It's not just a case of "minimum pages", but more "minimum spine width". We require a minimum of 3mm spine for PUR Binding. If you have a 56-page book, which is the lowest page count we offer as standard, that equals 28 sheets of paper excluding the cover. Let's say you intend to print on 100g/m² gloss which is typically around 78µm each so you're looking at a spine of ±3.04mm. If you increase the paper to 148g/m² gloss which is around 95µm, you can PUR bind 48 pages which is just 24 sheets, but still has a spine of more than 3mm.
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It should be noted that this does not always apply and decreasing the number of sheets also carries a risk as there are fewer sheets for the glue to stick to.
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If you have any uncertainty you are welcome to reach out to us via email.