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Molded holes are preferable to drilled holes because of the strong, denser surface of the molded hole. For straight holes, the taper should not exceed a 1° included angle, and tapered holes should have an 8° included angle.
The weakest sections of many plastic part designs are the joints and assembly points. During screw assembly of mating components, the screw must be tightened with sufficient torque to produce the recommended axial tension load between the host component and the threads of the screw to prevent loosening. A common problem with bolted joints is that plastics are susceptible to creep or stress relaxation. Under loads well below the elastic limit, plastics will lose their ability to maintain a load. When this occurs, the threaded connection becomes loose.
Metal threaded inserts can improve joint strength in plastic parts and are not themselves susceptible to creep. The larger body diameter and body design of the insert allows the appropriate installation torque to be applied to the screw. These joints do not become loose over time since brass provides permanent creep resistance for the entire load path of the thread. Additionally, the inserts enable unlimited assembly and disassembly of the components without compromising the integrity of the threads. Ultimately, it is often the metal insert that lets you replace cast or machined metal components with less expensive plastic without sacrificing performance.
Typical performance requirements for assemblies using inserts involve tensile strength, rotational torque, and pull-through strength. Tensile strength, or pull-out, is the axial force required to pull the insert out of the plastic material. Torque is the rotational force required to rotate the insert in the plastic material. Lastly, pull-through is a combination of rotational torque and tensile force applied opposite the head of the insert.
The following factors affect insert performance:
Start with the performance require-ments of the assembly, and then select the appropriate insert. The objective is to choose an insert with sufficient torque resistance to accommodate the tightening torque necessary to achieve sufficient axial tension load on the threaded joint to keep it together and prevent loosening, while also achieving pull-out values necessary for the load conditions that the insert will be exposed to while in service. Generally, resistance to torque is a function of insert diameter, and resistance to pull-out is a function of insert length.
For more information about thread inserts be sure to give a call. We have a very good inventory of products from Alcoa Fastening Systems ReCoilproducts.