It's in the bag!

Said to be more user-friendly and cost efficient, Alan Harper, director of Alan Harper Composites explains how the use of reusable vacuum bags are fast becoming a more attractive proposition for open composite mould workshops. The world of composite moulding encompasses many processes. The open mould hand lay method is the most universal approach, whereby fibre mat is placed upon a pre gel coated mould and manually laminated with brushes and rollers in an open mould process.

The very nature of the resin being spread over large areas, whilst the worker consolidates the fibre to eliminate air voids, means they can be subjected to a high degree of volatile organic compounds (VOCs), such as styrene vapours for example. This isn’t good news for the worker - or the industry - and various industrial countries are attempting to enforce more stringent worker exposure limits, resulting in a change for the better for workers in this field of manufacture.

Saltash, Cornwall-based Alan Harper Composites specialises in the production of closed moulding innovations and believes in protecting the environment while providing the closed mould and composite plastics sector with cost-effective and efficient technology.

The company has designed and produced a reusable vacuum closed moulding system that permanently removes the need for costly and environmentally unsound use and disposal of one-time use composite consumable bags.

“Closed mould laminating by resin injection, vacuum infusion and press, all using a second matching mould membrane of various types, have evolved to minimise worker exposures to VOCs,” states company director, Alan Harper. “This means that the mixed resin is pressure- or vacuum-infused into the fibre pack within the closed cavity mould leading to little or no worker VOC exposure.

“Naturally there is a cost implication as moulders now need to make a second matching mould membrane to the first open mould. This can take the form of another accurate composite mould with seals and steel frames, or more simply, a film of nylon sheet stuck down around the mould periphery to allow vacuum sealing. Unfortunately both are met with resistance from the average moulder who sees them as yet another cost, reducing profit margins and more complication to what has always been a very low tech hand layup manual process.”

As secondary matching mould surfaces, reusable vacuum bags (RVBs) are now becoming attractive as they represent a much lower cost entry for the average open mould workshop into these potential advantages and meet the legislation demands that have been satisfied through using the closed mould manufacturing process.

“Basically, a flexible RVB is produced to tailor fit the corresponding mould,” Harper concludes. “It needs no maintenance of release agent or constant re- sealing, as edge seals are a moulded feature of the bag flange design. Raw material cost per m2 at best is only £60 and can be manufactured and ready for repeated production use within two hours. These features are a considerable saving of resin transfer moulding (RTM) or light resin transfer moulding (LRTM) counter moulds and even if consumable vacuum bags are considered, they are paid for with a RVB within six moulding cycles. Able to operate up to 240°C and with self-release, they offer considerable advantages over both semi-rigid and one time use film membrane moulds

“Although the RVBs employ the use of silicone in the closed mould process, we’ve undertaken independent scientific analysis and studies which show no silicone material contamination. This has been done by infrared and x-ray spectrometry, and also lap shear testing to Airbus standards on samples made by silicone RVBs.”

www.alanharpercomposites.co.uk