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 centerline of the thread, Figure 3.17. 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.
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 handloaded
threaded inserts that are removed from the
mold with the part.
Stripped Threads
When threaded parts are to be stripped from the mold,
the thread must be of the roll or round type. The
normal configuration is shown in Figure 3.18 where
R = 0.288 ´ 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. Figures
3.19 and 3.20 show the method of ejection from the
mold.
Thread Profile
The Unified Thread Standard, shown in Figure 3.21,
is best for molded plastic threaded parts as it eliminates
the feathered edge at both the tip and root of the
thread. Other thread profiles such as acme or buttress
can be used with good results.
The Unified Thread Standard is divided into three
categories:
• Class 1A, 1B—Adequate for most threaded nuts
and bolts
• Class 2A, 2B—Offers a tighter fit than Class 1 with
no looseness in the thread
• Class 3A, 3B—Used in precision work and requires
extreme care during the molding operation
Note: “A” refers to external thread, “B” to internal.
Threads finer than 32 pitch are difficult to mold
successfully; where possible, avoid them. Sometimes
a small interference placed between two threaded
parts will prevent loosening under mechanical
vibration.
Parts should be designed so that threads terminate
a minimum of 0.78 mm (1⁄32 in) from the end (see
Figures 3.22 and 3.23). This practice helps reduce
fretting from repeated assembly and disassembly, and
eliminates compound sharp corners at the end of the
thread. It also prevents cross-threading of finer threads
when assembled to a mating metal thread.
Threads—Effect of Creep
When designing threaded assemblies of metal to
plastic, it is preferable to have the metal part external
to the plastic. In other words, the male thread should
be on the plastic part. However, in a metal/plastic
assembly, the large difference in the coefficient of
linear thermal expansion between the metal and
plastic must be carefully considered. Thermal stresses
created because of this difference will result in creep
or stress relaxation of the plastic part after an extended
period of time if the assembly is subject to temperature
fluctuations or if the end use temperature is
elevated. If the plastic part must be external to the
metal, a metal backup sleeve may be needed as shown
in Figure 3.24.
Walsh's Plastic consulting. 