Tornillos Autoroscables

Self-Tapping Screws: How To Choose
and Use The Right One
By Charles T. Keller

Self-tapping screws provide an economical means of assembling components,
especially where dissimilar materials
must be joined together. They offer a particular advantage where occasional disassembly may be necessary for maintenance or repairs.

psi) or less can. Included in this category
are most unreinforcedZytel® nylon resins
(the new family of crystalline nylons with
amorphous-like properties being specifically excepted). Use of conventional
thread-forming screws with these resins
produces strong joints.

Thread-forming and thread-cutting are the
two major types of self-tapping screws.
Thread-forming screws deform the plastic
into which they are driven, producing a
permanent thread;thread-cutting screws
physically remove material, in the same
fashion as a machine tap, to form the
thread path.

Figure 1. Types of Thread-Forming Screws

In specifying the self-tapping screw that
will work best, designers will find that a
knowledge of the material’s flexural modulus—a measure of a plastic’s stiffness—is
the best guide to its ability to absorb stress
exerted by varyingscrew designs. Recommendations that follow are based on the
suitability of a screw type or design for
engineering plastics in four stiffness categories, values being determined at 23°C
(73°F) and 50 percent relative humidity.

Thread-Forming Screws
Thread-forming screws are ordinarily less
expensive than thread-cutting screws and
the threads they produce offer the highest
resistance tobackout. Their advantages
are offset to a degree, however, by the fact
that they can cause a high and concentrated hoop stress. While the stiffer plastics
usually cannot absorb this stress, resins
with flex moduli of 1,380 MPa (200,000

Type C

Type BP
Type AB

Type B

Stress generated by thread-forming screws
generally is too great for resins in the second category, those plastics withflex moduli ranging between 1,380 and 2,758 MPa
(200,000 to 400,000 psi), and thread-cutting screws should be employed. Delrin®
acetal resin is an exception, however, as are
those grades of Zytel® based on 612 nylon.
The low coefficients of friction offered by
these two materials literally smooth the
way, permitting threads to be formed with
a minimum of stress.
When using a thread-formingscrew with
material in this stiffness group, however,
consideration should be given to Trilobe™
or Hi-Lo™ screws, specifically designed to
reduce radial pressure.

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Figure 3. Hi-Lo™ Thread-Forming Screw

The Hi-Lo™ is available in both threadforming and thread-cutting types, with the
latter recommended for use on materials
with an even higher flex modulus. The
HiLo™ thread formingfastener has a double lead thread where one thread is high
and the other low. A sharp 30 degree thread
angle allows for a deeper cut into the material and reduces the radial stress that would
be generated by a conventional 60 degree
thread angle form. Its smaller than conventional minor diameter increases the material in contact with the thread. The result is
a stronger fastener with greaterresistance
to pull out.

Thread-Cutting Screws
A third group of resins, with flex moduli in
the 2,758 to 6,895 MPa (400,000 to
1,000,000 psi) range, gain strength—and
stiffness—from glass fiber and/or mineral
reinforcement. Resins in this category
include glass-filled Delrin® 570; those
compositions of Zytel® nylon reinforced
with up to 13 percent (by weight) glass
fibers; and Minlon®mineral reinforced
nylon. Rynite® 430, a toughened, 30 percent glass reinforced member of DuPont’s
family of PET polyester resins, although it

has a slightly higher flex modulus—7,590
MPa (1,100,000 psi)—also fits in this
grouping because of its ability to accept
conventional thread-cutting screws.

2. Molded-in inserts, shaped to minimize
stress, are prepositioned directly in the...