Written by Kerryn Caulfield, Executive Director, Composites Australia Inc.
Towpreg—continuous carbon fibre tows pre-impregnated with resin—serves as a high-precision feedstock for automated composite manufacturing. Unlike prepreg fabrics, which are supplied as woven sheets, towpreg is an impregnated tow in a semi-cured ‘B-staged’ state that can be placed, wound or braided at speed. Control of resin content, fibre distribution and consistency is fundamental. Accuracy at this stage determines reliability in filament winding and automated fibre placement used in aerospace, hydrogen storage, defence and high-performance marine structures. The primary technical challenge is not simply wetting fibres with resin but achieving uniform impregnation across thousands of filaments at line speed. Resin viscosity, bath temperature, tow tension and roller pressure all must be tightly controlled. Any deviation risks voids, uneven resin content or compromised fibre alignment.
Consistency in resin content and fibre bandwidth is equally critical. CST’s closed-loop feedback and control system, integrated into the resin impregnation line and proprietary software, monitors these parameters in real time to ensure a lower co-efficient of variation than traditional sampling methods. The result is verifiable uniformity throughout the production run, giving designers confidence in tighter tolerances and leaner structures.
Imported material typically maintains a resin tolerance of ±3%. CST’s proprietary line has more than halved this variability, consistently achieving ±1%. “Each line is an engineered precision instrument,” says Dixon. “We design them to run with the accuracy of a laboratory system but at an industrial scale. That’s what ensures every metre of towpreg meets the standard required for high-performance winding.”. CST’s bandwidth tolerances approach those achieved with slit tow — a far more expensive product. With a 12K tow, the company achieves a standard deviation of ≤0.1 mm, and with a 24K tow, ≤0.2 mm.
This level of accuracy directly translates to fewer defects in automated manufacturing, reduced downtime and scrap, and greater structural reliability in demanding applications such as pressure vessels. It is also essential for use in highly automated processes, such as Automated Fibre Placement (AFP), paving the way for applications in aerospace, hydrogen storage, and defence.
CST’s move into towpreg production was as much necessity as ambition. During COVID-19, imported feedstock faced 16+ week lead times., unpredictable delivery, high refrigerated freight costs and limited shelf life. “We found ourselves in a position where we couldn’t guarantee supply of a material that is fundamental to our business,” Dixon recalls. “So we decided to engineer our own solution.”
By impregnating resin only when required, CST has established a just-in-time system that eliminates the cost and risk associated with importing shelf-life-limited materials. The company is very vertically integrated — from raw tow through to tooling, finished tubes, spars and pressure vessels — ensuring supply continuity and control. The company currently operates two Towpreg production lines, with two more under construction and a further expansion planned to bring the total to six by late 2025. This scaling delivers redundancy and throughput, while allowing resin chemistries and tow widths to be tailored for different industries. “Having our own line means we can control the interaction between fibre and resin at the most fundamental level,” Dixon explains. “We are not just buying feedstock; we are engineering the feedstock to optimise the downstream process with each line a precision instrument.”
Towpreg production is one part of a broader precision environment at CST. The company designs and builds its own filament winding machinery, enabling fibre angles from 0° to 90° and variable wall thickness in tapered tubes. Finite Element Analysis (FEA) predicts stiffness, weight and thickness within 2–3% accuracy before manufacture, with results validated against in-house testing.
To support this, CST invests in centreless grinding and metrology technologies capable of finishing tubes within ±30 microns — and in some cases as tight as ±5 microns. Laser-based sensors verify tolerances. This combination of material control, process control and measurement creates a closed-loop system rarely matched in global markets.
“Automated composite manufacturing depends on towpreg,” Dixon says. “Without it, you are dependent on imports, with all the cost and uncertainty that implies. With it, you can scale production, improve quality and compete globally.”
CST Composites’ towpreg initiative represents the convergence of material science, machine design and process control. Strategically, it serves as a foundation for sovereign resilience in advanced composite manufacturing.
