Assembly Considerations
| Molded Threads |
The molding process can create threads in the part as it is being molded, thereby avoiding the expense of secondary, thread-cutting operation. Injection molded threads tend to be stronger than tapped threads. They can usually be produced with tighter tolerances and have a smoother and denser molded skin than that left by the thread cutting operation.
External threads centered on the mold parting line add little to the mold cost as they can be molded in both mold halves. However, because both halves of the mold are involved, they are prone to parting-line flash or mismatch. Threads that do not lie on the parting line require slides or side actions, which add to the mold costs.
Internal threads, which require unscrewing devices such as collapsible or unscrewing cores, significantly increase mold cost and complexity. To reduce unscrewing time, keep the length of the thread as short as possible. Sometimes multi-start threads are deployed to help reduce the unscrewing time. Unscrewing at a separate station usually involves two sets of cores and special molding equipment, which also increases cost.
Occasionally, shallow threads in flexible materials can be stripped off the core while they are still hot and soft. This option usually requires generous rounded threads with a thread diameter to wall-thickness ratio greater than 20 to 1.
Some alternative thread designs, as shown in figure, offer good functionality as well as ease of molding.
