Beginners' Guide to Out-of-Autoclave Prepreg Carbon Fibre

Part 1 of 3

Explanation of this Guide

One of the most advanced techniques for making composite components is by using prepreg carbon fibre reinforcement, cured under heat and pressure to produce professional quality parts with a high quality surface finish, low resin content and excellent structural performance.

Uncured prepreg carbon fibre is easy to handle and can be cut and laid precisely into detailed and intricate moulds making the process ideal for smaller, complex parts that might be very difficult using alternative processes such as traditional laminating, vacuum bagging or resin infusion. The ease and accuracy with which the material can be templated and cut means that material can be used very efficiently; significantly reducing waste compared to other manufacturing techniques. Prepreg laminating is also a very clean, odourless process.

Despite the many advantages to prepreg production and its potential for delivering excellent results on difficult mouldings, it is a process that is widely misunderstood and often overlooked or dismissed by smaller composites companies or individuals who do not realise that with the right materials and advice it is a process that can be undertaken and mastered with very little specialist equipment. The purpose of this guide is to show you how.

Introduction to Prepreg

What is ‘prepreg’?

Prepreg is the name given to composite reinforcement materials, such as carbon fibre, that have been pre-impregnated with activated resin. The resin system used is almost exclusively epoxy resin, which has already been mixed with its hardener at the manufacturing stage, before being impregnated into the dry reinforcement fabric to make the ‘prepreg’ reinforcement.

The most common way to make a composite part from prepreg reinforcement is to layer the uncured prepreg reinforcement into a mould, vacuum bag the mould and laminate and then cure it in an autoclave or an oven. Although autoclaves are beyond the budget of individuals and most small businesses, curing pre-pregs in an oven, known as ‘out-of-autoclave’ or ‘oven-only’ curing, is a very effective and accessible way to use prepreg technology that almost anyone can use.

Shelf-Life and Out-Life

Special formulation of the epoxy ensures that at ambient temperature the curing process is incredibly slow (often several weeks) and that at freezing temperatures (typically -20°C) the process is halted almost completely. The amount of time the resin system can spend at room temperature before noticeable partial curing of the resin takes place is known as the material’s ‘out life’ whilst the time that the prepreg can be stored in the freezer and remain useable when thawed-out is known as its ‘freezer life’ or ‘shelf-life'.

Handling Prepreg

At room temperature, prepreg resin systems have such a high viscosity that, even though they are made of uncured resin, they can easily be handled. Such is the firmness of prepreg resin systems when they are handled that prepreg carbon fibre is sometimes referred to as ‘dry carbon’; although as descriptions go, this one is particularly confusing.

Tack Level

The viscosity at room temperature and other properties of a specific resin system determine the level of ‘tack’ of the prepreg. Tackier pre-pregs require more delicate handling but will easily stick to mould surfaces, less tacky pre-pregs are easier to handle but may not stick as easily to a mould’s surface.

Curing Prepreg

Only at elevated temperatures, known as the cure temperature, does the resin really start to react and cure properly which is why pre-pregs must always be cured in an oven of some description. Typical cure temperatures for pre-pregs range from 60°C up to around 180°C with the most common cure temperature for out-of-autoclave pre-pregs being around 100°C. Temperatures such as these can easily be reached by domestic and commercial cooking ovens as well as other types of oven including paint drying and powder coating ovens. Although it is possible to successfully cure a prepreg part without having an active vacuum line inside the oven, it is certainly preferable.

Getting Started

As with many composites processes, the key to successfully making prepreg carbon fibre parts is in using the right materials and techniques throughout the process. Changing something as simple of the type of release film or release agent and certainly changing something significant like the type of prepreg carbon fibre used can have a big impact on the end result and so we would suggest that for the best results, particularly at first, you stick with all the materials we list in this guide.

Pre-requisites

Our out-of-autoclave prepreg carbon fibre process requires very little by way of specialist equipment but will require an oven of some description to cure the parts in, an appropriate mould and some basic tools.

Curing Oven

To follow this guide it is essential that you have an oven of some description which will allow you to set and maintain a temperature of 100°C. The oven may be a domestic or catering oven designed for food, or an industrial oven designed for paint, powder coating or specifically for composites. The most important thing is that the oven is safe and reliable and that it can reach and maintain the required temperature. You will only be able to make parts as big as those that can comfortably fit inside the oven so if you are only using a standard domestic oven you will be limited to making quite small parts. As you develop your use of pre-pregs you can upgrade to larger or more sophisticated equipment but to start off, most ovens will do.

Although it is not essential, it is certainly advantageous (and standard practice) to have a hole through the side or back of the oven through which a vacuum hose can be passed enabling a vacuum connection to the parts to be maintained during their cure. If this is not an option then it is possible to cure a part without a vacuum connection inside the oven providing that the part has been vacuumed-down outside of the oven and that the vacuum bag is perfectly sealed. It is then also essential that the bag does not develop a leak or lose vacuum pressure during the full curing cycle.

Mould

Because the pre-pregs used in this guide need to be cured at, or around, 100°C, it is essential that the mould the prepreg will be laminated into is able to withstand this temperature without softening, distorting or deteriorating. The resin system used in the prepreg is epoxy and so it is also important that the mould material is compatible with epoxy resin. For these two reasons, standard polyester moulds are not suitable for use in making prepreg carbon fibre parts and should not be used. Instead, moulds can be made from high temperature epoxy, metal (such as aluminium or stainless steel) or a high temperature vinylester tooling system such as Uni-Mould.

If you are in any doubt we would suggest Uni-Mould™ because of its excellent compatibility, low cost and polishability. For the purposes of this guide we will be using a mould made using the Uni-Mould system.

Tools

To work with pre-pregs, you will also require:-

Marker pen

  • Stanley knife
  • Scissors
  • Heat gun
  • Masking tape

Materials & Equipment

To ensure the best possible results from your out-of-autoclave prepreg carbon fibre we will be using carefully selected products from the Easy Composites range.

click to enlarge Easy Composites’ Easy-Preg surfacing ply has a special solid resin film on one side only which is laid against the mould surface

Easy Composites’ Easy-Preg surfacing ply has a special solid resin film on one side only which is laid against the mould surface

Most pre-pregs are intended for curing at very high pressure in an autoclave and under these conditions will yield an excellent, pin-hole free, surface finish. Unfortunately however, these same pre-pregs, when cured under vacuum pressure only in an oven will result in a pin-holed surface finish that many people would find unacceptable. For this reason, Easy Composites’ purpose developed ‘Easy-Preg’ out-of-autoclave surfacing prepreg is used as the surface ply. This special prepreg has a solid resin film layer on one side of the material which makes full and intimate contact with the mould surface and a dry fabric on the reverse to facilitate complete air removal under vacuum. This special surface layer is then combined with one or more layers of our Vari-Preg prepreg which is designed to co-cure perfectly with the Easy-Preg with a perfectly clear resin system further enhancing the appearance of the cured laminate.

With this special combination of Easy Composites pre-pregs it is perfectly possible to achieve professional finish quality carbon fibre parts using only vacuum pressure and an oven cure.

In this project we will be using a single layer of the 240g Easy-Preg surfacing prepreg backed up with a single layer of 400g Vari-Preg. This will result in a cured laminate thickness of around 0.75mm which is an ideal thickness for many none-structural or semi structural parts such as scoops, ducts, interior trim, cases and covers. For thicker, more structural parts simply add additional layers of the backing ply to achieve the desired thickness.

Materials:

  • Easy-Preg 240g Surfacing Prepreg Carbon Fibre
  • Vari-Preg 400g Backing Prepreg Carbon FibreM
  • Unperforated Release Film
  • Vacuum Bagging Film
  • Vacuum Bagging Gum Tape
  • Vacuum Bagging Breather Cloth
  • Easy-Lease Chemical Release Agent

Equipment:

  • Composites Vacuum Pump
  • Through-Bag Connector
  • 8mm ID Silicone Vacuum Hose
  • 2x 8mm ID Hosetail Barb Connectors

For the easiest and most reliable start possible to your prepreg carbon fibre laminating, all of the above materials and equipment can be purchased together in our Prepreg Carbon Fibre Starter Kit.

Step by Step Guide

  1. Mould Preparation
  2. Creating Cutting Templates
  3. Cut Prepreg Material
  4. Putting Down the Surfacing Ply
  5. Adding the Backing Ply/Plies
  6. Release Film
  7. Breather Cloth
  8. Make the Vacuum Bag
  9. Loading the Parts and Positioning the Through-Bag Connector
  10. Complete the Vacuum Bag
  11. Vacuum Bag Pull-down
  12. Vacuum Leak Drop-Test
  13. Loading into the Oven for Curing
  14. De-moulding
click to enlarge Out-of-Autoclave Prepreg Carbon Fibre

Throughout the guide you will see photos showing us making two different parts; both genuine parts taken from our manufacturing division, Carbon Mods’ range of in-house performance parts. The two parts are a 12” Bonnet Scoop and a 50mm NACA Duct. The bonnet scoop is a relatively flat shape with some sharply defined detail whereas the NACA duct is a very contoured shape which shows how easily prepreg carbon fibre can conform to difficult shapes. Both of these parts are difficult or impractical to manufacture using any other technique.

1. Mould Preparation

click to enlarge Easy Composites’ Easy-Preg surfacing ply has a special solid resin film on one side only which is laid against the mould surface

Easy Composites’ Easy-Preg surfacing ply has a special solid resin film on one side only which is laid against the mould surface

Other than ensuring that the mould you will be using is made from a suitable material, mould preparation for prepreg laminating is much the same as for other processes. Because curing will take place at around 100°C it is very important to use a release agent that will be effective at this temperature. Most mould release waxes will not perform well at this temperature and so we recommend never using mould release wax with pre-pregs. Instead, a chemical release agent such as Easy-Lease should be used. Having ensured the mould is clean and free from contaminants. Apply the release agent according to the product’s instructions.

2. Creating Cutting Templates

One significant advantage of working with prepreg material is that the reinforcement for a specific part can be accurately cut from the roll with minimum wastage. The way this is done is by creating cutting templates for the different plies of material. You only need to create a template (or set of templates) for a specific part once; accurately made templates will make it much simpler the next time you make the same part again.

Cutting templates can be created in any of a number of ways. The first option is to start off with an oversized piece of prepreg and start laying it down onto the surface of the mould, softening and pulling it as required until it completely covers the surface of the mould before cutting off the excess all the way around and then removing the material from the mould, flattening it out again and tracing round the shape onto a piece of cardboard or similar to preserve the template for future use.

click to enlarge A soft material like Coremat is excellent for making templates for very contoured shapes

A soft material like Coremat is excellent for making templates for very contoured shapes.

click to enlarge Simple shapes like the bonnet scoop can easily be templated using a piece of cardboard

Simple shapes like the bonnet scoop can easily be templated using a piece of cardboard.

Alternatively, templates can be created using a malleable fabric material (thin Coremat works well) which can be smoothed and stretched over the mould surface and then cut or marked to final size. This soft template can then either be used as it is or transferred to a more durable material. For more basic shapes, simple cardboard templates are easy to make.

click to enlarge This close-up shows how later in the guide we will ensure that the backing ply/plies are slightly smaller than the surfacing ply, ensuring air paths to the surface are maintained

This close-up shows how later in the guide we will ensure that the backing ply/plies are slightly smaller than the surfacing ply, ensuring air paths to the surface are maintained.

In most cases, when laminating a part the surface ply should be made a few millimetres larger than the backing ply/plies. This is done to ensure that the air path that is created by the special dry side of the Easy-Preg Surfacing Prepreg is not maintained all the way out of the laminate ensuring that any air trapped in the reinforcement can be removed when the part is vacuum bagged. In practice, it is not necessary for the surfacing ply to extend beyond the backing ply all the way around the part but it should be aimed for as a matter of good practice. Although you could make templates, one for the surface layer and one for the backing layer, it is usually sufficient to simply mark out the surface layer by drawing slightly wider than the template when you transfer the template to the prepreg material, making a separate template for the surface layer unnecessary.

3. Cut Prepreg Material

Once you are happy with your template (or templates if the part will be laminated in multiple pieces) you are ready to transfer the templates to the prepreg material itself. If you have been storing your prepreg in a freezer, before using it, ensure that you have allowed it to thaw to room temperature in a sealed bag before removing it from the bag.

click to enlarge Instead of cutting, a Stanley knife blade is used at a right angle to ‘score’ the Easy-Preg to mark it

Instead of cutting, a Stanley knife blade is used at a right angle to ‘score’ the Easy-Preg to mark it.

click to enlarge The backing ply can easily be marked using a permanent marker pen on the removable film

The backing ply can easily be marked using a permanent marker pen on the removable film.

Transfer the outline of the cutting template(s) first to the surfacing prepreg and then to the backing ply. The Easy-Preg surfacing prepreg only has a removable film on one side and the film itself is waxy and difficult to mark by pen so the recommended means of transferring the template to the surface ply is by scoring the dry side of the fabric with a blade or some other sharp object. Remember that the surface layer should be larger than the backing layer by a few millimetres all the way around to ensure that air can be fully evacuated from the laminate. To do this, simply mark slightly wider than the cutting template when marking the surfacing ply.

The Vari-Preg bagging ply has a removable film on both sides and so can easily be marked out using a marker pen directly onto the backing film. Unlike the surfacing ply, the backing ply is not ‘sided’ and so either side can be marked.

click to enlarge The Easy-Preg surfacing ply is easily cut with a pair of composites shears or normal scissors

The Easy-Preg surfacing ply is easily cut with a pair of composites shears or normal scissors.

click to enlarge The backing ply is best cut using a Stanley knife on a cutting mat to avoid clogging shears

The backing ply is best cut using a Stanley knife on a cutting mat to avoid clogging shears.

Once you’ve marked out the two pieces of material, cut them using a Stanley knife or pair of shears.

4. Putting Down the Surfacing Ply

Unlike most prepreg materials, Easy-Preg surfacing prepreg is ‘sided’ which is to say that it needs to be used a specific way round. The resin film side of the material must be laid down against the mould’s surface with the dry side facing up.

click to enlarge Carefully peel the backing paper away from the Easy-Preg surfacing ply. Only one side has a resin film

Carefully peel the backing paper away from the Easy-Preg surfacing ply. Only one side has a resin film.

click to enlarge Gently lay the Easy-Preg down onto the mould, film side down

Gently lay the Easy-Preg down onto the mould, film side down.

click to enlarge Start conforming the prepreg to the mould by pressing and smoothing it down on the flatter areas of the mould

Start conforming the prepreg to the mould by pressing and smoothing it down on the flatter areas of the mould.

click to enlarge Continue to work the prepreg, gently smoothing and shaping it into the contours of the mould

Continue to work the prepreg, gently smoothing and shaping it into the contours of the mould.

To begin, peel the backing away from the prepreg carefully and then gently drape the material, film-side down, into the mould. Start by smoothing the prepreg gently onto a larger, flatter area of the mould and then gently pull and smooth the material over the more complicated areas of the mould. You may find it helpful to use a heat-gun or hair drying to soften the prepreg slightly so that it can be distorted and formed more easily. You may actually be surprised how malleable and resilient pre-pregs are in this respect, compared to dry fabrics, and with a little patience you should be able to put the Easy-Preg down over the whole surface of the mould.

click to enlarge Use blunt tools (often home-made) to press the prepreg firmly into any tight corners or angles of the mould

Use blunt tools (often home-made) to press the prepreg firmly into any tight corners or angles of the mould.

click to enlarge Once down, trim off any excess material so that the prepreg does not reach the edge of the mould’s flange

Once down, trim off any excess material so that the prepreg does not reach the edge of the mould’s flange.

The essential thing to ensure when putting down any layer of prepreg reinforcement (surfacing or backing) is that the material is firmly into all corners without any ‘bridging’ whatsoever. Because the prepreg has a precisely controlled amount of resin impregnated into it, there is no surplus resin to flow into and fill any gaps or voids so it is vitally important that when the prepreg is laid into the mould it is in intimate contact with the mould surface in all areas. The most likely problem you may encounter when you first work with prepreg is ‘pitting’ on tight corners where the prepreg itself or the vacuum bag has ‘bridged’.

You may find a range of different shaped blunt tools useful to help you press the prepreg firmly into tight corners on your mould. Objects such as credit cards, ice scrapers, and even grouting tools are often used.

click to enlarge A very contoured shape like this NACA duct requires gentle application of heat to make the prepreg more pliable

A very contoured shape like this NACA duct requires gentle application of heat to make the prepreg more pliable.

click to enlarge To create a perfectly straight seam at the back of the duct where the material meets, masking tape is used to mark a vertical line

To create a perfectly straight seam at the back of the duct where the material meets, masking tape is used to mark a vertical line.

click to enlarge The surface ply can then be gently peeled away from the mould and cut perfectly along the tape line

The surface ply can then be gently peeled away from the mould and cut perfectly along the tape line.

click to enlarge The other side then overlaps the cut edge, resulting in a perfect seam

The other side then overlaps the cut edge, resulting in a perfect seam.

click to enlarge The natural tack of the Easy-Preg makes even a difficult shape like this NACA duct quite straightforward

The natural tack of the Easy-Preg makes even a difficult shape like this NACA duct quite straightforward.

click to enlarge Composites Snips are useful for snipping off small excesses of material

Composites Snips are useful for snipping off small excesses of material.

Because both our Easy-Preg and Vari-Preg have an excellent natural ‘tack’ you will find that it will stick to the mould surface without the need for any spray-tack or other adhesive. Warming the prepreg slightly will improve its tack, especially if you are laminating in cooler conditions.