Our mold designers often see threads on a plastic part, they are called internal threads and external threads,and mold designer absolutely know cores for internal threads and sleeves for external threads,in order to get high quality threads and to be molded economically and in large quantities,unscrewing device will be used inside a mold. Threads are critical in Injection Molding. Not only do they require more complexity in the tool design, the resulting plastic part must work perfectly, with next to no warpage and align perfectly when assembled. It’s important that you get it right the first time, so in this article we’ll explore what you need to know about threads in Injection Moulding.

There are two options for creating threads in your part. You can either overmold a metal threaded insert or you can use a rotating core to form the thread in the mould. The overmolded insert provides slightly better strength and better alignment, however it costs more as it is overmolded and requires an Injection Moulding operator at all times. Molded Threads are cheaper and do not require supervision, but the still need a rotating core to properly form the thread. This means an increase in cycle time and. Most moulded threads produce undercuts, though this can be avoided if the thread is located on the parting line of the mold. If you do decide to move the parting line of the product to include the thread, be extra cautious about molding it as it may create flashing.

In general, try use threads that have radiused crests and roots, a thicker cross section and wider spacing between threads. This gives good engagement to the plastic part, which is more likely to break or fail rather than the screw or bolt you intend to use. The Unified Thread Standard is commonly used in the design of plastic threads, however acme or buttress thread profiles also produce good results. Extremely fine pitched threads are difficult to mold, if you need a finely pitched thread, the overmoulded thread insert is your best option. It’s best practice to leave 0.8mm clearance at the very end and beginning of threads. Also if you are connecting plastic to metal, it’s best to have the plastic as a male thread (external), and metal as the female thread (internal). When required, external and internal threads can be automatically molded into the part, eliminating the need for mechanical thread-forming operations.
External Threads
Parts with external threads can be molded in two ways. The least expensive way is to locate the parting line on the centreline of the thread. It should be considered however that it is generally not possible to avoid an undercut in the parting line. This should lead to deformation of the thread on ejection. If this is not acceptable, or the axis of the thread is in the direction of mold-opening, the alternative is to equip the mold with an external, thread-unscrewing device. Moulding external threads without side core
Internal Threads
Internal threads are molded in parts by using automatic unscrewing devices or collapsible cores to produce partial threads. A third method is to use hand-loaded threaded inserts that are removed from the mold with the part.
Stripped Threads
When threaded parts are to be stripped from the mould,
the thread must be of the roll or round type. The normal
configuration is R = 0,33 pitch. Requirements for thread stripping are similar to those for undercuts. Threaded parts with a ratio of diameter to wall thickness greater than 20 to 1 should be able to be stripped from a mold.