Composite Laminating
Composite laminating lays reinforcing fiber into a mold and consolidates it with resin, curing into a stiff, light monocoque part.
- Part
- Forming
- Variants
- 3
- Revised
- 2026-08-11
At a glance
- Family
- Composites
- Typical tolerances
- ±0.010 in (±0.25 mm) on tool-side surfaces. Laminate thickness varies ±10–20% on open (bag-side) surfaces; matched tooling or RTM is required to control both faces.
- Surface finish
- Tool-side surfaces replicate the mold and can be near-optical with a polished tool or gel coat. Bag-side surfaces carry peel-ply or breather texture and normally require filling and finishing if cosmetic.
- Typical volumes
- 1–5,000 parts per year, depending on route: wet layup at the low end, RTM at the high end
- Lead time
- 2–6 weeks including tooling for prototype work; 1–5 days per part once tooling exists, dominated by layup labor and cure cycle.
- Materials
- Composite
What it is
Composite laminating builds a part by placing layers of reinforcing fiber into or onto a mold and consolidating them with a thermosetting resin, so the finished laminate is a single monocoque structure whose stiffness and strength follow the fiber directions the designer chose. It is the only process in which the engineer specifies the material and the part simultaneously.
Three routes dominate. Wet lay-up brushes or rolls liquid resin into dry fabric and reaches 30–40% fiber volume fraction by hand, 45–55% under vacuum. Pre-preg lay-up uses fabric pre-impregnated with B-staged resin, cured under vacuum and typically 90 psi (6 bar) of autoclave pressure at 250 °F or 350 °F (120 °C or 177 °C), reaching 55–65% fiber volume. Resin transfer molding (RTM) injects resin into dry fiber in a closed tool, giving two finished faces and better repeatability.
Fiber volume fraction is the number that matters: laminate stiffness scales almost linearly with it, which is why an autoclaved pre-preg part can be roughly 50% stiffer than the same layup done wet by hand.
How it works
- Prepare the tool. The mold defines the finished surface. Composite tooling is used for prototypes and low volume; machined aluminum or steel for production; invar where the tool's thermal expansion must match the part at cure temperature. Release agent or release film is applied, and gel coat where a finished cosmetic surface is required.
- Cut and kit the plies. Plies are cut to a nested pattern, usually on a CNC ply cutter, and kitted in layup sequence with orientations marked. Typical cured ply thickness is about 0.005 in (0.13 mm) for a 150 gsm unidirectional pre-preg and 0.010 in (0.25 mm) for a 200 gsm woven fabric.
- Lay up. Plies are placed to the ply book, orientation by orientation. Compaction — debulking under vacuum every three to five plies for pre-preg — removes trapped air before it becomes porosity.
- Bag. Peel ply, release film, breather, and vacuum bag are applied and the bag is drawn to 25–29 inHg (0.85–1.0 bar). A leak here is the most common cause of a porous part.
- Cure. Wet lay-up epoxies cure at room temperature and are usually post-cured at 120–180 °F (50–80 °C). Pre-pregs cure at 250 °F or 350 °F (120 °C or 177 °C) for roughly 2 hours, under 90 psi (6 bar) in an autoclave or vacuum-only for out-of-autoclave systems. RTM injects resin at 30–150 psi (2–10 bar) into a heated closed tool, with cycle times from 10 to 60 minutes.
- Demold and trim. Edges are trimmed with diamond or carbide tooling, or water jet cut, and holes are drilled with specialized bits to avoid delamination on breakout.
- Inspect. Void content, ply orientation, and thickness are verified; structural parts are ultrasonically inspected for delamination and porosity.
Design guidelines
Balance and symmetry
Lay up symmetric about the mid-plane and balanced in ±θ pairs. An unsymmetric laminate warps as it cools from cure temperature because the resin shrinks against an unbalanced fiber arrangement — the part comes out of the tool the wrong shape and no amount of tooling accuracy fixes it.
Minimum ply count and the 10% rule
Use at least four plies in any structural laminate, and include a minimum of about 10% of fibers in each of the 0°, ±45°, and 90° directions unless the load case is genuinely uniaxial. All-0° laminates split under any off-axis or bearing load.
Corner radii and spring-in
Inside radii should be 0.12 in (3 mm) minimum and 0.2 in (5 mm) preferred; outside radii at least twice the laminate thickness. Cured corners spring in by roughly 0.5–2° because through-thickness resin shrinkage exceeds in-plane shrinkage — production tooling is compensated for this, and prototypes should expect it.
Thickness tolerance
The tool side is accurate to about ±0.010 in (±0.25 mm); the bag side is not, and laminate thickness typically varies ±10–20% unless matched tooling or RTM is used. Never design a fit against a bag-side surface.
Ply drops
Taper thickness changes gradually: drop no more than one ply at a time and stagger drops by at least 0.4 in (10 mm). An abrupt ply drop is a delamination initiator.
Joints and inserts
Bonded joints outperform bolted ones in composites, because a bolt hole cuts fibers and concentrates load in a material with no yield mechanism to redistribute it. Where fasteners are unavoidable, use a bonded metallic insert or a locally thickened bearing pad, and follow standard edge distance of at least 3× hole diameter.
Galvanic compatibility
Carbon fiber is cathodic to aluminum and steel. Any carbon-to-aluminum joint needs an insulating glass ply or sealant between them, or the aluminum corrodes rapidly.
| Feature | Recommended | Limit | Why |
|---|---|---|---|
| Laminate symmetry | Symmetric and balanced | — | Unsymmetric laminates warp after cure |
| Ply count | 6+ | 4 | Fewer plies cannot balance |
| Inside radius | 0.2 in (5 mm) | 0.12 in (3 mm) | Fiber bridging and resin-rich corners |
| Outside radius | 2× thickness | 1× thickness | Fibers cannot turn a sharp corner |
| Ply drop stagger | 0.8 in (20 mm) | 0.4 in (10 mm) | Abrupt drops delaminate |
| Fastener edge distance | 3× hole diameter | 2.5× | Bearing failure at the free edge |
| Fiber volume fraction | 55–60% (pre-preg) | 30–40% (hand wet) | Stiffness scales with fiber content |
Cost drivers
Labor dominates almost every composite laminating operation. Hand layup is measured in plies placed per hour, so ply count, part size, and geometric complexity map almost directly onto cost. Automating with a ply cutter and kitting the layup sequence is the first and largest saving available.
Material cost is next and varies widely: E-glass fabric is inexpensive, standard-modulus carbon considerably more, and intermediate- or high-modulus carbon and aramid more again. Pre-preg carries a cost premium plus frozen storage and a finite out-life, so a shop paying for freezer capacity and tracking out-life has overheads a wet layup shop does not.
Cure equipment is the third. An autoclave is a large capital item with a per-cycle cost, which is why out-of-autoclave pre-pregs and RTM have taken so much of the market. RTM adds matched tooling cost but converts labor into cycle time, which is what makes it viable at volume.
Volume breakpoints: wet layup for 1–50 parts, vacuum-bagged pre-preg for 10–500, RTM from roughly 500 to several thousand a year, and compression molding of SMC above that where the mechanical requirement allows chopped fiber.
- Reduce ply count and part count before optimizing anything else.
- Design for a single-piece layup rather than bonded sub-assemblies.
- Use glass where carbon's stiffness is not needed — much of a typical laminate is there for handling, not stiffness.
- Accept a bag-side surface wherever cosmetics allow; matched tooling doubles tool cost.
- Design bonded joints instead of bolted ones; every hole is machined labor plus a stress concentration.
Variants
3 named
Wet Lay-up
Pre-preg Lay-up
Resin Transfer Molding
Questions
7 questionsWhat fiber volume fraction should I expect?
30–40% for hand wet layup, 45–55% for vacuum-bagged wet layup, and 55–65% for autoclaved pre-preg. Laminate stiffness scales close to linearly with fiber volume fraction, so the consolidation method matters as much as the fiber choice.
Why must a composite laminate be symmetric?
Resin shrinks as it cures and the laminate cools from cure temperature. If the ply stack is not symmetric about its mid-plane, that shrinkage is unbalanced through the thickness and the part warps off the tool. Symmetric and balanced layups are a hard rule, not a preference.
What is spring-in and how much should I allow?
Corners of a cured laminate close slightly because through-thickness resin shrinkage exceeds in-plane shrinkage. Typical spring-in is 0.5–2° per corner. Production tooling is machined with the angle compensated; on a prototype tool, expect the part to come off the mold slightly tighter than nominal.
Should composite parts be bolted or bonded?
Bonded, wherever possible. Drilling a hole cuts load-carrying fibers and concentrates stress in a material with no yield mechanism to redistribute it. Where fasteners are unavoidable, use a locally thickened bearing pad or a bonded metallic insert and keep edge distance at 3× hole diameter or more.
What is the minimum inside radius for a laminated corner?
0.12 in (3 mm) as an absolute minimum, 0.2 in (5 mm) preferred. Tighter radii cause fiber bridging, where plies span the corner instead of conforming, leaving a resin-rich pocket that is the weakest point of the part.
Can carbon fiber be bonded directly to aluminum?
Not without isolation. Carbon is strongly cathodic to aluminum, and a direct joint drives galvanic corrosion of the aluminum in any damp environment. Interleave a glass ply or an insulating adhesive layer, and seal the joint edge.
When does RTM beat hand layup?
Roughly from a few hundred parts a year upward, and sooner if both surfaces must be finished. RTM trades tooling cost and cycle discipline for a large reduction in layup labor, more consistent fiber volume fraction, and two molded faces instead of one.