Sensors for short run moulding

Tom Shelley reports on a development that greatly improves the production of major fibre reinforced parts A Cornish company has developed pressure sensors specifically designed to optimise the control of 'Light' Resin Transfer Moulding (RTM). Made with PTFE bodies, they use only an air pocket to separate the resin from the pressure sensing apparatus, rather than an expensive foil diaphragm susceptible to damage. In Light RTM, the process steps being by applying gelcoat as usual. The reinforcing media is then placed in the mould. The two halves of the mould are then brought together and a vacuum applied to seal their contact areas. Resin is injected into a feed channel round the part perimeter while vacuum is applied near the mould centre to draw resin in through the reinforcing fibres. The part is then cured, demoulded and finished as usual. Because the resin is drawn in by vacuum, the pressure needed to inject the resin is lowered. Also, the mould needs only to be lightly constructed, relying on natural atmospheric air pressure to provide the clamping force. The process produces lower environmental emissions and ensures improved quality and part to part consistency at reduced cost per part as compared with processes relying on resin injection at elevated pressures requiring much heavier tooling. It does, however, require sufficient pressure to ensure that the resin penetrates all parts of the mould and reinforcement, without that pressure causing the mould walls to flex, making the part thicker than it should be. Plastech, which is based in Gunnislake has developed two types of what it calls, 'PVSensors'. MD Alan Harper tells us that the first screws into a 3/8 in BSP mould insert positioned in the peripheral resin feed channel. The all PTFE body and tip design provides compatibility with resin systems, while the chip sensor is deeply embedded within the body to be well isolated from the resin exotherm and its chemical nature. In service, the sensor signal is fed to the system electronics providing accurate digital pressure readings and signals to ensure optimal control of the delivery of material from the meter mix machine. As well as ensuring optimal mould quality, the system ensures very little mould overfill. The second type of sensor is designed for film infusion applications. Similar in design, it also has an all PTFE body, but is adapted to fit directly on the film of the infusion mould face without an additional insert. It has a large sensor body base to provide sufficient contact with the film, enabling the addition of tack sealant to ensure vacuum integrity. After use, it is simply lifted away from the cured resin without any need for cleaning. Users include a major European wind turbine blade producer which uses several PVSensors over a large mould surface to monitor vacuum levels and resin pressure increases as the mould fills with several hundred kg of resin. Enter Plastech TT Pointers * Sensors allow the better control of Light Resin Transfer Moulding, ensuring improved part quality at lower cost * Sensors rely on use of PTFE bodies which do not stick to the resin and air to separate the resin from the chip sensing device