early history of the resin transfer moulding proces for aerospace applications

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The early history of the resin transfer moulding process for aerospaceapplications

K.D. Potter

Aerospace Engineering Department, University of Bristol, Queen’s Building, University W alk, Bristol BS8 1TR, UK

Received 3 July 1998; accepted 16 September 1998

Abstract

The resin transfer moulding (RTM) process has been the subject of a great deal of practical and theoretical development for aerospace

applications since the early 1980s. This article looks at the very early developments of RTM in an aerospace setting. This development took place over a few years at the start of the 1950s. By 1956 almost all the features of RTM for aerospace applications had been introduced in a

series of six patents. This achievement was made without any of the theoretical infrastructure now considered critical and was the work of a

small group within a single company. The developed technology dropped from view in the general aerospace composites community and had

to be redeveloped 25 years after the last patent was applied for. 1999 Elsevier Science Ltd. All rights reserved.

Keywords: Aerospace applications; E. Resin transfer moulding (RTM)

1. Introduction

It is generally assumed that Resin Transfer Moulding

(RTM) is a process that was developed for general aero-

space applications in the 1980s, following on from somelimited earlier applications such as aircraft radomes. The

development of the technology of RTM is understood to

have been underpinned and made possible by a substantial

research effort aimed at explicating fundamental aspects of

the process and turning these into effective process models.

The state of the art can be described as noted in the list

given below.

1. Proven and patented process.

2. Demonstrated for complex integrated components

including curved and stiffened panels.

3. Demonstrated for closely toleranced components.

4. Resin flow lengths up to 3.6 m achieved, although flowlengths up to 1.8 m preferred.

5. Metallic inserts may be used.

6. Combination processes using an element of prepreg

with additional resin introduced under pressure are

possible.

7. A variety of resin types may be used.

8. A variety of reinforcement types used including shape

woven preforms and other preform types.

9. Tools sealed and evacuated, tools made of metal or

composite, importance of adequate stiffness in tools

understood.

10. Multiple injection ports, sequential injection to control

the flow front path, flow monitored during injection.

11. Resin premixed or catalyst incorporated in preform

pack.

12. Resin can be mixed on line in RTM specific machinerywith solvent flushing.

13. Pre-drying of reinforcement by passing warm air

through the tool cavity.

14. Automatic shut-off valves at outlets when a predeter-

mined through flow has occurred.

15. Feed-back from outlet valves to shut off inlet valves.

16. Automation of the process based around a four-station

carousel for the various stages of the process.

The 16-point list shown above will be familiar to those

currently working with the RTM process. What may be less

familiar is that each and every one of the points actually

represents the state of the art in RTM prior to 1960.

2. Origins of the RTM process

The earliest approach to RTM appears to arise from a US

navy contract for the development of 28-ft long personnel

boats [1]. This was let in 1946 and specified the use of glass

fibre/polyester to be moulded using a vacuum injection

method. In this method the reinforcement was held between

two mould halves with the resin poured into a trough in

which the lower edge of the tool sat. The resin was drawn

Composites: Part A 30 (1999) 619–621

1359-835X/99/$ - see front matter 1999 Elsevier Science Ltd. All rights reserved.

PII: S1359-835X(98) 00179-1


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up by vacuum from the top of the tool. This process was also

later used in the manufacture of aircraft and missile nose-

cones, and versions were utilised in which pressure rather

than vacuum was used to drive the resin. The basic vacuum

injection process was very problematical and unsuitable for

complex geometries or the majority of potential applications

in aerospace.

What we would now recognise as RTM was described in

a series of patents with priority dates between 1952 and

1956 [2– 6], by Harold John Pollard and John Rees (Pollard

and co-workers for the first two patents) of Bristol Aircraft

Limited. They did this at a time when the only reinforce-

ments offering a better stiffness to weight ratio than steel

were plant fibres and asbestos, and the resin of first choicewas phenolic, although polyester and epoxy resins had been

commercially available for some years. One of the spurs to

the development was clearly the difficulties being encoun-

tered in moulding complex shapes with asbestos/phenolic

prepregs. Unless hydrostatic pressure could be guaranteed

the water evolved in the condensation reaction would

damage the laminate properties. One way to guarantee the

pressure is to inject resin at high pressure to flood the tool

cavity and act as a pressure medium. The patent to cover this

was applied for in November 1952 [2] and included a claim

relating to moulded-in metal inserts. In this case the injected

resin was intended to be filled with short asbestos fibres bothto provide a pressure medium and to build up stiffening ribs

on the moulding, which was intended as an integrally stif-

fened wing skin. In November 1953 [3] another patent was

applied for, this extended the idea to an integrally stiffened

airfoil to be made in one shot from reinforced phenolic or

(significantly) polyester resin. Fig. 1 shows just how ambi-

tious this proposal was. In this case the whole geometry is

intended to be filled with reinforcement plies, although

these are still said to be pre-impregnated. The baseline

process is still seen as the high pressure injection of filled

phenolic resin over felts of preimpregnated asbestos pheno-

lic, but the patent also discusses the use of polyester resin at

low injection pressure and the use of woven reinforcementsrather than felts.

The patent applied for in February 1955 [4] describes a

process that would be immediately recognisable as RTM.

The applications quoted for the invention are aircraft fuse-

lages and automobile bodies and the illustrative component

is an integrally stiffened shell with double curvature. There

is sufficient detail given in the patent to demonstrate that this

is no armchair patent and that real parts had been made and

problems in their manufacture had been overcome. For

example the maximum resin flow length is quoted as

12 ft, whilst 6 ft is said to be a preferable limit. The patent

also gives an adequate description of the manufacturing

route to a GRP mould and the importance of mould stiffness

in RTM is recognised by the addition of external stiffeners

in the tooling. The whole thrust of this patent is how the

limitations of the previously known vacuum infiltration

techniques, suitable for small shell parts, could be elimin-

ated to permit the manufacture of large and complex

structures.

The story does not quite end there as in October 1955 [5]

patents were applied for on an injector mechanism for

mixing two part resins in a controllable way prior to injec-

tion into a tool. The injector also incorporated a solvent

flush for the mixing cavity and injection lines. Another

refinement was an air drying mechanism so that reinforce-

ments could be dried with warmed air prior to injection of

resin (the lesson that reinforcements must be rigorously

dried prior to resin injection was re-learned the hard way

by the author, 30 years later). Lastly, the suggestion is made

that automation is possible by utilising timers to operate the

various valve sequences required.A year later another patent application [6] described

devices that could be attached to each outlet point of a

tool such that they automatically shut off the outlet valves

when a pre-determined amount of resin has passed through

them. When all the outlet points are shut the inlet valve can

be automatically shut. This patent also described the use of a

four station carousel for automating moulding, station 1 for

mould loading, station 2 for air drying, station 3 for resin

injection, station 4 for cure and demould.

The work to develop the RTM process seems not to have

reached the technical literature of the time; except for a

short mention of the patenting of the ‘resin injectionprocess’ in ‘Engineering Materials and Design’ of 1960,

which set the author off looking for the patents [7]. At the

time of the developments Bristol Aircraft Ltd was part of the

same group as the Bristol Car Company and the developers

of the process were clearly seeing little distinction between

the two potential markets.

In 1959 Fothergill and Harvey introduced shape-weaving

capability into UK [8]. Preform manufacture by lay-down of

chopped glass onto shaped formers was already known [7].

By 1960 all the major features of modern aerospace RTM

had been demonstrated; with the possible exception of

press-forming preforms from lightly bound, but dry,

woven cloth [9]. The word ‘possible’ is used here as,although the author is confident that this technology was

developed later, the little research he has carried out to

date in the early history of RTM leaves him uncertain in

making any hard claims. Even allowing for the fact that

some of the proposed solutions to RTM automation may

not have worked very well, and the usual tendency to try

to patent a little more than has actually been carried out the

development of this technology in the infancy of the compo-

sites industry was a remarkable achievement.

In 1960, RTM, including the variant ideas expressed in

the first two patents, was the most generally capable

K.D. Potter / Composites: Part A 30 (1999) 619–621620

Fig. 1. Cross-section through integrated single shot wing moulding

described in Ref. [3].


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composite moulding process available and would have been

expected to be developed into full use for aerospace parts.

Clearly this did not happen (although radomes, and later

propellers, continued to be made in this way [10,11]) and

RTM had to be rediscovered for the generality of aerospace

applications two decades later. Fortunately, back in the

1950s there was not such a gulf between aerospace and

industrial moulders as there is today and RTM continued

to be used in the industrial moulding sector until it was once

more identified as useful in an aerospace setting [12]. The

process that was generally used in industrial moulding was

considerably less sophisticated than that revealed in the

patents applied for in 1955– 1956, requiring much develop-

ment effort to re-establish the process for aerospace.

It is widely assumed that the development of RTM tech-

nology was made possible by the development of a theore-

tical basis for understanding how resin flows through a tool.

That this is not entirely true is demonstrated by the early

development of the RTM process. In the 1950s the only

parts of the theoretical basis for RTM that were published(at least as far as the author knows) were Darcy’s Law [13]

and the Kozeny–Carman equation [14]. There is no

evidence in the patent to determine whether or not these

equations were known to the inventors. Equally, the compu-

ters thought essential to modelling the process were unavail-

able at that time. The fact of the matter is that, as in so many

areas, it was and is not necessary to understand the science

to develop the technology. The author’s own developments

(or more properly re-developments) in RTM carried out

from 1981 onwards shared many of the characteristics of

the earlier work; and at no time in the next decade did the

ability to analytically predict flow front shapes catch up withthe ability to construct more and more complex mouldings.

There are very real problems with using RTM to produce

complex three-dimensional mouldings with the stiffening

ribs shown in the 1955 patent. Pollard and Rees quite prop-

erly identify the ability to monitor the flow front and then

control inlet and outlet positions to control the flow as one

effective route to the manufacture of high quality mould-

ings. Today an array of resin detector types could be utilised

connected to a PC that controlled the switching of valves to

control the flow. In the early 1950s a simpler solution was

utilised; at least one side of the mould was made of translu-

cent GRP and the flow could be directly visualised. That this

would not necessarily be an acceptable solution today does

not invalidate the approach taken in the 1950s. Pollard and

Rees identified technical problems and produced engineer-

ing solutions rather than science-based solutions. There is a

tendency among researchers to assume that technical

progress follows scientific understanding, as often as not

the reverse is true as is shown in the development and

re-development of RTM.

Acknowledgements

Occasionally one is lucky enough to be able to trace one’s

debts to previous workers in the field and so this article is

respectfully dedicated to Messrs Harold John Pollard and

John Rees.

References

[1] Spaulding K. Fibreglass boats in naval service. US Naval Engineers

Journal 1966;78:333–340.

[2] British Patent Specification, 778, 683.



[5] British Patent Specifications, 849, 351, 2.


[7] Pickthall D. New design possibilities with Glass Fibre Reinforced

Plastics. Engineering Materials and Design 1960;3(2):96–100.

[8] Product announcement. Engineering Materials and Design 2 (1959) 4.

[9] Potter K. Fabrication techniques for advanced composite compo-

nents. Proc Inst Mech Engrs 1989;203.[10] T. Cook, H. Bertram, P. Bini, Development of the MRCA radome. In:

Proceedings of the third international conference on electromagnetic

windows, Paris, 1975.

[11] R. McCarthy, G, Haines, R. Newley, Polymer composite applications

to aerospace equipment, Composites manufacturing 5 (2) 83–93.

[12] K. Potter, Advances in the resin injection process for the reliable

production of complex structural parts. In: Proceedings of the sixth

international conference, SAMPE European Chapter, 1985.

[13] H. Darcy, Les Fontaines publiques de la ville de Dijon, Delmont,

Paris, 1856.

[14] Carman P. Fluid flow through granular beds. Trans Int Chem Engng

1937;15:150–166.

K.D. Potter / Composites: Part A 30 (1999) 619–621 621

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