Automated solution: New presses for SMC molding process

Using glass/polyester sheet molding compound (SMC) that it formulates and manufactures in-house, Stahlin Non-Metallic Enclosures (Belding, Mich.) produces composite boxes and enclosures for electrical systems and components. Outfitted with eight compression molding presses, CNC cutters and SMC compounding equipment, Stahlin’s plant was running three shifts, six days per week to meet demand. Capital investment in new presses became a necessity.

Stahlin sought a partnership with an equipment vendor that could respond to the technical challenges its team had encountered in compression molding processes, including how to minimize part cycle time and reduce scrap. The company eventually selected Greenerd Press & Machine Co. (Nashua, N.H.) and purchased two custom-designed 400-ton, four-post hydraulic presses.

The large tonnage and bed size of the presses accommodates the majority of Stahlin’s existing mold tools, and the four-post configuration minimizes deflection and ensures repeatable die alignment. A die lift allows faster setups so that operators can quickly change molds without having to set the stroke, says Stahlin press operator Chuck Edmonds, and the press can be loaded from any of its four sides, further increasing efficiency by accommodating multiple work cell designs. Tonnage can be adjusted to accommodate different part molds for production flexibility. Dean Childs, Stahlin’s composites supervisor, adds that the presses meet all relevant rules for worker safety, and the presses’ open bed design also means that operators do not have to bend down to retrieve molded parts and have less chance of contacting hot surfaces.

The new presses feature a degassing cycle: Gasses generated during cure are evacuated from the mold, which greatly reduces the chance of small voids or pinholes on the part surface. Although the company’s older presses require manual, mechanical adjustment, the new presses feature computer touch screens. “With the computer controls, the parts aren’t pressed too quickly, which can cause the resin to squeeze out of the mold, or too slowly, which causes premature cure of the SMC,” Childs notes. In addition to improved part quality and productivity, the scrap rate reportedly has been reduced by at least 20 percent.