- Upstroke press: 36,000 kN and 3.6 x 2.4 m
- Employment of HP-RTM process
- High closing speeds and short pressure build-up times
The National Composites Centre (NCC) in Bristol, UK, is operating an open-access facility for the development of new composite materials. The facility, owned and hosted by the University of Bristol, is equipped with an innovative press from company Schuler for the high-volume manufacturing of composites.
Upstroke press: 36,000 kN and 3.6 x 2.4 m
The upstroke short-stroke press has a clamping force of 36,000 kN and a clamping surface of 3.6 x 2.4 m. “We have installed the world’s largest openly accessible high rate manufacturing press for composites,” says NCC Business Development Director Tom Hitchings. The press is aimed at supporting sectors such as automotive, where developing high rate, low cost manufacture of composite components is critical. This press is producing parts within less than five minutes, according to NCC.
Employment of HP-RTM process
The NCC uses the HP-RTM process (High Pressure Resin Transfer Molding) in which carbon fiber fabrics are placed in a die, filled with resin and hardened by applying heat and the pressure of the press. High-pressure RTM presses not only enable short cycle times for complex parts, but also deliver consistently high part quality. Voids, i.e. resin-free vacuum pores or gaps within the part or along its edges, are virtually eliminated.
In the high-pressure RTM process, resin is injected as quickly and smoothly as possible into the vacuum mold which is opened by just a few tenths of a millimeter. This gap injection process enables the resin to spread over the mat with far less flow resistance and thus with low injection pressure. It then quickly infiltrates the mat before polymerisation is started by heat induction.
Curing process begins with the wetting of the last fibers
The vacuum assistance, fast resin injection, high resin pressures and tempered dies mean that the gelling process virtually begins with the wetting of the last fiber. The thicker the part, the longer the curing. Depending on the part, the necessary resin pressures also vary between 60 and 150 bar.
Depending on the part geometry or cavity surface, the die’s center of loading is not necessarily in the middle of the press. There are also off-center forces from the injection positions. The parallelism control prevents the slide or upper die from tilting during gap injection and thus ensures smooth and even injection over the entire surface.
High closing speeds and short pressure build-up times
Conventional downstroke machines work with a fixed bed and moving bolster, and a slide whose press force is transmitted via cylinders in the press crown. Parallelism is ensured by four servo-controlled counter-pressure cylinders located at the bed corners. These are also responsible for the break open force needed to counter the adhesive forces and open the die.
In the upstroke short-stroke press, the slide only acts as support during the pressing process. From top dead centre, the slide is moved by a drive cylinder to its support position and locked there. The actual working stroke is performed by the bed plate, driven by several short-stroke cylinders. Parallelism is ensured by the servo controlling of these cylinders. The breakout force in upstroke presses is achieved by the withdrawal of the bed plate. The benefits of the upstroke short-stroke press compared to downstroke designs are the high closing speeds of 1,000 mm/s, the shorter pressure build-up times of under 0.3 s and the significantly lower construction height.