Forming two materials with one stone
May 11, 2009
Many companies in the aerospace, automotive, sporting goods, and wind energy industry segments are using composites and thermoplastics to augment and support metal stampings.
During compression molding, material is preheated and placed in a mold cavity that is heated by hot platens in the press. The mold is closed, pressure is applied to force the material together in the mold, and heat and pressure are maintained until the molded material has cured.
By Michael Riehn
Many companies in the aerospace, automotive, sporting goods, and wind energy industry segments are using composites and thermoplastics to augment and support metal stampings. These nonmetallic materials also are used in industrial, electronics, oil and gas, and military applications.
Historically, composites were more expensive than their steel and aluminum counterparts. Demand for composites resulted from light weight requirements or for use in demanding applications requiring high strength-to-weight ratios, corrosion resistance, and so forth. Composite materials can be glass, thermoplastic, phenolic, or epoxy. Lower composite costs and rising steel prices have driven additional demand for composites, and volatile fuel prices have created an increased need for lighter material for increased efficiency.
Composites' structures differ from metals'. Metals' structures are isotropic, having uniform values along axes in all directions. Alternatively, composites have different property values when measured in different directions (anisotropic). They combine two or more different materials for increased strength and other properties and applications.
When thermoset resins are melted, they form a permanent, heat-resistant material that cannot be modified without degradation.
When thermoplastic resins are melted they form a material that can soften or fuse when heated and harden again when cooled. Thermoplastics provide strong, durable binding at normal temperatures, and can be softened by heating without losing strength or undergoing degradation.
The main process used to form thermoset composites and thermoplastics is compression molding. Thermoplastics are most often heated at lower temperatures than composites.
Compression molding is a way to form large, fairly intricate parts using pressure and heat. Generally, the application requires consistent pressure requiring a dwell period to form the part. The material may be preheated and placed in a heated mold cavity, which often is heated through heated platens in a press, and may be cooled at a specified rate (depending on the requirement). The mold is closed with the material inside, pressure is applied to force the material together into the mold, and heat and pressure are maintained (to varying degrees) until the molded material has cured (see lead image).
Several heating and cooling methods are available per the application's specification. These methods include electric heating rods, steam/hot water, and hot oil. The optimal choice is contingent upon each user's specific application requirements.
Electric heating rods have fast heat- up and recovery times with a maximum operating temperature of approximately 1,200 degrees F (dependent on press insulation space). Single- or variable-watt heaters are available and can be controlled with varying types of temperature control over the span of the bed.
Steam/ hot water is used for accurate heating (±5 degrees F) with a maximum temperature of approximately 400 degrees F. Serpentine cooling circuits are often used with adjustable water valving for cooling following the heat cycle.
Hot oil has temperature accuracy comparable to steam/ hot water, but has an increased maximum operating temperature of approximately 700 degrees F. A heating element, similar to a Calrod® heater, is immersed into a vessel of high-temperature oil (will not degrade at high temperatures). The oil is pumped through a series of coils to maintain a setpoint temperature and circulates through platens in the press, which in turn heat the tooling through thermal transfer. This provides more even heating and uses less energy than inserting the Calrods directly into the platen.
Because compression molding uses heat and pressure to change the material properties, it is suitable for forming complex, high-strength, adhesive compounds for customized applications. Many times these parts are lighter than metal and have an equal or greater strength-to-weight ratio.
Advanced thermoplastic composites can also be compression-molded to produce fewer knit lines and less fiber-length degradation than injection molding.
Compression molding applications require precise pressure and position, speed control, and parallelism throughout the working stroke (see Figure 1).
A hydraulic press, used to form metal, 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 longer than for metals. A hydraulic press is well-suited to maintain energy-efficient pressure over an extended dwell period. The press system can be with variable-volume pumps, accumulators, lock valves, and so forth, with several control packages to monitor and manage the system requirements.
The press can be engineered with sizable daylight to allow enough space to install both the male and female tooling of large molds. Often increased daylight is required to form large contoured panels or shapes inherent in 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 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 inch to 1 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.