Resin Infusion
A cleaner way to acheieve vacuum bagging perfection.
This process is essentially the same as for vacuum bagging, except that you lay up the fabric dry and add a special resin at the end. The resin is drawn through the mould under vacuum and is absorbed by the dry fabric as it passes. Because, in theory, the fabric absorbes enough resin to fully wet it out but no more the finished part should always have the optimum resin content. It is also known as Resin Transfer Moulding (RTM).
Resin infusion is much cleaner than wet lay up methods.
Step 1 – Moulds And Set Up:
Follow the same process as explained for vacuum bagging, setting up the mould and preparing a vacuum bag. The difference here is you are not under any pressure of time at any stage, so you can take your time making the vacuum bag and leave it until after you have laid up the carbon fabric in the mould if you choose.
You will have to consider how the resin lines will flow through your part when designing/making your moulds so read the full guide below before starting.
You will need a more sophisticated workshop set up to master resin infusion.
Step 2 – Lay Up:
Lay up dry fabric in the mould, adding multiple layers as per your lay up schedule. You can use a spray on adhesive to tack the fabric in place if necessary. On top of the dry carbon fibre fabric, add a layer of peel ply. On top of the peel ply add a single layer of infusion mesh and attach it in place on the mould with some tape.
A mould with dry fabric laid up and stuck in place. Without any resin at this stage the fibres can become frayed.
Step 3 – Set Up The Resin Lines:
Then you will need to plan how the resin will flow through the part. This may inform your mould design and you can either incorporate some resin ‘ports’ into the mould or at least allow space for the resin lines on either end of the part. Under vacuum the resin will seep through the part, using the pockets created by the infusion mesh, taking a route from the resin line in to the vacuum line out.
Spiral tube is narrow plastic tube you can buy which has a spiral cut along it's length - which means it leaks very efficiently. You can use spiral tube to disperse resin from the end of the input line to a wider area, for example running spiral tube across the length of a mould. You should then position the output on the opposite side of the part, so the resin is drawn through the mould and across the whole part.
A resin infusion set up with resin input lines running around the outside of the mould and the output line from its centre.
Step 4 – Set Up The Vacuum Bag:
Once you have placed the resin input line with cut spiral tube and the vacuum output line and secured them with some tape, you can place the mould assembly inside your vacuum bag. Seal the bag and connect the vacuum output line to the catch pot and the catch pot to the vacuum pump.
There is no substitute for a proper composites vacuum pump with this process because you need to be able to maintain a full vacuum for well over an hour and ideally longer.
You need to clamp the resin input line closed and then you can then pull a vacuum on the bag using the pump. You need to ensure the full mould is under pressure from the vacuum bag as before, taking care to avoid bridging and also sharp edges that may pierce the bag. At any point up until now you can take a break in the process, even leaving things for a few days if necessary.
A close up of the mould showing infusion mesh on top of peel ply as well as resin lines and cut spiral tube taped in place. Also shown is the resin output line.
Step 5 – Add The Resin:
Mix up your resin, follow the instructions for the specific type of resin you are using and leave it to settle to ensure the air bubbles can dissipate. Once the resin is ready to use you can drop the resin input line into the pot of mixed resin, switch the vacuum pump back on to ensure a complete vacuum, before you unclamp the resin input line.
Part way through the infusion, resin enters to the right hand edge of this mould and is drawn across the part to the output line on the left edge. The peel ply turns dark as it is wet out showing the progress of the resin.
Step 6 – Open The Resin Lines:
This is the most rewarding part – watch as the resin is drawn through the input line and the cut spiral tube and then bubbles and seeps across the part. The peel ply will turn dark as it is wetted out with the resin and the ‘tide’ of resin should move gracefully across the whole of your mould until resin starts to be drawn out of the output line and through the tube to the catch pot. Depending on the flow lines you have devised and the size of your mould and part it can take anything from a few minutes to over an hour to wet out the whole part. Once the peel ply has all darkened and you are satisfied the whole part is fully wetted out you can close the input line again and dispose of excess resin.
A close up of the resin 'tide' making its way through the mould.
Two notes of caution at this point. Firstly be careful to ensure the level of the resin in the input resin pot does not fall below the level of the input line so that air is drawn into the input line and so the mould. If necessary you can clamp the input line closed halfway through the process and mix up a new batch of resin to add to the pot. Secondly, be aware that the curing of the resin gives off heat, therefore avoid leaving any significant quantities of mixed resin in a pot to cure. If it gets too hot it could melt through the pot and spill over the floor, possibly melting other things on the way.
The resin is drawn through the part by the force of the vacuum in the pockets of the infusion mesh and is absorbed by the peel ply and, more importantly, the dry carbon fibre fabric as it passes through the part. Therefore, if you have a full vacuum to begin with you need only open the resin line and let the resin flow, however, there will inevitably be a small amount of air in the input line ahead of the resin and there may well be some small leaks in the bag. Therefore it is best to leave the vacuum pump running until at least the full part has been wetted out and you have clamped the input line closed.
A fully wetted out part, left to cure under vacuum as excess resin is sucked out through the output line (in the centre).
Step 7 – Cure And Demould:
Next, leave the part to cure according to the temperature and time requirements of the particular resin you are using. Typically a room temperature curing resin will take around eight hours to cure.
After the cure cycle is complete you can remove the mould from the vacuum bag and as before remove the peel ply (and infusion mesh) before removing the part from the mould(s).
The inside of a cured part showing the pattern from the (removed) infusion mesh.
Done correctly resin infusion is a good way to achieve consistent resin content throughout a part and reduce air or resin filled voids.
It is possible to achieve a part quality that closely matches that of pre-preg. For the very best results though, the easiest way to ensure optimum resin content is to actually use pre-preg materials. Click here to learn about that.