Two Clocks: Shelf Life and Out-Time
Prepreg has a freezer shelf life, commonly twelve months or more at about minus 18 degrees Celsius, and an out-time, also called mechanical or handling life, that runs only while the material is warm. Out-time limits vary by resin system, from a few days to a month or more, and are set by the supplier and often tightened by the customer's process specification. Every removal from the freezer, every hour on the cutting table and every shift on the layup tool adds to the total.
Thawing is not free time: the material must reach room temperature in its sealed bag to avoid condensation, and that period usually counts. Record the data sheet limit and the governing spec limit separately, and enforce the stricter one.
Where Out-Time Records Break Down
Paper logs fail in predictable places. A roll goes back to the freezer on Friday and the return time is not written down. A partial roll is cut into kits, and the leftover inherits the wrong total.
Two kits from one roll are used on different days. A shift change loses the context of what is already on the tool. Operators also round, and a few hours rounded across five handling events can push a roll over the limit unnoticed.
Another common trap is applying one limit to all rolls of a material even when different batches carry different remaining life from the supplier. The most reliable fix is to put a unique ID on every roll and every kit and to log in and out events, not elapsed times typed by hand.
- Unique ID per roll and per cut kit, printed at the moment of cutting
- Scan events for freezer out, freezer in, tool in and cure start
- Automatic cumulative total, not a manually added number
- Hard stop when the remaining life is below the planned layup time
Plan Layup Time Against Remaining Life
The check that saves material is forward-looking. A kit with 40 hours of out-time left is not usable for a part that needs three days of layup and a cure start after that. Planning should compare remaining life with the time to complete layup, bag and enter the cure, with a safety margin.
This is a scheduling problem as much as a records problem: if the autoclave is booked after the kit expires, the schedule is wrong even if the log is perfect. Schedule thaw and cutting just in time, keep an eye on partial kits waiting for a delayed tool, and decide in advance which parts get priority for aging material.
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Prepreg Out-Time Tracker Worksheet
Track every freezer exit and return per prepreg roll, with cumulative out-time, remaining out-life and percent used calculated for you.
Records the Customer and Auditor Expect
For aerospace work under AS9100 and NADCAP composites requirements, expect to show material receiving inspection, freezer temperature monitoring, out-time logs by lot, and the link between the lot and the finished part. Auditors tend to sample a part and trace backward, asking for the roll, cut date, cumulative out-time at layup and the freezer chart for the storage period. For non-aerospace composites, the same evidence protects you in a warranty dispute.
Keep the record in the ERP or MES lot history where possible, because a spreadsheet beside the system gets out of date, and keep the freezer alarm logs with it.
When a Simple Tool Is Enough
A small shop with two or three prepreg materials and a handful of rolls can run a sound system with printed labels, a scan or sign-out sheet and a daily check. The case for software strengthens with more materials, partial kits, multiple shifts and customer audits. Before buying anything, test whether your current ERP can keep a cumulative field per lot and block issues past a limit.
If it can, a barcode printer and a few rules may be enough. If it cannot, a lightweight tracker that writes back to the lot record is usually cheaper than customizing core inventory logic.
Frequently asked questions
What is prepreg out-time?
Out-time is the cumulative time prepreg spends above its storage temperature, usually at room temperature, from the first freezer removal. It consumes the resin's handling life. When the limit set by the supplier and your customer specification is reached, the material cannot be used for structural parts, even if freezer shelf life remains.
Does thaw time count toward out-time?
In most specifications yes, because the resin is warming and advancing in cure. Some processes define the clock from removal from the freezer, others from opening the bag. Follow your material specification and customer process requirements, and write the rule into the procedure so every shift counts it the same way.
Can expired prepreg ever be used?
Only for non-structural uses such as tool proving, training or process trials, and only if the part is not delivered. For production parts, using material past its limit needs a customer-approved deviation, and many customers do not allow one. Mark expired material clearly and quarantine it.
Netray can add a private monitoring layer over your ERP lot data that flags kits whose remaining out-time is shorter than the scheduled layup and cure window.