7/25/2019 1-s2.0-S0034361701802083-main

http://slidepdf.com/reader/full/1-s20-s0034361701802083-main 1/3

LFT: the future of

reinforced thermoplastics?

The application of long fibre reinforced thermoplastics has been mainly limited to

the automotive industry where large production runs offset the high cost of

commercially available processing equipment. Ferrie van Hattum and Sjef van

Breugel, Centre of Lightweight Structures, the Netherlands, discuss the potential

advantages of LFT over GMT and introduce a more economical production machine.

F

bre reinforced plastics (FRP) are

used in products where weight

savings, reduced production costs,

low maintenance costs and freedom of

design are an issue. In these cases,

thermosets are traditionally used, rein-

forced with glass or carbon fibres. In

many cases the properties of these mat-

erials allow their application as a

construction material where they replace,

for example, metals.

A second group of FRP is fibre rein-

forced thermoplastics, where thermo-

plastic material is used as the matrix for

the fibre reinforcement. Compared to

thermosets, these materials often show

better impact properties, increased

toughness and they can be processed in

a fast and clean way.

Large-scale application of these mat-

erials has so far been limited to short

fibre reinforced injection moulding

materials. The material properties in

these cases are about two to three times

higher than the unreinforced material.

In the automotive industry glass mat

reinforced thermoplastic (GMT) (often

glass reinforced polypropylene) is widely

used in semi-structural, compression

moulded parts. ‘High end’ applications

can be found in, for instance, the use of

high quality thermoplastic matrices like

polyetheretherketone (PEEK) with car-

bon fibre reinforcement. In spite of the

excellent properties of this group of

materials, their high cost and processing

limitations allow only limited applica-

tion in, for example, aerospace products.

Continuous fibre

reinforced plastics

Price

Figure I: Price property relat ionship between dif ferent f ibre reinforced thermoplast ics.

42

REINFORCEDplastics

J u

n e

2 0 0 1

I

In terms of price and properties there

is a huge gap between short fibre rein-

forced thermoplastics and GMT on the

one hand and continuous fibre rein-

forced thermoplastics on the other hand.

This keeps fibre reinforced thermoplastics

from wider application as (semi-)structur-

al materials in relatively cheap and fast

processes for large production series.

LFT

A relatively new group of materials,

long fibre reinforced thermoplastics

(LFT) can exactly fill this gap. LFTs

are thermoplastics reinforced with

discontinuous long fibres (a few cen-

timetres in length) and (unlike GMT)

are available in different fibre and

matrix combinations. As a result, for

every application the right material

combination can be used. This charac-

teristic enables LFT to fill the gap

between GMT and continuous fibre

reinforced thermoplastics (see Figure 1).

Furthermore some specific advantages

make LFT a serious competitor in

terms of price for products in which

GMT is traditionally used. In addition,

the compression moulding process for

these materials has not yet been fully

optimised. Better control of the effect of

the process on fibre distribution in the

final product, and therefore on the

material properties, can result in con-

siderably improved product properties

using the same materials.

0034-3617/07/

see front matter 2 1

Elsevier

Science Ltd. All rights reserved.

7/25/2019 1-s2.0-S0034361701802083-main

http://slidepdf.com/reader/full/1-s20-s0034361701802083-main 2/3

LFT: the future of reinforced thermoplast ics?

The potential cost reduction of LFT

compared to GMT and improved

product quality has been demonstrated

in the front-ends of the Volkswagen

Passat series which are currently made of

LFT, at the expense of the GMT previous-

ly used. In this example some advantages

of LFT over GMT are:

the possibility of working without

semi-finished materials (in-line extru-

sion compression moulding): less

labour intensive; greater freedom of

materials; easier to recycle;

lower compression forces due to

better flow which results in cost sav-

ings with respect to moulds and

machinery;

better surface quality;

shorter cycle times; and

less rejected products.

In the West European automotive indus-

try alone, it is expected that by 2008 LFT

will account for 20% of the total amount

of fibre reinforced thermoplastics used.

Products in other markets are also suit-

able for the application of LFT. Examples

include street furniture, self-supporting

housing and machine parts.

The process

The processing of LFT (see Figure 2) is

very similar that of GMT and, to a lesser

extent, sheet moulding compound

(SMC), and can be described in a simpli-

fied way by four steps:

melting the material;

ejection and insertion in the mould;

compression moulding; and

cooling under pressure and release of

the product.

In the processing of LFT the conserva-

tion of fibre length (and hence the

mechanical properties of the final prod-

uct) is vital. The first step, the melting

of LFT, is therefore not done using a tra-

ditional extruder. Traditionally extrud-

ers work with high shear forces in the

mat-erial, which causes considerable

fibre breakage. Several companies have

developed extruders especially for LFT,

which melt the material whilst conserv-

ld I

One of the reasons why the application

approximately O.SS/kg of material can

of LET has so far been mainly limited to

be re&sed. This implies that using such

the automotive industry lies in the high

a process, one has to take into account

equipment costs of commercially avail- approximately 2SQ 000 of capital ar

able LPT processing machines. Large royalty expenses annually. This is main-

production series are required to coun-

ly caused by the high equipment costs

terbalance these costs.

of such a machine,

CPI indicates that using its (direct) The machine developed by the CLS

LFT process for glass/polypropylene (PP)

costs only a fraction of its commercially

LPT can be economically justified if the

available counterparts. This reduces

equipment is used to process approxi-

required investments and associated

mately 450 000 kg of material annually.

risks to a minimum. This makes the LPT

This can usually only be attained with technology accessible for new markets,

applications in the automotive industry. where much smaller production series

By using its in-line process, savings of

are common.

Economic advantages of LfT

[a) Melting the material

(b) Material ejection and

insertion in the mould

(c) Compression moulding

and cooling

(d) Release product

Figure 2: The

process ing

teps

for

LFT

ing the fibre length. However, these

machines require a significant invest-

ment (about go.51 million), one rea-

son why their application has so far

been mainly limited to the automotive

industry.

In processing LFT two different

methods are used, mainly differing

in the initial materials used. Indirect

LFT processing uses semi-finished mate-

rials supplied by several manufacturers

in pellet form which are then melted

in the extruder. Manufacturers that

supply LFT with different fibre/matrix

combinations include DSM, LNP, Asahi,

-

RTP and Ticona. To take advantage

of the economy of the LFT process to

the greatest extent fibres and matrix

material can also be mixed in the

extruder, so called direct LFT process-

ing. This eliminates the need for semi-

finished materials. As a result of this

cost reduction this is the preferred

method in the automotive industry

(still only in combination with glass/

polypropylene), where traditionally

cost plays a dominant role.

After melting, the desired quantity of

material is ejected from the extruder and

cut-off by a cutting device. The molten

June 2001

RElNFoRCEDplastics 43

7/25/2019 1-s2.0-S0034361701802083-main

http://slidepdf.com/reader/full/1-s20-s0034361701802083-main 3/3

LFT: the future of reinforced thermoplastics?

LFT material is then inserted in a ‘cold’

mould, which then closes at relatively

high speed. On closing the LFT material

will start to flow to fill the mould cavity.

During cooling the mould is kept closed

t

at a pressure of at least SO-100 bars, after

which the mould is opened and the

product can be taken out.

New developments

Because of the large investments that

go with the purchase of the required

extruder and press for the processing

of LFT, the material’s use has mainly been

limited to the automotive sector, but it’s

evident that there are also many opportu-

nities for the material outside of the auto-

motive sector. At the Centre of

Lightweight Structures (CLS TUD-TNO), a

merger of Delft University of Technology,

faculty of Aerospace Engineering, and

TN0 Industrial Technology, research is

being directed LFT. As part of this work a

patented piston-blender has been devel-

oped for melting the LFT while conserv-

ing fibre length (see Figure 3). Because of

25000

PP-GF

20000

t PA12-CF 25

mm)

GMT

s

15000

E

w 10000

0

5000

A(

0 5 10 15 20 25 30 35

v, W

Ygure 4: Stif fness v ersus f ibre

fraction for different

LFh.

300

PP-GF

j =?Ix

t

0 0

D

100 0 . - . _

50

0 5 10 15 20 25 30 35

v, W

r igwe 5: Strength

versus fibre fraction for different

L s.

the machine’s simple construction, its

cost is only a fraction of that of conven-

tional LFT extruders and it offers the pos-

sibility of processing LFT at only minor

investments.

Using this machine (Figure 3) LFT

with different fibre/matrix combin-

ations ranging from glass fibre rein-

forced polypropylene (PP) to carbon

fibre reinforced polyamide (PA) have

been processed and tested. The possibil-

ities that LFT offers are clearly shown in

Figures 4 and 5, where the tensile stiff-

ness and bending strength of the LFT

(in this case the two extremes: 12.5 mm

glass fibre reinforced PP and 25 mm

carbon fibre reinforced PA 12) versus

fibre volume fraction are plotted. It is

clearly shown that by modifying mat-

erial combinations and fibre fractions a

wide range of stiffnesses can be attained

(the shaded area). In this way properties

can be tailored to the final desired

product properties. Extension of this

area is possible by further change of

the parameters mentioned. For comp-

arison, the properties of GMT are also

given, which clearly shows the position

of LFT relative to this material.

Note that in Figure 5 at higher fibre

fractions the strength of the glass/PP

material decreases. Too high a fibre

fraction does not necessarily result in

better properties.

Apart from possible applications of

LFT and attainable properties (Figure 1:

New material combinations) the research

at the CLS also aims at process control

(Figure 1: Control): the relation between

processing parameters, fibre orientation,

fibre length

and distribution and

product properties. W

CLS TUD -TN 0 is looki ng for companies

int erested in t he appli cation of LFT in

products or i n furt her development of the

piston- blender o an ndust ri al evel. Al dert

Verheus, CLS TUD-TNO, Delft, the

Net herl ands; t el : 31-l 52781778.

44

REINFoRCEDplastics I u

n e

2

1