By: Glenn Nausley
President of Promess, Inc.
Composite materials consist of fibers — in the aerospace industry typically glass, carbon or kevlar — suspended in a matrix of epoxy resin. They are lighter than traditional aerospace materials and incredibly strong, corrosion resistant, can withstand wide variations in temperature and exposure to severe weather and can be made into just about any required shape.
With all these advantages the decision to use composite materials should be a no-brainer, except for one thing: the complicated fabrication process required to produce a finished part reliably makes them very expensive.
They are produced in molds and the process requires very tight controls over pressure and temperature which vary over time.
In greatly simplified terms, a sheet of the base fiber pre-impregnated with the resin, called the “prepreg,” is placed in a heated mold. As the prepreg begins to melt and become viscous the upper mold half is moved down to apply a constant load as the sheet progressively conforms to the mold contours. This force has to be carefully controlled to ensure proper heat transfer as the prepreg/mold contact area changes continually during the process.
Once the mold is completely closed as the prepreg begins to cure and harden it also expands. But the load has to remain constant so the actuating force must be gradually reduced until final dimensions are reached. At that point the press must stop and hold position not allowing the material to expand any further while the chemical curing process is completed.
That level of control is extremely difficult to obtain with a traditional hydraulic press system that depends on physical pressure adjustments and mechanical stops to make process changes. The kind of hydraulic systems needed to achieve that kind of control are expensive and so, therefore, are the parts they produce.
The application is a natural for a system consisting of a programmable electric servo press and a controller that can monitor temperature, pressure and position sensor inputs in real-time and make adjustments on-the fly to keep the process within pre-defined acceptable limits. Not only is the level of control of the fully electric servo system much more precise, the movement itself is very smooth compared to the abrupt start and stop of a hydraulic system responding to a valve opening and closing.
The all-electric servo press solution offers several other advantages. A major one is cleanliness since there is no hydraulic fluid present to leak and contaminate the composite. Programmability also facilitates rapid changeover from part to part with few to no mechanical adjustments required.
Electric press/actuators are much more energy efficient because unlike hydraulic power units that must run constantly, they only consume electricity when they are actually performing work. Finally, all of the force, temperature and position data generated during the process can be captured and saved to meet quality control and product documentation requirements.
A number of electric servo-controlled systems are in use producing aerospace composites today. Most are single press systems limited to relatively small parts due to the difficulty of coordinating the simultaneous movement of multiple press/actuators in real-time that is required to produce a system capable of processing large parts.
In reality this is not a mechanical issue, but rather one of processing power and software in the control. One solution is the Joined Axis Technology developed for the Promess UltraPRO control that links multiple press/actuators together and commands them as if they were one while monitoring and controlling the force and position of each one individually. This technology makes large presses powered by multiple electric servo press/actuators a practical solution for aerospace composites of virtually any size.
Someone once said “the future is electric” but they probably weren’t thinking about aerospace composites when they said it. But, with the electric servo-press system technology that exists today, the statement is nonetheless likely to have been prophetic.