulding process and to withstand the mechanical impact during the locking process Table 4 compares the mechanical properties of some common mould steels in the industrial market During mould opening and closing in injection moulding the mould plates are loaded by the clamping force Figure 31 indicates the stress distribution of a typical injection mould The pressure inside the cavity is considerably less than the Fig 26 Workflow of meltflow analysis by MPI 3 1 Table 2 Specifications for meltflow analysis Specification s Units Materials mould steels P20H13A6 Mould temperature C 55 Injection pressure Nm 2 190 Tolerance of accuracy mm 0 01 Table 3 Cooling channel parameters Cooling parameter s Descriptions Cooling channel diameter mm 8 Cooling pitch mm 16 Mould materialsTool steel P20 Tool steel H13 Tool steel A6 506Int J Adv Manuf Technol 2007 34 496 515 Fig 27 Geometries of two dif ferent cooling channel design a traditional cooling and b scaf fold cooling Fig 28 Performance of maxi mum mould temperature be tween a traditional cooling and b scaffold cooling Fig 29 Performance of in cavity residual stress between a traditional cooling and b scaffold cooling Fig 30 The performance of volumetric shrinkage between traditional cooling a and b scaffold cooling Int J Adv Manuf Technol 2007 34 496 515507 injection pressure at the injection nozzle The stress is investigated by the following equations The maximum stress Smaxunder load W is Smax WL 4Z 3 where Z is the section modulus in mm3 Z 1 d2 6 4 where the unit width is 1 Smaxmust be equal to or less than the critical fatigue stress developed by the steel mould plate As the porous structure of the scaffolding element provides less regular support than a solid volume the mechanical strength of the injection mould have to be high enough to withstand the force and stress from mould opening closing and locking Here mechanical CAE software provides the insight to the non linear dynamical analysis Thermal stress temperature distribution and mechanical strength are investigated to determine the mould s mechanical performance FEA package of COM SOS Works 24 is used as it integrates tightly with the SolidWorks CAD software Figure 32 shows the scaffold ing assembly to be tested Table 5 shows the results of CAE simulation to evaluate the mechanical properties of different cooling methods The injection pressure in the simulation is set at 1 Nm 2 The analysis results indicate that scaffolding architecture has a higher residual stress value 2 216 Nm 2 than the solid assembly structure 1 407 Nm 2 The injection pressure is accumulated near the vertical columns or bone like config uration as shown in Fig 33 for the porous structure The protection of the mould cavity against injection pressure highly depends on the mechanical strength of the mould material and the arrangement of the bone like configura tion Scaffolding assembly can provide the extensive mechanical properties for rapid plastic injection mould with proper mould material selection Table 4 Material properties of typical mould materials Material s Hardness Rockwell C Tensile strength N mm2 Thermal conductivity W m K Thermal expansion 10 6 K Wear resistance Compressive stress Dimensional stability in heat treatment Mould steel P20 30 366402912 7247 Mould steel H13 50 521 170 1 95024 612 13678 Mould steel A2 56 6074562 314 9999 Properties rankings on scale of 1 to 10 10 best Fig 31 Stress distribution of the injection mouldFig 32 Import of scaffolding assembly 508Int J Adv Manuf Technol 2007 34 496 515 5 3 Thermal management and heat transfer in an injection mould Within the duration of the injection mould cooling process a three dimensional cyclic transient heat conduction and convection problem on the cooling channel and mould surfaces is involved Figure 34 shows how coolant flows through the cavity mould half with scaffold cooling system configuration A cooling fluid or coolant such as water is pumped with coolant flow rate Vfviathe scaffold cooling configuration entering at temperature Toand leaving at temperature Te Assume that the coolant flow rate Vfis maintained as a high pumping pressure so as to facilitate high local heat transfer from the solid surface into the coolant The efficiency of heat removal depends on the offset distance between the scaffold cooling system and mould cavity surface The heat transfer to the coolant increases as either the offset distance decreases or the surface to volume ratio increases by increasing the amount of scaffolding elements Figure 35 indicates the heat transfer within the mould and the cooling channel The local heat transfer coefficient at the surface of the scaffold element is h Wm 2K 1 During the injection moulding process heat is removed from the mould surface and through the injection mould plate with heat conductiv ity Wm 1K 1 Then the heat is removed by the coolant via conduction and forced convection The scaffold cooling configuration provides a network ing system that transfers heat from the hot surface into the coolant The performance of thermal management is based on the heat transfer coefficient heff Wm 2K 1 which is related to the heat flux per unit area q Wm 2 from the hot surface q heff T 5 where T is the temperature change between the mould surface and the coolant The heat transfer coefficient heff for metal can be obtained from theoretical derivation and experimental results The rate of convective heat transfer is the diffusion of energy according to random molecular motion with energy transfer due to bulk motion Once the heat transfer coefficient for a given geometry and the flow conditions are known the rate of heat transfer can be expressed by the Newton s law of cooling 25 qc hcA Ts Tf 6 where qc W is the rate of heat transferred from a surface at uniform temperature Ts K to a fluid with temperature Tf K A is the surface area m hcis the mean coefficient of heat transfer Wm 2K 1 dqc hxdA Ts Tf 7 Table 5 Comparison of mechanical properties between solid assembly and scaffolding assembly ModelMaterialTemperature K Pressure N m2 Restraints N m2 Residual stress N m2 Displacement m e 014 Strain e 012 Solid assemblySteel H13373111 4071 3664 919 Scaffolding assemblySteel H13373112 2169 9692 912 Fig 33 Comparison of thermal stress with static finite element analysis of mould material H13 a Scaffolding assembly and b solid assembly Int J Adv Manuf Technol 2007 34 496 515509 Fig 34 Cross sectional views of the cavity mould half with scaffold cooling system config uration a XZ plane cutting and b YZ plane cutting Fig 35 Direction of heat transfer from mould surface via scaffolding architecture of cooling channel Fig 36 CAD model of scaffold cooling architecture for COSMOS FloWorks analysis Fig 37 3D profile of coolant flow analyzed by COSMOS FloWorks Fig 38 Flow trajectories of coolant analyzed by COSMOS FloWorks 510Int J Adv Manuf Technol 2007 34 496 515 Fig 40 a Strain and b defor mation of tool steel H13 Fig 39 a Strain and b defor mation of tool steel P20 Fig 41 a Strain and b defor mation of tool steel A6 Fig 42 a Strain and b defor mation of tool steel P20 at solid structure Int J Adv Manuf Technol 2007 34 496 515511 where dqcis the rate of heat transferred from a differential surface area dA and hxrepresents the local coefficient of heat transfer While the mean and local coefficient of heat transfer are related by Eq 8 h 1 A Z A hxdAs 8 5 4 The pressure distribution of coolant flow within the scaffolding architecture The scaffolding architecture provides a more uniform cooling surface over the mould cavity surface as every location of the mould cavity surface and scaffold cooling surface can experience an even rate of heat transfer However the volume of coolant flow increases within the scaffolding structure as well as the pressure drop In order to investigate the effect of pressure drop of coolant within the scaffold cooling configuration computational fluid dynamics CFD simulation software of COSMOS Flo Works is used 26 A CAD model of the cavity mould half with internal scaffolding architecture coolant inlet and outlet is designed and analyzed by internal fluid flow analysis Figure 36 illustrates the CAD model of scaffold cooling architecture for COSMOS FloWorks analysis By setting the boundary conditions of inlet mass flow rate 80 kg h the results of pressure distribution can be obtained and illustrated graphically Figures 37 and 38 show the pressure drop distribution along the scaffolding architec ture From the results of the coolant flow from the inlet to the outlet within the cavity mould half the pressure reduces constantly from 6 294e12Pa to 1 801e12Pa It indicates that the large contact area of the coolant flow will lead to a large pressure drop of the coolant flow The turbulent flow cannot be maintained within the cavity mould half To maintain the cooling performance it is necessary to modify other cooling parameters in order to compensate effect of the pressure drop 5 5 Dimension stability of scaffold cooling channel design Dimensional stability reflects the change in length of an unrestrained film sample subjected to a specific elevated temperature It depends on the properties of plastic material and tool steel Units are reported as percentage change from the original dimension During the injection moulding process the increase in mould temperature from normal to critical will cause physical change in the tool material The dimensional accuracy will decrease due to thermal expan sion of the tool materials Figures 39 40 41 42 43 and 44 show the difference in strain and deformation of common tool steels between solid and scaffold structures at injection mould temperature from normal 328K to 483K Fig 43 a Strain and b defor mation of tool steel H13 at solid structure Fig 44 a Strain and b defor mation of tool steel A6 at solid structure 512Int J Adv Manuf Technol 2007 34 496 515 From CAE results by COSMOS Works 7 0 in Tables 6 and 7 thermal stress and thermal