Molded-In Inserts

Molded-in insert If metal inserts are to be molded into a plastic product consideration should be given to wall thickness around the insert (Table 3-18) and their shape. The shape should present no sharp edges to the plastic, since the effect of the edges would be similar to that of a notch. A knurled insert... 

All these type assembly methods usually require the same time to perform as placing inserts in the mold, but they also lower IMM time. There are several other means of accomplishing the desired result that depend on the circumstances at hand. In any event, conventional molded-in inserts usually prove costlier. There are highly automatic injection molding machines available designed just for insert molding that will reverse the cost. 

Molded-in inserts are commonly used with thermosetting materials. However, since the screw-injection process is automatic, it is desirable to use postassembled inserts rather than molded-in inserts because molded-in inserts require that the mold be held open each cycle to place the inserts. A delay in the manual placement disrupts an automatic cyclic operation, affecting both the production rate and the product quality. 

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. 

However, molded-in inserts could increase cycle time while the inserts are manually placed in the mold. 

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. 

Many plastic parts are produced with molded-in inserts such as a screw driver with a plastic handle. Molds for such items are designed... 

Many plastic parts formed in molding operations will have some excess material, called flash, at the parting lines and on molded-in inserts. Gates are also excess resin material resulting from the flow path of the plastic into the mold. It is usually necessary to remove this excess material for cosmetic or functional reasons. 

Figure 1.5 Molded-in inserts. (Ref Hull, J.L., Design and Processing of Plastic Parts , Handbook of Plastics, Elastomers, and Composites, C.A. Harper, ed., McGiaw-HiU, New York, 1996)...

Molded-in inserts are inserts that are placed in the mold before the plastic resin is injected. The resin then flows around the part geometry and locks the insert into its body as it hardens. Molded-in inserts provide very high strength assemblies and relatively low unit cost. However, molded-in inserts could increase part cycle time while the inserts are manually placed in the mold. 

Ultra-Mold, Mold-in insert threaded, Dodge, Emhart Fastening Teknologies... 

Despite the minimal ductdity and capabUity for flexure, snap fits and interference fits are possible with reinforced PPS compoimds, since their high modulus makes it possible to attain adequate holding strength with minimal interference. Snap-fit applications typically involve only onetime assembly, and press-fit components should not induce more than about 0.5% hoop strain. Considerable force is often required for assembly and must be applied with care to prevent part cracking. Molded-in inserts are preferred over press fitting for larger components such as hubs or bearings. 

Undercuts, intricate contours, molded-in inserts, and double-wall construction are routinely included wall thicknesses are uniform. 

A self-tapping screw with boss cap can be torqued 40-100% higher than pressed-in or molded-in inserts. It will also limit thermal expansion of the boss. 

Generally, a part containing molded-in inserts will function properly at normal room conditions. Pretesting parts under anticipated service conditions is recommended to determine their suitability for chemical, high temperature, and thermal cycling environments. 

---