Selecting a Process

7/28/2019 Selecting a Process

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26 EnginEErEdCastingsolutions November/December2006

Shannon Kruse,Associate Editor

Choosing metalcasting is just the start. This article will help you navigate the castingprocess palette and nd the optimal one for your part.

Make a Selection

Design engineers must choose amongseveral manuacturing methods toind the best it or a particular compo-nent. This ft takes into account manuac-turability, required properties, time andcost. Every method, including abricating,orging, machining, and powder metal-lurgy has unique advantages, but metal-

casting has the ability to manuacture thewidest range o engineered components

by alloy, size and geometry.Much o metalcastings appeal comes

rom the geometries achieved throughshaping molten metal. Design engineerscan use geometry to attain better proper-ties rom their chosen metal because shapecontrols stress points.

While youre considering metalcasting,

youll need to whittle down the list oavailable casting processes or the one that

best fts your part. With the wide rangeo casting choices available, selecting aprocess can be daunting, but runningthrough the ollowing checklist can makethe task approachable.

CastabilityWhen you are choosing an alloy, take

note o the properties you are lookingor. I you are choosing an alloy based

Choose an

alloy and

correlating

casting process

Compare properties of casting

processes and narrow your choice


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November/December2006 FordEsignEnginEErs&PurChasErs 27

on structural considerations, be awarethat elongation and yield strength canbe traded o with geometry. Perhapsyou can build that structural piecewith aluminum rather than a heaviersteel using the geometric reedom thatmetalcasting provides.

The metal you choose and its castabilitywill narrow your metalcasting options.

Some processes, such as sand casting orinvestment casting, are exible enough toaccommodate almost any type o metal.However, other casting processes, such asplaster, permanent mold, lost oam anddiecasting, work best with a handul ometal choices. The chart in Fig. 1 shows

the metals to which each casting processis best suited.

MicrostructueIn some part designs, the eect o an

alloys microstructure on the propertieso the cast component is a major actor inproducing a successul part, particularlyor iron and aluminum parts. The rateo solidifcation can either positively ornegatively aect the metals desired proper-ties. Aluminum gains strength rom small,

Compare lead times and cost

Shopfor

quotes high tooling cost

low tooling lead time

low casting cost

eliminated machining

medium tooling cost

low tooling lead time

low casting cost

reduced machining

Sand Plaster Permanent Centrifugal LostFoam Investment Die

Metal

Iron

Steel

Aluminum

Copper

HighAlloySteel

Magnesium

Zinc Titanium

Thickness(in.)

downto0.025

downto0.03

downto0.1

downto0.125

Tolerances

excellent

good

fair

SurfaceFinish(RMS)

300-200

200-100

lessthan100

Draft(degrees)

2

1

0.5

0

Complexity

complex

moderate

simple

Fig. 1. Use this chart to compare the main casting processes by part specifcation.


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CastingSize

large

medium

small

verysmall

Toolingleadtime

high moderate

low

Toolingcost

high

moderate

low

Castingleadtime

moderate

low

CastingCost

high

moderate

low

FinishingCost

high

moderate

low

Economicalquantities

verylarge large

medium

small

Sand Plaster Permanent Centrifugal LostFoam Investment Die

tight dendrites in its microstructure. Thequicker the metal solidifes, the smaller thedendrites and the stronger the aluminum.Conversely, this same rapid solidifcationalters irons microstructure to adverselyaect its machinability.

Molding processes with a high thermal

gradient, usually rom metal tooling, such aswith permanent molding, are well-suited oraluminum structural parts. But iron com-ponents perorm better with sand moldingdue to the slower solidifcation rate.

SizeSize matters when youre choosing a

casting process. For instance, i you plan ondesigning a 1,000-lb. part, investment cast-ing is a less likely candidate. Although airlylarge investment castings do exist, the invest-ment casting process has complexities that

are best suited to very aggressive net shaperequirements and/or tight specifations orsolidifcation integrity and surace fnish.

Other molding processes oer aspects oinvestment casting capabilities. Frequently,one o those alternative processes can meetpart requirements by matching specifc ca-pabilities to specifc component unctionalneeds. For example, that 1,000-lb. partmight have its needs met with precision

air set molding with careully engineeredcores and chills. Another part might haveits needs met in the diecast, lost oam, per-manent mold or resin shell processes.

DimensionalRequirementsAter youve narrowed down your list o

casting process candidates based on metaland size, you can dive into the dimensionalrequirements and surace fnish you areseeking. Because there is such a wide varietyo casting processes, you have the ability to

tailor a process and metal to ft your needs.I you are looking or a smooth surace,diecasting will be your best bet, ollowedby plaster and investment casting. I elimi-

Before you start piling stacks of castingprocess books on your desk, check the leson the opposite page. Each casting processis listed with a photo of a representativecasting, features of the process, such asachievable dimensions and surface n-

ish, and a description of suitable jobs.This quick reference is a good place to

Narrow Your Choicesstart when you are trying to narrow down

your choices.If youre still struggling to make a choice,

call up a metalcaster from each process.Ultimately, they will have the best knowledgeof a process capabilities, and often, they

will be able to show you additional tricksthat will help you achieve your goals. ECS

Fig. 2. Use this chart to compare casting processes by cost and size.


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November/December2006 FordEsignEnginEErs&PurChasErs 29

DIECASTING

SAND CASTING PLASTER CASTING PERMANENT MOLD

CENTRIFUGAL CASTING

LOST FOAM CASTING

INVESTMENT CASTING

Suitable for:

most types o metal (except titanium) moderate or simple complexity large, medium or small casting sizes (less than

1 lb. to 10,000s o lbs.) quantities in the large, medium or small ranges

Features: thicknesses down to 0.125 in.;

base tolerances o +/- 0.4-1 in. or greensand and +/- 0.01-0.5 in. or resin-coated;

surace fnish o 120-350 RMS; minimum drat o 0.25-5 degrees; moderate to low tooling lead times; low tooling cost ($800-$4,000);

moderate to low casting lead times; moderate to low casting cost; moderate to high fnishing cost.

Suitable for:

aluminum, copper, magnesium andzinc alloys

simple to moderate complexity medium or small casting sizes

(ounces to 200 lbs.) quantities in small or medium ranges

Features: thicknesses down to 0.03 in.; base tolerances o +/- 0.005-0.01 in.; surace fnishes between 63 and 125 RMS; drat o 0.5-2 degrees; low tooling lead times; low to moderate tooling cost

($3,000 to $15,000)

low casting lead time; low to moderate casting cost; moderate fnishing cost.

Suitable for: aluminum, copper-base,

magnesium and zinc alloys simple complexity small to medium casting sizes (ounces

to more than 100 lbs.) quantities in medium to large ranges

Features: thicknesses down to 0.125 in.; base tolerance o +/ -0.015 in.; surace fnishes between 150-250 RMS; drat o 2-4 degrees; moderate tooling lead times; moderate tooling cost ($5,000-

$20,000); low to moderate casting lead times; low to moderate casting cost; moderate fnishing cost.

Suitable for: steel, aluminum, high-alloy steel and cop-

per-base alloys simple complexity (must be cylindrical) small to large casting sizes (3-55 in. in diam-

eter; 1-20 t. long) quantities in the medium range

Features:thicknesses down to 0.1 in.;base tolerances o 0.1-0.15 in.;surace fnishes between 100-300 RMS;drat o 01 degree; low to moderate tooling lead time;moderate tooling cost ($10,000-$20,000);moderate casting lead time;moderate casting cost; low to moderate fnishing cost.

Suitable for: iron and aluminum alloys simple to moderate complexity small to medium casting sizes

(ounces to 1,000 lbs.) quantities in the small to medium

range

Features: thicknesses down to 0.125 in.; base tolerance o +/- 0.005 in.; surace fnishes between 100-300

RMS; drat o 1 degree; moderate tooling lead time; moderate to high tooling cost

($1,000-$200,000); low to moderate casting lead time; low to moderate casting cost; low to moderate fnishing cost.

Suitable for: iron, steel, aluminum, copper-base, high-al-

loy steel, magnesium and titanium alloys moderate to high complexity very small to medium casting sizes

(ounces to 50 lbs.) quantities in the small to medium range

Features: thicknesses down to 0.025 in.; base tolerances o +/- 0.003-0.008 in.; surace fnishes between 63-125 RMS; drat o 0-1 degree; moderate to high tooling lead time; moderate to high tooling cost

($3,000-$10,000); moderate casting lead time; low to high casting cost; moderate to high fnishing cost.

Suitable for: aluminum, magnesium and zinc alloys simple to moderate complexity very small to medium casting sizes (ounces to 30 lbs.) quantities in the medium to very large range

Features: thicknesses down to 0.025 in.; base tolerance o +/- 0.002 in.; surace fnishes between 32-90 RMS; drat o 0.5-3 degrees; high tooling lead time; high tooling cost ($5,000-$500,000);

low to moderate casting lead time; low casting cost; low fnishing cost.


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nating machining is a chie concern, knowthat the sand processes are going to reducethe machining less than others. Similarly, iyou require thicknesses down to 0.025 in.,minimal drat and excellent tolerances, di-ecasting and investment casting are the topchoices. However, these high-end tolerancesand surace fnishes come at a cost.

SpecialConsiderations

Depending on the requirements o yourpart, other considerations might aectyour fnal casting choice. For instance,

the exibility to use cores when needed tocreate internal passageways or the abilityto create cavities without cores may be adeciding actor. Most permanent molds arenot well-suited or cored-parts, but manyshops use semi-permanent molding (metaltools, sand cores) in order to produce thenecessary geometry o a part. On the otherhand, investment casting and lost oam canproduce highly complex parts without the

use o cores at all.Secondly, the type o rapid prototypingyou use may naturally steer you to a certain

casting process. Stereolithography patternslend themselves to investment casting be-cause the prototype can be attached directlyto an investment tree and used to producethe metal product. But used depositionor laminated object manuacturing rapidprototypes are rigid enough to be used aspatterns or sand molds.

Metalcasting holds a ew other quirksthat can help you optimize your parts de-

sign. Investment casting shells can be hotwhen the molten metal is poured, whichimproves the castability o the alloy. Post-casting processes, such as hot isostatic press-ing (HIP), can lend additional or improvedproperties to a casting. HIP can heal deectsin solidifed metal by using high tempera-ture and high pressure to squeeze the part.Titanium castings that are HIPed, or in-stance, have isotropic structural propertiesthat can be preerable to anisotropic orgedtitanium properties. Semi-solid casting, inwhich the metal poured isnt completely

liquid, results in less solidifcation shrinkageand entrained porosity.

CostWith varying processes and capabilities

come varying costs. In general, the actualcasting cost or most processes is airly low,depending on the part you are designing. Alarge portion o your initial cost will comerom the tooling or the mold and fnishingthe component.

Sand casting generally comes withthe lowest tooling cost, while investment

and diecasting have the highest toolingcost. It is important to remember thatquantity also will be a actor in toolingcosts. I the component is a high volumejob, tooling wil l be more expensive inorder to handle the wear and tear oproduction. But, the higher the quan-tity, the more economical it becomes toront a higher tooling cost or a speediercasting process.

At irst glance, a casting process mightseem too pricey or the part youre de-signing, but a more expensive process can

cut your total manuacturing costs in theend. Remember to actor in tooling costand the cost o the inal assembled part(including machining assembly, etc.), aswell as the total value o the casting. Fig.2 shows the guidelines or economicalquantities or each process.

The fnal value o the part also shouldactor in weight savings and quality. I youare able to sell the product at a higher pricebecause it is o higher quality, spendinga little more in casting production mightbe worthwhile. When reducing weighting

is an important design actor, castings arethe most powerul orm o engineeredmetal component. ECS

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Selecting a Process