Material that arrives already impregnated at a precise ratio, and is curing from the day it was made. Freezer life and out-life, thawing without condensation, kitting and templates, bias cutting for drape, debulking, the cure bagging stack, oven cycles controlled on the part rather than the air — and the bridging problem that accounts for nine out of ten prepreg defects.
Before you start
Prepreg moves the resin decision off the shop floor and into the factory. The reinforcement arrives already impregnated at a precise ratio, so you are no longer metering resin at all — you are managing a cold chain, a clock, and a cure cycle. That is a genuinely different discipline, and it comes with a statistic worth memorising before you start: Easy Composites report that nine out of ten defects on prepreg parts are caused by bridging of the laminate or the bagging stack. Fourteen chapters, 80% on the final.
The material starts curing the day it is made. Freezer life and out-life are the two clocks you are managing.
Nine out of ten defects. Everything about prepreg technique is really about not letting anything tent across a corner.
Controlled temperature, air circulation and full vacuum throughout — to the schedule in the material data sheet, not from memory.
Chapter 1
"It is just carbon that comes wet." Then it sits on the bench all weekend and it is scrap.
Easy Composites define it precisely: prepregs are "a type of composite reinforcement such as carbon fiber or glass fiber that has already had the activated resin matrix impregnated into the reinforcement." The resin is already mixed with hardener at the prepregging stage, which has one consequence you must internalise: it is — theoretically — curing right from the point of manufacture.
| Decided at the factory | What that gives you |
|---|---|
| Resin system and hardener | No mixing, no ratio error, no catalyst measurement |
| Fiber volume fraction | CKN: "Prepregs typically have a very precise fibre volume ratio and thus do not require bleeding of the resin" — though they add "this depends on the specific material used" |
| Resin distribution | Uniform through the ply. No wet-out judgement, no dry spots from technique |
| Cure schedule | Published in the material data sheet. Yours to follow, not to invent |
That is why prepreg produces the most repeatable parts of any process short of an autoclave: the largest sources of operator variation have been removed from the shop floor entirely.
Easy Composites put it in one sentence: "The difference between these two pieces of equipment is pressure; an autoclave is a pressure chamber... whereas a curing oven only has normal atmospheric pressure inside."
That is it. Both need "precisely controlled temperature (typically 100-150 °C), good air circulation and vacuum lines inside the oven or autoclave so that vacuum can be maintained to the part for the duration of the cure." The autoclave adds pressure on top of the one atmosphere a vacuum bag can deliver — and charges heavily for it. Easy Composites note it is "rare to find an autoclave for less than around £50k (60k euro, $70k)," before you run it, service it or inspect it.
Out-of-autoclave means exactly what it says: prepreg cured under vacuum and heat only. Composites 6 covers the autoclave side.
Chapter 2
A roll comes out of the freezer and gets unwrapped straight away. Condensation forms on every ply.
Because the resin is activated, prepreg has a shelf life measured in two different clocks, and you have to track both. This is the discipline that separates a prepreg shop from a shop that owns a freezer.
Easy Composites: "the length of time the prepreg can be safely stored at -20 °C." Cold slows the reaction dramatically — "By storing the prepreg in a freezer at -20°, the life of the uncured prepreg can be extended to many months or even years." Their XPREG XC110 is quoted at at least 12 months freezer life.
Easy Composites: "the time the prepreg can be stored and handled at room temperature," possible "because of the very low reactivity of the resins typically used in prepregs." XC110 is quoted at at least 6 weeks out-life.
Cold material in a warm shop condenses moisture out of the air, exactly like a cold drink can. Water on the surface of a prepreg ply becomes voids and poor interlaminar bonding in the cured part, and you cannot see it happening.
Thawing procedure above is standard industry practice; the specific thaw times and any material-specific instructions are given in the prepreg technical data sheet and processing guide. Work to that document for the exact material.
None of the above works without a record. Every roll in the ESI freezer carries:
| Field | Why |
|---|---|
| Material and batch | Traceability, and out-life figures are material specific |
| Date received | Start of freezer life |
| Freezer life expiry | The outer limit regardless of use |
| Out-life allowance | From the data sheet for that material |
| Every removal and return, with times | Cumulative out-life used. This is the entry people skip and it is the one that matters |
| Out-life remaining | Calculated, visible on the roll |
If a roll is out of life or shows these signs, it does not go in a customer part. That decision is not a judgement call made under schedule pressure.
Chapter 3
Scissors gum up on the third ply. Half an hour goes on cleaning them instead of laminating.
Easy Composites identify accuracy as a headline benefit of the material: "One major advantage to working with prepregs is that the material can be cut extremely accurately into different sections." That accuracy only exists if you develop the pattern properly.
Easy Composites give the whole sequence, and the key idea is that you develop the pattern in cheap material first:
The outline is transferred by tracing round the template with a marker pen.
The whole point of prepreg is that layup runs cleanly, and layup runs cleanly when the plies are cut, labelled and stacked in order before anything goes into the mold.
Chapter 4
The ply will not go round the curve. Somebody stretches it, and the fibers end up somewhere else.
Composites 1 Chapter 2 established that fibers only carry load along their length. In prepreg that becomes a precision matter, because the whole reason for using the material is repeatability — and a ply put in at the wrong angle destroys exactly that.
Woven fabric has almost no ability to stretch along the fiber directions, but it can shear — the yarns can pivot relative to each other. Cutting on the bias puts that shear where you need it and makes a dramatic difference on compound curves.
Note the consequence: better drape means fewer relief cuts, and every relief cut is a small discontinuity in the laminate. Drape is not just about convenience.
Where drape is not enough — a tight inside corner, a deep recess, a sharp change of direction — you relieve the material rather than stretch it. Easy Composites use composite snips to make small cuts "to allow the material to lap and conform to the mold."
Chapter 5
Nine out of ten. Not most. Nine out of ten.
Bridging is material tenting across a corner or a step instead of laying into it. Under vacuum the bag presses on the tent, not into the corner, so you get a void exactly where the geometry concentrates stress. It applies to the laminate and the bagging stack equally — a perfectly laid laminate under a bridged bag still bridges.
The logic: if you tack the edges down first, all the excess material is trapped in the middle with nowhere to go, and it has to bridge something. Starting at the lowest point and working outward pushes the excess ahead of you toward the open edge where it can be dealt with.
Easy Composites use dibbers — laminating tools — to press the prepreg into the mold. They note these can be "home-made, bought ready-made, or improvised (e.g. the handle on shears)."
Easy Composites describe a small cosmetic part built with a surface ply of 210g (6.2oz) 2×2 twill 3k carbon plus one backing ply of 416g material, and note that "for bigger and thicker parts you may use extra plies as necessary." They observe the heavier 416g ply "is a bit harder to laminate and get to conform," but the process is otherwise identical.
The general rule that follows: lighter, more drapable material near the surface where conformity and finish matter, heavier material behind it for bulk. The same logic as the skin coat in Composites 3 Chapter 8.
Chapter 6
The part comes out with pinholes across the whole visible face. One debulk would have prevented it.
Debulking is an intermediate vacuum cycle during layup, before the part is finally bagged for cure. You compress the plies laid so far, hold them, then unbag and carry on.
Easy Composites: "Debulking will help with improving the fabric consolidation helping to avoid pinholes and voids on the part surface. It is especially important for larger parts or parts with multiple plies." They report the visible result plainly: "Once debagged, you can see the material is much better consolidated against the mold surface."
The important distinction to fix in your head now, because it recurs in Chapter 7:
| Film used | Why | |
|---|---|---|
| Debulk | Perforated release film | You want air to get out through the film into the breather |
| Cure | Non-perforated release film | You do not want resin leaving the laminate — the fiber volume is already set |
Getting these the wrong way round on the cure bag is a way to bleed resin out of a material that was never designed to be bled.
Easy Composites debulk after the surface ply, before the backing ply on a two-ply cosmetic part — so even a very thin laminate gets one. They state it is "especially important for larger parts or parts with multiple plies" but do not publish an interval such as every N plies.
Chapter 7
Breather goes over the whole part. It fights the bag out of every corner and the detail bridges.
The cure bag is different from both the debulk bag and the infusion bag, and the differences all follow from one fact: the fiber volume is already correct, so nothing should leave the laminate.
| Layer | Job | Technique |
|---|---|---|
| Non-perforated release film | Separate the laminate from the bag without letting anything through | "It is essential that this layer, just like the laminate itself, is carefully pressed onto the mold surface without any bridges." Rub it down with a cloth, and use flash release tape to hold it in place. |
| Breather — underside and edge only | Provide a continuous air path | See below. This is the counter-intuitive one. |
| Envelope bag | The airtight enclosure | Chapter 8 |
| Through-bag connector | Vacuum connection | On a corner, sitting on the breather, "to ensure there is a continuous air path" |
Note what is absent compared with a wet-layup bag: no peel ply against the laminate (the non-perforated release film does that job here), and no bleeder.
This is the detail that surprises people who learned wet-layup bagging first, and the reasoning is worth understanding rather than memorising.
Breather is a thick cloth. On a complex shape it has its own bulk and its own stiffness, and it will bridge corners in exactly the way Chapter 5 warns about — and then hold the bag out of the corner too. Since nothing is bleeding through, the breather does not need to be on the surface at all. It only needs to complete an air path from the bag to the port.
CKN describe two methods, chosen by whether the material system bleeds:
Release → laminate → sealant perimeter → peel ply over the laminate → small off-cuts of bleeder 1 to 2 cm wide, or bleeder strings, at the edge of the peel ply, long enough to almost reach the sealant tape, to create a contact point with the breather → non-perforated release film covering the laminate but not fully covering the bleeder strings → breather inside the perimeter with a 1 cm gap to the tape → ports on extra breather → bag.
Release → laminate → edge dam around the laminate → sealant perimeter → peel ply → porous release film → bleeder cloth extending past the dam to contact the breather → non-perforated film covering the laminate but leaving bleeder edges exposed → breather → ports on extra breather → bag.
The pattern in both: a deliberate, controlled contact point between the laminate area and the breather, and nothing else.
In plain terms: if you leave a route for resin to escape and the material was not designed to bleed, you will lose resin you needed — and under autoclave pressure you will lose more of it. The non-perforated film and the controlled bleeder contact are how you decide exactly where, and how much, resin may go.
Chapter 8
One tool in a five-part bag has a leak. All five parts are scrap.
Prepreg bags are usually smaller and often enveloped rather than sealed to a flange, and the vacuum requirement is at the top of the range.
Easy Composites bag the whole mold inside the bag rather than sealing to the flange, and explain why: "This is common with production of smaller pre-preg parts as it is perfectly possible to place several small parts into one big vacuum bag and cure them all together."
CKN state the rules and the reasoning — the same three reasons as Composites 4 Chapter 2:
The same technique as everywhere else in the ladder, and Easy Composites state it for prepreg explicitly:
"Start by pulling only a partial vacuum, stopping the pull as necessary to position and move the bagging film... Use creases of film to achieve this, as the vacuum increases the spare film in the crease will be pulled into the corner thus avoiding any bridging of the film. Once you are happy the bag is positioned correctly, a full vacuum can be pulled."
Chapter 9
The bag snags on the oven rack going in. Nobody notices until the part comes out porous.
Prepreg is less forgiving about vacuum than any process before it, for one reason: there is no resin flow to watch. In an infusion you can see the front advancing and you know something is happening. Here the bag goes into the oven and the next information you receive is a cured part.
| Requirement | Figure | Source |
|---|---|---|
| Vacuum level | "essentially full vacuum; e.g. 29.9"Hg or better" | Easy Composites — needed "to get good fibre consolidation and also ensure a high quality surface finish (free of pin holes)." They add that a vacuum cleaner, "even a shop-vac, won't get anywhere near." |
| Leak test duration | "Once a full vacuum is pulled, carry out a leak test for at least 10 minutes." | Easy Composites |
| Drop criterion | No more than 2 in-Hg over 5 minutes | CKN, the same rule of thumb as Composites 4 Chapter 6 |
The pump runs for the entire cycle, including cool-down. Vacuum can be lost at temperature in ways it never would on the bench:
| What happens | Why |
|---|---|
| Sealant tape softens and flows | Tape used above its temperature rating |
| Film thins or splits at a corner | Film stretched over a sharp feature, then softened by heat |
| A line degrades or a connector loosens | Lines not rated for the cure temperature |
| A small bench leak becomes a large one | Everything is softer and more mobile at temperature |
Chapter 10
The mold is a beautiful polyurethane board pattern. The epoxy never cures against it.
Composites 3 Chapter 13 covered this from the tool builder side. Here it is from the operator side, because you are the last person who can stop a tool going into an oven it cannot survive.
| Tooling material | Compatibility | Note |
|---|---|---|
| Tooling prepreg | Fully compatible | Easy Composites made the mold in their tutorial from XT135 tooling prepreg |
| High temperature epoxy | Fully compatible | Post-cured above the intended part cure temperature |
| Toughened glass | Fully compatible | Flat sheets and panels |
| Aluminium | Fully compatible | Heats and cools quickly, high thermal expansion |
| Stainless steel | Fully compatible | Stable, heavy, slow to heat |
| Vinyl ester (e.g. Uni-Mold) | Semi-compatible | "can affect surface finish and need to consider maximum cure temperature" |
| Polyurethane tooling board | Not compatible | "at elevated temperature polyurethane inhibits the cure of epoxies" |
Compatibility list quoted from Easy Composites out-of-autoclave prepreg guidance. Confirm against the tooling and prepreg data sheets for the specific products.
Tool and part heat together and expand at their own rates. If the rates differ significantly, the part is stretched or compressed against the tool for the whole cycle. That shows up as dimensional error, surface defects, or a part that grips the tool hard at temperature and releases badly.
This is the main reason tooling prepreg is the natural choice for prepreg parts: a composite tool made from similar material expands at a similar rate to the part, and the mismatch largely disappears. Metal tools are used where their durability, heat transfer and machinability justify managing the mismatch.
Composites 3 Chapters 5 and 6 covered the release system. The one item that matters most here is the maximum service temperature, and it varies enormously between products — from a working ceiling of 175 °C on one product up to 400 °C and beyond on others.
Chapter 11
"Just set it to 120 and leave it." The part is under-cured and nobody has a record of anything.
Easy Composites are explicit about where the numbers come from, and this course follows them: "We recommend following the cycles described in the Pre-preg TDS and processing guide." The only numbers they publish generally are the typical range — 100 to 150 °C — and the requirement for precisely controlled temperature, good air circulation and vacuum lines inside the oven.
| Stage | What it achieves | What goes wrong |
|---|---|---|
| Ramp up | Brings tool and part to temperature. The resin viscosity falls before it gels, which lets it flow and consolidate | Too fast: thermal gradients, distortion, uneven flow, and the tool lags the part. Too slow: wasted time and out-life |
| Dwell / hold | The cross-linking happens here, at temperature, for a stated time | Too short or too cool: under-cure, low glass transition, poor properties |
| Cool down | Brings everything back under control, still under vacuum | Too fast: thermal stress, distortion, surface defects |
| Post-cure (if specified) | Drives cross-linking to completion for full thermal and mechanical properties | Skipped: the part never reaches its specified performance |
An oven controller reads air temperature. What matters is the temperature of the laminate, and those two are not the same — especially with a thick or heavily framed tool that lags behind.
Easy Composites: the vacuum line is connected inside the oven with the pump outside, "allowing the bag to be maintained under full vacuum throughout the cure."
Chapter 12
The part gets pulled while it is still warm. Print-through across the whole visible face.
The cycle is done. This chapter is about not losing the part in the last hour.
The part and tool are at different temperatures, the resin has not reached its full room-temperature stiffness, and the two are expanding and contracting against each other. Pulling a warm part is the single easiest way to write off a perfectly cured cosmetic panel.
| Check | Looking for |
|---|---|
| Surface, in raking light | Pinholes, porosity, print-through, resin-starved or resin-rich areas, waviness |
| Every corner and radius | Voids from bridging — the nine-out-of-ten defect. This is where they are |
| Edges | Ply drop-offs, dry edges, delamination at the trim line |
| Tap test | Internal delamination and voids that the surface does not show. Composites 2 Chapter 1 |
| Cure state | Hard, not tacky anywhere, particularly at the surface and at any tool contact point |
| Dimensions | Against the drawing or the reference part |
| The tool | Damage, release condition, anything that will affect the next pull. Composites 3 Chapter 12 |
| Defect | Cause | Prevention |
|---|---|---|
| Void at a corner or radius | Bridging of the laminate or the bagging stack — nine out of ten prepreg defects | Centre-out layup, dibbers, relief cuts, bias cutting, partial-vacuum bag positioning, breather kept off complex surfaces. Chapters 4, 5, 7 |
| Pinholes on the surface | Poor consolidation — no debulk, or insufficient vacuum | Debulk at least 10 minutes at full vacuum; a pump reaching 29.9 inHg or better. Chapters 6 and 9 |
| Porosity through the laminate | Vacuum lost or never achieved; a leak; moisture from bad thawing | Leak test, vacuum monitored through the cycle, thaw sealed. Chapters 2 and 9 |
| Soft or tacky surface | Under-cure, or cure inhibition — classically polyurethane tooling at temperature | Correct cycle from the TDS, thermocouples on the part, compatible tooling. Chapters 10 and 11 |
| Resin-starved area | Uncontrolled resin pathway out of the laminate | Non-perforated film, controlled bleeder contact only. Chapter 7 |
| Print-through / surface defects at demold | Demolded before the part had cooled to room temperature | Cool fully. Always |
| Creasing or material lifting in tight corners | Drape ran out and no relief cut was made | Bias cutting and composite snips. Chapter 4 |
| Wrinkles in a ply | Excess material trapped with nowhere to go — usually from tacking the edges before the centre | Centre-out sequence, and a template that actually fits |
| Distortion or dimensional error | Ramp or cool too fast, thermal expansion mismatch, or an unbalanced ply schedule | Correct cycle, appropriate tooling, symmetric layup |
Chapter 13
The bonding face gets wiped with solvent to clean it up. The bond fails in service.
Prepreg parts are usually made as pieces and assembled. The assembly is where the performance you paid for is either kept or thrown away.
Composites 2 Chapter 7 applies in full: this is a secondary bond and it depends entirely on adhesion to a clean, mechanically prepared surface. Three prepreg-specific points:
Easy Composites state their default: "We generally choose to bond carbon fibre parts (to each other or to other fixings/fittings etc.) using a methyl methacrylate adhesive such as VuduGlu VM100. Depending upon the requirements, other structural adhesives, particularly epoxies, can also be used."
| Adhesive | Character | Use where |
|---|---|---|
| Methyl methacrylate (MMA) | Fast, tough, tolerant of less-than-perfect surface preparation, good gap filling | General structural bonding of composite parts and fittings |
| Structural epoxy | Highest strength and durability, more demanding on surface prep and cure conditions | Where the requirement or specification calls for it |
Product references quoted from Easy Composites guidance. Select and apply adhesives to the current technical data sheet for the specific product and joint.
Chapter 14
The freezer log has three months of gaps. Nobody knows what is usable.
Prepreg is the most controlled process in the ESI ladder short of an autoclave, and that has a specific consequence for improvement: because the manufacturer removed most of the operator variation, anything that varies is coming from the process, the equipment or the material handling — and all three are systematically fixable.
Run down the Chapter 12 defect table and notice what is not there: nothing that says "the tech was careless." Every entry has a mechanism — a bridged corner, a missed debulk, a vacuum level, a cure schedule, a tool material, a demold temperature. That is unusually good news, because a mechanism can be designed out. A skill problem can only be trained, and training does not survive turnover.
| Metric | What it tells you |
|---|---|
| Out-life used versus out-life expired unused | Pure inventory waste. Ordering to usage and oldest-first are direct savings |
| Freezer temperature record | Material validity. A freezer failure can write off everything in it with no visible sign |
| Kit cut time per part | Where durable templates pay back |
| Debulks performed versus specified | Whether schedule pressure is eroding the standard |
| Leak test pass rate first time | Bagging technique and tool condition |
| Thermocouple trace versus programmed cycle | Oven health. Nobody knows the oven overshoots until somebody compares traces |
| Defects by cause, from the Chapter 12 table | Which mechanism to design out next |
| Parts per oven cycle | Cycle time is a bottleneck. Envelope bagging several parts together is a real gain |
The single highest-return improvement in a prepreg cell. It converts the most time-consuming and most error-prone part of the job into a lookup.
Photographed bagging stacks per repeat part, kept with the tool record. Zero ongoing cost, removes the most common bagging error.
Padded loading, spacing for circulation, thermocouple positions. Cheap, and it prevents punctured bags and uneven cures.
A tool radius too tight for the material. Fixable in the tool, permanently, for everyone.
In lean terms, prepreg out-life is one of the clearest cases of inventory as waste there is: material that expires unused is money that was spent and produced nothing. The improvements are unglamorous and effective — order to actual usage, use oldest first, cut kits close to when they will be laminated, label and use offcuts, and keep the log honest so nobody has to guess.
Prepreg has a different hazard profile from open molding — far less styrene, because the resin is contained and cured under a bag — but it adds ovens, hot tooling, and a lot of carbon dust at trimming. Composites 1 applies throughout: extraction on every cut, particulate and eye protection, hearing protection, and carbon dust kept away from electrical equipment.
Final certification
Thirty-six questions drawn from all fourteen chapters. You need 80% to pass. Put your name on the certificate and your completion is recorded to ESI training automatically.