Polyurethane dies improve aircraft repairs

Manufacturing Engineering, Jul 1999

While conducting periodic inspections and heavy maintenance checks on a variety of aircraft including DC-8s, FAA-approved repair facility TIMCO (Greensboro, NC) often must replace damaged aluminum components with new parts. To help get planes back in the air as quickly as possible, TIMCO repair technicians turned to a new metalforming polyurethane to cast a die and press one or two 0.025" (0.64 mm) thick, highly contoured 2024 aluminum alloy parts in less than 40 hours.

Before switching to the RP 6470 R/H polyurethane from Ciba Specialty Chemicals, Performance Polymers (East Lansing, MID, TIMCO used an epoxy system for the dies that often cracked under the 20-ton (178 kN) pressures needed to form the aluminum parts.

"The Ciba polyurethane works well for short-run forming, even when we work with thick aluminum fairings that have compound contours," according to toolmaker Alfonso Garrido at TIMCO's Composites Center. The RP 6470 R/H polyurethane with DT 082 filler features a Shore hardness of 91D, ultimate compressive strength of 13,900 psi (96 MPa), ultimate tensile strength of 4700 psi (32.4 MPa) and notched Izod impact strength of 0.42 ft-lb/in.

In preparation for pouring a polyurethane die for a part like a flap fairing, TIMCO toolmakers manufacture a master model using loft data. After applying sheet wax to the model surfaces to represent part thickness, they make a plaster splash. Technicians then build a plastic-faced plaster (PFP) tooling intermediate on the splash by putting down a layer of Ren RP 1118 universal surface coat backed by two layers of Ren RP 1710 epoxy and fiberglass. To build up the bulk of the tooling aid wall, moldmakers then use Ciba's TDT 177-62 paste compound to produce a fast curing, lightweight PFP. After the PFP fully cures, surfaces are sanded to remove about 5% of the thickness to allow for springback of the metal when it is pressed.

TIMCO builds a metal frame around the PFP to begin the first die half and all surfaces are sealed and released. Ciba's RP 6470 R/H polyurethane is then mixed with DT 082 filler to reduce costs and minimize exotherm as thick sections are cast. The mixture is poured into the PFP cavity until the frame is full. After the polyurethane cures overnight, the die is removed from the PFP, sheet wax is added to allow for thickness of the metal parts to be formed, and the second die half is poured following the same process.

"The RP 6470 polyurethane is easy to mix and pour to form a highquality tool with a good surface texture," says Garrido.

To form the fairings, TIMCO installs the die in the press. A precut piece of aluminum is placed between the two halves and the dies are clamped shut to form the sheet. After forming, the aluminum is put in an oven and soaked for 25 minutes at 920 deg F (493 deg C) to temper it. The part then is quenched in a water solution to reduce the temperature to approximately 77 deg F (25 deg C). After age-hardening at room temperature for 96 hr, the fairing is tested to verify that it has attained the needed hardness before being bonded in place on the aircraft. Circle 225