Resin infusion

Resin infusion is an advanced laminating technique that greatly improves the quality and strength of fibreglass parts versus conventional hand lay up. The technique allows for strength and weight improvements in the final parts. Resin infusion has been in use since the 1960s and is now the standard for applications requiring lightweight but SUPER-strong end products. Resin infusion is the standard technique used for high-performance yacht hulls and UAVs to name but two examples.

How IT Works

Inside the mold, after the usual mold release wax is applied, the gel coat, and skin coat of thin fibreglass reinforcement are applied in the conventional manner and allowed to cure.

The fibreglass material being used for the outer skin in the infusion process are carefully fitted into the mold over the skin coat. They are put in dry and held in place with a spray contact adhesive, allowing the material to be orientated precisely. Because this part of the job does not have the time constraints that are imposed by wet lay up techniques, technicians can improve the strength of the final piece in various ways. In addition, attention can be paid to quality and the conscientious [...], fitting and orientation of the fabric's fibres and core.

Next, in the case of a part with a structural core, the core materials are cut, fitted and adhered into place. Then the inner skin of reinforcement fabric is carefully fitted over the core to form a sandwich. After this, a layer of peel ply, which allows it and the layers above it to be removed from the finished piece easily, is applied. Then a layer of flow medium is applied to enable the resin to flow throughout the job easily and not leave any dry patches. Flow medium is a plastic mesh that can be structured in a honeycomb or crosshatch pattern or even in a random swirl pattern. It allows easy movement of the resin.

Finally, the resin distribution hoses and vacuum lines are laid out on top of the fibreglass and the entire inside of the mold is covered with a large sheet of loosely fitting plastic sheeting that is sealed onto the mold’s perimeter using sealant tape. Using the vacuum pump, all the air is removed from the job which compresses the dry stack of reinforcement fabrics.

Through the series of feed hoses sealed into the bag, catalysed resin is then sucked via the vacuum from large mixing containers. The resin travels through the mold, saturating the entire stack of dry laminate material. The vacuum is maintained until the resin has cured several hours later. The vacuum bag, feed hoses, flow medium and peel ply are removed, and the lamination is complete.

The Result

The quality of an infused part is stronger, lighter and superior to a piece done using conventional hand lay-up.

Traditional wet lay up methods of bonding the core to the skin call for a polyester paste that is manually spread onto the cured surface of the fibreglass, with the core being bedded into it. Clamping pressure is applied until the bonding material is cured. The integrity with this method relies upon the technician’s skill level, the performance of the bonding paste and its ability to adhere evenly to the cured skin. The process almost always leaves air gaps in the bond layer. This, in conjunction with the inherent relative weakness of the bonding material, all constitute significant strength losses.

With resin infusion the numerous benefits and significant strength gains are due to the method of consolidating the materials within a vacuum all at once. The huge clamping pressure of the vacuum (approximately 1 ton per square foot) helps fuse the materials together and any air gaps are replaced by resin. Due to the reliability of high quality results with this process, and the reduction of potential errors by the skill of the laminator, the engineer can afford to specify less material in the structure. This, along with the vacuum compressing the fibreglass and reducing the amount of resin absorption, results in a typical weight saving of over 30% over traditional cored fibreglass laminate while improving its strength.

In addition the emission of vapours and pollutants is greatly reduced, because they stay sealed within in the vacuum bag. It is also a much less messy process than wet lay up techniques.

Going one Step Further

For ultra high strength and increased weight savings, some companies are creating parts with Carbon Fibre as opposed to glass fibre. Carbon Fibre is stronger and much stiffer than glass for the same weight. While carbon is not itself lighter than glass, a part can be engineered to be lighter by using less resin and fibers. Kevlar fibers are lighter than glass or Carbon and have good structural properties that also allow for stronger and lighter parts than glass.