The primary function of a diaper making machine for sale is to automate the production of diapers, including the assembly of various components like the absorbent core, top sheet, back sheet, and elastic bands. 

These machines efficiently convert raw materials into finished diapers through processes such as:

1. Material Feeding: Feeding raw materials like fluff pulp, nonwoven fabric, and elastic into the machine.
2. Processing: Cutting, folding, and shaping the materials to form the diaper components.
3. Assembly: Joining various parts together, such as attaching the absorbent core to the top and back sheets.
4. Quality Control: Ensuring that each diaper meets specific standards for size, absorbency, and safety.
5. Packaging: Preparing the finished diapers for shipment, often through automated packaging systems.

Overall, diaper making machines enhance production efficiency, reduce labor costs, and ensure consistent quality in the final product.