Vacuum Bagging
A step closer to creating professional parts with a wet lay up.
This process uses the same wet lay up technique as set out here but instead of compressing the wet fabric with tape you use a vacuum bag. This is a very common technique for composite fabrication but it is still relatively crude and easy to learn. The wetting out still requires some skill and can result in inconsistencies in the quality of your finished part but because of the uniform and (relatively) high pressure compression from the vacuum bag it is possible to achieve excellent results.
Vacuum bagging is a step closer to producing professional parts with a wet lay up.
Step 1 – The Mould:
By using a vacuum bag to compress the carbon fibres and squeeze out excess resin you can use a wider variety of mould shapes compared to using tape. It is possible to design an ‘external’ concave mould, where the mould forms the outer surface of the part and is not a trapped core inside it. This can open many more possibilities for moulding your final part, for example you can create hollow tubes by making them in two open half moulds and joining them after, either by bonding or wrapping additional carbon as a second fabrication step. If you intend on bonding finished parts together it may be possible to incorporate some kind of tabs to the parts to make this easier.
Vacuum bagging opens up a wider range of mould shapes compared to using compression tape.
When designing the mould system you will need to bear in mind how you will remove the rigid, cured carbon part at the end. You will need to be aware of the direction the part will move as it is removed from the mould and the angle of that to the various faces of the mould. The easiest release will come from lower angles, i.e. flatter surfaces, where you can prise the part out.
The biggest advantage of using an external mould system is that the mould will provide the outside surface of your finished part and therefore you can better control the appearance and outside dimensions of your parts. Also, bear in mind that the final part will take on the exact surface finish of the mould it is cured in, therefore you can finish and polish your mould to create the finish you want. After finishing the mould you will need to treat it with a mould release agent to help you remove the part after the resin has cured, this can be wax or a type of polish.
Using a mould that will define the external face of your part gives you the advantage of having more control over the surface finish of your cured part.
Step 2 – Make The Vacuum Bag:
You can buy everything you will need for vacuum bagging from a range of specialist composite suppliers. There are several different systems but essentially they all achieve the same end, leaving you with an air tight bag that you can seal your part inside, drawing the air out through a pipe/tube to create a vacuum inside.
A common technique uses two thin plastic sheets which are joined around the edges with double sided gum tape to make an air tight bag. Often you can buy the plastic sheets (or film) in a tube or sleeve, which is already joined down two edges which saves time. There is plenty of guidance elsewhere online to help you make a bag and any technique you use will benefit from practice.
An empty vacuum bag made with double sided gum tape along three sides. The inset shows how you can overlap the tape at the bag edges to get a better seal.
At this stage you want to make a vacuum bag that is sealed along three edges and remains open along the fourth, so you can place your mould inside later. It helps if the open edge can be sealed quickly once the mould is inside, so for example you can add double sided gum tape to one side of the open edge. Once you start mixing up the resin it will start to cure, so it is important you can seal the wet mould inside the vacuum bag and compress it as soon as possible.
You will also need to have added the air tube to the bag, this can be either through a professional metal valve system screwed into a small hole in the bag or more crudely by poking some plastic tube through the bag and sealing it with gum tape on the inside and outside. You will also need to connect the tube to your vacuum pump, again either a professional vacuum pump (recommended) or something else capable of drawing air through a pipe (e.g. a domestic vacuum cleaner). Again, at this stage you want the bag and pump to be primed and ready to use as soon as you have placed the mould in the bag and sealed it.
Two common methods of connecting the vacuum tube to your bag, left is a professional full metal connector which screws into the bag creating a seal, right the tube is pressed through a hole in the bag and sealed with double sided gum tape inside and outside the bag.
If you end up with a perfect seal on the vacuum bag you will only need the pump to draw the air out of the bag and create the vacuum, you can then close off the air tube and leave the sealed bag under vacuum. If you don’t achieve a perfect seal (which is likely to begin with) you will need to run the pump whilst the resin cures, which could be as long as eight hours. You need to bear this in mind when deciding what to use as a pump, professional composite vacuum pumps usually operate in two stages, to draw air out in high volume and then to maintain a (close to) full vacuum for a long time. They can safely be run for several hours as long as the leaks in your vacuum bag are very small, the same may not be true for your hoover. If your bag loses pressure before your part is fully cured it will not be sufficiently compressed which will lead to voids inside the part, filled with either air or resin, which cause weaknesses and added weight.
An alternative to using a full bag is to use a single sheet ‘half bag’ which attaches directly to an open mould. Rather than preparing a bag as described above you can prepare your mould with double sided gum tape around the edge. You will need to add surplus bagging film, to allow the bag to drape inside the mould fully, by adding pleats, as pictured. This can take longer to finish and therefore is more suitable to a resin infusion technique rather than wet lay up, where you are under a pressure of time as the resin cures.
A half bag set up, where the vacuum bag consists of a single sheet attached directly to the mould. Adding pleats at various points provides enough slack for the bag to spread into all the troughs and channels in the mould.
Step 3 – Prepare All Your Materials:
You will need to prepare all your materials before you mix any resin. This will involve cutting to size all carbon fabric or tape you will need as well as cutting to size all the peel ply and breather cloth you will need. Peel ply is a thin, course fabric that is treated with a non-stick chemical to allow excess resin to soak through it but without it become bonded to the final part. Breather cloth is a cheap fluffy fabric that will soak up the excess resin as it is squeezed out of the part.
It will help to lay all of your pieces of fabrics out so that each piece is easily to hand, as well as laying out all your other equipment and your lay up schedule. Once you mix up the resin it will start to cure and you will need to complete the lay up before it reaches its ‘gel’ stage. The lay up can be a very messy process and therefore it is helpful to be organised with setting everything out at this stage.
Once you mix the resin you need to move fast. It is essential to have all your materials organised in advance.
Step 4 – Wet Out Your Fabric:
Using the same technique as here wet out the carbon fabric and lay it on your mould/mandrel in accordance with your lay up schedule.
Mix up resin and wet out the dry fabric in the same way as for a standard wet lay up.
Step 5 – Add Your Vacuum Bagging Consumables:
After you have laid all of the fabric in/on your mould(s) you need to add a layer of peel ply, to cover all of the carbon fibres. There should not be any carbon fibre visible it must all be covered by peel ply, it is okay to overlap the peel ply if it is necessary to use several pieces. On top of the peel ply add a layer of breather cloth.
A mould with carbon fabric and then added peel ply (left) and breather cloth (right)
Step 6 - Compression:
Place the mould with the wet carbon fibre, peel ply and breather cloth inside your vacuum bag and seal it. Draw the air and create the vacuum to compress the part. It is important to ensure that all of the wet carbon fibre is under pressure from the vacuum bag, to achieve this monitor the bag as you draw the air out and manipulate the bag as necessary. A common problem at this stage is where the bag ‘bridges’ over tight concave mould shapes and creates a space where it is not compressing your part. You can solve this by pulling the bag and making a pleat over the affected area, to give the bag some slack. Another problem can be caused by the edges of the mould piercing the bag as the bag is pulled tightly around it (which can be loud), you can mitigate this by adding a layer of breather cloth or some other padding to any sharp edges of the mould, or by smoothing any sharp edges as part of step 1.
Depending on where you place your air tube and how much excess resin will be squeezed out of the part you may need to take care to ensure resin is not drawn up the air tube and into the vacuum pump. The best way to avoid this is to use a catch pot, which is a sealed container placed along the air tube, between the bag and the pump, which catches any resin which makes its way up the tube. A cheaper alternative is to place a folded piece of breather cloth at the end of the air tube inside the bag.
A catch pot can help prevent resin being sucked into your vacuum pump (pictured with blue line in from the vacuum bag and clear line out to the vacuum pump).
Step 7 – 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.
A part is left to cure under vacuum.
Step 8 – Finishing:
Once cured, you can remove the part from the vacuum bag and rip out the breather cloth and peel ply. If the vacuum bag worked and kept the whole part under pressure the peel ply will have become bonded into the carbon fibre. This is the best way to tell if the compression worked – the exposed carbon should have a matt patterned finish (from the peel ply), any areas with a glossy finish or where you can see the carbon weave pattern have not been compacted fully. The peel ply should rip off the carbon with moderate force and take any breather cloth with it. You should also see the excess resin which has soaked in to the breather cloth, making it rigid.
The part will also be bonded to the mould and might require some force to remove it, depending on how successful the mould release agent was. Depending on the surface finish of the mould the cured part may not require any additional finishing. You may want to sand the surface and add a layer of wet resin to seal the part and give it a glossy finish. 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.
A cured part, with peel ply removed.
If you are fed up with the mess of wetting out fabric with resin or the time constraints of the resin cure cycle (for example if you are making a complicated or large parts) or if you want a more reliable way to achieve the optimum resin content click here to learn more about resin infusion.