Pieces on plastics: here are some things that you should know regarding useful development in the plastics arena

Automotive Design & Production, Feb, 2003 by Gary S. Vasilash

MINIMIZING MICROCRACKS

Although sheet molded composite (SMC) plastic is finding not-insignificant use in vehicle applications, including for Class A surfaces, Michael F. Dorney, vice president, Sales & Marketing, The Budd Company-Plastics Div. (Troy, Ml), says, "We've hit a ceiling for SMC applications." At least so far as the Class A jobs go. When it comes to the other applications--valve covers, skid plates, etc.--there is no roof. But Dorney goes on to explain that they've pretty much busted through that ceiling thanks to work that was done by Probir K. Guha, manager, R&D for Budd Plastics, as well as people from AOC (Collierville, TN), which, according to Michael H. Dettre, AOC's business manager, Closed Mold Resins, is the largest resin supplier in North America.

Consider the Ford Thunderbird. Certainly a car with classic, flowing lines. But the last thing that a customer would tolerate would be what's known in the business as an "edge pop." Odds are, a vehicle with edge pop would not make it out of the Wixom Assembly Plant. It is an issue that's addressed in the plant. It's an issue that means that first-time throughput numbers aren't what they should be. And it all comes down to the use of SMC.

The Thunderbird uses SMC for its hood (as well as its fenders and decklid). Dettre explains that a typical SMC consists of 25% resin, 25% glass reinforcement, 45% calcium carbonate, and 5% "minor" ingredients. Fundamentally, a company (in this case, Budd) takes the material and places it in a chromed steel toolset and molds it at 300[degrees] F and at 1,200 psi. At its Kendallville, Indiana, plant Budd primes the T-Bird hoods and sends them to Wixom. At Wixom, the SMC panels are assembled right along with the steel body panels, and like the steel, are put through e-coat, primer surfacer, base coat, and clear coat. After the vehicles leave the oven, edge pop sometimes occurs.

Essentially, edge pop is what it sounds like. Simply, there is a bubble that comes up through the paint and pops on the surface, leaving a round dimple (or crater, so far as those who are concerned with Class A surfaces go). Typically, these edge pops are near, well, the edge of the surface. Guha explains that this is a result of the molding process. As mentioned, there's glass reinforcement. Generally, the glass fibers are about 1-in, long. During molding, the fibers don't propagate to the edges as well as they do throughout the rest of the surfaces, to say nothing of the fact that when you consider that the edge is "folded," there are stress risers created in that vicinity.

Now, edge pops are nothing new. And Guha says that they've been working on solving them for about the last five years: "We've done DOE on DOE, Taguchi on Taguchi." They evaluated all of the parameters (e.g., the material constituents; the settings on the equipment). They instituted best practices. And over time, they made incremental improvements. Still, there were problems with popping. They cataloged over 500 different types of defects. They did a Pareto analysis of the defects. They determined that microcracks were responsible for more than 70% of the popping. What happens is that the solvents in the color and clear coat were going through the surface of the primer, and collecting in these micro-cracks. Then, when the vehicle was put through the oven, solvents would vaporize and reemerge with a pop!

Dorney explains that the reason for the aforementioned ceiling on the implementation of SMC for body panels is that the assembly plants would be overwhelmed with rework if they were to have to deal with still more panels with pop. Clearly, for a company like Budd that wants to sell SMC panels (and which also, curiously enough, sells material to competitors: "We have some overcapacity in our compounding plant," Dorney explains, "so if we don't get the job and a competitor does, then at least we're still getting something"), popping is an intolerable problem.

One of the things that they hadn't investigated was the matrix alone. That is, they figured that the problem could be resolved by doing things like adjusting the amount of glass put in the matrix. But Guha says that they came up with a test that permitted them to actually measure the toughness of the material. Consequently, they discovered that the way to minimize microcracks was to create a tougher matrix, a material that would be more resistant to cracking. (There are other alternatives that have been developed, such as employing a UV-cured sealer in place of the conventional primer but that, Dettre notes, involves additional costs to the process and special paint lines). While they were at it, formulating a tougher material, engineers from Budd and AOC worked at developing a material that also exhibited a 50% reduction in its surface waviness index (i.e., it is smoother).

As a result of this work, a new resin, Atryl TCA, was developed and is available from AOC. This material is more resistant to microcracking during various steps along the way, from demolding to material handling. The material is said to achieve the improvement in surface waviness and to be 69% tougher than traditional SMC resins. They've determined that there is a 90% reduction in paint pops.