Dodge Intrepid and Dodge Viper: Why they switched from plastic to steel
by Bob Sheaves
One objective for the 1998 Concorde and Intrepid was to eliminate weight wherever possible; the body shells were stronger and stiffer than their predecessors, with added integrated side impact protection, but weighed little more. This was the result of lightweight materials and optimizing the structure, with extensive use of high-strength steels for areas affected by impact requirement.
One may ask, then, why Chrysler changed from lightweight, dent-resistant plastic fenders to steel fenders.
LXbuilder wrote: "On the plant level I can tell you those "plastic" LH fenders were a mess. They struggled in body shop to fit them and by the time they hit assembly alot of them needed refitting. Maybe the heat of the paint ovens caused some warpage, I don't remember if that was the problem or not. What I do remember was before we got the much nicer fitting steel fenders, you could drive a Neon through the fender to door gap."
Cost was a very small part of the change. Bigger effects were:
1. Lack of dimentional stability
2. Inability to hold consistant curvature
The LH front fenders were made with the RIM or Reaction In Mold process, which uses a temperature stabilized mold and a matte cloth (not woven cloth), laid into the heated mold halves. The halves are closed on a clamping fixture, and resin is injected throught the feed network in the mold.
The resin is a multiple part liquid which cures in the mold. When the mold is released, the objective is to have all the resin uniformly cured, so the release pin (used to eject the part) does not damage or deform the molded part. This hot part actually has a number of soft spots and hard spots in the part, causing the part, when fully cured, to warp and have internal stresses.
RIM part processing was invented by General Motors
for the Pontiac Fiero. It was a fast and relatively simple process at the time, but it was almost immediately shown to have flaws (which also affected the LH fenders and other parts) which constantly slowed down the assembly line.
Parts would not be uniformly cured, leading to warps and discontinuities in the sight line of the parts causing immediate part rejects. At assembly on the space frame PLP's, this warpage caused panel misalignment and "out of level" conditions. OOL conditions are when 2 panels are not tangent and contigous in their curvature. Think of a door that does not align the skin to the front fender when completely latched. This is the #2 item from the previous list.
Item #1 is, to reiterate, a lack of dimensional stability: the part changes shape as the temperature and humidity changes. Think of this problem as series of clamps (the PLP's) that hold the part in a fixed place on the spaceframe. Now, due to the difference in the co-effecient of expansion between the RIM plstic and the steel or other metal substructure, as the day warms up, the plastic is going to expand more than the metal structure, but it does not have anywhere to expand to. The panel has to buckle...which the "Dustbuster" minivans and Fiero were known for. In the U.S. Southwest, a black Fiero would grow almost 2 inches in length on a 110 degree day. All the plastic body panels would warp so badly that many hundreds of thousands in warrenty costs were spent before GM discovered the problem and stopped paying the warranty on warped body panels.
As a sidebar ...the largest RIM panel in the automotive industry was the hood for the first generation Viper. This part was so bad that the manufacturer rejected 1 out of 3 parts before
shipping to the plant for assembly. This was so bad for quality control, it was eventually changed to a smaller multi-piece design to try to provide some control over the problems.
The reason I used the Corvette and the Ferrari 308 was because neither of these cars (nor the Trabant) were affected to the degree of literally every
other attempt at using any kind of plastic. The 308 was available in both steel bodies and reinforced fiberglass bodies; the steel bodies are the more valuable, the more fiberglass bodies were built. The Corvette or Ferrari fiberglass bodies were not without their problems.
Regarding the first carbon fibre and Kevlar bodied vehicle ever put into production, the Consulier GTP
, take a close
look at the waves and warps in that black car (specifically the front picture). It had never been in an accident...all the waves are from simple heat and cooling cycling. There was no structural metal
in that body at all.
In the Consulier, foam coring used between the layers of S-glass, E-glass (2 types of fibreglass clot, roving, and matte) distorted under the heat of the sun and introduced unacceptable waves in the body surface. As this heating during the day, and cooling during the night (heat cycling) warped the originally smooth mold surfaces, the composite structure "cooked" into a contour that was far from the original, intended contour. Corvette also had the issue in the early days (1953-1982). (Consulier also used Kevlar and carbon fibre in the structural parts of the body, along with epoxy resins and polyester resins.)
The only option that can eliminate this warpage is to "cook" the panels in an autoclave, until the composite material matrix (which is what moves around) is stabilized. This is the process used by the production aerospace companies to fix the issue. Homebuilt aircraftparts do
warp and get out of shape when they are not autoclaved. Same with boats....look down the hull of a fiberglass $5 million dollar hull that was built more than 3 years ago. The hull will be as warped as the rolling hills of Kentucky (yes I am exaggerating for effect). Unless the composite parts are cured by heat cycling in the molds, under pressure in a male and female mold set, or under vacuum in a single side mold set, the parts are going to warp. Which brings up point 2..
The cycle time for a current body side aperture, one of the largest body stampings, is about 30 seconds for one panel. To completely stamp out all the parts for an old LHS is a total of just under 37 minutes. Now- this is only
to make the parts, no dies setup time, no maintenance, no bad parts, no nothing out of the ordinary.
By comparison, the Corvette body panels take approximately 49 minutes to cycle, but
the Corvette body is not monocoque, so there is additional time required to stamp the various metal substructure components.
The Consulier, for all its warts, was
a composite monocoque and it took the Rivera Beach body shop between 4-5 days
to manufacture just 1 body shell comparible to the LHS in the original example.
Boeing's 777, which has composite wings which total almost the same surface area in their molds, takes almost 47 hours
to make 1 wing (not both, just one) using an autoclave to cure the parts to maintain their shape.
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