Molded-In Threads

The most common applications for molded-in plastic threads are containers and caps, molded plastic hardware, and fluid handling applications.  Integrally molded threads are often used for detachable fixed joints in applications such as housings, filters, valves, pumps, fittings, and locking rings.  They are also used for movable joints to convert rotary motion into linear motion, or torque into linear forces and vice versa.  Examples of this type can be found in ventilation windows, lift jacks, vending machines, and automotive locks, mirrors, and seats.  Pipe threads are commonly used in pipe joints in plastic plumbing and pneumatic devices.

Several standard thread profiles such as ANSI Unified and Metric ISO threads have relatively large loaded thread angles that produce large tangential (hoop) stresses and radial deflections.  Standard thread profiles such as buttress and trapezoidal threads have small loaded thread angles and are more favorable for plastic threads.  In addition, coarse pitch threads, which have larger thread depths that increase thread stripping shear strength, are also recommended for plastic threads.  In all cases, the roots and crests of the threads should be rounded to reduce stress concentration and to provide increased strength.

Fixed joints:
Threaded fixed joints must be designed to prevent breakage or loosening.  The axial, tangential (hoop), and torsional stresses may break the weakest cross-section of the threaded joint during initial assembly.  Weld lines, which are usually present around molded holes, will further reduce the joint strength.  Additionally, the threads themselves may shear and cause the joint to fail.  Over time, axial creep deformation may loosen the joint.  Likewise, radial creep deformation in the threaded section will reduce the thread shear area and may cause the threads eventually to fail.

It is also important to remember that molded-in plastic threads should not be terminated with a sharp feather edge, but rather with a complete thread in order to reduce the possibliity of cross threading.  Similarly, threads should not be terminated abruptly at the base of the part to form a sharp notch, as this will contribute to increased stress concentration.  Instead, they should be blended with a generous radius.

Movable joints:
Threaded movable joints should permit smooth, jolt-free transmission of motion.  Trapezoidal thread profiles have proved most successful for movable joints.  These threads are designed as either single- or multi-start threads.  Single-start threads are often 'self locking' and have low efficiency.  In addition to the design considerations for molded-in plastic threads in fixed joints, the thread flank surface pressure and rotational speed must be examined in movable joints to ensure the plastic thread flanks do not melt.  Low-wear plastics will improve the performance of threaded movable joints.  Lubrication will also improve performance and should always be used if possible.

Pipe Threads:
Properly designed plastic pipe threads usuallly require only hand tightening to create a good seal.  Assembling a tapered externally threaded pipe into a n internally threaded plastic part is analogous to driving a cone into a round hole.  Many split bosses are the result of improper installation.  The use of straight threads and an O-ring seal can often avoid the need for pipe threads.  

When pipe threads must be used, the following recommendations should be followed whenever possible:

• When mated with metal, make the externally threaded component plastic so it is in compression.
• When torque can be controlled, specify hand-tighten only.
• When torque cannot be controlled, consider using an external or molded-in hoop ring.
• Avoid flats in a plastic threaded part, as this encourages over-tightening with tools.  If necessary, add wings, ribs, or texture for improved grip.

As with all threaded assemblies, long-term testing under the extremes of operating pressure, temperature, and installation procedure is essential to ensure reliability.