Molded-in thread

Machine Screws, Bolts. Parts molded of thermoplastic resin are sometimes assembled with machine screws or with bolts, nuts, and washers (Fig. 7.6), especially if it is a very strong plastic. The threads used to engage the mechanical fastener may either be molded into the part (molded-in threads or molded-in inserts) or attached after the molding operation (post-molded inserts). 

When the application involves infrequent disassembly, molded-in threads can be used. Molded-in inserts provide very high-strength assemblies and relatively low imit cost. 

Coarse threads can be molded into most materials. Threads of 32 or finer pitch should be avoided, along with tapered threads, such as pipe threads. These types of threads can cause excessive stress. Other factors that should be considered when designing molded-in threads are ... 

Thermoformed decorative inserts and labels have been molded into cold-press molded parts however, threaded metal inserts cannot be molded-in. Threaded inserts can be glued or tapped in place, but not into bosses, which are impractical. Molded-in holes, core pulls, slides, undercuts, and ribs are, likewise, imfeasible. Holes need to be drilled and their location can be held to 0.030 in. The same assembly methods used for parts made firom compression molding can be used for those made from this process. 

The high modulus and creep resistance of reinforced PPS compounds make them more suitable than many other plastic materials for fastening with bolts, screws, or rivets. For one-time bolted assemblies, tapped or molded-in threads with bolts screwed in three to four diameters may be used. However, with bolted assemblies where repeated assembly and disassembly of parts are expected, or if torque retention at elevated temperature is a concern, ultrasonic or molded-in metal inserts may be used. Molded-in metal inserts should be preheated to the mold temperature before being placed in the tool for molding. For assembly with self-tapping screws, BF-type or BT-type steel thread cutting screws work well, using hole sizes as recommended for phenolformaldehyde thermosets by the... 

Figure 11-65. Holes having molded-in threads or ones that are to receive tapped threads should have molded-in countersinks to avoid chipping and burrs. If a threaded pin is being used to mold threads in a through hole, the lead thread on the mold pin should have a pilot thread for support.

Molded-in inserts represent inserts that are placed in the mold before the plastic resin is injected. The resin provides is then shaped to the part geometry and locks the insert into its body. Molded-in inserts provide very high-strength assemblies and relatively low unit cost. However, molded-in inserts conld increase part cycle time while the inserts are manually placed in the mold. When the application involves infrequent disassembly, molded-in threads can be used successfully. Coarse threads can also be molded into most materials. Threads of 32 or finer pitch shonld be avoided, along with tapered threads, because of excessive stress on the part. If the mating connector is metal, overtorque will result in part failure. 

Along with the possibility to mold-in threaded inserts, threads can be shaped directly inside the plastic material. Specific thread formers made from brass, steel, or aluminum are used to shape either internal or external threads. These have to be manually screwed off to allow the demolding of the plastic products. 

Perhaps you can design a plastic part with molded-in threads, and eliminate the need for machining a metal part, and for the cleanup (and disposal) of the metal chips and the solvents and lubricants used in the machining process. 

Mechanical fastener connections are used when there is the need to disassemble the components. The most common reason for disassembly is for repair of the internal components of a device, which can be electronic, mechanical, or both. There are two main types of mechanical connections for assembling polymer components that will be considered. One type is the threaded inserts and bolts. The other common situation is when screws are threaded directly into the unthreaded polymer for mechanical fastening of another component. Threads can also be molded into the polymer component. Molding in threads increases the cost of the molding operation. 

Internal plastic thread The strength of plastic threads is limited, and when molded in a product involving either an unscrewing device or a rounded shape of thread similar to bottle-cap threads, they can be stripped from the core. Screw threads, when needed, should be of the coarse type and have the outside of the thread rounded so as not to present a sharp V to the plastic that can produce a notch effect. 

With plastic materials, the designer also has a greater choice of bonding techniques than with many other materials. Thermosets must be adhesively bonded or mechanically joined, but most thermoplastics can also be joined by solvent or heat welding. Additionally, plastic parts can be designed for assembly by means of molded-in, snap-fit, press-fit, pop-on, and threaded fasteners so that no additional adhesives, solvents, or special equipment is required. 

Stripping torque n. (1) Of a self-tapping screw, the twisting moment in Nm required to strip the threads formed by the screw in a softer material. (2) Of a molded-in insert, the torque required to break the mechanical bond between the plastic and the insert s knurled surface. 

When two pieces of thermoset plastic must be joined together in service, but must also be suited to simple disassembly, threaded joints are possible. If metal screws are to be used, female threads can be molded in a plastic part to accept the metal screw. Metal female threaded inserts may also be molded into plastic parts to accept metal screws in a subsequent assembly operation. 

The requirement for assembly of castings is limited since casting can make complex shapes. The most common methods are adhesives and fasteners. Self-tapping screws can be used with most of the materials used for casting. Threaded inserts can be molded in place, however they are difficult to salvage in the event of a reject moldment because the material is a thermoset and the inserts cannot be melted out. Threaded inserts can also be glued or tapped in place. 

It is possible to strip undercuts for parts molded of these processes in some cases and threads can be molded-in. However, side actions or split cavities, as would be required for details A and G, are more commonplace. Threaded inserts, like those at E and H, can also be molded-in. However, it may be more advantageous to emplace them after molding with adhesives since the molding cycle will be shorter and the risk of one misplaced insert ruining an entire moldment is ehminated. The other common methods of assembling compression, transfer, and thermoset injection-molded parts are self-tapping screws (holes can be tapped), adhesives, snap-fits, press-fits, and the usual screw and bolt techniques. None of the plastics welding techniques can be used with thermoset parts. 

Holes can be molded-in rotational molded parts by using core pins to which the resin does not adhere. Bosses can be molded by molding a raised cylinder such as that at T. The tip of the cylinder is then cut off to leave an opening. The diameter of this type of hole should be at least 5 times the nominal wall thickness (D > 5W). Threads, both inside and outside, such as the one at S, are readily rotational molded. Recommended tolerances are provided in Table 8.17 (see Fig. 8.37). 

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