Wet Lay Up With Compression Tape
This is a simple but effective technique to get started making composite parts.
It is possible to create fully functional and top quality parts using a wet lay up technique, although to do so requires a lot of care and it can be a very messy process. Consequently, after some initial experience of a basic wet lay up technique you may prefer to graduate to more advanced processes. That being said, this technique allows you to get a good idea of material strength and the effect of different lay up configurations and it is a worthwhile introduction for that reason alone. It’s also possible to master this technique without any professional equipment or workshop space and it is therefore cheap to learn. Set out below is the basic, most crude, process and as you read it you can adapt and develop each stage to fit your specific project.
Wet lay up is a good introduction to composite materials.
Before starting any composite process you will need a clear idea of the part you are making and the lay up schedule (the shape and size of each piece of carbon fabric you will be using including when, where and how it will be placed on/in the mould).
What you will need to try out a simple wet lay up project.
The Basics:
The key feature of any composite part is the ratio of carbon fabric to resin. Too little resin and there may be small air voids trapped inside the part which makes it weak. Too much resin can also impact the strength of the part and will also make it heavy. Wet lay up uses dry carbon fibre fabric and a resin which cures at room temperature. The resin consists of two parts which are mixed in specific quantities (usually measured by weight) and once mixed, the resin begins to cure.
You will wet the fabric with resin and scrape any excess out, then wrap your mould with the wet fabric. It is important to ensure the fabric is fully wetted out but that you also scrape off as much excess resin as possible. The wet fabric is then compressed using basic electrical tape perforated with pin holes (you can use normal tape and stab it with a pin, or find some kind of perforated tape). As the carbon is compressed more excess resin will squeeze out of the pin holes in the tape as the part cures. After the resin has cured, remove the tape and you are left with a carbon part. You may need to tidy the part up afterwards and then repeat the lay up process to get the desired number of layers.
Achieving the correct ratio of resin to fibres is the key to producing good quality composite parts.
Step 1 – The Mould:
The first stage is to make a mould/mandrel to form the shape of your part. The mould will form a solid core for you to lay the wet fabric around, so it needs to be fairly strong (strong enough to hold its form under mild compression from the tape afterwards).
Moulds are commonly made from polystyrene foam, which you can buy in a sheet or block and cut/shape by hand, or for a repair, carbon can be laid straight on to the original frame tubes. Open, convex shapes work best for this technique, because you will need to wrap the electrical tape tightly around the mould whilst the carbon cures and ensure that the tape is able to compress the whole of the part. You may need to design your mould(s) in such a way that you make your part in two or more component pieces, each fabricated separately and joined once cured.
For a wet lay up curing under compression you need to start with a mould or mandrel to wrap carbon fibre over.
Step 2 – Set Up:
Lay out all your materials on the workspace. Cut the dry carbon fabric to the right size for each of the pieces you will need and set them out so you can easily grab them during the lay up. Once you mix the resin, it begins to cure. Typically a resin has a working time (pot life) in it's liquid state before it gels and starts to cure into a solid piece. Gel times / pot life depend on the resin you are using and most brands offer 'fast' and 'slow' compounds depending on how long you will need to complete the lay up. For larger or more complex parts you may need to do the lay up in several stages, mixing a fresh batch of resin for each.
Good preparation and an organised work area will help with a wet lay up.
Step 3 – Wet Out:
Wear gloves and eye protection. Mix the resin in a pot (follow the instructions for your specific resin for the precise quantities of each component to mix and note the work time it allows). Take each piece of carbon in turn and lay it on your protected work surface, pour a small amount of resin on to the fabric and spread it out before scraping off any excess. Ensure the whole of the fabric is completely wetted out then try to scrape off any excess. You may have to flip the fabric over and repeat the wetting out process on the other side. The key to wetting out is to saturate the fabric and then remove as much as the resin as you can.
Wetting out the fabric is a crucial stage of the process.
Step 4 – Lay Up:
Carefully wrap each piece of carbon fabric around your mould/mandrel following your lay up schedule. You can wet out each piece of fabric as you lay it, or wet out several pieces or all of the fabric and then wrap the mould. Make sure the fabric is wrapped tightly around the mould and work it around tight corners or into dips with your (still gloved) hands to ensure the carbon covers the mould properly. Any slack areas will be compressed in the next stage and create wrinkles or voids which will compromise strength and add unnecessary weight. You can apply more than one layer of fabric to any specific area as necessary, but the more layers you apply the harder it becomes to control the quality of the finished part. As a guide, lay up one or two layers at a time to begin with. With more experience you can try more than this, depending on your fabric weight.
Take care to ensure you wrap the fabric tight enough to ensure a smooth finish but not so tight that you deform your mould/mandrel.
Step 5 – Compression:
After laying up all of the fabric you need to wrap the whole part in tape to compress it. The aim of this stage is to squeeze out any excess resin with uniform pressure across the whole mould. A low tack perforated tape is best for this, for example by using the non-sticky side of some electrical tape with pin holes added. The small holes allow the resin to escape.
The holes in the compression tape allow the excess resin to seep out as the part cures, ensuring you achieve the optimum resin to fibre content.
Step 6 – Cure:
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 and is best left overnight.
Once you have compressed the whole of your part you can leave it to cure, according to the requirements of your resin system.
Step 7 – Finishing:
Once cured, you need to remove the tape to reveal the cured carbon part beneath. The carbon takes the form and finish of the surface it contacts during cure (in this case the underside of the tape). Depending on how careful you have been the part may have some wrinkles from where the tape compressed unevenly or where the mould deformed slightly under pressure. You may need to sand the part to get your desired surface finish. You may also need to repeat the process to add more layers of fabric if necessary.
The cured carbon part will always take the finish of the surface it contacts during cure - here you can see the thick dark lines where carbon fabric has been wrapped and the thin lighter lines where the tape has been applied.
Bear in mind that most resins degrade to some extent in UV light and therefore you may want to finish your part with paint or a UV protecting clear coat. It is possible to find resins with added UV protection which you can either apply at this stage or use from the start.
If you’re fed up with pricking holes in electrical tape, click here to learn about vacuum bagging.
A finished carbon part made with a wet lay up technique.