special feature

Laminar gasflowsensure'clean sweep'in sintering

Powder injection moulding (PIM)is a manufacturing technique thatencompasses metal powder injec-tion moulding (MIM), ceramic

powder injection moulding (CIM) andcemented carbide powder injection mould-ing (CCIM). All three processes combinethe attributes of plastic injection mouldingwith the engineering and performanceproperties of metals, ceramics, andcemented carbides.

In this article Claus Joens of PVAMIMtech's Elnik Systems looks at some ofthe problems associated with the thermaldebind and sinter technology of metalinjection moulded parts - and their solu-tions.

MIM is a process where fine metalpowders are mixed with a variety ofbinders to create thermoplastic feedstocksthat can be injection moulded. They arethen debound and sintered to full densitiesto attain the desired mechanical and phys-ical properties.

The advantages of MIM are:• Excellent shaping possibilities;• Complex shaped parts can be manu-

factured with very little secondary finish-ing. For example, undercuts in parts,which are not possible with conventionalsintering processes, can be easily achieved;

• Excellent surface quality when com-pared to precision cast parts. Finishing andpolishing costs can be eliminated or great-ly reduced;

• Excellent material properties;• Parts reach densities of 96 per cent to

100 per cent of the theoretical materialdensity;

• Very close tolerances. Parts are dimen-sionally accurate by a range of better than+/- 0.05 per cent; and

• A wide material selection. The greatvariety of metal powders and bindersavailable cater for a broad spectrum ofdesign needs. The harder it is for a part to

be machined, the more advantageous theMIM process becomes.

The MIM process exhibits great costeffectiveness in producing complex parts.In fact, the more complex the part, themore suited this process becomes (andtherefore the greater the cost savings.)Additionally, this process allows mixing ofdifferent metal powders with binders sothat it is possible to engineer a part withvery specific thermal, wear, magnetic andstrength properties.

The MIM process also yields net shapecomponents with little or no secondaryoperations. This simplifies production,

Metal injection moulding can provide designengineers with economic solutions to otherwiseapparently insoluble part production problems.But although MIM's 'can-do' abilities open theway to design freedom, care is needed at allstages of the process. The debinding and sintering of parts are critical steps…

52 MPR March 2005 0026-0657/05 ©2005 Elsevier Ltd. All rights reserved.

L

P

BinderParticle

Open pore

Vapor

Samplesurface

Po

Figure 1. Critical issues in thermal debinding

Debinding takes place by diffusion and permeation of thevapourised binder via the pores.

The thermal debinding processconverts the binder to a vapour.

The transport distance 'L' of thevapourised binder to the surfaceincreases over time.

Laminar gas flow ensures aneven binder evolution and noredeposition of binder on thepart.

The thickest sections of the partlimit the debind cycle time.

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increasing yields and lowering the manu-factured cost of the part.

The MIM process has four key manu-facturing elements:

• The creation of the feedstock;• The injection moulding cycle;• The solvent/chemical debind cycle;

and • The thermal debind and sinter cycle. During thermal debind and sinter, the

component part is heated and may be sub-jected to undesired binderreactions with the powderalong with shrinkage andthermal stresses. Theseare contributors tocracked, warped, chemi-cally incorrect parts withpoor density and of vary-ing and incorrect sizesbecause the entire processrequires very accurate andrepeatable control.

The goal of the ther-mal debind step is toremove the majority of thebinder which evaporatesat low temperatures, leav-ing the backbone polymerto hold the powder parti-cles in place so that thepart can be sintered. Thisbackbone polymer evapo-rates when the furnacegoes up to sintering tem-peratures, then the partstarts to sinter just beforeit densifies.

The thermal debindprocess converts thebinder to a vapour thatdiffuses and permeatesthrough the pores to thesurface of the part. Thecritical issue here is aneven binder evolution anda sweep of gas flowaround the part to ensure no re-depositionof binder on the part.

An even flow all around that partensures that all of the binder is extracted;during the sinter phase, when the poresbegin to close, there is no binder leftbehind. With no excess binder to contaminate or distort the part nounwanted physical effects such as blister-ing are seen and no difficulties arise withdensity and chemical composition, such ascarbon control in stainless steel.

In the early days of MIM processing itwas quite usual to thermally debind thepart in an air, vacuum or atmosphericnitrogen or hydrogen oven, depending onthe material and the binder.

Today, there is another binder materialwhich catalytically erodes the polyacetalused as the binder via nitric acid vapour.This requires a specially designed oven.The process works from the outside of thepart to the inside, unlike the wax/polymer

binder. However, both debinding times area direct function of the thickness of thepart. This means that the thickest partgoverns the total cycle time.

The problem with the ovens used origi-nally was that there was no even gas flow.They also exhibit large thermal gradients. Both conditions create poorlydebound parts through re-deposition of binder components on the parts and also contribute to contamination onthe inside of the furnace.

This is not a problem with polyacetalbinders since their removal is a chemicalreaction. This binder is not offered forsome metal powders and its removalrequires a special catalytic debind oven.Parts after catalytic debinding are veryfragile and their brown strength is a criti-cal issue not found with wax/polymerbinders. Damage to the brown parts dur-ing their transfer to the sinter furnace is anissue that merits consideration.

The double debind-sinter cycle is time con-suming and expensive interms of power used,since the parts area heat-ed to debind tempera-ture, cooled, then trans-ferred to the sinter fur-nace, heated again tosinter temperature andcooled.

To overcome theproblems of long processcycle times and break-age, a furnace with par-tial pressure operation inthe range of 1 to 760Torr was built to com-bine the thermal debindand sinter cycle in onestep. This furnacedesign, with built-inretort and gas manage-ment system allows pro-cessing the injectionmoulded part under"laminar gas flow". Thiseliminates the contami-nation issues associatedwith the debind ovens ofearlier days.

In this furnace design,the process gas flowsfrom the distributionholes across the parts tothe centre of the retort

through the gas manifold with its inlet andoutlet passage ways.

The gas is preheated by the heating ele-ments, which guarantees the flow to thecenter of the retort.

While the physical attributes of the furnace are important in directing the gas flow, the type of flow is critical to the success of the laminar gas flow design. The three different principal gas flows are turbulent, laminarand molecular.

Figure 2. The process gas flows from the gas distribution holes across the shelvesand parts to the centre of the furnace. It is preheated by heating elements andflows into the furnace at a higher temperature than the internal furnace tempera-ture. The design ensures short gas flows to the centre of the furnace giving a constant clean gas flow across the parts. The centre area where gas is evacuatedthrough the manifold vacuum ports is a few degrees colder, guiding the gasacross the parts.

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At an atmospheric pressure of 760 Torr,(typical for air or controlled gas ovens) thegas molecules flow at high pressure andvelocity, colliding with each other. Thiscreates uneven flow and shadow effects onthe parts with consequent uneven debind-ing.

At a molecular flow of 1 Torr or less,(typical for vacuum ovens) the gas mole-cules collide with each other randomly andgas flow becomes unpredictable. The gasflow also escapes to the cold walls of theoven, creating random flow on the partsand contamination inside the furnace.

At a laminar flow of around 300 Torr,the gas molecules flow at sufficient veloci-ty to flow smoothly and evenly over sur-face irregularities. This creates an evenflow and no shadow effects, much as if theparts were submerged in a liquid.

What exactly is laminar gas flow, andwhat are the benefits?

Figure 3 shows turbulent flow, whichresults in greatly separated flow at theback side of the part. This uneven flow cre-ates an uneven temperature distribution onthe part resulting in different debindingand sinter results.

Figure 4 shows that by lowering thedensity of the gas via partial pressure, weprovide a greater chance of laminar flowwith less separation at the back side of thepart. This can be measured by calculatingthe Reynolds number, indicative of turbu-lence, which decreases under partial pres-sure. And by lowering the density, weachieve a higher gas velocity. This in turncreates a thinner viscous boundary layer,which allows for greater thermal transfer.

Figure 5 shows the ideal laminar flow,which guarantees an even and continuousdebinding of the part, since all gas movesin a predictable way. Additionally, laminargas flow moves the process gas to the cen-tre of the retort. This is where the bindercontaminant is pulled through the gasmanifold, ending up in an easily cleanabledebind trap. The binder contaminantscannot redeposit on either the parts or thecold walls of the furnace as in atmospher-ic of vacuum furnaces. The advantage ofa furnace equipped for partial pressureoperation is the flexibility it creates. Theprofile parameters can be tailored specifi-cally to the concerns of the material.Three such concerns are: the slow ramp-ing during specific temperature phases, thevariation of partial pressure, and the mixof gas to obtain surface finishes. All three

contribute to the great variety of materialssuch a furnace is capable of running. Therange extends from iron through steel,stainless steel, tool steel, high-temperaturealloys such as Inconel 718 and HastelloyX, to titanium and tungsten compoundsincluding tungsten carbide.

The heating inside this furnace isaccomplished partially by radiation andpartially by convection. This depends onthe partial pressure and the resulting tem-perature uniformities. The temperatureuniformity varies greatly under the condi-tions from vacuum to different partialpressures as well as different temperaturelevels and type of gas.

The ability to change the partial pres-sure, gas type and flow allows this furnaceto process any MIM material with anybinder component. Some materialsrequire a higher partial pressure during

sintering to prevent the evaporation ofmaterial components such as copper instainless steel.

Additionally, laminar gas flow allowsdifferent sized parts to be processed in thesame furnace envelope in the same processrun. This is because gas flows evenly fromboth sides to the centre, which guaranteesperfect product results every time, regard-less of whether the furnace is fully loadedwith identical parts or differently sizedones. There is no shadow effect under lam-inar gas flow.

Several case studies show the benefits oflaminar gas flow. In the first case, an elec-tronic housing was being produced in aseparate debind oven. By using a laminarflow partial pressure furnace a cycle timereduction of 45 hours from 60 hours to 15hours was achieved, saving considerableamounts in terms of consumables costs -

54 MPR March 2005 metal-powder.net

W

W

Woo

Figure 3. Low gas velocityresults in a high Reynoldsnumber (indicating turbulence)and greatly separated flow.An uneven gas or temperaturedistribution at the productleads to different debindingresults, most common in vacu-um furnaces with a sweep gasoption.Only even flow round the prod-uct causes a continuous andeven debinding as shown inFigure 5.

Figure 5. Lowering theReynolds number overall cre-ates conditions that approachideal laminar flow, as shownin Figure 5, creating even andcontinuous debinding.

Figure 4. Lowering the densityof the gas via partial pressureresults in a lower Reynoldsnumber, providing a greaterchance of laminar flow withless separation. Higher gasvelocity creates thinner vis-cous boundary layers thatallow for greater thermaltransfer and a lower Reynoldsnumber

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electricity and gas consump-tion. At the same time, thecarbon control and the dimen-sional characteristic of thepart were greatly improved.

In the second example aring mount was made fromcarbon steel made in the tradi-tional way. It had high poros-ity (7.04 g/cc), a high oxygencontent (0.60 per cent), poorcarbon control (+/-0.02 percent), and poor dimensionalcontrol and concentricity.When the same part was madein a partial pressure furnaceunder laminar gas flow condi-tions, that part exhibited verylow porosity (7.76 g/cc), lowoxygen content (0.11 percent), acceptable carbon con-trol (+/- 0.01 per cent) and

excellent dimensional controland concentricity.

Batch furnaces with lami-nar gas flow capability haveshown to be advantageous inall respects. It provides com-plete debinding. It ensurestemperature uniformity dur-ing debind and sinter. It elimi-nates evaporation of key con-stituents of the part and itprovides versatility and flexi-bility in producing the largestvariety of PIM/MIM parts.And importantly, it eliminatesa separate thermal debindstep. All of these benefitsprove that the batch furnace is able to improve all proper-ties of the part, and to re-duce cycle time and utilityconsumption.

Figure 6. A typical partial pressure furnace can handle applicationsup to 1650oC with six individually controlled heating zones. Themakers say this results in temperature uniformity better than ± 3oC.

Reprinted fromMetal Powder ReportMarch 2005

PVA MIMtech, LLLCElnik Systems Division107 Commerce RoadCedar Grove, New Jersey 07009, USAwww.elnik.comEmail: cjoens@elnik.comTel: 973 239 6066Fax: 973 239 3273

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