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Composite parts within less than five minutes

News International-French

13 May 2015

High volume manufacturing composite press by Schuler in operation at National Composites Centre in Bristol, UK.

The National Composites Centre (NCC) in Bristol, UK, is a joint working between industry, academia and government in order to build the future for composites. The unique open-access but secure facility which is owned and hosted by the University of Bristol provides a flexible approach with the aim of delivering truly innovative composite solutions. Part of this story is an innovative Schuler press for high-volume manufacturing of composites that was put into operation about half a year ago.

The upstroke short-stroke press has a press force of 36,000 kN and a clamping surface of 3.6 x 2.4 m.

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 shorter cycle times for complex parts with high requirements regarding geometry and rigidity, but also deliver consistently high part and surface quality. This virtually eliminates voids, i.e. resin-free vacuum pores or gaps within the part or along its edges.

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.

Due to the geometry of the part 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.

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.

High closing speeds and short pressure build-up times
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.

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