Threads are important additional elements on plastic parts. There are external  threads (screws) and internal threads (nuts). Just as different as the threads are, is  also their production and the injection mould. Plugs, nipples (external threads),  screw caps and screw-on points for holders (internal threads) are typical uses for a  thread on a plastic part. On our cover in Figure 2.18 an external and an internal thread were designed. In  this example, the external thread offers the possibility to close the opening on the  cover with a screw cap. The internal thread in the second cover can for example be  the screw connection for screwing in a screw coupling for a hose or pipe.

1. External Thread

The injection mould for the external thread belongs to the category moulds with  moving elements. In our example, the contour of the thread is also an undercut.  What is special about this is that the whole of the thread is an undercut and therefore it must be fully incorporated in both slides. Both slides go exactly to the middle of the thread. With the thread halves, the entire contour of the cover which is  in the demoulding direction of the thread is incorporated in the slides. The thread  itself is round and therefore it does not matter in which direction the slide moves.  In such a case, the shortest distance for the slide to move inside the injection  mould is chosen.

In Figure 2.20 the injection mould for the cover with external thread is shown. The  slides are open and are force-controlled by means of inclined pins. The overall  contour is incorporated in the slide. The distance covered by every slide is half the  width of the plastic part plus a few millimetres safety margin. Note also the size of  the injection mould in relation to the size of the plastic part.

In Figure 2.21 the slide with all functional surfaces is displayed. Right in the front  there is the split line surface in the middle of the injection mould, where both  slides and the mould contour—where the thread geometry belongs—meet. In  addition, the inclined hole is shown in which the inclined pin for moving the slide  is immersed. On the sides the slide guides, in which the slides are embedded and  are driven in the demoulding direction, are displayed. Only exception: the contour of the thread lies in the mould and is right in the main  demoulding direction. The threaded plug in the top left corner of Figure 2.19 is  such a plastic part. It is produced with an open/close mould.

2.Internal Thread

The injection mould for the internal thread also belongs to the category moulds  with moving elements but is a completely different mould concept. The thread itself  is also an undercut for this injection mould, but this time it cannot be demoulded  with slides. There are two significant options for the demoulding of these internal threads: ƒ Option 1 is the collapsible core. This is explained in detail later in Section 4.3.6. ƒ Option 2 is the unscrewing of the thread. Specialists call this de-spindling. Here  the mould core, where the thread is incorporated on the front, unscrews from the  plastic part before it is ejected. The difficulty when demoulding the internal thread is that there is no room to  move the slide inwardly with a linear movement such as with external thread. The  question is, how do you release the thread or undercut? The solution is to unscrew  the core from the mould contour using the thread. Previously, all movements that were important for demoulding were linear movements in either the main demoulding direction of the machine or the secondary  demoulding direction through a moving slide. But for unscrewing, a circular or  screwing movement is required. The screwing movement for a core can be produced in different ways. The three  common types of operating a mould core are: the unscrewing unit, the drive via a  gear with gear rack or a gear with a high-helix lead screw.

In Figure 2.22 the plastic part is shown with the mould core which is injection  moulded with the thread.

3.Drive Types for De-spindling

Hydraulic Unscrewing Unit

The hydraulic unscrewing unit is an additional device that either is screwed onto  the injection mould or to the machine. It is connected to the injection moulding  machine and driven and regulated from this. It can either be electrically or hydraulically operated. These unscrewing units are highly flexible in their handling. They  can be used for demoulding threads which are on the fixed half or the moving half  or on the split line face. The unscrewing unit produces the screwing movement  and drives either a small gear or the mould core directly. Unscrewing units can be  obtained in different types and options from the manufacturers of standard mould  components or manufacturers of injection moulding machines.