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Fastening With Metal Screws
Metal screwed joints are commonly used to join plastic components.
They provide a high-strength joint capable of withstanding continuous stresses safely even at
relatively high service temperatures. Screwed joints are usually easy to produce and cost-effective.
Self-Tapping Screws
Screws
that create their own threads are widely used with all thermoplastic parts, because they are economical
and require no extra operations or special unscrewing cores. A cutting screw actually removes
material, like a thread-cutting tap, and works with most materials. The forming screw, which displaces
material, must be used with caution, since very high hoop stresses can be developed in high-modulus,
low-creep materials. Some screw designs create the threads through a combination of cutting and
forming. More and more specialty types of screws are being designed to provide better holding power
and lower levels of induced stress, for example screws that are not round but have multiple lobes and
screws with alternating thread heights.
Practical trials with screws from a major US supplier of self-tapping
screws have shown that joints with thread-forming screws can be used in parts made from Ticona engineering
plastics. Our design engineers are able to give recommendations regarding good boss design, proper
assembly torques and achievable pullout forces. Ticona also has data for pullout forces and assembly
torques for one main European screw supplier.
Thread angle, pitch and core geometry of these screws are designed
to conform to the specific meshing conditions in plastics. Since the threaded fasteners are usually
made from metal, the plastic parts being assembled can likely be overstressed well before these fasteners
break or strip. Good design practice dictates that certain precautions be taken to prevent excessive
stresses when bolted assemblies are used.
With all self-tapping screws, certain guidelines
should be followed: - The diameter
of the molded hole in the plastic part must be properly sized for the screw design and grade of plastic
material. An undersized hole can lead to excessive hoop stresses in the boss and eventual fracturing.
- The depth of the molded hole should be sufficient to prevent bottoming
of the screw. A planned clearance is usually recommended, especially for thread-cutting screws.
- The wall section of the boss must be sufficient to resist the
stresses created by the screw. As a general rule, the outside diameter of the boss should be at
least twice the major diameter of the screw.
- Avoid the use of thread-forming screws on glass-reinforced and
other very high modulus thermoplastics.
- To avoid stripping or high-stress assemblies, torque-controlled
drivers should be used on assembly lines.
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