Compression molding was one of the original plastics processing methods developed at the dawn of the commercial plastics industry. As the inherent cost/performance advantages of this process gained acceptance in the marketplace, the resins in the compression molders� gamut gradually expanded � from the early thermoset resins (phenolic, epoxy, melamine and urea) to unsaturated polyester (the ideal material for glass fiber-reinforced composite parts). With the development of PE & PP in the 1950s, compression molding process was modified to accommodate these low-cost commodity thermoplastics as well.

Yet as the North American economy grew, hiking the corresponding demand for engineering materials, manufacturing industry increasingly sought fast-cycling plastic materials and processing methods to accommodate their mass-production methods. Compression molding and other thermoset processing methods fell into disfavor in comparison to thermoplastics molding methods that advanced. The compression molding business underwent massive corporate consolidation as players determined to remain in the business sought economies of scale. By 2000 the number of companies with compression molding operations had been drastically reduced, many long-term players in machinery manufacturing had exited the business, and the share of compression molding in total plastics production ebbed close to 1%. Then came the recession of 2001 and the events of 9/11, and the compression molders, like all other plastics processors, experienced a further production downturn.
The compression molding business, driven by a range of domestic and global economic pressures, has seen a revival since 2001. Energy as well as resin prices have soared. The impact on thermoset resin pricing has been more benign relative to the pricing of thermoplastics. Moreover, compression molding has been �rediscovered� as a highly cost-effective production process with low-cost molds and low-maintenance machinery. The car and heavy truck OEMs are intensifying their search for lightweight materials to produce exterior, interior and under-the-hood parts to enhance vehicle fuel efficiency. Builders and homeowners are converting from steel to lightweight and creative composite-skinned exterior residential doors. And although much of US electrical component and electronic equipment industries has gone offshore, the OEMs continue to regard compression molding as a critical component of their operations.

Compression molding as a plastics processing method has undergone numerous advances: In the 1990s, Minnesota based processor Composite Products Inc. (CPI) unveiled a new process for compression molding automotive and other structural composites from long-glass-reinforced thermoplastics which offered advantages in economics, processability, and part quality over processes using glass-mat/thermoplastic (GMT) sheet. This novel molding technique involved compression molding an extruded "hot log" of reinforced compound. The hot log compound was produced directly from glass and resin in a single step thereby eliminated the expense of buying pre-compounded material. CPI process offered several advantages over GMT sheet, including ability to mold more complex parts. Superior flow inside the mold resulted in more uniform distribution of fibers and got rid of resin-rich areas in complex parts. The process could result in substantial scrap reduction and enabled to reuse scrap material, much like in injection molding. The inherent cost advantages over GMT, resulting from scrap reduction and the elimination of the value-added step of sheet preparation, were to be drivers for future applications and licensees. The two most proprietary aspects included the extrusion compounding system and the preform accumulator. The compounding system needed a single-screw extruder built to CPI's proprietary specifications while the preform accumulator was designed and built by CPI. The control package also included a variety of dedicated microprocessor-driven software systems and monitors.

Most recently in K 2007 show in Dusseldorf, Italian company Sacmi Imola S.C. unveiled a new continuous compression molding process called Preform Advance Molding (PAM) for making PET bottle performs which resulted in cost reduction, higher quality and productivity, and the potential for weight reduction. The PAM system was an outgrowth of the company�s established machinery for continuous compression molding of beverage-bottle caps and other closures, which now can turn out up to 100,000 caps/hr. The PAM starts with an extruder. The system uses a 120-mm model with the processing capacity of 1760 lb/hr of PET. It continuously extrudes a cylindrical parison, which descends from a vertical die and is cut by a knife into a slug. The gob is grabbed by an insertion carousel and dropped into one of 48 mold cavities on a spinning carousel. As it turns (9.5 to 10.5 rpm), the cavities rise to meet the cores and thread splits and mold each preform under 2 tons of force. The molding carousel processes standard 0.5L carbonated soft-drink performs (23 gm weight, 120 mm long, PCO 1810 neck finish) at up to 450 to 500/hr. After a 6.5-sec molding cycle (approx. 90% of which involves actual pressing), the core and neck splits lift the preform out of the cavity and another robot transfers the preforms to a post-cooling carousel. The carousel has 256 sleeves in which preforms are air-cooled inside and out for the equivalent of 5.3 molding cycles. The preforms leave the system aligned on a conveyor, which permits integration of 100% vision inspection or direct delivery to a stretch-blowing machine. PAM system will be competitive with a preform injection system of similar output capacity. Molding at lower temperature and tonnage saves energy, Sacmi notes. But the biggest savings come from reduced scrap and potential lightweighting.

In 2003, Swiss manufacturer of aerospace, automotive, and medical parts - Icotec AG developed a novel compression molding process called Composite Flow Molding (CFM) that delivered net-shape thermoplastic composites with strength per unit weight reportedly competitive with those of machined steel, aluminum, and titanium. This process transferred up to 62% by volume of carbon fiber into a thermoplastic, yielding high-strength and abrasion-resistant screws, bolts, inserts, studs, anchor nuts, and other fasteners. CFM composites were increasingly being used to make medical implants and small (up to 0.5-in long) structural bearings and other parts. The favored matrix polymer was PEEK, supplied by Victrex PLC. The CFM process started with a pultruded PEEK/carbon fiber rod made by one of the German suppliers who employed a proprietary method of incorporating high levels of carbon-fiber tow into PEEK with virtually no damage to the integrity of the reinforcements. Icotec then cuts the PEEK rod into a blank whose volume is precisely equal to the volume of the final part. The blank was transferred by robot to a heating chamber where the PEEK melted and wets-out the continuous fibers, minimizing voids. The blank is then transferred to a compression mold cavity where it is pressed at precise speed, temperature, and pressure to form a part with a predictable fiber orientation to enhance part strength. The vibration-resistance of CFM fasteners had immense appeal for marine applications like rig fittings and the like. The company then worked to broaden the capability of CFM processing with additional resins such as nylon 6 and PBT and then to permit fibers other than carbon to be used, in particular ceramic, glass, and tantalum.
After this, Greenerd built the highly customized, up-acting compression molding press for filled urethane polishing pads used in semiconductor manufacturing. It had �2�F temperature control and �0.001 in. parallelism over 34 in. This press boasted temperature control of �2� F. It was specifically built for molding high-precision polyurethane polishing pads used in semiconductor manufacturing. This press was developed for a company which required temperature accuracy within �5� F and thickness variation of no more than �0.001 in over a 34-in span. The press was to be fed manually with batches of high-viscosity (280,000 cp) liquid thermoset polyurethane plus in addition to proprietary filler. There would be limited time to get the catalyzed material into the press, so the company designed a 22-in stroke to provide enough daylight for loading. An even complicated job was developing the controls and documentation needed for the semiconductor industry. The mold halves each have three temperature-control zones, whereas a 42 x 42 inch platen typically had only two zones.
In conclusion, recent advancements in compression molding processes has resulted in the improvement in quality, high perform consistency, faster molding cycles and a significant reduction in cost at a higher output. These factors have heralded a new era in which this process is gaining its ground once again.