Int. J. Mach. Tools Manufact. Vol. 32, No. 1/2, pp. 51-56 , 1992. 0890-6955/9255.00 + .00 Printed in Great Britain Pergamon Press plc

ASSgSSING THE SURFACE FINISH OF POLYMER COMPOSITE COMPONENTS

G R DICKSON and R McILHAGGER

DEPARTMENT OF MECHANICAL AND INDUSTRIAL ENGINEERING

UNIVERSITY OF ULSTER AT JORDANSTOWN, NEWTOWNABBEY, CO ANTRIM, BT37 OQB

SUmmARY

T h i s p a p e r e x a m i n e s t h e s u r f a c e c h a r a c t e r i s t i c s o f some c a r b o n f i b r e r e i n f o r c e d p l a s t i c c o m p o n e n t s and i n v e s t i g a t e s t h e i n t e r a c t i o n s b e t w e e n c u r i n g c o n d i t i o n s f o r t h e c o m p o s i t e c o m p o n e n t , t h e t o o l and t h e r e l a t e d c o m p o n e n t s u r f a c e c h a r a c t e r i s t i c s . The r e s u l t s show t h a t t h e p r o c e s s i n g c o n d i t i o n s de h a v e an i m p o r t a n t b e a r i n g on t h e s u r f a c e c h a r a c t e r o f t h e c o m p o n e n t a n d t h a t t h e n a t u r e o f t h e t o o l i n g m a t e r i a l h a s a l s o a s i g n i f i c a n t i n f l u e n c e . CFRP t o o l i n g i s shown t o be an e f f e c t i v e and e c o n o m i c a l t e r n a t i v e t o m e t a l t o o l i n g t h o u g h t h e r e s u l t i n g c o m p o n e n t s u r f a c e f i n i s h i s s l i g h t l y p o o r e r t h a n f o r c o r r e s p o n d i n g m e t a l t o o l s .

INTRODUCTION

The e v e r i n c r e a s i n g u s e o f c o m p o s i t e m a t e r i a l s i n e n g i n e e r i n g p r o d u c t s , p a r t i c u l a r l y i n h i g h - s t r e n g t h a p p l i c a t i o n s h a s l e d t o a c o n s i d e r a b l e i n t e r e s t i n a l l a s p e c t s o f t h e p r o p e r t i e s o f c o m p o n e n t s p r o d u c e d i n t h i s w a y . New a n d s t r o n g e r m a t e r i a l s a n d m a t e r i a l c o m b i n a t i o n s , e x e m p l i f i e d b y c a r b o n f i b r e r e i n f o r c e d p l a s t i c s (CFRP) h a v e s h o w n d r a m a t i c a l l y i m p r o v e d s t i f f n e s s a n d s t r e n g t h - t o - w e i g h t r a t i o s . S o p h i s t i c a t e d t e s t i n g m e t h o d s s u c h a s C - S c a n h a v e b e e n e v o l v e d t o p e r m i t n o n - d e s t r u c t i v e e v a l u a t i o n o f c o m p o n e n t q u a l i t y i n t e r m s o f h o m o g e n e i t y a n d a b s e n c e o f v o i d s , e t c . , t h o u g h l i t t l e h a s b e e n d o n e t o a s s e s s t h e p a r a m e t e r s c o n t r o l l i n g t h e s u r f a c e f i n i s h o f CFRP c o m p o n e n t s .

C o m p o s i t e m a t e r i a l s e n c o m p a s s a w i d e r a n g e o f c o m b i n a t i o n s o f r e i n f o r c e m e n t s a n d r e s i n s y s t e m s . E s s e n t i a l l y t h e s e c a n b e i n t h e f o r m o f a f i b r o u s m a t , w o v e n o r n o n - w o v e n w h i c h i s i m p r e g n a t e d w i t h a v i s c o u s r e s i n i n a n u n c u r e d s t a t e . D u r i n g i m p r e g n a t i o n , r e s i n p e n e t r a t i o n i s o f c r u c i a l i m p o r t a n c e i n e s t a b l i s h i n g t h e q u a l i t y o f t h e r e s u l t i n g c o m p o n e n t . M o s t r e s i n s y s t e m s a r e t h e r m o s e t t i n g , t h o u g h i n t e r e s t i s c u r r e n t l y b e i n g s h o w n i n t h e n e w t h e r m o p l a s t i c systems.

To produce a component patterns of these

'prepreg' material.~ are cut out and assembled in

a tool to form the component. The fabric layers are orientated in different directions to (i):

(i) resist the p:~incipal stresses carried by

the componen% in service.

(ii) minimise spring-back during manufacture.

In the latter case a balanced lay-up is essential

to reduce spring-back which occurs after the thermal treatment of the materials during the curing cycle (2).

Where components of complex 3-dimensional shape

are required difficulties often arise as a result of laying them up using 2-dimensional

p r e - p r e g m a t e r i a l s . R e s e a r c h w o r k a t t h e U n i v e r s i t y o f U l s t e r i n t o t h e p r o d u c t i o n a n d u s e o f 3 - d i m e n s i o n a l w o v e n ' p r e f o r m s ' f o r s u c h a p p l i c a t i o n s i s c u r r e n t l y i n p r o g r e s s ( 3 ) .

A f t e r l a y - u p i s c o m p l e t e d a v a c u u m b a g g i n g p r o c e d u r e i s u s e d , f o l l o w e d b y a n a u t o c l a v e c u r e w h e r e h i g h - p e r f o r m a n c e c o m p o n e n t s a r e r e q u i r e d . S u c h p r o c e d u r e s r e s u l t i n g o o d c o m p a c t i o n a n d h e n c e c o n s o l i d a t i o n w i t h l o w v o i d c o n t e n t ( 4 ) . T h e e p o x y r e s i n s y s t e m s c u r r e n t l y i n u s e i n a e r o s p a c e a p p l i c a t i o n s r e q u i r e a c u r e t e m p e r a t u r e o f a r o u n d 1 7 9 o c a n d p r e s s u r e s up t o 7 b a r ( 7 0 0 k N / m 2 ) . A t y p i c a l a u t o c l a v e c y c l e ( F i g . 1) c o m m e n c e s w i t h a s l o w r a t e o f h e a t i n g . D u r i n g t h i s p e r i o d r e s i n f l o w i s e s s e n t i a l t o e n s u r e c o n s o l i d a t i o n t h r o u g h o u t t h e c o m p o n e n t . O n c e c r o s s l i n k i n g , a n e x o t h e r m i c r e a c t i o n , h a s c o m m e n c e d t h e v i s c o s i t y o f t h e r e s i n i n c r e a s e s , f l o w d e c r e a s e s o r c e a s e s a l t o g e t h e r a n d t h e r e s i n m a t r i x s e t s , c o n f o r m i n g t o t h e s h a p e o f t h e t o o l a n d t o t a l l y e n c a p s u l a t i n g t h e r e i n f o r c i n g m a t e r i a l . A f t e r c r o s s l i n k i n g o f t h e r e s i n h a s b e e n c o m p l e t e d t h r o u g h o u t t h e t h i c k - n e s s o f t h e c o m p o n e n t , c o n t r o l l e d c o o l i n g i s c a r r i e d o u t s o t h a t t o o l a n d c o m p o n e n t c o o l a t t h e s a m e r a t e i n o r d e r t o r e d u c e i n t e r n a l stresses.

The use of composites in marine, automotive and

150

50

0 0 60 90 120 150 '~80

time O~in)

F i g . 1 . T y p i c a l A u t o c l a v e D u t y C y c l e

51

52 G . R . DICKSON and R. MC|LHAGGER

p a r t i c u l a r l y a e r o s p a c e a p p l i c a t i o n s h a s m e a n t t h a t s u r f a c e f i n i s h , a s w e l l a s s t r e n g t h a n d stiffness has assumed a new importance. In

aerospace and marine applications a smooth

surface finish on external surfaces is important

to reduce skin friction and hence drag and fuel

consumption. In the automotive industry a Class

A surface finish is a cosmetic necessity. In

order to produce Class A surface finishes in

hand laid-up components 'gel' coats are often

used. This further step in the production

process introduces additional costs, both direct

and indirect, which are not insignificant and

also technical difficulties of ensuring uniform

application and thickness. It is suspected that

there may be an 'optimum' surface roughness and

character for surfaces of CFRP sub-assemblies

which are to be bonded together into a

finished assembly.

This project has extended the work at the

University of Ulster in this field (5),(6) and

examines further the relationships between the

surface character of composite materials and the

tools and processing conditions used.

MEASUREMENT AND INTERPRETATION OF SURFACE FINISH

component geometry could be eliminated. Tools

used in the work described here were produced

from mild steel, CFRP and aluminium. The surface

characteristics of each of the plate tools

(Master Plates) were measured and assessed.

Sample Preparation

Samples of carbon fibre prepreg fabric (plain

weave and S-harness satin weave) were taken from

the refrigerated store and glowed to reach room

temperature. Square sections 150 x 150 nun were

cut out, care being taken to ensure that no

contamination of the material occurred. These

layers were cut from the rolls of prepreg in

such a way that the layers had the warp threads

aligned in different directions to the

longitudinal axis of the tool (0 ° orientation).

The orientation of the lay-up of these samples

was 0/90/45/-45 degrees to the tool axis, each

layer being placed carefully in the correct order

in the tool, which had previously been

thoroughly cleaned and coated with a uniform

layer of release agent to assist removal of the

plaque after curing.

The complete assembly was covered with release

film, a bleeder cloth and then a bagging film

A l l s u r f a c e s e x h i b i t s o m e d e g r e e o f r o u g h n e s s a n d , w a s s e a l e d t o t h e t o o l s u r f a c e u s i n g a n a d h e s i v e

in general, most measurements made are concerned

with the primary texture of surfaces. Addition-

ally, surfaces usually exhibit directionality,

i.e. differing surface characteristics in two

orthogonal axes, with one direction being

'rougher' than the other. This difference is

due to the manufacturing methods employed in

producing the surface. Fully isotropic surface

characteristics are not very common being

produced by only a few production processes such

as EDM and shot blasting.

The surface analysis equipment employed in this work consisted of a stylus type instrument, a

Rank Taylor Hobson Talysurf 4 head and gearbox

unit connected to a PC-based 2-dimensional

surface analysis system developed by Whitestone

Business Communications for research purposes.

This system permits a very wide range of data

manipulation methods and statistical parameters

to be used in assessing the surface in question.

The parameters used in assessing surface

characteristics in this work were the arithmetic

parameters Ra, Rt and Rv for roughness measure-

ment and spatial parameters of Skewness (Rsk) and

Kurtosis (Rku) for surface profile character.

From previous work it was considered that spatial as well as arithmetic parameters should be

employed as composite component surfaces are

changed from those of the tool since:

(i) the surface of the component is an

approximate 'mirror image' of that of the t o o l .

( i i ) t h e r e i s u s u a l l y i n c o m p l e t e p e n e t r a t i o n b y t h e r e s i n o f t h e t o o l s u r f a c e i r r e g u l a r i t i e s a s a c o n s e q u e n c e o f v i s c o - e l a s t i c e f f e c t s .

SAMPLE PREPARATION AND TEST METHODS

Tool

As t h e r e l a t i o n s h i p s b e t w e e n s u r f a c e c h a r a c t e r a n d p r o d u c t i o n m e t h o d s a r e c o m p l e x , s i m p l e f l a t - plate tools were used to produce flat 2-

dimensional plaques so that variables due to

tape. A debulking vacuum was applied for I0

minutes to this complete assembly to ensure

removal of all air and consolidation of the

layers of prepreg material.

The tool and encapsulated layers of carbon fibre

material were cured in a 300 mm diameter x 600 mm

long Reaves Industrial Furnaces Ltd laboratory

autoclave under the cycle shown in Fig.l. The

controlled heat-up rate (3°C/min) was used to

ensure uniform temperature throughout the thick-

ness of the plaque, the cure being completed at

a temperature of 179°C. Controlled cool-down

(3°C/min) was also required to ensure stress-

free plaques.

Including heat-up and cool-down phases, the

complete cycle took from 2.5 to 5 hours. The

controlled heat-up and cool-down rates were

essential to ensure that the plaques remained as

flat and uniform as possible, i.e. minimum

differential expansion between tool and plaque,

uniform cure throughout the sample thickness and

stress-free components.

Variable conditions used in the tests were:

(a) Autoclave pressure

(b) Autoclave hold time

(c) Carbon fibre fabric and lay-up (d) Tool material

The c o m p l e t e d s a m p l e p l a q u e s w e r e r e m o v e d f r o m t h e t o o l a n d s u r f a c e c h a r a c t e r m e a s u r e m e n t s m a d e .

S u r f a c e A n a l y s i s M e t h o d s

R e a d i n g s on e a c h t o o l a n d p l a q u e w e r e t a k e n a t p o i n t s on a 10 mm s p a c e d g r i d , w i t h a t l e a s t 25 r e a d i n g s b e i n g t a k e n i n e a c h c a s e . The r e s u l t i n g d a t a w a s t r a n s f e r r e d t o a s p r e a d s h e e t f o r f u r t h e r a n a l y s i s . V a r i a b i l i t y o f s u r f a c e f i n i s h a c r o s s t h e p l a q u e s w a s e x a m i n e d a s i t w a s f e l t f r o m e a r l i e r w o r k t h a t q u i t e l a r g e r a n g e s m i g h t b e e x p e c t e d i n a n y o n e s a m p l e .

DISCUSSION

T h e b a s i s r e s u l t s o f t h e c u r r e n t s e r i e s o f t e s t s

Polymer Composi te Surfaces 53

REMARKS ON TESTS:

All measurements made in the cross-lay direction. Date: 29/03/91

TEST VARIABLES PLAQUE MEAN VALUES

IDENT Ra Rt Rv Rsk Rku Ra Range

PRESSURE (psi)

60.0 PLII 0.394 3.114 1.904 0.003 3.992 2.170 85.0 PLI2 0.376 2.446 1.176 -0.274 4.256 0.739

HOLD TIME (hrs)

0.5 PL8 0.219 1.323 0.595 0.245 3.150 0.750 2.0 PLI2 0.376 2.446 1.176 -0.274 4.256 0.739 3.0 PL5 0.263 1.691 0.813 0.094 3.055 0.584

PLAQUE MATERIAL Plain - 4 layers PL2 8-Harness Satin - 4 layers PLI2 8-Harness Satin - 8 layers PL3

TOOL MATERIAL CFRP Mild Steel Aluminium

MASTER PLATES Mild Steel CRFP Mild Steel Aluminlum

0.269 1.764 0.913 -0.089 3.131 0.360 0.376 2.446 1.176 -0.274 4.256 0.739 0.231 1.562 0.826 -0.097 3.317 0.303

PL5i 0.553 3.274 1.508 0.065 2.775 0.725 PL52 0.530 3.103 1.493 0.066 2.859 0.419 PL53 0.271 1,405 0.683 0.058 2.667 0.565

M1 0.270 1.733 0.880 0.051 2.787 0.i17 M51 0.407 2.311 1.385 -0.142 2.745 0.296 M52 0.526 3.042 1.598 -0.019 2.552 0.275 M53 0.078 0.720 0.258 0.840 5.733 0.064

Table. 1, Summary of Sur face Finish Data for P laques

and Master Plates

] ROUGHNESS OF MASTER PLATES (Ra, Fit, Rv) /

/ (Tools M1 M51 M52 M53)

s.5

3

2.5

! ' >

i 1.5

o.s

0

[ Master Plates: l f M i l d 5~eel #1 (M1); 2=CRFP (M51);

/ 3 = M i l d Sleet #2; (M52): 4 = A l u m i r t l u m [M55 J

0 E7

Ig

o o

• n

_ _ s i _ _ _ _ 2 . . . . 1 2 3 4 5

Master Rates

~----~-._~-~] [ i l e MA.% I [R 1

o

F~v D

Fig.2. Surface Roughness of Different Master Plates

a r e s u m m a r t s e d i n T a b l e 1 , w h i c h s h o w s Ra, R t , Rv, Rsk and Rku f o r e a c h o f t h e t e s t v a r i a b l e s . F i g . 2 s h o w s t h e r o u g h n e s s o f t h e m a s t e r p l a t e s .

MTM 32.1/2--E

A u t o c l a v e P r e s s u r e

Lower a u t o c l a v e p r e s s u r e s r e s u l t i n l a r g e r s c a t t e r s o f d a t a and a g e n e r a l l y l o w e r q u a l i t y o f p r o d u c t . T h i s was n o t e d f o r Ra, Rt and Rv. I n c r e a s i n g p r e s s u r e d l e a d s t o i m p r o v i n g s u r f a c e finish and also less sharp peaks and flatter valleys as noted from Skewness and Kurtosis measurements. The effect of release agents is not currently well understood and it is postulated that at higher pressures the release agent is partially squeezed out allowing direct tool/plaque contact in places which may cause removal of peaks during release of the component from the tool.

Autoclave Hold Time (Fig. 3)

Increasing hold time first produces a worsening of surface finish and then an improvement as hold time increases further. Skewness and Kurtosis measurements indicate that between 0.5 and 2 hours hold time there is a change in the character of the surface, i.e. the surface becomes less peaky and more negatively skewed. Above this time the reverse is true. These effects are believed to be caused by incomplete cross-linking at shorter hold times with some d i f f e r e n t i a l c o n t r a c t i o n s b e t w e e n t o o l and p l a q u e o c c u r r i n g d u r i n g c o o l - d o w n . At l o n g e r h o l d t i m e s t h e s u r f a c e c h a r a c t e r may be b e g i n n - i n g t o be a f f e c t e d by t h e r m a l d e g r a d a t i o n o f t h e r e s i n . O b v i o u s l y t h i s s h o u l d be a v o i d e d b o t h f o r r e a s o n s o f p r o d u c t q u a l i t y and a l s o because of the economic implications of over- long autoclave cycles.

Fabric and Lay-up Sequence

An i n d e f i n i t e p i c t u r e h a s e m e r g e d w i t h no c l e a r

54 G . R . DICKSON and R. MCILHAGGER

VARJAI:ION OF ROdQH~SS WiTH AU~:0~^V~ I HOLD T,ME I L (CRFP Plaques - Mild Stool Tool M1) J

3 . . . .

~ 2 . 5

2

i

't: ~ o 5

0 0

0

Q

[]

o.s i I s 2 2.s 3 a.5

Autoclave Hold Time (hours)

F')t

f,

Rv LJ

Fig.3. Surface Roughness of Plaques v Autoclave

Hold Time

I MASTER PLATE MATERIALS I (CRFP Plaques) L _ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . I

3.5

3

2.5

| 2 >

~: 1.5

1:

0.5

o o

Tool Motel iols: l I=CRFP (M51); 2=Mi)d Steel (M52); 5=Alumin iu m (1455)

O U O

o

i ~--%___ I 1 3

Tool Materials

Ro

R t o

Rv B

Fig.4. Surface RoughnessofPlaques v Master

Plate Materlal

o r l o g i c a l c o n c l u s i o n s . I n v i e w o f t h e f a b r i c g e o m e t r y i t is s u r p r i s i n g t h a t t h e 8 - h a r n e s s s a t i n i s n o t b e t t e r t h a n t h e p l a l n w e a v e . A d d i t l o n a l l y , t h e r e i s n o r e a s o n w h i c h c a n b e p u t f o r w a r d a t t h e moment f o r t h e d i f f e r e n c e s

in surface finish between plaques produced using

4 and 8 layers of harness satin. Clearly more

work is required to establish if this finding

is normal or anomalous.

Tool Material

The r o u g h n e s s o f t h e p l a q u e s f o l l o w e d t h e g e n e r a l t r e n d o f r o u g h n e s s o f t o o l s u r f a c e s b u t t h e c o m p o s i t e t o o l p r o d u c e d a b i g g e r v a r i a t i o n o f r o u g h n e s s o v e r t h e p l a q u e t h a n d i d t h e m i l d s t e e l t o o l . A g e n e r a l l y w i d e r s p r e a d o f r o u g h n e s s i s f o u n d on p l a q u e s t h a n on t h e m e t a l t o o l s w h i c h p r o d u c e d t h e m .

T h e r e f o r e a CFRP t o o l w i l l n o r m a l l y h a v e a l a r g e r s p r e a d o f r o u g h n e s s a n d w i l l i m p a r t t h i s c h a r a c t e r t o p l a q u e s m o u l d e d f r o m i t . The w o r s t p e r f o r m a n c e o f a l l i n t e r m s o f r o u g h n e s s and r a n g e o f r o u g h n e s s was f o u n d f o r t h e s m o o t h a l u m i n i u m t o o l . I t i s b e l i e v e d t h a t t h i s may be due t o s u r f a c e d a m a g e c a u s e d t o t h e a l u m i n i u m

a s p l a q u e s a r e r e m o v e d f r o m i t . F u r t h e r t e s t s a r e , h o w e v e r , n e c e s s a r y t o c o n f i r m t h i s c o n c l u s - i o n a n d , i f v a l i d a t e d , m i g h t l i m i t t h e u s e o f s o f t a l u m i n i u m t o o l s i n p r o d u c t i o n .

TOOL ECONOMICS

T y p i c a l l y t h e c o s t o f o p e r a t i n g an a u t o c l a v e i s i n t h e r e g i o n o f £ 1 2 0 / h o u r a n d w i t h a t h r e e t o f i v e h o u r c y c l e t h i s c a n p r o v e t o b e e x t r e m e l y e x p e n s i v e . I n many i n s t a n c e s , i t i s t h e s l o w h e a t - u p a n d c o o l - d o w n r a t e s w h i c h f o r m t h e m a j o r p o r t i o n s o f t h e c y c l e a n d t h e s e s l o w r a t e s a r e u s e d t o e n s u r e u n i f o r m h e a t - u p a n d c o o l - d o w n o f b o t h t h e t o o l a n d c o m p o n e n t b e i n g m a n u f a c t u r e d . D i f f e r e n t i a l t h e r m a l e x p a n s i o n b e t w e e n t o o l a n d c o m p o n e n t c a n b e a f u r t h e r s o u r c e o f i n t e r n a l s t r e s s e s w i t h i n t h e c o m p o n e n t .

F o r p r o d u c t i o n o f a s p e c i f i c c o m p o n e n t , t h e t e m p e r a t u r e r i s e w i l l be f i x e d and h e n c e t h e h e a t c a p a c i t y o f t h e t o o l w i l l be d i r e c t l y p r o p o r t i o n a l t o t h e p r o d u c t o f t h e mass and t h e s p e c i f i c h e a t . The m a s s o f t h e t o o l w i l l be controlled by its stiffness which must be

constant regardless of the tooling material used.

Using elementary bending theory in analysing the

production of a 'flat' sample, it can be shown

that the thickness of the tool is inversely

proportional to the cube root of the flexural

modulus and in turn the mass of the tool is

directly proportional to density and inversely

proportional to the cube root of the flexural

modulus, since the surface area of the tool

remains independent of material type.

Hence the heat capacity of the tool can be shown

to be proportional to the product of the density and specific heat divided by the cube root of the flexural modulus.

This heat is supplied through the tooling

material by conduction. Using uni-directional

heat flow theory, it can be further shown that the heat flow is proportional to the ratio of

the material thermal conductivity and its

thickness. But since the thickness has already

been shown to be inversely proportional to the

cube root of the flexural modulus it follows that the heat flow through the tool is

proportional to the product of the thermal

conductivity and the cube root of the flexural modulus.

Polymer Composi te Surfaces 55

Material Relative Time Based on Cost of Time Factor Current Practice* Operation +

(arbitrary units) (minutes) (£)

Mild Steel 0.00068 i00 200.00 Aluminium 0.00030 44.3 88.60 Carbon Fibre 0.00035 51.5 103.00

whL~re relative time factor = ( s p e c i f i c g r a v i t y x s p e c i f i c heat)

( thermal conduc t iv i t y x [ f l e x u r a l modulus] 2/3)

and * typically for a steel tool the heat-up and cool-down would be I00 minutes

+ based on £120/bour for operation of a commercial autoclave

Table.2. Relative Heat-Up Times and Costs for Different Tooling Materials

S i n c e t h e t i m e f o r t h e t o o l t o h e a t up ( o r c o o l down) c a n be d e t e r m i n e d by t h e r a t i o o f t h e h e a t c a p a c i t y t o h e a t f l o w i t f o l l o w s t h a t t h e h e a t i n g t i m e i s d i r e c t l y p r o p o r t i o n a l t o t h e p r o d u c t o f d e n s i t y and s p e c i f i c h e a t and i n v e r s e l y p r o p o r t i o n a l t o t h e p r o d u c t o f t h e t h e r m a l c o n d u c t i v i t y and t h e c u b e r o o t o f t h e f l e x u r a l m o d u l u s s q u a r e d .

I t i s t h e r e f o r e p o s s i b l e t o u s e t h e p h y s i c a l p r o p e r t i e s ( S p e c i f ~ . c G r a v i t y , S p e c i f i c H e a t C a p a c i t y , T h e r m a l C o n d u c t i v i t y and S p e c i f i c Modu lus ) t o e s t i m a t e t h e r e l a t i v e h e a t - u p t i m e s and o p e r a t i o n c o s t s u s i n g d i f f e r e n t t o o l m a t e r i a l s , T a b l e 2

T h e s e s a v i n g s w i l l be p r e d o m i n a n t l y d u r i n g h e a t - u p and c o o l - d o w n w h i c h i n a t y p i c a l c y c l e , u s i n g s t e e l t o o l i n g , may be u p w a r d s o f 100 m i n u t e s . The s a v i n g s c a n be e q u a t e d t o a r e d u c t i o n i n t h e c o s t o f t h e o p e r a t i o n o f an a u t o c l a v e o f t h e o r d e r o f £ 9 7 / c y c l e f o r a c o m p o s i t e t o o l and E 1 1 1 / c y c l e f o r an a l u m i n i u m t o o l , c o s t r e d u c t i o n s w h i c h a r e n o t i n s i g n i f i c a n t .

However i t h a s t o lie r e m e m b e r e d t h a t i n c e r t a i n i n s t a n c e s , e . g . t h : . c k c o m p o n e n t s , s l o w h e a t - u p r a t e s and c o o l - d o w n r a t e s may s t i l l be n e c e s s a r y t o e n s u r e t h e p r o d u c t i o n o f s t r e s s - f r e e c o m p o n e n t s .

A f u r t h e r f a c t o r w h i c h o f f - s e t s t h i s a d v a n t a g e i s t h e c o s t o f t h e raw m a t e r i a l s u s e d i n t h e t o o l i n g . C o m p o s i t e m a t e r i a l s a r e e x t r e m e l y e x p e n s i v e a l t h o u g h i t m u s t be p o i n t e d o u t t h a t t h e more c o m p l e x t h e t o o l , t h e g r e a t e r w i l l be t h e m a c h i n i n g t i m e and w a s t e g e n e r a t e d t o p r o d u c e a f i n i s h e d m e t a l t o o l . F o r c o m p o s i t e tooling the waste will essentially remain the

same regardless of complexity since

sophisticated computer controlled Gerber cutters

a r e now u s e d t o ' n e s t ' t h e r e q u i r e d s h a p e s w i t h t h e r e s u l t a n t i n c r e a s e i n e f f i c i e n c y o f m a t e r i a l utilisation.

F u r t h e r a d v a n t a g e s o f c o m p o s i t e t o o l i n g l i e i n t h e c o m p a t i b i l i t y o f t h e t o o l and c o m p o n e n t m a t e r i a l i n t e r m s o f t h e r m a l e x p a n s i o n , t h e r e b e i n g a s i g n i f i c a n t d i f f e r e n t i a l b e t w e e n t h e 1. m e t a l l i c t o o l s and t h e c o m p o s i t e c o m p o n e n t , w h i c h c a n l e a d t o s i g n i f i c a n t i n t e r n a l s t r e s s e s i n t h e c u r e d p r o d u c t . C l e a r l y t h e t h e r m a l e x p a n s i o n o f a c a r b o n f i b r e t o o l i s c o m p a t i b l e w i t h a c a r b o n f i b r e c o m p o n e n t . I n a d d i t i o n , 2 . the expertise for composite tooling exists in-

house since clearly handling of prepreg

materials and their processing is a pre-

requisite for composite component production.

In summary it is concluded that composite

tooling produces CFRP components of satisfactory

surface finish, offers a significant saving in

autoclave operation costs, gives a hard,

durable tooling material and, with careful

processing, can provide relatively stress-free

components using the expertise already available

in the production unit.

CONCLUSIONS

From t h e f o r e g o i n g i t w i l l be s e e n t h a t t h e r e l a t i o n s h i p b e t w e e n t h e s u r f a c e f i n i s h a c h i e v e d on CFRP c o m p o n e n t s i s c o m p l e x , b e i n g r e l a t e d t o t h e s u r f a c e c h a r a c t e r o f t h e t o o l i n g and t o t h e m a n u f a c t u r i n g and a u t o c l a v e p a r a m e t e r s . I n g e n e r a l , h i g h e r a u t o c l a v e p r e s s u r e s and m o d e r a t e h o l d t i m e s a r e t o be p r e f e r r e d .

CFRP tooling offers significant savings in

terms of shorter autoclave cycle times, but at

the cost of slightly poorer and more variable

surface finish on the components. This effect

is probably marginal or unimportant in terms of

aerospace components but may be significant in

Class A automotive body components.

ACKNOWLEDGEMENTS

The a u t h o r s w i s h t o a c k n o w l e d g e t h e u s e o f f a c i l i t i e s p r o v i d e d by t h e U n i v e r s i t y o f U l s t e r , t h r o u g h t h e D e p a r t m e n t o f M e c h a n i c a l and I n d u s t r i a l E n g i n e e r i n g and t h e P o l y m e r C o m p o s i t e s Group and t o t h a n k P r o f e s s o r s D McCloy and P P M i l l e r f o r t h e i r c o n t i n u i n g s u p p o r t .

REFERENCES

D u n l o p , G. ' T o o l i n g f o r A e r o s p a c e C o m p o s i t e s - a S m a l l Company V i e w ' . P r o c e e d i n g s o f 2rid C o n f e r e n c e o f IMC 1985 , pp 4 7 7 - 4 9 2

F r a m e , C S. ' I n t r o d u c t i o n t o C o m p o s i t e M a t e r i a l s ' , D e s i g n i n C o m p o s i t e M a t e r i a l s C o n f e r e n c e , 7 - 8 March 1989 , pp 1 -12

56 G . R . DICKSON and R. MCILHAGGER

3.

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HilI,B J; McIlhagger, R; Harper, C M; Wenger, W; Goksoy, M. 'Manufacturing Engineering Preforms' International

Conference on Fibre and Textile Science, Ottawa, Canada, 1991, Paper 10

Brain, N. 'Vacuum Bag and Autoclave Moulding', IPRI, 1985, pp 4-8

6.

Dickson , G R; M c I l h a g g e r , R; M i l l e r , P P.

'The Effects of Mould Surface Finish on the Surface Finish Characteristics of Polymeric Injection Mouldings', Surface Topography I, 1988, pp 455-467

Wenger, W; Diekson, G R; McIlhagger, R; Miller, P P. 'The Surface Finish Characteristics of Composite Components', Proceedings of 7th Conference of IMC, 1990, pp 590-606