Undercut insert

Where applicable an undercut insert in the mold can remain as an insert in the finished part. Such inserts can be held in place by the undercut. If possible, they should be made from the same material used in forming the part so there will be no difference in thermal expansion and contraction. 

Different actions in molds occur such as using ejector pins to remove molded parts from their cavities. Side actions of molds may be required to remove parts that have undercuts. Other actions may be required such as unscrewing threaded parts, including inserts, and so on (Chapter 17). 

Compression molding For intricate parts containing undercuts, side draws, small holes, delicate inserts... 

PS can be readily molded with metallic inserts, however the inserts should be pre-heated to 80 to 120° C (176 to 248° F) prior to being placed into the mold in order to avoid molded-in stresses occurring. All metallic inserts should be thoroughly degreased and have some form of undercut feature on their external surface (i.e. in the form of a knurl or groove). 

On the wall opposite the one with the side hole in Fig. 8.9a, there is an indentation on the core side of the wall that does not protrude all the way through the wall of the part. That indent, known as an undercut, would also interfere with ejection of the part from the core. However, when the undercut is shallow, its edges are adequately radiused and the material is sufficiently flexible, an undercut can be stripped from the mold once the cavity has been removed. If the undercut is too deep to be stripped, removable core inserts or a collapsing core will be required. Undercuts can be placed in the cavity as well however, a split cavity, slides, or removable core inserts will be required if the undercut is too deep to be stripped. Cavity undercuts can be deeper than core undercuts by the amoimt of the shrinkage since the part shrinks away from the cavity wall. Removable core inserts add a considerable amount of time to the molding cycle split cavities or slides add substantial cost to the mold and collapsing cores are, generally, expensive to tool when they are feasible at all. 

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. 

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. 

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. 

Holes (A, D) are possible both perpendicular and parallel to the parting line if they are large. Small holes must be drilled or routed. Metal inserts (E, H) can be molded into the part, however bosses F), while possible, can be difficult. Cores are possible to a limited degree as they must be pulled by hand before demolding. Undercuts (G), however, can be performed with slides in the mold or with a split mold. Corrugated sections and ribs (5) are possible. The principal means of assembling RTM parts are fasteners and adhesives. 

Not suitable for intricate parts, undercuts, or dehcate inserts. Close tolerances difficult to achieve. High equipment costs. 

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

Factors—part shrinkage, dimensional tolerances, warpage, drafts and undercuts, location of holes, fillets and radii to promote resin flow and reduce stress concentration, wall thickness, weld and parting lines to improve appearance and avoid areas of high stress concentration, and the introduction of ribs, bosses and studs, and metal inserts to improve strength—are treated. Recommended safety factors as a function of load are given. 

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