25-tooling for composites

7/30/2019 25-Tooling for Composites

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TOOLING FOR COMPOSITES 25Jerry L. Cadden and Paul F. Sadesky

25.1 INTRODUCTIONThe manufacture of composite details andassemblies requires that some kind of accuraterepeatable tool surface be provided, indexedto an engineering database or reference modeland be capable of withstanding repeated expo-sures to the cure cycle environment of hightemperatures and pressures. Once the specificmanufacturing process has been selected (i.e.vacuum bag lay-up or resin transfer molding),decisions regarding tolerances, heat up rates,coefficients of thermal expansion, toollongevity etc. influence the construction of thetool from an engineering design and materialselection standpoint. Individual compositeparts or details will require a variety of sup-port tooling beyond the initial cure tool, suchas master model reference patterns, trim orrouter tools, precision hole location drill tools,assembly fixtures, ply locating templates andother shop aids. Planning must ensure that apoint of reference is established that will con-trol all tooling in any one part family. This willguarantee that critical dimensional tolerancesare maintained within the relationshipbetween different tools supporting the fabrica-tion of one composite detail or assembly. Inaddition, coordination between various com-posite details will ensure thatinterchangeability or replacement is main-tained throughout entire structures. There isan extensive list of materials which can be uti-

Handbookof Composites. Edited by S.T. Peters. Publishedin 1998by Chapman&Hall, London. ISBN 0 412 54020 7

lized for tooling, but no one material solves allof the problems, particularly when factorssuch as cost, longevity and tolerances are con-sidered. The primary objective of any tool forcomposite fabrication is to make an accuraterepeatable part, within the confines of theprocess parameters defined by the compositematerial supplier and the detail performancecharacteristics of the end use customer. Designof the initial tool becomes the most pressinginitial issue of tooling for composites.

25.2 TOOL DESIGN BASICS25.2.1 COEFFICIENT OF THERMAL EXPANSIONOne of the most critical parameters in thedesign of tooling for composites is the differ-ence between the coefficient of thermalexpansion (CTE) of the tool being designedand of the composite detail being fabricated.During the cure cycle of the composite lay-upon a tool, the lay-up expands during the heatup cycle. The specific rate of expansion isdirectly related to the type and combinationof resin or matrix and fibers or reinforcementused. The tool will also expand and contractat a specific rate determined by the materialand construction techniques utilized. If theCTE values for the laminate and the tool dif-fer significantly, stresses may result in thelaminate causing the occurrence of dimen-sional, strength and part stability problems.The greater the difference between the CTE ofthe composite detail and the tool, the morepronounced the effect will be.


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Tool design basics 557One of the effects that occurs as a function of

these dimensional differences is called spring-back. Composite details, when cured, hold thespecific molded shape, as defined by the tool,as a result of the cured combination of resinand reinforcement. The springback, or moreaccurately defined as a warpage condition,occurs when the composite detail is cured intoa tool, that at a specific temperature has onedefinite dimensional tolerance and then uponcooling to ambient temperature, contracts to itsoriginal ambient dimensions. The compositedetail, based on the resin chemistry, cures dur-ing the specific period when tool expansion isat it greatest. Warpage occurs when stresses areinduced to the composite as the tool begins toreturn to the ambient dimensions, because thecomposite detail is being forced to conform tothe new dimensional range against the dynam-ics of the state it reached during cure. Thiscondition will increasingly become greater asthe temperature difference between ambientand cure temperature increases and the dimen-sional size of the tool increases. A commonmethod of minimizing the effects of springbackor warpage of quasi-isotropic compositedetails during and after cure cycling is to deter-mine the CTE of the composite part beingfabricated and the CTE of the tooling materialselected. During the design of the tooling, care-fully match as closely as possible theappropriate tooling material CTE to that of thecomposite detail.

Other conditions that might lead to awarpage of the laminate include an unbal-anced laminate orientation where the numberof layers or plies of material are more domi-nant in one direction than another. Thiscondition is separate from any function of thetool and must be considered during the designof composite detail.

25.2.2 USING CTE IN THE DESIGN OFTOOLING FOR COMPOSITETwo methods are commonly used to minimizethe effect of CTE when designing tooling for

the fabrication of composite details. Onemethod is careful selection of the appropriatetooling material. Each of the commonly usedtooling materials available has a specific CTEvalue (Table 25.1). When selecting the appro-priate tooling material based on the issue ofCTE compatibility only, first determine theCTE of the composite detail being fabricated.The specific expansion rate will be determinedby the combination of the resin and reinforce-ment utilized along with the particular fiberorientation that is incorporated into the lami-nate. For example, the CTE of the morecommon unidirectional carbon fibers used inmost composite epoxy laminates is approx. 4.5x 104/OC (2.5 x 10"/OF ). The strength of thereinforcement material lies along the directionof the fiber, not perpendicular to it. If a lami-nate is balanced, quasi-isotropic, withindividual laminate layers or plies equally dis-tributing loads throughout the laminate, theCTE of the laminate will be consistently equalin all directions. If one direction is dominatedby more material plies than any other direc-tion, the CTE value will vary, with thedominant direction having the lower CTE.Once the CTE of the laminate is determined,using the appropriate chart select the toolingmaterial with the closest match to the laminatevalue.

The other method of accounting for CTEvariations between the detail being fabricatedand the associate tool, besides material selec-tion, is the use of shrink factors in thecalculation of dimensions prior to tool fabri-cation. If requirements dictate that whenfabricating the tool, a material with an incom-patible CTE to the detail being manufacturedmust be used, steps may be taken to mini-mize the effect of this variation. During thedesign phase of the tool, accurate estimates ofthe actual tool size at its greatest expansionpoint or at the highest temperature duringthe cure cycle must be made. The percentagedifference between this calculation and thedimensions at ambient must be applied to thebase design as a 'shrink factor' reducing the


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558 Tooling o r compositesTable 25.1 Propertiesof commonly used tooling materialsMaterial Coefficient of thermal expansion, Density,

x 10-fipc g/cm3(x 1c 6 / a F ) ( i w t 3 )Thermal conductivity,cal cm/s cm2"C(BTU in/hft2O F )

Aluminum

Electroformed nickelCarbon/epoxy prepregGlass/epoxyInvar 36

Invar 42

Monolithic graphiteHigh carbon cast steelSteel

Wood (mahogany)Urethane board stock

23.7 2.7 0.48(13.6)13.3(7.4)

(2.5)

( 7 4

(1.9)

(3.0)(1.2)

(5.4)

4.5

13.1

3.4

5.4

2.16

9.7

12-146.7-7.8

22(12)

(28)50

(170)

(540)8.6

1.61.5(87-94)1.6-2.0

(100-125)8.0

(504)

(507)

(110)

(456)

(490)

(44)(48)

8.1

1.76

7.3

7.8

0.7

0.77

(1400)0.19(564)

16 x 10-4(4.6)

(1.7)

(48)

(48)

(840)(360)

(300)

6 x

0.016

0.016

0.28

0.12

0.10

0.902600n.a.

size of the tool by that same percentage. Forexample, if a specific tool at 177C(350F) hasa growth factor equaling 1.27 mm (0.050 in) ofgrowth over ambient dimensions, this samefactor would be applied as a reduction of theoverall tool dimension while at room temper-ature. This method of applying a shrinkfactor to allow for variations of CTE betweentools and details must be approached cau-tiously when complex shaped surfaces areinvolved. This is due to the potential for thecomposite details to become entrappedwithin the geometry of the tool as the toolreturns to the ambient temperature dimen-

sions. Since the composite detail was curedwhile the tool was at the larger dimension, ifthe detail is confined to the tool surface orrestricted from movement due to complexityin the tool surface, the composite detail couldbecome entrapped, resulting in dimensionalabnormality in the laminate and possibledamage to the composite detail or the toolingsurface.A variety of other factors should alsobe considered such as tool durability, toolusage rates, thermal conductivity of the mate-rial and machinability or fabrication cost.Each of these factors must be weighed indi-vidually before final selection is made.


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Tool design basics 55925.2.3 MATERIAL CHOICES IN THE DESIGN OFTOOLING FOR COMPOSITE machining the final tool surface directly froma computer model can be accomplished usingThe use of CTE is not the sole determinant forselection of a tooling material. For example, asdefined in Table 25.1 monolithic graphitewhich as a tooling material has a CTE value of2.2 x 104/OC (1.2 x 1O4/'F) has good machin-ability at a relatively low cost compared toother tooling materials, but exhibits poordurability when utilized in the high usage ofproduction environments. InvaP 36, however,displays a similarly low CTE value, displays amuch higher level of damage resistance buthas poor machinability and also has a highacquisition cost. Careful selection of theappropriate material for tool use must includereview of the following criteria:0 anticipated tool usage (expected life of tool);0 cost available for tool fabrication;0 materials available for tool construction;0 available methods of tool manufacturing;0 level of dimensional tolerances required

from composite detail.

Anticipated tool usageThe life expectancy of any tool fabricated forthe lay-up and cure of composite details isdependent on a variety of factors. Materialselection, shop handling procedures and curecycle times all affect the ability of the tool towithstand long usage. Certain materials dis-play characteristics that allow longer tool life,however the advantage and disadvantage ofeach material must be analyzed prior to selec-tion. In addition, each of the tooling materialspresently used is sensitive to damage specificto that material. If short term usage is antici-pated, temporary tooling such as wet lay-upepoxy or polyester tooling (dependent ondetail cure temperature) may be acceptable,however master models and intermediatetransfer tooling would be necessary to main-tain the correct surface tolerances. Tominimize the costs associated with mastermodels and intermediate transfer tooling,

va material capable of the final cure tempera-tures. Monolithic graphite and a variety ofepoxy and polyester tooling boards wouldallow this with monolithic graphite offeringthe highest quality and lowest CTE at a com-parable cost to the board stocks available.These costs would be similar to laminatedtooling without the intermediate and timeconsuming steps necessary to complete thelaminated tool. However, in a high usage pro-duction program, these materials can bedamaged more easily than the composite lam-inate or metallic tools currently in use.Continued advances by the suppliers of com-posite tooling prepregs have drasticallyincreased the ability of composite laminatetools to support a high number of cycles atcure temperature.

Another factor directly affecting thelongevity of tooling fabricated from compositematerials is the effect of proper employeetraining in the care required of such tooling.Improper handling techniques will drasticallyshorten the expected life of composite tooling.Employee caused damage such as cutting onthe surface of the tool, using sharp instru-ments to facilitate the removal of details aftercure and improper application of releaseagents are the most readily identified causes ofshop induced damage. Special handling pro-cedures and employee indoctrination canminimize this type of damage. While all tool-ing, both metallic and non-metallic, issusceptible to damage, tooling fabricated fromcomposite materials is especially sensitive tosurface damage caused by employee careless-ness.Cost available for tool fabricationCost of tool fabrication is difficult to quantifysince material procurement and labor costvary widely throughout the industry.However, comparisons can be made betweenthe different tooling materials and methods


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560 Toolingfor compositesof manufacturing to assist the user in deter-mining the optimum approach based on acost/performance comparison. Comparisonsmust include all necessary steps to deliver acompleted tool family. There is a temptationto compare tooling cost for just the lay-upmold in vacuum bag processing and ignorethe procedures and tooling necessary toobtain that final surface. In direct compari-son, a machined lay-up tool will appear to besubstantially more expensive when com-pared to a lay-up tool fabricated from acomposite prepreg. However, when the asso-ciated costs of reference patterns, additionalsplashes to obtain the correct surface andpossible autoclave or oven processing time,are factored in, the tool machined directlyfrom numerical control (NC) data, which willeliminate the need for all the intermediatesteps mentioned, will become more compara-ble in price.

Materials available for tool constructionBecause of the advent of computer-aideddesign and manufacturing systems(CAD/CAM) to support tool fabrication, thereare more materials now available. Baselinemethods such as plaster master models andplastic-faced plaster transfer tooling willalways have a specific application within tool-ing, but as the use of CAD/CAM increasesbeyond aerospace into all regimens (e.g. sport-ing goods, medical and transportation), suchtooling practices that depend more on the skilllevel of shop personnel than on the accuracy ofa machine tool are rapidly being replaced.

Available methods of tool fabricationMethods of manufacturing vary, dependent onequipment and personnel resources available.Plaster type master models, intermediate trans-fer tooling and wet lay-up type molds takeminimum facility requirements. Basic shop skilllevels require a familiarity with resins andgeometry in order to support simplistic wet lay-

up fiberglass or graphite tooling. Equipmentobligations can be as minimal as a calibratedsurface table and measuring instruments forthe accurate setup and weighing of mixedresins and a general knowledge of the systemsinvolved. Use of composite prepreg materialsrequires additional employee skills levels plusthe additional equipment expenditures of anoven or autoclave to cure the materials accord-ing to manufacturers instructions. Increasinglystringent facility requirements involving theinstallation of a controlled environment, suchas a clean room, have been recommended bymaterial suppliers. As the complexity of thetooling rises, so does the requirement for ade-quate employee training and engineeringsupport.

To support the fabrication of complex,highly accurate machined tooling, machinetools with sufficient work envelopes arerequired in the correct axis of motion to sup-port the complexity of tooling surfaces.Three-axis machine tools will support basicsimple contours, with 4 ,5 and 6 axis machinescapable of machining the much more complexcompound contour surfaces.A sufficient com-puter-aided design and manufacturing(CAD/CAM) system is required to supplysoftware commands in order to drive machinetool directional and cutting speeds. In addi-tion, if board stock materials are chosen forreference patterns, the skill and equipmentnecessary to assemble, bond and seal thematerial after machining are required.

Level of dimensional tolerances requiredfrom composite detailBased on the type of manufacturing methodand the type of material selected, different lev-els of dimensional tolerances are possible.Initially, the designer determines the level ofdimensional tolerance for the composite detailbeing fabricated. Compliance to this toleranceis critical in meeting structural demands andconformance to any form, fit or functionrequirements.


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Tool design basics 561In the early days of advanced composites,

applications were limited to aerospace, whichinvoked strict dimensional requirements forboth tooling and detail parts. These require-ments continue within aerospace today. Withthe expansion of composite usage into otherareas, such as sporting goods and automotiveapplications, the range of acceptable dimen-sional variations has increased but visualrequirements are much more stringent thanbefore. The variation of CTE between differ-ent materials for tooling has a major effect ondimensional tolerances. In addition, somematerials display sensitivity to environmentalconditions that have an adverse effect ondimensional stability. Tooling, such as refer-ence patterns or master models manufacturedfrom urethane board stocks or plaster models,are hygroscopic and may absorb moisturefrom the atmosphere. This condition will, at aminimum, cause dimensional changes relatedto the level of moisture absorbed. Also, thiscondition could be excaberated if the moisturecontamination is extensive and the model istaken to elevated temperatures. At highertemperatures, the moisture will expand andmay result in possible significant changes instructural integrity. Urethane or epoxy boardstock materials have greater resistance tomoisture. However, if machined and used forreference patterns they are both still suscepti-ble to contamination, which could result indimensional changes and possible failures inthe bond joints between the block surfaces.Additional steps must be taken to protect allmodels manufactured from these materials toprevent these types of contamination. Sealersmust be applied and the items must be segre-gated from potential sources ofcontamination. Monolithic graphite offersadvantages over these materials because it isinert and resists contamination from the envi-ronment. Lay-up molds manufactured fromboth ferrous and nonferrous materials mustbe protected from oxidation. Failure to main-tain a nonoxidized lay-up surface will requirerestoration of the tooling surface which could

result in dimensional changes occurring dur-ing this process.

The highest level of tolerances available areobtained by machining the tooling surfacedirectly from the computer model. Certainmaster model materials such as plaster andsome board stock materials have a limitedtemperature exposure level which inhibits theability to pull composite laminates directly offtheir surface. Intermediate tooling must befabricated either to obtain the correct surfacelevel from the master or to be capable of with-standing elevated temperatures within anautoclave or oven, above that of the originalmodel. By machining directly from the engi-neering database, the need for intermediatesurface splashes to obtain the correct surface iseliminated. Each time a splash or the originalsurface model is duplicated, a 'stackup' oraccumulation of the tolerances for each modelis combined, resulting in a much greater rangeof tolerances in the final tool. For example, if aplaster master is fabricated to k0.25 mm(a.010 in) tolerance and each of two addi-tional splashes have the same tolerance range,prior to fabrication of the final lay-up mold,the beginning tolerance range is now d.76mm (4.030 in). If a tool is machined directlyfrom NC data, the tolerance stackup is elimi-nated and only the range of the individualmachine tool applies. Machine tools, depen-dent on the condition and environment ofmachining, are capable of providing 4.1 28mm (4 .005 in) accuracy or greater.

25.2.4 DESIGNING TOOLS FOR RESISTANCE TOFAILUREBecause of the abusive environment experi-enced by tooling during the fabrication ofcomposite details, life expectancy of toolingwill always be short of anticipation. Repetitivecycling from ambient to over 177C (350"F),inadequate care and handling procedures,incorrect fabrication techniques have all led toa variety of defects resulting in prematuretemporary or permanent failure of the tool.


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562 Toolingfor compositesFailure modes common to composite lay-up

tools fabricated by both wet lay-up andprepreg methods generally involve fiber sepa-ration. This is due to a variation in CTEbetween the resin matrix and the fiber.Generally, the neat resin systems used in mostcomposite tooling systems have a CTE of 65 x104/"C (36 x 104/"F) The graphite fibers usedin most prepreg tooling systems have a CTE ofaround 4.5 x 104/"C (2.5 x 104/"F). Duringexposure to cure cycles where temperatureswill vary from ambient to 177C (350F) andabove, the difference in CTE between thefibers and the resin will eventually cause dis-bonds between laminate layers resulting inleaks internally within the tool. In addition,the expansion of the resin is somewhat con-trolled by the fiber reinforcement in the x andy axis. Because no reinforcement exists in the zplane linking the individual layers together,the difference in CTE between the resin andreinforcement becomes more pronounced.Failures between the individual plies increasebecause of the lack of reinforcement restrain-ing the resin from the repeated expansion andcontraction.Furthermore, when laminating layers ofeither prepreg or wet lay-up tooling, by cut-ting each of the plies into pieces 304-457 mm(12-18 in) square, no continuous fiber pathwill pass through the tool. By discontinuingthese pathways, leaks occurring along thefiber path will be minimized. In addition,because each layer or ply consists of sectionswithout any continuous fiber path, stresseswithin the laminate will be lower, minimizingwarpage during use. A majority of failures incomposite tooling may be directly tracked toleakage around tooling holes or plumbing fit-tings. Tooling hole fittings are exposed torepeated shocks during the removal of curedcomposite details. If steel bushings are used,the difference in CTE will possibly lead tocracks in the tool surface which will becomepotential leak paths. One solution is to installInvar 36 bushings in laminated tools which arecloser in CTE to the parent tool. Also, when

laminating the initial tool, apply additionalplies of pregreg in the area of the bushings toincrease support in those areas to resist move-ment during part removal.

Another possible solution to the problem ofdelamination between layers of prepreg toolingis the application of glass transition tempera-ture values (T J to extend the life expectancy ofa tool. Most tooling resin systems are formu-lated with a Tg value at or slightly above themaximum use temperature of the resin system.As a function of the resin chemistry, glass tran-sition temperatures decay or reduce with eachexposure to the cure temperature that the sys-tem was designed for. This decay, inincremental steps will continue until wellbelow the cure cycle temperature that the toolwas intended to be cycled at. When this point isreached, the resin will begin to break downwith a mechanical failure of the bond betweenthe resin matrix and the fiber reinforcement.

The solution to this problem is to use aresin system with the highest possible T gvalue available. For example, if the tool isintended to be cycled repeatedly at 177C(350"F), a T value of the resin system in the220C (425"h) range will allow more cycles. Itis common among some aerospace compa-nies to now fabricate composite tooling forepoxy laminates from a bismaileimide or acyanate ester resin system with Tg valueshigher than 260C (500F). This allows theinevitable decay of the Tg value to span agreater difference allowing the life of the toolto be extended. The same principle may beapplied for any prepreg system from poly-esters to the higher temperature resinsystems. One consideration in using thismethod requires the selection of a mastermodel material capable of exposure to theelevated temperatures that the higher tem-perature systems require during cure. Plasterand most of the board stocks available are notcapable of these higher temperatures andintermediate splashes or surfaces would haveto be provided. Monolithic graphite does pro-vide a surface capable of exposure to higher


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Master models 563temperatures in addition to having the lowestCTE available, allowing tools fabricated fromthese resin systems to be taken directly off themodel surface without need for intermediatesurfaces.

Employee-induced damage of tooling canplay a much greater part in the reduction ofexpected tool life. Correct indoctrination intothe importance of the tool to the fabrication ofaccurate details must be stressed. Most of theemployee-induced damage will occur eitherduring the lay-up procedures, the removal ofcured composite details from the tool surface,or preparation of the tool surface prior to thenext lay-up. During lay-up of details, damagewill be the result of employees using knives orother sharp objects during the trimming of thecomposite material. If proper care is nottaken, the employee will not only cut thematerial but also cut into the tooling surface.While not as detrimental to a metallic tool,damage of this kind may be catastrophic to atool fabricated from a composite material. Thecut will allow a breach in the vacuumintegrity in addition to allowing resin to pen-etrate beyond the surface of the tool. Also,when laminates are removed from toolingafter completion of the cure cycle, damageoccurs when personnel use sharp equipmentto force the completed detail from the tool.The greatest care must be given whenattempting to remove the detail, to preventinadvertent damage if the detail fails tocleanly release from the tool surface. Damage,not only to the tool, but also to the detail mayresult. To prevent this damage from occur-ring, proper steps must be taken. Employeesmust be indoctrinated in the proper tech-niques of tool maintenance and lay-upprocedures and must be provided withacceptable tooling aids to assist in the saferemoval of cured details from the tool surface.Soft wood or plastic wedges must replacehammers and hard-faced chisels for detailremoval and tool surface preparation. Toolsmust be designed with adequate laminatethickness to prevent damage to the tool if

struck with a hard object. Carriers designed totransport the tool to and from work stationmust also function as a protective barrier toprevent the tool from striking walls or beamswithin the shop environment. Support tool-ing, where applicable, must be designed to beas lightweight as possible to prevent injury tothe employee and damage to the tool surfacewhen handled.

25.3 MASTER MODELSA master model is considered to be just that -a master source identified with holes, scribelines, trim lines or any other feature of thepart that requires duplicating to other tools.The master model is the physical representa-tion of the design or a point of reference towhich all support tooling, both for fabricationand inspection, would be indexed. Becausethis surface will provide the reference patternfor all subsequent operations beyond initialfabrication, such as assembly fixtures index-ing a variety of details from differentlocations, extreme care must always be takenin protection of the master model. Mastermodels may be fabricated from a variety ofmaterials. Common materials include plaster,machined urethane or epoxy board stock,monolithic graphite or most ferrous and non-ferrous metals. Each material offers distinctadvantages and disadvantages. To determinewhich material is the most feasible, the entiretooling family philosophy must be reviewed.Master models are generally stored indefi-nitely so that they may be referred to over thelife cycle of the manufactured parts. In caseswhere cost and/or time schedules are impor-tant, temporary models are produced andthen destroyed once they have been used.However, because of the hygroscopic natureof plaster, care must always be taken to pro-tect the master model from the environmentto maintain accuracy. Adequate storage con-tainers, allowing for complete protection,must be utilized throughout the lifeexpectancy of the model.


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564 Tooling or composites25.3.1 PLASTER MASTERSOne of the oldest methods of producing a mas-ter is from plaster. Plaster is made from themineral gypsum (CaSO,). which is finelyground and calcined (dehydrated) to producea fine powder with uniform properties. Withthe addition of water to form a workableslurry, a reaction occurs which produces heatand the inert gypsum on drying. Plaster ismanufactured in various textures or gradeswhich support the level of detail required onthe model. Coarse grades are used to build upthe master model surface and then followedby the fine grades which allow precise detailssuch as trim lines or other identifications to bescribed into the surface. Depending on thegrade being used, plaster has a setting expan-sion of approximately 0.080% and a thermalexpansion in the dried state of a maximum of0.027/ "C (0.0156/ OF).

25.3.2 TEMPLATE! METHODThere are several techniques of building aplaster master determined by the shape of thepart. If the part is not symmetrical and doesnot have a constant cross section or the size islarge, the master model is made from a seriesof templates secured to a flat base to form athree dimensional full scale model of the part.Space between the templates is relative to thedegree of abruptness of the contour. For nor-mal gentle contours a space of 15.24-20.32nun(6-8 in) is common. Templates are usuallymade from 0.317 mm (0.125 in) thick alu-minum to prevent corrosion. For temporarymasters, steel is sometimes used, but, becauseof the amount of moisture used in the mixingand application of the plaster, steel templatesmay rust (Fig. 25.1).

If electronic data is available, the templatescan be NC machined or cut with a water or

REFERENCE LINEREFERENCE L INE

Fig. 25.1 Skeletal structure for template plaster master.


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Master models 565laser jet directly from the flat pattern generatedby the data. Section cuts taken at specific sta-tion lines from two-dimensional blueprints canbe used to saw out a template. Except for theNC machining method, deburring is generallyrequired to remove spurs or sharp edges fromthe templates prior to use. Holes are drilledinto the templates for threaded rod spacers andscreen support rods. For larger models, air pas-sages are cut into the bottom of the template toallow for even curing of the plaster. Once suffi-cient templates have been prepared, bluing isapplied to a flat ridged steel table and scribedwith an awl to denote the location of each tem-plate. Flatness of the table is critical and shouldbe within 0.127 mm (0.005 in). Tooling ballswhich indicate the x , y and z direction aresometimes placed on the table corners as refer-ence points for the system. Tooling balls canvary in size but a common size is 12.7mm(0.5 in) diameter on a 6.35 mm (0.250 in) diam-eter x 12.7 mm (0.5in) long pin. The pins, eachwith a 'ball' on top are placed into locationholes and optically sighted relative to the posi-tion of the each ball location. Location can alsobe treated relative to a position on the mastersuch as a station line. Each template is attached90" to the base table with angles and held towithin 0.127 mm (0.005in)of the reference lineat the base, the face square to the base to within0.076 mm (0.003 in) in 304.8 mm (12 in) andwithin 0.127 mm (0.005 in) of the base referenceline. Threaded rods are secured with sheet nutson each side of the template to provide rigidityto the template face. Wire mesh is placedbetween the templates and secured to thethreaded rod with wire hooks approximately101.6 mm (4 in) below the top surface of thetemplate. This is used to hold the plaster inplace. Plaster is mixed with hemp and placedagainst the screen to approximately 9.5 mm(0.375 in) below the template surface.A secondlayer without hemp is added to this surface toapproximately 12.7 mm (0.5 in) A sawtoothscraper is used to build a striated surface andallowed to dry. A final mix is made with the

steel blade, the plaster is 'faired' or swept flushbetween the templates to form a smooth accu-rate surface. Because of the propensity ofplaster to absorb moisture, it should be sealedafter the surfacehas had adequate time to cure.Commercially available lacquers can be usedto seal the surface and provide a suitable pro-tection within the shop environment.25.3.3 FOLLOW BOARD METHODA method widely used when a constant crosssection is to be built is the follow board. A flatsurface s required with an accurate side surfaceto act as a guide rail.A template of the contouris prepared from a rigid 3 mm (0.125 in) mini-mum sheet of aluminum or steel and attachedto a wooden guide support. Plaster is mixedand built up on the surface to within 3mm(0.125 in) of the final contour. Partial drying isrecommended before the final plaster mix isapplied. This will prevent shrinkingand crack-ing of the plaster surface which would affectaccuracy. Using the template and guide sup-port, the plaster contour is formed by pushingthe template evenly over the surface (Fig. 25.2).

FOUOWBOARD- /

FOLLOW BOARD METHODFOR PRODUCING PLASTER MASTER

Fig. 25.2 Follow board method for producing plas-ter master.

25.3.4 SWEEP METHODA third method called a sweep is best utilized

fine grade of plaster and using a flat spring when a symmetrical surface s;ch as a cone or


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566 Tooling or compositeshemispherical shape is involved. As with thefollow board method, a flat surface from whicha frame can be constructed of the shape to beproduced is required. For large shapes, inter-mittent templates should be placed within theframework to allow support for the sweep andprovide adequate support for the sweep to fairagainst. The sweep itself is usually made fromsheet metal 3 mm (0.125 in) minimum thick-ness and supported by a wooden guide orother mechanical guides that can ride the sur-face of the flat surface table. Smaller shapes, ofcourse, do not require this extent of rigging.Plaster can be reinforced with saturated hempfibers, mixed into the slurry and applied toform rough shapes and to form strengtheningribs on the back surfaces of casts. All mastermodels fabricated from plaster require, inaddition to sealing with commercial grade lac-quer, suitable storage containers if the model isrequired to be stored for any period of timeoutside the shop environment.

25.3.5 NC MACHININGBecause of the widespread use of CAD (com-puter aided design) systems, older methodswhich utilized two-dimensional prints tobuild master models are now used less fre-quently. With CAD systems, a great deal ofaccuracy can be transferred into the mastermodel via the NC machining operation. Table25.1 lists various materials widely used todayfor NC machined master models. From a CADmodel of the part, a tool manufacturer mustdesign a tool from the surface data supplied.Advances with CAD/CAM systems seek tominimize the operator input to the system andtransfer design responsibility to the computer.One example of this technology is demon-strated in a system developed by amulticompany team lead by the NorthropGrumman Corporation for the US Air ForceManufacturing Technology Directorate atWright Patterson Air Force Base. This system,while not totally removing the tool designerfrom the design process, does streamline the

operation significantly. The system, known asAutomated Tool Manufacture for CompositeStructures (ATMCS), is an expert system withmacros which dramatically speeds up the tooldesign process. ATMCS takes the compositedetail surface model into either IBM Catia orEDS Unigraphics I1 and creates the toolrequired around the part model.

The system, acting through a series ofinquiries made to the tool designer, selects theoptimal configuration, material, manufactur-ing process and design. The design is thencreated around the part model, with signifi-cant savings in time. Although the system wasdeveloped initially for the aircraft industryand is presently used for basic open-faced lay-up molds, it could be expanded for manydifferent types of tools and processes such asresin transfer molding and injection moldingin other industries.

25.4 COMPOSITETOOLSComposite tools are usually made from epoxyresin matrix and either E-glass or carbon fibersas reinforcement. Depending on the life cyclerequired, tools can be made from prepreg orby 'wet' lay-up procedures. Prepregs generallyrequire curing within an autoclave because ofthe elevated pressure specified by the manu-facturer. Because of the increased compactionavailable when curing in an autoclave, toolingfabricated from prepregs are capable of agreater number of cure cycles than the wet lay-up method. In addition to greater compaction,autoclave curing offers better control of resincontent and uniformity of reinforcement.However, for shop aids such as trim tools,room temperature curing epoxy systems arerecommended.25.4.1 LAY-UP MOLDSLay-up molds are used to form the shape ofthe part to be produced and have the partperiphery scribed on the surface as well asthe location of any required cross hairs and


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Composite tools 567tooling holes. Tools can be made directlyfrom a NC machined master model or from aplastic faced plaster splash taken from a mas-ter model not capable of elevatedtemperatures and pressures. The choice ofglass or carbon fiber/epoxy for the mold isgenerally governed by the complexity andCTE of the part to be fabricated. Lay-upmolds must be capable of maintaining a vac-uum tight environment while being subjectedto high temperatures and pressures.

25.4.2 PREPREG METHODThere are a considerable number of prepregsavailable as epoxy 'B' staged glass or carbonreinforced cloth. Prepregs can be obtained inrolls or as precut squares or rectangles. Theweave style can vary depending on theamount of drape to be encountered but gener-ally plain or satin weaves are readily available.The resins are tailored for tack, out time andglass transition temperatures at a minimumand are around 40%by volume of the prepreg.(Tooling prepreg manufacturers have verydetailed procedures that they recommend fortheir specific system. These comments are notmeant to supersede the recommendation of amanufacturer, but rather to place emphasis onimportant steps that should not be overlookedfor tool fabrication.)

Within the last several years, an innovationfor tooling prepregs is the capability for lowtemperature curing 61C (145F) in an auto-clave, followed by a free standing post cure at177C (350F) after removal of the tool fromthe master. This has allowed the use of plasticfaced plaster and urethane based toolingboards for direct lay-up of composite tools.Monolithic graphite with a low CTE and capa-bility to withstand 315C (600F) underautoclave pressures can be a good choice.

The first step prior to prepreg application onthe master surface is to ensure that the prepregand the master surface are absolutely clean andfree of debris and that the surface is smoothand without pin holes. A quick vacuum check

is always a good idea at a minimum of 6.2 kPa(25 in Hg). A loss of 500 Pa (2 in Hg) within 5min with the pump nonoperating is acceptable.Apply masking tape around the tool peripheryfor later application of the sealant tape. It isabsolutely necessary, regardless of prior his-tory of the master surface, that it can bereleased with a suitable hard wax or otherrelease agent. The prepreg manufacturer mayrecommend a specific release agent for hisprepreg system and it is advisable to followthose instructions due to the possibilities ofchemical reaction occurring between the resinsystem of the tool and the release agent used(Table 25.2). After the cleaning and releasingprocesses have been completed, release coatedtooling pins should be placed into the holes ofthe master. These are generally index andlocating holes that have bushings and are usedto position or align one tool to another, or to aproduction part. Bushings can be installed dur-ing lay-up of the tool or potted in after finalcure of the tool.

In general, a face or gel coat layer is notused by most manufacturers today. The reasonis associated with the difference in CTE of aneat resin on the face and the CTE of theunderlying reinforced prepreg which over thelife span of a tool can cause cracking and craz-ing of the face and subsequent loss of vacuumintegrity. If a gel coat is used it should be ofminimum thickness to minimize these effectsover time.A lightweight (style 7781) cloth is the firstlayer applied to prevent mark through to thesurface from heavier cloth. Carefully lay eachply onto the surface and work out wrinkles orair bubbles and maintain the warp directionofeach ply in the 0" direction. An overlapbetween the plies should be 3-6 mm(0.125-0.250 in). Some manufacturers recom-mend a debulking step at this point to ensureno air entrapment at the interface and asmooth surface on the tool. Debulking isaccomplished by application of a peel ply netto the edge of the laminate and working outwrinkles and air bubbles.A resin dam (sealant


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568 Toolingfor compositesTable 25.2 Cures for common mold release problemsProblem Cause SolutionNon-adherance of paint oradhesive to part

Poor release with small particlesof visible on mold surface

No paint or adhesive sticks topart

Multiple release not possible

Failure to obtain any release eventhough release was applied

Generally poor release withpatches of white transferring tothe part

Build up of mold release, layersof incompatible mold releases, ortoo much release in formulation

Porosity in mold

Check for the presence of siliconesin the area. Silicone mold releasescan be transferred over longdistancesPoor release in some areas,particularly in severe contoursInadequate cleaning of the moldbefore application of release hasinterfered with the ability of therelease to bond to the mold or therelease has been improperlycured. Shelf life of release mayhave expiredMold surface not properly cleanedwhich results in poor bonding ofrelease to mold

Use manufacturersrecommendations for layers andcure schedules. Avoid addingincompatible layers such assilicone, wax and flurocarbon.Check with manufacturer forpossible revision or customformulation to allow multiplereleases and adequate paintadhesionThoroughly clean mold with anappropriate solvent and thenadd a mold sealer before therelease coatRemove silicones from plantwhere painting or adhesivebonding is performedApply one or two additionalcoats of release to severe contourareasStrip out the part and thoroughlyclean the mold, then apply andfully cure release agent. Alsocheck shelf life of release

Remove all release andthoroughly clean mold with anappropriate solvent beforereapplication of release. Followmanufacturersrecommendations for cure cycletape) can be placed aro und the perimeter toprevent resin flow (Fig. 25.3). Next, lay-up oneply of Teflon@ eyond the resin d am and attachto the resin dam. Using the manufacturersrecommendation, holes should be placed intothe Teflon ply to allow for resin bleed. Pre-per-forated film can be obtained for this purposeand provides greater control over the size andspacing of the holes. If only one lightweightply has been applied, no holes are required to

permit a higher resin content on the tool sur-face. Over this layer, one ply of polyesterbreather cloth or 7500 style glass cloth isapplied. A nylon vacuum bag is placed overthe entire stack and a vacuum of at least625mm (25 inHg) is applied for at least onehour. Removal of the bag, breather, separatorfilm and peel ply should be done very carefullyto avoid lift up or shifting of the prepreg lay-ers. The orientation for each additional ply


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Composite tools 569Vacuum Line

Vacuum Vacuum Bag Sealant,,Breather

T d l Laminate \Release FilmFig. 25.3 Laminate pre-bleed stackup.should be such that a balanced system is main-tained to minimize stress build up in thelaminate. Prepreg manufacturers will clearlystipulate the lay-up sequence. After the secondor third ply has been laid down, knurled bush-ings should be placed onto the tooling pinsand pressed down to seat them. Subsequentplies will be placed over the bushings to inte-grated them into the laminate. After theseventh or eighth ply, the pins can be removedso that later plies can cover the bushing com-pletely to prevent vacuum leaks. In someinstances a pad or build up of plies over thebushing is recommended. An alternatemethod is to pot the bushings into the lami-nate after the final cure. To do this a taperedwax or rubber plug should be placed over thepin to allow space for the potting compoundafter the final cure. Each ply should be care-fully worked into corners and radii makingsure all entrapped air is removed. Wrinklesshould also be carefully worked out beforeanother ply is placed over it. If a persistentwrinkle or air bubble can not be rolled out,then carefully slit the pockets with a sharpknife and work it down into the surface. In theexcess area of the tool, thermocouples can bestrategically located between the plies forrecording during the autoclave run. As a ruleof thumb, debulking should be performed

after every 4-5 plies. Final build up of the lam-inate should be at least 0.013mm (0.375 in) orwhatever is recommended by the prepregmanufacturer. Final vacuum bagging is per-formed in the same manner as for debulkingwith a layer of peel ply, perforated Teflon,polyester breather and vacuum bag. Prepregmanufacturer will provide a detailed heat uprate and cure temperature for their system andthis should be carefully followed. Most sys-tems can be initially cured at up to 63C(145F) and 586-689 kPa (85-100 psi) of pres-sure for 14 h.

After the autoclave cycle, carefully removethe bag and films from the laminate to avoidlift up from the master surface. Attachment ofthe egg crate structure (support or back-upstructure, Fig. 25.4) to the laminate is veryimportant to minimize any potential residualstresses built into the laminate or stresses fromthe egg crate itself. Leave the laminate on themaster surface and attach board structure ofthe same material to the surface of the lami-nate. If the laminate is glass/epoxy, the eggcrate material can be made from glass/epoxyor aluminum honeycomb sandwichedbetween glass. The point is to avoid stressescaused by the difference in CTE between theegg crate and laminate by using similar mate-rials.


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570 Toolingfor composites

Fig. 25.4 Supportstructure details.The egg crate should be cut to the contour ofthe laminate with a standoff of 3.17mm(0.125 in). This prevents hot spots during pro-duction part curing and also mark off into thetool laminate. The egg crate should be con-structed so that it will lie flat on the surface.The intersections of the board stock are heldtogether with cloth and resin with at leastthree strips per junction. To ensure minimalstress to the laminate from the egg crate struc-ture, it is advisable to remove the structurefrom the laminate and cure the strips holdingthe structure together at 177C (350F).After curing, the structure can be placedback onto the laminate and 'tied' into the lam-inate with at least three strips of cloth and resinaround the periphery of the egg crate. Shimscan be used to provide for the standoff. If thetype of master used for the lay-up permits an

oven cure at 177C (350F) (Le. metal or mono-lithic graphite) the post cure can be performedwithout removal of the tool from the master.However, if the master material will not toler-ate this temperature, careful removal of thetool from the master must be done prior to thepost cure.

Separation of the tool from the mastershould be done carefully to avoid damage tothe master or the tool itself. Tooling pinsshould be removed prior to separation. Toolswith severe contours may require plasticwedges to be inserted around the tool periph-ery until it releases. Once the tool is separated,the surface should be inspected for pinholes orroughness. Pinholes can be filled with resinand the roughness can be smoothed out with afine grit sandpaper. Edges of the tool can besawed to even up the periphery and then


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Composite tools 571sanded lightly with 220 grit sandpaper toremove any loose fibers. Care must be takenthat no fibers are lifted by sanding along thelength of the fibers.

Once the tool has been cleaned up, therequired check for vacuum integrity is accom-plished by placing a layer of polyesterbreather cloth on the surface and a vacuumbag over it. The acceptance criterion is gener-ally that there be no loss greater than 500 Pa(2inHg) in 5 min at a minimum of 6.2 kPa(25inHg) at the start of the test. If possible,depending on the complexity of the tool, placethe tool back onto the master and check forany warp or out of contour problems.

25.4.3 WET LAY-UP METHODThe wet lay-up of composite tools can be forroom or elevated temperature use. The differ-ence is in the resin selection. Procedurally, theprocess is the same except for the cure cycles.The master or tooling aid should be cleaned ofall defects and debris such as scratches andloose fibers. Solvent clean the surface toremove any residual resin or oil. Check forvacuum integrity using a criteria of a maxi-mum loss of 500Pa (2 in Hg) at a minimum of6.2 kPa (25 in Hg) starting vacuum.

Place masking tape around the peripheryfor later use for sealant tape. Regardless of theprior history of the tool, it should be releasedwith a suitable hard wax or release agent.Tooling pins should be released and placedinto the holes provided on the tooling aid.For wet lay-ups, two resins are used, one forthe gel or face coat and one for laminating. Thegel coat is generally the same as the laminatingresin but with additives to thicken it to make itadhere to the contour of the master or toolingaid. Resin manufactures can supply both roomtemperature and high temperature systems.Apply the gel coat to the surface using ashort bristled brush or squeegee. Work thecoating as evenly as possible over the surfaceat a thickness of approximately 12 mm(0.030 in). Do not allow excessive build up to

occur in corners or the bottom of contours. Toomuch resin will result in cracking and crazinglater in the tool life cycle. Also, ensure that allair bubbles have been worked out by repeat-edly applying the brush back and forth acrossthe surface. To ensure that all air has come tothe surface while brushing, pause occasionallyand allow the air to rise to the surface where itcan be brushed out. Air that remainsentrapped either on the tool surface or withinthe layers of cloth could result in possible blis-ters and delamination later during tool usage.The resin supplier will provide mix ratios forresin and hardener as well as pot life and geltimes. Tooling cloth generally comes in rollsand is either a satin or plain weave with thewarp direction noted. Sufficient squares or rec-tangles can be precut from the roll prior tolaminating. Sections over 609 mm (24 in)become too cumbersome to work on the toolface, therefore smaller sections are advised.The first 2-3 plies should be from light weightcloth such as 7500 glass or 2534 carbon whichwill prevent mark through to the surface. Thegel coat should be advanced with time prior toapplication of the first ply. If enough tack isnot present, the ply will sink too deep into thegel coat and be visible on the tool surface. Onesimple test is to place a finger onto the surfaceand release. If the gel coat has not advancedadequately, the fingerprint will disappear. Ifthe fingerprint remains, the gel coat hasadvanced far enough to withstand therepeated pressing of subsequent layers ofcloth. Using the mix ratios provided by thesupplier,mix enough resin to cover the surfaceof the gel coat in the time allotted by the potlife or around 30min. Approximately 40g(0.088 lb) per 0.009 m2(1ft2)of tool surface foreach ply should be adequate. Application ofthe first several plies should be done carefullyto avoid pushing through the gel coat surface.

To ensure complete wetting of the ply, ashort bristled brush or squeegee can be used tocarefully work the ply into the laminating resin.All wrinkles and air entrapped areas should beworked out before another ply is added and if


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572 Tooling or compositesnecessary, use a sharp knife to slice through theply wrinkle in order to work it down. Bushingsshould be placed over the tooling pins at thispoint and worked into the resin to seat them. Aheavier cloth such as 7587 glass or 2548 carboncan be used for the remaining plies. Since thecloth was precut into squares or rectangles andthe warp direction was maintained, each plyshould be placed at 45" to the previous ply.Overlaps of 6.35 mm (0.250 in) between pliesshould be maintained but a seam should neverbe placed over a seam from a previous ply.After each ply, add additional resin to cover thesurface. Place the next ply and work the resinup through it by pressing the cloth with thebrush. If there is not sufficient resin to com-pletely wet out or saturate the ply, additionalresin should be used. Saturation of the clothwith resin on a table separate from the lay-upsurface and then transferring the saturated plyto the tool can cause air entrapment and bridg-ing of the ply. After the fourth ply, or prior tothe resin curing, apply a peel ply to the surfacefor a compaction cycle. The peel ply whenremoved before other operations will eliminatethe need for sanding the surface prior to bond-ing. This is followed by a Teflon filmwithperforations every 250-300 mm (10-12 in).Cover this with a heavy glass or bleeder cloth tobleed off excess resin during compaction.Finally, place a nylon vacuum bag over the sur-face using sealant tape to attach to the surfaceand apply a vacuum of at least 6.2 kPa(25inHg). Hold this vacuum for 10-12h orovernight or until the peel ply can be removedwithout disturbing the laminate layers.Following this cycle, the bag and peel ply canbe removed along with the tooling pins andlamination can commence as previouslydescribed. Debulking should be performedafter every 6 plies or before the resin begins tocure. Final laminate thickness should be 9.5 mm(0.375 in). It is probably a good idea to build upthe bushed hole area with additional plies toensure vacuum integrity.

After the final ply has been applied, thecompaction step is repeated with the peel ply,

FEP, bleeder and the vacuum bag. Dependingon the resin system and the tooling aid mater-ial used, a precure is recommended andshould be supplied by the manufacturer.Fabricate an egg crate structure using 9.25 mm(0.375 in) thick board stock of similar materialto the laminate to avoid stresses caused by thedifference in CTE between the egg crate andlaminate. If the laminate is glass/epoxy, theegg crate material can be made fromglass /epoxy or aluminum honeycomb sand-wiched between a glass laminate. The eggcrate should be cut to the contour of the lami-nate with a standoff of 3.17 mm (0.125 in). Thestandoff prevents heat differences or hot spotson the tool surface during production part cur-ing and also prevents mark off from the backup structure pressing upward into the toolsurface laminate. The egg crate should be con-structed so that it will lie flat on a surface. Theintersections of the board stock are heldtogether with cloth and resin with at leastthree strips per junction. To ensure minimalstress to the laminate from the egg crate struc-ture, it is advisable to remove the structurefrom the laminate and cure the strips holdingthe structure together at 177C (350F). Thiswill allow the tool surface to be tied into a sta-bilized support structure and minimizewarpage during subsequent cure cycles.

After the support structure is cured, thestructure can be placed back onto the laminateand attached or 'tied' to the tool laminate withat least three strips of cloth and resin aroundthe periphery of the egg crate. Shims can beused to provide for the standoff to preventwarpage. If the type of master used for the lay-up permits an oven cure at 177C (350F), thenthe post cure can be performed withoutremoval of the tool from the master. However,if the master material will not tolerate this tem-perature, careful removal of the tool from themaster must be made prior to the post cure.Allow the tool to stand at ambient temperaturefor a minimum of 24 h prior to post cure.

After the final 177C (350F) post cure,inspect the surface for pinholes and repair any


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Composite tools 573blemishes with gel coat resin. A final vacuumcheck at 635 mm Hg (25 in Hg) with a loss ofno more than 51 mm (2 in) is acceptable.

25.4.4 PLASTIC FACED PLASTERPlastic faced plasters (PFPs) are tooling aidsthat minimize the wear and tear on masters byduplicating the master surface with a suitableunit that can be used for a variety of purposes.PFPs allow for tooling to be directly fabricatedfrom the master surface without exposing themaster model to adverse environmental con-ditions, such as autoclave temperatures orpressures. If taken directly from a master sur-face, the PFP is the reverse of the mastercontour. An intermediate plaster splash isrequired to get back to the master contourwith a PFP. If the surface required is directlyfrom the master model, the PFP will be takendirectly from the master surface. If the surfaceis above or below the master surface, appro-priate steps must be taken by either takingadditional splashes with or without layers oftooling wax to achieve the appropriate dimen-sion.

Prepare the master surface by cleaning thor-oughly and removing blemishes, debris andpinholes by filling with a compatible fillercompound. Release the surface and any tool-ing pins with a hard wax or release agent. Ifthe master model is plaster, a hard wax can beused with a minimum of three applications,dried adequately and buffed between eachapplication per the manufacturers directions.

From the resin supplier, request a water-proof or hydrophobic resin system which willcure in the presence of water from the plaster.Apply the resin evenly 0.76 mm (0.030 in)thick to the master surface with a short bris-tled brush and work out air bubbles as theyappear. Ensure that no bristles are pulledfrom the brush to contaminate the resinAllow the resin to cure to a point that a fin-gerprint may be imprinted lightly into theresin and will remain for a period of timeafter touching. Then apply a second coat of

the same thickness and place any bushingsthrough the second layer and flush to the firstlayer. Place a ply of 7500 glass cloth onto thislayer and work in to impregnate the cloth.Allow to cure to the fingerprint test. Mix athird batch of the resin but add about 10-15%by weight of wet plaster to the mix and applyto a thickness of 2.5 mm (0.1 in). Do not waitfor curing but proceed with a layer of plasterapprox. 25.4 mm (1 n) thick. Allow this topartially dry and then finish the tool byadding plaster and hemp to the surface to athickness that will allow support for the sizeof the tool [50-76 mm (2-3 in) for a 914 mm x914 mm (4 ft x 4 ft) tool]. Support structurecan be built in for small tools using plasterand hemp to make strengthening ribs on theback surface. For large tools, steel pipe or tub-ing can be tied into the back structure withplaster and hemp ropes. Approximately 24 his required to dry and cure the system anddepends on the thickness and size. Drying inan oven up to 60C (140F) will provide a sta-ble system for use. PFPs can be used in anautoclave (with vacuum integrity) up toapproximately 105C (220"F), however a limitof one or two runs is all that can be expected.PFPs provide tooling aids for a variety ofother room temperature shop applications.

25.4.5 DRILL TEMPLATESDrill templates or fixtures are used primarilyto drill and locate precision holes in the pro-duction composite part. While their use islimited to hole location and drilling, theirfunction may also be combined with othersupport tooling, such as a trim/router fixtureto minimize tooling expenditures. Drill fix-tures are fabricated using a room temperaturecured fiberglass/epoxy laminating and facecoat system. Because the tool is used in theshop environment in ambient conditions, novacuum integrity or elevated temperaturerequirements are needed. Location of the holescan be obtained from the master model sur-face. In addition, to facilitate concurrent tool


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574 Tooling or compositesmanufacturing, a Mylar@ ilm sheet (0.010 in)or thicker may be used as a transfer mediumby relocating the position of the holes to thesurface of the Mylar and then using the Mylaras a temporary master surface while fabricat-ing the fixture. Prior to fabrication, it must bedetermined if the fixture is to mount on theoutside surface of the part (OML) or the inter-nal surface of the part (IML). Since mastermodels normally represent the OML surface,most support tooling such as drill and trim fix-tures may be fabricated directly off thissurface. Occasionally the surface required willbe a specific distance above or below the sur-face of the master model. If the surface isabove, tooling sheet wax at the specific dimen-sion required must be placed on the mastersurface prior to fabrication of the tooling aid.Sheet wax is commercially available in numer-ous thickness to accommodate mostrequirements. If the surface required liesinside the master model surface, a 'splash'consisting of plaster and hemp reinforcementmust first be pulled from the master surfaceand then the splash surface can be waxed tothe specific dimension inside the master sur-face. Tool pins are placed in the tooling aid.After the face coat and two layers of glass havebeen applied, the drill bushing is seated ontothe surface with additional lamination overthe bushing to provide an integral lock to thetool. Template thickness can vary dependingon use, but 9.5 mm (0.375 in) in thickness istypical.25.4.6 TRTM AND ROUTER TEMPLATESThese shop aids are used to trim and routcured composite parts to a specific dimen-sional tolerance. Accuracy is required for thesetools in order for the composite detail to fitprecisely with adjacent details. Trim androuter templates can be fabricated directlyfrom the master model, composite tool or atooling aid such as a PFP (plastic faced plas-ter). They are generally fabricated using roomtemperature cured epoxy glass systems. Since

trim and routing operations are always con-ducted at ambient temperatures, CTE is notconsidered in the design of this type of tooling.Procedures for laminating the room tempera-ture cured system are similar to those for drillfixtures. Periodic debulking is not requiredand the tool is not required to maintain anyvacuum requirements. Thickness can varydepending on final use but is usually about9.5mm (0.375 in). If the tool is to be used forrouting, a set back or offset will have to bedetermined as defined by the type of routingequipment used. This set back must be identi-fied on the surface of the tool to alertpersonnel to which equipment is acceptablefor use with the tool. Failure to use the correctset back will result in an under trimmed orover trimmed condition. A witness or verifica-tion line is usually scribed on trim fixtures as areference to which edges may be checked fordamage. This allows shop personnel toquickly verify the accuracy of the trim fixturewith minimal inspection equipment. With drillfixture tooling, a determination must be madeas to what surface the trim fixture is applied. Ifthe tool is to represent the same surface of themaster model, then the tool may be directlytaken from that surface. However, if the sur-face required is internal or external to themaster, appropriate steps including plastersplashes or waxing must be completed toobtain the correct surface.

25.4.7 PLY LOCATING TEMPLATESPly locating templates are used during the lay-up of the production part and designatelocations for the plies and indexing of detailparts. In addition, these templates may alsoshow individual ply orientation and designatespecial features of the part such as splice areasor hardware attachment points. Occasionally,honeycomb core placed within composite pro-duction details must be potted with a syntacticcore material to prevent core collapse whenhardware is attached. Reference locations ofall attachment hardware may be transferred


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Bibliography 575from the master model to produce a core pot-ting template. Core may then be accuratelypotted, laminated and then drilled after curingby referencing the location from the pottingtemplate. Similar templates may be producedto locate individual details or other assembliesthat have to be located during detail construc-tion. Slits or eyebrows are cut into the laminateto locate the edge of the production part plyand color coded and identified accordingly.All templates are fabricated from room tem-perature glass/epoxy cured systems and aredesigned to be light in weight with a thickness

Because some templates may be quite large,provisions must be made where possible toof 3.1-3.8 mm (0.125-0.150 in).

lighten the template to assist in handling.Lightening holes can be placed by removal ofsections of the template not serving a specificfunction. However, as material is removed toreduce weight, stiffeners must be added toprevent warpage that may affect dimensionalstability.

BIBLIOGRAPHYMallik, P.K., Fiber Reinforced Composites, New YorkFiberite Manufacturing Procedures, Toolrite ToolingUnited States Gypsum, Tooling Techniques.Morena, J.J., Advanced Com posite Mold Making, New

Marcel Dekker, 1968.Materials System.

York:Van Nostrand Reinhold.

25-tooling for composites

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