Thread Design - Plastic Injection Molding
Thread Design - Plastic Injection Molding
(OP)
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?
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?





RE: Thread Design - Plastic Injection Molding
RE: Thread Design - Plastic Injection Molding
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