F2000A106
Seoul 2000 FISITA World Automotive Congress
June 12-15, 2000, Seoul, Korea
Low Pressure Molding Compound Hood Panel for a Passenger Car
Chi-Hoon Choi1) *, Sang-Sun Park1), Kye-Won Ahn2) , Jeong-Eek Rhee2)
1)
Polymeric Materials Research Team, Hyundai Motor Company, Korea
2)
Development Department, Dayone, Korea
Low pressure molding compound (LPMC) is a new kind of composite material which can be used for automotive body
panels. LPMC has similar mechanical properties compared to conventional sheet molding compound (SMC) but excellent
moldability due to the different thickening system. In this paper, we prepared LPMC hood prototype for a passenger car
using a low cost tooling. Inner panel and outer panel were made of general-density and low-density grade LPMC,
respectively, in order to maximize weight reduction maintaining surface quality. Physical Properties containing tensile
strength, flexural modulus, notched Izod impact strength of those samples were investigated. In addition, CAE simulation
was also done for strength analysis of the hood assembly.
Keywords: LPMC, hood, low density, physical properties, glass microsphere.
INTRODUCTION
EXPERIMENT / MODELING
Sheet molding compound (SMC) based on liquid
unsaturated polyester (UP) is increasingly used for
automotive components including body panels, chassis
parts, and under-the-hood parts. Examples of SMC hood
panel application for passenger cars are Ford Lincoln
continental, Chrysler Sebring JX, Dodge Viper, GM
Corvette, Camaro, EV1 and so on. The composition of the
SMC generally includes UP resin (25-30wt%), fibrous
reinforcing material (25-30wt%), mineral filler (4045wt%) and other additives such as release agent, pigment,
thickening agent, and so on. Recently low pressure
moldable SMC, called LPMC, has been introduced by
Scott Bader company. They use unique thickening system.
A crystalline polyester called Crystic Impreg is used as a
thickening agent instead of metal oxide in conventional
SMC formulation. The crystalline resin is dissolved in the
liquid resins at 80℃, processed into a paste, converted
into LPMC at above 50℃. Other conditions like glass
fiber loading and resin feeding are much similar to
traditional SMC. At the end of stage the compound is
allowed to cool (room temperature) and the crystalline
resin in LPMC becomes insoluble almost immediately
inducing viscosity increase. Therefore, additional
maturation period is not needed and the handling
characteristics remain for much longer time than metal
oxide thickened conventional one. The advantages of
LPMC are mainly come from the different thickening
mechanism. They have superior flow characteristics than
conventional SMC. LPMC material can be compression
molded at much lower pressure (1-3 MPa), therefore
LPMC needs lower tooling and maintenance costs. In this
paper we developed two formulations, the one is general
density low profile Class ”A” LPMC for outer panel and
the other is low density hollow glass microsphere filled
LPMC. Effect of molding pressure on low density LPMC
also investigated. Prototype hood inner and outer panels
are molded by using zinc alloy molds and then bonded
together with an adhesive at room temperature.
MATERIALS
Resins
Ortho-type (OS108, Aekyung Chem.) and iso-type
(OS980, Aekyung Chem.) UP resins are used in LPMC
Compounding. Polyvinyl acetate(PVAc) type low-profile
agent (Q6016, Aekyung Chem.) is also used in this study.
Crystalline saturated polyester (Crystic Impreg C773,
Scott Bader) and crystalline unsaturated polyester (Crystic
Impreg C772, Scott Bader) are used as thickening agents
for general density and low density formulation
respectively. Crystalline saturated polyester is designed for
use as a combined thickener/low-profile agent so it is not
necessary to add any other low-profile agent but crystalline
unsaturated polyester is used with a low-profile agent.
Additional styrene is also included in LPMC fomulations
in order to achieve adequate crosslinking.
Reinforcement, Filler and Others
Glass fiber roving (RS4800-433, Owens-Corning
Korea) is fed and cut in 1-inch size, then the chopped
glass fiber is loaded on the resin paste. Calcium
carbonate is used as the primary filler to increase
stiffness of moldings and to reduce shrinkage / thermal
expansion and overall cost of material. Two types of
very fine calcium carbonate, surface-coated one
(Omyacarb 1T, Omya Inc.) and non-coated one
(Omyacarb 2, Omya Inc.) are incorporated in LPMC.
Their mean particle diameters are 1.7 and 2.7 μm,
respectively. To obtain lighter hood assembly calcium
carbonate is replaced by hollow glass microsphere (K37, 3M, 80 μm) as a filler. Cross-linking catalyst
used
* chchoi1@hyundai-motor.com
1
is a t-butyl perbenzoate. Inhibitor used is a pbezoquinone to prevent premature polymerization.
Internal release agent used is zinc stearate. A
methacrylate type adhesive (Plexus MA320, ITW) is
used for structural bonding of molded hood panels.
Low Density LPMC Recipe for Inner Panel
The recipe of newly developed LPMC compound for
hood inner panel is shown in Table 3. Iso-type UP
resin, PVAc, crystalline unsaturated polyester and
styrene monomer are used in this formulation. Direct
substitution of calcium carbonate in weight fraction
with microsphere can change the physical property
balances because of the differences between specific
gravity of calcium carbonate (2.7) and microsphere
(0.37). Thus we prepared low density LPMC on the
basis of volume fraction. The volume fractions of
resin and glass fiber are almost constant but mineral
filler is substituted by glass microsphere. It is noted
that the weight fraction of glass fiber is increased from
25 to 35 due to the presence of low-density filler. The
calculated specific gravity of the formulation (LPMC2) is 1.30.
COMPUNDING AND MOLDING
The LPMC compounds are prepared using an modified
SMC machine (Schmidt and Heinzmann) to maintain the
higher processing temperature.
Standard LPMC Recipe
The basic formulation of LPMC compound is shown
in Table 1. The formula was already developed for
class “A” automotive body panels. The calculated
specific gravity of the standard formulation (LPMC-S)
is 1.95, which is not suitable for our weight target, i.e.
30% weight reduction. Thus formulations for hood
outer panel and inner one are newly developed in this
paper. Resin consist of ortho-type UP, iso-type UP,
crystalline saturated polyester, and styrene monomer.
Table 3. LPMC Formulation for inner panel
Material
Resin
Glass Fiber
Mineral Filler
Microsphere
Additives
Total
Table 1. Standard LPMC Formulation
Material
Resin
Glass Fiber
Mineral Filler
Additives
Total
Volume
Fraction
40.6
18.4
34.5
6.5
100
Weight
Fraction
22.6
25.0
48.6
3.8
100
Test plaques in size of 300mm by 300 mm with 3mm
thickness are compression molded at a specimen mold
using a laboratory press at 145℃, 2MPa. Both hood
inner panel and outer panel are compression molded at
prototype using a 500 ton press at the same conditions.
The recipe of reformulated LPMC compound for hood
outer panel is shown in Table 2. The weight fraction
of glass fiber is maintained. However, the content of
mineral filler is decreased whereas that of resin is
increased. The calculated specific gravity of this
compound (LPMC-1) is 1.80.
Resin
Glass Fiber
Mineral Filler
Additives
Total
Volume
Fraction
46.3
17.4
29.1
7.2
100
Weight
Fraction
32.1
35.0
19.3
7.6
6.0
100
Specimen and Parts Molding
Class A LPMC Recipe for Outer Panel
Material
Volume
Fraction
39.9
17.6
9.4
26.7
6.4
100
TESTS
Tensile properties of the LPMC samples were
determined following the standard procedure
described in ASTM D638 with type I specimens. A
universal testing machine (UTM) was operated at a
crosshead speed of 5mm/min. Tests were made at
room temperature, and at least seven runs were made
to report the average. Flexural properties of the
samples were measured using the UTM at a crosshead
speed of 1.3mm/min (ASTM D790) and notched Izod
impact strength was also measured using a impact
tester (ASTM D256). Bending and torsion strength of
hood parts are measured using push pull gage, U-Cam
and mounting jigs.
Weight
Fraction
27.1
25.0
43.4
4.5
100
Table 2. LPMC Formulation for outer panel
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between calculated values and tested ones are existed,
LPMC-1 is 1.82 and that of LPMC-2 is 1.31. This means
that LPMC-1 and LPMC-2 are 7% and 33% lighter than
LPMC-S. The effect of molding pressure on specific
gravity for LPMC-2 is shown in Figure 2. The crush
strength of glass microsphere K37 used in this study is
about 20 MPa, which is much higher than molding
pressure of LPMC. Therefore, the specific gravity of low
density LPMC-2 is almost independent of molding
pressure in our experimental range. It is well known that
conventional SMC is compression molded at around
7～15 MPa. In that case, specific gravity of hollow
microsphere filled SMC increases with increasing molding
pressure. There must be particle size distribution of the
glass microsphere. The micosphere break-up may occur
more easily for large particles and during the high pressure
compression molding.
MODELING AND FEA
Redesign from sheet metal hood CAD files to LPMC was
done using CATIA at RS6000. This includes the structural
changes of inner and the outer hood component, part
thickness, and bonding area, etc. The CAD data was
translated into prototype tooling. Finite element analysis
(FEA) of part strength was performed to determine
optimum part thickness using Indigo 2 (Silicon Graphics),
in which Patran 8.0 software performed the analysis.
RESULTS AND DISCUSSION
Physical properties of newly compounded LPMC samples
were compared to standard LPMC (control). The strength
analysis and tests for both current steel hood and prototype
LPMC hood were performed.
Tensile Strength
The test results of tensile strength for LPMC-S, LPMC-1
and LPMC-2 are shown in Figure 3. Tensile strength of
reformulated LPMC-1 is 70MPa, which is slightly higher
than that of LPMC-S. The Strength of SMC depends on
many kinds of factors such as glass fiber type and content,
filler shape and size, interfacial adhesion between UP resin
and fiber or filler, the degree of filler dispersion etc. Glass
fiber is the most commonly used reinforcement in the UP
resin composites. Its fibrous structure imparts high tensile
strength and high impact strength at relatively low cost.
Calcium carbonate loading does not stiffen the composite
as much as other fillers such as talc and mica. Hollow
microsphere (glass microballoon) is used to lower the
density of composite and also reduce cost per unit volume.
The filler is not considered a true reinforcing filler. In our
experiments, volume fraction of glass fiber is almost
constant so filler type and content could become major
factors. The hollow microsphere K-37 used in this study
falls between 20 ～80 μm in particle sizes, which is much
larger than calcium carbonate. In addition, interfacial area
between UP resin and filler decreases with microsphere
content because of lager filler size as well as spherical
shape. Therefore, lower tensile strength of LPMC-2
indicates lower interfacial area among them. Disregarding
the shapes, generally the smaller the particle, the better the
reinforcement.
PHYSICAL PROPERTIES
Specific Gravity
The test results of specific gravities for LPMC-S, LPMC-1
and LPMC-2 are shown in Figure 1. Some differences
2.5
Sp. Gr.
2.0
1.95
1.82
1.31
1.5
1.0
0.5
0.0
LPMC-S
LPMC-1
LPMC-2
Fig. 1. Specific gravities for LPMC samples.
1.5
1.32
1.31
1.35
Tesile Strength(MPa)
SP. Gr.
2.0
1.33
1.0
0.5
0.0
1 Mpa
2 Mpa
4 Mpa
6 Mpa
Fig. 2. Specific gravities for LPMC-2 as a function of
molding pressure.
80
67
70
LPMC-S
LPMC-1
61
60
40
20
0
LPMC-2
Fig. 3. Tensile strength for LPMC samples.
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strength than LPMC-1 but has much similar impact
strength to LPMC-S. This is typical behavior in hollow
glass microspher filled system. It seems that the drop in
impact strength is due to the presence of microshere breakup on the notched edge acting as crack initiation sites.
Flexural Strength
200
170
172
160
Impact Strength(J/m)
Flexural Strength(MPa)
The test results of flexural strength for LPMC-S, LPMC-1
and LPMC-2 are shown in Figure 4. The trend is much
similar to tensile strength. Hollow microsphere filled
LPMC-2 shows lowest flexural strength among them.
160
120
80
40
0
LPMC-S
LPMC-1
1000
800
750
630
600
400
200
0
LPMC-S
LPMC-2
Fig. 4. Flexural strength for LPMC samples.
820
LPMC-1
LPMC-2
Fig. 6. Impact strength for LPMC samples.
THERMAL PROPERTY
All the samples showed heat distortion temperature(HDT)
higher than 250℃, which lies on maximum measuring
range in our equipment. This is probably acceptable heat
resistance for on-line painting. It is known that HDT of
MPPO/PA blend for thermoplastic fender is lower than
200℃. The higher heat resistance of LPMC could give
plastic body panels good on-line paintability.
Flexural Modulus
Flexural Modulus(GPa)
Flexural modulus for LPMC-S, LPMC-1 and LPMC-2 are
shown in Figure 5. LPMC-S shows the highest flexural
modulus. LPMC-2 has lower flexural modulus than
LPMC-1. It may come from the fact that reinforcing
efficiency of hollow glass microsphere is lower than
calcium carbonate. Reinforcing efficiency of filler in
LPMC is also dependent on filler content, type and size.
Lower modulus of LPMC-2 can also be explained by the
filler effect. That is, the modulus of LPMC decreases with
increasing glass microsphere volume fraction instead of
calcium carbonate.
1.5
1.20
1.13
1.0
MODELING AND FEA
Initial basic outlines for composite two-piece hood are as
follows :
1.01
0.5
-
Outside surface of the hood has to remain the same as
it is now
-
Inside “contact points” such as striker reinforcement,
hinges and overslam bumper also have to remain in
the same position
-
A shallow “egg-box” configuration is used with the
existing “contact points”. The design of the dimples is
very important. We need to get as little bond area
between inner and outer skins as possible while still
retaining the “sandwich” effect for stiffness and
strength
-
Outer panel has a small, downward flange that would
be used to complete the outer edge of the part and to
hide the outside joint condition.
0.0
LPMC-S
LPMC-1
LPMC-2
Fig. 5. Flexural modulus for LPMC samples.
Different thickness panels (either outer or inner) were
molded in their respective molds. The stops on each mold
were adjusted to create small changes in thickness of the
panels. The thinner panel thickness is the lighter the hood.
Therefore, a range of hood thickness/weight can be
molded and evaluated to determine the optimum weight
Notched Izod Impact Strength
Notched Izod impact strength for LPMC-S, LPMC-1 and
LPMC-2 are shown in Figure 6. LPMC-1 has higher
impact strength than LPMC-S. LPMC-2 has lower impact
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pronounced at torsion load condition. Bending strength of
steel hood is slightly better than LPMC Hood1, however,
torsion strength of steel hood is worse than Hood1. This
fact means that steel can be replaced by LPMC composite
without any significant loss of structural strength.
savings for this particular component. Both 2.0 mm and
2.5mm thickness hood panels were molded and bonded
together. FEA models for outer panel and inner panel are
shown in Figure 7 and 8.
Table 4. Displacement upon 10 kgf Load for Current Steel
Hood and LPMC Hood
FEA Analysis
Bending(mm)
Torsion(mm)
Steel(L/R)
0.96/0.96
2.16/2.16
Hood1(L/R)
1.24/1.24
3.26/3.26
Hood2(L/R)
0.90/0.90
2.37/2.37
Strength Test
Bending(mm)
Torsion(mm)
Steel(L/R)
1.7/1.9
14.0/14.0
Hood1(L/R)
2.1/2.4
10.4/10.1
Hood2(L/R)
1.9/1.8
5.9/5.4
Fig. 7. Outer panel model for FEA
Table 5. Weight Analysis for Current Steel Hood and
LPMC Hood
(unit ; g)
Steel
Fig. 8. Inner panel model for FEA
FEA analysis results for the hoods are summarized in
Table 4. Hood1 is composed of 2.0mm thickness inner and
outer panel and Hood2 is 2.5mm thickness panels.
Analysis procedure and test method are based on our
company’s specifications. Bending resistance upon
external load is higher than torsion resistance for all the
hood samples. When bending load is applied to the hoods,
Hood1 is more flexible than steel whereas Hood 2 is
slightly stiffer than steel. Hood1 has also weak resistance
to torsional load and Hood2 has almost same level of
torsion resistance. Actual test results for the prototype
hoods are also shown in Table 4. There are some
differences between FEA analysis and test results, that is,
true displacements upon bending and torsion load showed
higher values. Bending resistance of 2.0 mm or 2.5 mm
LPMC hoods are somewhat inferior to steel hood, on the
other hand, torsion resistance of LPMC hoods are superior
to steel one. Egg-box structure may contribute to the
increase in stiffness of hood assembly, which seems more
Hood1 Hood2
Inner Panel
4600
3280
3930
Outer Panel
3280
2370
2980
Sub Total
7880
5650
6910
Reinforcements
685
230
230
Adhesives
200
70
70
Total
8765
5950
7210
Weight analysis results for current steel hood assembly
and newly developed LPMC hoods with different panel
thickness are summarized in Table 5. When only panels
are considered 28% and 12% weight reduction are
obtained for Hood1 and Hood2, respectively. Two hinge
reinforcements and a striker reinforcement assembly are
contained in steel hood assembly. Those are cut and fitted
to LPMC hood inducing additional weight reduction. Steel
stamped hood needs sealer treatment with hemming
process for inner-outer panel assembly while LPMC hood
needs adhesive bonding with or without rivet. Total 32%
and 18% weight reduction are obtained for LPMC Hood1
and Hood2.
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CONCLUSION
Reformulated LPMC showed acceptable physical
properties for automotive body panels as compared to
standard LPMC. Addition of microsphere to LPMC
formulation is primarily aimed to decrease specific gravity,
leading to part weight reduction. It is noted that the
replacement of filler does not give significant effects on
physical properties of LPMC. Approximately 10%
decrease in physical properties was investigated. This may
imply that the glass microsphere is effectively dispersed in
matrix resin. Total 32% weight reduction was achieved in
the case of 2.0 mm thickness hood assembly due to the
application of newly developed low-density LPMC as well
as down-sized steel sub-parts. The comparison of the part
strength test results indicates that 2.0 mm thickness LPMC
hood assembly with design modifications could substitute
steel stamped hood.
REFERENCES
[1] Maxwell, J., Plastics in the Automotive Industry, p. 93103, Woodhead Publishing Ltd., 1994.
[2] Larson L. D., Robertson D. L., Ingham T. L., Shah V.
C., Botts T and Anderson R., SMC Glass
Microspheres as a Low Density Alternative to
Traditional Fillers, SAE 980982.
[3] Gregl B. V., Larson L. D., Sommer M. and Lemkie J.
R., Formulation Advancements in Hollow-Glass
Microspheres Filled SMC, SAE 1999-01-0980.
[4] James A. and Miller T., Performance Comparison of
Plastic Composites with Metals for Vertical Body
Panel Applications, SAE 1999-01-0848.
[5] Mallick P. K., 1988, Fiber Reinforced Plastics, p. 2334, Marcel Dekker.
[6] Atkins K. E., Seats R. L., Rex G. C., Reid C. G. and
Grandy R. C., Vertical Body Panels : Flexible Class A
Surface Composites Via Compression or Injection
Molding, SAE 920209.
[7] Margolis J.M., Advanced Thermoset Composites, p.
174-179, Van Nostrand Reinhold, 1986.
[8] Dick J.S., Compounding Materials for the Polymer
Industries, p. 58-63, Noyes Publications, 1987.
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