What Type of Joint for Sheet Metal?

Whatever may be the shape of the conduit conveying the sachets (or other packages), you should be able to accomplish your goals with flanged joints. Pay attention to the inner contours. Ream, grind, or otherwise machine the component pieces so that the insides widen slightly at the joints; no sharp edges for the sachets to "climb over" as they pass from one section to the next. The flanged sections can be screwed or bolted together, or can be joined by various clamps with one or two screws that can be disengaged quickly. Search "Tri-Clamp" or other sanitary joint systems to see the basic geometry. (I would do a drawing, but I haven't yet mastered how to insert drawings into these threads.)

Do you mean like this?

I was thinking more about flanged joints between sections of duct. If you want removable covers, Energygod's idea is good. The geometry you have shown is a bit different; it too could work.

Some types of covered electrical wireway are made in flanged sections, either solid or with covers. I don't know if the common sizes would fit your application, but you could look at some examples and get some further ideas.

Flake ice is sometimes blown through circular aluminum tubes with long-sweep elbows. The sections are joined with two-bolt clamp-on sleeves (rubber backed by metal). They come apart and reassemble pretty easily, and might be adaptable to your project.

With powders, you probably want to avoid sharp internal corners to avoid hangups.

did u mean powder coating to remove sharp edges? I am using MS sheet for this box....Is powder coating effective for MS sheet(ASTM 2062)?

The possible sharp edges I refer to would be the inside corners of bent sheet-metal forms, which may be undesirable because powder will tend to collect there. Not only that, but powder may abrade the paint, which could contaminate your product.

Note that this is a different issue from the original question.

If the length of the box is 3 meters or less, you can form the box as three-sided (base plus upright sides) with "Pittsburgh" seams at the tops, with the "S" formed outside the box- Use at least 25 mm deep "S" shapes.

Then, form a cap using a very flat "C" shape, with the short end legs vertical and sized to fit into the "S" section of the base with 25 mm long legs, minimum.

A few "D" handles spaced along the top of the cap plate will facilitate easy opening of the cap for servicing.

but frnd(energygod);

I am putting this box for a mass production, Will this pittsburg seam increase the cost.

It will be less expensive than flanged covers with bolted connections (whether nut and bolt or bolts with weld-nuts).

It is easy to fabricate- many sheet metal shops have automated machines that form the "S" bend- and it increases the rigidity of the assembly.