Mechanical Fastening

Mechanical fastening - screws, bolts and rivets - offer one of the least expensive, most reliable and commonly used joining methods for assemblies that must be taken apart a limited number of times. Common fasteners are discussed in this section.

SCREWS, BOLTS AND RIVETS

When using common mechanical methods for securing parts, pay special attention to the fastener’s head. Conical heads, called flat heads, produce undesirable tensile stress in the mating parts and should be avoided (see figure 1). Bolt or screw heads that have a flat underside called pan, or round, heads produce less harmful compressive stress. Use flat washers under both nut and fastener heads because these help distribute the assembly force over larger areas (see figure 2).

Figure 1 - Common head styles of screws and bolts

Figure 2 - Mechanical Fastenings with Bolts, Nuts and Washers

Self-threading or Self-tapping screws:

Self-threading screws classified into two categories for plastic parts - thread cutting or thread forming - are made in accordance with American National Standard ANSI B18.6.4. Various DIN and ISO specifications cover metric self threading screws.

Mechanical fasteners give you detachable connections that are both reliable and cost effective. Driving the proper screw directly into a thermoplastic part results in pullout force levels comparable to those using threaded metal inserts

Thread-cutting screws:

Thread-cutting screws cut away material from the boss inner diameter to form a mating thread. Compared to thread-forming screws, the radial and hoop stress in the boss wall are lower after installation, resulting in better long-term performance. Typically, thread-cutting screws are classified as ANSI BT (Type 25), ANSI (Type 23) and the Hi-Lo * screws with a cutting edge on its tip (see figure 3).

In multiple assembly/disassembly operations, thread-cutting screws must be reinstalled carefully to avoid damaging the previously cut threads. Alternatively, replace Type 23 thread-cutting screws with standard machine screws. Because Type 23 and Hi-Lo* screws have non standard thread pitches, you cannot substitute a standard machine screw for these types.

* Hi-Lo is a trademark of ITW Shakeproof.

Thread-forming screws:

Thread-forming screws do not have a cutting tip. They displace material in the plastic boss to create a mating thread. Because this process generates high levels of radial and hoop stress, avoid using these screws with less compliant materials, such as Makrolon polycarbonate resin or polycarbonate blends, such as Bayblend PC/ABS. As an alternative, use thread-cutting screws for these materials.

Stress caused during installation of thread forming screws can be reduced if sufficient frictional heat is generated in the contact area. Use an installation speed of 300-500 rpm for most screw sizes.

Figure 3 - Thread cutting screws can be used for Bayer thermoplastic resins.

For more information on self threading screws and their availability, contact:

ITW Shakeproof Industrial
Threaded Products
Rockford, IL 61115
(815) 654 1510

Textron Fastening Systems
Rockford, IL 61104
(815) 961 5000

ATF Incorporated
Lincolnwood, IL 60645
(847) 677 1300

Huck Fasteners
Park Forest, IL 60466
(708) 747 1200

Tightening Torque:

The torque required to tighten a screw should be at least 1.2 times the driving torque (Td), but should not exceed one half the maximum, or stripping torque (Ts) (see figure 4). Actual test data determines driving and maximum torques.

Use a thread engagement of at least 2.3 times the screw diameter for self threading screws.

Figure 4 - Suggested Tightening Torque

Self-piercing /self-drilling screws:

Generally, self-piercing or self-drilling screws that do not need a pilot hole, or screws that are force-fit into a receiving hole, should not be used with parts made of Bayer thermoplastics — these screws produce high hoop stresses.

Thread Lockers:

Generally, thread lockers can be chemically aggressive to plastics. If you are using a thread locking liquid to secure metal fasteners, fully test the liquid for chemical compatibility with the thermoplastic material before production use.
Request a copy of chemical compatibility resins from Bayer MaterialScience for further information.

RIVETS

Rivets provide a low cost, simple installation process that can be easily automated. Use them to join thin sections of plastic to sheet metal or plastic to fabric. To minimize stress, use rivets with large heads - three times the shank diameter is suggested - and washers under the flared end. Never use countersunk rivets (see figure 5). Calibrate the rivet setting tools to the correct length to minimize compressive stress and shear in the joint area.

Figure 5 - Four standard rivet heads for use with Bayer thermoplastic resins

Spring-steel fasteners:

Self-locking steel fasteners and push-on spring steel fasteners, such as Tinnerman* nuts offer another option for assemblies subjected to light loads. Usually pushed over a molded stud, these fasteners are frequently used in applications such as circuit boards. The plastic stud should have a minimum 0.015 inch (0.38 mm) radius at its base.

* Tinnerman is a trademark of Tran Technology Engineered Components, LLC, Brunswick, OH 44212 (330 220 5100)

Slotted tubular spring pins and spiral-wrapped (roll) pins:

Slotted tubular pins and spiral-wrapped pins (see figure 6) are typically used in shear-loading applications. Pressed into preformed holes with an arbor press or drill/hammer machine, these pins can cause high hoop stress similar to those in press fits. This may result in part crazing or cracking in some plastics.

Figure 6

JOINING DISSIMILAR MATERIALS

In a typical large plastic and metal assembly where movement is restricted, high compressive or tensile stresses can develop. Figure 1 shows a large plastic part fastened to a metal base or bracket. As the ambient temperature rises, the plastic will expand more than the metal because the plastic has a higher coefficient of linear thermal expansion. In this example, the plastic’s expansion coefficient is four to six times higher.

Figure 1 - Restricted fabrication technique is not recommended

Because the plastic part expands more, it develops a strain-induced compressive stress. An equal tensile stress develops in the metal part. In most cases, these stresses are more harmful for the plastic part than the metal part. An approximation for thermally induced stress in the plastic is:

σ_T = (α_m - α_p) · E_p · ΔT

Where:

α_m = Coefficient of linear thermal expansion of the metal

α_p = Coefficient of linear thermal expansion of plastic resin

E_p = Young’s modulus of elasticity for the plastic resin

ΔT = Change in temperature

(When performing these calculations, a consistent system of units is essential. Use the temperature units specified in “α “)

Typically, as the temperature rises, the stiffness of the plastic part decreases. With much higher temperatures, the plastic part will eventually buckle. The opposite occurs when the temperature decreases. The plastic part shrinks, developing strain-induced tensile stress. With much lower temperatures, stiffness increases even more, and the strain-induced stress approaches critical levels, leading to part failure.

To avoid these problems, use slotted screw holes in the plastic part for temperature sensitive designs. Figures 2A and 2B illustrate this concept. As shown in these figures, the slotted holes allow differential thermal expansion and contraction of the assembly’s plastic and metal parts.

When joining plastic and metal parts, tightening torque for the inserted screw has important implications. Do not tighten fasteners to the point where joint friction and compressive loads prevent relative movement. If the fasteners are too tight, the effect of the slotted holes will be negated, leading to possible part failure.

Figure 2A - Joining Dissimilar Materials

Figure 2B - Slotted holes would allow for relative movement in assemblies of dissimilar materials

Other factors to consider when joining plastic and metal parts include:

The span between mounting points
The magnitude of the temperature range; and
The relative thermal expansion coefficient of the material used in the assembly.

Consult the Bayer MaterialScience data sheet for the specific grade you’re using if it does not appear in Table 1. You can find Bayer data sheets in the Resources section of this web site.

Table 1 - Coefficient of Linear Thermal Expansion (CLTE)
Values for common materials

Material	CLTE (10^-5 in/in/°F)
Bayblend T-85	4.0
Bayblend 3030	4.1
Bayblend ET1000	3.9
Makrolon PC (Most)	3.9
Aluminum	1.3
Brass	0.95
Magnesium Alloys	1.5
Steel	0.8
Wood (W/Grain)	0.36
Wood (Acc/Grain)	2.9
Zinc Alloys	1.5
Glass	0.5

Worked Example:

Assume that an assembly made of Bayblend ET1000 resin and an attached aluminum stiffener will be exposed to a temperature range of -20 to 120°F. The outboard assembly fasteners are 48 inches apart and the part was assembled in an ambient temperature of 70°F. To determine the change in length, start with the basic formula:

ΔL = α · L · ΔT

Then substitute the difference of coefficients for α in the formula

α - for Aluminum is 1.3 x 10^-5 in/in/°F

α - for Bayblend ET1000 is 3.9 x 10^-5 in/in/°F

ΔL = (α_plastic - α_metal) · ΔT· L

ΔL = (3.9 x 10^-5 - 1.3 x 10^-5) x (120 - (-20)) x 48

ΔL = 0.175 inch

The total difference in thermal expansion is 0.175 inch. Because we have two assembly points, the movement between fasteners is 0.175/2 or 0.0875 inch.

In this example, you would have to plan for a range of movement of 0.0875 inch at each fastening site. You must allow for this expansion in your design to prevent stresses that could jeopardize the assembly, which can be estimated using the following formula: