January 27, 2009
The difference between a hydraulic press used to form metal stampings and a compression molding press used to form parts out of composite materials isn't that great. Could a metal former make the jump into composites forming? With a lesson in materials handling and some customers, a metal former might be able to make the jump.
Do you have a press that is not being utilized fully? It's not an uncommon situation in many shops.
To address this situation, many shops look to diversify their customer bases. For many companies, this means finding new customers in new industrial segments, such as aerospace or power generation. But what if diversification meant chasing after customers that didn't work with metals, but rather composite materials?
Unlike the metal forming industry, the composites industry has grown by leaps and bounds over the years—with some estimates suggesting it's at $35 billion already. The Boeing 787 and Airbus A380 programs are examples of "products" incorporating more composite materials in place of metal, but aerospace is not alone. Automotive companies are looking at composite parts to lighten vehicle weights, and even manufacturers of wind turbines desire blades made from durable but lightweight composite materials.
Of course, a shop may be apprehensive to jump into a new field of expertise. Composite materials have to be handled in different ways, and they react differently when they are formed.
Luckily, metal formers have two factors working in their favor if they are looking at moving into composite material forming:
In this application a 100-ton press with a 36-inch by 36-in. bed is used. The molding operation requires a press opening of 20 in. and a press capable of delivering a 14-in. stroke.
The press comes with advanced electronics—complete with a touchscreen—to control dwell on the ram, speed of the stroke, and temperature of the tooling. The control technology allows the operator to dial in a rapid advance of 44 IPM and a rapid opening of 87 IPM.
What do metal formers need to know? A hydraulic press can do all of the above—if it has the right control technology. In the past a hydraulic press might have had a dwell timer on it to guide the press for shorter dwell times, but that won't cut it for compression molding. PLCs and industrial computers are necessary to guarantee performance consistency and to create part files that are able to replicate stroke signatures of a particular part every time that job is done on a press. That type of repeatability is needed to serve customers in the aerospace and military industries.
Modern servo-driven mechanical presses have the capability to mimic the abilities of a hydraulic metal forming press and a compression molding press, but the focus for the duration of this discussion will remain on hydraulic presses because they are more commonly found in U.S. shops.
The composite material in this application is very similar to carbon fiber material. The strength of the material comes from the fiberglass strands mixed into the resin, which in this instance is in sheet form.
The machine operator pulls the composite material off a roll and cuts it into appropriate shapes according to a template. The shapes may be different sizes, but they are normally the same width.
What do metal formers need to know? This is where the differences between metal forming and compression molding are apparent. SMC material has a short shelf life—usually 30 days or less. Additionally, the material can't be stored in the back of the shop in an open-air environment. In some instances, it has to be refrigerated during storage to keep the material stabilized.
Because of the nature of the SMC, it is very pasty. Plastic liners are put in between the layers as they are rolled to keep the surfaces from sticking to each other.
Once the material is cut to size, the "charges" are placed into the mold cavity. A sort of topographical map guides the operator as he puts the charges into the mold.
In this application, metal is actually present in the mold form. Titanium-alloy hubs are incorporated into the part design and are sandwiched in the middle of the part.
What do metal formers need to know? For those metal formers who have dealt with tailor welded blanks—the combination of different steels into a welded blank—they will understand the need to stack SMC charges. The sheet, which usually comes in thicknesses from 1/4 in. to 1/2 in., will be stacked in the mold to strengthen certain areas of the final part and to ensure even flow throughout the entire mold once the forming process begins. In this application, the various thicknesses are necessary for machining and attachment requirements.
With the material strategically placed in the mold cavity, the compression molding process can begin. In this instance, the tool is below 100 degrees F.
The press has a settable platen temperature for each mold half. The control system is integral in gradually building up the temperature needed to complete the liquefication of the charges.
The press then closes, and the tooling temperature begins to build to 365 degrees F over a 30-minute period.
What do metal formers need to know? Sure, not all metal formers are involved in hot forming of steel, in which heated platens are incorporated into tooling design. That doesn't mean that SMC molds are that different from traditional metal forming dies.
For example, knockouts are designed in both metal forming tooling and SMC molds. In another example, holes can be accommodated just as they are in a typical punching operation in stamping, but instead of a punch, core cylinders are shot into the charges at the beginning of the molding process, and the material flows around these cores, leaving the desired holes.
The overall goal remains the same for both the tool and die designer and compression mold-builder—incorporate as many operations as possible into the tooling to eliminate secondary processes.
The compression molding process continues with growing pressures and increasing temperatures.
What do metal formers need to know? They already know this about hydraulic presses: It's all about the dwell time. The same goes for compression mold presses.
Simple hydraulic presses may have dwell systems that use a pressure lock valve and a small accumulator to maintain pressure. These types of systems do the trick, but they can provide consistent pressure for only a short amount of time, usually up to 10 minutes. That approach might work for a molder of a lighting fixture that requires 45 seconds of dwell time, but that's not going to get the job done for an aerospace part-maker who might require up to 16 hours of dwell time for structural parts.
For those applications that require longer dwell times, variable-volume pump systems with accumulators to turn the motor and pump off and on periodically and advanced electrical controls are needed.
Also during this time, a "bump" cycle is used to relieve pressures and evacuate gases from the mold. The mold is not opened to expose daylight; once the pressure is reduced, gases escape through channels incorporated into the tooling.
When the tool hits its exact end position and reaches its final temperature, the press's tonnage control takes over. Simultaneously, water is sent through the tool's cooling channels to lower the tool's temperature and encourage curing of the SMC material. In this application, the compression molding process is not complete until the material is cured, which requires the tool's temperature to be lowered to 100 degrees F.
What do metal formers need to know? Heated platens and cooling channels can be incorporated into the tool design. These types of additions don't have to be a permanent part of any hydraulic press, which might be an attractive option for shops doing both metal forming and compression molding.
In the end, metal formers don't have to be chemists to understand how to operate compression molding presses. They just have to understand how the process is supposed to work.
After all, most metal formers aren't metallurgists, even though they may be working with newer materials such as high-strength steel. Over time they become much smarter about the materials they work with. They just have to make that jump into working with something new.
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