RTM (Resin Transfer Moulding) is a composite process in which dry reinforcement is placed into a closed two-part mould and resin is injected under pressure. The resin spreads between the fibres inside the tool and cures to form the part.
What Sets RTM Apart: The Rigid Second Face
In vacuum infusion the upper surface is a flexible bag, so only the mould side comes out smooth. RTM uses a rigid tool on both sides. The result is parts with two finished faces, controlled thickness and dimensional repeatability.
That is decisive wherever both sides of a part are visible — hatches, bodywork, interior panels, seat shells.
Process Steps
- Preform: dry fabric plies are cut and usually pre-shaped into a preform.
- Loading: the preform is placed in the lower mould; gelcoat may already be applied to one or both faces.
- Mould closure: the upper mould is lowered and clamped by press or mechanical locks.
- Injection: resin is pumped in at typically 1–10 bar; displaced air escapes through vents.
- Cure: heated tooling shortens cycle time considerably.
- Demoulding: the tool opens and the part is removed with minimal flash.
RTM Variants
| Variant | Tooling | Pressure | Suited volume |
|---|---|---|---|
| Classic RTM | Two rigid tools (steel / aluminium / composite) | 1–10 bar | Medium–high |
| Light RTM | Rigid lower + semi-rigid composite upper | Low (vacuum assisted) | Low–medium |
| HP-RTM | Steel tool + press | 30–120 bar | Very high (automotive) |
Light RTM is the most common variant in the composite sector. The upper tool is lighter and cheaper and, because it works vacuum-assisted, it does not require a large clamping press. That makes RTM accessible to medium-sized manufacturers.
Advantages
- Two finished faces: no secondary surface work, a direct labour saving.
- Short cycle time: with heated tooling the cycle drops to minutes. RTM's short cycle and relatively low-cost tooling are its real advantages.
- Dimensional accuracy: thickness is fixed by the tool, so part-to-part variation is very low.
- Closed system: styrene emission falls dramatically against open moulding.
- Material flexibility: many reinforcement types can be used, and metal inserts, foam cores and embedded parts can be placed directly into the tool.
- High surface performance: which is why RTM is used for load-bearing parts in automotive and many other sectors.
Limits
- Tooling investment: two tools are needed, so the entry cost is higher than open moulding and only amortises over sufficient volume.
- Part size: on very large parts (a full yacht hull, say) tooling and clamping force become impractical — that is infusion territory.
- Design discipline: if injection and vent points are badly placed, dry areas or air traps result.
RTM Compared with Vacuum Infusion
| Criterion | RTM | Vacuum infusion |
|---|---|---|
| Tooling | Two rigid moulds | One mould + bag |
| Surface quality | Both faces finished | One face finished |
| Part size | Small–medium | Small–very large |
| Cycle time | Short | Long |
| Tooling investment | High | Low |
| Consumables | Minimal | Bag and media each cycle |
| Ideal volume | Medium–high | Low–medium, large parts |
RTM at Zenginler İmalat
Zenginler İmalat added RTM to its process range in 2016. It is used today especially in automotive and rail applications — panels visible from both sides, hatches and enclosures, and series production components. More about our RTM service.