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RTM hot tools drive the push for speed.

RTM tooling concepts are evolving to keep pace with the growing need for faster cycles in the automotive, aerospace and recreation industries.

The workman's adage "if you want to do the job right, you need the right tool" was never as true as it is for the growing field of resin transfer molding.

As RTM finds increased use in automotive, aircraft and sporting-goods fabrication, efforts are under way to develop tools that can not only speed up the process but can handle the expanding number of resins tailored for the process. Both trends are the result of awareness in automotive and recreation markets that RTM may be among the quickest, easiest methods to tool up for the thousands of new parts per year those industries require.

"Once more materials can be successfully molded, I think the use of RTM will grow," says Garrett C. Sharpless, president of Fiber Innovations Inc., a manufacturer of braided composites for aerospace and recreation markets. "Different resins have different processing requirements, and tool design can ensure the successful use of those resins."

Some even see RTM changing the face of composites manufacturing after it attains speedier cycle times. "I think there's a big opportunity to save on manufacturing costs with RTM," says Mark Thiede-Smet, technical development engineer at Heath-Tecna Aerospace Co. in Kent, Wash. "It will allow things like Just-in-Time manufacturing and can eliminate stockpiling of parts."

Yet, at least for aerospace fabrication, where the materials require long cure times and 10-15 parts a week is a full manufacturing schedule, the cost of tooling is a major hindrance to RTM gaining full acceptance. "We have to come up with ways of making less expensive tools," Thiede-Smet says. "If you're making automotive parts or sporting goods, you can afford these very expensive tools. But in our business it just takes too long to amortize the cost."

PUSHING THE SPEED LIMIT

Not everyone seems convinced that technology to dramatically speed up RTM is imminent. Dow Plastics, for example, recently abandoned its efforts to develop epoxies and vinyl esters for automotive RTM, says Mac Puckett, development leader at the company's Freeport, Texas, composites applications lab. The reason, Puckett says, was that Dow realized it would never be able to achieve the 2-2 1/2-min cycle times its automotive customers demanded for those materials. Instead, Dow is now focusing on modifying its resins for SMC and BMC molding, which can achieve the desired speeds. Still, Puckett concedes that RTM has a niche in the manufacturing of "reasonably complex" parts with total runs of no more than 100,000 parts. He adds, "It just isn't going to happen without a good tool design."

Tool design helps determine numerous important factors in RTM, including cycle speed, ease of mold-ability, surface quality and consistency of part thickness. No one tooling material holds a distinct advantage over any other in achieving all these goals, and processors agree that each material has its own pluses and minuses. Aluminum, for example, offers a thermal conductivity unmatched by any other commonly used material; but it's prone to porosity and distortion from repeated heating, so aluminum tools tend to have short lives. Tool steel offers the hardest surface and thus can produce the most parts per tool. But its mediocre thermal conductivity and coefficient of thermal expansion make it less suitable for high-speed applications.

"If you're looking for short cycle times, you want to be able to heat that tool up and cool it down in a relatively short time," explains Scott Beckwith, president of the Beckwith Technology Group, a composites consulting firm. That requires tool materials with higher thermal conductivity, such as aluminum, nickel or epoxy with carbon fibers and/or metal-bead fillers. Also, the lower conductivity of steel or some composite tools can be rectified somewhat through the use of an internal heating and cooling system.

More recently, mixed-metal alloys such as aluminum-bronze-copper and chromium-copper have been used to bring a better balance of properties to RTM molds. A few processors have gone to Invar, a high-nickel-content steel made by CTE Tooling, which has been used extensively in filament-winding mandrels and some hand-layup molds. Still others have opted for nickel-shell electroforming or vapor deposition, which is typically backed up with metal-filled epoxy.

Aero Detroit Inc., the molder of all of the body panels except the hood for Chrysler Corp.'s RTM-bodied Viper sports car, will employ nickel-shell molds this month when it begins the 1993 production of 3000 Vipers (see PT, Sept. '91, p. 40; March '92, p. 40; May '92, p. 63). Robert Fehan, Aero's general manager, says that depending on the part, the company will use both vapor-deposited and electroformed nickel tools, as well as machined aluminum, running at temperatures of 140-150 F to get cycles as fast as 5 min. In the initial phase of Viper production begun last year, Chrysler made only 300 cars using unheated epoxy tools. But with the call for stepped-up production volumes, faster cycles were mandatory and Aero developed the new tools. Fehan says the company is now working on developing molds and processes to manufacture body panels for other American cars. News of these methods is expected to be released late this spring.

Lotus Engineering in Norwich, England, has used nickel-shell molds for both its Vacuum Assisted Resin Injection (VARI) and Vacuum Press Molding (VPM) processes, achieving cycle times of 40 min with the former and 5 min with the latter. VPM is a hybrid of SMC and RTM processes, using a chopped-glass prepreg, which melts and flows under vacuum inside tools heated to 194 F(PT, Jan. '92, p. 27).

Electroformed nickel shells reportedly have the advantages of being harder and more durable than cast aluminum, and able to withstand extended use at higher temperatures (up to 200-240 F). Running hotter improves the effectiveness of most low-profile agents used to obtain Class-A surfaces; thus, Chrysler hopes to achieve Class-A surface quality on the Viper without gel coating.

Unfortunately, relatively few sources can provide high-quality matched nickel-shell tools, says Martin Baginsky of Fib-Chem Industries, which represents English mold maker Ex-Press Process Equipment.

TOYING WITH ALLOYING

Materials that are just beginning to be considered for RTM tools include copper and copper alloys. "With copper alloys, the key to success is selecting the right one for your application," says a spokesman at ISORCA Inc., an R&D consulting firm. "Most of all, you have to look at the tradeoffs between thermal conductivity and strength."

Chrome-copper, for instance, is very strong; copper-aluminum offers higher thermal conductivity but lower strength. Some molders are using copper alloys as inserts into steel molds. Last year, ISORCA and the International Copper Association began a program to develop copper-based RTM molds for high-volume automotive applications. ISORCA plans to work with a major truck manufacturer, custom molder and moldmaker on a production test of an RTM tool cast from C95400 aluminum bronze, said to be much tougher and harder than cast aluminum.

Copper is attractive, ISORCA says, because of its ability to be alloyed with several other metals, giving it the capability to make molds with varying degrees of structural properties, thermal conductivity and surface hardness. It's castability suits it to molding parts with complex geometries; and copper alloys reportedly have the potential for making near-net-shape parts and the ability to cast in place heating and cooling ducts. An ISORCA spokesman cautions that casting-in temperature control ducts is not always so easy. "The design of the cooling system is critical," he says, "but sometimes you can't get the cooling channel where you want it due to part configuration. That's where the copper alloy can help with the cooling."

Copper alloys have other benefits, ISORCA says. Their heat-transfer capability provides uniform mold temperatures and reduced cycle times. They are easily welded and thus damaged molds can be quickly repaired or reworked. Copper-alloy molds reportedly can be produced in shorter lead times than many other metals, and their surfaces are easily coated and polishable, leading to better surface quality in molded parts.

For most RTM parts, initial surface quality is not that important since many of the parts are either used in non-appearance applications or their surfaces are enhanced through gel coats or painting after molding. Still, getting the best surface quality possible right from the mold is something RTM processors would like to do. (Lotus has even sprayed two-component urethane paints directly onto the tool before introducing glass and resin.)

"Class-A finish is achievable," says Mark Perry, a doctoral candidate at Ohio State University who has been studying RTM since 1986 at the university's Engineering Research Center for Net Shape Manufacturing in Columbus, Ohio. Since the consensus among RTM users is that much of the damage that occurs to molds occurs in the demolding step, a key to achieving good surface quality is through use of special releases or waxes to ensure that the part pulls away easily and there are no nicks on the mold surface.

Nickel-shell tools like those used by Aero Detroit for the Viper panels are considered by most involved in RTM to provide the best surfaces. However, moldmakers warn that the thickness of the metal coating in these and other surface-treated tools has to be kept to a minimum. "If it goes beyond 20 mils," says Dimitrije Milovich, v.p. of moldmaker Radius Engineering Inc., "it can effectively change the shape of the tool."

EPOXY SHOWS ITS METTLE

Despite the current attention focused on metallic tooling materials for higher volume RTM jobs in areas like automotive, tried-and-true epoxy is still the workhorse, even for some very demanding process conditions. An example is RTM molds made for short runs of airplane propeller blades at the Hamilton Standard Group of United Technologies, Windsor Locks, Conn. The molds were made by Beverly Pattern Inc. using a Ciba-Geigy epoxy, Ren CGL 1310, which is able to withstand the mold temperatures of as high as 350 F. "This epoxy has the high glass-transition temperature needed for durable RTM tooling," says Richard Silva, tooling project manager at Beverly Pattern. "It is also easy to fill and cast around heating tubes to permit improved temperature control."

To help withstand such high temperatures, the tool face consists of layers of zinc and aluminum, which are nickel-plated after the mold is built to provide better surface gloss, hardness and durability. A complex tool can be built in about eight weeks versus the six to eight months it takes to make a similar tool from steel, Ciba-Geigy says.

THE HEAT IS ON

While some low-volume aerospace applications can still tolerate mold cycle times measured in hours, they aren't acceptable in automotive and recreational markets where making hundreds of parts a day is the norm. Instead of relying only on the slow build-up of exotherm heat to cure the part, more and more molders are going to some active form of temperature control. Heating can use various media, including steam, oil or air. There are three basic methods used to heat RTM molds:

* Platen heating, where the mold surface is heated through a series of heating channels on the platen.

* Integral heating, where the heating channels are internal, just behind the mold face.

* Oven heating, where the entire mold is placed in an oven or autoclave.

Each has its pros and cons. Platen heating can be used only where a mold has good thermal conductivity, since the heat will have to quickly penetrate the mold; and this method is restricted to tools that are relatively flat. Also, platen heating tends to be slow and doesn't speed up cycle times all that much.

Integral heating provides the fastest heating and cooling rates. It is compatible with press and perimeter clamping methods and several molds TABULAR DATA OMITTED can be heated simultaneously from a single source of hot air, oil or steam. However, integral heating is considered to be the most expensive method on a per-tool basis.

Oven heating is good for small tools with large thermal mass; it can also be used to heat several molds at once; and it is a relatively low-cost procedure. However, its main limitation is that molds must be filled and clamped outside the oven, and therefore will require considerable heat-up time.

Fluid-handling automation specialist EM|C.sup.2~ says it has used really hot molds to drive RTM cycles down to 1 min or less. The process uses highly reactive, heat-activated resins that kick over fast in steel molds heated to 300 F. (See Technology Newsfocus for more details.)

One new approach to RTM applies energy directly to the resin rather than conducting it through the mold. RP/C Machinery Corp. is working with heating specialist Nemeth Engineering on radio-frequency (RF) curing in RTM molds. It's still in the experimental stages.

SIMULATION: SCIENCE VS. ART

No less than 10 different computer modeling programs are being developed at the university level, and a few others are being explored by commercial software vendors. Although there is very little real-world use of such software yet, developers of computerized process simulation, such as AC Technology, say their programs will save both money and time in developing workable parts and tools--as has been the case with injection molding simulation. They say these programs will help molders visualize the in-mold pressure distribution and thereby tailor process conditions so as to minimize clampforce requirements. Molders will also see how to optimize locations of heating elements from the data on cavity temperature and degree of cure, predict the occurrence of pregel, avoid thermal degradation, and evaluate processing capability of a fiber preform.

Still, some who are involved in creating the modeling programs say that while they give mold and process engineers objective evidence of what will and won't work effectively, the software does have limitations.

"In our experience, low-volume manufacturers including high-quality aerospace firms, will probably manage without mold-flow analysis and process modeling," says M.J. Owen, a researcher at England's University of Nottingham who has developed his own finite-element RTM modeling software. "However, before higher volume manufacturers will commit to the RTM process for a new car model, they will want to be sure that the manufacturing route is viable and will want to have a clear idea of the manufacturing economics."

The Nottingham program does flow and cure analysis on the part and design analysis on the tool facing shell and backing structures. It can also determine pressure distributions at every stage of the filling process. The program has been used to design undershields and rear spoilers on two competition cars made by Ford Europe.

At least three commercial vendors of mold-filling and heat-transfer analysis software for injection molding are actively developing analysis packages for RTM.

* Plastics & Computer Inc. is doing joint RTM modeling work with Fiat's research center in Turin, Italy. Flow simulation has been performed on an automobile rear floor panel molded from glass mat, urethane foam core, and ICI Acrylics' Modar thermosetting acrylic (the same resin used in the body panels of the Dodge Viper) filled with 50% calcium carbonate.

Using 3-D finite-element flow analysis, the modeling technique provides a color-coded map showing predictions of filling patterns, filling time, and pressure, temperature, and shear stress throughout the part. Fiat says the predictions are accurate within 10% of actual molding times. (Fiat discussed some of its work with the software at last month's SPI Composites Institute conference in Cincinnati.)

* AC Technology is working with Ohio State University on an enhancement of its C-Set thermoset modeling software. Due for commercial release next January, C-Set 4.0 will include both 2-D and 3-D simulation of mold filling and curing in RTM/SRIM that predicts flow-front progression, cycle time, weldline locations, pressure and temperature profiles, peak exotherm, and degree of cure through the thickness of the part. Emphasis is placed on modeling the effects of heat transfer between the resin and reinforcement and the resin and mold wall, as well as the differences between epoxy and metal tooling of differing thermal conductivities. Calculations of porous flow through an anisotropic preform, determined by fabric structure and orientation, is a key aspect of the software design.

* Technalysis is working with two RTM processors and a compression molder who are using its Plastec/Injection software.

GE Corporate Research and Development in Schenectady, N.Y., which has done some work on injection mold-filling simulation, has also been looking into RTM. GE's work takes into account the time-dependent changes in viscosity of fast-curing materials. GE is using the program to design aerodynamic fan blades for aircraft engines.

On the academic level, modeling programs are being developed at such schools as the Universities of Delaware and Minnesota, Ohio State, Virginia Polytechnic Institute and State University, and the Center for Applied Polymer Research of the Ecole Polytechnique de Montreal. (The Montreal researchers discussed their work at last month's SPI conference in Cincinnati.)
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Title Annotation:reinforced thermoplastics
Author:Monks, Richard
Publication:Plastics Technology
Date:Mar 1, 1993
Words:2801
Previous Article:New equipment for RTM, SMC at Dusseldorf Show.
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