Where the equipment meters the resin instead of you. The consumable stack and what every layer is for, sealing and pleating a bag, leak testing to a real criterion, flow planning against pot life, resin calculation, the clamping order that decides whether a part is starved — and the defect table that shows why almost every infusion failure is built in before the resin moves.
Before you start
Vacuum bagging and resin infusion are the first processes in the ESI ladder where the equipment meters the resin instead of you. That buys consistency and fiber content, and it charges you in setup, consumables and the cost of a failure. A failed hand layup costs an hour. A failed infusion can cost the part, the consumables, the resin and the day. This course is therefore heavy on the two things that decide whether an infusion works: a leak-tight bag and a flow plan. Fourteen chapters, 80% on the final.
Easy Composites put it plainly: any leak will likely ruin the finished part. Leak testing is not optional and it is not a formality.
Feed and vacuum placement are decided before anything is cut. Flow distance versus pot life is the constraint that kills large infusions.
Consumables per part and leak time per bag are hard numbers. Improvements here are measurable.
Chapter 1
"Turn the pump up." There is no up. You are already using everything the atmosphere has.
A vacuum bag does not suck. It removes the air from inside a sealed envelope so that the atmosphere outside presses in. What you are using as a clamp is the weight of the air above the shop, and that sets a hard ceiling: you can never apply more than one atmosphere, no matter how good the pump is. Full vacuum is around 29.92 inches of mercury, and that is the whole budget.
This is exactly what an autoclave adds — it is a pressure vessel, so it applies pressure above atmospheric. That is the only difference and it is the reason autoclaves cost what they do (Composites 6).
Plies are pressed together so they act as one laminate. CKN: it removes air, "promoting better adhesion and reducing voids as well as removing other volatiles in the resin."
The same pressure everywhere, over any shape, without clamps or a matched tool. CKN: "A vacuum bag applies even pressure, facilitating resin flow and uniform resin distribution, for a uniform final part."
On a wet layup, the bag squeezes excess resin out into the bleeder. CKN: "It helps remove excess resin, optimizing the resin-to-fiber ratio."
CKN: it "creates a controlled environment, preventing moisture and contaminants from affecting the composite."
| Vacuum bagging a wet layup | Resin infusion | |
|---|---|---|
| When resin goes in | Before bagging — you laminate wet, then bag | After bagging — the reinforcement goes in dry |
| What vacuum does | Consolidates and squeezes out excess resin | Compacts the dry stack, then draws the resin through it |
| Consumables | Peel ply, release film, breather, bag | Adds flow mesh, spiral, feed lines, catch pot, clamps |
| Time pressure | You are racing the pot life from the moment you catalyse | The bag can be built and tested at leisure; the clock only starts when you open the feed |
| Failure cost | Moderate | High |
Chapter 2
Resin reaches the pump. The pump is dead and so is the schedule.
The system is simple — pump, catch pot, lines, gauge, port — and every element has one failure mode that costs real money.
CKN identify the key features as vacuum potential, displacement or air flow rate, and horsepower. Two practical consequences:
The catch pot is a resin trap between the bag and the pump. Easy Composites describe its purpose exactly: it "sits between the vacuum pump and the infusion project to prevent resin accidentally being drawn into the vacuum pump."
A two-part port clamps through the bag. CKN give three specific reasons for putting one or two extra pieces of breather underneath the base of the port, and they are worth knowing because they are not obvious:
CKN also direct that the port is placed outside the perimeter of the part, and that larger parts may need more ports to get consistent vacuum.
Chapter 3
Bleeder touches the laminate. It cures into the part and becomes structure you have to grind off.
Every layer in a bagging stack does exactly one job. Learn the job, and the stack order stops being something you memorise and becomes something you can reason about.
| Consumable | Its one job | The detail that matters |
|---|---|---|
| Sealant / gum tape | Seal the bag to the tool flange | CKN: thick rubberized tape, backing paper left on until the bag goes down. It has a temperature rating — tapes "will begin to flow if used at too high temperatures," and higher rating generally means higher cost. |
| Peel ply | A removable layer against the laminate that gives a clean, textured, bond-ready surface | CKN: "A smooth woven fabric that does not bond to resin, even when saturated." Extends at least 2 cm past the laminate perimeter, plus another 5 cm toward where vacuum is pulled. |
| Perforated release film | Separate, but let resin bleed through | CKN: "a grid structure of holes allows for resin to bleed through into the subsequent bagging material layers." Used for debulk and for bleeding. |
| Non-perforated release film | Separate, and let nothing through | Often Teflon FEP. Used when you do not want resin leaving the laminate. |
| Bleeder | Absorb and retain excess resin | CKN: it "must be in contact with the breather. However, it must not come in direct contact with the laminate as it will become saturated and bonded into the laminate." |
| Breather | Provide an air path across the whole part so vacuum is uniform | CKN: a thick cloth; it can also "pad sharp corners or thermocouples to prevent puncturing the vacuum bag." Placed inside the sealant perimeter with roughly a 1 cm gap to the tape. |
| Flow / infusion mesh | Give resin a fast path across the surface | Easy Composites: the gaps created when the bag pulls down "give a flow path across the laminate surface." Infusion only. |
| Infusion spiral | Distribute resin along a line | Easy Composites: resin flows freely through the centre "but just as easily out of the spirals in the side." |
| Dam | Give the laminate a vertical edge | CKN: "If not used, the edge can taper from vacuum bagging." Not needed for very thin laminates. |
| Bagging film | The airtight envelope | Nylon, Kapton or PVA. Rated by temperature. See the warning below. |
For infusion you conform to the shape with pleats and excess film (Chapter 5), not with a bag that stretches.
Ask one question: do I want resin to leave the laminate here?
CKN note that reusable silicone bags are an option "for production parts of high volumes where the initial investment... will make sense." Easy Composites make the same point about silicone connectors — resin cracks off them and they go again.
Chapter 4
The breather runs under the sealant tape. The bag never reaches vacuum and nobody can find why.
This is the stack for a wet layup that you have already laminated. Infusion is different and comes in Chapter 8.
This is the detail that catches people, and it is the reason for that 1 cm gap. If breather runs under the sealant tape, it makes a permanent air channel straight through your seal from outside the bag to inside it. The bag will never hold vacuum, the leak is invisible because it is buried under the tape, and you will spend an hour looking for it elsewhere.
Vacuum can only reach what has an air path to it. Trace it before you close the bag: from the laminate, through the peel ply and film edge, into the breather, across to the port, out through the line. If any link is broken — a release film that fully covers the bleeder edge, a breather that does not reach the port, a port sitting directly on the tool — that part of the bag is isolated.
Chapter 5
The bag is perfect except for one fold under the tape. It never holds vacuum.
Almost every leak in a boatyard is in the seal, not the film. This chapter is that seal.
CKN define it plainly: "A pleat is a folded piece of sealant tape that protrudes out vertically from the tape around the perimeter... pleats provide excess vacuum bag material to fully conform with the geometry of the mold."
Two ways to make one, and CKN are clear which is safer:
Fold a section of the perimeter tape as you lay it. CKN: "Ensure the bottom of the fold is pressed securely against the tool to eliminate any gaps that may cause vacuum leaks."
Stick folded sealant tape onto the already-laid perimeter tape. CKN: "This method has less chance of causing a vacuum leak as the sealant tape adheres well to itself... This method also allows for pleats to be added as necessary."
Easy Composites use the second approach — strips of gum tape folded back on themselves with small legs left at the bottom, stuck to the flange where the pleat is wanted — and note that "often it can be wise to put pleats into each corner of the flange to ensure there are no tight spots on the film in that area."
Every hole in a bag is a leak until you seal it properly. Easy Composites method for a hose through a silicone connector:
CKN describe the same principle for lines crossing the perimeter: add a collar of sealant tape where the hoses cross the sealant tape border.
Chapter 6
"It looks fine, let us just go." Four kilos of resin and a day, gone.
Easy Composites state the position without hedging: "It is essential to perform a leak test prior to the infusion as any leaks will likely ruin the finished part." CKN put the mechanism behind it: "If a leak is present then air is likely to be drawn into the laminate during resin flow and/or cure, resulting in porosity."
| Figure | Value | Source |
|---|---|---|
| Full vacuum | 29.92 in-Hg | CKN — "Vacuum infusion processes are typically run at full vacuum (29.92 in-Hg) but less is acceptable depending on the application/process." |
| Typical working range | 25–30 in-Hg | CKN |
| Prepreg / good consolidation | 29.9 in-Hg or better | Easy Composites |
CKN also give the first diagnostic: "If the desired level of vacuum is not achieved within a few minutes, the pump may not pull enough vacuum for the application, or there is a leak."
Method:
Clamp off the bag and allow 45 minutes before checking whether the gauge has moved. "If the gauge moves when you open the bag, that indicates that there is a leak."
The quicker method, 10 minutes. "Clamp off the bag and wait 10 minutes. Then open the clamp. Any air in the vacuum bag would rush up the pipework and through the leak flow indicator making the rotor spin and thus showing there is a leak." The criterion is zero movement: "Once the leak test is complete with no movement of the rotor... then you know the bag has no leaks."
| What the gauge does | What it usually means |
|---|---|
| Drops fast and keeps dropping to zero | A real hole or an open path. Usually audible. Find it. |
| Drops steadily within the criterion | Normal. No bag is perfectly tight and the pump keeps up with it. |
| Drops a little then stabilises | Often outgassing or a small trapped volume equalising rather than a leak. Repeat the test. |
| Never reaches working vacuum at all | Pump, catch pot or a large leak. Check the system before you blame the bag. |
Chapter 7
Forty minutes chasing a leak that was under the tape the whole time.
Leak hunting is where infusion time goes. Doing it systematically instead of hopefully is the difference between five minutes and an afternoon.
CKN name them: pleats, the vacuum port connection, and the sealant perimeter. Add the hose penetrations. Between them those four account for the overwhelming majority of leaks, so that is where you start — not with a slow crawl over the film.
CKN: "Large leaks can usually be heard as a small whistling sound of escaping air. Inspect the sealant perimeter, pleats and other possible leak locations closely and listen for the sound of air. Smaller leaks may be harder to locate."
| Leak | Fix |
|---|---|
| Gap or fold in the perimeter tape | Lift that section, clear the fold, re-lay the tape and press it properly. Do not just squash more tape on top of a fold — the tunnel is still there. |
| Pleat | Press the base of the pleat firmly to the tool. If it will not seal, add a fresh piece of sealant tape over the pleat base — tape sticks well to itself. |
| Port or hose penetration | Re-wrap the collar of tape, checking for creases in the film underneath. Confirm the hose end is not sealed flat against the tool. |
| Pinhole in the film | Patch with a piece of bagging film over sealant tape. Then ask why it happened — a sharp corner, a thermocouple, a tool edge. Pad it with breather so it does not happen again. |
Chapter 8
The mesh runs past the peel ply. Resin bonds the mesh into the part and it has to be ground off.
Easy Composites describe the process cleanly: reinforcement goes into the mold dry, is enclosed in the bagging stack and put under vacuum, and then — "once all the air has been removed from the bag and the reinforcement has been fully compressed under atmospheric pressure, liquid epoxy resin (mixed with hardener) is introduced to the reinforcement through a pipe which then infuses through the reinforcement under the vacuum. Once the resin has fully infused... the supply of resin is cut off (using a pipe clamp) and the resin is left to cure, still under vacuum."
Easy Composites give the order explicitly: gum tape → peel ply (against the laminate) → infusion mesh → resin feed, spiral and vacuum connectors → vacuum bag film.
Note what is not there compared with Chapter 4: no breather and no bleeder. Nothing is being bled out and the flow mesh plus the spiral provide the paths. Adding breather to an infusion stack is a way to fill it with resin.
| Layer | Purpose, in the source words |
|---|---|
| Release agent | So the part will not stick. Applied and fully cured per Composites 3 — Easy-Lease CR1 on a new mold is 6 coats, 15 minutes between, 1 hour after the last. |
| Gum / sealant tape | "Create perfect seals between the vacuum bag and the mold flange." |
| Peel ply | "The first removable layer between the composite and the vacuum bag. Peel-ply allows the resin to flow easily into the fabric and provides an easy way to peel off the bagging stack which, otherwise, would stick to the fabric." |
| Infusion mesh | "The gaps created once the bag is pulled under vacuum gives a flow path across the laminate surface for the resin." |
| Spiral | "A very free-flowing channel through which the resin is distributed near the point where the resin is first fed into the composite." |
| Silicone connectors | Join hoses to the bag. Reusable — the resin cracks off them. |
| Bagging film | The envelope. Rated by temperature — the film Easy Composites use is suitable to 160 °C. |
Two supporting details from the same source: flash release tape often will not hold springy mesh, so duct tape is used to hold it — and because "the tape is on top of the mesh, it will have no impact on the infusion." And ensure there is enough mesh that it will not bridge; cut and overlap it to conform to the shape.
Chapter 9
The infusion stalls two thirds of the way across. The resin gelled before it arrived.
Easy Composites state the principle: "Placement of the resin feed lines, spiral and vacuum ports are critical to a successful infusion." This chapter is the planning that happens before anything is cut.
This is the calculation that decides whether a large infusion is possible at all. Easy Composites describe a sledge that, "if infused down its length could take 4 hours or more, which is longer than the IN2 Infusion resin's pot life."
The resin gels while it is still travelling. The remedy is not a slower resin — it is a shorter flow path:
Easy Composites give a usable rule of thumb: "you would not want to flow more than 2kg of resin through a connector as it would then become restrictive." On the job described, "as approximately 4kg is to be infused, 2 silicone connectors are to be used" for feed, plus one more for the vacuum line.
CKN name them, and note their definitions carefully — they are the inverse of what the names might suggest:
CKN: "When resin feed line is located around the perimeter of the part, the vacuum line is placed in the center and the resulting flow front is called concentric."
CKN: "When a resin feed line is located in the center of the part, the vacuum line is placed on the perimeter of the part and the resulting flow front is called eccentric."
The universal rule underneath both, from CKN: feed lines "should be strategically placed to allow even resin flow through the part, ideally only reaching the vacuum port location(s) at the end of the infusion, once the part has been fully wet-out." It is typical to have multiple feed lines.
Easy Composites: "Ideally the spiral should be positioned on the edge of the part to avoid any print through onto the finished part. On a non-cosmetic part like this, it is not such a concern and the added spiral down the middle of the part is helpful in reducing the distance of the resin flow."
So there is a direct trade: spiral in the middle shortens the flow path and risks print-through; spiral at the edge protects the surface and lengthens the path. Cosmetic requirement decides it.
A neat and under-used technique. Easy Composites place the vacuum connector on the flange just off the material edge, and cut the mesh away around the connector:
In other words: it stops one fast-arriving edge of the flow front from short-circuiting to the vacuum port while the rest of the part is still dry.
Chapter 10
The resin runs out with a foot of the part still dry. There is no way to recover that.
Running out of resin mid-infusion is unrecoverable. Calculating it is arithmetic, and Easy Composites give the whole method.
| Component | Allowance |
|---|---|
| The laminate | At the 60/40 fiber/resin target: "for every, say, 100 grams of cloth you will need 66 grams of resin... so 1 square meter of [200gsm] cloth will need 133g of resin to achieve the 60/40 ratio." |
| The flow mesh | 700 grams per square metre of laminate — and note, "regardless of thickness." The mesh area is what matters, not the laminate thickness. |
| Feed lines and the bottom of the jug | Another 100 grams |
| Core | Add for it. Infusion cores absorb resin, and 3D and perforated cores absorb a lot. |
| Margin | Add a working margin. Mixing extra is cheap; running out is not. |
Figures quoted from Easy Composites infusion guidance. Confirm against the resin and reinforcement data sheets for the actual job.
Easy Composites describe their infusion epoxy as "ultra-low viscosity, ensuring that it is able to quickly infuse through a range of reinforcements," with mechanical strength suited to high-performance reinforcements. Three properties matter:
A general-purpose laminating resin is usually too thick and too short-lived. Use the infusion grade.
A small step that does two useful things at once. Easy Composites: "slightly crack open the resin feed line allowing the resin to go only as far as the clamp, then retighten the clamp. Then leave the bucket for a further minute. This allows any air drawn from the pipework into the bag to evacuate via the pump and also allows a little bit of settling of the mixed resin in the bucket to help it self degas any bigger bubbles."
Vacuum degassing before infusion removes dissolved air from the mixed resin. It is not always done, and the consequence is visible and alarming the first time you see it:
What is not normal is bubbles appearing steadily behind the flow front in already-wet reinforcement, or bubbles entering from a fitting or a hose end. That is air coming in from outside, and that is a leak.
Chapter 11
The feed gets clamped the moment resin reaches the vacuum port. The part is resin-starved.
By this point the difficult work is done. The bag is built, the leak test has passed, the resin is calculated and mixed. What follows is a sequence, and the order matters.
| What you see | What it means | What to do |
|---|---|---|
| Even front advancing steadily | Working as planned | Monitor and leave it alone |
| Front racing along one edge | Race-tracking — resin found an easier path | If you can reach it, clamping that feed briefly can let the rest catch up. Note it for next time. |
| Front slowing badly | Flow distance, viscosity rising as the resin ages, or a restriction | This is why flow distance was planned in Chapter 9. Mid-run there is little you can do. |
| Front stops entirely | Resin gelled, feed empty, or hose lifted out of the bucket | Check the bucket and the hose first. If it gelled, the part is lost — record why. |
| Bubbles behind the front | Air entering — a leak | Small leaks can sometimes be taped, but Easy Composites warn this "should never be relied upon." |
| Resin pouring into the catch pot early | Front short-circuited to the port | Resin break too small or absent. Note it for the next run. |
Chapter 12
The part comes out with a dry patch the size of a hand, right in the middle.
The consistent theme: almost every infusion defect is created during setup, not during the infusion. By the time resin is moving, the outcome is largely already determined. That is why Chapters 5 to 9 are where the quality is.
| Defect | What it looks like | Cause | Prevention |
|---|---|---|---|
| Dry spot | An area of unwetted reinforcement, white and opaque | Flow front never reached it — race-tracking took resin round it, the feed was clamped too early, resin ran out, or the resin gelled first | Flow plan and flow-distance calculation. Correct clamping order. Resin quantity with margin. |
| Race-tracking | Flow front running fast along one edge or channel and slow elsewhere | An easier path exists — a gap at the reinforcement edge, mesh running to the edge, a groove in the tool, a badly fitted core | Close the easy paths when building the stack. Cut mesh inside the peel ply. Fit core tightly. |
| Porosity / voids | Fine bubbles through the laminate | A leak. CKN: air "is likely to be drawn into the laminate during resin flow and/or cure, resulting in porosity" | Leak test to the criterion, every time. Chapter 6. |
| Bridging voids | A void at an inside corner or a step, often at a core edge | Fabric, peel ply, mesh or bag tenting across the feature instead of laying into it | Chamfer core edges. Lay everything into detail. Partial vacuum, reposition, then full vacuum. |
| Print-through | Spiral or mesh pattern visible in the finished surface | Spiral placed over the cosmetic area | Spiral on the edge of the part where finish matters. |
| Milky / cloudy cosmetic surface | Hazy patches on a clear or ungelcoated surface | Spray tack adhesive on the visible surface | No spray tack on the cosmetic face of a clear or no-gel infusion. |
| Resin-starved laminate | Fiber prominent, low gloss, weak | Feed clamped before the vacuum, so resin was drawn back out | Vacuum first, feed second, with a couple of minutes between. |
| Reinforcement out of position | Fibers displaced, distorted or wrinkled | A compliant, stretchy bag film dragging the dry stack under vacuum | CKN: do not use a compliant bag for infusion. |
| Mesh bonded into the part | Flow mesh cured into the laminate surface | Mesh cut larger than the peel ply | Cut mesh slightly smaller, just covering the material edge. |
| Excess resin in the catch pot | Large quantity of resin drawn through | Front short-circuited to the vacuum port | A resin break at the port. Chapter 9. |
| Tapered laminate edge | Edge thins out instead of finishing square | No dam | CKN: use a dam where a vertical edge is needed. |
| Pump contaminated with resin | Resin in the pump line or the pump | Catch pot hose not deep enough into the cup | Hose several inches into the liner. Chapter 2. |
It is the single most common root cause across bagging, infusion and prepreg. Something — fabric, peel ply, mesh or bag — tents across a corner or a step rather than laying into it, and under vacuum the bag presses on the tent instead of into the corner. The result is a void exactly where the geometry concentrates stress.
An infusion defect record is worth far more than a note that says "dry spot." Record:
Chapter 13
The peel ply gets pulled at the wrong angle. It takes a strip of surface with it.
The part is made. This chapter is about not damaging it in the last half hour.
Chapter 14
Every infusion in the yard is planned from memory. The same mistake gets made three times a year.
Infusion is the most plannable process in this yard and the least forgiving of improvisation. That combination makes it the natural home of standard work.
If two techs bag the same tool differently and get different results, the yard is missing a standard and that is worth saying out loud.
| Metric | What it tells you |
|---|---|
| Leak test result and drop rate | Bag quality, and over time, tool condition |
| Leak-hunt minutes per bag | Technique. A large number is a fixable problem. |
| Predicted versus actual flow time | The most valuable number in the course. It is what lets you plan a large infusion with confidence. |
| Resin calculated versus used | Builds a real allowance for your own materials instead of a generic figure |
| Consumables per part | The quiet cost in infusion work |
| Defects by cause | Turns expensive one-offs into permanent yard knowledge |
| Setup time per tool | Where kitting and templates pay back |
Mesh, peel ply, film and fabric cut to templates for repeat parts. Infusion setup is mostly cutting, and cutting is the easiest thing in the process to standardise.
Drawn before the first run, annotated after it. The second infusion on a tool should never start from scratch.
Not bad luck. Composites 3 — a porous flange or a porous laminate. Flag it with the leak test history.
Stack order card, margin card, leak test criterion posted at the station. Cheap, and it removes the most common defects.
Here is what makes infusion different from every process before it: you cannot inspect the laminate while you are making it. It is inside a sealed bag from the moment the stack is built until the part is demolded. There is no roller pass to catch a dry spot, no visual check between plies.
Infusion is a genuine exposure improvement — styrene and solvent stay inside a sealed bag rather than evaporating off an open laminate, which is the elimination and engineering end of the hierarchy in Composites 1 Chapter 10. But nothing else changes: catalysed resin in a bucket is still a fire risk, leftovers still go to the cure-out station, rags still go in the metal can, and trimming still needs extraction.
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.