Pieces on Plastics

Gary S. Vasilash

Here are some things that you should know regarding useful developments in the plastics arena.

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, MI), 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° 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 microcracks. 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.

Dorney says that he thinks that this new material should help increase the number of exterior applications for vehicles in the range of up to 150,000 units per year.

Done In One

Although the name of the TRW Automotive operation is “Engineered Fasteners & Components,” Ken Kernen, product line group manager, North America, within the operation says, “Injection molding is a core competency.” So this is not just about nuts and bolts. Rick Schmidt amplifies Kernen’s observation by noting that one of the areas that the Farmington Hills, Michigan-based group specializes in is interior trim components, which leads to the performance of injection molding and has led them to having the ability to perform in-mold assembly. And when they’re talking “in-mold assembly,” they are not talking about putting things together in the mold with nuts and bolts. Instead, they are actually able to perform injection molding in a single tool such that they are able to produce a fully functional part with movable components. Think, for example, of an automotive air register. Rather than having to make this with multiple components (vents, housing, connecting rod) that would have to be individually molded and then assembled, this can be produced by TRW as a finished, functional part in a single tool. Note that this is not a case of over-molding or encapsulating.

As Kernen points out, this process is being done with conventional injection molding equipment. And while multiple materials can be used, including dissimilar materials (although there must be a certain amount of thermal and processing parameter commonalities so as to maintain a reasonable cycle time, and non-compatible materials cannot be used), there is no need for the use of exotic materials. All common thermoplastics can be employed. And he notes that TRW Automotive is supplying parts that are produced with this process, not the process.

One of the key advantages of the in-mold assembly process is the limitation of variation. For example, when there is the need to produce parts in different molds and then to put them together, there is variation that can result from both the molds as well as the manual assembly operation. What ends up happening in the in-mold assembly process is that one shot of material is used, in effect, as the tooling for the subsequent shot. Consequently, tolerances are more readily managed by paying careful attention to the single tool.

A related benefit is faster ramp up to part production. That is, consider the aforementioned air register and three separately produced parts. During the tooling development, during the debugging stage, if pieces don’t assemble properly, it is necessary to figure out where the adjustment needs to be made (e.g., to the vents, housing, rod—or a combination there of). Because the tooling engineers are dealing with a single tool with the in-mold process, the debugging can occur much more quickly.

To be sure, the tool for the in-mold process is somewhat more complex than a single tool for a component. In the case of the air register, for example, there are three cavities for each tool: one for the vents, one for the rod, and one for the housing. There are three gates. So first the vents are injected. The parts are indexed to the second station, where the connecting rod is molded over the vents’ pivot points. There is a third transfer, and the outer housing is produced around it.

According to Kernen and Schmidt, the assemblies that result from the process tend to be far less prone to rattling or sticking than conventionally produced parts. What’s more, because a completed assembly is made during each cycle, there are reduced handling, inventory, and logistics issues related to the operation than is the case when multiple parts are produced for subsequent assembly.

Kernen suggests that the process has plenty of applications for vehicle interiors, including such things as radios and other components where there are assembled plastic pieces.

Looks And Performance

The interior of the 2003 Lincoln Town Car is accented by trim that includes panels of walnut burl appliqué. When you move along the instrument panel to the passenger-side airbag door location, you can see that the look continues. Which is somewhat tricky because the airbag deploys through that door. In order to produce it, two thermoplastic polyolefins (TPOs) developed by Solvay Engineered Polymers (Auburn Hills, MI) were employed.

First of all, there’s the walnut-appearing material, which is a sheet stock of DEXFLEX E1501TF that’s coated by Avery Dennison with the wood grain treatment. The sheet is thermoformed into shape by Summit Polymers in its Portage, Michigan, plant. Next, Summit puts the shape into a mold and injects DEXFLEX 756-67 behind it. This then forms the cover, which includes both an integral hinge and a tear seam. Key to the use of the sheet stock material are its stiffness through the manufacturing processes, its ability to provide adhesion to the substrate and resistance to warpage during the differential cooling during the insert-molding process. An important attribute of the TPO is low-temperature ductility: Ford has a requirement that the part not fragment at test deployment at -30°C.

Insert Molding in Small Spaces

Looking for a compact press that can perform insert molding? Check out the recently introduced Magna V 130 and 280 units from Milacron, Inc. (Batavia, OH). Consider the bigger of the two. Although this 280-ton vertical machine has a maximum daylight of 35 in. and can handle molds up to 24 x 36 in. (in the standard two-station configuration), it has a footprint of just 15 x 7 ft, which is said to be less than half that of comparable tie-bar machines.

The Magna V insert molding machines have a C-frame design, which means that there is ease of accessibility for operators. In order to protect operators, there is a standard infrared light curtain that surrounds the perimeter of the table and shuts down table motion if breached. Other features of the machine include an open-architecture, PC-based control with a Windows NT operating system.

These new molding machines join a family that also includes 30-, 50-, and 80-ton machines.

Drop In & Save

Tyco Electronics, which makes things like connectors, found that there was a potentially expensive proposition ahead of it as the company to which it provides connectors to had a process change that required a switch from the PBT (polybutylene terephthalate) components that it was making to something that provides a higher heat-distortion temperature. There was the potential that it would need to create new molds in order to accommodate, say, high-temperature nylon to handle the job—new molds because of a different shrinkage factor.

However, a solution was found in the form of Thermx CGT 33 from Eastman Chemical (Kingsport, TN), a PCT (polycyclohexylene dimethylene terephthalate) glass-filled polyester that has a melting point of 290°C, and is therefore capable of withstanding the effects of the IR soldering application that the customer would be employing for the component. Although the polyester was a bit more expensive than the PBT resin system that it was being used to replace, as David Revak, Tyco Electronics product engineering leader puts it, “By using a drop-in material, we saved tooling money and better utilized existing capacity.”

NPE Is Coming

One of the biggest events in the plastics industry is going to be held June 23-27 at McCormick Place in Chicago: NPE 2003. This event, sponsored by The Society of the Plastics Industry, is held every three years and is a massive event by any measure: some 2,000 exhibitors are expected to cover 1.1-million-ft2 of space. Everything from equipment to processors will be on display at the event. As it is held every three years, you can be confident that there will be plenty to learn about that’s new. In addition to which, a conference will be held on 16 topics, ranging from design through materials, that will be covered in 68 different presentations. To learn more and to make arrangements to attend, check out the event’s website at www.npe.org.

A Thoroughly Technical Look

Nowadays, as the “technical” look becomes more appealing in exterior and interior applications (and continues under the hood), there are more and more pieces of plastic that are made to resemble metal. In many cases, this metallic appearance is the result of painting the parts. Which, of course, presents a few problems. For one thing, this means that there is a secondary operation performed. For another, in the case that the painted surface gets scratched, then the underlying plastic material, which may be, say, black, is exposed, which can be incredibly annoying to the proud vehicle owner. This, perhaps, is one reason why vehicle manufacturers including BMW, VW, and Mercedes have been using a patented plastic formulation from ALBIS Plastics (in the U.S. in Rosenberg, TX) that is compounded with various plastic materials (e.g., nylon, ABS, polycarbonate) such that the silver metallic color is thoroughly molded into the resin. This means that there’s no need for a secondary operation (i.e., painting) and that if the surface is scratched, the metallic color isn’t eliminated. What’s more, ALBIS has developed the formulation such that there are said to be no flow lines nor weld lines. The applications have been varied, from IP bezels to diesel engine covers.