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Selective reinforcement of LLDPE components produced by rotational

molding with thermoplastic matrix pultruded profiles

A. Greco , G. Romano, A. Maffezzoli

Department of Innovation Engineering, University of Salento, Via per Arnesano, 73100 Lecce, Italy

a r t i c l e i n f o

Article history:Received 26 April 2013

Received in revised form 25 June 2013

Accepted 12 August 2013

Available online 21 August 2013

Keywords:

A. Glass fibers

B. Adhesion

E. Pultrusion

E. Thermoplastic resin

Rotational molding

a b s t r a c t

This work is aimed to study the use of pultruded profiles for the selective reinforcement of linear lowdensity polyethylene (LLDPE) parts produced by rotational molding. A preliminary screening on different

types of pultruded profiles was performed, highlighting the relevance of adhesion to LLDPE in order to

prevent debonding of the reinforcing pultruded profiles. As expected, high density polyethylene (HDPE)

matrix pultruded tapes are characterized by a very high adhesion to rotomolded LLDPE. Therefore, HDPE

matrix pultruded tapes, fastened on the inner surface of the mold, are incorporated into LLDPE during

rotomolding. Plate bending tests performed on reinforced rotomolded plates and pressurization tests per-

formed on the box shaped prototypes showed a significant increase of the stiffness with a negligible

amount of reinforcement and increase of the weight of the component.

2013 Elsevier Ltd. All rights reserved.

1. Introduction

Rotational molding is a process for manufacturing hollow or

double-walled plastic products, in the absence of any external

pressure. Specific material requirements limit the polymers avail-

able for the fabrication of products by rotational molding. In partic-

ular, low melt viscosity is required in order to achieve an efficient

sintering of polymer powders and void-free products. Further, an

adequately high toughness is required in order to allow the extrac-

tion of components from molds[1]. Nowadays, only few classes of

thermoplastic polymers are processed by rotomolding, and most of

them are different grades of polyethylene, in particular linear low

density polyethylene (LLDPE). The mechanical properties of these

polymers are relatively poor, and rotomolded products find appli-

cations in fields where mechanical requirements are not particu-

larly critical. Therefore, in recent years, in order to improve the

mechanical properties or rotomolded parts, incorporation ofnanofillers [2,3], particulate reinforcements [4,5] or short fibers

[6,7] were considered. However, such approaches involve some

drawbacks, either in terms of mechanical properties, either in

terms of processability. In facts, incorporation of any type of filler

is always associated with an embrittlement of the material [8],

as well as with an increase of the viscosity of the polymer melt,

which in turn reduces the sinterability of the material, thus

increasing the fraction of voids [3,9]. Further, the presence of

powders of different size and weight must be carefully managed,

in order to avoid segregation, with finer and heavier particles being

predominantly dispersed on the outer surface of the rotational

molded products, and coarser and lighter particles being located

at the inner surface[1,4,10]. Other approaches developed in order

to increase the mechanical properties of the rotomolded products

involve the use of different types of polymers, such as polyamides

[3,11], polypropylene[12], high density polyethylene[13], or com-

bination of different materials in multi walled products [14,15].

Recently, it was shown that the rotational molding equipments

can be readily adapted to the production of long fiber reinforced

composites by the use of thermoplastic prepreg in a bladder mold-

ing process[15,16]. Such approach requires pressurization of the

mold, and in general, poses severe limitations to the geometric

complexity of the part. Further, the glass fibers are uniformly dis-

tributed in the rotational molding product, even where loading

conditions would not require any reinforcement.

On the other hand, pultruded rods are used for the reinforce-ment of polyethylene beams produced by in mold extrusion (or

intrusion), in order to optimize the distribution of reinforcement,

thus limiting the presence of the glass fibers in the zones of the

part which are subjected to higher loads [17]. This involves a

reduction of the weight of the components, and of the costs asso-

ciated with production, compared to an uniformly distributed rein-

forcement. Despite the significant increase of the stiffness of the

material, the reinforcement is effective only if high matrix/rein-

forcement adhesion occurs.

The aim of this work is the production of LLDPE prototypes,

reinforced with thermoplastic matrix pultruded profiles, by rota-

tional molding. Initially, the adhesion between LLDPE and different

1359-8368/$ - see front matter 2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.compositesb.2013.08.047

Corresponding author. Tel.: +39 0832297233.

E-mail address:antonio.greco@unisalento.it(A. Greco).

Composites: Part B 56 (2014) 157162

Contents lists available at ScienceDirect

Composites: Part B

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http://dx.doi.org/10.1016/j.compositesb.2013.08.047mailto:antonio.greco@unisalento.ithttp://dx.doi.org/10.1016/j.compositesb.2013.08.047http://www.sciencedirect.com/science/journal/13598368http://www.elsevier.com/locate/compositesbhttp://www.elsevier.com/locate/compositesbhttp://www.sciencedirect.com/science/journal/13598368http://dx.doi.org/10.1016/j.compositesb.2013.08.047mailto:antonio.greco@unisalento.ithttp://dx.doi.org/10.1016/j.compositesb.2013.08.047http://-/?-http://-/?-http://-/?-http://-/?-http://crossmark.crossref.org/dialog/?doi=10.1016/j.compositesb.2013.08.047&domain=pdfhttp://-/?-

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pultruded profiles was analyzed. Then, box shaped prototypes

were built by rotational molding, using HDPE matrix pultruded

tapes as reinforcement. The improvement of mechanical properties

showed the potential of the developed approach for the production

of continuous fiber reinforced LLDPE parts by rotational molding.

2. Materials and methods

The materials used are a linear low density polyethylene

(LLDPE), Clearflex RM 50 by Polimeri Europa (Italy), characterized

by a density equal to 0.936 g/cm3 and a melt flow index equal to

4.2 (measured by ASTM D1238). The melting peak temperature,

as measured by DCS analysis, is about 129 C.

Different pultruded profiles were used for the characterization

of adhesion properties to LLDPE and production of rotational mold-

ing prototypes:

Thermosetting matrix pultruded rods with circular cross section

(kindly supplied by Polystal Composites, Germany) characterized

by a nominal diameter of 3 mm, a tensile modulus of 50 GPa,

composed of about 85% by weight of glass fibers and 15% vinyl

ester matrix. A picture of the rods is reported in Fig. 1. Besides

longitudinal fibers, the Polystal rods are also characterized by the

presence of helicoidally winded fibers, which are expected to

improve the adhesion to LLDPE.

Thermoplastic matrix pultruded rods with circular cross sec-

tion, kindly provided by Jonam Composites (United Kingdom),

characterized by a polypropylene (PP) matrix. The rods have a

diameter of 6 mm, and a fiber weight fraction equal to 0.5 (corre-

sponding to 0.25 by volume). PP matrix rods are actually made of

dry glass bundles surrounded by the PP matrix. The measured ten-

sile modulus of pultruded rods is about 6 GPa and the strength is

about 120 MPa.

Thermoplastic matrix pultruded tapes, with rectangular cross

section (0.2 10 mm), supplied by Phoenixx TPC inc (USA). They

are characterized by the presence of a high density polyethylene

matrix (melting temperature of 127.5 C), a glass fiber weight frac-tion equal to 0.5 (corresponding to 0.25 volume fraction). The mea-

sured tensile modulus of the pultruded tapes is equal to 12.4 GPa

and the tensile strength is equal to 280 MPa. The higher modulus

and strength of the HDPE matrix tapes compared to the PP matrix

rods can be explained by the good impregnation of fibers observed

in the former case, as evidenced by the SEM images of pultruded

tapes, reported inFig. 2.

The adhesion strength of pultruded rods to LLDPE matrix was

measured by means of pullout tests. Samples were obtained by a

double stage compression molding process, using a Campana hot

press. At first, 20 40 6 mm samples of LLDPE were obtained

by compression molding under 200 bar and plate temperature of

30C, after preheating the material at 180C. Then, the LLDPE plate

was divided in two parts. The rods were enclosed between the two

LLDPE plates and compression molded at 200 bar and plate tem-

perature of 30C. Rectangular specimen 20 20 12 mm wereobtained. Samples with thermoset matrix rods were obtained after

preheating at 170 C, whereas samples with polypropylene matrix

rods were obtained after preheating at different temperatures,

ranging between 150 and 170 C. In facts, in the case of thermoset

matrix rods, the temperature of the process is expected to be of lit-

tle relevance, while in the case of the thermoplastic matrix rods,

the processing temperature is expected to have a significant influ-

ence. Pull-out tests were performed on a Lloyd instruments, series

LR5K, according to ASTM D1871-98 standard, using the mentioned

rectangular specimens. Each specimen has a single reinforcing rod,

which protrudes 30 mm from the cross section area of the plastic

mass. The crosshead speed was 50 mm/min.

Polyethylene prototypes were built by using a two axes lab

scale rotational molding machine designed and produced by Salen-tec srl (Italy). A box-shaped mold was used to fabricate samples

characterized by an edge length equal to 148 mm. HDPE matrix

pultruded tapes were used for the reinforcement of LLDPE proto-

types. The tapes were bonded at the inner surface of the aluminum

mold by means of silicon adhesive. One single tape was placed on

each of the six square faces of the mold. Then, a standard rotational

molding cycle was run using LLDPE powders, setting the oven tem-

perature at 280C, and the rotation speed of the primary and the

secondary axes at 6.1 rpm and 1.6 rpm respectively. The mold

was held inside the oven for 25 min, after which it was cooled by

forced convection in air for about 30 min. A picture of a prototype,

including a tape on each face, is reported in Fig. 3a. The prototype

is characterized by an average wall thickness equal to

4.2 0.05 mm. A scheme of the side view of each face of the proto-type is reported inFig. 3b.

The mechanical properties of samples extracted from rotomol-

ded prototypes were measured by LLOYD LR 5K dynamometer.

For characterization purposes, beams were extracted, according

to the scheme ofFig. 3b, in correspondence of the red1line, and are

therefore composed of a stack of LLDPE (about 4 mm thick) and pul-

truded strip (about 0.2 mm thick), characterized by a width of

10 mm. Beams were characterized by means of flexural tests, double

lap shear tests and short beam tests.

Beam flexural tests were performed using a length to thickness

ratio equal to 16 and a crosshead speed of 2 mm/min.

Fig. 1. Microscopy of thermoset matrix pultruded rod.

Fig. 2. Microscopy of HDPE matrix pultruded tape.

1 For interpretation of color in Fig. 3, the reader is referred to the web version ofthis article.

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Double lap shear tests were used for evaluation of the average

adhesion strength between LLDPE and pultruded tapes, by measur-

ing the maximum force during the tests:

sA Fmaxbh

1

whereb is the width of the samples and h is the distance between

the two laps.

Short beam tests were performed according to ASTM D 2344with a span to thickness ratio equal to 4 and a crosshead speed

equal to 0.5 mm/min.

Plate bending tests were also performed on the faces extracted

from the rotomolded prototypes. Sheets 100 100 mm wide,

which, due to the high edge to thickness ratio, can be approxi-

mated as thin plates, were simply supported on their perimeter,

and loaded with a square punch (16 mm edge) in their center. In

reinforced samples, the tape was placed at the extrados. With ref-

erence to Fig. 4, being a=b= 90 mm (edge of the plate),

x1=y1= 37 mm, x2=y2= 53 mm (edges of the loading punch), the

deflection at a generic point x ,y on the plate is given as:

v 4

p6BF

x2 x1y2 y1

Xm

Xn

senmpxa sennpy

b

mn

m2

a2

n2

b2 2

cosmpn1a cos

mpn2a

cos

npg1

b cos

mpg2

b

2

64

3

752

whereB is the flexural stiffness of the sheet.

Finally, the box-shaped prototypes were pressurized by an

hydraulic system up to 10 bar. In absence of water leakages, the

volume change of the prototype was assumed to be equal to the

total volume of water injected inside the prototype. In order to per-

form these tests, the prototype was produced by insertion of a con-

necting device, according to the sketch of Fig. 5. The connection

device is made of a cylindrical duct for water inlet and a flange

disk, which is fixed on the surface of the mold at the beginning

of the rotational molding process. After processing, the disk

remains included in the rotomolded sample wall. The cylindrical

part is threaded on its outer surface in order to tighten the LLDPE

wall on the disk with a nut.

3. Results and discussion

A typical force displacement curve obtained by pullout tests of

pultruded rods is reported inFig. 6. In the initial stage of pullout

test, the stress is elastically transferred from the rod to the poly-

ethylene[18]. The peak in the initial part of the curve corresponds

to the break of the adhesive bond between rod and polymer, whichtakes place when the interfacial shear stress reaches the adhesion

strength, ss. As the rod is further pulled out, the load gradually de-

creases with the reduction of the contact area between the two

components. The measured load at this stage is mainly due to

the friction stress acting between rod and polymer.

The values of ss can be calculated fromFig. 6. Assuming a uni-

form shear stress along the length of rod, the adhesion strength

can be obtained from the maximum force Fmax attained during

the test:

ss FmaxpDL0

3

where D is the rod diameter and L0 is the contact length between

rod and polymer. The adhesion strength calculated by Eq. (3) and

(a)

4.2 mm

0.2 mm

LLDPE

Pultruded tape

(b)

Fig. 3. (a) Photography of reinforced prototype processed by rotational molding and (b) scheme of the side view of each face of the prototype.

y

x

a

b

x2x1

y1

y2

Fig. 4. Scheme of the sample geometry and loading device for plate bending tests.

polyethylene

Cylindrical duct

for water inlet

Flange disk

threaded nut

Fig. 5. Scheme of the connection device for pressurization tests.

0 2 4 6 8

0

5

10

15

20

25

force(N)

displacement (mm)

FMAX

Fig. 6. Forcedisplacement curve from pull-out tests.

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reported in Table 1, indicates that the thermoset matrix rods are

characterized by a lower adhesion strength compared to PP matrix

rods. The adhesion strength of the latter increases when the pre-

heating temperature is increased from 150 to 160 C whereas no

relevant difference can be observed heating up to 170 C. This result

highlights that full melting of the PP matrix of the rod, occurring

only above 160 C, is capable to promote a proper adhesion with

LLDPE.

A typical force-displacement curve from double lap shear tests

is reported in Fig. 7. The adhesion strength between LLDPE and

HDPE matrix tape, calculated from the maximum force according

to Eq. (1), is 8.4 0.21 MPa. This value is very high, and of the same

order of magnitude of the interlaminar shear strength measured

for all high density polyethylene composites processed by

compression molding[19], despite the fact that that the composite

processed by rotational molding is obtained by a virtually zero-

pressure process. HDPE matrix tapes are characterized by a much

higher adhesion with LLDPE than PP or thermosetting matrix rods

(Table 1).

Even if thermoset matrix rods are characterized by a higher

modulus, and therefore by a higher stiffening potential, debonding

at LLDPE/rod interface can be responsible of a loss of the stiffening

effect and eventually of yielding of reinforced plates. In contrast,though being characterized by lower modulus, and therefore by a

lower stiffening efficiency, the HDPE matrix tapes are likely to pre-

serve their stiffening effect even at high values of the applied

stress, due their higher adhesion to LLDPE. Therefore pultruded

tapes were used for the reinforcement of LLDPE processed by rota-

tional molding.

A comparison between flexural stressstrain curves of unrein-

forced and reinforced beams extracted from rotational molded pro-

totypes is reported in Fig. 8. Addition of the HDPE matrix tape

involves an increase of the equivalent modulus of the material,

from 0.6 0.02 GPa, which is the value of LLDPE, up to

1.2 0.14 GPa.

In a previous work[15]it was shown that the flexural stiffness

of double wall composites can be efficiently represented by the fol-lowing set of equations:

yG;PUL - tLLDPE

2

yG;LLDPEtPUL

2

yNA1 - n

2

tPULtLLDPEtLLDPEntPUL

ILLDPE bt3LLDPE

12 ALLDPEyG;LLDPE - yNA

2h i

IPUL bt3PUL

12 APULyG;PUL - yNA

2h i

KXi

EiIiEPULIPULELLDPEILLDPE

8>>>>>>>>>>>>>>>>>>>>>>>>>>>:

4

whereyGis the position of the baricenter across the thickness of

each layer,Ithe moment of inertia, tthe thickness, andA the area,and the pedices LLDPE and PUL refer to LLDPE and pultruded tape.

Further, y NA is the neutral axis position across the thickness, n is

the ratio between the moduli of pultruded tape and LLDPE, and K

is the total flexural stiffness of the beam.

Therefore, it is possible to obtain the equivalent modulus of the

material as:

EEQ K

b tLLDPEtPUL3

12

5

Combining Eqs. (4) and (5) with the geometric characteristic of

the beam extracted from rotational molded prototypes (b= 10 mm,tPUL= 0.2 mm, tLLDPE= 4.0 mm, EPUL= 12 GPa, and ELLDPE= 0.6 GPa),

an equivalent flexural modulusEEQ= 1.28 GPa was obtained, which

is value in very good agreement with the experimental value.

Short beam tests performed on samples extracted from rota-

tional molded prototypes did not show any deboning at the inter-

face between LLDPE and pultruded tape. On the other hand,

tension break of the pultruded tape was observed for a load value

of about 600 N. In such case, the interlaminar shear stress between

LLDPE and pultruded tape can be evaluated to be [15]:

s 1

b

Sx;LLDPEELLDPEF

2K

6

where Sx,LLDPEis the static moment of the LLDPE area with re-

spect to the neutral axis, Fis the applied force, and Kis given by

Eq. (4). Combining the geometric characteristic of the beam, for

an applied force of 600 N, which is the maximum force attained

during short beam tests, it is possible to estimate an interlaminar

shear stress equal to 6.4 MPa, in correspondence of tension failure.

This value is lower than the adhesion strength measured by double

lap shear tests, and explains the absence of any delamination dur-

ing short beam shear test. It is worth observing that, in the case

that PP or thermoset matrix rods were used, the interlaminar shear

stress would exceed the adhesion stress measured by pullout tests

(Table 1), causing delamination.

The force displacement curve during plate bending tests on

sheets extracted from rotational molded prototypes are reportedin Fig. 9. From the slope of the force displacement curve, by

Table 1

Adhesion and friction stresses for thermoset and thermoplastic pultruded rods. In

parentheses the pre-heating temperature.

Rod ss (MPa)

Poystal (170 C) 0.36 0.16

Jonam (150 C) 1.90 0.68

Jonam (160 C) 2.52 0.42

Jonam (170 C) 2.40 0.39

0.0 0.3 0.6 0.9 1.2 1.50

150

300

450

600

750double lap shear test

displacement (mm)

force(N)

Fig. 7. Forcedisplacement curve from double lap shear tests.

0.000 0.005 0.010 0.015

0

2

4

6

8

10

12 not reinforced

reinforced

strain (mm/mm)

s

tress(MPa)

Fig. 8. Stressstrain curves from flexural tests on beams extracted from rotational

molding prototypes.

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inversion of Eq. (2), a flexural stiffness of 3.96 0.15 Nm can be cal-

culated for neat LLDPE plates. On the other hand, the flexural stiff-

ness for the reinforced plates was calculated as 5 0.15 Nm.

Therefore, a stiffening ratio SR = 1.26 was obtained.

The results of pressurization tests are reported inFig. 10. Even

in this case, the higher stiffness of reinforced prototype is high-

lighted by the lower volume increase. The stiffening ratio (obtained

as the ratio between the volume increase measured using neatLLDPE and reinforced LLDPE boxes) is 1.25, very close to the value

obtained by plate bending tests.

The structural mechanics problem of a reinforced plate

subjected to an uniformly applied pressure, was solved by finite

element (FEM) software (FlexPDE). For the same lay-up of the

reinforcement used for the production of the prototypes, compris-

ing one single tape parallel to edges, a stiffness ratio between

reinforced and unreinforced plates of 1.22 was calculated. Never-

theless, it must be highlighted that more than one reinforcing

tape can be used at different positions. For example, by placing

two tapes parallel to the square edges, a stiffness equal to

5.3 Nm was estimated by FEM simulation. Two tapes placed on

the diagonals of the square face of the rotomolded box yielded

a stiffness of 5.8 Nm. Therefore, the use of two tapes in different

positions could lead to a stiffness ratio of 1.32 and 1.53,

respectively.

Such increase of the stiffness of the plate can be obtained by

adding a very low amount of glass fibers (about 0.3% by volume

for one single tape, 0.6% for two tapes), with a negligible increase

of the weight of the component (0.5% and 1% respectively).

For comparison purposes, other approaches aimed to increase

the stiffness of the rotomolded box are analyzed:

(a) Addition of glass spheres to LLDPE.

(b) Increase of the thickness of the prototype.

The first approach can be studied by assuming an elastic mod-

ulus of 72 GPa for glass and 0.6 GPa for LLDPE. Therefore, the vol-

ume fraction of the reinforcing phase required to attain the samestiffness of the plate can be calculated by coupling the HalpinTsai

model for isotropic material[20](with an aspect ratio of particles

equal to 1) with the expression of the plate bending stiffness, valid

for homogeneous materials:

B Es3

121 m2 7

For the three different simulated values (one single tape, two

tapes parallel to square edges, and two tapes on the square diago-

nal) the corresponding values of the glass spheres volume fraction

needed to reach the same stiffness, B, is reported in Table 2.

Assuming a LLDPE density of 935 kg/m3 and a glass density of

2540 kg/m3, the corresponding weight increase was obtained.

The values reported in Table 2 are much higher than those ob-

tained by using pultruded tapes.

Following the second approach, the thickness of the prototype

should be increased by a factor of SR1/3, as reported inTable 2. In

this case, the weight increase would be almost equal to the thick-

ness increase. Even in this case the weight increase, though lower

than the weight increase estimated for glass spheres reinforce-ment, is much higher than that calculated for the pultruded tapes

reinforcement.

Besides the increase of the weight of the components, both the

alternatives are characterized by severe processing limitations in

rotational molding. The drawbacks of adding glass spheres were al-

ready discussed in the introduction section. Instead, the increase of

wall thickness causes higher temperature gradients across wall

thickness [21], which in turn involves more severe degradation

phenomena on the outer surface of the part, in direct contact with

the mold.

4. Conclusions

A new design and processing route was developed for the pro-duction of reinforced LLDPE components processed by rotational

molding. A preliminary evaluation of the adhesion properties of

different types of pultruded profiles to LLDPE was performed. De-

spite their lower modulus compared to thermosetting matrix pro-

files, thermoplastic matrix profiles are characterized by a higher

adhesion to LLDPE. Further, HDPE matrix tapes were characterized

by an improved adhesion to LLDPE compared to PP matrix rods.

Debonding was not observed between LLDPE and HDPE matrix

tapes even during short beam tests. As a consequence, HDPE ma-

trix tapes were used for the reinforcement of LLDPE prototypes

processed by rotational molding. To this purpose, HDPE matrix

tapes were fastened on the inner surface of the mold, before run-

ning a standard rotational molding cycle. During processing, melt-

ing of LLDPE powders and of HDPE matrix is responsible of theincorporation of the tape in the component walls.

0 2 4 6

0

100

200

300

400not reinforced

reinforced

displacement (mm)

force(N)

Fig. 9. Force displacement curves from plate bending tests on sheets extracted fromrotational molding prototypes.

0 2 4 6 8 10

0

200

400

600

800

1000not reinforced

reinforced

vo

lumechange(cm

3)

pressure (bar)

Fig. 10. Volume change vs pressure from water pressurization tests.

Table 2

Comparison between different approaches for stiffening of rotational molded LLDPE.

Pultruded tape

reinforcement

Glass spheres

reinforcement

Thickness

increase

SR Weight increase

(%)

Vf Weight

increase

(%)

Weight

increase

(%)

One tape 1.23 0.5 0.1 17 7

Two tapes

parallel to

edges

1.31 1 0.14 24 9.5

Two tapes on

the diagonal

1.46 1 0.21 36 15

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Plate bending tests performed on the faces of prototypes

showed that the incorporation of HDPE matrix tapes into LLDPE

is responsible of a significant increase of the stiffness, even for very

low amount of glass fibers. A similar stiffness increase was ob-

tained by pressurization of the box-shaped prototypes. In both

cases, it was shown that incorporation of a single tape leads to

an increase of the stiffness of about 25%, in good agreement with

FEM simulation. Finally, FEM analysis was used for the optimiza-tion of the layout of pultruded tapes on the surface of the box-

shaped prototypes. A comparison of different materials choices

leading to the same stiffening effect was also proposed, showing

the potential of the developed approach for selective reinforce-

ment of rotomolded parts.

Acknowledgements

Mr. Francesco Montagna is acknowledged for the support to

experimental activity, and Dr. Andrea Salomi for his useful

suggestions.

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