In injection molding, parts can be molded with either male or female threads.

Male threads are less complicated to produce than female threads.  Male threads are cast as an open and shut injection mold except in some instances where the rest of the part design dictates where the parting line will be located.  If male threads need to be cast in the vertical position, the use of cam action or hydraulic action slides is required because of the undercut that is produced from the thread.

On the contrary, casting female threads always present an undercut situation.  These type of threads need to be unscrewed in order to have the part removed from the injection mold.  There are different ways to achieve this.  The most common way used is by hydraulic or electrical motors that automatically unscrew the threaded core from the molded part.  The hydraulic motor is the least expensive option.  Threaded cores have wurm gears attached which mate with the main drive gear.  There is a chain that connects the mold drive gear to the gear on the drive motor shaft.  There are a few different options of unscrewing the cores which are sometimes dictated by the part design.  The most common application is after the injection mold opens, the cores are unscrewed by the actuation of the unscrewing motor.  The time can be set so the motor stops once the part is unscrewed off the core.   In this application the part needs to be held in place so the part does not turn during the unscrewing cycle.  This can be achieved by a variety of designs.  Again, this is always dictated by the part design.   In some instances the threads need to be unscrewed before the mold opens.  This is achieved by timers and/or the machine control.  Usually, this option of molding female threads is incorporated for high production parts because of the expense involved.

In the case of low volume and prototype injection molded parts, the female threads are cast with hand-loaded cores.  In this process threads are commonly ground on three hand-loaded cores.  One core needs to be loaded into the mold by the means of locating pins to position the core into the mold open position.  The mold then goes through the injection molding cycle. After the mold opens, the part is ejected with a hand-loaded core attached.  Another hand-loaded core is inserted into the mold. During the next molding cycle the hand-loaded core that was ejected with the part after the previous cycle is manually unscrewed from the injection molded part.  This process usually doubles the cycle time of the injection molded part which in effect raises the part price but is very cost effective in producing prototype parts.

Another option to cast female threads is by the use of collapsible cores, but this is a very expensive option. By the use of these collapsible cores essentially the injection mold is considered an open and shut mold.

These are some of the most common options of producing threads on injection molded parts.  These threads are able to be cast to a +/- .002 tolerance.