I'm an engineer with many years of experience designing manufacturing machinery, among other things. Designing and building your own diaper production machine is certainly not impossible, but please be aware that it's going to be a very, very large and expensive undertaking. I would not tackle this level of design without an engineering team (two mechanical, one controls, and a draftsman as needed, at a bare minimum) and significant support from vendors or outside experts to solve the challenges of adhesives, heat sealing, pad production, and web cutting.
The first challenge is how to produce the pad. I understand that most current manufacturers use a spray deposition of SAP and cellulose fibers into a vacuum mold, so you'll have to work out how the spray nozzle work, how to entrain the materials in the feed lines, and how to handle the bulk materials.
Once you have the pad production portion designed, you'll need to deal with the inner liner. This is generally done as a hot spray/hot press operation with hydrophobic synthetic fibers such as polypropylene, with the sheet density designed such that you get a permeable but continuous sheet.
Next up is web handling for the PE backsheet rolls. To minimize waste, though, you may want to cut them asymmetrically from the web, which will add to the challenges of handling the material since you'll no longer have a continuous web. Also, you'll have to work out cutting methodology - steel rule dies are easy and cheap but need the web started and stopped, while rotary die cutters are more expensive and complicate your changeovers but handle the web better.
Now that you've got your primary raw material feed, you'll have to work out how to assemble it. Adhesives, heart sealing, impulse sealing, and ultrasonic sealing all come to mind, and it's likely that various parts will require various solutions. What holds the liner and backsheet together might be very different from what holds the elastic cords for the legs and waist.
Once you get past all that, you'll just need a folder and a packaging stage, ideally feed from a pre-printed PE web that can be impulse sealed at the edges to form a bag, then held open while the stack of complete diapers is compressed and dropped in.
Beyond these basic steps, you'll want prices controls, quality inspections, and likely a vision inspection of the finished goods prior to the folder. There are going to be significant controls that go into this machine - I can visualize at least half a dozen servo drives, and banks of Mac valves running various air cylinders and nozzles. Off the church, if expect a machine of this magnitude to require at least a hundred square feet of panel space for the PLC and all the I/O.
You'll need a plant with a good air supply, probably on the order of a rotary vane compressor and a large receiver tank. You'll need equipment to handle master rolls and supersacks of raw materials. Depending on the material handling systems chosen, you may need a mezzanine with flow bins, or you may need an air entrainment transfer system. Don't forget to keep OSHA happy with how you handle airborne didn't and explosion hazards.
Over you get done with all this, the real money is going to be in selling machines to manufacturers. There's going to be a tremendous amount of NRE cost to absorb, and if you're primarily interested in manufacturing diapers you might do well to let a machine supplier handle the NRE.
FWIW, my company is busy with a capex project to add a new line of products at our existing plant. The machines are much simpler, just vertical fill/seal units similar to what we've been running, maintaining, and modifying in out existing line for many years. Even with four machines that we have considerable expertise with, it's much cheaper to source then from a supplier whose whole business is building vertical fill/seal machines.
