Powder Metallurgy

NANO54

Foothill College

Overview

• History

• Definitions

• Benefits

• Process

• Applications

Introduction

• Earliest use of iron powder dates back to 3000 BC. Egyptians used it for making tools

• Modern era of P/M began when W lamp filaments were developed by Edison

• Components can be made from pure metals, alloys, or mixture of metallic and non-metallic powders

• Commonly used materials are iron, copper, aluminium, nickel, titanium, brass, bronze, steels and refractory metals

• Used widely for manufacturing gears, cams, bushings, cutting tools, piston rings, connecting rods, impellers etc.

Powder Metallurgy

• . . . is a forming technique

Essentially, Powder Metallurgy (PM) is an art & science of producing metal or metallic powders, and using them to make finished or semi-finished products. Particulate technology is probably the oldest forming technique known to man

• There are archeological evidences to prove that the ancient man knew something about it

Powder Metallurgy• Producing metal or metallic powders• Using them to make finished or semi-finished

products.• The Characterization of Engineering Powders• Production of Metallic Powders• Conventional Pressing and Sintering• Alternative Pressing and Sintering Techniques• Materials and Products for PM• Design Considerations in Powder Metallurgy

Powder Metallurgy (P/M)

• Competitive with processes such as casting, forging, and machining.

• Used when• melting point is too high (W, Mo).• reaction occurs at melting (Zr).• too hard to machine.• very large quantity.

• Near 70% of the P/M part production is for automotive applications.

• Good dimensional accuracy.• Controllable porosity.• Size range from tiny balls for ball-point

pens to parts weighing 100 lb. Most are around 5 lb.

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Process CapabilitiesCon’tional HIP Injection

Molding (IM)Precision IM Preform

Forging

Metal All All (SA, SS)

All (Steel, SS) All Steel, SA

Surface detail B B-C B A A

Mass, kg 0.01-5(30) 0.1-10

10-7000 (e)

0.01-0.2 0.005-0.2 0.1-3

Min. section, mm 1.5 1 0.1 3

Min. core diam. mm 4-6 1 0.2 5

Tolerance +/-% 0.1 2 0.3 0.1 0.25

Throughput (pc/h) 100-1000 5-20 100-2000 100-2000 200-2000

Min. quantity 1000-50,000 1-100 10,000 10,000 100,000

Eq. Cost B-C A A-B A-B A-B

A: highest, B: median, C: lowest

Design Aspects

(a) Length to thickness ratio limited to 2-4; (b) Steps limited to avoid density variation; (c) Radii provided to extend die life, sleeves greater than 1 mm, through hole greater than 5 mm; (d) Feather-edged punches with flat face; (e) Internal cavity requires a draft; (f) Sharp corner should be avoided; (g) Large wall thickness difference should be avoided; (h) Wall thickness should be larger than 1 mm.

Advantages / Disadvantages P/M

• Virtually unlimited choice of alloys, composites, and associated properties. – Refractory materials are popular by this process.

• Controlled porosity for self lubrication or filtration uses.• Can be very economical at large run sizes (100,000 parts).• Long term reliability through close control of dimensions and

physical properties.• Very good material utilization. • Limited part size and complexity • High cost of powder material.• High cost of tooling. • Less strong parts than wrought ones. • Less well known process.

History of Powder Metallurgy

• IRON Metallurgy >

• How did Man make iron in 3000 BC?

• Did he have furnaces to melt iron air blasts, and

• The reduced material, which would then be spongy, [ DRI ], used to be hammered to a solid or to a near solid mass.

• Example: The IRON PILLER at Delhi

• Quite unlikely, then how ???

History of P/M

• Going further back in Time . . .

• The art of pottery, (terracotta), was known to the pre-historic man (Upper Paleolithic period, around 30,000 years ago)!

• Dough for making bread is also a powder material, bound together by water and the inherent starch in it. Baked bread, in all its variety, is perhaps one of the first few types of processed food man ate.

• (Roti is a form of bread.)12

Renaissance of P/M

• The modern renaissance of powder metallurgy began in the early part of last century, when technologists tried to replace the carbon filament in the Edison lamp.

• The commercially successful method was the one developed by William Coolidge. He described it in 1910, and got a patent for it in 1913.

• This method is still being used for manufacturing filaments.

13

Renaissance of P/M

• The Wars and the post-war era brought about huge leaps in science, technology and engineering.

• New methods of melting and casting were perfected, thereby slowly changing the metallurgy of refractory materials.

• P/M techniques have thereafter been used only when their special properties were needed.

P/M Applications► Electrical Contact materials► Heavy-duty Friction materials► Self-Lubricating Porous bearings► P/M filters► Carbide, Alumina, Diamond cutting tools ► Structural parts► P/M magnets► Cermets► and more, such as high tech applications

Hi-Tech Applications of P/M

Anti-friction products Friction productsFiltersElectrical Contacts Sliding Electrical Contacts Very Hard MagnetsVery Soft MagnetsRefractory Material ProductsHard and Wear Resistant ToolsFerrous & Non-ferrous Structural parts etc . . .

THESE COMPONENTS ARE USED IN AIR & SPACE CRAFTS, HEAVY MACHINERY, COMPUTERS, AUTOMOBILES, etc…

Powder Metallurgy Merits

o The main constituent need not be meltedo The product is porous - [ note : the porosity can be controlled]o Constituents that do not mix can be used to make composites,

each constituent retaining its individual propertyo Near Nett Shape is possible, thereby reducing the post-production

costs, therefore:

Precision parts can be produced The production can be fully automated, therefore, Mass production is possible Production rate is high Over-head costs are low Break even point is not too large Material loss is small Control can be exercised at every stage

Powder Metallurgy Disadvantages

o Porous !! Not always desired.

o Large components cannot be produced on a large scale [Why?]

o Some shapes [such as?] are difficult to be produced by the conventional p/m route.

• WHATEVER, THE MERITS ARE SO MANY THAT P/M,

• AS A FORMING TECHNIQUE, IS GAINING POPULARITY

Powder Metallurgy

• An important point that comes out :

• The entire material need not be melted to fuse it.

• The working temperature is well below the melting point of the major constituent, making it a very suitable method to work with refractory materials, such as: W, Mo, Ta, Nb, oxides, carbides, etc.

• It began with Platinum technology about 4 centuries ago … in those days, Platinum, [mp = 1774°C], was "refractory", and could not be melted.

Powder Metallurgy Process

• Powder production

• Blending or mixing

• Powder compaction

• Sintering

• Finishing Operations

Powder Metallurgy Process

1. Powder Production

(a) (b) (c)

(a) Water or gas atomization; (b) Centrifugal atomization; (c) Rotating electrode

• Many methods: extraction from compounds, deposition, atomization, fiber production, mechanical powder production, etc.

• Atomization is the dominant process

Powder Preparation

(a) Roll crusher, (b) Ball mill

Powder Preparation

2. Blending or Mixing

• Blending a coarser fraction with a finer fraction ensures that the interstices between large particles will be filled out.

• Powders of different metals and other materials may be mixed in order to impart special physical and mechanical properties through metallic alloying.

• Lubricants may be mixed to improve the powders’ flow characteristics.

• Binders such as wax or thermoplastic polymers are added to improve green strength.

• Sintering aids are added to accelerate densification on heating.

Blending• To make a homogeneous mass with uniform distribution of particle

size and composition– Powders made by different processes have different sizes and

shapes– Mixing powders of different metals/materials– Add lubricants (<5%), such as graphite and stearic acid, to

improve the flow characteristics and compressibility of mixtures• Combining is generally carried out in

– Air or inert gases to avoid oxidation– Liquids for better mixing, elimination of dusts and reduced explosion

hazards

• Hazards– Metal powders, because of high surface area to volume ratio are explosive,

particularly Al, Mg, Ti, Zr, Th

Some common equipment geometries used for blending powders(a) Cylindrical, (b) rotating cube, (c) double cone, (d) twin shell

Blending

ME 355 Sp’06 W. Li 28

3. Powder Consolidation

Die pressing

• Cold compaction with 100 – 900 MPa to produce a “Green body”.– Die pressing

– Cold isostatic pressing

– Rolling

– Gravity

• Injection Molding small, complex parts.

Compaction

• Press powder into the desired shape and size in dies using a hydraulic or mechanical press

• Pressed powder is known as “green compact”• Stages of metal powder compaction:

• Increased compaction pressure– Provides better packing of particles and leads

to ↓ porosity– ↑ localized deformation allowing new contacts

to be formed between particles

Compaction

• At higher pressures, the green density approaches density of the bulk metal

• Pressed density greater than 90% of the bulk density is difficult to obtain

• Compaction pressure used depends on desired density

Compaction

W. Li

Friction problem in cold compaction

• The effectiveness of pressing with a single-acting punch is limited. Wall friction opposes compaction.

• The pressure tapers off rapidly and density diminishes away from the punch.

• Floating container and two counteracting punches help alleviate the problem.

• Smaller particles provide greater strength mainly due to reduction in porosity

• Size distribution of particles is very important. For same size particles minimum porosity of 24% will always be there– Box filled with tennis balls will always have open space between

balls– Introduction of finer particles will fill voids and result in↑ density

• Because of friction between (i) the metal particles and (ii) between the punches and the die, the density within the compact may vary considerably

• Density variation can be minimized by proper punch and die design

(a)and (c) Single action press; (b) and (d) Double action press

(e) Pressure contours in compacted copper powder in single action press

Compaction Pressure of some Metal Powders

Metal Powder Pressure (MPa)

Al 75-275

Al2O3 100-150

Brass 400-700

Carbon 140-170

Fe 400-800

W 75-150

WC 150-400

(a)Compaction of metal powder to form bushing

(b)Typical tool and die set for compacting spur gear

A 825 ton mechanical press for compacting metal powder

Cold Isostatic Pressing• Metal powder placed

in a flexible rubber mold

• Assembly pressurized hydrostatically by water (400 – 1000 MPa)

• Typical: Automotive cylinder liners →

• FFT: Advantages?

4. Sintering

• Parts are heated to 0.7~0.9 Tm.

• Transforms compacted mechanical bonds to much stronger metallic bonds.

• Shrinkage always occurs:

sintered

green

green

sintered

V

VshrinkageVol

_3/1

_

sintered

greenshrinkageLinear

Sintering – Compact Stage

• Green compact obtained after compaction is brittle and low in strength

• Green compacts are heated in a controlled-atmosphere furnace to allow packed metal powders to bond together

Carried out in three stages:

• First stage: Temperature is slowly increased so that all volatile materials in the green compact that would interfere with good bonding is removed– Rapid heating in this stage may entrap gases and

produce high internal pressure which may fracture the compact

Sintering – Three Stages

• Promotes solid-state bonding by diffusion.

• Diffusion is time-temperature sensitive. Needs sufficient time

Sintering: High temperature stage

• Promotes vapor-phase transport• Because material heated very close to MP, metal atoms will be released in the vapor phase from the particles• Vapor phase resolidifies at the interface

Sintering: High temperature stage

Sintering: High temperature stage

• Third stage: Sintered product is cooled in a controlled atmosphere– Prevents oxidation and thermal shock

Gases commonly used for sintering:

• H2, N2, inert gases or vacuum

Sintering: High temperature stage

Sintering Time, Temperature, and Indicated Properties

Liquid Phase Sintering

• During sintering a liquid phase, from the lower MP component, may exist

• Alloying may take place at the particle-particle interface• Molten component may surround the particle that has

not melted• High compact density can be quickly attained• Important variables:

– Nature of alloy, molten component/particle wetting, capillary action of the liquid

Hot Isostatic Pressing (HIP)

Steps in HIP

Combined Stages

• Simultaneous compaction + sintering

• Container: High MP sheet metal

• Container subjected to elevated temperature and a very high vacuum to remove air and moisture from the powder

• Pressurizing medium: Inert gas

• Operating conditions– 100 MPa at 1100 C

Hot Isostatic Pressing

It may sound like some new, exotic dry cleaning process and though many have heard of "HIP", Hot Isostatic Pressing, few of us understand the many benefits of this materials process. Since it's largely misunderstood, many conservative engineers are reluctant to adopt HIPping as an element in their manufacturing designs, thus missing a valuable process tool.

HIP is a process that subjects a material simultaneously to both high temperature and high gas pressure, usually Argon, in vessels equipped with sophisticated control systems and telemetry.

Typically, the temperature is selected to permit limited plastic deformation of the material being processed in the solid state at an argon gas pressure of 15,000, 30,000, or at times, 45,000 psi (1,000 to 3,000 atmospheres) is isostatically exerted on the heated parts for a period of time. The chamber is then slowly cooled, depressurized and the parts removed.

• Produces compacts with almost 100% density

• Good metallurgical bonding between particles and good mechanical strength

• Uses– Superalloy components for aerospace

industries– Final densification step for WC cutting tools

and P/M tool steels

Combined Stages

(i) Slip is first poured into an absorbent mould(ii) a layer of clay forms as the mould surface absorbs water(iii)when the shell is of suitable thickness excess slip is poured away(iv)the resultant casting

Slip-Casting

• Slip: Suspension of colloidal (small particles that do not settle) in an immiscible liquid (generally water)

• Slip is poured in a porous mold made of plaster of paris. Air entrapment can be a major problem

• After mold has absorbed some water, it is inverted and the remaining suspension poured out.

• The top of the part is then trimmed, the mold opened, and the part removed

• Application: Large and complex parts such as plumbing ware, art objects and dinnerware

5. Finishing

• The porosity of a fully sintered part is still significant (4-15%). • Density is often kept intentionally low to preserve

interconnected porosity for bearings, filters, acoustic barriers, and battery electrodes.

• However, to improve properties, finishing processes are needed:– Cold restriking, resintering, and heat treatment.– Impregnation of heated oil. – Infiltration with metal (e.g., Cu for ferrous parts).– Machining to tighter tolerance.

Special Process: Hot compaction

• Advantages can be gained by combining consolidation and sintering,

• High pressure is applied at the sintering temperature to bring the particles together and thus accelerate sintering.

• Methods include– Hot pressing– Spark sintering– Hot isostatic pressing (HIP)– Hot rolling and extrusion– Hot forging of powder preform– Spray deposition

Characterization of Powders

Size of powders 0.1 um – 1 mmSieve size quoted as mesh numberParticle D = 15/mesh number (mm)325 mesh 45 um

Atomization

• Produce a liquid-metal stream by injecting molten metal through a small orifice

• Stream is broken by jets of inert gas, air, or water

• The size of the particle formed depends on the temperature of the metal, metal flowrate through the orifice, nozzle size and jet characteristics

Electrode Centrifugation

Variation: A consumable electrode is rotated rapidly in a helium-filled chamber. The centrifugal force breaks up the molten tip of the electrode into metal particles.

Finished Powders

Fe powders made by atomization Ni-based superalloy made by the rotating electrode process

Reduction• Reduce metal oxides with H2/CO• Powders are spongy and porous and they have uniformly

sized spherical or angular shapesElectrolytic deposition• Metal powder deposits at the cathode from aqueous

solution• Powders are among the purest availableCarbonyls• React high purity Fe or Ni with CO to form gaseous

carbonyls• Carbonyl decomposes to Fe and Ni• Small, dense, uniformly spherical powders of high purity

P/M Process Approaches

P/M Process Approaches

Comminution• Crushing • Milling in a ball mill• Powder produced

– Brittle: Angular– Ductile: flaky and not particularly suitable for P/M

operationsMechanical Alloying• Powders of two or more metals are mixed in a ball mill• Under the impact of hard balls, powders fracture and join

together by diffusion

P/M Summarizing:

• Powder Metallurgy is sought when -

a) It is impossible to form the metal or material by any other technique

b) When p/m gives unique properties which can be put to good use

c) When the p/m route is economical

• There may be over-lapping of these three points.

Summary

• Powder metallurgy

• Metals and ceramics

• Particles and heat

• Compaction and fusion

• Interesting chemistry

References• Wikipedia Powder Metallurgy (

http://en.wikipedia.org/wiki/Powder_metallurgy)• Wikipedia Sintering (

http://en.wikipedia.org/wiki/Sintering)• All about powder metallurgy http://www.mpif.org/ • Powder Metallurgy -

http://www.efunda.com/processes/metal_processing/powder_metallurgy.cfm

• John Wiley and Sons – Fundamentals of Modern Manufacturing Chapter 16 (book and handouts)

POWDER METALLURGY TEXTAppendix 1

Powder Metallurgy (PM)Metal processing technology in which parts are produced from metallic powders• In the usual PM production sequence, the powders are compressed (pressed) into the desired shape and then heated (sintered) to bond the particles into ahard, rigid mass− Pressing is accomplished in a press-type machine using punch-and-die tooling designed specifically for the part to be manufactured− Sintering is performed at a temperature below the melting point of the metal

Powder MetallurgyJohn Wiley and Sons

Why Powder Metallurgy is Important• PM parts can be mass produced to net shape or near net shape, eliminating or reducing the need for subsequent machining• PM process wastes very little material - about 97% of the starting powders are converted to product• PM parts can be made with a specified level of porosity, to produce porous metal parts− Examples: filters, oil-impregnated bearings and gears

More Reasons Why PM is Important• Certain metals that are difficult to fabricate by other methods can be shaped by powder metallurgy− Example: Tungsten filaments for incandescent lamp bulbs are made by PM• Certain alloy combinations and cermets made by PM cannot be produced in other ways• PM compares favorably to most casting processes in dimensional control• PM production methods can be automated for economical production

Limitations and Disadvantageswith PM Processing• High tooling and equipment costs• Metallic powders are expensive• Problems in storing and handling metal powders− Examples: degradation over time, fire hazards with certain metals• Limitations on part geometry because metal powders do not readily flow laterally in the die during pressing• Variations in density throughout part may be a problem, especially for complex geometries

PM Work Materials• Largest tonnage of metals are alloys of iron, steel, and aluminum• Other PM metals include copper, nickel, and refractory metals such as molybdenum and tungsten• Metallic carbides such as tungsten carbide are often included within the scope of powder metallurgy

Engineering PowdersA powder can be defined as a finely divided particulate solid• Engineering powders include metals and ceramics• Geometric features of engineering powders:− Particle size and distribution− Particle shape and internal structure− Surface area

Measuring Particle Size• Most common method uses screens of differentmesh sizes• Mesh count - refers to the number of openings perlinear inch of screen− A mesh count of 200 means there are 200openings per linear inch− Since the mesh is square, the count is the same inboth directions, and the total number of openingsper square inch is 2002 = 40,000− Higher mesh count means smaller particle size

Interparticle Friction andFlow Characteristics• Friction between particles affects ability of a powder to flow readily and pack tightly• A common test of interparticle friction is the angle of repose, which is the angle formed by a pile of powders as they are poured from a narrow funnel

Observations• Smaller particle sizes generally show greater friction and steeper angles• Spherical shapes have the lowest interpartical friction• As shape deviates from spherical, friction between particles tends to increase

Particle Density Measures• True density - density of the true volume of the material− The density of the material if the powders were melted into a solid mass• Bulk density - density of the powders in the loose state after pouring− Because of pores between particles, bulk density is less than true density

Packing Factor = Bulk Densitydivided by True Density• Typical values for loose powders range between 0.5 and 0.7• If powders of various sizes are present, smaller powders will fit into the interstices of larger ones that would otherwise be taken up by air, thus higher packing factor• Packing can be increased by vibrating the powders, causing them to settle more tightly• Pressure applied during compaction greatly increases packing of powders through rearrangement and deformation of particles

PorosityRatio of the volume of the pores (empty spaces) in the powder to the bulk volume• In principle, Porosity + Packing factor = 1.0• The issue is complicated by the possible existence of closed pores in some of the particles• If internal pore volumes are included in above porosity, then equation is exact

Chemistry and Surface Films• Metallic powders are classified as either− Elemental - consisting of a pure metal− Pre-alloyed - each particle is an alloy• Possible surface films include oxides, silica, adsorbed organic materials, and moisture− As a general rule, these films must be removed prior to shape processing

Production of Metallic Powders• In general, producers of metallic powders are not the same companies as those that make PM parts• Virtually any metal can be made into powder form• Three principal methods by which metallic powders are commercially produced1. Atomization2. Chemical3. Electrolytic• In addition, mechanical methods are occasionally used to reduce powder sizes

Conventional Press and Sinter• After the metallic powders have been produced, the conventional PM sequence consists of three steps:1. Blending and mixing of the powders2. Compaction - pressing into desired part shape3. Sintering - heating to a temperature below themelting point to cause solid-state bonding of particles and strengthening of part• In addition, secondary operations are sometimes performed to improve dimensional accuracy, increase density, and for other reasons

Blending and Mixing of Powders• For successful results in compaction and sintering,the starting powders must be homogenized• Blending - powders of the same chemistry butpossibly different particle sizes are intermingled− Different particle sizes are often blended to reduceporosity• Mixing - powders of different chemistries arecombined− PM technology allows mixing various metals intoalloys that would be difficult or impossible toproduce by other means

CompactionApplication of high pressure to the powders to form them into the required shape• The conventional compaction method is pressing, in which opposing punches squeeze the powders contained in a die• The workpart after pressing is called a green compact, the word green meaning not yet fully processed• The green strength of the part when pressed is adequate for handling but far less than after sintering

SinteringHeat treatment to bond the metallic particles, thereby increasing strength and hardness• Usually carried out at between 70% and 90% of the metal's melting point (absolute scale)• Generally agreed among researchers that the primary driving force for sintering is reduction of surface energy• Part shrinkage occurs during sintering due to pore size reduction

Densification and SizingSecondary operations are performed to increase density, improve accuracy, or accomplish additional shaping of the sintered part• Repressing - pressing the sintered part in a closed die to increase density and improve properties• Sizing - pressing a sintered part to improve dimensional accuracy• Coining - pressworking operation on a sintered part to press details into its surface• Machining - creates geometric features that cannot be achieved by pressing, such as threads, side holes, and other details

Impregnation and Infiltration• Porosity is a unique and inherent characteristic of PM technology• It can be exploited to create special products by filling the available pore space with oils, polymers, or metals• Two categories:1. Impregnation2. Infiltration

ImpregnationThe term used when oil or other fluid is permeated into the pores of a sintered PM part• Common products are oil-impregnated bearings, gears, and similar components• An alternative application is when parts are impregnated with polymer resins that seep into the pore spaces in liquid form and then solidify to create a pressure tight part

InfiltrationAn operation in which the pores of the PM part are filledwith a molten metal• The melting point of the filler metal must be belowthat of the PM part• Involves heating the filler metal in contact with thesintered component so capillary action draws the fillerinto the pores• The resulting structure is relatively nonporous, andthe infiltrated part has a more uniform density, as wellas improved toughness and strength

Alternative Pressing and SinteringTechniques• The conventional press and sinter sequence is themost widely used shaping technology in powdermetallurgy• Additional methods for processing PM parts include:− Isostatic pressing− Hot pressing - combined pressing and sintering©2002 John Wiley & Sons, Inc. M. P. Groover, “Fundamentals of Modern Manufacturing 2/e”Materials and Products for PM• Raw

Materials and Products for PM• Raw materials for PM are more expensive than for other metalworking because of the additional energyrequired to reduce the metal to powder form• Accordingly, PM is competitive only in a certain range of applications• What are the materials and products that seem most suited to powder metallurgy?

PM Materials – Elemental PowdersA pure metal in particulate form• Used in applications where high purity is important• Common elemental powders:− Iron− Aluminum− Copper• Elemental powders are also mixed with other metal powders to produce special alloys that are difficult to formulate by conventional methods− Example: tool steels

PM Materials – Pre-Alloyed PowdersEach particle is an alloy comprised of the desiredchemical composition• Used for alloys that cannot be formulated by mixingelemental powders• Common pre-alloyed powders:− Stainless steels− Certain copper alloys− High speed steel

PM Products• Gears, bearings, sprockets, fasteners, electrical contacts, cutting tools, and various machinery parts• Advantage of PM: parts can be made to near net shape or net shape− They require little or no additional shaping after PM processing• When produced in large quantities, gears and bearings are ideal for PM because:− The geometry is defined in two dimensions− There is a need for porosity in the part to serve as a reservoir for lubricant

PM Parts Classification System• The Metal Powder Industries Federation (MPIF) defines four classes of powder metallurgy part designs, by level of difficulty in conventional pressing• Useful because it indicates some of the limitations on shape that can be achieved with conventional PM processing

Design Guidelines for PM Parts - I• Economics usually require large quantities to justify cost of equipment and special tooling− Minimum quantities of 10,000 units are suggested• PM is unique in its capability to fabricate parts with a controlled level of porosity− Porosities up to 50% are possible• PM can be used to make parts out of unusual metals and alloys - materials that would be difficult if not impossible to produce by other means

Design Guidelines for PM Parts - II• The part geometry must permit ejection from die after pressing− This generally means that part must have vertical or near-vertical sides, although steps are allowed− Design features such as undercuts and holes on the part sides must be avoided− Vertical undercuts and holes are permissible because they do not interfere with ejection− Vertical holes can be of cross-sectional shapes other than round without significant difficulty

Design Guidelines for PM Parts - III• Screw threads cannot be fabricated by PM; if required, they must be machined into the part• Chamfers and corner radii are possible by PM pressing, but problems arise in punch rigidity when angles are too acute• Wall thickness should be a minimum of 1.5 mm (0.060 in) between holes or a hole and outside wall• Minimum recommended hole diameter is 1.5 mm(0.060 in)