Sandwich structures on track for continued growth

Shut it! - moving into closed moulding Increasingly more stringent emissions legislation

may mean that the future of the

reinforced plastics industry lies with closed moulding wet lay-up techniques.

rather than traditional

spray or

George Marsh reviews the various options available and some

of their applications.

G

eneral

that

that

open moulders growing

may feel

pressure

to ‘shut

it or die’ is unjust considering their achievements have under-

tion

legislation (OSHA, MACT, local regulations etc) mean that the future may lie with closed moulding rather than traditional wet lay-up. spray or

transition. A good

polyesters

were subject

to at that

to in

The

next

level

of

sophistication,

notes Scott Bader’s Mark Cooper,

Encouragement encompasses educain print, audio-visual (such as the

company’s Processing’),

pinned the growth of the reinforced plastics industry, but it is no less real for that. Ever more stringent emissions

Suppliers are positioning themselves meet the growing needs of moulders

that time.)

CD on ‘Closed and word of mouth

Mould during

seminars, workshops, site visits and other customer contact. All the methods championed involve transfer of liquid resin from outside the mould to inside, under varying degrees of control. For open moulders

seeking the most

accessible option Scott Bader recommends vacuum injection, an extension of wet lay-up in which cure takes

duces

the

use

of

matched

Glass reinforced polyester adequate for the Vat-Flo

introtooling.

moulds are process so

new capital investment is still modest. For the lower tool a normal hand layup mould can be used, though with a widened and stiffened flange. The light matching GRP upper tool is reusable. equipped

The rigid tool set is normally with a double concentric seal

enabling a clamping vacuum of one bar to be pulled between the two seals,

Bader, a company which has built a leading position at the low-technology end of the market by supplying poly-

place under a plastic film (vacuum bagging). Since flexible film can be applied to existing minimally modified open

and an infusion vacuum of some 0.6 bar to be drawn inside the mould cavity. This results in faster spread of the resin front, speeding up production. After the

ester resin to moulders of boats and other products for half a century. This organization, loath to see the fortunes of its traditional customers decline, is

moulds, allowing a vacuum of about 0.60.8 bar to be drawn by evacuating the mould cavity, previous capital invest-

resin has permeated the dry reinforcement fully, vacuum can be relaxed and held at about 0.3 bar while cure takes

ment does not have to be written off. Pre-catalysed liquid resin injected at a central point or via a peripheral channel

place. Cosmetic quality of the finished part benefits from the fact that both surfaces are moulded, while surface veils can be

example

is the

UK’s Scott

actively encouraging them to adopt closed-mould solutions and has assembled a logical series of these along with the necessary conversion expertise. Techniques supported range from basic ‘close it with a flexible film’ to the hightech matched-mould resin transfer moulding (RTM) system. Naturally, Scott Bader also supplies a range of its Crystic resins optimized for these solutions. (A point of interest is that the company was one of the world’s first to develop a viable closed mould process. Its Marco system, which appeared in 1946, was necessary until the company had solved the air inhibition problem 14

REINFDRCEDpfasfics

Buyers’

Guide

permeates dry reinforcement previously arranged in the mould. Despite the time and care needed to ensure a vacuum-tight set up, some loss of laminate thickness that can occur under atmospheric compression and the need to dispose of film, tape and other materials after use, vacuum injection does offer a relatively painless transition. In application the process is almost as flexible as open moulding since it can produce components of up to very large sizes and can accommodate design changes easily.

2002/2003

applied to prevent glass print-through. The fact that gel-coat can be applied to both faces is proving a key advantage for the many moulders which are adopting the system. Fabrics used may be woven, stitched or combination types. Core material can also be included in the dry lay-up, though it is crucial to cut and place the pieces carefully to allow the necessary edge clearances for resin flow. Despite the fact that Vat-flo requires accurately matching moulds, the 0034-36 17/02J$ - see front matter 0 2002 Elsevier Science Ltd. All rights reserved.

Shut it! - moving into closed moulding

company

can still justly

reasonably

cost-effective

present

it as a

entry to closed

r

1 To vacuumpump

moulding.

T-pvacuumgawgs

At the next step, pressure is brought into the SeRkInt

equation.

At the

next

tape

step,

into the equation. ing

pressure

is brought

In vacuum mould-

(VM), sometimes

referred

to

as

‘RTM Light’, impregnation

is acceler-

ated by the use of pressure

as well as

Vacuum impregnation/bagging in which the mould’s open side is closed off by a flexible membrane, is highly accessible to open moulders in transition, being virtually an extension of the wet lay-up process. @iograms c0urtesy of Sf Systems.)

vacuum. A machine connected to the inlet injects resin in shots to a maximum being

pressure of 1 bar. Resin, already drawn into the reinforcement by Resin drawnacross andthrow reinforcement8 by vacuum

vacuum, is also pushed round a peripheral channel and into the fibres towards the centre. Reduced cycle times make the method suitable for series pro-

Peel ply end/orresin distributiotlfabtic

duction and better control of resin flow can be achieved. However, because pressures used are low compared with those of composite

classic RTM, the necessary matched tooling tends still

to be affordable. All the closed-mould

technologies

mentioned so far operate at ambient temperature. As a result, the speeds of resin permeation and cure are limited and the level of control that can be exercised over the cure cycle by manipulating the thermal conditions is lacking. Improving cycle times to the levels required for competitive series production requires that resin be transferred rapidly into the mould and through the reinforcement, then cured fast. This demands the application of significant heat and pressure, under close control to maintain safety margins. Aerospace manufacturers achieve this with autoclaves, matched metal tooling and preimpregnated materials, but this is an expensive high-technology route only appropriate where there is a premium on laminate quality and performance. An alternative finding favour with many moulders is resin transfer moulding.

Processes such as RET. VARTM, SCRIMP (Ire variations on the resin/vacuum

infusion theme.

I

I

I

PressorcJanqtohoM hMofWtqether

RTM with its precision closed matched moulds can deliver high production rates and products of a

quality second only to those achievable using prepreg and autoclave technology.

REINFORCEDplastics

Buyers’

Guide

200212003

15

Shut it! - moving into closed moulding

In

RTM,

matched

heat

via

the

- which

are

is applied

moulding

tools

flat surfaces should be checked to ensure that

they have not gone concave,

substantial

and rigid so that some pres-

shrinkage

sure

also

out surface corrections

can

deforming metal,

be

applied

them. Tools, though

can

be composite,

latter are often stiffened port frames. Precision tightly

without

to 40°C (typical

polyester). soon

as resin

at 2-4 bar

mal

be used

starts to appear

around

the mould,

closed

off.

machines

and

for

number at vents injection and

flushed through with solvent ready for the next cycle. The high fibre volume fractions that can be achieved with RTM, along with two finished sides and Class A surfaces, can result in a quality of product that is acceptable in automotive and consumer markets. The rapid inject, permeate and cure potential is a boon to manufacturing departments, which can turn out in minutes parts that would have taken hours or days by traditional open moulding methods. The RTM process

and cure process, and these requirements make detailed preparatory work necessary. This, along with generation of waste consumables, means that RTM can require considerable initial investment. The process best suits experienced moulders contemplating medium-to-high volume composite applications. A notable pioneer of RTM and enthusiast for the process, Alan Harper of Cornwall, UK-based Plastech T”I stresses the importance of achieving a high surface finish quality on the master pattern (plug) for the eventual female mould. “Any imperfections are bound to be replicated in the finished part,” he says. “Surface finish should be to at least 1000 grit smoothness, the ‘flatness’ of

REiNFORCEDpiastics

Buyers’

Guide

chemical

longer.

highest

resistance

for the mould perform

epoxy Mould

tooling

better

by laminating

cloth

into

ding

pipes

should faces,

that vinyl than

systems

heaters

rated

ther-

working

who has found

ester resins

tradi-

and

last

can be incorpoelectrical

the structure

heating

or by embed-

for hot liquids.

(The latter

has the advantage that coolant can subsequently be circulated for the cooling cycle). Vinyl ester based tooling can, according to Plastech, last for over 18 000 parts but lives of low-cost tools can be prolonged by periodically replacing the tool faces or skins. Plastech’s application of Multiple Insert Tooling (MIT) has the further advantage of allowing some of the time consuming fibre lay-up, to take place

‘off-line’, fully loaded and prepared skins being fed to the mould as often as necessary. RTM injection has been extensively automated and certain companies (Plastech and Aplicator System AB of Sweden to name two) market injection machines which pump, mix and accurately dispense resin at a controlled with minimum waste and pressure, high consistency. Current-generation machines have the in-built intelligence to inject the appropriate quantity of resin at optimum pressure whilst progressively adjusting the catalyst ratio. Automatic resin injection points, or sprue valves, which can close off after injection and flush themselves ready for the next cycle, are critical to a successful interface between machine and mould. One issue for moulders has been the expense of these injection machines, so Plastech has just developed a new model utilising innovative technology

200212003

75%.

previous The

machine, to

price

Sprinter

6

by over

being

injection introduced

can pump at 150 g/min

kg/min

adjustable

levels RTM

currently

into the market,

rates the

processes (gel-coating, release agent application)

can be automated to run with minimal intervention and high repeatability. But design of robust, well sealed tooling is critical, as is control of the injection

16

and

tional

can be

can then be disconnected

it is more

and may go too far and

having

says Harper,

and as

the mould

Pipework

Materials

values

After the calculated

on the tool face

damage the mould.”

are closed

of resin shots has been injected

and

for. Carrying

itself is a bad idea because labour intensive

the

with steel sup-

moulds

while resin is injected

and heated

often of

though

must be allowed

to slash

with

catalyst

ratios

from 0.5 to 3.0%. It incorpo-

a

two-stream

solvent

flush,

instrumentation to protect

the

mixing

emergency

head,

stop,

and

a pressure

mould

from

full guard

excessive

pressure. High

specification

aerospace, as an claving,

use higher

peratures metal

which

affordable

sectors

such

have developed alternative pressures

and so may specify

dies. All the standard

as

RTM

to autoand temmatched resin

sys-

tems - polyester, vinyl ester, epoxy, phenolic, bismaleimide and polyetherimide - may be used, typically at 350-450 cp viscosity, while all the normal reinforcements - glass, aramid, carbon etc - are utilised in various mat, woven, stitched or knitted forms or as dry bonded preforms. Gel-coats may be applied to one or both mould halves before FRP layers and core (if used) are laid up. Process simplification is a constant quest and the introduction of single-component epoxy resins that require no mixing and merely need temperature to activate them (eg from Cytec and 3M) are a step in this direction.

Variations These

fundamental

closed-mould

sys-

tems have many variations, some of which (it has to be admitted) are more Ire-badged’ existing processes than substantially new ones. Vacuum-assisted resin injection (VARI), otherwise known as vacuum-assisted resin transfer moulding (VARTM), is a form of RTM in which vacuum is applied to help draw resin into the fabric. In reaction injection moulding (RIM), a rapid-cure resin and a catalyst are delivered to the mould at high pressure in two streams so that mixing and the resultant chemical reaction occur in the mould rather than in the dispensing head as with RTM. Resin infusion under flexible tooling (RUT), originally

Shut it! - moving into closed moulding

conceived

in the 195Os, can be viewed as

a combination

of RTM and vacuum

bag-

On the downside, (though

ging in which one mould face is replaced

bagging

by a flexible polymeric

core

thin

bag or a relatively

GRP ‘splash’ tool.

Resin is drawn

not

skills are needed.

materials

60-100°C such

The fundamental systems

volume

serving

process referred injection, from

automotive enclosed

vac-

pultrusion

closed-mould

extrusion

methods

being

with

forced

used

thermoplastics

Application

such process, known as SCRIMP’” (Seeman Composites Resin Infusion Manufacturing Process) is patented in the USA. A promising variation which similarly achieves through-thickness resin infusion, is most highly developed as SP Resin Infusion Technology SPRINT’” from SP Systems and the equivalent HexFIT’” system from Hexcel. In these materials dry fabrics are laid up interleaved with layers of semisolid resin film supplied on a release paper. The lay-up is vacuum bagged to remove air through the dry fabrics, and then heated to allow resin to first melt, and then flow into the air-free fabrics and then, after a certain time, to cure. This system has proved able to infuse very thick laminates fast and, because it can yield high fibre volumes with low void content, delivers high mechanical properties. Cure at elevated temperature further enhances these. High surface quality can be achieved on one side and complex components can be moulded by using preforms. Core and laminate can be infused in one hit.

Guide

(the

long

because it incorporates flow channels, is quickly permeated by the resin and then serves as a layer source from which resin can infuse across the dry fibre stack from one face to the other. One

Buyers’

moulders, or

latter

and now

glass fibre-reinwith

which

incompatible),

other systems generally scope of this article.

outside

according

layers

(such

as a copper

ous refractory ing to

be

required, modest

halved

lay-up.

The

the

explaining

why

conversion. UK shipbuilder Vosper Thornycroft, initially a licensee for SCRIMP but now referring to its process simply as vacuum infusion, has enlisted this technique in producing minehunters, for which using GRP as the main structural material secures low magnetic signature. As well as containing all the emitted styrene, the process has proved able to infuse up to 30 plies together and can deal with the heaviest plies, such as a three-kilo shipbuilding fabric. Complex dry structures complete with cores, inserts, stiffeners and fasteners, can be infused in one shot. Another

larger

tool-

costs

labour

travel being

are

is said

with

hand

become

more

the

moulding,

SCRIMP is used

The SPRINT infusion is already

per canoe at this company’s production volumes was no more than before

and

where in producing railcars, ers and other large items. direct

incor-

matched

economics

attractive

resin,

time and waste were both reduced by a third. Despite extra production costs, including bagging for instance, cost

plane or vari-

set-up

compared

and

canoes, 30% less resin was used than with the previous hand lay-up method, and labour was halved. Cycle

ground

screening,

Since precision

is not

is

materials

fabrics) are readily

and

variation of vacuum infusion. Existing open moulds were used, closed with a reusable bag membrane. In moulding

mesh

for electromagnetic

short

the

to the company,

of non-polymer

the

The following examples illustrate some of the applications and benefits of closed mould options. A Sydney company eliminated emissions by adopting a styrene

2002/2003

that

relatively

for runs of over about 10 000 parts; con-

process was hitherto

REiNFORCEDplastics

of

such as com-

techniques

uum infusion, is a system that has several established offspring. In a number of these infusion is assisted by a layer of non-structural fabric which,

18

the

have also adopted

semi-enclosed

tinous

distinguishable

and

to with-

pression moulding, which can pay off

have many variations.

scarcely

Tooling

be able

advantage,

porated.

high

as those

industry,

to by Scott Bader as vacuum

vacuum

typically.

Specialist

The simple vacuum-bag

must

is required

and

stand resin film process temperatures

into the mould using only vacuum.

closed-mould

an oven

autoclave),

else-

dock fend-

system, with its route

for the

used in several

‘super’ cars as well as in marine,

aero-

space, energy and other sectors (see Reinforced Plastics, February 2002, page 44). Automotive business manager at SP, James Austin, says that SPRINT offers manufacturers an entry path into closed moulding which is less demanding than that for other closed mould techniques or prepregs. SPRINT and HexFIT are attractive for large composite mouldings, requiring less time and skill to laminate to high quality standards. Hexcel product manager Claude Despierres sees potential for HexFIT as a direct low-emissions replacement for open moulding in the marine industry, and for producing land transport items like truck fairings and rail cars. NEG Micon Rotors believes it has achieved the ‘largest single-shot infusion process to date’ after adopting a proprietary form of resin infusion for its latest glass/wood hybrid wind turbine blades. Drawing epoxy into the mould by vacuum avoids exposing surrounding areas to epoxy, says the company, and enables resin to be distributed accurately, so saving weight. The Advanced Composite Materials Centre at Plymouth University in the UK helped develop the process. NO1 Scotland is also developing onehit infusion for very large moulded blades, using a ‘SCRIMP-like process’. Designers of land and air vehicles are attracted to RTM by its potential to

Shut it! - moving into closed moulding

maintain

tight

stability

tolerances,

and Class A surfaces on all sides, over metal. weight

while

saving

According

to a spokesman

cialist Intellitec, new

combat

Lockheed-Martin aircraft:

many of

parts used in F-22 Raptor

“Our RTM equipment

very simple. The complexity design

for RTM spe-

which produces

the 350 plus RTM-formed the

Helicopter

dimensional

of the mould

is

comes in the

and proper

place-

turned

manufacturer

to RTM when it wanted

to cut the

costs of hand lay-up production

of much

of the 50% composite military

helicopter.

helicopter’s assemblies

more

better beams.

quality

than

ing on robotic

enables

flexible

us to give our customers

good turn-around Reinforced

and both

times and low cost.” plastic

faces for Raytheon’s

wing

control

extensively

sur-

compos-

in yielding Multiple Boeing RTM

wings

durable

and

of

cross-

is currently

work-

of the pre-

it considers

acceptable

important

economics. tooling of

by

economic

centre

fuselage

ite Premier 1 business jet are made using RTM and internal pressure moulding (PM), which together yield near-net parts that significantly reduce finishing

frames Apache

and assembly time. The parts are produced fast and consistently. Anodised aluminium tooling is used for its ability

facturing cost and conventional tooling results in prohibitive cycle times. By enabling a range of different aluminium inserts to be used with common mould

to withstand temperatures

repeated cycling and pressures.

to high

and bulkheads for its AH-64 helicopter. According to Dan

Davenport of sub-contractor Intellitec, tooling represents some 20% of manu-

bodies, the modular

tooling

the cost of conventional

tooling

saves 70% of

for each

component.

Tooling represents some 20% of manufacturing cost.

Polymer Engineering RTM to produce

Ltd in UK has used

nearly

all the exterior

panels for the aluminium

is seen

as key to achieving production

using

that the new

production

insert

the

even prepreg

The company

forms, a capability

and

stub

reported

were

of its Tiger

After fabricating

robust

ment of ports, vents, gates and channels. is lean

content

RTM, Eurocopter

The

process

Eurocopter

body shells of

Strasbourg’s 27 new Eurotrams ced materials. plete

with

in advan-

Parts were moulded shaped

polyurethane

comcore

inserts, integral steel reinforcements and threaded inserts for panel attachments. Examples like these show that rewards over and above compliance with emissions legislation await moulders who take the plunge and transition closed mould processes. This feature was first published issue of Reinforced Plastics.

RElNFOR~Dplastics

Buyers’

Guide

to H

in the May 2002

200212003

19