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TPUs are surfacing in automotive exteriors

March 27, 1999

7 Min Read
TPUs are surfacing in automotive exteriors

As the light truck market takes off, so do the vehicles' aesthetic requirements. Ford's popular F-series and Ranger trucks feature these ditch moldings supplied by Guardian Automotive and molded from Estaloc 60155 RTPU.

It doesn't take much observation to conclude that car exteriors have changed dramatically from the days when chromed trim dominated. Today's sleek body styles are complemented by molded body-color trim, rocker panels, cladding, and even grilles. While these applications have traditionally been the domain of lower-cost plastic materials, two factors are changing that picture.

Consumers are demanding better aesthetics, and OEMs want better dimensional stability to correspond with narrower gaps and tighter designs. Thermoplastic polyurethanes (TPUs) are increasingly selected as the answer, according to Jerry Blayne, manager of applications engineering at BFGoodrich Estane TPU Div. (Cleveland), the largest global supplier of TPUs (Estane) and reinforced, alloyed RTPUs (Estaloc).

Urethanes come in several forms-adhesives, paint, foam, casting, sheet, extrusion-and are used in many applications because they have a wide variety of properties. TPUs, the molding compounds, are no exception. Traditionally this material hasn't been a heavy favorite in automotive because of competitive pricing situations-TPU is at the high end of the cost/performance scale compared to competing thermoplastics. However, Blayne explains, "Incoming resin prices are becoming less important now than the total cost or out-the-door finished piece cost for most OEMs and Tier Ones. TPU typically runs faster than competitive materials, and yields are better. These resins also have low viscosity and can fill 6-ft-long parts without difficulty." The end result is they are more cost competitive and are, again, becoming the material of choice for demanding applications.

Design Freedom
Most importantly, TPUs tend to have low, controllable shrinkage rates and, with RTPUs, excellent dimensional stability over time. Of course, dimensional stability is a big issue with large car parts that have to fit exactly. Some RTPU compounds have the same coefficient of linear thermal expansion as sheet metal, which helps maintain the fit on door pieces from Alaska to Phoenix.

Because of low shrinkage rates, TPUs can handle different thickness sections for fit and style purposes. Parts that are impossible to mold can be done in TPU. In the area of body side cladding, which often combines thick and thin sections in one piece, non-TPU parts shrink and expand significantly because of their lower dimensional stability while RTPUs are unaffected. Blayne confirms he is currently working with a supplier on production parts with a nominal 3-mm wall and an adjacent 10-mm wall. "The RTPU parts have no sink marks. One of the areas in which TPU shines is this design freedom. With other materials, you can inject gas to eliminate sinks, but part yields go down and expense goes up," he adds.

Painless Painting
According to Blayne, painted parts represent some of the most exacting molding requirements in the industry because even a minor defect is noticeable on a high-gloss, painted exterior body part. "And there is no texturing allowed, obviously, when you are trying to achieve a Class A surface." The absence of sink marks is a plus for TPU.

In addition, with other materials, additives are required to make the resin accept paint and make it stick. "These are two totally different chemistries that are not compatible, so additives and primers are a necessity," adds Blayne. Not so with TPUs. "Paint is a urethane and adheres to TPU very well. No primers, adhesion promoters, or additives are required," he says. By cutting out the primer step in the production process, TPU's overall per part cost is further reduced.

Also, as a result of the tremendous adhesion between a urethane paint and a TPU substrate, the parts are more resistant to paint chips. Damage resistance is superior as well. Blayne explains, "TPU chemistry allows for rubber-like properties even at very low temperatures. This elastomeric backbone imparts durability that is well suited to damage-prone applications. Overall, these materials excel in abrasion and impact resistance."

Tier One supplier Plastcoat produced these body side moldings for the Mercury Grand Marquis from BFGoodrich Estaloc 59010, a reinforced TPU. More Tier Ones and OEMs are turning to TPU for body-color trim because the material accepts paint without needing any intermediate steps. Other reasons for the switch include dimensional stability over a broad temperature range and design freedom.

Flexibility and Appearance
Automotive applications for TPUs are growing, especially in luxury-class models, according to Blayne. They are used to make rocker panels, cladding, body side trim, and ditch covers.

"The area where the roof joins the side of the vehicle, called the ditch, used to be spot welded, filled with filler, and painted," he says, "a very labor intensive process. Today, entire sides are stamped out of one sheet of metal. Some truck models simply weld the side of the car to the roof. Low-cost, low-aesthetic plastic strips are used to cover many of these ditches."

Because this area of the car appears at eye level when standing next to a vehicle, automakers looking for better aesthetics on this high appearance part turned to TPU. "Our customers would like the ditch cover to look more like the vehicle," adds Blayne. "Urethane is ideal because it is flexible and paints well. Also, it has the ability to conform to the ditch, which varies in width and depth because sheet metal tolerances are not exact."

Dos and don't for TPUs

Jerry Blayne often conducts training seminars for customers to introduce them to the right part design, mold design, and processing parameters for TPUs. "Many molders and designers are not familiar with the material," he tells IMM, "so education is an important component in successfully molding parts. Also, the most efficient jobs usually result from early teamwork between part purchaser, molder, toolmaker, and material supplier. In turn, it helps if all of these team members have the same information." Here are a few of the main guidelines that Blayne recommends.

1. Design the part and mold with TPU in mind. No one set of part and mold design guidelines can begin to cover the various applications in which TPU is used because its physical properties span the range from soft, highly rubbery materials to rigid, engineering-grade plastics. Get guidance on part design and mold construction from your material supplier early in the game-before tooling is complete-to avoid costly mistakes.

2. Properly dry TPU materials. TPUs, like other plastics, are hygroscopic and will absorb moisture from the environment. They should be dried to target moisture content of .02 percent or less. Poor material drying is the number one cause of emergency technical service visits-excessive moisture can cause processing problems such as sticking or sinks and can adversely affect physical properties. Remember also the absence of splay is no guarantee the material is properly dried. Rely on correctly sized, well maintained drying equipment.

3. Vent the mold. Long fill lengths, large volume parts, and fast fill rates are a fact of modern injection molding. Poor mold venting is the most common mold defect found in the field. It is also the root cause of many processing problems. Semicrystalline TPU has excellent shear and temperature stability. Its low melt viscosity and quick solidification enables large parts with long fill lengths to fill quickly, often through one small gate. A poorly vented tool can negate these advantages.

4. Pay attention to part ejection details. Soft, elastomeric TPU grades require particular attention to mold design, both to insure part retention on the ejector half of the mold and to eject parts without damage or extensive distortion. Draft angles, surface finishes, air blow off, part pullers, and other know-how details often make the difference between smooth production and costly overtime. In short, make sure the mold design matches the material.

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