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Zhu extended liquid phase laser sinteringtechnique for high purity copper-based powder, with dendriteshape and mean particle size of 40 mm, and pre-alloyedpowder SCuP with spherical shape and particle size rangingfrom 5 to 20 mm with 60:40 by volume ratio (Zhu et al.,2003). It has been shown that the surface finish, density andcompressive strength of DMLS RT can be improved byelectroless nickel plating and semi-bright nickel electroplatingtechniques, which enable closing most of the pores of theparent metal (Tay et al., 2002).In summary, several RT methods have been explored forproducing molds for injection molding. The non-metallic andnon-ferrous materials involved impose several constraintsrelated to mold life, injection molding cycle, part quality andaccuracy. The DMLS molds are widely used for conventionalinjection molding, but there appears to be no published workon studying the effect of DMLS molds on injection moldingcycle and part quality. This has been investigated in this work.3. Experimental studyFigure 2 shows the sequence of study and the key aspectsassociated in each steps. The product CAD model of anindustrial component “Hub Gear” was used as an input formold design, the mold flow filling simulation was performedto decide on runner and gating, the sprue gate was found tobe more ideal. The largest diameter of the product was76.36mm and minimum section thickness was 2.16mm. Thecavity inserts shown in Figure 3 were fabricated using Cu-Ni-Sn based alloy (EOS direct metal 20) powder by DMLSprocess on EOSINT-M250 machine. The cavity inserts of110 £ 110 £ 54 and 110 £ 110 £ 34mm3(thicknessincludes 20mm thick steel base plate) were builtsimultaneously in 16 h. The inserts were inspected forvisible defects and dimensional errors, before assembly intoa standard mold base. The key observations made with theseDMLS molds are listed below:.Rough parting surface. Small spherical cavities wereobserved at parting surface, requiring surface grinding toensure proper matching between the two halves. .Distortion. There was significant distortion in the moldinsert; the circular cavity (larger) lost its circularity by^0.2-0.5mm. Moreover, parting surfaces (top surface ofsintered mold inserts) were found to have a taper towardsthe outer periphery..Stair-step effect. The stair-step effect was not significant,since the majority of the cavity walls were either horizontalor vertical..Surface roughness. The measured Ra values varied from 1.6to 2.1 mm, which were beyond the standard acceptablelevels for injection molding. Hand polishing was done tofacilitate part ejection..Dimensional accuracy. Most dimensions were within 0.3-0.6mm from CAD dimension. Thicker sections had largerdeviations.The following secondary machining operations wereperformed before mold assembly:.surface grinding of mold parting surface;.machining of runner, gates and vents for the optimizeddimensions as per the results of filling analysis performedusing Mold Flow software;.drilling and reaming of ejector holes and two guide pinholes; and.drilling and tapping of four holes on base plate (steel) ofthe insert for fastening purpose.The assembled mold was finally used in microprocessor-controlled DGM-150 injection-molding machine. The effectsof four injection molding parameters on molded partshrinkage and shot/part weight has been studiedexperimentally, while producing glass filled Du Pont ZytelNylon-66 moldings shown in Figure 4. Experiments wereconducted based L-9 orthogonal array, and “Grey relationalanalysis” technique was used to estimate the weights thatrepresent significance of different molding process variables.These steps represented in Figure 2 are discussed next.3.1 Selection of injection molding process variablesThe quality of plastic part produced in injection moldingprocess largely depends on polymerization process. Themolding parameters have a great influence on part qualityafter the thermo-mechanical properties of mold material. Fora fixed combination of mold material, geometry and plasticmaterial, the process variables such as cooling time, injectionpressure, melt temperature, injection speed, injection time,filling time, and mold temperature parameters have influenceon the quality of injection molded parts (Shen et al., 2002).However, considering the difficulties in controlling processvariables at different levels for the experiments four keyvariables: injection pressure, melt temperature, injectionspeed and injection times were selected. Cooling time,filling time and mold temperature were maintained constantafter the initial mold testing to produce the moldings withoutany short shot and burn marks and visible weld lines. The L-9orthogonal array based experimental design shown in Table IIwas used to conduct experiments using injection moldingprocess variables designed at three levels, denoted by 1, 2, and3 in Table III. These levels for variables were selected basedon the preliminary tests conducted using the parameterssuggested in Du Pont Zytel nylon 66 resins data sheet. 3.2 Selection of molding quality parametersAlthough, injection molding process is widely used inindustries, molder’s experience play a critical role, processsimulation software such as Mold flow, C-mold, Cadmold,and polymer material data sheets provide first handinformation for mold trial runs. Therefore, effective qualitycontrol enables simple yet accurate testing of mold-materialand parts for desired specifications and functional behavior.
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