Molded-In Holes

Many products, particularly the often very complex near-net or net shape parts (Fig. 6.11-9), which also include simpler gears, self-lubricating or high temperature bearings, bushings, retainer hubs, roller guides, thrust washers and discs, wire guides with molded-in holes, wear strips, and many more, are made on punch-and-die... 

Nails and staples are applied by simply hammering or stapling. Other fasteners may be inserted by drill press, ultrasonics, air or electric gvm, hand tool. Special molding— that is, molded-in-hole—may be required. 

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. 

Posts and bosses can also have a wall thickness at the base equal to the nominal wall thickness, and their height dimension can be permitted to reach 4 times the wall thickness. These processes can make hollow bosses, blind holes, and through holes both parallel and perpendicular to the parting line. However, the knit line side of the hole is likely to have less reinforcement, a condition known as resin rich. It is best to avoid placing molded-in holes within a two diameter distance of the outside edge of the part. If holes are needed in those... 

As a closed-mold process, both surfaces of compression-molded parts can be finished. Draft for long-fiber compression-molded parts should be 3° per side, but less can be used for depths under 6 in—down to 1° per side in extreme cases. Molded-in holes are feasible in the plane of the parting line as are bosses, corrugated sections, molded-in labels, and raised lettering. Molded-in holes within a two-diameter distance from the outside edge of the part will need to be drilled as a secondary operation. The solid material thickness between two holes should not be less than one diameter (the largest). Core pulls, slides, split molds (for external undercuts), and metal inserts are recommended only for the SMC and BMC varieties. 

Mechanical fasteners (staples, screws, molded-in inserts, snap fits and variety of proprietary fasteners) Typical mechanical fasteners are listed at left. Devices are made of metal or plastic. Type selected will depend on how strong the end product must be, appearance fac-tors. Often used to join dissimilar plastics or plastics to nonplastics. Adaptable to many materials low to medium costs can be used for parts that must be disassembled. Some have limited pull-out strength molded-in inserts may result in stresses. Nails and staples are applied by simply hammering or stapling. Other fasteners may be inserted by drill press, ultrasonics, air or electric gun, hand tool. Special molding (i.e., molded-in-hole) may be required. 

Molded-in holes, both blind and through possible, but difficult. 

Bosses, ribs and lettering possible, but at large added cost. Parts with molded-in holes not possible. 

Molded-in holes, bosses, finishes and lettering all possible at added cost and limited accuracy. With rotation speed variation, can build up thicker layers at key points in the mold. 

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