Fibre Reinforced Facade Encyclopedia: Manufacturing
Vacuum infusion is a closed mould fibre reinforced polymer process in which the dry reinforcement is laid into the mould, sealed under a vacuum bag, and resin is then drawn through the fibre stack by the pressure difference rather than being brushed on. Because atmospheric pressure compacts the laminate before any resin enters, an infused panel carries far more fibre and far less resin than the same plies laid by hand, and the result is lighter, stiffer and much more consistent from panel to panel.
An infused laminate reaches 50 to 65 percent fibre by volume with void content usually under 2 percent. The same plies hand laid reach 25 to 35 percent fibre with 2 to 5 percent voids.
At a glance
| Property | Typical value | Note |
|---|---|---|
| Fibre volume fraction | 50 to 65 percent | Woven and stitched fabrics at the upper end |
| Void content | 0.5 to 2 percent | Low, provided the bag holds full vacuum |
| Vacuum level | typically 80 to 95 kPa below atmospheric | Full vacuum is the objective; leaks are fatal |
| Resin to glass ratio | about 1:1 by weight | Against 2 to 2.5:1 for hand lay-up |
| Weight saving against hand lay-up | 20 to 35 percent for equal strength | Less resin carried for the same fibre |
| Thickness consistency | plus or minus 5 percent | Set by fibre compaction, not by operator pressure |
| Styrene emission | Very low during infusion | Closed process; the resin is never open to the shop |
| Consumables per part | Bag, peel ply, flow media, spiral, tacky tape, tubing | Single use; a real per panel cost |
| Tooling requirement | Airtight, vacuum rated mould | More expensive than an open mould of the same shape |
How the process runs
The mould is prepared and gel coated as for any other moulded FRP part, then the dry reinforcement is laid in and any core material, inserts or local reinforcement placed. Over the stack go the process consumables: a peel ply that releases from the cured laminate and leaves a bondable surface, a flow medium that lets resin run quickly across the part, spiral tubing for the feed and vacuum lines, and the vacuum bag sealed to the mould flange with tacky tape.
The bag is evacuated and held to prove it is leak tight. Only then is the resin feed opened, and catalysed resin is pulled through the stack from the feed line towards the vacuum line, wetting the fibre progressively. When the flow front reaches the vacuum line the feed is clamped and the part cures under vacuum. The bag and consumables are stripped after cure.
Why the laminate is better
Compaction happens before resin arrives, so the fibre bundles are already nested tightly together and the resin has only to fill the space left between them. That is the whole reason the fibre volume fraction rises. Since fibre carries the load and resin carries very little, a higher fibre fraction gives more strength and stiffness per kilogram of panel, which on a facade translates directly into longer spans, fewer fixings and lighter lifting.
Void content falls for the same reason. Air is removed before infusion rather than rolled out afterwards, and any volatiles are drawn towards the vacuum line rather than left in the laminate. Low voids matter for long term durability as much as for strength, because voids are where water collects and where interlaminar failure starts.
Resin control and consistency
In hand lay-up the amount of resin in the panel is decided by the person holding the roller. In infusion it is decided by the fibre architecture and the vacuum, both of which are measurable. The practical consequence is that panel mass becomes a reliable quality indicator: two panels made from the same stack should weigh within a few percent of each other, and a panel outside that band signals a process fault rather than a variation in workmanship.
- Vacuum decay test before resin release, holding the sealed bag for a set period with the pump isolated
- Recorded resin batch, catalyst ratio and ambient temperature, since infusion time competes with gel time
- Flow front observed and timed; a stalled front means a dry spot
- Panel mass compared to a calculated target
- Glass content confirmed by burn-off to ISO 1172 on a witness coupon
- Barcol hardness to confirm the resin has reached full cure
The limits, and where hand lay-up wins
Infusion is a single attempt process. If the bag leaks, the flow front stalls or the resin gels before the part is full, the result is a dry patch that cannot be repaired invisibly and the panel is usually scrapped. Risk rises with part size, with geometry that has tight radii and deep returns, and with anything that traps air. Tooling has to be airtight and stiff enough to resist a full atmosphere of pressure, so moulds cost more than the open moulds used for the same shape.
For that reason infusion suits repetition, large gently curved surfaces, sandwich construction with foam or honeycomb cores, and any panel where weight is the governing constraint. Deeply sculptural one-off geometry, local reinforcement added at an engineer’s instruction, and site repairs remain the territory of hand lay-up.
Standards and references
- ISO 1172, glass content of fibre reinforced plastics by burn-off
- ASTM D2734, void content of reinforced plastics
- ISO 14125 and ISO 527, flexural and tensile properties of composite laminates
- ASTM D2583, Barcol hardness for degree of cure
- ISO 12215 and BS EN 13706, laminate specification conventions used in architectural composites
- AS 5637.1 and AS 5113 where the finished panel is used on an external wall (check the edition of the NCC adopted in your jurisdiction)
Related terms
Questions specifiers ask
How much lighter is an infused panel?
For the same structural capacity, typically 20 to 35 percent lighter than a hand laid equivalent, because the panel carries roughly half the resin for the same amount of fibre.
What fibre volume fraction does infusion reach?
Commonly 50 to 65 percent by volume with stitched or woven fabrics, against 25 to 35 percent for hand lay-up. That is the main reason for the weight and stiffness gain.
What happens if the infusion fails?
A dry spot forms where resin did not reach the fibre. It cannot be repaired invisibly in a structural laminate, so the panel is normally scrapped. This is why vacuum decay is tested before resin is released.
Does infusion make an FRP panel non-combustible?
No. Infusion is a manufacturing method, not a fire performance pathway. FRP is a polymer composite and is not non-combustible. Fire performance comes from the resin system and must be demonstrated by test, for example AS 5637.1 for a group number or AS 5113 for a full-scale facade test.
Is infusion always the better process?
No. It suits repetition, large surfaces, sandwich construction and weight critical panels. Complex sculptural geometry, short runs and locally reinforced panels are usually better hand laid.
Weight critical facade or a long run of repeating panels? ShapeShell-RT is our FRP system, published at 5 to 32 kg per square metre. See ShapeShell-RT specifications
Reviewed by Richard Halvorsen, General Manager, ShapeShift Technologies. Last updated September 2026.