resin transfer moulding for aerospace structures || manufacturing and tooling cost factors

Manufacturing and tooling cost factors

Teresa Kruckenberg

11.1 INTRODUCTION

11

The need to reduce the cost of composite parts is critical in these economic times. The traditional manufacturing method of prepreg lay-up is labour intensive and requires autoclave curing. Reducing product cost while maintaining quality is imperative. Resin transfer moulding (RTM) is one method that can reduce the cost of composites.

Additionally, RTM designs can compete with metal designs when prepreg designs are not even considered for these applications. The type of structure, complexity, primary structural application and quality requirements of the RTM design allow it to compete with the metal design at a competitive cost.

Some of the benefits and disadvantages of RTM significantly impact the cost, and these factors will be discussed further in this chapter. The benefits of RTM include:

• low-cost materials; • non-autoclave cure; • reduced lay-up time; • improved reproducability; • excellent surface finish; • net or near-net shape manufacturing; • improved laminate quality; • improved tolerance control; • integration of parts.

Resin Transfer Moulding for Aerospace Structures. Edited by T. Kruckenberg and R. Paton. Published in 1998 by Chapman & Hall, London. ISBN 0412731509.

Recurring cost factors 389

The disadvantages include:

• high tooling costs; • difficulty of using certified prepreg resins; • reduced cost benefits for low-volume applications.

Product design teams must assess these cost factors when concurrently designing a product prior to manufacturing. A large percentage of the final cost of a part is determined in the early phases of the product lifecycle [1,2]. The cost factors for traditional autoclave-cured prep reg manufacturing have been well documented [1, 3]. Since RTM is new in the aerospace industry, the cost factors are not yet well established. This chapter will present the cost factors and attempt to quantify some of the costs involved with RTM.

11.2 RECURRING COST FACTORS

11.2.1 EFFECT OF MANUFACTURING QUANTITY

Manufacturing quantity has a major impact on whether RTM is a costcompetitive process. A new production programme of less than 100 parts may need to be analysed closely to determine if RTM is cost-effective, as usually initial tooling costs are high.

To determine the feasibility of RTM for a low production program (less than 100 parts) the following cost factors should be considered:

• mould costs; • facility costs; • qualification costs; • recurring savings.

When switching an existing autoclave-cured prep reg program to RTM, the pay-off quantity is usually calculated by dividing the non-recurring costs by the cost saving per part. A pay-off quantity of 100 units is normally the acceptable maximum for a 500 unit program.

The learning curve is generally less steep for RTM parts than for prepreg parts. A complex-shaped prepreg part can have a steep learning curve whereas the RTM learning curve is much lower. In the beginning, this is because hand lay-up of prepreg onto complex shapes requires more skill than most RTM preforming techniques. The learning curve beyond 15 to 20 parts has more to do with design changes or system efficiencies. There are usually mould adjustments required, but since RTM is a more automated process the need for system and design changes is usually less than that required for autoclave-cured prepreg parts. The more the RTM system is automated, with a smart injection system, automated preforming and advanced mould assembly and

390 Manufacturing and tooling cost factors

clamping techniques, the flatter the learning curve. Generally, the learning curve for prepreg parts is the steepest (c.75%) for the first 100 parts, less steep (c.80%) for the second 100 articles, and from the 200th to 250th article the curve is generally at about 95% because learning has been completed [2]. For RTM parts the learning curve could be expected to be the steepest (c.85%) for the first 100 parts, then reach 95% around 100 units, depending on the amount of automation used in the process. However, since RTM is relatively new to the aerospace industry, learning-curve profiles are not well established. Figure 11.1 shows a comparison of an expected learning curve for RTM and a typical learning curve for a prep reg part.

11.2.2 MATERIAL COST FACTORS

RTM has the potential for lower material costs than prepreg. If the fabric is supplied dry and the resin supplied in bulk then material cost is saved by eliminating the prepregging operation. This cost saving may not be realised with some materials because RTM is relatively new to the aerospace market and high volumes of resin and fabric are not being ordered compared with prep reg.

The material costs shown in Table 11.1 have been supplied by Hexcel, 3M, and other resin and fabric suppliers. All costs are in US dollars. The majority of high-temperature epoxy resins cost between $70 to $145 per kilogramme. Low-temperature resins are very inexpensive, at $6 to $40 per kilogramme; however, these are two-part systems which require

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,

1000 10000

Figure 11.1 Learning curves for resin transfer rnoulding (solid line) and prepreg (dashed line) in terrns of hours of labour per number of units produced.

Table 11.1 Raw material costs

Product

Epoxy Resins: one or two part, 177 °C cure two part, 120°C cure film, 600 gsm, 177°C cure

Bismaleimides: one part Carbon Fabrics:

carbon tackified carbons

Prepreg Fabric: epoxy, 177°C cure, carbon fabric


Quantity

500 kg

500 kg 500 m 500 kg

50000 m2

50000 m2

3000 m2

Cost (US dollars)

70-145 per kilogramme

6-40 per kilogram me 40-55 per square metre 145-165 per kilogramme 25-50 per square metre

26-58 per square metre 80-100 per square

metre

mixing before injection. Depending on the fibre type, fabric weight and weave style, most carbon fabrics cost in the range of $25-$50 per square metre for a quantity of 50 000 m2. Fabric with 12 k tows will fall in the low end of this range, and fabric with 3 k tows will fall in the high end. Application of tackifier to the fabric will increase the dry fabric cost by approximately 5% to 15%, depending if the tackifier is applied to one side or both sides of the fabric.

Fibre tow is the least expensive form of fibre material, and if used in robotic winding can significantly reduce the overall part cost. Most carbon fibre tow costs between $45-$90 per kilogramme. E-glass roving costs between $10-$20 per kilogramme.

Material cost reduction can be significant for RTM. For example, if prep reg for a 120°C cured part costs $50 per square metre, and the part volume is 600 cm3, then the material cost for this part is $75. This cost does not include material scrap. If this part can be robotically wound using $55 per kilogram me fibre and $25 per kilogramme resin then the material cost for this part is 47% less than the prepreg part. If $35 per square metre fabric is used instead, then the material cost for this part is 20% less than the prep reg part. See Table 11.2 for a comparison of the costs.

One of the benefits of RTM is that advanced textiles can be used. These textiles may have a high material cost, but the savings realised from reduced labour can be substantial. To determine the real cost savings from using advanced textiles it is desirable to perform a global cost study that includes materials, labour, trimming and inspection operations.

Expendable materials are a significant cost for the autoclave process. These include bagging film, release films, porous release film, bleeder cloth, breather cloth and sealant tape. RTM expendable materials include tubing ($1-$2 per metre), fittings (50 cents to $4), and containers (50 cents to $2) for mixing and/ or heating the resins. These materials can be reduced further by the following techniques:


Table 11.2 Example part material costs: a comparison of prep reg and resin transfer moulding (RTM)

Preprega RTM

Raw Material Cost $50 $75

$25 $55 Part Material Cost (US$)

a Raw material cost in US dollars per square metre. b Raw material cost in US dollars per kilogramme.

$40

RTM

Resinb Fabrica

$25 $35 $60

• modify the fittings to run tubing through the fittings or purchase bored-through fittings - this requires replacing only the olive for each moulding;

• use hose damps for low-temperature and low-pressure mouldings; • use polyvinykhloride (PVC) or nylon tubing for low-temperature

mouldings; • use annealed copper tubing for high-temperature mouldings; • reuse tubing from dispensing equipment to oven wall for some one

part resins (the applicability of this technique depends on the resin chemistry and line temperature);

• manufacture dispensing equipment suited for the application; • use a pail unloader when working with high-temperature one-part

resins already degassed.

11.2.3 MANUFACTURING COST FACTORS

The major manufacturing steps for the RTM process are shown in Figure 11.2.

Preforming

The most significant cost benefit of RTM is the reduction of lay-up time. Lay-up of dry fabric is usually faster than lay-up of prepregged materials

Prefonn

Trim/ Inspect

Load into Tool

Demould

Inject Resin

Cure

Figure 11.2 Major Manufacturing Steps Required in resin transfer mOUlding.


because of the hand-consolidation and wrinkle-smoothing time involved with prep reg lay-up. Automated preforming or use of advanced textiles can further reduce this draping time. Advanced textiles include multilayer non-crimp fabrics, three-dimensional (3D) braids, angle interlock weaves, 3D knits, and woven-to-shape preforms, which all reduce preforming time by reducing the number of layers and by fabricating net shape preforms. Automated techniques for making preforms are essential for reducing costs. Automated systems have the potential to increase overall part quality while decreasing human labour content and material scrap [1].

Preforming of dry fabrics and fibres can be automated by using similar techniques to those used for thermoplastic components such as filament (robotic) winding, press (stamp) forming, vacuum forming and diaphragm forming. The fabric or fibre tow is usually coated with 3%-5% epoxy binder or tackifier. Since the tackifier is normally epoxy that has been b-staged (Appendix A, Glossary) to a point where it is a solid at room temperature but flows again upon heating, the tackifier acts like a thermoplastic. However, there is a limit on the amount of heating the tackifier can be submitted to without curing. The forming temperature is substantially less than that required for high-performance thermoplastics such as polyetheretherketone (PEEK) or polyphenylene sulphide (PPS). Most preforms can be formed at SO°C-IOO°C for a couple of minutes, which makes the forming equipment substantially less expensive than that required for thermoplastics. The majority of preforming tools can be manufactured with wood or plastic. Inexpensive heat sources such as infra-red lamps, heat blankets, hot-air guns, heated dies and ovens can be used to melt the binder or tackifier for consolidation of the preform.

Some fabric suppliers provide fabric with the tackifier already applied. This normally costs an additional 5%-15%, depending whether the fabric is coated on one or both sides. Resin suppliers may provide RTM resin in a powdered form to be used as a tackifier. In this situation the fabric will require coating by manual application, machine powder coating, spraying of tackifier dissolved in solvent or some other automated method to apply the powder. The powder needs to be distributed and then heated for a short time to ensure that it adheres to the fabric. The cost of producing tackified fabric will need to be assessed in the cost study.

Preform complexity is a significant factor in recurring costs. Inserts, moulded holes, complex shapes and 3D fibre architecture significantly add to the preform complexity. However, the more automation in the preforming process, the less impact on costs. Inserts and moulded holes can be used to minimise subsequent bonding or drilling operations and to improve part quality.


Preform costs can be minimised by using a hybrid preform with high modulus fibres in the principle stress direction and lower cost materials in the secondary stress directions [4]. Stress optimisation programmes can be utilised to design the fibre architecture for placement of fibres where essential. Automated preforming methods such as 3D weaving, or fibre placement, can be used for selected placement of fibres.

Automated preforming or advanced textiles can be used to manufacture net or near-net shape preforms. These preforms are then loaded into a mould for manufacture of a net or near-net shape part. The cost of a trim fixture and the recurring cost of the trimming operation can be eliminated for net shape manufacturing and minimised for near-net shape manufacturing.

Gerber, die or ultrasonic knife cutting of individual plies or ply stacks is essential to an efficient production programme. The cutting process can be optimised to produce tackified fabric with a neat edge for net preforming. In some instances, depending on the shape, it is more cost effective to trim on the preforming tool. Depending on the part geometry and mould configuration, net shape moulding may not be appropriate. Plies that shear in opposite directions around complex contours may be difficult to preform net. For example a sine-wave rib with 00 /90 0 and ±4S0 plies would be difficult to preform net.

Resin Transfer Moulding with Prepreg Resins

Some companies are considering using prepreg resins for RTM to eliminate the cost of a materials allowables programme. However, this strategy introduces its own problems and costs as it can be difficult to use RTM with prepreg resins. Resins designed for use in prepregs generally have a much shorter pot life and a much higher viscosity than RTM resins. However, these difficulties can be overcome for some parts. The trade-off is that the cost of the prepreg resin may be higher than the cost of an RTM resin, so recurring costs may be increased even though the non-recurring allowables cost is eliminated. The out-time for a prepreg resin is short and it is likely to have more costly storage requirements. More material wastage and/ or requalification tests of expired shelf-life resin may be expected when one uses a prepreg resin. There is also a higher potential for an increased reject rate with prep reg resins because of the tighter processing window.

Quality

RTM has the benefit of improved reliability. When a matched-metal mould is used, and processing parameters are constant, the parts produced will be dimensionally almost identicaL However, adequate and consistent clamping forces must be used for closing the mould. If the


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2-... 1/1 0 ()

0 I I Steel Aluminium

Material

Figure 11.3 Mould Material Cost Comparison.

I I Cast Iron (includes pattern)

processing parameters are tightly controlled then the part porosity level will remain constant. This will reduce the scrap rate and rework time involved.

Figure 11.4 Resin transfer moulded F22 sine-wave spars and frames. Reproduced courtesy of DOW-UT, Wallingford, CT.


200,---------------------------------------------,

$166 U53

150

~

'" "0

" ill 100 ::l 0 .s ~

05 u 50

Advanced B-stage auto automated tow placement, tape lay-up autoclave autoclave cure cure

Fabrication process

• LABOUR

~ MATERIAL

Uniwoven dry fibre stitched preform-RIP

Warp-unit dry fibre stitched preform-RIP

Figure 11.5 Estimated cost summary for 2.4 m x 6.1 m blade-stiffened panels. Reproduced from [7) Copyright © 1990 by McDonnell Douglas Corporation. All rights reserved under the copyright law.

For thin RTM parts almost all significant defects appear to be detectable by visual inspection of the surface. If and when this can be verified it may be possible to reduce non-destructive examination (NDE) costs substantially.

The finish on the mould determines the finish that appears on the part. It is worth the cost to spend more time on mould polishing to eliminate the surface preparation necessary for some prepreg parts to obtain a good surface finish. The initial tooling may be more expensive, but significant recurring cost savings can be realised in this situation. Generally, a good tool produces a good part.

Part Complexity

With increasing part complexity tooling costs rise for all processes. A small cost study of various moulds with increasing complexity is given later in this chapter. However, with RTM, the fabrication cost rise is less steep, and so RTM becomes more cost effective with increasing part complexity. For prepreg parts, the recurring costs will rise more rapidly because of the increased lay-up time and added debulking cycles. The average total recurring part cost for RTM is considered to be 25% less than the total recurring costs for autoclave-cured prepreg for good RTM applications. These savings will rise substantially with increasing part complexity and preform automation. Chapter 1 gives good examples of RTM applications.

(a)

Four nine-ply elements


Stitch two 36-ply elements in web area of blade stiffener (72 plies total)

Fold blade web ends open

Fold open flanges

(b) '-____________ -----Y

(e)

Seal and eliminate sidewise resin flow

bag seal

Figure 11.6 (a) Stitching concept for stiffeners; (b) stitching T-flange to skin; (c) vacuum impregnation of stiffened panel. Reproduced from [7J Copyright © 1990 by McDonnell Douglas Corporation, All rights reserved under copyright law,


Cost Studies

There are several methods for modelling the cost of composites. Most companies have their own internal method for estimating costs, based on their experiences of previous programmes. In one method cost data is plotted on a log-log graph versus indicators such as performance, weight, manufacturing method and part complexity: a first-order equation can be fitted to the plot. Another method is to use detailed manufacturing time step estimates. One well known example of this is the Advanced Composites Cost Estimating Manual (ACCEM) developed by Northrop for the US Air Force [3]. The following is a list of detailed manufacturing steps that may be used for estimating RTM recurring labour:

• mould surfaces cleaning; • preform mandrel cleaning; • release agent application to surfaces; • fabric cutting; .. preform lay-up; • preform assembly onto preform mandrel; .. preform consolidation; .. preform assembly into mould; .. mould assembly and clamping; • tubing assembly; • injection equipment assembly; CD vacuum trap assembly; .. thermocouples installation; " leak check; .. mould preheating; .. resin preheat and degassing; CD resin injection; .. injection completion; .. injection equipment disassembly; .. part curing; .. tubing disassembly; ., mould disassembly; ., part trimming.

To make a global cost model comparing the RTM process with prepreg, the following factors need to be included [5]:

., capital equipment costs; " maintenance of equipment, tooling and facilities; <} equipment power consumption; ., quality assurance personnel; " machine cycle times; ., machine downtime;


(a)

Central ExH

~

Mould Top

Spacer Perimeter Injection

(b) Mould Base

Figure 11.7 Concept 1: (a) flat panel laminate; (b) flat panel mould.

• facilities expenses; • programming time; • supervisory labour; • equipment downtime; • human cycle times; • part demoulding and tool cleaning; • tooling costs; • process yields; • scrap; • rework; • set-up.

Usually the best approach to model fully the cost of the application is to use the basic manufacturing steps and some global factors. This should include the savings arising from part complexity (elimination of details, and inventory) and subsequent assembly as well as tooling costs of the details and the assembly operation and cost avoidance achieved by close tolerance control (reduced shimming during bonding, or elimination of


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....I

(a)

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2

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80

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0.2 0.4 0.6 0.8

Area (sqm)

~ ~ -

.~ ~

---0.2 0.4 0.6 0.8

Area (sqm)

1

1

Figure 11.8 Concept 1: (a) manufacturing costs for resin transfer moulding of flat panels - . - . - . - = 16-ply; - = 8-ply; - - - - = 4-ply); (b) manufacturing costs of flat panel mould.

laminate thickness matching regarding blind fastener grip length). The saving from good, repetitive quality is significant. The cost of rework, standard repairs and scrap, so significant in prepreg manufacturing, is virtually eliminated in RTM. The true potential of RTM is not shown until the cost avoidance and cost savings factors are incorporated into the model (Allen Samuel, DOW-UT, 1996, personal communication).

11.3 NON-RECURRING COST FACTORS

11.3.1 INJECTION EQUIPMENT

Injection equipment can cost from $200 to $100 000, depending on what type of system is required. A simple paint pressure-pot can be used for injection of low-temperature resins and will cost approximately $200 to $300. A heated pressure-pot will cost between $3000 and $7000. A heated pail unloader (typically used with one-part resin systems) will cost between $10000 and $20000. The more expensive piston injection systems

Non-recurring cost factors 401

(a)

Central Vent

Perimeter Injection

(b)

Figure 11.9 Concept 2: (a) curved panel; (b) curved flat panel mould.

include comprehensive data acquisition systems. These systems can record the following data:

• volume injected; • volume remaining; • flow rate; • pressure (in-line and on-tool); • flow front position; • degree of cure; • line temperature; • pot temperature; • tool temperatures; • tool deflection.

The average pneumatic-driven system which contains most of these capabilities will cost around $40 000 to $50 000. Electric-driven or hydraulic-driven systems will cost more than a pneumatic-driven system


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(a)

400

- 300 ... .c: -... 200 ~ 0 .c 100 cu -I

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Area (sqm)

1.5

2

2

Figure 11.10 Concept 2: manufacturing costs for resin transfer moulding of curved panels (- - - - = 16-ply; - = 8-ply; - - - - - = 4-ply); (b) manufacturing costs of curved panel mould.

but will allow the flow rate to be closely controlled. The volume of the injection cylinder will also affect the cost substantially.

Vacuum is used with RTM to increase the injection driving force and to remove air during the flow process. Shop vacuum may not be adequate for air removat so a vacuum pump may need to be purchased. A doublestage vacuum pump will cost between $1000 and $5000, depending on the capacity.

11.3.2 HEAT SOURCE

RTM parts can be cured in an integrally heated mould, in a press or be oven cured. Each of these methods requires less investment and less energy than the traditional autoclave cure approach. An autoclave can cost over 10 times the cost of an efficient oven for the same capacity. When designed properly, integrally heated moulds provide the most energy-efficient heat source. Integrally heated moulds may also reduce the recurring cost by eliminating mould transportation and minimising injection set-up.


(a) L,====:=!£::=~==,-,,=====:y Inlet Ports

I J \

;-----------, \

(b) /-, ~~=:§§~§§~~=~(f Locating Pin

Figure 11.11 Concept 3: (a) stiffened panel; (b) stiffened panel mould.

11.3.3 MOULD COSTS

The moulds for RTM are usually more expensive than tooling used for prepreg. This is because RTM requires a matched mould with a much increased mould stiffness compared with prep reg tooling. This disadvantage may prevent an RTM research and development project from transferring into production, even when the recurring cost saving is substantial. More effort is needed to reduce tooling costs. However, it is important not to sacrifice quality because a poor mould design results in increased recurring costs and decreased part quality.

Almost all RTM moulds are manufactured from metal. Composite tools can be used, but the life of the tool is significantly reduced and any wear on the surface shows on the part. Composite tools may be preferred in some cases because of the similar coefficient of thermal expansion to the part. Generally, well-designed matched-metal moulds should last well over 800 parts and will allow much tighter tolerances than composite moulds. Spheroidal graphite cast iron is a preferred material for a complex-shaped mould. This type of cast iron is durable,


No. Stiffeners

3 4 5 6 7 8 -.. 6 .c -.. 4 :::J

0 ,g 2 CIS

...I 0

0 0.25 0.5 0.75 1 1.25 1.5

(a) Area (sqm)

No. Stiffeners

300 3 5 6 7

- 250 .. .t:. 200 -.. 150 :::J

-I ---J..--

---~ 0 100 ..Q CIS 50 ...I

0 o 0.25 0.5 0.75 1 1.25 1.5

(b) Area (sqm)

Figure 11.12 Concept 3: manufacturing costs for resin transfer moulding of stiffened panels (- - - - = 16-ply; - = 8-ply; ------ = 4-ply); (b) manufacturing costs for stiffened panel mould.

and copper tubes may be embedded into the mould for integral heating. The cost of a cast iron mould is usually less than that for steel for a complex shape. A model is fabricated for casting to a rough shape, then less than 1.2 cm is machined away to give the final contour, minimising machining time. Aluminium is often used for inserts to take advantage of its larger thermal expansion. Aluminium is also suitable for prototype moulds because of its lighter weight, but is not usually used for production because it is not as durable. Figure 11.3 illustrates the comparative costs of tooling materials (costs provided by Marand Precision Pty. Ltd, Melbourne, Australia). Aluminium raw material costs are significantly more than those of steel or cast iron; however, final tool costs may be comparable because aluminium is easier to machine.


(a)

Inlet ports on leading edge

Vent ports on trailing edge

(b) o Inlet ports on leading edge

Figure 11.13 Concept 4: (a) flap; (b) flap mould.

11.3.4 CERTIFICATION COST FACTORS

Chapter 14, on certification, addresses some of the issues involved with certification of a new RTM part. In some cases it is more cost-effective to perform a materials allowable programme with limited part structural testing. In others, minimal materials testing and an extensive structural test programme may be a better option.

Materials Allowable Programme

Some companies have developed certified allowables with RTM resins. Typically, a new resin is used with an existing qualified fabric to make specimens for testing. Materials allowable test programmes usually require three batches of resin and three lots of fibre or fabric for testing. There may be over 20 tests to perform at different temperatures and moisture contents. This is an expensive process which can cost over a


million US dollars. However, the approach qualifies the resin for use with this fibre or fabric on other applications, perhaps justifying greater initial certification costs.

11.4 APPLICA nONS

The Airforce F22 fighter programme has over 300 RTM parts which directly replace metal designs. DOW-UT produces 44 detail sine-wave spars (Figure 11.4) for Boeing. The RTM process for making these carbon fibre epoxy sine-wave spars results in a $250 000 savings per aircraft. Additionally, the process results in greater quality, increased production rates and the capability of manufacturing highly complex parts [6].

McDonnell Douglas Aircraft Company has been evaluating resin infusion for large wing skins [7]. Stitching of dry fabric has been incorporated into the preform to improve damage tolerance. Figure 11.5 shows the cost study comparison of various processing methods for a 2.4 mx by 6.1 m blade-stiffened panel. These estimated cost numbers do not reflect the total panel cost - only materials and fabrication labour cost are included. The aluminium structure is estimated from large-scale production records. The composite cost estimates are conservative as shown by the use of automated equipment - 50% efficiency factors are added to the processing time. The efficiency of the automated methods would be expected to improve considerably during production, providing a substantial cost saving.

The stitched warp-knit fabric with the resin infusion process is cost competitive with the aluminium design. Nine-ply warp-knit fabric is the baseline material, with the skin consisting of six nine-ply elements stitched together; each stiffener angle consists of four nine-ply elements stitched together. Two of the 36-ply angle elements are stitched together in the web and the flanges are folded open [Figure 11.6(a)]. The stiffeners are then stitched to the skin as shown in Figure l1.6(b). The preform is positioned on top of the resin for infiltration by vacuum under a bag [Figure 11.6(c)]. Traditional RTM is not appropriate for this application as the size and weight of a matched mould would create handling difficulties and the cost would be prohibitive.

11.5 CASE STUDIES

The intent of these studies was to evaluate the effect of size and complexity on the mould and manufacturing cost. Mould cost estimates were provided by Marand Precision Engineering, Melbourne, Australia. The following four part concepts were selected for the study: flat panel, curved panel, stiffened panel and a flap. Mild steel was used for the study since it is preferred over aluminium for high-quantity parts.

Case studies 407

No. of Stiffeners

15 3 4 5 6 -... .c 10 -... :::J 0 5 .a

1-----.. ---- .. -- .. .. .. :-:-:- .. .. .. .. -..

co ..J

0 i

0.4 0.6 0.8 1

(a) Area (sqm)

No. Stiffeners

600 3 4 5 6

-... .c 500 -... :::J 0 400 .a CO

..J 300

0.4 0.6 0.8 1

(b) Area (sqm)

Figure 11.14 Concept 4: (a) manufacturing costs for resin transfer moulding of a flap (- - - - =16-ply; -- = 8-ply; ----- = 4-ply); (b) manufacturing costs of the flap mould.

Aluminium inserts were used where their higher coefficient of thermal expansion was of benefit. The recurring costs were estimated using detailed manufacturing time-step estimates excluding cure time. Material costs were not included.

11.5.1 CONCEPT 1: FLAT PANEL

Concept 1 is a flat panel laminate [Figure ll.7(a)]. Figure 11.8(a) shows the recurring cost estimates for the RTM of various thickness laminates. The flat panel mould design is a simple two-piece steel mould with a picture-frame spacer to obtain the desired laminate thickness [Figure 11.7(b)]. The clamping method, using a hollow beam back-up structure with bolts, is not shown. Figure 1l.8(b) shows the non-recurring cost of manufacturing the mould.


11.5.2 CONCEPT 2: CURVED PANEL

Concept 2 is for a curved panel and is shown in Figure 11.9(a). Figure 11.10(a) shows the recurring cost estimates for the RTM of various thickness curved panels. The mould design is a two-piece mild steel mould for a curved panel [Figure 11.9(b)]. A press is used to clamp the mould. Figure 11.10(b) shows the non-recurring cost of manufacturing the mould.

11.5.3 CONCEPT 3: STIFFENED PANEL

Concept 3 is for a stiffened panel and is shown in Figure 11.11 (a). Figure 11.12(a) shows the recurring cost for resin transfer moulding of various thickness stiffened panels. The stiffened-panel mould is a two-piece mould design with inserts for forming the T -sections [Figure 11.11 (b)]. There are three injection ports through the top of the stiffeners, with four vents in the corners. A press or integral bolts can be used for clamping the mould. Figure 11.12(b) shows the non-recurring cost of manufacturing the mould.

11.5.4 CONCEPT 4: FLAP

Concept 4 is for a flap and is shown in Figure l1.13(a). Figure 11.14(a) shows the recurring cost for the RTM of various thickness stiffened panels. The flap mould design is a two-piece steel mould with aluminium inserts used for the ribs [Figure 11.13(b)]. The injection ports are located on the leading edge with a small weir for flow down the length of the edge. The vent ports are located on the corners of the trailing edge. Integral bolts are used for clamping the mould. Figure 11.14(b) shows the non-recurring cost of manufacturing the mould.

11.5.5 SUMMARY

Figure 11.15(a) summarises the manufacturing costs for all concepts. As expected, the graph shows that increasing part complexity increases labour. Figure 11.l5(b) compares mould costs for all concepts. Perhaps surprisingly, the stiffened panel mould should be less expensive to manufacture than the curved panel mould.

Figure 11.16 compares the recurring costs for manufacturing an eightply flat panel laminate by RTM and by autoclave-cured prep reg. A 5%-11 % recurring cost advantage is indicated for RTM over the prepreg option. This slight advantage is primarily a result of the reduction in wrinkle-smoothing time.

Figure 11.17 compares RTM and secondarily bonded prepreg for an eight-ply flap. A recurring cost advantage between 30%-36% is indicated

12

10 -;:-

8 :S .. 6 ~ 0 .D 4 ~

2 0

0

(a)

600 'i:' 500 :5. 400 ~ 300 .8 200 ~ 100

o

Case studies 409

No. Stiffeners > ..... -.........

............. .".. .. - .- ----

_----:-.. r······ . .-. ..... ~: ..........

0.2 0.4 0.6 0.8 1

Area (sqm)

N Siff o. t eners '-....... /

.-' --- ---. -

: ~ . :.: - ---- --:..: .::- --

o 0.25 0.5 0.75 1 1.25 1.5

(b) Area (sqm)

Figure 11.15 Comparison of (a) manufacturing labour for all concepts; (b) mould costs for all concepts. - = concept 1; ---- = concept 2; - - - = concept 3; - - - -= concept 4.

5

'i:" 4 .c - 3 ... 5 2 .c ~ 1

o

-o

.. .. ~

~ .. .. .. I

0.5

Area (sqm)

1

Figure 11.16 Comparison of recurring costs for the flat panel for resin transfer moulding (--) and prepreg (- - - -).


20

- 15 ... .t:. -... 10 ~ 0 ,Q CIS 5 -I

0 0.4

No. of Stiffeners 345 6

. . - --.. .. -.. - - ~--' --' ------' ::--::..:----I

! I

0.6 0.8

Area (sqm)

1

Figure 11.17 Comparison of recurring costs for the flap for resin transfer moulding (-), co-cured prepreg (- - - -) and secondarily bonded prepreg (----).

for RTM. This is to be expected because RTM is more likely to be cost effective for complex shapes where significant lay-up time can be reduced and bond time eliminated. Figure 11.17 also compares RTM with a co-cured prepreg flap. The co-cured flap shows a cost advantage of 20% -26% when compared with the secondarily bonded prepreg flap. However, RTM still indicates a 10% cost advantage over the co-cured flap.

References

1. Foley, M. and Bernardon, E. (1990) Cost estimation techniques for the design of cost effective automated systems for manufacturing thermoplastic composite structures. 35th International SAMPE Symposium, Society for the Advancement of Material and Process Engineering, 1161 Parkview Drive, Covina, CA 91724-3748, April 2-5, pp. 1321-35.

2. Noton, B. (1987) Cost drivers in design and manufacture of composite structures, in ASM International Engineered Materials Handbook, Volume 1, ASM International, Metals Park, OH, pp. 419-24.

3. T. Gutowski, D. Hoult, G. Dillon et al. (1994) Development of a theoretical cost model for advanced composite fabrication. Composites Manufacturing, 5(4),231-9.

4. Curtis, P.T. and Browne, M. (1994) Cost-effective high performance composites. Composites, 25(4), 273-4.

5. Foley, M. (1990) Techno-economic automated composite manufacturing techniques. 22nd SAMPE International Technical Conference, Society for the Advancement of Material and Process Engineering, 1161 Parkview Drive, Covina, CA 91724-3748, November 6-8, pp. 764-5.

6. Material News (1996) RTM for aircraft. Sampe Journal, 32(4), 21. 7. Palmer, R. (1995) Techno-economic requirements for composite aircraft com

ponents. Presented at New Fabric Technologies, New Manufacturing Processes for Advanced Composite Structures Course, 29 March, Cooperative Research Centre for Advanced Composite Structures, Fishermens Bend, Vic., Australia.

resin transfer moulding for aerospace structures || manufacturing and tooling cost factors

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