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I would like to understand what's the best optimization in terms of design for a single mold, dual cavity of a two-part box (top and bottom parts) which should be hold together with 4 screws without any clip. For other reasons not drawn, both parts should be roughly the same height - therefore the box is cut in the middle.

enter image description here

From a manufacturing point of view only, which following injection molding option is better for a single mold with two cavities?

Option A:

  • to have 4 pillars on both bottom and top parts, that are at same height of each part. Basically, the 4 screws will be inside the pillars in the middle of the box.

enter image description here

Option B:

  • to have 4 pillars on one part ONLY, that are at the double height of that part. The 4 screws will be close to the main wall of the other part.

enter image description here

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  • $\begingroup$ How will it be loaded? $\endgroup$
    – Solar Mike
    May 31, 2022 at 6:33
  • $\begingroup$ The shorter pillars will likely be easier to machine, and easier to eject from the mold. I'm not an injection molding expert though, so I'm not going to post this as an answer. $\endgroup$
    – Drew
    May 31, 2022 at 7:44

1 Answer 1

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The optimal solution is usually thus:

You move each of the cylindrical features into its nearest corner and mold it in flush with the side walls.

You extend all eight of these bosses (that's what those tall cylindrical features are called) flush with the parting line at the center. You make four on one side into hollow tubes where the tube goes all the way through the boss to the outside of the part and the tube has enough diameter to pass a screw without interference.

The bosses and the side walls of the part are then relieved by tapering them by about 1 to 1.5 degrees, so the molded part will free itself more easily from the mold cavity.

On the mating half of the box, you put small shallow holes into the ends of the bosses that are the right diameter to engage the screw threads.

To assemble the box, you fit the two halves together and run a long screw into each of the thru-holes on one half so the threads engage the holes in the tips of the bosses on the other side, and tighten the screws.

Note also that it is standard design practice to pilot the mating surfaces together so that they interlock when pressed together. This will automatically bring the screw features into proper alignment during assembly, so the screw point will "find" the hole it is supposed to screw down into.

To save a few pennies on each assembly, you can step the diameters of the thru-holes so as to pass the head of a screw and then stop the screw head near the bottom of the thru-hole with a short section of smaller diameter hole. This lets you use a shorter (cheaper) screw.

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  • $\begingroup$ Is there an example/image somewhere on the web for the following "it is standard design practice to pilot the mating surfaces together so that they interlock when pressed together"? $\endgroup$
    – gregoiregentil
    May 31, 2022 at 18:20
  • $\begingroup$ Boxes with these features can be bought off the web. look up "die cast aluminum boxes" on the MPJA,com webpage for some good images of this. They sell plastic boxes too but the images of the aluminum ones are better. $\endgroup$
    – niels nielsen
    May 31, 2022 at 19:14

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