[机械模具数控自动化专业毕业设计外文文献及翻译]【期刊】选择性激光烧结间接制造注塑模具的研究-外文文献

ORIGINAL ARTICLEInvestigation into manufacturing injection mold via indirectselective laser sinteringJinhui Liu & Zhongliang Lu & Yusheng Shi & Wenwu Xu &Jing ZhangReceived: 20 January 2009 /Accepted: 16 August 2009 /Published online: 8 September 2009# Springer-Verlag London Limited 2009Abstract Cooling system of an injection mold is importantfor the promotion of production rate and the quality ofinjection plastic components. Conformal cooling channelsare newly developed temperature-adjusting method topromote the efficiency of cooling system. They can be madein the injection mold inserts via the method of indirectselective laser sintering combined with traditional powdermetallurgy. This work discussed some processes such asthermal transmission, powders removing, and metal meltinfiltration during the manufacturing of the mold inserts indetail. The result showed that redundant powders outside oflaser-scanned areas might be sintered together with thesintered parts owing to the accumulation of laser energyduring sintering process. This was solved by switching thetemperature to initial level after one layer had been sintered.A limit length corresponding to some certain power vacuumsystem was found when the removal of unsintered powdersin the cooling channels was carried out. Therefore, somesubsidiary channels leading the cooling channels outsideweremadetohelptheremovalofpowderswithinthecoolingchannels. Dripping method was adopted during metal meltinfiltration process, which was proven to be relevant formaintaining of the final shape of infiltrated inserts.Keywords Selectivelasersintering.Injectionmold.Conformalcoolingchannels.Manufacturingdetail1 IntroductionThe production rate and the quality are much of a concerncurrently when more and more plastic components withcomplex structures are demanded. The adjusting of coolingrate and thermal arrangement through conformal coolingchannels(CCC)provideaneffectivewayto satisfy these tworequirements. Nowadays, rapid manufacturing methodsoffer the opportunity to create CCC in the core/cavity insertsof injection mold. Among these methods, selective lasersintering (SLS) is the appropriate one. Metal powdersblended with polymer material are heated by laser beamlayer by layer to generate a green shape with CCC firstly.Then,thepolymermaterial isburntoutwhenthegreenshapeis heated up to degradation point of the polymer material,and it is presintered with the increasing of temperature fromthe degradation point. Finally, the presintered shape withmetal material (infiltration metal) of low melting point isheated at a higher temperature than the melting point of theinfiltration metal to finish the infiltration densification.Although several previous works have discussed themanufacturing of injection mold insert with CCC by usingindirect SLS, they focused only on the surface treatmentmethod of SLS green insert 1, the thermal properties ofSLS hybrid materials and the optimization of cycle timeof the mold 2, and the CCC arrangement for the purposeof achieving the temperature homogeneity within the insert3. However, the manufacture cycles of CCC inserts wereJ. Liu (*)Modern Manufacture Engineering Center, HeilongjiangUniversity of Science and Technology Harbin,150027 Harbin, Chinae-mail: Z. LuState Key Laboratory for Manufacturing Systems Engineering,Xian JiaoTong University,Xian 710049, ChinaY. Shi:W. Xu:J. ZhangState Key Laboratory of Materials Processing and Die and MouldTechnology, School of Material Science and Engineering,Huazhong University of Science and Technology,Wuhan 430074, ChinaInt J Adv Manuf Technol (2010) 48:155163DOI 10.1007/s00170-009-2272-8not mentioned in these investigations. Lakshminarayan et al.4 indicated in 1995 that ironcopper composite part can beproduced via indirect SLS combined with traditional powdermetallurgy method employed in injection tooling. But theyonly discussed the mechanical properties of the copperinfiltrated iron parts and their application performance.Ikonen and his coworkers in 1997 studied the manufacturingof injection mold through indirect SLS, but they also brieflypresented the several cycles such as sintering, debinding,and infiltration processes 5. Similar with Lakshminarayan,they employed polymer-coated metal powders as formingmaterials and did not discuss the key points of theseprocessing cycles. For an example, the thermal transmissionbetween the powders, air, and scanning layers during lasersintering, and its controlling were not found in their works.It is known that a remarkable difference from sintering andposttreatment processes will result from different powdermaterials used in SLS process. Unlike the pretext investiga-tions, polymer powder mixed composite elemental metalpowders were used for forming materials in this work, whichhad been provento be flexible and convenient in literature 6.This work is concerned mainly on how the green part wasformed withpolymerpowdermixed withcompositeelementalmetalpowders,theinfluencesfromheattransmissionwhenthepowders were heated by laser beam, the removal of redundantloose powders from the cooling channels, the detailedinfiltration method, the microstructure of infiltrated materials,and the injection plastic part generated by the final mold.2 Experimental2.1 MaterialsEpoxy powders mixed with composite elemental metalpowders presented in literature 6 were chosen as SLS-forming powders in this work. The content of epoxy in themixed powders was 4%. The composite elemental metalpowders were composed of iron, copper, nickel, and carbonas shown in Table 1. Bronze powders were chosen forinfiltrating materials in the posttreatment process.2.2 Experiment pathElementalmetalpowdersandepoxypowdersweremixedinathree-dimensional motional blending machine initially. Thenthe composite powders were sintered in an SLS systemdeveloped by Huazhong University of Science and Technol-ogy. According to dimensional and structural characteristicsof the insert, forming path and process parameters weredesigned to guarantee the precision and strength of the SLSgreen insert. Two cubic green parts (120 mm20 mm15mm)werealsoformedtobeemployedascomparativespecimens with the inserts. The green inserts and the cubicparts were both heated at 80C/1 h for strength enhancementin a furnace. Then, they were infiltrated by bronze melt underthe protection of H2in a graphite cabin after they had beendebinded and sintered at 1,000C/1 h. The infiltratingtemperature was set at 1,080C and then, the infiltratedinserts were cooled naturally in the furnace. Debindingprocess followed what was expressed in literature 6. Theinfiltrated inserts and one of the cubic parts were treatedfollowing the process listed in Table 2, and then, two groupsof tensile test samples (Fig. 1) made from the cubic partsbefore and after thermal treatment were processed. Therewere five tensile test samples in each group, and the final testresults were the mean value of five samples in each group.SEM samples made of the cubic parts before and afterthermal treatment were also produced to analyze thevariation of alloys microstructures.3 Results and discussion3.1 Forming of SLS green insert3.1.1 Forming processFigure 2 is the three-dimensional model of CCC insert inthis work, and the darker pipes are the cooling channels.Table 1 Composition of SLS metal powdersElements Fe Cu Ni CContent (%) Balanced 8 4 0.5State 0.2/MPa b/MPa Youngs modulus (GPa) Elongation (%)Before 328 419 43 T2, andthe direction of heat flux between the two successive layerswill point to the previous one. Increasingly, heat will beaccumulated layer by layer, and the temperature of thewhole SLS green part will become high enough to raise therisk of binding the extra loose powders outside the processcross-sections.Shown as Fig. 4, cooling channels have three positionsaccording to the relationship between the axel of the channelandtheZ axis. Their positions are perpendicular, parallel, andthe position which is between other two ones, respectively.Cross-sections of the channels are surrounded by heat fluxfrom circumference of sintered sections (Fig. 4ac), and it isdifferent according to the different relationship betweenchannels and laser beam. Such heat accumulation andtransmission will induce the excessive temperature augmentover the softening point of epoxy powders within the closedzones where the unsintered powders are. Among these threeposition relationships in Fig. 4, loose powders in the channelwhose axel is in parallel with the laser beam has the mostprobability of being binded. That is because the heat accu-mulation and transmission will become larger under suchconditions that the area of the channel cross-section issmaller in Fig. 4c than those of the other two ones in Fig. 4band Fig. 4a, respectively, and there are more layers needed tobe built-up to finish the whole channel in Fig. 4c than thosein Fig. 4a, b. The channels are blocked when loose powdersare binded additionally.The precision of the green part will be damaged if theextra loose powders are binded on the surfaces of the part.Therefore, the forming procedures and the correspondingparameters should be adjusted to avoid the extra bindingproblem. One creative method is to change the combinationaba core b cavityFig. 2 CCC insert CAD model. a Core, b cavitySSqPrevious layer Previous layerNext layer12T1TTLaying and scanningSGqSGqConvectionFig. 3 Heat transmissionbetween the successive layers(T1T2T0)Int J Adv Manuf Technol (2010) 48:155163 157SGq -heat flux density of convection a perpendicular b between perpendicular and parallel c parallelSSq - heat flux density of conduction SGSSqZabcFig. 4 Relationship between theaxel of the channel and Z axis.a Perpendicular, b betweenperpendicular and parallel,c parallel. qSGheat flux densityof convection, qSSheat fluxdensity of conductionlimLoutVinVoutVf f 2121Powderab21pp Fig. 5 Skeleton of powderremoving in channelsabPositioning pin Fig. 6 Green inserts andcorresponding graphite boxes.a Core insert and its graphitebox, b cavity insert and itsgraphite box158 Int J Adv Manuf Technol (2010) 48:155163space core cavity Fig. 8 Assembling of greeninserts and graphite boxes5mm 3mm 10mm Fig. 7 Graphite coverInt J Adv Manuf Technol (2010) 48:155163 159of processing parameters in real-time, i.e., to change theinput amount of laser energy with the variation of the layerstemperature. For example, laser energy should be controlledin a low level if the temperature of the previous layer isrelatively higher than what the sintering needs. The purposeof this method is to maintain the temperature of the layersunder a certain level. Another method is that scanningintervals between two successive layers should be longenough until redundant heat is lost into the air throughconvection completely. Thus, the temperature of the currentlayer can return to the environmental one, and the temper-atures of each layer are maintained nearly equal, that is eachlayer can be sintered under similar temperature conditions.The former method is a highly intelligent one, and muchenergy and time can be saved through this method. But therelationbetweenthelayers temperature and the input amountof laser energy decided by the combination of processingparameters should be clearly understood and matched. Itis difficult indeed for simple structures, and it is nearlyimpossible for the complex one at present. Compared withthe former one, the latter one is a practical way to achievethe temperature uniformity in the aid of air convection, and itis controllable. The SLS system in this work is equippedwith a group of heating lights in the forming chamber, andtheir heating power and onoff are controlled by a computerprogram. When the temperature in the chamber is below therequired one, the lights will be on to heat the chamber or elsethey will be off. As a temperature information collector, aheatelectriccoupleisalsoassembledinthechambertoobtainthe variation of environmental temperature. It continuouslyprovides the temperature information to the controllingprogram which further governs the heating systems. Thetemperature of the chamber must have returned to the initialenvironmental one while the heating lights which are beingoff are onagain.The variation of the lightsisthe signal whichtells thatthe temperature isappropriate for the nextlayer to beprocessed.3.1.2 Powder-removing processThe unsintered powders should be removed from thesurfaces of the SLS green inserts after they are formed.ffNNGR + GabNNTFig. 9 Force state of molten bronze when infiltrating. a Within thehole b outside the hole and touching the insertFig. 10 Inserts after infiltrationFig. 11 SEM microstructure of the alloy quenched at 890C160 Int J Adv Manuf Technol (2010) 48:155163So, an aspirator is used to clean the powder in this work.Loose powders in channels are hard to remove clearly forthey are almost sealed in the channels which have thecharacteristic of bending and extending. The muzzle of theaspirator is fixed to the entrance of a channel. Instantaneousnegative pressure in the channel comes into being when theaspirator starts to work. The powders near the entrance ofthe channel will be initially driven out via the air turbulenceresulted from the negative pressure. Figure 5 depicts themechanism of the removing process where pnis thepressure difference between the outside atmosphere andthe point n in the channel. At the starting time, pnis largeenough for powder to be driven out. But with the motion ofthe powderair interface to the deep site in the channels, thevacuum degree will decrease, and pnat the interface willalso decrease with this motion because of the powerlimitation of the aspirator. The tendency of the air pressurevariation on the interface can be described with theinequality p1p2in Fig. 5, which results in the fallingof collision force (f) between the air and the powders in thechannel (Fig. 5). Additionally, the wall of the channel is notcompletely airtight, and the external air will penetrate thechannel through its seams. All these factors will cause thecollision force (f) to decrease to a limit, and the powderswill be still instead of moving to the entrance of the channelwith the advancing of interface. So, there exists a maximumlength (Llim) for a certain powder aspirator. It is impossiblefor the powders in a channel whose length is over Llimto bedriven out clearly. In this work, subsidiary channels areproduced to divide the cooling channels into severalsections, and the length of one section is below Llim(Fig. 2), which solves the above problem. Finally, theentrances of subsidiary channels are blocked with plugssintered with the same mixed composite powders after theremoving process is finished.3.2 Post-treatment of SLS green insertGreen inserts are heated at 1,000C/2 h in the same furnacefor presintering after they are debinded as in literature 6.Then, there comes the infiltration process. Two graphiteboxes are processed firstly corresponding to the two insertsbefore infiltration, and they each have a cover with cone-shaped holes in them. Figure 6 is the picture of graphiteboxes while Fig. 7 shows the graphite cover of the boxesand the sizes of the holes.Sintered inserts and graphite boxes are assembled aswhat is shown in Fig. 8. The spaces between the inserts andthe boxes are filled with alumina powders leaving the topsurface of the inserts naked. Bronze powder-pressed ingotsare placed in the covers of the boxes. The assembled boxesare heated up to 1,080C with the heating rate of 100C/h protected with H2. When the temperature goes up over865C (the melting point of bronze), bronze ingots start toabsorb the latent heat of fusion and melt. Bronze melt is alittle viscous initially and cannot wet the wall of graphitebox. The force relation of this section can be depicted asFig. 9 where f is the surface tension of bronze melt, N is thesupporting force from the wall of holes to the melt, G isgravity of the melt, R is capillary force when the melt isinfiltrating, and T is the dragging force from the upper partFig. 12 SEM microstructure of the alloy quenched at 890C andnormalized at 200CFig. 13 Assembled plastic in-jection mold with the inserts init. a Before combination and bafter combinationInt J Adv Manuf Technol (2010) 48:155163 161of the melt. Figure 10a, b shows the state of forces beforeand after the melt flows out of the holes, respectively. f, N,and G are balanced along Z axis according to the Eq. (1)before the melt flows out of the holes.N cosq0 f sinq G 1After the melt flows out of the holes, the capillary forceworks during the infiltration, and the forces along Zdirection change into a new state which can be expressedas inequation (2).R G N cosq02Inequation (2) gives that the bronze melt will infiltrate thepores of the inserts at a certain speed. The relation ofsurface energy (), temperature (T), and enthalpy (S) undera certain pressure (p) is shown in thermodynamics Eq. (3).g=TpC0S 3It is indicated that the surface energy of the melt falls withthe decrease in temperature under a constant pressure.Combining the whole foreground analysis, the infiltrat-ing process can be divided into three sections. Firstly, theviscosity of the bronze melt is high at starting time when itstemperature is a little higher than its melting point. So, it ishigh enough for the surface tension (f) of the melt tobalance the gravity (G) with the help of supporting force(N). Secondly, the balanced relation of f, N, and G is brokenwith the decrease of the viscosity of the melt when thetemperature is rising, which results in the rapid decreasingof f so that it cannot counterweigh the gravity. Therefore,melt flows out of the hole to touch the surface of the insertto induce the capillary force (R). Finally, much meltincreasingly forms a pressing force to enhance the capillaryforce (R) to accelerate t