Fastener for thermoplastics

A thread-forming fastener and method of installation thereof are provided for use with a workpiece constructed of a thermoplastic material. The fastener includes a threaded shank having a thread of desired form and a driver head portion. A metallic coating of a predetermined composition is applied to the threaded shank. The composition of the coating is selected for its effective Curie temperature for heating the fastener during application of the fastener to the thermoplastic workpiece. The heated surface enhances plastic flow of the thermoplastic material during thread forming and effectively reduces the required drive torque for thread forming by the fastener while increasing the holding torque. The fastener is preferably also formed with a spaced thread defining an unthreaded surface portion running adjacent to the thread at substantially the root diameter of the thread. At least one radially outwardly extending ramp-like ratchet tooth is formed in at least one part of the unthreaded shank portion oriented for resisting rotation of the fastener relative to the workpiece.

BACKGROUND OF THE INVENTION 
This invention relates to the thread-forming fastener arts and more 
specifically to thread-forming fasteners for use with thermoplastics. 
Thermoplastics are extremely important in manufacturing because of this 
relatively low cost, high degree of formability, corrosion resistance, and 
ease of conversion from raw material to final finished product. 
Thermoplastics are useful in applications including automotive, medical, 
recreational and disposable products. New compositions of thermoplastics 
are constantly being developed including materials which are reinforced 
with glass, carbon or other strengthening fibers. Often it is desirable to 
attach other structures to a body formed of thermoplastic or to attach the 
thermoplastic body itself to another structure. 
When installing thread forming fasteners to thermoplastic materials, 
inherent problems of the thermoplastic materials are encountered, 
including a relatively high drive torque required to drive such fasteners 
combined with a low holding torque upon installation in such materials as 
well as the notch sensitivity of such materials at ambient temperatures. 
The combination of high drive torque and notch sensitivity results in 
thermoplastic materials being highly susceptible to crack formation upon 
the installation of thread forming fasteners. Such cracking is extremely 
prevalent when fastener installation is required in thin wall sections of 
thermoplastic material where material thickness and notch sensitivity 
combined with a high drive torque results in crack propagation. 
Attempts have been made to overcome the cracking problems and/or subsequent 
loosening resulting under high drive torque, low holding torque and notch 
sensitivity. One solution to the problem of crack propogation and thin 
wall sections was to apply various sealants and/or adhesives. However, 
many times an adhesive does not prove sufficiently reliable and may not be 
able to tolerate either tensile or torsional forces. Further, such 
measures require additional materials as well as added time and labor in 
installation. In cost effective automotive or other assembly operations, 
time and labor must be optimized, and therefore such additional steps are 
undesirable. 
Another attempt to overcome the problems of utilizing thread-forming 
fasteners with thermoplastics employed drilling and threading or tapping 
each individual hole into which a fastener was to be applied. However, 
such procedures are extremely expensive due to the additional time and 
tooling required for this additional tapping operation. Further, since the 
thread-tapping operation is very similar to a thread forming fastener 
being driven into a pilot hole, crack propagation and attendant problems 
may still occur. 
Yet another attempt to overcome the problem of fastener attachment to 
thermoplastics was to mold fastener inserts or nuts directly into the 
thermoplastics body. However, this solution is impractical in many 
applications because of assembly requirements which are not conducive to 
preassembly of inserts, or because of the size or type of part to be 
manufactured, or the necessity to specifically locate each fastener, 
depending on the specific part, assembly and application. Further, even if 
the application was conducive to molding inserts into the thermoplastic 
body, such an operation can be overly costly. Molding inserts into 
thermoplastic requires additional set up and worker time. Additionally, 
such operations require special equipment and tooling to accommodate the 
inserts. Such measures may not prove cost effective. 
Finally, even if the fasteners were applied to a drilled and tapped 
thermoplastic body or to inserts molded into the body, a fastener may to 
vibrate loose from the thermoplastic body since these methods may not 
reliably securely retain the fastener within the thermoplastic body. 
Therefore, it is highly desirable to provide a fastener and method for 
applying such a fastener to a thermoplastic body which will not promote 
crack, propagation and preferably will reduce drive torque in applying 
such a fastener to a thermoplastic body and yet increase holding torque 
following installation. Heretofore, it is believed that no prior art 
device or method has resolved the above-noted problems associated with 
thermoplastics. 
OBJECTS AND SUMMARY OF THE INVENTION 
A general object of the present invention is to provide a thread-forming 
fastener and method for installing such a fastener in a thermoplastic body 
which prevents crack propagation, reduces drive torque and increases 
holding torque. 
Yet another object of the present invention is to provide a fastener and 
method for installing such a fastener which results in relieving residual 
stresses in the thermoplastic body when such a fastener is installed. 
Still another object of the present invention is to provide a fastener and 
a method for installing such a fastener which results in securing the 
fastener in a thermoplastic body such that it resists counterrotation once 
it is installed in a thermoplastic body. 
Briefly, and in accordance with the foregoing objects, the present 
invention comprises a thread-forming fastener and a method of installing 
such a fastener in a workpiece constructed of a thermoplastic material. 
The fastener comprises a threaded shank portion having a thread of a 
desired form and a driver head attached thereto. In accordance with one 
embodiment of the invention, the threaded shank portion is coated with a 
coating material which has a preselected Curie temperature and which may 
be heated during the application of the fastener to the workpiece to 
enhance the flow of the thermoplastic material around the fastener while 
the fastener is being installed in the workpiece. The heating of the 
coating through Curie temperature heating reduces the drive torque and 
promotes plastic flow of the thermoplastic around the fastener. However, 
heating of the fastener by other means during driving is also within the 
scope of this embodiment. In another embodiment of the invention, the 
fastener is formed with a spaced thread having one or more ramp-like 
ratchet teeth in the spaces between the thread courses which resist 
counterrotation of the fastener once it is driven into a thermoplastic 
workpiece.

DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENT 
While this invention may be susceptible to embodiment in different forms, 
the drawings illustrate specific embodiments which will be described 
herein in greater detail with the understanding that the present 
disclosure is to be considered an exemplification of the principles of the 
invention and is not intended to limit the invention to that which is 
illustrated. 
Referring now to the drawings wherein like elements are designated by the 
same numerals throughout the figures, a thread-forming fastener 20 is 
shown in FIG. 1. The thread-forming fastener 20 is intended for use with a 
workpiece constructed of thermoplastic material. The fastener 20 is formed 
with a threaded shank portion 22 and a driver head portion 24. A spaced 
apart thread 26 is formed on the shank portion 22. A crest angle 28 of 
substantially on the order of from 30.degree. to 60.degree. is defined by 
the thread 26. An unthreaded surface portion 30 is defined in the area 
between the spaced apart thread 26, (i.e., running adjacent the thread 26) 
at substantially a root diameter 32 (see also, FIG. 2) of the thread 26. 
As shown in the cross-section of FIG. 2 taken along line 2--2 in FIG. 1, a 
metallic coating 34 of a predetermined composition is applied to the 
threaded shank portion 22. The metallic coating 34 is a material of a 
predetermined composition which is selected for a resultant heating effect 
when energy is applied thereto. The thickness of the coating is based on 
the desired heating effect to be achieved and the type of material used. 
The coating 34 can be applied to the thread forming fastener 20 through 
any method which results in a thin surface coating, however, plating is a 
method which is employed in the preferred embodiment. Energy may be 
applied to the surface coating 34 in any suitable form, including but not 
limited to RF, ultrasonic and selected frequency AC induction. 
When a suitable amount and form of energy is applied to the thread-forming 
fastener 20 to obtain a suitably elevated temperature of coating 34, the 
thermoplastic material of the workpiece with which it comes in contact 
will tend to plastically flow around the features of the threaded shank 
portion 22 due to the increased energy level and molecular excitation. 
Since the energy is applied only to the surface coating 34, the entire 
fastener 20 does not act as a heat sink retaining energy. Therefore, once 
the energy applied to the threaded fastener 20 is removed, the plastic 
flow ceases and the thermoplastic material tends to set up very rapidly. 
Dissipation of energy is enhanced since the thermoplastic material 
typically dissipates energy more quickly than the coating material 34 used 
on the thread-forming fastener 20. 
The coating material 34 used in forming the thread-forming fastener 20 is 
specifically adapted for a particular type of thermoplastic material or a 
range of thermoplastic materials, and certain characteristics of the 
thermoplastic material such as hardness. The coating material 34 is 
generally a metallic alloy which is selected to achieve a desired 
energization characteristic which is related to the thermoplastic material 
to be energized. Goals of the present invention include reducing the 
driving torque, increasing the plastic flow of the thermoplastic material 
around the threads 26 and to relieving residual stresses in the workpiece 
once the fastener 20 is inserted therein. The present invention achieves 
these goals and additionally provides a fastener which is securely locked 
into the workpiece until and unless a sufficient removal force is 
deliberately applied thereto. 
The present invention is applicable to those grades of thermoplastics which 
because of chemistry and/or reinforcement are extremely hard, tough and 
non-ductile at ambient temperatures. These factors contribute to cracking 
and checking, because of the notch sensitivity of the thermoplastic 
materials at ambient temperatures. The present invention overcomes these 
problems as described herein. 
As illustrated in FIG. 3 the fastener 20 is positioned so that at least a 
lead-in portion 38 of the fastener 20 is positioned in a pilot hole 40 
being formed of a predetermined diameter 42 in a thermoplastic workpiece 
44. Energy is applied to the fastener 20 either through conduction or 
induction. An energy source 46 as illustrated in FIGS. 3 and 4 is intended 
to diagrammatically represent a range of energies to be applied to the 
fastener 20 and is not limited to alternating current type energy sources. 
The energy source 46 is switched on to apply energy to the fastener 20 to 
cause heating of the surface coating 34. Once the energy source 46 is 
applied, the fastener 20 is rotated to drive the threaded shank portion 22 
into the pilot hole 40 forming threads therein. Since the surface coating 
34 of the fastener 20 is energized by the energy source 46, energy is 
transferred to the thermoplastic material of the workpiece 44 which 
plastically flows around the fastener 20 engaging the threads 26 formed 
thereon. 
The method of applying energy to the fastener 20 includes applying 
electromagnetic energy at a predetermined frequency which is related to 
given properties of the surface coating 34. Energy transfer to the 
fastener causes heating of the surface coating 34 to a predetermined 
temperature resulting in plastic flow of the thermoplastic material in 
which it is inserted. This method of installing the fastener 20 in 
thermoplastic also relieves stresses created in the thermoplastic material 
by the advancing insertion of the fastener 20 therein resulting in 
preventing crack propogation. Preferably, the surface coating 34 is 
comprised of a material having a predetermined effective Curie 
temperature, and the predetermined frequency of the energy applied to the 
fastener 20 is selected to cause the surface coating 34 to heat to its 
effective Curie temperature. 
Once the fastener 20 has been inserted into the thermoplastic workpiece 44 
to a desired depth, the energy source 46 is removed from the fastener 20 
whereupon the plastically flowing material immediately surrounding the 
threads 26 sets up rather quickly due to the rapid dissipation of heat 
from the surface coating 34 through the thermoplastic workpiece 44. 
However, energy source 46 may be removed before the fastener 20 is 
completely inserted with residual heat of the fastener being adequate to 
maintain material flow for complete insertion. The fastener 20 of the 
present invention engaged with a workpiece by the method of the present 
invention is securely retained or embedded in the workpiece 44 and is not 
easily removable therefrom unless and until an energy source 46 is applied 
thereto for removal of the fastener 20 from the workpiece 44. As shown in 
FIG. 4 the plastic material of the workpiece flows into the areas between 
the threads 26. 
As shown in FIG. 5 and with reference to the cross sections in FIGS. 6 and 
9, a fastener 20a is formed with at least one radially outwardly extending 
ramp-like ratchet tooth 48 extending from the unthreaded surface portion 
30. Each such tooth 48 extends generally vertically between adjacent turns 
of the thread 26 formed on the threaded shank portion 22. As shown in 
FIGS. 6 and 9, each tooth 48 has a tooth face 50 oriented on the tooth 48 
for resisting rotation in a direction for removal (as indicated by arrow 
51) of the fastener 20a from the workpiece 44. As a result of formation of 
the teeth 48, underfill areas 52 are formed in thread crest 54, which 
under fills 52 are abutted by a tooth 48 on either side or both sides 
thereof. The thread crest 54 as shown in FIG. 6 follows an undulating 
pattern with the underfills 52 being formed in the crest generally 
adjacent each tooth 48. The underfills 52 help in thread forming and 
relieving stress in the workpiece material by enhancing radial inward flow 
of material. The pressure of the teeth 48 causes inward displacement of 
the heated material forcing it into the underfills 52 and toward the 
unthreaded shank 30 of the fastener 20a thus enhancing the degree of 
contact between the fastener 20a and the workpiece. 
As shown in FIG. 6, an insertion rotation direction is indicated by arrow 
56 and the tooth face 50 is formed on the side of the tooth 48 opposite 
the direction of rotation 56. A tooth face angle 58 is defined by an angle 
relative to a radius 60 extending from a center 62 of the fastener 20a 
intersecting the point on the unthreaded surface portion 30 which the 
tooth face 50 intersects. Each tooth 32 extends from a root radius 64 a 
radial projection distance 66 from between substantially on the order of 
1/5 to substantially on the order of 1/2 of a thread depth 68 of the 
thread 26. 
FIG. 7 shows a fastener 20b similar to the fastener 20a as shown in FIG. 5 
in which the ramp-like ratchet teeth 48b are formed at a slope or spiral 
orientation to a central axis 70b. Similar to the teeth 48 as illustrated 
in the cross section of FIG. 6, underfill areas 52b are formed on the 
thread crest 54b between the teeth 48b. FIG. 8 provides a view of the 
fastener 20b formed with the teeth 48b which has been inserted into a 
workpiece by employing the Curie effect in driving the fastener 20b into 
the workpiece. FIG. 9 provides a cross sectional view of a shank portion 
22 of a fastener which has been formed with five generally equally spaced 
apart teeth 48. 
The dimensions used in forming the thread-forming fasteners 20a, 20b are 
adaptable for a particular type of thermoplastic material or a range of 
thermoplastic materials. For example, the radial projection distance 66 of 
the tooth 48, and the crest angle 28 are chosen depending on the 
characteristics of the thermoplastic material such as hardness. A goal of 
the present invention is to reduce the driving torque, increase plastic 
flow of the thermoplastic material around the threads 26 and the teeth 48 
and to relieve residual stresses in the workpiece once the fastener 20, 
20a, 20b is inserted therein. The present invention achieves this goal and 
additionally provides a fastener 20a, 20b which is securely locked into 
the workpiece until and unless force is deliberately applied thereto for 
removal. The teeth 48 prohibit rotation of the fastener 20 thereby 
resisting removal and prohibit operation of the threads 26 which results 
in increased pull-out resistance of the threads 26. 
While a particular embodiment of the present invention has been shown and 
described in detail herein, it may be obvious to those skilled in the art 
that changes and modifications of the present invention, in its various 
aspects, may be made without departing from the invention in its broader 
aspects, some of which changes and modifications being matters of routine 
engineering or design, and others being apparent after study. As such, the 
scope of the invention should not be limited by the particular embodiment 
and specific construction described herein, but should be defined in the 
appended claims and equivalents thereof. Accordingly, the aim of the 
appended claims is to cover all such changes and modifications as fall 
within the true spirit and scope of the invention.