Structural plastics combines form and functionStructural plastics combines form and function

Application by application, plastics continue their relentless campaign to replace metals, but structural components remain a stronghold for steel and other metals. Even there, however, polymers have made inroads as OEMs, designers, and processors push existing resin technologies to their limits, coupling pleasing aesthetics and style with ever-improving mechanical properties.

Specifically targeting its exterior panels that served structural and decorative functions, global farm/industrial equipment manufacturer CNH, which distributes Case New Holland, first considered a radical redesign for the new generation of its mammoth Case IH AFX7010 agricultural combines four years ago. As it began its initial inquiry, CNH left no process off the table.

?Typically for parts of this size, OEMs are looking at a number of different processes,? GI Plastek program manager Ryan McNamara explains. ?They?re looking at metal, which they?ve used for years, but their design engineers are trying to gravitate nowadays to more of a styled look that you can really only get from plastics.?

Meeting Structural Needs with RIM

Since design engineers at CNH were targeting greater style and a plastics processor who could provide it, they considered GI Plastek (Newburyport, MA), a company with extensive, large-part RIM experience, and an innovative inmold coating process that could replace postmold painting.

Anticipating a volume of around 3500 units/year, CNH weighed the cost effectiveness of RIM, SMC, and fiberglass processes for its newly refurbished combine. It eventually settled on the package offered by GI Plastek, which included that company?s ProTek inmold coating process.

ProTek is GI Plastek?s patented answer to inmold decoration. A red urethane paint is manually sprayed onto the surface of the nickel shell tool, then the mold closes as a two-part RIM material system is shot into the tool. Since the coating and the RIM system are chemically similar, a molecular-level bond is formed and the materials crosslink, creating a red surface with molded-in-color qualities that help resist dents, scratches, or other marring.

The company has used the process in the past to successfully impart a number of properties to products, including static dissipation, impact resistance, class A finishes, and overall ruggedness. In the case of the combine, ProTek was used for its toughness and visual appeal.

?Opposed to what can happen with a post-painted SMC or fiberglass substrate, in ProTek there?s an absence of what we call orange peel in the part,? McNamara says, ?because we?re putting the coating down onto the mold and really the part that?s setting up is the layer that?s between the substrate and the exterior surface.?

Processes like ProTek further augment the appeal of RIM versus more traditional methods and materials for structural parts, due in a large part to what happens after part fabrication and the level of style it achieves.

?[With] metal you?re pretty limited as to what you can do,? McNamara says. ?I think that the inmold coating was really the entry point to being able to do large panels like this. The problems that you have with a lot of other plastics, especially with large parts, is painting them. There aren?t too many paint lines that are geared towards parts in the 7- by 6-ft range. With a lot of processes, you have to manufacture the part, then clean it, and then paint it, so there?s a lot of handling there. [Structural parts] are all large surfaces so there are a lot of areas to get scratched or nicked.?

Low-Pressure Process, Low-Cost Tools

In addition to postprocessing cost savings, McNamara says RIM offers substantial savings to OEMs needing structural components?even before any resin is shot?through decreased tooling costs. Applying substantially lower pressures than injection molding (process pressure of only around 100 psi for the combine?s structural panels), RIM parts can be molded in tools made from composites and materials other than the P-20s and hardened steels commonplace in the high-speed-injection world of molding. Because of this, what amounts to a casting process can be used to create the molds for parts with deep draws. With traditional tools, hours of cutting would be needed to core out large sections of hardened steel that would essentially be wasted.

?The tooling advantage that we have with RIM is pretty pronounced,? McNamara says. ?For large, deep parts, this tends to be very economical, as opposed to a high-pressure process where you?re having to machine a tool out of a solid billet of aluminum or steel.?

Instead, GI Plastek builds a male model of a part and places it into an electroforming bath. There, a thin coating of nickel adheres, and this male part is taken out and turned around to become the mold?s cavity.

Underneath the coating, the tool is constructed of a honeycomb structure and epoxy composite backup. The core portion for the tool is a slightly oversized cast kirksite that is machined down to the final dimensions.

GI Plastek?s VP of sales and marketing, Steve Trapp, says that in spite of using composite materials to construct the tools rather than a hardened steel, the molds themselves offer a long service life, due in large part to their nickel coating.

?What we?ve found with the nickel shell tooling,? Trapp says, ?is that we have run as many as 100,000 parts without any failures. Most of our applications are under 10,000 shots on an annual basis, so we?ve run 10 years and are going longer on some of the original nickel shell tools.?

Starting Anew

?Where you?re pushing the envelope with this [process] is the size,? according to Trapp. ?This is the first time you?ve seen these type of large panels in the RIM process. Heretofore, it?s been metal or SMC. This is bigger than we had ever done before?bigger than anybody had ever done before in terms of exterior body panels, and at the same time, we?re building a plant with the largest equipment that?s available in the U.S. We were juggling a lot of activities here.?

Ten tools were used to create the 12 panels, with one mold providing two parts in a family tool. The largest of these parts, the left and right front panels, weigh 56 lb each and measure over 100 by 76 inches. A pair of rear panels weigh in at 41 and 54 lb respectively, and overall the average weight of the 12 panels is 31 lb. Still, RIM provided a 25 percent weight reduction compared to their steel predecessors and tool savings compared to SMC of 50 percent. All of this is being constructed in GI Plastek?s new Dewitt, IA plant. To be built in three phases, the plant is still in its initial design and encompasses 35,000 sq ft. It will eventually cover 100,000 sq ft, but for now the facility is running two new RIM systems for the panels, with new business on the horizon.

RIM Looking Good

The program is an illustration of the acceptance RIM is gaining among processors needing structural parts. They now feel that it can provide top-of-the-line parts in terms of appearance and strength.

?RIM was typically used for structural noncosmetic parts,? McNamara says, ?and we?ve kind of taken that process and put a little bit of a twist on it and ventured into the highly cosmetic applications. I would say that about 80 percent of our RIM business is inmold coated or highly cosmetic surfaces.?

In spite of companies like GI Plastek, RIM remains a mystery to many?making the inherent benefits it brings to large structural parts lost on the masses. RIM was developed in the late 1960s by the material company Bayer. As it heads towards its mid-30s, it remains a virtual infant compared to more established technologies.

?The RIM process is still relatively new and relatively unknown,? Trapp says. ?When you go through engineering, if you?re interested in plastics, clearly they teach you about injection and the thermoplastics industries, but to be familiar with the urethane RIM process [is unusual]. In my mind, people don?t appreciate the benefits and don?t naturally think about the utilization of it.?

McNamara has a first-hand recollection of the overall neglect paid to RIM from his days in the polymer engineering department at UMass Lowell. ?I think that one day we probably had about a half-an-hour lecture on RIM,? McNamara says, ?as opposed to probably 90 percent of the course teaching injection molding.?

But now McNamara, and companies like GI Plastek, have taken to educating OEMs on the benefits of RIM for structural molding, and gaining some surprising converts.

?[At CNH] you had several camps,? McNamara explains. ?You had the guys who had been with Case for years, and those were the metal guys, and then you had the younger guys with fresh ideas and new concepts [who backed plastics]. One of the gentlemen who was in the metal camp was one of the most outspoken, biggest proponents of the plastics camp by the end of the program?that turned some heads.?