Material And Design Guidelines For Reinforced ...

SOCIETY OF AUTOMOTIVE ENGINEERS, INC.
Two Pennsylvania Plaza, New York, NY. 10001

Material And Design
Guidelines For

Reinforced Thermoplastic
Body Panels

C.R. Grimball and N.J. Jackson
Guide Lamp Div. .General Motors Corp.

SOCIETY OF AUTOMOTIVE ENGINEERS

Automotive Engineering Congress 740262 wm
%\ Detroit, Mich.
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February 25-March 1, 1974
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ABSTRACT MATERIAL AND DESIGN GUIDELINES FOR
REINFORCED THERMOPLASTIC
This paper describes new reinforced BODY PANELS
thermoplastic materials that have been
developed for rigid exterior, painted auto- C. R. Grimball and N. J. Jackson
mobile panels. Traditionally these panels Guide Lamp Div., General Motors Corp.
have been fabricated from zinc die cast,
steel, or more recently from sheet molding
compound (SMC). Guide Lamp Division of
General Motors is injection molding mineral
filled nylon, fiber glass reinforced nylon,
and fiber glass reinforced thermoplastic
polyester rigid exterior automotive panels
that are painted car color for 1974 Chevelles,
Cadillacs, Pontiacs, Oldsmobiles and Buicks.
The applications, material properties, paint
performance, and design recommendations
for these three reinforced thermoplastic
materials are reviewed.

Guide Lamp Division of General Motors and characteristics similar to sheet molding
has produced exterior automotive panels and compound (SMC) and bulk molding compound
fender extensions that are painted car color (BMC), materials previously used for exterior
from die cast zinc, sheet molding compound, automotive panels. These reinforced thermo-
or bulk molding compound (polyester premix) plastic materials were introduced for the first
for several years. A program was begun in time as exterior, car color body panel mate-
1964 at Guide Lamp to develop injection rials for the 1973 and 1974 car models. Guide
molded thermoplastics for these types of Lamp has been a major producer of reinforced
applications, with initial concentration pri- thermoplastic automotive components for the
marily on nylon and acetal materials. The past several years--using over 13 million
program concentration shifted in 1969 to pounds of reinforced thermoplastics per year
fiber glass reinforced and mineral-filled for lamp housings, consoles, instrument panel
nylons. The study of fiber glass reinforced inserts, and fan shrouds. This extensive en-
thermoplastic polyesters for these applica- gineering and production background and expe-
tions was also begun in 1972. A number of rience with reinforced thermoplastics was
experimental tools were built for applica- important and essential to the successful intro-
tion development, and thousands of parts duction of these major, new body applications.
were molded and tested. Some of the dif-
ferent types of experimental parts are Fiberglass filled nylon was used as a
included in Figure 1. painted exterior body panel material for the
first time for the 1973 Oldsmobile Omega
Laboratory and service testing indicated rear panel as pictured in Figure 2. Mineral
that fiber glass reinforced nylon, mineral- filled nylon was used as a painted exterior
filled nylon, and fiber glass reinforced body panel material for the first time for the
thermoplastic polyester each have properties 1973-1/2 and 1974 Buick Apollo rear panel and

rear fender extensions (Figure 2). Fiber- temperatures of up to
glass reinforced thermoplastic polyester was 350°F for curing and
used for painted exterior rear body panels reflowing exterior
for the first time in the 1974 Pontiac LeMans acrylic lacquers without
and Catalina-Bonneville rear license pocket part distortion.
panels and the 1974 Chevelle-Laguna rear
panel (Figure 3). The replacement of die High rigidity - Must be able to support self and
cast metals and steel in exterior body panels components without dis-
by these engineering reinforced thermo- tortion. Must not distort
plastics has been due to a combination of the in paint bake ovens up to
following properties—ease of processing, 350°F.
limited finishing steps, reduction of com-
ponent parts, cost reduction, weight reduc- Good surface finish - Must have good surface
tion, and broad styling capabilities. without defects as
molded, minimal sinks
Reinforced nylon and thermoplastic poly- opposite bosses or ribs
ester materials will be reviewed in this paper on appearance surfaces,
in terms of their physical properties, appli-
cations, design criteria, and paintability for Good paint adhesion - Excellent paint adhesion
exterior body panels. must not exude internal
lubricants to the surface
MATERIALS AND APPLICATIONS that could affect the
paint.
A. MATERIAL PROPERTY REQUIREMENTS

The following property requirements are B. COMPARATIVE PROPERTIES OF EXTERIOR
necessary for plastic materials for painted BODY PANEL MATERIALS
exterior body panels:
The mechanical properties of zinc die cast,
High tensile strength Hold fasteners, with- polyester premix or bulk molding compound,
stand assembly han- mineral filled nylon, fiberglass reinforced
dling, support other nylon, and fiberglass reinforced thermoplastic
components such as polyester are listed in Table 1. All of these
lenses and lamp materials are being used for car color painted
bezels. exterior body panels. Zinc die cast is much
stronger and more rigid than any of the engine-
High flexural strength - Must withstand ering plastics. However, the mineral filled
severe flexing in nylon, glass filled nylon, and thermoplastic
assembly and across- polyesters have been proven by extensive
car torsional flexing testing to have adequate properties for exterior
and impacts in service. body panels and surpass other exterior body
Parts must bend and materials in many ways. The reinforced
conform to mating thermoplastics are more flexible and ductile
sheet metal. than other body panel materials and bend to
conform to mating sheet metal. Flash free,
Good impact Must withstand assembly, defect free parts are injection molded that
rock impacts, field require no trimming or repair. These mate-
rials permit fast molding cycles and automatic
abuse. machine operation. They are lower cost than
other body panel approaches, particularly in
High heat distortion - Must withstand oven

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larger, more complicated parts. The thermo- the 1973-1974 Oldsmobile Omega rear applique
plastic materials do not corrode. Part weights panel (Figure 2). As can be seen from Table 1,
with these materials are reduced by 60 per cent this material has excellent tensile and flexural
versus zinc die cast and 20 per cent versus bulk strengths as compared to bulk molding com-
molding compound or sheet molding compound. pound. Bulk molding compound has been used for
These attributes made these engineering thermo- painted exterior fender extensions and headlamp
plastics important and definitely advantageous housings in previous production years. Rein-
over other body panel materials. forced nylon has excellent rigidity and good re-

TABLE 1 - Comparative Mechanical Properties of Exterior Body Panel Materials

Mechanical Test Zinc Bulk Molding Mineral Glass Glass
Property Method Die Cast Compound or Filled Filled Filled
SAE 903 Polyester Nylon Nylon Thermo-
Pre-mix plastic
Polyester

Tensile strength ASTM-D688-64 41,000 5,000 11,000 11,000 17,000
(Psi) (2)

Flexural strength ASTM-D790-66 12,000 22,000 20,000 27,000

(Psi) (2)

Flexural Modulus ASTM-D790-66 10,000,000 1,000,000 750,000 790,000 1,200,000

(Psi) (2)

Unnotched Izod ASTM-D256-56 43 ft. -lb. 4-6 20 15
(Charpy)
Impact (2)

Ft. lb. /in. ,+75°F

Heat Distortion ASTM-D648-56 Over 500°F 400°F 275 °F 370 °F 420 °F

Temp., °F, 66Psi (2)

Specific Gravity ASTM-D-792 6.6 1.8 1.47 1.25 1.52

Fiber Content-% 20% 40% 15% 30%
(fiber- (mineral) (fiber- (fiber-

glass) glass) glass)

Material Cost- 7.85 1.68 3.70 2.25 3.85
Cents/cubic inch

C. FIBERGLASS FILLED NYLON sistance to part distortion at up to 350°F. Its
impact performance is good in the dry state
Fiberglass reinforced nylon 6 was the first and excellent after absorbing moisture. Nylons
reinforced thermoplastic material to be used develop more ductility or flexibility and tough-
for a painted exterior automobile body panel-- ness as they absorb moisture--a phenomenon

that occurs in part service. In the dry-as- state. Mineral filled nylon also absorbs water
molded state, glass reinforced nylon is some- and becomes more flexible. These factors
what notch sensitive or will break in sharp should be considered in part design.
corner areas. Proper part design can alle-
viate notched conditions. Part length is not TABLE 2 - Sink Measurement Opposite A
significantly affected by moisture absorption. Mounting Boss of Various Body
Appearance sinks opposite mounting bosses Panel Materials
can also be a problem. This is not a problem
in the Omega rear panel because all mounting Materials Sink Depth (Mils)
bosses are designed opposite non-appearance
surfaces. This is the lowest cost material 1. 40% mineral filled nylon 4.2
of the three reinforced thermoplastic materials
reviewed. 2. 15% glass filled nylon 17.0

D. MINERAL FILLED NYLON 3. 30% glass filled thermo- 13.5

Guide Lamp is injection molding mineral plastic polyester

filled nylon in three applications for 1974: glass filled nylon 11.5
(1) 1974 Cadillac Calais, Brougham, DeVille

front fender extensions, (2) 1973-1/2, 1974 Readings were made on a precision "contour
Buick Apollo rear panel, and (3) 1973-1/2, reader" on a 20X magnification. The boss
1974 Apollo rear fender extensions (Figure 2). size was for a 1/4-20 fastener.
The 1973-1/2 Apollo applications were the

first exterior automotive applications of min-

eral filled nylon. E. FIBERGLASS FILLED THERMOPLASTIC
This engineering thermoplastic was chosen POLYESTER

for the Cadillac front fender extensions and

the Buick components because it exhibits, as

shown in Table 1, high tensile strength, high Guide Lamp is injection molding three exte-

rigidity, high unnotched impact strength, and rior rear panels in 1974 from fiberglass filled
adequate heat distortion resistance. Although thermoplastic polyester: (1) 1974 Pontiac LeMans

its heat distortion in Table 1 is listed as 275°F, rear license panel, (2) 1974 Pontiac Catalina -

this material will pass through paint reflow Bonneville rear license panel, and (3) 1974 Chev-

oven temperatures of up to 350. F without dis- rolet Laguna rear body panel(twopieces)(Figure3).

tortion, if processed without stress and sup- The reinforced thermoplastic polyester mate-

ported properly. A further consideration for rial is the strongest, most rigid and highest heat

having chosen this material was its ductility resistant of the plastic materials listed in Table 1.

and flexibility. This material has the least It is not affected by moisture, as is nylon. It is,

sink opposite mounting bosses or appearance therefore, the best of the materials being con-

surfaces of the three materials reviewed in sidered in structural strength and is also the

this paper, as indicated in Table 2. It also most costly. Problems can exist with appearance

has the least warpage tendency in molded parts sinks opposite mounting bosses and part warpage

of the three materials discussed. These are as molded, if the part and tool are not properly

the reasons that it was selected for the cur- designed.

rent 1974 production applications.
The main limitation in using mineral filled DESIGN CRITERIA

nylon is its notch sensitivity--or tendency to The reinforced nylons and thermoplastic
break at sharp corners in the dry, as molded

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3T MINIMUM RADIUS:
MINIMUM

RADIUS:
.030" MINIMUM

3-T MINIMUM-J

FIG. 4 - Transition Area Geometry for (A) wall thickness changes and (B) boss areas.
A gradual transition length of three times the wall thickness should be used
with a minimum radii of .030".

FILLETS FILLETS-

NORMAL WALL NORMAL WALL
xr~7 THICKNESS THICKNESS

FIG. 5 -Design locations to help minimize sinks opposite bosses. Bosses should be
located opposite the apex of angles where the surface changes abruptly.

polyesters have proven their practicability in fail or break first in sharp corners or notched

existing automotive applications. Each mate- areas. Sharp corners in part should be elim-

rial offers unique combinations of properties inated by use of radii and fillets. Internal

to give the essential performance requirements corners should have fillets of .025" to .075"

needed in automotive body panel applications. and external corners should have radii of .015"

The design engineer should be familiar with to .040" which are adequate for most body panel

these combinations of properties in order to applications. This will.reduce the effect of

select the best material for a specific applica - stress concentrations at sharp corners.

tion. In this way, he can produce a high Minimal sinks opposite bosses will result

quality part at minimum cost. The most im- with the correct design of parts with filled

portant factors in designing plastic body panels nylon or thermoplastic polyesters. In general,

are wall thickness, ribs, radii and fillets, the boss height should not be more than twice

bosses and studs, attachments, and design for the boss diameter. A generous radius or fillet

minimum warpage. These will be discussed should be provided where the boss joins the wall

in detail. section. The bosses should be located at the

Wall thickness for automotive body panels apex of angles where the surface contour of the

should be kept uniform and as thin as is pos- part changes abruptly, as shown in Figure 5.

sible as dictated by the requirements of the This will help hide any sinks opposite bosses

part. A nominal wall thickness of .090"-. 110" present.

is most desirable for use with the filled nylons

and thermoplastic polyesters. Thicknesses The mineral filled nylon, as is indicated in

below .050-. 060 inch are difficult to fill in Table 2, gives the least sink opposite mounting

molding. Where changes in wall thickness bosses of the reinforced chermoplastic materials,

are required by part design, a gradual trans- Another method of eliminating sinks opposite

ition should be used between thick and thin bosses in body panel applications where aesthetics

sections, as shown in Figure 4. are of prime concern is by means of add-on bosses.

This type of separate boss could be fabricated

Ribs are designed into automotive body from zinc die cast, steel, or plastic, such as

panels to strengthen parts without an increase shown in Figure 6. The bosses are then

in wall thickness. This provides additional attached to the body panel by means of adhesives,

strength and rigidity with minimal increase in sonic welding or metal fasteners.

weight and cost. Ribs may be added to pre-

vent part warpage. Rib thickness should be A third method of minimizing sinks opposite

limited to a maximum of 50% of the wall mounting bosses is to core out or eliminate mass

thickness of the adjacent wall because thicker from the base of the boss as is shown in Figure 7.

ribs will cause sink marks on appearance

surfaces of body panels. Nominal wall thick- In body panel applications, mechanical as-

ness may have to be locally increased in the sembly to other parts is required. Screw and

rib areas to minimize sinks. A .010"-. 020" stud stripping torque performance for plastic

radius should be located at the intersection of body panel materials are listed in Table 3.

the rib and the adjacent wall. Ribs should Proper pilot hole and boss diameters are

conform to exterior surface contour and rib important in order to obtain maximum torques

height should be as short as possible. A 1° with filled resins. The pilot hole is molded into

draft angle should be used for all ribs so the body panel parts. The pilot hole size should

that they will release from the die. be maintained within a tolerance of+.001" for

Filled nylons and thermoplastic polyester the best fastening properties. Boss diameter

resins are notch sensitive materials. These must be of the correct size.

materials, under high stress conditions, will Undersized boss diameters may crack or

TABLE 3 - Screw and Stud Stripping Torque Performance
of Thermoplastic Body Panel Materials

Screw or Minimum Polyester Fiberglass Fiberglass Mineral
Stud Size Requirements Premix Reinforced Reinforced Reinforced
Thermoplastic Nylon
In. -Lbs. In. -Lbs. Nylon Polyester In. -Lbs.
In. -Lbs. In. -Lbs.

No. 8-18 25 25 40- 45 40- 45 35- 45
Hi-Lo Shank
80 80-100 90-120 100-140 100-120
1/4" - 20
Hi-Lo Shank 80 100-150 90-120 105-140 80-110

9/32 - 16
Hi-Lo Shank

break during driving or torqueing. Oversized plastic body panels must match the finish of the
bosses will add weight, increase surface sinks adjacent sheet metal body parts. With proper
opposite bosses, and increase molding cycles. mold finish and molding conditions, each of
these three materials will give excellent sur-
Reinforced nylons and thermoplastic poly-

esters are all prone to warpage as molded. face finish and defect free parts as molded.

Mineral filled nylon gives the least warpage, The filled nylons and thermoplastic poly-

and glass filled polyester causes the greatest esters require special primer systems so that

problems. Warpage may be prevented by paint adhesion may be obtained to the substrate

selecting the best gate location and correct materials. These primers are formulated to

gate size. Parts should be as uniform in wall accept thermoplastic acrylic topcoat lacquers

thickness as is possible by design. Generous used on all General Motors automotive sheet

radii should be provided in all sharp corners metal. The primer-topcoat systems undergo

to-minimize differential shrinkage problems. certain performance testing before approved
Incorrect processing of filled thermoplastic for production use. Laboratory paint system

resins such as improper mold temperatures, test requirements are listed in Table 4.

may also lead to warpage problems. Correct With the proper primers and processing,

pressures in molding are also required to car color painted glass filled nylon, mineral

mold quality, defect free parts. filled nylon, and glass filled thermoplastic

/ PAINTABILITY OF BODY PANEL MATERIALS polyester have passed all of the required labora-
tory tests. Excellent paint adhesion and dura-

bility performance comparable to steel have

Paintability of glass filled nylon, thermo- also been obtained after hundreds of thousands

plastic polyester, ^nd mineral filled nylon is of miles of service testing on test cars and one

an important factor in body panel applications. year exposures of painted panels in Florida

The finished painted surface of the thermo- and Arizona.

FIG. 6 -Separate metal mounting bosses attached
to reinforced thermoplastic panels by
screws give no sink opposite bosses.

FIG. 7 -Coring on the 1974 Chevrolet Chevelle-
Laguna rear panel to eliminate sinks
opposite bosses.

FIG. 8 - Mounting of prime-painted reinforced
thermoplastic body panels to steel
body. Body and primed panel have
been painted car color in the positions
indicated and processed through 300-
350CF paint bake ovens. The plastic
part on the right is supported loosely
at the studs and bosses (arrow) and
permitted to float in elongated holes in
the rear body sheet metal panel on the
left in the paint bake ovens.

TABLE 4 - Paint System Test Requirements for
Thermoplastic Body Panels

Type of Test Test Test Description Test Result Requirements
Procedure 100% R. H. at 100°F No loss of adhesion
96 hours
humidity GM-4465-P

2 hours GM-4466-P Water immersion at 100°F No loss of adhesion
water immersion
GM-4298-D Neutral salt spray test at No loss of adhesion
336 hours 95°F-100°F
salt mist SAE J400 Fair rating - 7 minimum
at 0°F Chip resistance of on 10 scale
Gravelometer GM-9071-P coating No loss of adhesion

Tape adhesion —

In the paint processing of plastic body tended exposures to paint oven temperatures
panels, certain requirements are necessary because of better heat stability.
to obtain qual ity painted parts. Paint racking
of plastic body panels is a very important SUMMARY
factor. The plastic body panels have a much
higher coefficient of thermal expansion than 1. Guide Lamp has invested years of
steel. When the parts are exposed to oven materials and product development and has
paint bake temperatures of up to 350°F, the had extensive previous production experience
reinforced thermoplastic panels expand up to with reinforced thermoplastics. This back-
. 5 inch in length greater than steel on an ground was necessary to extend reinforced
across-car panel and may permanently warp thermoplastic materials usage for the first
or distort if attached firmly to the steel body. time into rigid exterior body panel applica-
To prevent this warpage, the part must be tions that are painted car color.
racked in a manner so that it is free to float
on the paint rack. This type of floating paint 2. The specific strength and heat resistance
racking is shown in Figure 8. of filled nylon and thermoplastic polyesters have
enabled these resins to replace such materials
The filled nylon and thermoplastic poly- as steel, zinc die cast, and sheet molding com-
esters are able to withstand prime bake oven pound in body panel applications. Cost savings,
temperatures of 300 °F and topcoat and reflow weight reduction, and added corrosion resist-
temperatures of 350 °F for one hour. The time ance have been gained.
in the paint bake ovens should be minimized
for reinforced nylon materials. Over exposure 3. Mineral filled nylons have the least sink
to bake oven temperatures may cause filled opposite bosses of the thermoplastic resins.
nylon materials to degrade. The filled thermo- Sinks may also be eliminated by means of add-
plastic polyester resin will not degrade at ex- on bosses or reduced by cored-out thick areas.
Proper design is essential to obtain suitable

the least warpage as molded of the three re-
inforced thermoplastics discussed.

4. Glass filled nylon is the lowest cost
and lowest specific gravity or weight material
of the three materials in use.

5. Glass-filled thermoplastic polyester
has the highest structural strength and rigidity
of the materials reviewed.

6. Reinforced nylons and thermoplastic
polyesters are painted with standard acrylic
thermoplastic topcoats after priming. Proper
primers and processing will give excellent
paint adhesion and durability--equivalent to
car color painted steel.

7. Proper part design is necessary to give
successful filled thermoplastic polyester and
nylon exterior body panels. These materials
have been proven satisfactory by their success-
ful use without any field service problems for
1973 or 1974 Chevelle, Cadillac, Oldsmobile,
Buick and Pontiac car color, exterior body
panels.

REFERENCES

1. "Celanex Design Manual" Summit, N.J.,
Celanese Plastics Co., Publication SBN-72-224
<1972), pages 2-4, 11, 17.

2. "Cost/Performance-Winning Formula For
Zinc Over Plastics" SDCE International Die
Cast Congress Transactions, 1972, Paper3172,
D. C. Herrschaft, L.S. Lazar, F. Pepe.

3. "Development of Plastic Lamp Housings"
SAE Transactions, Vol. (1970), Paper 70018,
F. W. Johnson.

4. "Injection Molding of Valox" Pittsfield,
Mass., General Electric Plastics Department,
pages 3-13.

5. "Vydyne R-Mineral Reinforced Poly-
amide Engineering Data Manual" St. Louis,
Mo., Monsanto Polymers & Petrochemical Co.,
Publication No. 6331, pages 8-20.