00c Introduction RTM

8/8/2019 00c Introduction RTM

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RsinTanfer

Mudng

KevinPotter

Department of Aerospace Engineering

University of Bristol

UK

CHAPMAN & HALL

London • Weinheim • N e w York • Tokyo • Melbourne • Madras



Publ i shed b y Chapman & Hal l , 2-6 Boundary Row, London SE l 8 H N , U K

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First edit ion 1997

© 1997 Kevin P o t t e r

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Pr in t ed in Grea t Br i t a in by S t E d m u n d s b u r y P re ss , B u r y S t E d m u n d s , S u f f o l k

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Preface

The science and technology of composite materials has generated a large

number of processes by which components can be manufactured. These

range f r o m the contact moulding approach of rolling resin into the rein-

forcement on simple tools to the use of capital-intensive automatic tow

placement machinery. All of these processes have a single aim in common;

the cost-effective meeting of the design requirements for the part to be

made. To meet this aim the strengths and weaknesses of the manufac-

turing route must be reflected accurately in the design of the part.

There is a tendency to treat design and manufacture as two boxeswhich, while they overlap, can be handled separately. For processes that

have a large experience base that can be called upon there may be some

j u s t i f i c a t i o n for this belief, although maximum effectiveness w i l l always

arise f r o m a concerted design and manufacture approach. For emerging

processes there is a danger that carrying over design practices f r o m other

processes w i l l be, at best, non-optimal.

In order to utilize a concerted design and manufacture approach it is

essential that access is available to sources of information to guide bothdesign and manufacture. In the case of resin transfer moulding (RTM) a

great deal of information is available in the academic literature. Perhaps

the m a j o r i t y of this information is concerned with details of the s c i e n t i f i c

underpinnings of the process and less is available on the apparently more

mundane aspects. It is o f t e n these less immediately exciting issues that

have the greatest i n f l u e n c e on matters such as detailed design and, v i t a l l y ,

production costs.

This work aims to provide an adequate understanding of the basic sci-

ence of RTM and to provide much needed information on the technolog-

i c a l aspects of the process. The aim throughout is to equip both designers

o f components and those entrusted with their manufacture with the tools

to make the best of the opportunities that RTM presents to them.

Lastly, the word advanced w i l l be f o u n d throughout the text. This is a

d i f f i c u l t word as it seems to mean very d i f f e r e n t things to d i f f e r e n t people.



In this work it can be taken as a shorthand way of saying any or all of

the following: highly loaded; complex in geometry; intended for use in

s a f e t y critical applications; of high moulded quality and f r e e of defects;

optimally cost-effective and so on.



Introduction

It must be stressed at this early point that RTM is not a single manu-

f a c t u r i n g process that can be dealt with in a monolithic manner. RTM is

better thought of as a philosophy of manufacturing in which the resin

and fibres are held apart until the last possible moment. In this it can be

contrasted with those manufacturing methods where the resin and fibre

are combined prior to use.

In many ways the development of an aerospace or, more broadly,

advanced composites industry was permitted by the development of

preimpregnated reinforcements (prepreg). This development permitted

real structures and components to be designed and manufactured that

could r e f l e c t the properties of high-performance fibres. A variety of indi-

vidual processes, such as autoclave moulding, vacuum bag moulding,

compression moulding, expanding bladder moulding, and silicone rubber

expansion moulding were developed that utilized the new form of semi-

finished material known as prepreg. As people became comfortable and

experienced with the new material form, a design and manufacturing data-

base grew up such that the strengths and limitations of the materials andprocesses were reflected in design philosophies and detailed designs.

These design philosophies and detailed design features have become the

norm for advanced composite products. They largely r e f l e c t the capabil-

ities of the dominant aerospace manufacturing route, autoclave moulding.

Despite the similarities between the various processes it is unusual to

refer to prepreg moulding. Each process has its own literature and the

commonalities between them are sometimes lost. By contrast, RTM has

as many d i f f e r e n t processes under the RTM umbrella as there are prepreg

processes; but it is more or less commonplace to speak of RTM rather

than, for example, rigid tool RTM with semi-rigid preforms.

All the process variants have common features. Unresinated fibres are

held within a tool cavity and a d i f f e r e n t i a l pressure is applied to a supply

o f resin such that the resin flows into the reinforcement completely

wetting it out. The tool may be essentially rigid, semi-rigid, or contain



flexible elements. Any consolidation pressure required to give the

required reinforcement volume fraction ( V f % ) may be applied by

mechanical clamps, f r o m a tooling f r a m e or press, or by the use of an

internal vacuum or external applied pressure in non-rigid tooling.

Reinforcements may be of any fibre, and the use of all f o r m s has been

reported, f r o m unidirectional (UD) through woven or knitted cloths to

needled and random mats and f u l l y three-dimensional reinforcement

preforms. Volume fractions f r o m below 20% to above 60% have been

reported. The reinforcement may be laid onto the mould by hand, formed

to shape by the mould closure, assembled by a wide variety of preforming

techniques or may utilize specially woven or braided constructions. The

resin can be of a very wide range of chemistries and formulations, so long

as the basic process requirements are met. Cure times can be f r o m a few

minutes to many hours. Resin injection machines can be of a very widevariety of types and production line design can be just as varied.

The focus here will be on those RTM techniques that are intended to

produce components to high-quality standards for structural applications.

Thus material combinations such as random glass mats and polyester

resins will not be discussed in any depth. Concentration w i l l be on the

materials that can produce advanced structural components and the

processes for their conversion into such products.

The age of RTM as a manufacturing process is, despite its apparentlyrecent origins, much greater than that of any prepreg-based system. RTM

can be traced back to the Marco process of the 1930s,[l] and in the 1960s

work was done on the pressure injection of a high-performance matrix

into an organized fibrous preform. [2] The f a c t that the matrix was

aluminium does not detract f r o m the f a c t that the process was clearly a

variant of the RTM methodology.

The use of an RTM approach for the manufacture of advanced polymer

matrix composites is more recent. Even so RTM was used to manufac-

ture radomes in high- and low-temperature matrices as early as the mid

1970s.[3] Later in the 1970s RTM was used for other components such

as aeroengine compressor blades. [4] Most of these early applications were

driven by the need for high levels of geometrical accuracy and this is still

a major driving force behind many RTM component developments. By

1980 many groups were attempting to devise manufacturing methods that

could step beyond the cost and geometrical complexity limitations

imposed by the baseline aerospace manufacturing processes.

At that time RTM was f a i r l y well developed as a niche process in the

general engineering composites area, and some of the early advanced

RTM work was carried out at the top end of the general products area

rather than in aerospace. This sort of work is exemplified by the devel-

opment by British Petroleum of high-speed flywheel system components

b y RTM.[5, 6] The materials used were glass fibre cloths and polyester



resins, but the geometrical accuracy and mechanical reliability require-

ments were very high, while costs had to be constrained for the proposed

transport application. Many of the approaches to tooling and preforming

that are in use today can be traced back to such early work in the field,

and this work established the suite of advantages that RTM can bring to

the design and manufacturing processes. This period also saw the intro-

duction of a i r c r a f t propeller blades manufactured by RTM.[7]

From the mid 1980s interest in advanced RTM began to pick up. A

search of one of the major databases could only find two references to

Advanced RTM prior to 1986, with references increasing very rapidly

a f t e r 1987. In this period there was much more interest f r o m manufac-

turers of aerospace components, leading to programmes resulting in series

production of f l i g h t hardware outside the previous niche markets by the

end of the 1980s.[8]Interest in advanced RTM has been steadily building for more than a

decade. In this period reinforcements, matrices, preform techniques, injec-

tion equipment, process and flow models have all been subject to great

improvements. One of my first involvements with RTM was in 1981. A

high-temperature demonstrator component of complex geometry was

made, but the only bismaleimide (BMI) matrix available for RTM work

had all the toughness of shellac. Enormous e f f o r t s had to be put in to

the design of both component and reinforcements to overcome this f u n d a -mental weakness of the matrix. Today, while BMIs are still somewhat

lacking in toughness compared to epoxies for RTM, the same component

would be very much easier to deal with.

Currently many applications of the technology are in operation or

development and both resins and reinforcement forms s p e c i f i c a l l y tailored

f o r RTM are widely available. This upsurge in developments is directly

attributable to the advantages that RTM can bring and these are outlined

below.

ADVANTAGES OF RTM

1. For rigid tool RTM all dimensions including part thickness are

directly controlled by the tool cavity. Surface finish replicates that of

the tool, generally a smooth finish is chosen for advanced work, but

matt or decorative finishes could be utilized.

2. Net shape parts can be produced, eliminating some finishing oper-

ations.

3 . Many reinforcement types, such as thick or 3D wovens, stitched

assemblies and braids, are d i f f i c u l t to mould by conventional means.

All of these f o r m s can be handled via RTM and no problems have

been reported w i t h any s p e c i f i c fibre types.



4. A wide variety of resin systems can be utilized. Much epoxy resin

formulation for prepreg is related to flow control during consolida-

tion, this is not required for RTM resins although the need for a low

viscosity can be d i f f i c u l t to reconcile with toughness requirements.

Resins that cure by condensation reactions or contain volatiles are

not ideal. Even so, good results have been reported with acid cata-

lysed phenolics.[9]

5. As noted above, the prepreg process stages relating to flow control

and consolidation in autoclaves are not required in RTM. This can

lead to simplicity in cure scheduling, faster heat-up rates for tools

that are not injected at the cure temperature and generally leads to

shorter overall cure cycles.

6. Because prepreg is not used the s h e l f - l i f e and refrigerated storage

costs associated with the use of prepreg are avoided. The use of unim-pregnated reinforcements can also lead to cost savings as the cost of

the prepregging itself is avoided.

7. For fixed cavity tooling, fibre volume fractions can be very well

controlled, leading to very consistent mechanical properties.

8. The factors leading to porosity and voidage in RTM are somewhat

d i f f e r e n t to those in prepreg moulding. With correct mould design

and good process control very low or zero voidage levels are routinely

achieved.9. Experience with operating production lines has shown that defect

rates in RTM production of aerospace parts can be lower than those

experienced in autoclave moulding production lines. While the posi-

tioning of quality control inspection points may be d i f f e r e n t for RTM

and autoclave work, good control can be imposed on RTM-based

production lines. For additional security and quality control, samples

o f both laminate and neat resin can be obtained within an RTM

mould.

10. Very complex components can be produced via RTM. Many compo-

nents have shown high levels of parts integration, leading directly to

cost savings. Some of the usual geometrical limitations on autoclave

moulding, such as the use of bend radii several times the laminate

thickness, can be eliminated through the use of RTM. Bend radii

down to h a l f the laminate thickness have been reported without

evidence of interlaminar cracking.[10]

It is v e r y d i f f i c u l t to make an assessment of which of these factors is the

most important. They can all make a contribution to minimizing costs,

but the exact mix would depend on the s p e c i f i c component being con-

sidered.



REFERENCES

1 . M o u n t f i e l d , J. ( 1 9 6 9 ) F o r m i n g p r o c e s s e s for g l a s s fibre and r e s i n - o t h e rm e t h o d s , Composites 1 , 41-9.

2 . C o o p e r , G . ( 1 9 7 0 ) F o r m i n g p r o c e s s e s f o r m e t a l m a t r i x c o m p o s i t e s ,

Composites 1, 153-9.3 . C r a y , M . ( 1 9 8 0 ) D e v e l o p m e n t o f a p o l y i m i d e r e s i n i n j e c t i o n p r o c e s s f o r

a d v a n c e d c o m p o s it e s t r u c t u r e s , Proc. Symposium: Fabrication Techniques fo r

Advanced Reinforced Plastics, A p r il , S a l f o r d : I P C S c ie n c e a n d T e c h n o l o g y ,35-9.

4 . J o n e s , W . a n d J o h n s o n , J . (1980) A r e s i n i n j e c t i o n t e c h n i q u e f o r t h e f a b r i -

c a t i o n o f a e r o - e n g i n e c o m p o s i t e c o m p o n e n t s , Proc. Symposium: Fabrication

Techniques fo r Advanced Reinforced Plastics, A p r i l , S a l f o r d : IP C Sc ience a n d

T e c h n o l o g y , 4 0 - 7 .

5 . P o t t e r , K . D . ( 1 9 8 6 ) T h e d e v e l o p m e n t o f a G R P c a s i n g c o m p o n e n t a n d i t sm a n u f a c t u r e b y t h e r e s in i n je c t i o n m o u l d i n g p r o c e s s , Proc. 15th Reinforced

Plastics Congress Br i t i sh P l a s t i c s F e d e r a t i o n , 123-5.

6 . M e d l i c o t t , P . A . C . a n d P o t t e r , K . D . (1986) T h e d e v e l o p m e n t o f a c o m p o s i t eflywheel r o t o r for v e h i c l e a p p l i c a t i o n s - a s t u d y of the i n t e r a c t i o n s b e t w e e nd e s i g n , m a t e r ia l s a n d f a b r i c a t io n . I n K . B r u n s c h , H . D . G o l d e n a n d C . M .H e c k e r t ( e d s ) , High Tech - The Way into the Nineties, A m s t e r d a m : E l s e v i e rS c i e n c e P u b l i s h e r s , 29-42.

7 . M c C a r t h y , R ., H a i n e s , G . a n d N e w l e y , R . ( 19 9 4 ) P o l y m e r c o m p o s it e a p p l i-

c a t i o n s t o a e r o s p a c e e q u i p m e n t , Composites Manufacturing 5(2 ) , 83-93.8 . M o r g a n , D . (1 9 89 ) D e s ig n o f a n a e r o - e n g i n e t h r u s t r e v e r s e r b lo c k e r d o o r ,

Proc. 34th International SAMPE Symposium, 2358-64.

9 . F o r s d y k e , K . ( 1 9 8 4 ) P h e n o l i c r e s i n s f o r f i r e and h i g h t e m p e r a t u r e a p p l i c a -t i o n s , Proc. 2nd International Conference. Fibre Reinforced Composites 84,

Pl a s t i c s a n d R u b b e r I n s t i t u t e , P a p e r 1 .1 0 . P o t t e r , K . D . a n d R o b e r t s o n , F . C . (1987) B i s m a l e i m i d e f o r m u l a t i o n s f o r

r e s i n t r a n s f e r m o u l d i n g , 32nd International SAM PE Symposium, A p r i l .

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00c Introduction RTM