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CHINESE JOURNAL OF MECHANICAL ENGINEERING Vol 23 aNo 3 a2010 312 DOI 10 3901 CJME 2010 03 312 available online at Permeable Steel and Its Application in Plastic injection Mould ZHOU Zhaoyao CAO Wenjiong WU Zhengqiang YU Minqiang and LI Yuanyuan School of Mechanical and Automotive Engineering South China University of Technology Guangzhou 510640 China Received July 13 2009 revised March 30 2010 accepted April 27 2010 published electronically April 29 2010 Abstract The gas in plastics mould has great influence on performance appearance and lifespan of the injection molded parts Venting channel and its appendix system should be used for gas exhausting in general However the dependence on the venting system complicates the mould design Furthermore in certain condition it is difficult to integrate the venting system into the mould Currently a kind of mold material which has gas permeability has been developed in abroad but the applications of this mold material were restricted by its higher cost and smaller size In this research a porous material which was made by powder metallurgy was applied to plastic mould to replace the venting system Permeability of the steel with different secondary processing was tested and compared with a special apparatus The metallographic samples of the steel with different secondary processing were prepared and investigated Finally an actual injection set was established to investigate the applications of permeable steel The metallographies indicate that the micro holes inside permeable steel were interconnected Moulds made of permeable steel exhibit good permeability in the plastic injection experiments and gas generated in the mould cavity was smoothly exhausted The melted plastic did not penetrate into the mould or block in the micro holes After several times of plastic injection experiments the mould still retained good permeability The strength of this permeable steel is between 200 250 MPa and suitable for industrial applications The venting systems simplified by permeable steel in plastic injection have simple structures which can be applied into any place that requires gas exhausting Key words powder metallurgy permeable steel plastic injection mould venting system 1 Introduction The plastic injection process is one of the important manufacturing methods to polymers which provides products with high dimensional steadiness low manufacture cycles and costs 1 One of the problems affected on the quality of the final product is the gas entrapments occurred in the mold filling stage Generally the residual gas in mould is exhausted through the venting system 2 However the dependence on the venting system complicates the mould design Furthermore in certain condition it is difficult to integrate the venting system into the mould Now this problem can be avoided by using permeable steel to make mould or mould inserts With permeable steel gas generated in the mould cavity can be exhausted directly through the micro holes of the mould and the venting system is unnecessary anymore Moreover the mould structure and design becomes simple whereas the venting and aspirating function is preserved 3 Many researchers have been attracted by permeable metal s forming measurements and applications ZHANG et al 4 studied the effects of warm compaction on fabrication of porous iron and adopted macromolecule polymer to generate porous in permeable iron NORMITUS et al 5 Corresponding author E mail wenjiong cao This project is supported by Guangdong Hong Kong Key Project of China Grant No 2007Z010 and National Basic Research Program of China 973 Program Grant No 2007CB616905 investigated the effects of partially alloyed and prealloyed powders with different compositions of alloying elements and compacting pressure on the Young s modulus and Poisson s ratio of sintered low alloy steel ZHANG et al 6 chose NH 4 HCO 3 to generate porous in NiTi shape memory alloys and formed gradient porosity by changing the amount of NH 4 HCO 3 added LIN et al 7 analyzed the effects of sintering processing parameters and compaction pressure on the filtration performances of stainless steel porous metal ZHOU et al 8 investigated the reason of porosity formation effect of pressure on porosity and linking of pores in metallic porous material However though permeable steel is very important for making plastic injection mould how to obtain proper permeable steel apply it into plastic injection process and retain the permeability after secondary machining haven t been well studied In this work a kind of permeable steel was obtained by controlling the compact and sintering processes The details of these processes were presented in section 2 Section 3 demonstrates the permeability of this material through macroscopic experiments microscopic metallographs and scanning electron microscope SEM images Since most plastic injection moulds require secondary machining the effect of different machining methods on the permeability of porous material is also presented in section 3 In section 4 the permeable mould was used along with an injection set to make real plastic injection parts The high quality of the parts further validated the good permeability and venting function of this CHINESE JOURNAL OF MECHANICAL ENGINEERING 313 permeable steel 2 Compact and Sintering Process 304L stainless steel powder was applied in this work Before compaction granularity grading was conducted for stainless steel matrix powder 9 Green compacts with certain porosity were obtained by controlling the compacting pressure 10 11 Table 1 shows the chemical composition of this powder and Table 2 shows the composition of particle size Table 1 Chemical composition of 304L steel powder Composition Mass fraction w t C 0 03 Si 0 89 Cr 18 51 Ni 10 15 Mn 0 25 Fe Bal Table 2 Composition of different size of particles Mesh Mass fraction w t 100 0 4 100 200 Bal 200 325 29 325 46 In this work 250 g including 1 zinc stearate of 304L stainless powder was used for each compact Cylindrical samples were obtained under the compaction pressure of 100 MPa followed by sintering in cracked ammonia atmosphere in the RSJ 13 sintering furnace The sintering temperature was 1 100 1 300 and the duration was 1 h The average porosity of the sample was about 17 and the tensile strength was between 200 MPa and 300 MPa 3 Tests of Permeability 3 1 Macroscopic tests of the permeability Fig 1 shows the permeability of the steel with different secondary processing Fig 1 a shows the wire cut slice of the compact sample The micro holes on the surface were too small to see with the naked eye Fig 1 b shows another slice processed by grinding Before the permeability tests oil infiltration process was conducted on these two slices In the atmosphere environment the infiltration won t happen due to the surface tension caused by the air stored in the micro holes of the compact Therefore this process was conducted in the vacuum environment After vacuumization air was dispelled from the holes and surface tension was reduced so infiltration would take place under certain pressure After oil infiltration the cleaned sample was placed in the apparatus for experiment as shown in Fig 2 a Compact slice I processed by wire cut b Compact slice II processed by grinding c Sweating phenomenon of slice I d Sweating phenomenon of slice II Fig 1 Sweating phenomenon of the permeable steel 80 mm Fig 2 Schematic view of the apparatus used for permeability test A special apparatus see Fig 2 was designed to test the permeability of the compact slices It is composed of two chambers named chamber A and B respectively The slice sample was placed between these two chambers Chamber A is made of rigid dense material and was connected to an air pump Chamber B is a sealed shrinkable vessel and inside maintains the constant atmospheric pressure all the time When the air pump begun pressure in chamber A would decrease If the compact slice placed between the two chambers has certain permeability chamber B would shrink due to the air penetration from chamber B to YZHOU Zhaoyao et al Permeable Steel and Its Application in Plastic injection MouldY 314 chamber A through the porous slice The two compact slices shown in Figs 1 a and 1 b were tested in this apparatus Chamber B shrunk in both tests which indicated that both slices had certain permeability But the shrinkage in the test for slice I was quicker than that for slice II During the permeability tests oil inside the slices sweated out of the surface as shown in Fig 1 c and Fig 1 d Sweating phenomenon of slice I was much more obvious than that of slice II Comparisons of the shrinkage and sweating phenomena indicated that slice I had better permeability than slice II 3 2 Microscopic proofs of the permeability The connectivity of the micro holes in the permeable steel was further verified by the micro structure using metallographs and SEM images Fig 3 a shows the unetched metallograph of the sintered sample The irregular black areas in this figure are the cavities on the surface Fig 3 b is the etched metallograph and reveals the pores on the surface and the granular boundaries of the matrix material The average granular size of the original material can be determined by the statistic analysis of this photo After compaction and the subsequent sintering boundaries of the powders connected to each other and formed a three dimensional connective network and these cavities left functioned as gas channels a Unetched metallograph of the surface b Etched metallograph of the surface Fig 3 Metallographs of the permeable steel sample SEM micrographs of the permeable steel are shown in Fig 4 Fig 4 a is the surface topography of the pre machined green compact The irregular black areas indicate the pores on the surface Fig 4 b shows the fracture topography of the compact The SEM micrographs proved that the porosity existed not only on the surface but also inside the material The porosity enabled this material with good permeability and exhibited the macroscopic behaviors just as the tests shown in Fig 2 a Surface topography b Fracture topography Fig 4 SEM micrographs of the sample 3 3 Effects of machining on the connectivity of the cavities In the plastic mould industry mould manufacturing demands various machining processes But for the permeable steel used in plastic injection mould the machining processes may cause problems Although the permeable steels can sustain machining it is hard to retain the high quality porous surface After machining the porous structure on the surface will be damaged or the micro holes may be blocked Therefore the permeability will decrease In this work the effects of the wire cutting and grinding on the surface porosity were analyzed Fig 5 shows the section topograph of the wire cut slice I which was shown in Fig 1 a This topography differs from that shown in Fig 3 a significantly In this case the porous structure was weakened and some of the micro holes in the section were blocked The amount and size of micro holes decreased Moreover many spherical particle groups can be seen in this topograph These particles would possibly be formed after the material suddenly suffered from melting 30 m 30 m 50 m 50 m CHINESE JOURNAL OF MECHANICAL ENGINEERING 315 and solidification which was caused by the temperature effect during wire cutting Some of the micro particles piled up in the cavities and some micro holes in the section were blocked But there were still many connective micro holes left in this slice and the permeability remained As for slice II the high speed grinding left plastic deformation on the surface Thus most of the micro holes were blocked and the permeability was almost destroyed Fig 5 SEM micrograph of the wire cut sample Usually the cavity of the mould needs polishing after wire cutting and other additional processes The micrograph of the section dealt with grinding using fine sand paper under water environment is shown in Fig 6 The irregular black areas represent the porosity of the material Grinding under water environment avoided the blocking of the micro holes on the surface The water cooling effect on the surface prevented the melting of matrix material and the slow grinding of the fine sand paper also avoided the friction effect Although with low efficiency this process effectively guarantees the connectivity of the porous structure Fig 6 Micrograph of the section processed by grinding in water and polishing 4 Plastic injection Experiments To examine the practicability of the permeable steel in the plastic injection mould an injection set was developed see Fig 7 There was no additional venting and aspiring system in this unit and venting was achieved completely through the permeability of permeable steel mould Fig 7 Schematic of the injection mould 1 Bottom board 2 Connection screw 3 Discharge cavity 4 Mould 5 Top board 6 Sealing rubber 7 Mould cavity 8 Connection bolt 9 Plastic barrel 10 Injection pole The permeable material used in this work was made of 304L stainless steel powder including 1 mass percent of zinc stearate Green compacts were obtained under the compaction pressure of 400 MPa After compaction compacts were sintered in cracked ammonia atmosphere in the RSJ 13 sintering furnace Sintering temperature was around 1 300 and the sintering process lasted 1 3 h The average porosity of this permeable steel is about 25 and the mechanical properties were improved by oil quenching The mould cavity was processed by electric discharged machining and the cavity surface was polished by grinding as shown in Fig 8 Parameters used in plastic injection experiments are listed in Table 3 Fig 8 Mould cavity cut by EDM 80 mm The plastic injection experiments demonstrated the permeability of this permeable steel mould The gas in the mould cavity was well exhausted thus the filling of the plastic in this permeable mould was completed and the surface of the part was smooth see Fig 9 The viscous plastic didn t penetrate into the micro holes in the mould 0 02 mm 100 m YZHOU Zhaoyao et al Permeable Steel and Its Application in Plastic injection MouldY 316 material and block these gas channels After over ten plastic injection experiments this mould still retained good permeability This experiment again validated the good permeability and venting function of this porous steel Table 3 Parameters in plastic injection experiments Plastic material Plastic injection temperature T in Mould cavity temperature T m Injection pressure P in MPa Injection duration t in s Holding pressure P h MPa Pressure holding duration t h min ABS resin 170 40 15 20 10 15 5 Fig 9 Plastic Injection part 5 Conclusions 1 Green compacts made of stainless steel powder with certain porosity were obtained by controlling the compaction pressure after granularity grading 2 The sintered compacts were porous with connective inner holes and the strength was around 200 250 MPa Experiments showed that this permeable steel has good permeability 3 Plastic injection moulds were made by this permeable steel After many times of plastic injection experiments the plastic parts produced in this permeable steel still had good surface quality Therefore it can be concluded that this permeable steel retains good perme ability and satisfies the use in industrial applications References 1 CHEN Ching Piao CHUANG Ming Tsan HSIAO Yun Hsiang et al Simulation and experimental study in determining injection molding process parameters for thin shell plastic parts via design of experiments analysis J Expert Systems with Applications 2009 36 7 10 752 10 759 2 GUO Xinling The design of exhaust system in injection mould J Journal of Electromachining E mail zhyzhou CAO Wenjiong born in 1983 is currently a PhD candidate in School of Mechanical and Automotive Engineering South China University of Technology China His research interests include mould technology and CFD analysis Tel 86 21 87112948 E mail cao wenjiong WU Zhengqiang born in 1980 is currently a PhD candidate in School of Mechanical and Automotive Engineering South China University of Technology China His research interest is mould technology YU Mingqiang born in 1985 is currently a master candidate in School of Mechanical and Automotive Engineering South China University of Technology China His research interests include mould technology and CAE LI Yuanyuan born in 1958 is currently a professor in School of Mechanical and Automotive Engineering South China University of Technology China His research interests include foundry and powder metallurgy