Some of you asked me to give some hints to design a core for a diaper.Here you are: below you can find some indications to start design your own diaper core. But stay tuned for additional info and tools to improve your nappy!!
A core is made of a mix of fluff and SAP. Usually this mix is “homogeneously blended” (HB): it means that SAP and fluff are in the same ratio everywhere along the core.
The reason to have an HB core is because SAP works better when it is well mixed with fluff and this improves core general performances. Of course other type of constructions are possible although, with current cores available on the market today, they do not reach performances that HB cores can. Therefore my recommendation is to design an HB core.
Good products have core with a three-dimensional shape. It means that a core with uncompressed fluff show a typical increased thickness (it means more absorbent material) in the crotch area and especially in the front side.
See simplified core design in Pic1 on the side.
You can see there is a first layer along the whole core and a second one positioned in the center/front area. These two layers can be two distinct cores but generally it is a single core with a 3D profile (usually it is obtained through a profiled mould filled by absorbent material).
You can identify 3 different zones
shallow zone: it is the lightest area mainly the back of the core
deep zone: it is the heaviest area and it is located where you need more absorbency
transition zone: it is the area between shallow and deep
Typical basis weights are what you see in Pic1. As you can imagine depending on deep zone shape and basis weights you choose for your core you will have a specific distribution of absorbent capacity from front to back.
See Pic2 hereunder.
Since we choosed to design an HB core it means SAP will follow fluff distribution (if our equipment is able to do it!!). More SAP we add more absorbent capacity you have distributed along the core with same “capacity diagram” that fluff defined.A simple spreadsheet will help you to obtain the proper “capacity diagram” and modify your core geometry accordingly.
Of course you can design a more stylish cores, rounded and narrow at crotch area but the basic rules I have described here above are the same.
Just their application requires more sophisticated tools to predict core performances. We will see them in a next post.
