Assembly Considerations
| Pipe Threads |
Pipe threads, common in plumbing for fluid tight applications, are slightly conical and tapered and can place excessive hoop stress on the internal threads of a plastic part. Moreover, they are v-shaped, which forms sharp notches that act as stress concentrators and reduce the ability of the plastic to bear loads and resist impacts. Bayer does not recommend the use of tapered internal pipe threads molded in Bayer thermoplastics.
If you have to mate the plastic and metal pipe threads, design the external thread on the plastic component to avoid hoop stress in plastic. However, to mate with the internal pipe thread in metal, the external thread in plastic will have to conform to the standard v-shaped profile in the metal part. The disadvantage associated with the sharp notch at the root of the thread, will still exist. Hence, even this combination is not ideal and should be avoided if possible.
To achieve fluid-tight seal, a better alternative would be to make both components in plastic, use modified straight threads with rounded roots and crests and use an "O" ring as shown in figure.
An additional caution, if using pipe dopes or thread lockers (plumbers routinely use pipe dopes during assembly), make sure that they are compatible with the plastic resin. Polycarbonate resins, in particular, are susceptible to chemical attack by many of these compounds.
