by Michael Riehn
Compression molding applications require precise pressure and position, speed control, and parallelism throughout the working stroke.
A hydraulic press, is well-suited for these requirements because of its ability to dwell under pressure for any length of time, vary ram speed, and provide a constant force over a large area—precisely the press characteristics compression molding requires.
In addition, many compression molding applications require large bed sizes but comparatively low tonnage. A hydraulic press bed size can vary independent of tonnage capacity, so there is no need to order a large press with unneeded press tonnage to obtain a large bed size.
Dwell times for composites are typically considerably longer than for metals. A hydraulic press is perfectly suited to maintain energy efficient pressure over an extended dwell period. The press designer can engineer the system with variable volume pumps, accumulators, lock valves, etc, with a multitude of control packages to monitor and manage the system requirements. — anywhere from two to 15 minutes per component. .
The press can be engineered with a sizeable daylight to allow enough space to install both the male and female tooling of large molds. Often times, increased daylight is required to form large contoured panels or shapes inherent in the size and shape of an aircraft or automobile, for example.
Specifications for aerospace, wind turbines, and many automotive components are critical, and even small part defects would create quality problems. A bump cycle capability is a standard typical requirement for forming thermoplastic and composite materials because the materials create gas that must be released. The press closes up on the material, applies some tonnage, then opens a ¼ inch to one inch, and then closes again immediately. Without it, many parts would have large voids in them.
Hydraulic press features that can optimize the compression molding process include control of ancillary process equipment, actuation of internal die components (such as core knockouts, ejectors, and punches), and upacting or multiple-action rams. Precision-heated platen integration with specified heating tolerances and die controls usually are standard.
Additional features such as double- and triple-action rams and part ejection systems can be incorporated into an integrated press system.