For all the honors heaped on single sheet pressure forming, no conventional thermoforming can make a hollow part unless two halves are made separately and bonded together. Twin Sheet Pressure Thermoforming can make that same part in a single operation. Depending on part size and quantities, it can actually do the job much more economically than blow molding.
At Grimm Brothers, two separate processes are employed to Twin Sheet Pressure Thermoform. In the "SST" (Single Station Twin Sheet Thermoforming) process, two material sheets are simultaneously clamped into one frame. A metered air probe is inserted between the two sheets to prevent them from sticking together during the heating cycle to come. The frame is shuttled into an oven so the material can be heated to the proper forming temperature. Then it's shuttled back out of the oven and positioned between two mold halves which are mounted on a top and bottom plate that come together, sandwiching the two halves close, a powerful vacuum literally "sucks" each sheet into their respective mold half and high pressure is applied through additional air probes. The high pressure along with a special pinch off design and high heat effectively "welds" the two pieces together into a single hollow, molded part. The final product advances through an appropriate series of fully automated secondary operations, delivering an end result as polished, refined and sophisticated looking as any blow molded or injection molded part. And to add either rigidity, insulation or sound deadening qualities, SST parts can be urethane foam filled.
We call the other process Sequential Twin Sheet Pressure Forming. This process requires a specifically designed four station rotary thermoforming machine. Rather than starting with two plastic sheets, one sheet is heated and formed in one half of the mold. Then a second sheet is heated and formed in the other half of the mold. The two tool halves are then closed to bond the two sheets together. The same "pressurizing and welding" that happens in the SST also happens with this process. The time lapse of the first sheet to the forming and mating of the second sheet, about 15 to 20 seconds, allows enough time to insert a metal, wood, foam or other structure between the two sheets. This insert gives a designer opportunity for a super structural part. CAD-assisted design services are available to help make sure costs are kept to a minimum and your finished parts do exactly what you want them to do.