A process that runs cleanly from October to April and gives trouble from May to September is a familiar report. The causes are usually three separate mechanisms, and telling them apart is mostly a matter of noticing which symptom appears.
Heat: things get softer and move faster
Polypropylene loses stiffness as temperature rises, and the change over a realistic range is not negligible. A flange that is comfortably stiff at 15 °C is measurably less stiff at 35 °C. Nothing dramatic happens, but anything that was marginal stops being marginal.
At the same time, everything that involves time gets faster:
- Creep accelerates, so packs settle sooner and permanent set develops faster — the mechanism discussed in our article on long-term wound storage, running at maybe five times the rate
- Blocking gets worse where it is a risk at all
- Product takes set faster wound under heat, per our article on warm product
The symptom is usually increased flange movement and more set in the product, appearing after shorter dwell than in winter.
Humidity: water arrives and stays
High humidity does less to the spool, which is unaffected by water, and more to everything around it:
- Condensation on anything that cools below the dew point — cold product in a humid building, or a container going through a temperature cycle
- Corrosion on steel shafts, fasteners and any metal component
- Blocking in products prone to it
- Dust and grit behaving differently, since slightly damp dust is stickier
The symptom is surface problems — rust, staining, layers sticking — rather than structural ones.
Light: ultraviolet does its slow work
UV degrades polypropylene progressively, and the loss is cumulative rather than seasonal. What is seasonal is the dose: a summer in an open yard delivers a large fraction of a year’s exposure.
The symptom is surface chalking followed by embrittlement, and it shows up as cracking at the end of summer rather than during it. Our UV article covers the material side.
Telling them apart
A rough guide:
- Flanges moving more, packs settling faster — heat
- Rust, staining, sticking layers — humidity
- Chalking, then brittle cracking — light
- More handling damage — not seasonal at all, but people are outdoors more and the ground is harder; worth checking whether the real variable is behaviour rather than weather
That last one is genuine. We have seen summer damage attributed to heat that was simply more spools being moved around outside.
What helps, by cause
Heat: shade, ventilation, lower stacks, and specifying spools knowing the peak temperature rather than the average. If your warehouse reaches 40 °C, that is the design condition.
Humidity: keep product and spools above the dew point, do not wrap anything that is cooler than the surrounding air, and maintain shafts and fasteners.
Light: cover, or specify UV-stabilised material. There is no clever alternative.
The specification point
Most spool specifications describe an average condition. If your environment has a season, the specification should describe the worst month, not the median one. That is a small change to how you specify and it prevents a predictable category of summer complaint.
What to tell us
Peak temperature, whether spools see direct sun, and whether they are indoors or out. With that we can specify grade and construction for the worst case on our hollow board spools. Without it, we are designing for a condition that does not exist for four months of the year.