Temperature control on compression molding presses for thermoset sheet and bulk molding compounds typically does not have to be all that precise. Control within ±5° F is usually adequate for BMC and ±10° F for SMC (with automatic shutoff for temperature deviations over 18 to 20°). Compression molding long-fiber thermoplastics, which uses cold rather than hot molds, has even broader thermal tolerances.
But specialty thermoset molding for the semiconductor industry is different. Greenerd Press and Machine Co. has just built a press that boasts temperature control of ±2° F. It is custom built for molding high-precision polyurethane polishing pads used in semiconductor manu facturing.
Greenerd, which has built manual and hydraulic presses for metal forming since 1883, is a relative newcomer to plastics applications. It had built custom clamps for BMC and SMC molding 20 years ago, but pulled out of the market because of its lack of expertise in composites, says Thomas Lavoie, applications manager. Two years ago, however, Greenerd began to get back into composites and built several large clamps, such as a 600-tonner with 10 x 3 ft platens for SMC lighting fixtures and a 400-ton, 7 x 7 ft press for a windmill propeller support. The new high-tech press for semiconductor polishing pads is much smaller—120 tons and 42 x 42 in. mold bed. It was developed for Freudenberg Nonwovens Div. in nearby Lowell, Mass., which wanted temperature accuracy within ±5° F and thickness variation of no more than ±0.001 in. over a 34-in. span. The polishing pads come in sizes from 10 to 34 in. diam. and 55 to 80 mils thick.
Freudenberg was entering a new market with only one major competitor and many smaller ones, and it wanted to get into production fast. It chose Greenerd based on its press design and proximity—being 20 minutes away made collaboration on product development easy. Greenerd recently delivered the first two model 25R3 presses to Freudenberg and has two more on order.
“We work very hard to get to know what the customer wants to do,” Lavoie says. The press was to be fed manually with batches of high-viscosity (280,000 cp) liquid thermoset polyurethane plus a proprietary filler. There would be limited time to get the catalyzed material into the press, so it needed a 22-in. stroke to provide enough daylight for loading.
The machine also has an up-acting stroke because Freudenberg’s plant has low ceilings. Large daylight and up-acting closure made the platens trickier to keep level, Lavoie says. Greenerd used four posts to reduce deflection across the platens.
A bigger 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 in. platen typically would have only two zones. (Temperature has to be higher around the edges to keep thickness uniform in a flat part.) There are 15 electrical heating elements in each platen. Opening speed and position are controlled independently for a profiled stripping stroke.
For quality assurance, Freudenberg’s semiconductor customers want electronic records of mold temperatures, clamp-speed profiles, and cycle times. So Greenerd modified the clamp’s PLC control to interface with Freudenberg’s data-collection and remote monitoring system. Freudenberg manufactures the polishing pads in a Class 7 clean room, so the hydraulic power unit had to be 20 ft away from the clamp station. Some hydraulic pressure is lost in the long connecting hose, which meant Greenerd had to deliver an initial 2500 to 2600 psi of hy draulic pressure to maintain clamp pressure of 2300 to 2400 psi.