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Handling Prepregs Correctly = Better Parts, Less Waste
Prepregs are at the heart of high-performance composites. They bring consistency, strength, and precision to your parts. But to get the most value from them, you need to handle and process them according to best practices.
Even the best prepreg can lose its edge if stored improperly or laid up with careless technique. Taking simple but effective steps in handling and processing ensures your composite parts deliver on strength, reliability, and quality.
Here are some top tips to help you work smarter with prepregs and keep your projects on track.
Store With Care to Preserve Performance
Prepregs are sensitive materials. They need cool, stable storage to stay fresh and ready. Most require refrigeration below 40F. Lower temperatures such as 0F freezers can increase shelf life in many cases. Room temp storage prepregs are trade-off, often sacrificing strength, working tack, drape, clarity, or variable / low temp cure cycles for the Room Temp storage benefit. Consult your Mako technical sales rep to learn more.
Always keep prepregs sealed tightly in their original packaging. Use a zip tie or other method to re-seal the bag. Moisture and temperature swings can cause early curing or add volatiles during cure, leading to pinholes and more scrapped parts.
It is important to keep the prepreg in the sealed bag each time you thaw it. This prevents moisture from forming on the prepreg itself (it will form on the outside of the bag instead). Moisture on the prepreg will evaporate during the curing process, which could cause early delamination in the part.
Keep Your Workspace Clean and Controlled
Dust, oils, and humidity can sabotage your layup. A clean, dry environment with stable temperature and humidity is key. Keep tools and gloves clean and reserved for prepreg-use only.
Organize your workspace to minimize the time prepregs spend out of controlled storage. The out-time accumulates over multiple trips in and out of the freezer. This is particularly important for tooling prepregs and other short outlife prepregs which have additional catalyst allowing the low-temp cure cycle. Some high-temp and high-performance prepregs have a formula that is more prone to moisture-uptake, temperature swings, and the thaw-freeze cycle - and it may not be immediately apparent from the product number on the box or core tag. Look for the STORAGE TEMP on the box label, and carefully review the TDS for Outlife (or Working Life), and Shelf Life. The Outlife/Working Life is the total accumulation of time allowed at room temperature before the material is boardy, and potentially low in flow. The Shelf Life is the time in freezer at the specified temperature for which the Certificate of Conformance applies. If you’ve had a freezer power-outage, please consult your Technical Sales Representative or Quality Manager at the prepreg manufacturer.
Handle With Care and Precision
Touch prepregs with clean gloves only. Oils and dirt from hands can weaken bonds and cause surface blemishes.
Lay prepreg sheets gently to avoid wrinkles or fiber misalignment. Take your time to follow fiber orientation and stacking order exactly as specified. Precision in layup means precision in performance.
There are many tricks to learn from the textiles industry regarding prepreg layup. For example cutting and darting the material can aid in layup around corners. Care must be taken to avoid bridging, which can result when the tool expands more than the part during heat up, due to a mismatch in thermal expansion properties. A gap is formed between the prepreg and tool surface, and the resulting part will appear to have chunks of resin missing from the surface. Allowing for slippage between plys, and clever ply cutting can help with this problem - or minimize by matching CTE between part and tool.
Follow Cure Instructions Exactly
Every prepreg system has a specific cure temperature and time. Follow these instructions carefully. Under-curing can leave parts weak. Over-curing can cause brittleness and cracking.
Use proper equipment like autoclaves or ovens with good temperature control. Monitor the cure cycle for even heat and pressure distribution. The cure cycles are written to apply to all plys in the prepreg stack, keeping in mind that the tool may take a long time to reach temperature. A thermocouple at the coldest zone, the lagging thermocouple, should be used to ensure all plys see adequate heat for full cure.
Inspect and Test for Quality
Once cured, inspect parts for defects. Visual checks are essential, but consider non-destructive testing methods like ultrasonic scans to catch hidden flaws.
Testing mechanical properties on sample parts can confirm your process is producing the strength and durability your application requires.
Work Closely With Your Supplier
Your prepreg supplier is a vital partner. Share your project goals and manufacturing environment to get tailored advice. Training and detailed handling guides can save time and reduce errors, especially when using new materials or scaling up production.
Final Thoughts
Handling prepregs correctly is a small step that makes a big difference. It preserves the material’s integrity, minimizes waste, and ensures your composite parts perform as expected.
With the right storage, clean workspace, careful layup, precise curing, and solid supplier support, you can unlock the full potential of prepregs for your next project.
Have questions about handling prepregs or need help optimizing your process?
Our experts are here to guide you through best practices and help you get the most from your composite materials. Our technical team can walk you through product selection, processing guidelines, and material options to match your performance and production requirements. Let’s build something that lasts.
Speak with our Technical Sales Team
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