Thread Design - Plastic Injection Molding

[rctong]

When designing threads for a plastic injection molded part, say a jar with threads at the top and its corresponding mating lid, is the threaded portion of either piece on a drafted surface? I know that you are supposed to add 1-2deg of draft to aid ejection from the mold in general but are the threaded portions on both parts also drafted? Or are they on orthogonal surfaces to the pull direction (assuming its a straight pull part)?

If the draft runs the entire length of each part then wouldnt that mean when you screw the lid onto the jar the draft of the lid opposes the draft of the jar? Wouldnt this be bad for sealing?

 

[plasgears]

Consider two flats on the screw at the separating line. The screw is not complete, but the majority is threaded. This will facilitate making the screw in regular dies.

 

[MikeHalloran]

Molded internal threads in a lid are ordinarily made with no (or vanishingly small) taper or draft in the direction of the central axis. You might think you'd need some, because the core is unscrewed from the part (or the part is unscrewed from the core). ... but the part is still rubbery when the mold opens, so it can open a little if needed. The unscrewing is why the exterior of the lid always has some kind of wrenching surface.

The external threads on the jar are not typically made the same way; they are pulled radially with the mold halves that form the jar's exterior. Their intersection with the parting line is made much simpler by providing flats at the parting line. There is usually some draft in those flats, so a cross section would be subtly diamond-shaped. The flanks of the threads provide enough draft. (unless you try to do without the flats, in which case there won't be enough draft on the thread flanks near the parting lines)





Mike Halloran
Pembroke Pines, FL, USA