It is often desirable to assemble plastic components to each other or to different materials using bolts, screws, or rivets.
Bolts
In most applications where repeated assembly and disassembly is not required, Ryton® PPS moldings with tapped or molded-in threads work very well. For maximum holding power, bolts should be screwed in three to four diameters rather than the two diameters normally recommended for metals. When repeated assembly and disassembly of parts is required, or when torque retention at elevated temperature is a concern, ultrasonic or molded-in metal inserts may be used. Molded-in metal inserts should be pre-heated to the mold temperature before being placed in the tool for molding.
Tapped Threads
For maximum strength using tapped threads, holes should be drilled at least one drill size larger than that recommended in the “Machinery’s Handbook” for steel threads. Tests using various drill sizes demonstrated that a threaded oversize hole required more force to pull the bolt than threads in a regular hole or in an undersize hole. Holes should be drilled slightly deeper than the expected thread depth to provide adequate space to collect cut material, and lead-in holes should be chamfered to prevent cross-threading. In tests where threads were tapped in this way, the pull out force for bolts screwed in to a depth of four bolt diameters equaled or exceeded the strength of brass and mild steel bolts.
Molded-In Threads
Because of the excellent processability of Ryton® PPS, molded-in threads can often be used to avoid the need for secondary machining operations. Molded-in threads should also provide superior performance compared to machined threads, because a smooth, resin-rich molded surface has less tendency of crack propagation than a machined surface.
Self-Tapping Screws
The excellent creep resistance of Ryton® PPS also makes it well suited for assembly with self-tapping screws. Due to the hardness of Ryton® PPS compounds, thread cutting types, rather than thread forming types, perform best. The hole size for BF type and BT type steel thread cutting screws recommended by the “Machinery’s Handbook” for phenolformaldehyde thermosets should be used. The depth of the hole should be slightly deeper than the screw to provide room to hold the cut material.
Rivets
For rivet assembly of Ryton® PPS parts, semi-tubular rivets having a mean hole depth that does not exceed 112% of the mean body diameter are recommended. Optimally, the distance between the hole and the underside of the head should be the same as the combined material thickness. This design will put less stress on the molded parts. Alternatively, hollow rivets may be used, but solid rivets are not recommended. The rivet setter should be adjusted to exert the minimum impact required to clinch the rivet to ensure that minimum stress is placed on the molded parts during the riveting operation. The (Tommel) rivet setting machines that round over the metal on the bottom of the rivet are more successful.
